for TWEED RIVER MANAGEMENT PLAN ADVISORY COMMITTEE
November 1998
1.1 GENERAL
The Tweed River is an important natural resource of the far north coast of New South Wales. Since European settlement of the region in the mid to late 19th century, the river has undergone a series of natural morphological changes and human-made morphological changes, particularly in the lower estuary region.
Bank erosion is presently occurring throughout the tidal reaches of the Tweed Estuary, which result from both natural processes and human-induced factors. At several sites along the estuary, current riverbank erosion threatens private land and public property. Such erosion has been identified by Council as a major environmental and social concern.
In 1996, Patterson Britton and Partners (PBP) completed a Management Plan for the Upper Tweed Estuary, which addressed management issues in the river from Barneys Point to Murwillumbah1. One of the primary management issues identified by the Plan was continued erosion of the river’s banks and foreshores. As part of the estuary management process, PBP carried out a riverbank assessment for the Upper Tweed River and summarised the findings in a Management Plan reference document (PBP, 1995)2. The assessment included identifying areas of bank erosion and accretion through air photo interpretation, historical hydro-survey comparisons, riverbank inspections, and underwater diving inspections. From this information, the morphological processes of the river were assessed in the context of the Upper Tweed River Management Plan. This riverbank investigation expanded on a previous, preliminary riverbank assessment, which was carried out by WBM Oceanics3 in 1992.
In the context of holistic management of the Tweed Estuary waterway, Tweed Shire Council, through the Tweed River Management Plan Advisory Committee (TRMPAC), commissioned Patterson Britton and Partners (PBP) to carry out a Bank Management Study and prepare a formal Management Plan to address the issues of existing and on-going bank erosion and morphological changes of the Tweed Estuary, including the Rous River up to Kynnumboon, Terranora Inlet, Terranora Creek, and the entrance to Cobaki Broadwater.
1.2 BROAD GEOMORPHOLOGICAL SETTING
A review of evolutionary models of late Quaternary evolution of the coastal catchments and rivers in NSW shows that they can be characterised as sediment-starved systems occupying ancient bedrock valleys excavated by fluvial processes over millions of years. A relatively stable tectonic setting and low rates of terrestrial denudation have delivered only modest volumes of sediment from the catchments throughout the Quaternary, and today many coastal lowlands are infilled with marine and estuarine deposits.
The geomorphic and stratigraphic character of the non-tidal catchment reaches appear to be influenced by climatically-induced changes in river discharge interacting with factors such as inherited bedrock valley configuration, antecedent floodplain textures and low rates of sediment supply. Within the tidal reaches, geomorphic and stratigraphic relationships have been profoundly influenced by late Quaternary sea level fluctuations. Bedrock valleys excavated to base levels well below present sea level became local sinks or traps for marine, estuarine and fluvial sediments during periods of high sea level. Stratigraphic and geomorphologic investigations have shown the coastal lowlands to be infilled to varying degrees by estuarine muds and fluvio-deltaic sediments which accumulated during the mid-late Holocene in protected estuarine basins landward of barrier and tidal inlet deposits.
Druery and Curedale4 mapped the sediments of the lower Tweed floodplain and showed that the Tweed River flows through a dual barrier system of marine sands. A Pleistocene Inner Barrier is located between Chinderah and the eastern shoreline of Terranora and Cobaki Broadwaters. The Holocene Outer Barrier constitutes the active beaches, dune ridges and marine delta sand flats of the present day shoreline.
Behind this dual barrier system, the Tweed River has infilled its palaeo estuarine basin and is prograding across the dual barrier as a low gradient, leveed channel and floodplain. The floodplain deposits constitute a thin mantle of alluvial deposits on top of estuarine clays and muds, ie estuarine basin infill.
Tidal water bodies such as Cobaki, Terranora and Tweed Broadwaters are relicts of the palaeo tidal basin. Because the Tweed palaeo tidal basin is essentially infilled, coarse grained sediments are now accumulating within the lower reaches of the river. Old estuarine and deltaic deposits are being exposed as the channel cuts into the underlying fluvio-deltaic deposits. From a geomorphological perspective, fluvial reworking of the basin deposits is predicted to increase over time as the volume of coarse grained sediment to be accumulated within the lowland plain grows.
The extension of fluvial increase across the lowland plains has the potential to alter patterns of channel migration and floodplain sedimentation with clear implications for erosion hazard management on systems like the Tweed, ie as fluvial processes build-up the Tweed floodplain gradually over time, bend migration and thalweg adjustment can be expected to increase bank erosion. Whilst the timeframe involved is likely to be great, the impact is likely to be manifest suddenly by severe and extreme floods.
1.3 EUROPEAN SETTLEMENT: IMPACT ON TWEED RIVER BANK EROSION
The Tweed River has undergone a number of changes since European settlement some 150 years ago. These changes are described below and are summarised in the following schematic:
Figure 1.1 History of Changes to Tweed River
As described by the first European explorers, the Tweed Valley was covered in "the big scrub".
"…the banks are generally very high on their foundations, covered with thick forest – giant figs, red cedars, Moreton pines and palms and eucalypts of various kinds, in many places impenetrable for the thick foliage of the native vines"
– Henry John Rouse observations of the Tweed River in 1828.
This would have been a virtually "wall-to-wall" forest of dense vegetation including palms, casuarinas, mangroves, and other indigenous species. Only small remnants of "the big scrub" are present today – one such example is Stott’s Island, which has been preserved as a nature reserve.
With European settlement, the lower sections of the valley were cleared of all vegetation to make way for small farms. Apart from a few urban centres, small farms still dominate the valley floor, however, horticultural practises have progressed to predominantly sugar cane farming.
Prior to clearing of the valley floor and foothills, the sediment yield from the catchment would have been relatively low. The river channel would have been tuned to the low sediment supply, ie the channel would have been in a state of "dynamic equilibrium". That is, the river morphology was approximately stable based on the catchment runoff flows and sediment loads. Subsequent deforestation of the catchment has resulted in altered catchment runoff flows and sediment loads. In addition to this, the physical characteristics of the river have been altered by dredging in both the upper and lower reaches, and training of riverbanks in the lower estuary by construction of rock training walls. The river is presently responding to these changes by way of new accretion and erosion patterns. The significance of catchment clearing on river channel morphology is described in the following figures.
Rain running off a catchment carries sediment into the network of tributaries and channels that feed the main channel of a river system. Rivers expend considerable energy as they erode, move and deposit the sediment.
When the upstream sections of the catchment are undisturbed and heavily forested, there is a relatively low sediment yield to the river. In these conditions fine sediments (eg. sand) are in scarce supply and gravel pools and riffles are common. The banks of the river are relatively stable.
The bed slopes of NSW coastal rivers tend to be steep in the upper reaches, yet nearly flat as they flow across the coastal floodplains to the sea. Due to these relatively flat gradients, the coastal portion of the rivers has a much lower capacity to move sediment. In this natural state the channels tend to have a broad, meandering coarse.
Once the vegetation is cleared from the upper catchment, the whole system is dramatically changed. Enormous amounts of sediment are released into the river channel. Rainwater runs off the catchment quickly and flows in the river channel are increased. The banks erode and collapse, adding more sediment.
The sediment is moved downstream as long as the river channel has the capacity to carry it. When flood flows reach the coastal floodplain, sediment movement decreases and sediment is deposited in the channel as pronounced sand bars and on the floodplain as alluvial deposits. Over aeons this sediment builds up the floodplain and increases the river gradient. Eventually, the river discharges sand into the sea.
The river is also subject to relatively heavy boating traffic, particularly in the lower estuarine reaches. Boats affect the river by increasing the occurrence of short period waves (similar to wind generated waves), which can cause erosion of the riverbanks at the waterline. Areas of the river which are subject to significant boating traffic, such as Terranora Creek and around Tumbulgum, or areas which have banks that are particularly susceptible to wave attack, such as the Rous River, have experienced significant fretting and undercutting of the riverbank.
Five different methods were used to assess the morphological changes in the Tweed River over the last 100 years. These methods were:
- Photogrammetric analysis
- Comparison of aerial photographs and comparison with the original Tweed River survey
- Comparison of hydrographic surveys at defined cross-sections
- Visual inspection of the river from a boat
- Underwater diving inspections of the riverbanks
Results from these methods of analysis were incorporated into the final identification of accretion and erosion in the Tweed River estuary, and provided information such as rates of change, and mechanisms of bank failure.
These methods are described in more detail below.
1.4.1 Photogrammetric Analysis
A photogrammetric analysis of the Tweed River was carried out by the Department of Land and Water Conservation (DLWC) based on aerial photographs taken in 1962, 1971, 1985 and 1994. The analysis mapped riverbanks for each of the years investigated, and overlayed results, identifying areas where riverbanks have moved during this period. The results of this photogrammetric analysis are presented in Appendix A. Because of difficulties associated with dense vegetation in places, not all the riverbank could be analysed, and in some cases, the results of the photogrammetry had to be confirmed by visual bank inspections.
The photogrammetric analysis provided a valuable insight into current bank movement rates, which in turn, provided an insight into the relative priorities of the bank management measures.
1.4.2 Air Photo Comparisons and Comparison with Original Hydrosurvey
In addition to the photogrammetric analysis, comparisons of air photos were carried out to assess the magnitude of riverbank erosion and accretion in the Tweed River estuary. In particular, historical air photos, prior to 1962, were compared to more recent photos to provide an insight into the long term changes in the riverbanks.
A comparison was also made between recent aerial photographs of the Tweed River and the original hydrographic survey of the estuary complied by the Royal Navy in 1884. The survey was reduced to the same scale as the aerial photographs and then overlaid. The shorelines from the two records were then compared, and the differences noted. The results of the assessment are presented in PBP2.
The Royal Navy first surveyed the Tweed River in 1884 under the supervision of Commander Howard. This original survey mapped the existing shoreline at the time, and sounded the entire estuary, including the shallow bays and broadwaters. This survey provides an excellent picture of what the river was like before extensive urban development of the catchment. By comparing this original survey with recent aerial photographs, an assessment of the movement in shorelines was able to be made.
Contemporary hydrographic surveys are generally carried out by depth sounding, and the exact location of the shoreline tends to be disregarded. Changes in the location of the shoreline are therefore not generally picked-up by contemporary hydrographic surveys. The river foreshore as defined by the 1884 survey replicates the existing foreshore as defined by recent aerial photographs with remarkable accuracy. This allowed minor differences in the two foreshores to be identified easily, and noted as areas of active riverbank change.
1.4.3 Cross-Section Comparisons
Since the original hydrosurvey in 1884, the Tweed River has been surveyed many times, usually involving cross-sections at the same locations. Because the survey lines have been consistent for most of the surveys, a comparison of these sections could be made, however, care was required during interpretation of the results, as some surveys have different datums and methods for measuring water depth. Comparisons were made between cross-sections surveyed in 1884, 1954, 1974, 1979, 1990 and 1992. WBM3 prepared cross-section comparisons of the surveys 1954 - 1992. The 1884 survey data was added to these by PBP2. The results of the cross-section comparisons are presented in Appendix A.
Areas of channel erosion and shoaling were identified. Where progressive changes of the two could be linked in any one section, this was interpreted as an indication of general river bend meandering. In some locations this could be discerned in relatively straight sections of the river, implying a natural meander propensity of the channel thalweg.
The 1884 hydrosurvey soundings were measured in feet, which were reduced to Low Water Ordinary Springs (LWOS). A conversion to metres below Australian Height Datum (AHD) was necessary to compare this survey with the later survey documented by Oceanics. For the purpose of converting to AHD, a correction of -0.4 metres was applied to all the soundings.
Visual inspections of all the riverbank of the Tweed River estuary were carried out, focussing mainly on the impacts on the banks between water level and the top of bank. Photographic records were taken of most of the riverbanks, including those sites affected by erosion.
1.4.5 Underwater Diving Inspections
Underwater inspections were carried out at a number of sites. The geological classification of the natural bed material was described and if any bank protection material was present, its general condition, coverage and competency was described. Notes from the underwater diving inspections are provided in Appendix A.
The underwater inspections provided a unique insight into the role of riverbank stratigraphy at various local areas of bank erosion, and also invaluable insight into the root causes of riverbank erosion.
1.5 OTHER TRMPAC MANAGEMENT PLANS
TRMPAC has taken an initiative to develop a number of Management Plans addressing the long term management of the Tweed River waterway. These Plans work together to provide a total management framework for the future sustainability of the estuary and considers the likely future demands placed on the waterway.
Other TRMPAC Management Plans which address the long term sustainability of the Tweed River estuary are:
- Upper Tweed River Estuary Management Plan
- Lower Tweed River Estuary Management Plan
- Terranora Broadwater Management Plan
- Cobaki Broadwater Management Plan
- Ukerebagh Passage Management Plan
- Tweed Estuary Water Quality Management Plan (in prep.)
2 FAILURE MECHANISMS AND REMEDIATION OPTIONS
2.1 CONTRIBUTING FACTORS TO BANK FAILURE
Since the original survey in 1884, the Tweed River has displayed gradual geomorphic development, manifest as continued infilling of Tweed Broadwater and Chinderah Bay, and thalweg meandering at some bends, particularly in the downstream reaches.
The sedimentology of the river also gives an indication of the present state of its development. Between Bray Park Weir and Murwillumbah, the river is actively accreting with coarse sands and gravels. Town reach, adjacent to Murwillumbah, has prograded substantially this century and extensive gravelly sand bars are exposed at low tide. This is symptomatic of the sediment supply from the upper catchment of the river. The sand is transported further downstream and before extensive river dredging was carried out, it was responsible for the gradual emergence of point bars and mid reach shoals (ie "crossings") between Murwillumbah and the general area of Dodd’s Island.
As the fluvial sand is carried downstream, it gets finer in grain size. Fine fluvial sediments extend downstream to approximately Dodd’s Island. Before extensive sand extraction, the river above this location was closer to a state of equilibrium than the river below. The dredged areas of the upper river have created a bedload trap which will slowly fill with fluvial sands and silts, and the process will take many decades to return to the pre-dredging conditions. The river below Dodd’s Island is still actively moving towards a more stable state of existence. The result is localised pockets of bank recession and accretion, and riverbed shoaling where the river channel is much wider than the widths upstream of Dodd’s Island.
Factors contributing to the river bank erosion on the Tweed River estuary include:
· altered flow patterns due to natural changes and human activities;
· tidal currents;
· flood velocities;
· wind and boat waves; and
· saturated bank soils.
As outlined in Section 2.1, the Tweed River has not yet reached a mature state of river geomorphology, and as such, is still undergoing natural geomorphic change. For example, as sediment is washed off the catchment, point bar and mid-channel shoals can become established. This accretion within the channel can redirect natural river flows closer to river banks and / or increase river flood velocities (refer Section 2.1.3), resulting in erosion and recession of the banks.
However, in addition to the natural processes, a number of human-induced factors have influenced the flow patterns of the Tweed River, which may have impacted on the natural erosion rates of the river banks. Significant clearing of the catchment associated with European settlement, followed by rural and then urban development, would have released large (and unnatural) quantities of sediment to the river. The additional sediment load would have compounded the natural shoaling in the river, and possibly compounded the related river bank erosion. As the catchment is now significantly different from its natural state, the river morphology would be moving towards a state of equilibrium which is related more to its present catchment condition.
Flow patterns within the river would also have been altered by humans through dredging and construction of flow training walls. Although not specifically investigated, it is possible that these human activities may have caused, or at least accelerated, river bank erosion at some sites.
During the clearing of the catchment, natural riparian vegetation would also have been removed. Riparian vegetation plays a significant role in the overall stability of a river bank5,6. The removal of such vegetation may have destabilised some of the river banks, and hence may also have caused, or at least accelerated, river bank erosion at some sites.
Tidal currents, or velocities, are a function of the river cross-section and the upstream tidal prism volume (ie, volume between high and low tide). In the lower reaches of the estuary, the upstream tidal prism is larger, which results in higher tidal currents. Previous studies7 have shown that cross-section averaged tidal velocities are in the order of 0.6 m/s in the lower estuary. Given that local effects could increase these velocities by twofold (eg on the outside of bends), tidal currents could be a contributing factor to bank erosion in the lower estuary. Although the magnitude of tidal currents would not be as high as flood velocities (refer Section 2.1.3), tidal effects would impact on the river for several hours per day, as opposed to significant flood effects, which last for a couple of days every year or so.
In the upper reaches of the estuary, cross-section averaged velocities are lower (in the order of 0.1 m/s 7). As such, tidal currents are unlikely to have a significant contribution to bank erosion in the upper estuary.
During flood conditions, water is discharged down the river at high velocities. These velocities are further increased on the outside of bends as the river tries to conserve momentum. As a result, shear stress on the outside of bends is often sufficient to erode the underwater bank material and destabilise the slope.
The erodability and stability of the underwater slope is dependent on the characteristics of the bank material. For example, erosion of indurated sands, which typify the lower reaches of the estuary, pose a more serious hazard than erosion of the estuarine muds of the upper estuary, because the sand is more erodable, and hence, erosion rates are higher (refer Section 3.2 for examples of sites and erosion rates).
It is also possible that changes to the catchment associated with rural and urban development (refer Section 2.1.1) have increased the flood discharge rate of the river. This increased discharge may result in increased flood velocities, which may further compound the problems associated with bank erosion along the river.
Erosion of river banks due to wind waves is largely dependent on the exposure of the site to significant wind fetches. Although the wind climate has not changed, the removal of riparian vegetation may have impacted on the wind fetches, thus exposing the banks to higher wind generated waves.
Waves generated from boats (ie boat wake) are similar to waves generated by winds (similar wave heights and periods). However, boat wake waves impact on all the river bank over which the vessel is travelling, including those sites which are both exposed and protected from wind generated waves. Hence, sites which are vulnerable to wave action, but protected from the wind, may start to erode due to boating activities, while erosion at sites which are already affected by wind waves may be compounded by boat wake. Banks in the vicinity areas which are heavily trafficked, such as Terranora Inlet and around the main boat ramps would be particularly at risk of boat wake erosion.
Erosion of river banks can be exacerbated by large differences in level between the groundwater and the free river surface level. Larger water level differences (or heads) result in larger groundwater pressures, which can subsequently dislodge bank material through seepage processes.
The greatest difference in ground and water surface levels, and hence the period when the banks are most susceptible to seepage failure, would occur immediately following a flood event, when the ground would be saturated (ie high groundwater level) and the river level has dropped to normal tidal levels.
Floodplain and riparian vegetation also has a significant impact on groundwater levels. Surrounding vegetation uptakes and lowers groundwater levels via the process of evapotranspiration. Hence, removal of floodplain and riparian vegetation could exacerbate the susceptibility of river banks to seepage failure.
2.2 MECHANISMS OF BANK FAILURE
The contributing bank erosion factors (Section 2.1) result in various mechanisms of erosion including:
· surface scour of bank material;
· toe scour and subsequent oversteepening and collapse through bank failure;
· loss of internal strength through excessive pore pressure; and
· wave induced erosion at the waterline, causing undercutting and bank collapse.
These mechanisms are described in more detail below:
2.2.1 Surface Scour of Bank Material
Surface scour is the erosion of material from the bank surface resulting from non-river based activities. A typical example of this would be the formation of a stock path, and the subsequent soil erosion caused by gullying of this devegetated path. Stock with hard hoofs are particularly responsible for surface scour, especially when riverbanks are steep, or the bank material is friable.
2.2.2 Toe Scour of Bank and Subsequent Oversteepening
Banks with vertical faces or banks with little or no vegetation and often with overhanging tree roots, are the most common visible symptom of river bank erosion. Although they appear dramatic, they are not necessarily evidence of rapid and immediate bank recession. Lengthy sections of unstable banks are indicative of extensive damage resulting from a significant flood event and/or a regime shift in a reach.
Oversteep banks offer a less dramatic, but often similar symptom to the vertical escarpment. They reflect a more gradual rather than sudden change as the toe is scoured or the bed recedes. Oversteep and undercut banks tend to evolve into vertical escarpments or scalloped banks as they become unstable and fail.
Figure 2.1 Bank Erosion due to Toe Failure
2.2.3 Excessive Pore Water Pressure
When riverbanks are high, the seepage of groundwater may cause localised weakening of the bank structure, depending of the geotechnical characteristics of the bank. Excessive pore water pressure associated with surface runoff seeping through the bank may dislodge bank material, resulting in a "piping failure" of the bank.
These symptoms are caused by wind and boat wave action which is only applied to a narrow interval of the bank above and below the waterline. A bench is created through bank failure after undercutting and washing away of talus.
Figure 2.2 Bank Erosion due to Wave Action
Bank protection works or hazard management measures that are installed solely in response to symptoms, rather than addressing underlying causes, may have a limited life or be ineffective or worse still, lead to new problem areas downstream. It is therefore crucial that the causes of any erosion problem be understood and addressed, especially in the case where the stream channel is continually adapting and reacting to fluvial energy.
A management plan can adopt a number of approaches varying from structural protection through to non-structural adaptive land use planning controls, viz:
- Construction of protective works
- hard structural options
- soft structural options
- combined hard & soft options
- Non-structural options
- improved catchment management
- adaptive hazard management
The appropriateness of a particular management measure depends to a large extent on the level of existing development likely to be affected and the financial resources of the community. For instance, a threatened area characterised by intense urban use and substantial infrastructure may require heavy bank protection, whereas rural areas may approach the issue with a much larger range of management options. Whatever the case, the plan should be flexible and allow for conditions to be varied subject to monitoring of the hazard.
Hard structural alternatives are primarily applied to banks requiring significant stabilisation and erosion protection. A physical barrier is constructed on the riverbank to absorb the erosive energies of the river and physically hold back the toe or slope of the bank.
If properly designed, many of the "hard" structures catalogued below can potentially eliminate erosion problems along the protected area. However, they can also alter the hydrodynamics of the river and thus modify patterns of erosion and accretion along its banks. This is especially true immediately downstream of the structure, where increased flow and sediment load due to the works can exacerbate already present erosion problems or in fact create new ones.
Also from a design perspective, structural protection must be able to cater for the application of surcharge loads and the dissipation of pore pressure, as well as protecting the bankline from surface and toe erosion. Similarly, erosion at the ends of structural protection (outflanking) is a common failure mode and must be guarded against.
Besides downstream problems, the potential negatives in an area proposed for hard structure stabilisation are numerous, including structure failure due to unexpected toe scour, loss of aesthetics, reduction in the amount of viable habitat and often the high cost of construction and maintenance. In particular, any major change to the river bank could have a significant affect on the local terrestrial and aquatic habitats, and may inhibit future vegetation development. The riparian habitat is an important interface between the aquatic and terrestrial environments, and its establishment and development should be encouraged. Hard structural modifications to a river bank would require a permit under the Fisheries Management Act, 1994. All factors needs to be carefully examined when weighing the costs and benefits of any proposed hard structural option.
Where the scale of erosion processes is large, the size and cost of works required to provide effective protection usually cannot be justified on economic grounds. The main attraction of structural works is that by containing the hazard, they present the least community hardship. Hence, the structural works option requires economic costs to be set against social benefits.
Hard stabilisation options are outlined below:
Full protection of the bank usually involves covering the effected bank with an erosion resistant surface, be it rock or some other material. Full bank protection options are outlined below.
Rock Revetment with Buried Toe Apron
Figure 2.3 Rock Revetment with Buried Toe Apron – Option A
- Directly protect affected bank with a sloping rock revetment.
- Wall design typically incorporates armour layer, under layer of smaller units and geotextile to prevent fines washing through the revetment.
- A toe apron is required to protect against toe scour and is buried to minimise impacts on stream conveyance.
- Upper bank landscaping improvements could be incorporated.
ADVANTAGES:
- flexible and durable in design
- simple and low maintenance
- provides some new habitat value
DISADVANTAGES:
- high cost
- difficult to construct below water level
- artificially hardens soft banks
- decreases access to water
- most vulnerable area (toe) is out of sight
- changes existing habitat by excluding riparian vegetation
- does not allow for significant vegetation to be incorporated
- if not properly designed hydraulically, may result in changes flow patterns and erosion problems elsewhere
Rock Revetment with Self Launching Toe Apron
Figure 2.4 Rock Revetment with Self-Launching Toe Apron – Option B
Directly protect affected bank with a sloping revetment.
Wall design typically incorporates armour layer, underlayer of smaller units and geotextile to prevent fines washing through the revetment.
The toe apron is placed directly on the current bed level to facilitate underwater construction. In the event of bed scour the toe apron will collapse into the scour hole forming a natural inverse armour and filter layer.
Upper bank landscaping improvements could be incorporated.
ADVANTAGES:
- flexible and durable in design
- simple and low maintenance
- self-positioning
- provides some habitat value
DISADVANTAGES:
- high cost
- difficult to construct below water level
- impedes into conveyance area
- possible hazard to water users (eg. swimmers, boaters)
- artificially hardens soft banks
- decreases access to water
- most vulnerable area (toe) is out of sight
- changes existing habitat by excluding riparian vegetation
- does not allow for significant vegetation to be incorporated
- if not properly designed hydraulically, may result in changes flow patterns and erosion problems elsewhere
Gabion Wall on Reno Mattress Toe Apron
Figure 2.5 Gabion Wall on Reno Mattress Toe Apron – Option C
- Stone filled wire baskets which enable a smaller size and thickness of rock than conventional rock protection.
- Baskets are stacked to form a stable, protective wall.
- Reno mattress is used to provide a flexible toe protection.
- Upper bank landscaping improvements could be incorporated.
ADVANTAGES:
- mainly used where conventional armour rock is not readily available
- greater accuracy of placement possible
- some habitat value
- can be designed to have long life with minimal maintenance
DISADVANTAGES:
- construction difficulties below water level
- artificial hardening of soft banks
- reduced access to water
- damaged structure may be costly to repair
- changes existing habitat by excluding riparian vegetation
- does not allow for significant vegetation to be incorporated
- if not properly designed hydraulically, may result in changes flow patterns and erosion problems elsewhere
- wire baskets may corrode in an estuarine environment
Structural Membrane
Figure 2.6 Structural Membrane – Option D
eg reno-mattress or fabriform A flexible rock filled mattress or a concrete mattress consisting of a grout injected nylon cushion. Mattress forms a laminate over the regraded embankment. Reno mattress is lined with geotextile to prevent fines washing through the mattress. Upper bank landscaping improvements could be incorporated.
ADVANTAGES:
- can be used where conventional rock armour is not readily available
- semi-flexible protection (ie. not solid structure)
- does not intrude into waterway
- can provide scope for upper bank landscaping improvements
DISADVANTAGES:
- does not provide structural stability to bank slope (ie. scour protection only)
- construction difficulties below water level
- hardening of currently soft banks
- regraded slope may intrude onto private land
- any loss of covering may lead to more severe erosion
- not suited to areas of wave activity (eg. long wind fetches and/or heavy boat action)
- changes existing habitat by excluding riparian vegetation
- does not allow for significant vegetation to be incorporated
- if not properly designed hydraulically, may result in changes flow patterns and erosion problems elsewhere
Table 2.1 below outlines comparative costs for the abovementioned full bank protection options over typical river bank heights (~ 5 metres).
Cost ($/m)
A. Rock revetment with buried toe apron 1500 – 2000 B. Rock revetment with self launching toe apron 1200 – 1800 C. Gabions on a reno mattress toe apron 2500 – 3500 D. Structural membrane 1500 – 2000
Table 2.1 Indicative costs for full bank protection options
2.3.1.2 Repair Existing Bank Protection
Reconstruct Revetment using Existing Materials – Option E
In some sections of the river, historic rock revetments are in evidence. However, for a number of reasons, mostly due to poor design and/or poor construction, some sections of this bank protection is no longer effective. As such, options have been investigated for repairing existing bank protection. In some locations, the existing bank protection materials are inappropriate, and construction of an entirely new revetment, or similar, would be necessary to achieve the desired bank protection.
Figure 2.7 Reconstruct Revetment using Existing Materials – Option E
This option can be adopted where the existing revetment utilises the correct materials, but the construction has been poor. For example, where large armour stone have been placed directly on banks with no sub-layer or geotextile protection, the existing stones can be removed, an appropriate substructure constructed, and the armour stones then replaced.
In this way, savings can be made as primary armour stones do not need to be purchased.
Top-Up Existing Revetment with Additional Material – Option F
Where the existing revetment is effective to some degree, but simply doesn’t have enough material to be completely effective at preventing bank erosion, additional armour material can be placed directly onto the bank without the need for reconstruction of the complete revetment.
This option would involve minimal cost, as the additional rock could simply be dumped into the required location without need for precision placement.
Figure 2.8 Top-up Existing Revetment with Additional Material – Option F
Table 2.2 outlines comparative costs for the abovementioned revetment restoration options over a typical river bank height.
Cost ($/m)
E. Revetment reconstruction with existing materials and a new underlayer / geotextile 800 – 1200 F. Top-up of existing revetment with additional rock 300 – 500 Table 2.2 Cost comparisons for existing revetment repairs
2.3.1.3 Local rock protection and anchoring of valuable trees / vegetation
(option g)
- Protect valuable threatened trees with rock armour surrounding the exposed roots. The rock armour should include a toe apron to prevent undermining and should be cut into the bank at the sides to prevent outflanking.
Figure 2.9 Local Rock Protection and Anchoring of Valuable Trees – Option G
ADVANTAGES:
- protects valuable trees
- can provide additional habitat value
DISADVANTAGES:
- hardens previously soft banks
- not a long-term solution (ie. high maintenance costs and risk of failure)
- if not properly designed hydraulically, may result in changes flow patterns and erosion problems elsewhere
Indicative cost for localised protection of valuable trees or vegetation is approximately $8,000 - $10,000 per tree.
Soft structural alternatives may also be employed for erosion protection but can be additionally targeted at foreshore enhancement for flora and fauna and for recreational access, particularly where there is the opportunity to combine bank protection with foreshore landscaping / habitat enhancement. They can be extremely useful in areas where erosion problems exist at or above the water line, though these methods will not generally aid in the prevention of deep seated bank failures. However, methods such as regrading combined with re-vegetation can significantly improve the stabilisation of existing steep and unstable banks.
Vegetation, in particular, can play a significant role in bank stabilisation. The binding root systems of riparian vegetation act to prevent soil erosion from river banks by increasing bank strength. Vegetation also lowers natural groundwater levels, and thus reduces the susceptibility of banks to seepage failures. Surface runoff can be intercepted and slowed by vegetation and its associated natural debris. This acts to dissipate the energy of the flow allowing sediments to be deposited before entering the waterway. United States studies8 have shown that vegetated floodplain channel banks are up to 20,000 times more resistant to erosion over a 500 year period than unvegetated banks.
A range of different soft structural management options are outlined below:
(option h)
Cutting of trees and roots which are under immediate threat of erosion
- Cut and trim trees that have become undermined or are leaning, to remove the threat to public safety and to avoid the potential for local bank scour or failure. Any tree cutting should be complementary to a program of riparian vegetation enhancement.
- Cutting or lopping of river bank trees requires a permit from DLWC under the Native Vegetation Conservation Act, 1997, while lopping or removal of mangroves requires a permit from NSW Fisheries
Figure 2.10 Cutting of Trees and Roots – Option H
ADVANTAGES:
- can minimise scour in the area where the trees were located
- reduces threat to public safety due to falling trees
- eliminates threat of navigation hazard
- logs and brush from tree could be utilised in landscaping / habitat restoration programs
- significantly lower cost than the removal of a fallen tree from the river.
DISADVANTAGES:
- reduces aesthetic value
- damages habitat
2.3.2.2 Gravel or cobble fillets on berm
(option j)
Where wave action has eroded the riverbank at the water level, and bank recession has resulted in a shallow underwater bench, a fillet of cobbles or gravel could be placed on the bench to prevent further wave attack of the bank. It is important that the cobble fillet extend to above the wave run-up level, otherwise the overall integrity of the option would be compromised.
Figure 2.11 Gravel or Cobble Fillet on Berm – Option J
- Fill existing sub- or inter- tidal benches with a cobble/gravel fillet to absorb wave energy.
ADVANTAGES:
- simple but effective solution to wave erosion problem
- vegetation can be planted to provide habitat and further stabilisation, eg mangroves (salty), and phragmites (brackish)
- create valuable intertidal habitat
- can improve access to water
DISADVANTAGES:
- only applicable where there is a significant low tide bench in front of the bank
- appropriate only where banks are relatively low
- not suited to deep-seated failures
- flood velocities must not be high (ie, less than 1.5 m/sec)
2.3.2.3 Gravel or cobble fillets behind a rock toe
(option k)
For sections of the riverbank which are affected by wave erosion at the water level but an underwater bench is not present, a gravel or cobble fillet can be adopted, as above, but supported by a small rock toe.
Figure 2.12 Gravel or Cobble Fillet behind Rock Toe – Option K
- Create an artificial bench supported by a rock berm or and possibly vegetate to absorb wave energy.
ADVANTAGES:
- simple but robust solution to mild wave erosion problem
- vegetation can be planted to provide habitat and further stabilisation, ie mangroves or phragmites
- create valuable intertidal habitat
- can improve access to water
DISADVANTAGES:
- requires shallow water at low tide
- appropriate only where banks are relatively low
- must be no toe scour potential
2.3.2.4 Phragmites planting behind rock toe
(option l)
As an alternative to cobble fillets, phragmites can be planted in front of the eroding bank to provide protection against wave attack. Phragmites provide an effective resistance against waves reaching the bank. The effectiveness of stands of lush phragmites in protecting the adjacent bank can be seen at a number of locations around the Tweed estuary. A layer of granular material would be required to plant the phragmites into. This material would then require support by way of a small rock toe.
Intertidal phragmites plantings could be carried out in combination with additional revegetation of the riverbank. Specific species should be planted in combination with phragmites so that the phragmites would not become over-shadowed. Alternatively, if existing river bank vegetation is retained, overhanging trees would need to be trimmed in order to maintain phragmites. A list of riparian plant species appropriate for river bank stabilisation has been compiled by Tweed Shire Council and is provided in Appendix B.
Figure 2.13 Phragmites Planting behind Rock Toe – Option L
ADVANTAGES:
- simple solution to wave erosion problem
- create valuable intertidal habitat
- aesthetically pleasing
- can improve access to water
DISADVANTAGES:
- appropriate only where banks are relatively low
- flood velocities must not be too high (ie, up to 2 m/s)
- not suitable in areas of pronounced river bed scour
2.3.2.5 Phragmites planting behind wave wall
(option m)
Where the underwater bank is relatively steep, or the wave energy relatively high, phragmites can be planted behind a wave wall. The lower portion of the wave wall would provide an effective toe for the granular material in which the phragmites are planted, while the upper portion of the wall would provide some energy dissipation of waves approaching the riverbank.
Intertidal phragmites plantings could be carried out in combination with additional revegetation of the riverbank. Specific species should be planted in combination with phragmites so that the phragmites would not become over-shadowed. Alternatively, if existing river bank vegetation is retained, overhanging trees would need to be trimmed in order to maintain phragmites. A list of riparian plant species appropriate for river bank stabilisation has been compiled by Tweed Shire Council and is provided in Appendix B.
Figure 2.14 Phragmites behind Wave Wall – Option M
ADVANTAGES:
- simple solution to mild wave erosion problem
- can improve access to water
- timber wall protects phragmites more than rock toe option – assists establishment of phragmites
- can be used for steeper underwater slopes
- create valuable intertidal habitat
DISADVANTAGES:
- appropriate only where banks are relatively low
- flood velocities must not be too high (ie, up to 2 m/s)
- not suitable in areas of pronounced river bed scour
(option n)
Figure 2.15 Plant Riparian Vegetation – Option N
Riparian vegetation has significant potential to reduce bank erosion5,6. As stated earlier (refer Section 2.3.2), the root system of riparian vegetation acts to bind together the bank soil structure, making it more resilient to scour, seepage and slumping. In addition to the structural benefits, planting riparian vegetation introduces significant habitat values, aesthetic values and a valuable buffer / filter strip between the land and the waterway. Such vegetated filter strips have been shown to reduced surface runoff pollutants (ie, sediment and nutrients) entering the waterway8.
At suitable locations, appropriate tree species could be planted to improve the overall stability of the river bank along the riparian corridor. A list of appropriate riparian plant species suitable for bank stabilisation has been compiled by Tweed Shire Council and is shown in Appendix B.
ADVANTAGES:
- aesthetic improvement
- relatively inexpensive
- provides increased protection through the following processes:
- leaves and branches of low-growing foliage help to dissipate flow at the surface of the soil, thereby reducing the erosive flow along the riverbank. This can ultimately result in sediment deposition around plants, eventually increasing further colonisation and diverting flow
- a shallow root mat (as provided by most grasses) can armour the soil surface, preventing disaggregation and subsequent loss of sediment
- deeper roots can reinforce soil as well, aiding in the prevention of deep-seated geotechnical failures
- deep roots can also provide an effective path for drainage, thereby reducing the "drawdown" effect and slumping (as a result of falls in river water level)
- extensive tree planting helps to control riverbank soil moisture, reducing the risk of excessive pore pressure in wet conditions and reducing the likelihood of soil shrinkage and cracking during extended dry conditions
- many vegetative types are self maintaining, regrowing after damage and filling any gaps in the protection
- commercial products such as reinforced turf are also available
- surface runoff is intercepted and dissipated by vegetation, leading to potential deposition of sediment (soil building) and filtering of soil-bound pollutants (nutrients, pesticides, petrochemicals etc)
DISADVANTAGES:
- Care needs to be taken with species selection to ensure appropriate native vegetation for any given situation
- Will require maintenance for up to three years to ensure adequate establishment of vegetation
- could be damages by extreme event
- not applicable to oversteepened banks
- may require river bank regrading
- could be damaged by storm events or bushfire
- may obscure views of the waterway
(option p)
Overly steep banks have a greater tendency to collapse, under the weight of surcharge loading, or through groundwater seepage. Regrading of an upper slope can also be adopted for banks experiencing toe scour, however, under such circumstances, the toe of the slope would need to be additionally protected (refer previous options).
Figure 2.16 Bank Regrading – Option P
- Vegetate bench for habitat or provide for recreation, eg walkway.
- Width to suit function of bench.
- Can incorporate planform features such as a small beach front for boating access.
- Cost depends on height of bank to be regraded.
ADVANTAGES:
- relatively simple and inexpensive process
- reduces the slope of the bank, thereby decreasing its likelihood of failure.
- provides area for vegetation
- aesthetically attractive
DISADVANTAGES:
- may initially damage habitat
- requires sufficient strip of riparian land
(option q)
Sandy beaches can be created in areas of existing bank erosion, particularly areas of wave induced erosion. Firstly, banks are regraded on shallow slopes (typically 1 in 7 to 1 in 10), and a layer of sand, approximately 0.5 metres thick is placed from below low water to above the wave run-up level. Groynes are usually required to prevent the sand from being transported away from the site under combined wind wave and tidal / flood current conditions.
Figure 2.17 Create Sandy Beaches – Option Q
ADVANTAGES:
- reduces the slope of the bank, thereby decreasing its likelihood of failure.
- provides area for recreation – multi-functional
- aesthetically attractive
- can be adopted in more erosive sites than other non-structural options
- cheaper than full bank revetment options
DISADVANTAGES:
- may initially damage habitat
- requires sufficient strip of riparian land
- associated costs can be high
Table 2.3 outlines comparative costs for the abovementioned soft structural bank protection options for typical river bank heights.
Cost ($/m)
H. Cutting of trees and roots along riverbank 200 – 2000 ea depending on size and accessibility J. Gravel or cobble fillet on natural wave berm 100 – 200 K. Gravel or cobble fillet behind a rock toe 200 – 300 L. Phragmites planting behind a rock toe 200 – 300 M. Phragmites planting behind a wave wall 300 – 500 N. Vegetation in riparian corridor 50 per tree P. Regrading of riverbank 20 – 200 plus revegetation Q. Creation of sandy beaches 60,000 – 100,000 ea typically ($1000–1500/m)
Table 2.3 Cost comparisons for soft structural options
2.3.3 Combined Hard & Soft Options
A combination of hard and soft options can be an effective and aesthetic way to provide erosion protection. It provides the instant fix of hard structural works whilst allowing vegetation integral to river health and function to establish and improve long-term bank stability. In general, there are two primary configurations for these combinations:
- hard structural options below normal water levels and soft options along the upper banks
- hard structural options on entire bank, through which vegetation can grow
With the former, the hard structure acts as a foundation upon or behind which bank regrading and/or revegetation can be undertaken. The latter provides a stable matrix for the direct planting of vegetation.
An important action to be taken by all communities is improved catchment management, that is, identifying all the catchment land use activities which generate sediment and nutrients, identifying changes in land use practices, and employing strategic management plans which will lead to a reduction in the rate of runoff and the input of both water and sediment to the river channel. This may involve changes in farming and logging practices, drainage from car parks and roads, water usage, stormwater outlets, retro-fitting of nutrient control ponds and revised development controls and building codes etc.
Improved seepage and drainage control can reduce the rate of runoff from developed areas. Many areas in the United States are establishing permeable car parks and roadsides, thereby decreasing the pressures exerted on rivers by reducing urban flow to stormwater drains. Catchment management can also result in decreased rural and agricultural runoff, subsequently lowering the sediment load in a river thereby potentially reducing erosion problems.
Improved catchment management should embrace the concept of rehabilitation of riparian corridors – areas along rivers and creeks where established vegetation creates a healthy, viable habitat and helps stabilise the riverbanks. Vegetation corridors, or buffer strips, can also aid in the assimilation of nutrients and sediment and reduce the rate of runoff into the river system. This, in conjunction with regrading and occasional bank protection to improve the overall resistance of the bank, will help to establish healthy and stable river banks.
Catchment management practices will not deliver immediate benefits in reducing the in-stream sediment load and addressing related erosion problems. Rather, they are a long-term environmentally sustainable management practice.
3 EROSION AND ACCRETION ALONG THE BANKS OF THE TWEED RIVER ESTUARY
Areas of bank erosion and accretion on the Tweed River have been mapped using the methods of analysis discussed in Section 1.4. These areas have been identified and numbered on a site by site basis, and cover all of the Tweed River estuary, from the entrance to Bray Park Weir, the Rous River up to Kynnumboon, and Terranora Creek up to the entrances to Cobaki and Terranora Broadwaters.
3.1 UPPER ESTUARY (BRAY PARK WEIR TO TUMBULGUM)
3.1.1 General Features of Area
The waterway, in this section of the estuary, is typically a meandering channel with high natural riverbanks. Land immediately adjacent to the river is predominantly farming land (grazing and sugar cane). The urban centre of Murwillumbah flanks the river for a reach of approximately two kilometres. The main town centre is protected from overbank flooding of the river by an encircling concrete wall and levee system.
Rock protection is present at a number of locations in the upper estuary, which indicates that on-going bank erosion has been an issue for many years. Unfortunately, the historic rock protection offers limited resistance against current and wave action, as firstly, underwater inspections have shown that competent rock layers occur only within the intertidal zone, and secondly, no filter material was placed between the rocks (d50~0.3m) and the alluvial river banks. Fortunately, the rocks within the intertidal zone have stabilised the shoreline so that waterplants such as phragmites and juncus have been able to establish. It is now these waterplants which are preventing ongoing erosion of the riverbanks by dissipating the energy of the waves before they strike the banks.
Bedrock outcrops also limit the migration of riverbanks at a number of locations, particularly in the upper reaches around Murwillumbah. The high riverbanks in the upper estuary are alluvial in nature, and are susceptible to natural slumping and wave erosion, particularly the reach upstream of Murwillumbah, however, the river in the upper estuarine reaches has become established within a stiff estuarine clay basin. The stiff estuarine clay has a low erosivity potential, thereby significantly slowing any long term morphological riverbank erosion in this section of the river.
This section of the river is presently used for both passive and active forms of boating. Boatramps are located at Murwillumbah, Condong and Tumbulgum, with intermediate reaches frequently used for water skiing. It is considered that an increase in boating traffic could have detrimental effects on riverbank stability and destroy valuable intertidal habitats, particular in the river reach upstream of Commercial Road boatramp.
Bank erosion and accretion sites on the upper Tweed estuary, as well as general features of the area, are identified in the Figure,
3.1.2 Long-Term Morphological Changes
Although the upper estuary is well developed from a geological viewpoint, there are some locations within the upper reaches which exhibit tendencies for geomorphologic change. However, natural changes to the river have been affected by firstly, clearing of the catchment, which has altered the runoff and sediment load within the river, and secondly, dredging of the river bed for sand extraction purposes.
Morphological changes in the upper reaches of the Tweed estuary include continued point bar formation on inside of river bends, and some slow systemic recession of riverbanks on the outside of bends, as indicated in the Figure. Dredging appears to have generally lowered river bed levels in the reaches between Murwillumbah and Condong, and also between Condong and Tumbulgum, in the vicinity of Bartlets Creek.
3.1.3 Site-by-Site Identification
Site by site identification of erosion and accretion areas on the upper estuary reaches is provided in Table 3.1.
3.2 MIDDLE ESTUARY (TUMBULGUM TO CHINDERAH)
3.2.1 General Features of Area
The middle reaches of the estuary are typified by relatively long straight reaches, separated by a combination of sharp and gradual bends.
Like the upper reaches, the middle reaches of the Tweed River have become established within a stiff estuarine clay basin. This means that systemic erosion of riverbanks is a very slow process, and would not be considered significant from a typical planning viewpoint. However, this section of the waterway is still subject to substantial boating traffic, and as such, the banks have been affected by boat generated wave action.
The middle reaches of the estuary are more active with respect to geomorphologic development. Particularly around Stotts Island, some banks have prograded by up to 40 metres in the last 100 years, ie point bars are forming and then becoming vegetated. Likewise, inter-channel shoals have also developed into more substantial islands. An example of this is Rawson Island, which at the turn of the century, was known as "one-tree island". Rawson Island is now densely covered in vegetation over an area of approximately one hectare.
Bank erosion and accretion sites on the middle reaches of the Tweed estuary, as well as general features of the area, are identified in the Figure.
3.2.2 Long Term Morphological Changes
Geomorphic change is active within the middle reaches of the estuary, however, the rates associated with such change are low because of the stiff underlying estuarine clay. As for the upper reaches, natural changes to the river have been affected by clearing of the catchment and dredging, however, less historical dredging has been carried out in this section of the waterway than further upstream, or downstream (refer Section 3.3.2).
Morphological changes in the middle reaches of the Tweed estuary include continued point bar formation on inside of river bends, and some slow systemic recession of riverbanks on the outside of bends, as indicated in the Figure. Overall riverbed shoaling has also occurred in the reach between Tumbulgum and Stotts Island, as indicated by comparisons of historical cross-section surveys.
Site by site identification of erosion and accretion areas on the middle estuary reaches is provided in Table 3.2.
3.3.1 General Features of Area
The lower reaches of the Tweed River estuary, comprising the main arm up to Chinderah, and Terranora Creek up to the broadwaters, are typified by broad channels which are influenced by marine sand build-up, and heavy boating traffic.
There are a number of bank erosion sites within the lower estuary, which are mostly the result of boats and wind waves. However, deep seated bank recession also occurs at Chinderah, at a rate of approximately 0.2m /year, and the riverbank at the entrance to Cobaki Broadwater is collapsing due to pore water pressures.
The underlying geological stratum of this area is mostly fine to medium grained sand, with indurated marine sands at depth. A number of riverbanks have also been established through sidecasting of dredge material removed from navigation channels within the river.
A large amount of the riverbank along the lower reaches of the main arm, and to a lesser extent, Terranora Creek, have been lined with rock in order to train the river channels and maximise navigation through the river. This rock is well founded, and although dated back to between 50 and 100 years, shows few signs of deterioration.
Bank erosion and accretion sites, as well as distinguishable features of the lower reaches of the Tweed estuary are shown on the Figure.
3.3.2 Long Term Morphological Changes
Morphologically, the lower reaches of the estuary are the youngest, and are still undergoing change in order to reach a state of dynamic equilibrium. This was evident during the early times of settlement of the area, when the lower estuary contained a myriad of marine sand shoals and islands that would be moved around with each storm and flood event. Since that time, the banks of the lower estuary have been stabilised by rock training walls, and the river has been periodically dredged to ensure safe navigation.
Nonetheless, the lower reaches of the river still show signs of morphological change, most notably at Chinderah. At this site, ongoing recession on the outside of a river bend was stemmed by the construction of an upper bank profile rock revetment, however, since then, the underwater bank slope on the outside of the bend has steepened to the extent that the whole bank, including existing revetment, is in imminent danger of collapse.
3.3.3 Site by Site Identification
Site by site identification of erosion and accretion areas on the lower reaches of the Tweed estuary, including Terranora Creek, is provided in Table 3.3.
3.4 ROUS RIVER (Tumbulgum to Kynnumboon)
3.4.1 General Features of Area
The Rous River drains mostly agricultural lands to the north of Murwillumbah. The river has a meandering profile, and has a number of naturally formed levees flanking its banks.
Although somewhat removed from the main Tweed River estuary, the Rous River is still subject to boating traffic, by mostly recreational and commercial fishers. The Rous River is relatively narrow (approximately 30 metres wide), and as such, the banks receive significant wave energy from all vessels using the river.
The river exhibits typical meander section profiles, with shallow underwater slopes on the inside of bends, and steeper underwater slopes on the outside of bends. Phragmites have typically become well established on the shallow slope on the inside of the bend. The phragmites dissipates the majority of wave energy before reaching the bank. On the outside of the bend, it is more difficult for phragmites to become established due to the steeper underwater slope, and also because of shading by riparian vegetation. As such, the riverbank on the outside of bends receives more wave energy, and as a result, has generally become undercut by up to 2 metres. Undercutting of the bank means that slumping and collapse will be imminent, resulting in significant loss of riparian vegetation.
The lack of evidence of presently collapsed banks suggests that the problem is contemporary, and that significant bank collapse will occur in the future if wave energy is allowed to continue to attack the banks.
Bank erosion and accretion sites on the Rous River, as well as general features of the area, are identified on the Figure.
3.4.2 Long Term Morphological Changes
Shoaling has generally occurred on the inside of river bends in the Rous River, indicating an active meandering morphological process. However, corresponding long term recession of banks is not so obvious, indicating that point bar deposition is more a result of the sediment load of the river rather than overall geomorphic conditions.
3.4.3 Site by Site Identification
Site by site identification of erosion and accretion areas on the Rous River is provided in Table 3.4.
Table 3.4 SITE BY SITE IDENTIFICATION OF EROSION AND ACCRETION ON THE ROUS RIVER
4 TWEED ESTUARY BANK MANAGEMENT PLAN
The objective of the Tweed Estuary Bank Management Plan is to provide an integrated program of works and measures which will:
- identify areas of current bank erosion and prioritise them is order of severity and impacts on the community;
- identify the main causes of bank movement at these sites; and
- provide solutions to the bank erosion problems identified on the river.
The Management Plan has been developed to integrate with other Tweed River Management Plans, thus ensuring that compatible and achievable goals are maintained.
Bank erosion on the Tweed River occurs as a result of a number of different mechanisms, which are both natural and human-made. These mechanisms include:
· surface scour of bank material;
· toe scour and subsequent oversteepening and collapse through bank failure;
· loss of internal strength through excessive pore pressure; and
· wave induced erosion at the waterline.
Factors which result in these bank failure mechanisms include:
· altered flow patterns;
· tidal currents;
· river velocities;
· boat and wind waves; and
· saturated bank soils.
Some of these mechanisms have been compounded by human activities, including vegetation clearing of the catchment and riparian areas, rural and urban development, farming practices, dredging, and flow training works.
An assessment of the riverbanks for the entire Tweed River estuary, including Terranora Creek up to the broadwaters and Rous River up to Kynnumboon, was carried out using a variety of techniques, including:
· photogrammetric analysis;
· air photo and historic hydrosurvey comparisons;
· hydrographic survey cross-section comparisons;
· visual inspections; and
· underwater diving inspections.
From this assessment, the conditions of the riverbanks were determined and areas of erosion were prioritised, as indicated in the Figure, and Tables 3.1 to 3.4 of this report. Sites were prioritised based on the severity of the erosion, the erosion rate, and the risks associated with on-going erosion. Site 52, which is located at Chinderah, has a Very High priority as the riverbank is in imminent danger of collapse with temporary residences (caravan parks) located immediately behind the bank. High priority sites are generally sites where the risk of bank failure affects public safety (viz: major roads such as Pacific Highway or popular recreation reserves) and recession rates are significant. Remaining sites fall within a Medium or Low priority category, with the more severe erosion sites listed as a Medium Priority.
The most widespread erosion problem in the Tweed River estuary was wave induced erosion in the inter-tidal zone. Fortunately, this erosion problem can be addressed through a variety of soft structural options. Bank erosion solutions best suited for the Tweed River estuary include:
- phragmites planting behind a rock toe;
- phragmites planting behind a wave wall;
- gravel or cobble fillet behind a rock toe;
- gravel or cobble fillet on a shallow low tide berm;
- regrade riverbank and re-vegetate surface;
- cutting of overhanging trees and roots;
- creation of sandy beaches;
- full bank rock revetment;
- reconstruct revetment with existing material; and
- top-up revetment with additional armour;
Approximately 70 individual sites of bank erosion in the Tweed River estuary were noted. Not all these sites require immediate action, however, and some would not require any action, only regular monitoring. A decision first needs to be made whether or not protective works should be carried out at each erosion site. This decision will be based on a value judgement of the assets at risk, and the costs of remediation, over a reasonable planning period. Such value judgements would need to be made by Council and the local Tweed communities (ie, landowners, rivercare groups etc). Although priorities have been attached to the erosion sites, these are based on erosion rates and risks to public safety – no value judgements have been implied in prioritising the sites.
In addition to the physical bank remediation works described previously, the decision can be made to adopt a non-structural approach to river bank hazard, ie do nothing to the river bank, but reduce the assets at risk. This generally includes purchase of riparian land to relocate assets such as roads and houses, or changing planning policies (DCPs and LEPs) to prevent any increase in hazard in the future. Again, value judgements are required to make such decisions. Such value judgements are outside the scope of this study.
The Tweed Estuary Bank Management Plan is outlined in Table 4.1, below. Erosion sites are shown on the Figure.
Site
Description
Preferred Action
Performance Measures
Supplementary Works
Indicative Costs
Priority
Minor scarping with undercutting of trees Plant phragmites behind rock toe (refer Option L - Section 2.3.2.4) Reduced bank fretting Reduced loss of riverbank trees
Conserve existing and re-establish additional foreshore vegetation $114,000
LOW
Surface scour due to stock access to river Restrict stock access to river Reduced surface scour Re-establish foreshore vegetation $1,000
LOW
Fretting of flood deposit shoal Regrade bank to stable profile (refer Option P - Section 2.3.2.7) Reduced bank fretting $1,000
LOW
Major scarping of bank Sandy beach plus cobble fillet (refer Option K – Section 2.3.2.3 and Option Q – Section 2.3.2.8) Reduced bank fretting and loss of reserve Upgrade regional boatramp facilities and re-establish additional foreshore vegetation $100,000
HIGH
Scarping of bank Phragmites behind wave wall (refer Option M - Section 2.3.2.5) Reduced bank fretting Conserve existing and re-establish additional foreshore vegetation $60,000
MEDIUM
Overtopping of existing rock protection Top-up revetment with additional tipped rocks (refer Option F – Section 2.3.1.2) Reduced bank fretting Re-establish foreshore vegetation $130,000
LOW
Minor scarping of vegetated bank Plant phragmites behind rock toe (refer Option L - Section 2.3.2.4) Reduced bank fretting Re-establish foreshore vegetation $39,000
MEDIUM
Minor scarping with loss of riparian vegetation Plant phragmites behind rock toe (refer Option L - Section 2.3.2.4) Reduced bank fretting Conserve existing and re-establish additional foreshore vegetation $66,000
LOW
Collapsing private retaining walls Reconstruction of walls to engineering standard (refer Option A to D – Section 2.3.1.1) Reduced failed walls and bank slumps Upgrade Bullamakanka Rest river access facilities, including timber jetty $2,160,000
LOW
Inter-tidal scarping of bank Gravel fillet on natural berm or behind toe (refer Option J – Section 2.3.2.2 or Option K – Section 2.3.2.3) Reduced scarping and bank recession Conserve existing and re-establish additional foreshore vegetation $80,000
LOW
Scarping with loss of riparian vegetation Plant phragmites behind rock toe (refer Option L - Section 2.3.2.4) Reduced bank fretting Conserve existing and re-establish additional foreshore vegetation $660,000
LOW
Table 4.1 Bank Management Plan Actions and Priorities …cont’d
Site
Description
Preferred Action
Performance Measures
Supplementary Works
Indicative Costs
Priority
Scarping at waterline and resulting bank slumping Plant phragmites behind rock toe or wave wall (refer Option L - Section 2.3.2.4 or Option M – Section 2.3.2.5) Reduced bank fretting Conserve existing and re-establish additional foreshore vegetation $300,000
HIGH
Undercutting of riverbank at waterline Plant phragmites behind rock toe (refer Option L - Section 2.3.2.4) Reduced slumping and bank undercutting Re-establish foreshore vegetation and possible sandy beach for boats $72,000
MEDIUM
Undercutting of riverbank at waterline Plant phragmites behind rock toe (refer Option L - Section 2.3.2.4) Reduced slumping and bank undercutting Conserve existing foreshore vegetation $69,000
LOW
Scarping and recession at informal beach Formalise beach with appropriate anchors (refer Option Q – Section 2.3.2.8) Reduced shoreline recession and scarping Tumbulgum foreshore development plan $60,000
MEDIUM
Localised fretting at gaps in phragmites Formalise some beaches and re-establish phragmites at others (refer Option Q – Section 2.3.2.8 and Option L – Section 2.3.2.4) Reduced bank fretting Tumbulgum foreshore development plan $16,000
MEDIUM
Overtopping of existing rock protection Top-up revetment with additional tipped rocks (refer Option F – Section 2.3.1.2) Reduced bank fretting Re-establish riparian vegetation corridor between core habitat areas $70,000
LOW
Scarping, undercutting and slumping of banks Plant phragmites behind rock toe or wave wall (refer Option L - Section 2.3.2.4 or Option M – Section 2.3.2.5) Reduced slumping and bank undercutting Re-establish riparian vegetation corridor between core habitat areas and possible small sandy beaches for boats $60,000
LOW
Scarping, undercutting and slumping of banks Plant phragmites behind rock toe or wave wall (refer Option L - Section 2.3.2.4 or Option M – Section 2.3.2.5) Reduced slumping and bank undercutting Re-establish riparian vegetation corridor between core habitat areas $600,000
HIGH
Minor scarping with undercutting of trees Plant phragmites behind rock toe (refer Option L - Section 2.3.2.4) Reduced bank fretting Reduced loss of trees
$120,000
LOW
Undercutting and slumping of bank Plant phragmites behind rock toe (refer Option L - Section 2.3.2.4) Reduced slumping and bank undercutting Re-establish foreshore vegetation $102,000
HIGH
Table 4.1 Bank Management Plan Actions and Priorities …cont’d
Site
Description
Preferred Action
Performance Measures
Supplementary Works
Indicative Costs
Priority
Scarping and minor slumping of riverbank Gravel fillet on natural berm or behind toe (refer Option J – Section 2.3.2.2 or Option K – Section 2.3.2.3) Reduced scarping and bank recession Re-establish riparian vegetation corridor between core habitat areas $70,000
LOW
Scarping and minor slumping of riverbank Plant phragmites behind rock toe (refer Option L - Section 2.3.2.4) Reduced scarping and bank recession $96,000
LOW
Overtopping of existing rock protection Top-up revetment with additional tipped rocks (refer Option F – Section 2.3.1.2) Reduced bank fretting Re-establish foreshore vegetation $30,000
LOW
Major scarping and slumping of riverbank Gravel fillet on natural berm or behind toe (refer Option J – Section 2.3.2.2 or Option K – Section 2.3.2.3) Reduced slumping and bank undercutting Re-establish foreshore vegetation $368,000
MEDIUM
Undercutting and slumping of bank Gravel fillet on natural berm or behind toe (refer Option J – Section 2.3.2.2 or Option K – Section 2.3.2.3) Reduced slumping and bank undercutting Re-establish foreshore vegetation $30,000
HIGH
Minor fretting and scarping of bank Gravel fillet on natural berm or behind toe (refer Option J – Section 2.3.2.2 or Option K – Section 2.3.2.3) Reduced bank fretting Re-establish foreshore vegetation $100,000
LOW
Deep-seated bank recession Full bank rock revetment and sandy beaches (refer Options A to D – Section 2.3.1.1 and Option Q – Section 2.3.2.8) Reduced shoreline recession and bank steepening Re-establish foreshore vegetation and upgrade foreshore recreation facilities along Chinderah waterfront $1,040,000
VERY HIGH
Minor scarping and overtopping of rock Top-up revetment with additional tipped rocks (refer Option F – Section 2.3.1.2) Reduced bank fretting Possible planting of mangroves within existing revetment to enhance habitat $38,000
LOW
Scarping of banks with shoreline recession Gravel fillet on natural berm or behind toe (refer Option J – Section 2.3.2.2 or Option K – Section 2.3.2.3) Reduced scarping and bank recession $314,000
LOW
Table 4.1 Bank Management Plan Actions and Priorities …cont’d
Site
Description
Preferred Action
Performance Measures
Supplementary Works
Indicative Costs
Priority
Dilapidated rock revetment Reconstruct dilapidated sections of revetment (refer Option E – Section 2.3.1.2) Reduced bank failure and loss of sub-material Boyds Bay Bridge Foreshore Development Plan $350,000
MEDIUM
Major scarping and slumping of riverbank Sandy beaches and gravel fillet on berm (refer Option Q – Section 2.3.2.8 and Option J – Section 2.3.2.2) Reduced scarping and bank recession Upgrade regional boatramp facilities and re-vegetate riverbank reserves $100,000
HIGH
Minor localised scarping of bank due to waves and removal of bank vegetation Gravel fillet on natural berm or behind toe (refer Option J – Section 2.3.2.2 or Option K – Section 2.3.2.3) Reduced bank fretting Re-vegetation of riverbank reserves $506,000
LOW
Localised scarping beside bridge abutment Gravel fillet on natural berm (refer Option J – Section 2.3.2.2) Reduced scarping and bank recession $8,000
MEDIUM
Outflanking of existing rock protection Reconstruct rock revetment to engineering standard (refer Option E – Section 2.3.1.2) Reduced scarping and bank recession Possible planting of mangroves within existing revetment and re-vegetate riverbank reserves $720,000
MEDIUM
Piping failure of upper riverbank Full bank revetment to engineering standard (refer options A to D – Section 2.3.1.1) Reduced scarping and loss of vegetation Possible planting of mangroves within existing revetment and re-vegetate riverbank reserves $320,000
MEDIUM
Undercutting and slumping of riverbank Gravel fillet on natural berm or behind toe (refer Option J – Section 2.3.2.2 or Option K – Section 2.3.2.3) Reduced scarping and bank recession Conserve existing and re-establish additional foreshore vegetation to compensate for future losses $30,000
MEDIUM
Undercutting and slumping of riverbank Combination of intertidal cobble fillet and phragmites plantings (refer Option J – Section 2.3.2.2, Option K – Section 2.3.2.3 and Option L – Section 2.3.2.4) Reduced scarping and slumping of bank Conserve existing and re-establish additional foreshore vegetation to compensate for future losses $890,000
LOW
Table 4.1 Bank Management Plan Actions and Priorities
A breakdown of likely expenditure for each priority of works is given in Table 4.2, below.
VERY HIGH $1,040,000
HIGH $1,232,000
MEDIUM $2,043,000
LOW $5,575,000
TOTAL
$9,890,000
say $10 million
Table 4.2 Prioritised Costs of Bank Protection Works
4.4 RESPONSIBILITIES FOR BANK EROSION
The majority of riverbank affected by shoreline erosion of the Tweed estuary is within privately owned land. Hence, it is ultimately the responsibility of the land owner to address much of the erosion which is manifest along the river. However, there are a number of reasons why protection of the riverbank should be not be carried out in a piecemeal manner depending on adjacent property boundaries:
- need to ensure adopted bank protection is properly engineered and constructed;
- isolated sections of erosion protection can have adverse impacts elsewhere if the complete erosion-affected zone is not addressed;
- the majority of the erosion is largely affected by the general community’s use of the waterway, and hence the general community could be responsible in some manner for erosion protection.
There are much wider issues concerning bank management, other than just arresting erosion. For instance, the Upper Tweed Estuary Management Plan identifies areas for habitat enhancement, creation of vegetated riparian corridors to link key habitat areas, and improvements directed at passive and active recreation. As such, it is important that Council adopts a broad view in the implementation of this Plan. Council and the local community (through landowners, rivercare groups etc) need to make value judgements on the net benefits of bank erosion protection measures and weigh them against alternative actions, including the "do nothing" option, and the relocation of assets beyond the area considered to be at risk.
Once adopted by Council, the Bank Management Plan could be incorporated into a Development Control Plan (DCP) to provide added control on riverbank works, and ensure that works are carried out in a consistent manner through the river.
Some land holders in the Upper Tweed Estuary, upstream of Murwillumbah, have adopted an opportunistic approach to bank management, and have regraded and revegetated sections of inherently unstable riverbank. Such opportunistic bank management practices should be encouraged, which could include:
- regrading and revegetating riverbank;
- general riverbank tree management, such as trimming overhanging branches and roots;
- preservation of riverbank vegetation which helps stabilise the banks;
- planting of additional riverbank trees, such as casuarinas;
- restricting stock access to the water via appropriate fencing.
As indicated in Table 4.2, the total cost of bank protection works for the Tweed estuary is in the order of $10 million. It is unlikely that such a budget would become available for bank protection works, nor should an attempt be made to protect the entire river. Many of the low priority areas should be left to natural processes and monitored. Hence, alternative measures which stem further erosion, rather than addressing existing problems could be considered.
4.5.1 Ongoing Bank Monitoring and Assessment
Council and the wider community require a facility to determine if benefits of bank protection and remediation activities are being achieved, and the objectives of the Bank Management Plan being met. Such a facility could be provided through ongoing monitoring of the river banks, and regular review of the results in comparison with the findings presented in this report.
To be effective, a program for monitoring and assessment of the river banks should include the following:
- A photographic register of all sections of the river bank experiencing erosion should be established. Periodic (possibly annually) photos could be taken from identical locations and compared to previous photo sets. Ideally, photos should be taken at low tide and from the river. A survey staff within the field of view of the photograph would also assist in scaling-off any changes between the photos.
- Bank profile surveys of defined control sections down to the toe of the bank should be carried out regularly (eg annual and after major flood events). These sections would preferably be located at the sites of the worst erosion problem. Control sections would be defined using standard surveying practices ie, establish permanent bench marks (concrete plinths etc) and use bearing, offset and level records.
- In addition to the above, a simpler but more widespread technique could be adopted, where horizontal distance from a reference location to the top of the bank is recorded on a periodic basis (eg annual and after major flood events). This technique could be employed every 20 metres or so over the entire length of the identified river bank erosion areas.
- Council’s air photo library of the river should be maintained and updated. Comparisons using photograph overlays of erosion escarpment locations could be carried out whenever new air photography becomes available from Land Information Centre (Surveyor General’s Department).
- Repeat hydrographic surveys (as identified in Section 1.4.3) could be carried out every five years or so, and analysed for geomorphic patterns that can have an impact on bank erosion (refer Appendix A).
4.5.2 Community Awareness Program
In order to raise community awareness regarding river bank management, Council could give consideration to the implementation of a community awareness program on bank erosion processes. Conceptually, this program could involve the following elements:
- An information brochure based on a large scale map of the estuary showing erosion areas and outlining causes. Such a map could be adapted from the figures presented in this report (albeit a more simplified version).
- Regular newspaper articles on the restoration of river bank erosion sites and the enhancement of riparian vegetation.
- Establish a number of demonstration sites in different parts of the river which exhibit different erosion problems and require different management solutions. Educational signage could be established around these demonstration sites showing before and after images, and outlining the basic objectives of the Bank Management Plan
- Educational material (eg kits) could be developed for school children carrying out geography or environmental projects. Programs would need to be established and coordinated through local school principals.
Click here to view the figures.
APPENDIX A – SUPPORTING INFORMATION
Situated 400 metres upstream of Barneys Point Bridge, and opposite Chinderah Bay, cross-section 15 has undergone significant erosion on its right hand side (looking downstream) and accretion on its left hand side since the original survey in 1884, see figure. This pattern is consistent with typical meandering processes of river development.
As the river negotiates a relatively tight bend at this location, velocities increase around the outside of the bend, attacking unprotected bed and bank material. As the flow gyrates through the bend in the river, the eroded material is suspended, coming to rest where velocities reduce, some distance downstream. As the majority of the flow is directed towards the outside of a bend, the inside of the bend is relatively calm, forming a convenient place for material to be deposited - hence the formation of sand shoals and spits.
Located 800 metres upstream of cross-section 15, and at the apex of the river bend at Chinderah, cross-section 16 exhibits similar characteristics as cross-section 15, that is, extensive erosion on the right hand side and a continued accretion of sediment on the left hand side of the section - refer figure. This pattern of erosion and accretion appears to have been active since the time of the first survey in 1884.
By comparison of the five surveys performed at cross-section 16, it is apparent that the section has undergone a gradual change since 1884. For each subsequent survey after the original survey, the right hand bank has become progressively steeper, while the left hand side of the section has become progressively shallower.
Cross-section 17 is located approximately 500 metres upstream of cross-section 16, between the township of Chinderah and Dodd’s Island. Over the past 100 years, there has been little change in the general shape of this section as indicated in the figure. The only change has been the infilling of some deeper parts of the section by approximately 0.5 metres.
Situated just downstream of Chinderah Island, approximately 300 metres upstream of cross-section 17, cross-section 18 has undergone small changes over the past 100 years, as shown in the figure. Since 1884, there has been a tendency for the section to steepen its left hand bank, and shoaling to occur on the opposing side of the main channel. The reason for the main flow to move closer to the left hand side is likely to be linked with natural meander tendencies associated with flow patterns around Dodd’s and Chinderah Islands. On the right hand side of the section, which is protected from the main river flows by Chinderah Island, the surveys do not appear to be consistent, and hence a general trend analysis was not possible.
This cross-section is located towards the upstream end of Dodd’s Island, and approximately 1800 metres upstream of cross-section 18. Since 1884, a shoal in the middle of the section has grown about one metres - refer figure. The shoal is now intertidal, with the possibility of eventually becoming vegetated and stabilised, as Rawson Island and Goat Island have further upstream. Between the intertidal shoal and the mainland on the left hand side, a narrow channel has formed, and deepened approximately 1 metre since 1884. As this channel has deepened, the left hand bank has steepened.
The right hand bank of the river, adjacent to Dodd’s Island, has also steepened since the original survey. This part of the section has also become deeper, which is possibly the result of the channel attempting to maintain the same waterway area with an increasing central shoal.
Cross-section 21 is located approximately 2700 metres upstream of cross-section 19, and is just upstream of the Tweed Broadwater. This section has not undergone significant changes since 1884, however, some shoaling of the left hand side and a slight recession of the right hand side bank has occurred - refer figure.
Located only 500 metres upstream of cross-section 21, cross-section 21A has changed little since 1884 - refer figure. However, interpretation of this section is difficult, as only the 1884 and 1992 surveys were available for comparison.
Like the previous two sections, there does not appear to be any significant section changes at cross-section 22 over the past 100 years. Cross-section 22 is located approximately 500 metres upstream of cross-section 21A, and just downstream of Stott’s Island. Historical hydrosurveys for this section are shown in the figure.
Cross-section 26 is approximately 2200 metres upstream of cross-section 22, and is located just downstream of Rawson Island, adjacent to Stott’s Island. The figure shows that at this location, the river appears to be shoaling in the centre of the channel. This shoaling could be direct result of the formation of shoals surrounding Rawson Island immediately upstream, as velocities would have dropped off along the left hand side of the section, with a ‘dead spot’ in the water circulation patterns formed behind it allowing easy settlement of suspended material.
The shoaling in the middle of the channel is accompanied by a steepening of the right hand bank, adjacent to Stott’s Island. This process of erosion and shoaling is typical of a natural river meander process.
Like cross-section 26, 400 metres downstream, cross-section 27 has significantly shoaled in the centre of the channel over the past 100 years, as indicated by the figure. The channel thalweg does not appear to have deepened, however, to accommodate the reduction in waterway area associated with the shoaling, the banks of the river appears to have steepened.
Apart from some minor differences, the section appears to have remained in its present condition since 1954. Hence, the formation of the central shoal, which would possibly be associated with the formation of Rawson Island as it is so close, has occurred prior to 1954.
Located approximately 400 metres upstream of cross-section 27, cross-section 28 appears to have only experienced minimal shoaling over the past 100 years or so - refer figure. The centre of the channel has been elevated by only 1 metre in this time, however, on the left hand side of the section, this 1 metre rise in the river bed has halved the water depth.
There does not appear to be any counter-effects due to the reduction in waterway area associated with the shoaling of the channel.
Cross-section 29 is located approximately 800 metres upstream of cross-section 28, within the stretch of river between Tumbulgum and Stott’s Island. This stretch of the river experiences unusual flow patterns due to the bedrock outcrops on the northern bank limiting meandering patterns. It is therefore not surprising that the river has undergone some significant changes over the past 100 years.
The cross-section immediately downstream of this section (cross-section 28) has a rectangular profile of approximately 250 metres wide and 2 metres deep, on average. However, Section 29 has a triangular profile that is only 100 metres wide, but up to 10 metres deep. Although these sections are vastly different, they have the same waterway area, and thus convey river flows with approximately the same velocity.
Since 1884, cross-section 29 has become more triangular in shape as indicated by historical hydrosurveys presented in the figure. The centre of the channel has deepened, while the left hand side of the section has shoaled significantly. As a result of this change, the thalweg of the section has move closer to the centre of the channel by a distance of approximately 20 metres.
Apart from a subtle shallowing of the thalweg, and a deepening of the right hand bank, the surveyed profiles from 1954 onwards are approximately the same, indicating that the major divergence from the 1884 profile occurred in the 70 year period between 1884 and 1954.
Cross-section 30B is located on the bend of the river at Tumbulgum, and is approximately 1000 metres upstream of cross-section 29. Between 1884 and the present, there has been significant changes to this section. During this time, the left bank has steepened, while the right bank has shoaled, as shown in the figure. The crest of the outside bank of the river does not appear to have receded as with sections located on other river bends. This could possibly be the result of bedrock outcrops limiting the landward progression of the bank. The inside of the bend has shoaled approximately 1.5 metres in the last 100 years, and reflects a natural tendency to develop a meandering thalweg.
The differences between the 1954 & 1974 surveys, and the 1979 and 1992 surveys appear to be the result of an inconsistent survey line. Other sections, such as cross-sections 30A and 20 also exhibit apparent inconsistencies, and they were omitted from the analysis.
Situated approximately 800 metres upstream of cross-section 30B, and just upstream of the Rous River confluence, cross-section 32 has exhibited minimal cross-sectional change - refer figure.
Cross-section 34 is located approximately 2000 metres upstream of cross-section 32, in the vicinity of Bartlett’s Creek. Between 1884 and 1979, the survey profiles indicate that there was essentially no change to the section profile, however, since 1979, the left hand side of the section has lowered by approximately 1.5 to 2 metres. The abrupt reduction in the bed elevation of the river, particularly on the inside of a bend, is obviously the result of dredging activities in the river. Historical hydrosurveys at this section are presented in the figure.
Located approximately 1800 metres upstream of cross-section 34, cross-section 36 has changed little over the past 100 years. Between 1884 and 1954, there was a general tendency for the section to move to a more triangular shape, typical of what would be expected on the bend of a river. As a result, there was some accretion on the inside of the bend, accompanied by limited erosion on the outside of the bend - refer figure.
Cross-section 38 is located approximately 1700 metres upstream of cross-section 36, and is just downstream of the Condong Bridge over the Tweed River. As shown in the figure, the bed of the river at this section, as surveyed in 1884, appears to be about 0.5 metres higher than the more recent surveys indicate. However, the recent surveys appear to vary considerably, and it is not possible to be conclusive.
Cross-section 40 is located just downstream of a small intertidal shoal / island in the middle of the river, between Condong and East Murwillumbah, approximately 1200 metres upstream of cross-section 38. Between 1884 and 1954, there appears to be a general deepening of the section by up to 1.5 metres, particularly on the right hand side of the section, as indicated by the figure. On the left hand side, there appears to have been shoaling of the section, with a much reduced waterway width at mid and low tide levels.
It is understood that dredging has been carried out in this area. Natural river processes could also be the cause of the deepening. The shoaling on the left hand side of the section reduced the effective waterway area of the section, and to accommodate this, the section could have naturally scoured its bed to restore the previous waterway area.
This cross-section is located just 600 metres upstream of cross-section 40, and just upstream of the intertidal shoal / island in the river. The left hand side of the section accreted significantly between 1884 and 1974, however, after this time, the shoal was slowly reduced. By 1992, the shoal was completely removed, and the left hand side of the section was the same level as what it was in 1884 - refer figure. The centre and right hand side of the section remained relatively unchanged through the course of time until recently, when the bed was reduced by nearly 1 metre. This reduction, as with the reduction of the shoal on the left hand side of the section was the result of dredging in the river.
The formation of shoals on the left hand side of the section in the previous two cross-sections could have been the result of the formation of the intertidal shoal / island in the middle of the river. The two cross-sections were located on either side of this shoal.
Cross-section 42 is located approximately 500 metres upstream of cross-section 41, and apart from a recent reduction of the right hand side of the section, has not changed since the original survey in 1884 - refer figure. Within the last 15 years, the right hand side of the section has been reduced by between 0.5 and 1 metre, which would be the result of dredging.
Located at the junction between the Tweed River and Mayal Creek, and approximately 600 metres upstream of cross-section 42, cross-section 43 has not changed significantly over the past 100 years. As indicated in the figure, the only difference in the section between the original survey and the present condition is the left hand side of the section is about 0.5 metres shallower, and the right hand side is about 0.5 metres deeper. This variation is too marginal to be the result of dredging activities, and hence would be the result of natural river sedimentation processes.
Cross-section 44 is located approximately 500 metres upstream of cross-section 43, and approximately 500 metres downstream of the Murwillumbah Bridge. Between 1884 and 1979, the only change to the section was the formation of an island adjacent to the left hand side bank. In 1884, the left hand side of the section was relatively shallow (RL -1m AHD), however, in the following 95 years, a shoal grew on the left hand side to over RL 1m AHD. During this time the rest of the section remained unchanged, except for a steepening of the right hand bank. The steepening was possibly a natural response of the river to the formation of the left hand shoal, and was possibly trying to maintain a constant waterway area.
Since 1979, there has been significant dredging at this location reducing the channel bed by up to 4 metres. The shoal / island on the left hand side of the section was removed, and the central section was dredged an average of approximately 3 metres. The far right of the section has been untouched by the dredging recent activities.
The historical hydrosurveys for this cross-section are presented in the figure.
A.2 UNDERWATER DIVING INSPECTIONS
Detailed logs of dives that were carried out in this region are presented at the end of this Section.
Three underwater inspections were carried out in the vicinity of Chinderah, at Jenners Corner, approximately 150 metres upstream of Jenners Corner, and approximately 60 metres upstream of the riverside park beside the Caravan Park. All three inspections found consistent conditions along the riverbank on the outside of the river.
Generally, the inspections found that competent rock protection (ie 100% rock coverage) extended down the slope for a distance ranging from 4 metres at Jenners Corner (ie 70% of bank) to less than 0.5 metres between Jenners Corner and the park (ie <10% of bank). This means that the toe of the slope is still susceptible to scour by river flood flows. The material on the river banks was typically indurated sand, particularly at depth. The bed of the river comprises dense sand, with ripples on the surface.
The indurated sand is susceptible to surface scouring by the high velocities associated with flood flows. The rate of scouring is likely to be considerably damped by the indurated nature of the sand. Hence scouring below the level of rock has continued to scour the bed and bank on the outside of the bend leading to progressive erosion. It is considered that the erosion is ongoing. In the long run the bank could oversteepen, leading to sloughing of the rock and the possibility of deep seeded slumping failure. Without geotechnical analysis, the degree of instability in this regard cannot be quantified.
The rip-rap that protects the upper sections of the underwater bank is basalt with a typical size of approximately 200mm diameter (Mass=10-20kg). Two layers of rip-rap exist only in the intertidal zone, and the decreasing coverage and random scattered coverage with depth indicates that the rock has no competency with depth.
Detailed logs of dives that were carried out in this region are presented at the end of this Section.
Diving was carried out at two sites in the vicinity of Dodd’s and Chinderah Islands. These were outside the entrance to the Oxley Cove canal estate, and just downstream of the junction between the Dodd’s Island back channel and the Tweed River.
Just downstream of Oxley Cove, the underwater inspection showed that substrate material was considerably finer than that inspected at Chinderah. The bed sediment was sandy silt, which was slightly cohesive and contained worm tubules at depth.
In the upper section of the bank, rip-rap was again present, however, the cover significantly reduced at about 1.5 metres below the water surface. This armour was double layered in the intertidal zone, but only single layered below this. The rip-rap on the intertidal berm did not extend to the high water mark, allowing waves to overtop the rock and scour the bank behind. As a result of this wave action, the bank has receded in a series of vertical scarps, of up to 1 metre high. A shallow berm has formed in front of the scarps, on which the rip-rap protection lies.
Approximately 1800 metres upstream, at the next dive site near the Dodd’s Island back channel, the substrate was finer (siltier). At this site, there was no rip-rap, and there was little evidence of erosion. The only erosion on Dodd’s Island was the result of wave action which had caused localised vertical scarps where mangroves did not protect the foreshore.
The bed of the river at this site had a veneer of fine sand overlying finer siltier material. There was no ripples on the surface as there was at Chinderah, possibly due to the different sedimentology.
Detailed logs of dives are presented at the end of this Section.
Two dives were carried out in the Tweed River in the vicinity of Stott’s Island. The first dive was on the opposite bank to the northern tip of Stott’s Island, were the Tweed River turns sharply. Although the aerial photograph interpretation has indicated some bank recession has occurred at this location since 1884, the small amount of recession would suggest that the erosion is being resisted in some way. This dive site was chosen to determine why extensive erosion of this site is not occurring.
The second dive was off Stott’s Island next to the large fig tree on the foreshore of the island. This fig tree is in danger of collapsing into the Tweed River due to undermining of the soil by wave erosion. If the tree falls into the river, it would cause a significant hazard to boating traffic. Diving was carried out at this site to determine if any mechanisms could be implemented to prevent the tree from falling.
At the bend in the river, rip-rap in the intertidal zone prevents significant erosion resulting from wave attack. Below the rip-rap the bank was found to comprise very smooth and current swept firm to stiff grey to blue-grey, relatively dry estuarine clay. The lack of moisture in the sediment indicates that significant consolidation has occurred since the time it was first deposited.
As a result of the extensive consolidation of the sediment that would have occurred since its deposition, the sediment has a relatively high dry density, and shear strength; of the order of 5.7 N/m2. Local bed shears would exceed this only in extreme flood events. Hence the river bank can effectively resist the natural tendency of the river flow to scour the steep banks.
In places, the stiff estuarine clay is overlain by a thin deposit of contemporary organic mud, rich in organic detritus. This softer layer would be eroded easily by each significant flood.
At the second dive site in this region, the diving inspection found that the bulk of Stott’s Island comprises firm to stiff estuarine clay, of probable Pleistocene Age, overlain by a thin capping of oxidised B-horizon loam. The site exhibited an underwater shelf with a shallow gradient, approximately 1 to 1.5 metres below the water surface indicative of gradual bank retreat due to current action. This shelf could be used as a base to fill under the fig tree to prevent it from toppling into the river.
Detailed logs of dives that were carried out in this region are presented at the end of this Section.
Underwater inspections were carried out at two locations in the vicinity of Condong, these being opposite the existing downstream boatramp facility at Condong, and adjacent to the Condong sugar mill. Diving was carried out opposite the boatramp in response to local concerns about shoreline instability. The underwater inspection was aimed at determining the substrate material in order to evaluate feasible means for bank protection. The underwater inspection at Condong sugar mill was essentially carried out to assess the present situation with regard to the amount of rock protection present on the bank, and the level to which this rock protection extends.
The underwater inspection opposite the regional boatramp determined that the substrate material was essentially consolidated, soft to firm estuarine clay, similar to that found in the vicinity of Stott’s Island. Some rip-rap was present in the intertidal zone, however, this was a single layer of rock only.
Intermixed with the estuarine clay are lenses of fluvial sands and silts. A significant amount of detrital material was found at this site, both within the silty and clayey sediment, and in the fluffy surface material.
Adjacent to the Condong sugar mill, rocks, up to 400mm in diameter protect the bank against current scour. The rock coverage, however, is effective only to a depth of approximately 2 metres below the water surface. Below this level, the coverage of rock becomes well less than 100%. Below the rock is black estuarine clay, that is again soft to firm and dry, as determined further downstream. At depth, sandy silt overlays the estuarine clay, with significant amounts of shells and worm tubules.
A.2.5 East Murwillumbah Region
Detailed logs of dives that were carried out in this region are presented at the end of this Section..
Underwater inspections were carried out at one site in this stretch of the river, this being approximately 120 metres upstream of the Condong Bridge, and adjacent to a concrete retaining wall. The objective of inspection the bank at this site was to determine the condition of the substrate material.
The underwater inspection showed that the bank was littered with building rubble from the waterline to a depth of approximately 0.5 metres below the surface. The substrate material was estuarine clay with numerous bivalves to 1 centimetre. The surface of the bank was current swept, and contained a fluffy layer overlying the stiffer clay. On the bed of the river, black sandy silt, full of organic detritus, was present, which would be the result of recent mud deposits further up the catchment.
In general, the retaining walls constructed on the river bank are founded on firm estuarine clay. This clay, as seen further downstream, has a high resistance to scour, thus the structures have minimal chance of becoming undermined due to scour.
Detailed logs of dives carried out in this region are presented at the end of this Section.
Diving was carried out at two sites in the vicinity of Bartlets Creek, viz: 200 metres upstream, and 400 metres downstream of the Bartlets Creek drain. The underwater inspections showed that the underwater slope at both these sites was steep (ie, in the order of 1 in 2), indicating geomorphic processes are active. However, the slope has limited protection by scattered rubble with varying competency, overlying a stiff estuarine clay, which would be difficult to erode.
In the upper section of the bank, rubble with D50 of up to 0.3 metres lines the underwater slope. Near the surface, two competent layers of this rubble are present, however, coverage of the slope by the rock reduces with depth. Also, the rubble protection at the surface does not extend to above the high water level, resulting in localised undercutting and slumping of the upper bank surface behind the rubble.
Detailed logs of underwater inspections carried out in this region are presented at the end of this Section.
Inspections were carried out at two sites in the Rous River, viz: at Dinsey’s Dip, some 5 kilometres upstream of Tumbulgum, and in the vicinity of the confluence with Dulguigan Creek, some 2.5 kilometres upstream of Tumbulgum. The Rous River is typified by undercut banks on the outside of bends, and point bar accretion on the inside of bends. The diving confirmed that banks were only undercut in the intertidal zone, and that this undercutting extended up to 2 metres into the bank. Below the undercut, the underwater slope was relatively steep (approximately 1 in 2) and the stratum was stiff estuarine clay. In straighter sections of the river, the river bed and banks shows evidence of recent fluvial deposits of coarse and fine sediment and organic material.
Rip rap was not found on the underwater bank of the two dive sites in the Rous River, and based on the fact that there was no sign of rip-rap at the water surface, it is highly unlikely that any of the Rous River underwater banks have been protected against current scour or wave attack.
A.2.8 Cobaki Broadwater Region
A detailed log of the underwater inspection carried out in this region is presented at the end of this Section.
One underwater diving inspection was carried out at the entrance to Cobaki Broadwater, just upstream of the existing rip rap bank protection. The dive was made adjacent to a section of riverbank which is suffering extensive upper bank failure.
It was determined from the underwater diving inspection that the upper bank failure was totally independent of river and estuarine processes. The underwater slope at this location was relatively shallow (1 in 6) and showed no signs of current action. In fact, the underwater slope in front of the bank failure site was imbricated with large shells which would provide an effective resistance against tidal or flood current scour. The underlying bed stratum is a sandy silt.
A detailed log of the underwater inspection carried out in this region is presented at the end of this Section.
One underwater inspection was carried on the banks of Ukerebagh Island, on the eastern (main arm) side, adjacent to eroding sand dunes. The underwater inspections showed that the underwater bank slope was relatively shallow (1 in 5) and consisted of clean sand to depth. Slag material was present on the slope indicating that this particular area was an accumulation zone for submerged flotsam and is not an active erosion area.
DIVE SITE 1 - Chinderah
Location Chinderah, adjacent to Jenners Corner (Kingscliff turnoff) - approximate location of cross-section 16
Dive Time 1010 - 1026 (16 minutes), 27/10/94
Water Level (WL) 1.00 metres ISLW (RL 0.07 metres AHD)
Maximum Dive Depth 5.5 metres
Distance along tape (m)
Bed Level
(m below WL)
Notes / Remarks
RL
(m AHD)
0.7
0.0
well interlocked 300-400mm diam. basalt with extensive barnacles 0.07
2.3
1.0
300mm diam. max with shells in voids 0.15m rod penetration to rock and 5mm fluffy mud / grit
-0.93
4.3
2.0
200mm diam. max with some oyster shells 0.2m rod penetration to rock with less fluffy mud
subarmour appears to be well interlocked
-1.93
5.8
2.0
some large intermittent rocks - 400-600mm diam. <0.1m rod penetration to rock with gritty sand between units - well interlocked
-1.93
7.8
3.5
on a rubble ledge with well graded rock of various sizes -3.43
9.5
4.5
<100% coverage of rock - 200mm diam. typ. 0.8m rod penetration to sand and deeper silt ie no competent rock coverage
-4.43
11.1
5.0
only 30% rock coverage with floaters up to 400mm >0.9m rod penetration to sand and some fines
-4.93
11.9
5.5
tail of revetment - no more rock, only sandy bed that is flatter and has worm holes 0.1m rod penetration to dense sand (possibly indurated)
-5.43
14.8
5-5.5
no rock, only dense sand - current swept surface 0.2m rod penetration to sand - no fluffy mud
-5.43 to
-4.93
General Comments
Sample retrieved from the bed was a sub-angular, reworked marine sand with coarse lithics and basalt fragments. The sand was indicative of outcropping indurated sand of Pleistocene Age.
Effective rock revetment stopped approximately 4-4.5 metres below the surface. Hence, that the toe of the slope is not protected against current scour during floods. The material behind the revetment is most probably a dense indurated sand, relic of old beach ridges.
The rock revetment appears to be at its limit of stable slope (ie angle of repose) - less than 1 in 2.
DIVE SITE 2 - Chinderah
Location Chinderah, approximately 150m upstream from Jenners Corner adjacent to floodgates under the highway.
Dive Time 1106 - 1121 (15 minutes), 27/10/94
Water Level (WL) 1.07 metres ISLW (RL 0.14 metres AHD)
Maximum Dive Depth 4.5 metres
Distance along tape(m)
Bed Level
(m below WL)
Notes / Remarks
RL
(m AHD)
0.8
0.0
200-300mm diam. basalt rocks with oyster shells 0.4m rod penetration, units are not fully interlocked
0.14
2.9
1.5
80% rock cover with 200mm diam rock, rest of surface is 1" stone chips and gravel with some fluffy mud -1.36
4.1
2.0
1 layer 200mm diam. rock with some 400mm floaters 0.2m penetration to rock, with no fluffy mud
-1.86
5.3
2.5
30% coverage with 100mm diam. rock, remainder is bare sand and smaller material, little shell 0.25m rod penetration to dense sand
-2.36
6.9
2.5-3
mostly 100mm diam with some 300mm diam rock 0.22m penetration to dense sand, near base of slope
-2.86 to
-2.36
10
NR
10% coverage only with 200mm diam. rock 0.3m penetration to dense sand (possibly indurated)
NR
11
4.5
sandy bed with ripples 0.33m penetration to dense sand
-4.36
14
4.5
same as above -4.36
NR: Not Recorded
General Comments
The revetment is effective only to approximately 2.5 metres below the surface. Below this level, the slope is subject to current scour associated with flood flows. The slope appears to be near the critical stability limit. Further scour of the slope could result in sloughing of the rock and possible collapse, including the top road surface which is located only approximately 10-20 metres from the edge of the bank.
DIVE SITE 3 - Chinderah
Location Chinderah, approximately 60 metres upstream from the park, and adjacent to a 2 metre vertical escarpment at the edge of the highway
Dive Time 1147 - 1207 (20 minutes), 27/10/94
Water Level (WL) 1.13 metres ISLW (RL 0.20 metres AHD)
Maximum Dive Depth 7 metres
Distance along tape(m)
Bed Level
(m below WL)
Notes / Remarks
RL
(m AHD)
0.0
0.0
500-600mm diam. rock - 2 layers 0.20
1.6
0.5
70% coverage with up to 400mm diam rock - rock is random and most not engaged, with large sandy voids in between, 0.8m penetration to sand -0.30
2.8
1.5
single layer with no subarmour - odd 700mm diam floater, some fluffy mud and gritty sand between rocks, ie not effective revetment -1.3
4.9
2.0
10% coverage with 400mm diam and 50% coverage with 200mm diam rocks - remainder is sand >0.9m rod penetration to sand
-1.8
7.3
3.0
60% coverage of 200mm diam rock - maybe 2 layer 0.3m rod penetration, with less intersticies
-2.8
8.8
4.5
40-60% coverage with 200-300mm diam rocks with some 600mm diam floaters + oysters and fluffy mud -4.3
10.6
5.0
30% coverage total, 800mm diam floater 0.2m rod penetration to dense sand - no sub armour
-4.8
12.9
5.5
10% cover with 100mm diam rock 0.2m penetration to dense sand
-5.3
15.9
6.0
scattered rock - loose and random 0.2m penetration to dense sand
-5.8
19.2
7.0
Scattered rock on sand -6.8
General Comments
As with the previous two sites, the rock revetment is effective for only a short distance down the slope, leaving the toe of the slope open to current scour during times of flood. The revetment is effective only to 1.5 to 2 metres below the surface at this site, which is less than one third of the total river depth.
The vertical scarp at the surface is indicative of previous sloughing of armour on the top of the slope. The slope has a steep angle (1 in 2), and is considered to be very susceptible to collapse.
DIVE SITE 4 – Oxley Cove
Location Left bank of river under the powerlines, just downstream of the entrance to Oxley Cove canal estate
Dive Time 1240 - 1255 (15 minutes), 27/10/94
Water Level (WL) 1.20 metres ISLW (RL 0.27 metres AHD)
Maximum Dive Depth 4.5 metres
Distance along tape(m)
Bed Level
(m below WL)
Notes / Remarks
RL
(m AHD)
1.3
0.0
250-300mm diam rock with oyster shells - 2 layers in the intertidal section 0.27
4.1
0.5
80% coverage with 1 layer of 200-300mm diam rock >0.9m rod penetration to sand, measured from surface.
-0.23
5.4
1.5
80% coverage with 1 layer of 200mm diam rock - some mussel shells and seagrasses - bleached sand ie end of effective wave protection -1.23
6.5
1.5
<20% cover with some 300mm diam floaters >0.9m rod penetration to sand
-1.23
8.6
3.0
<20% rock on surface (200mm), with little more sand 0.5m rod penetration to gravel
-2.73
10.7
3.5
as above, with weak consolidated and slightly cohesive sediment 0.1m below the surface 0.6m rod penetration
-3.23
12.5
4.5
sediment is coagulated - flocc silts and few shells >0.9m rod penetration with tubules and some 5cm stones
-4.23
14.6
4.5
10% cover with 100mm diam stones 0.6m rod penetration into shell bed
-4.23
General Comments
Rip rap is only effective within the intertidal zone. The sediment appears to be finer with distance up the river. Undercutting above the level of the rip rap has resulted in a sandy intertidal beach below small escarpments resulting from small wave action.
In the same vicinity, poor rip rap construction, involving the simple dumping of stone on the intertidal berm, has resulted in reworking of fine material by wave action and collapse of the revetment which is now outflanked by waves at high tide.
There is a 1 metre vertical escarpments due to wave erosion of the banks. Pronounced beaches in front of the scarps indicates redistribution of the sandy component of the banks. Long term recession of the shoreline in this area is considered to be slow indicated by the exposed roots of a large fig tree in the erosion escarpment.
DIVE SITE 5 – Oxley Cove
Location Just downstream of the back channel at the upstream end of Dodd’s Island
Dive Time 1337 - 1350 (13 minutes), 27/10/94
Water Level (WL) 1.25 metres ISLW (RL 0.32 metres AHD)
Maximum Dive Depth 2 metres
Distance along tape(m)
Bed Level
(m below WL)
Notes / Remarks
RL
(m AHD)
0
0.2
2 metres from the water line – clean sand veneer overlying grey silty sand 0.12
2.9
0.5
>0.9m rod penetration with sand quite dense at depth -0.18
4.6
1.0
as above with silty sand 0.2m below surface - reasonably cohesive - siltier than dive site 4 -0.68
7.5
1.5
>0.9m rod penetration with little resistance, more cohesive with black sediment below surface, 2" mangrove branch detritus -1.18
10.5
1.75
greasy (finer) sediment, with tubules and sparse seagrass - no shells -1.43
14.0
2.0
medium-fine sand in veneer overlying finer sediment current swept surface with no ripples
-1.68
General Comments
The river bank is mostly fringed with mangroves, pacific hibiscus and casuarina, with dense pneumatophore mats on the intertidal bench. Occasional 1 metre escarpments exists on bank where the bank is fretting due to wave action - typified by lack of vegetation.
Does not appear to be a significant problem at this site.
DIVE SITE 6 – Stotts Island
Location Outside of sharp river bend at the northern tip of Stott’s Island
Dive Time 1527 - 1547 (20 minutes), 27/10/94
Water Level (WL) 1.30 metres ISLW (RL 0.37 metres AHD)
Maximum Dive Depth 9 metres
Distance along tape(m)
Bed Level
(m below WL)
Notes / Remarks
RL
(m AHD)
0.0
0.0
200-400mm diam rock in the intertidal zone only 0.37
1.7
1.0
0.8m rod penetration - soft refusal indicating soft mud -0.63
3.2
2.5
as above -2.13
5.1
2.5
cohesive, high plasticity mud 1.0m rod penetration
-2.13
7.8
4.0
1.0m rod penetration -3.63
11.4
6.0
1.4m rod penetration -5.63
13.0
7.0
loose veneer (approx 200mm) of fluffy mud (ie weakly consolidated) and current swept on top of high plasticity mud -6.63
14.4
6.5
1.5m rod penetration into high plasticity mud -6.13
18.8
7.0
1.5m rod penetration -6.63
20.7
7.5
>1.8m rod penetration, plasticine-like material below a fluffier surface -7.13
23
9
as above -8.63
General Comments
Rip-rap has been placed in the intertidal zone only.
Below the surface, the material is mostly estuarine clays.
From RL 0m to approx RL -3m, the material is a black organic silt, dry, soft to firm with some fibrous organic matter. The sample has no odour.
From Approx RL -3m to -8m, the material is a grey, with brown mottle, estuarine clay, which is dry and soft to firm indicating relatively low moisture content and therefore considerable consolidation. Brown mottle is suggestive of Pleistocene or early Holocene Age. The sample has no odour.
Below approx RL -8m, the material is a blue-grey estuarine clay containing bivalves and tubules. It is dry, soft to firm and has no odour, similar to the overlying clay.
This very sharp bend in the river has not undergone extensive bank recession because the estuarine clays appear to be Pleistocene (ie relatively dry and consolidated) with considerable cohesive strength.
Laboratory testing found a moisture content of 50% indicating a dry density of 1150 kg/m3 and indicating a shear strength (relates to current scour) of 5.7 N/m2. This is most likely to exceed the maximum shear stress exerted by the Tweed River in times of flood. Hence, the estuarine clays have easily resisted lateral channel migration and there is little long term bank recession.
This site is not seen as having a long term erosion problem.
DIVE SITE 7 – Stotts Island
Location Under roots of large overhanging fig tree on the western side of Stotts Island, behind Rawson Is
Dive Time 1618 - 1636 (18 minutes), 27/10/94
Water Level (WL) 1.29 metres ISLW (RL 0.36 metres AHD)
Maximum Dive Depth 6 metres
Distance along tape(m)
Bed Level
(m below WL)
Notes / Remarks
RL
(m AHD)
0.3
0.0
Very stiff and dry oxidised B-horizon loam 0.36
3.7
1.0
Fluffy surface, but stiffer 100mm below 0.6m rod penetration with stiffish refusal
-0.64
5.0
NR
Very fluffy mud surface - elbow deep to underlying stiff mud, with extensive root mat detritus NR
5.6
1.6
0.6m rod penetration into firm - stiff estuarine mud -1.24
7.2
3.0
1.2m penetration in shells, no real refusal -2.64
10.0
3.5
Very oily, current swept surface, material is black-grey with brown mottle estuarine clay, dry, soft-firm with some organic fibres - representative of 4-8 metres -3.14
12.7
5.0
Current swept surface 1.8m penetration into estuarine mud
-4.64
15.1
6.0
Softer material 1.0m penetration into estuarine mud
-5.64
18.5
6.0
Shelly or sandy horizon below the surface, bivalves present and bioturbated - dry, firm blue-grey estuarine clay with occasional sticks and organic matter - Pleistocene in age. -5.64
NR: Not Recorded
General Comments
Material is similar to dive site 6 with overlying B-horizon loam associated with the formation of Stotts Island.
Erosion of the bank in the intertidal areas would be associated with wave erosion, to the extent that the big fig tree is in danger of collapsing from undermining.
Could possibly fill under the fig tree as there is a bench in the bank which would support the fill material.
DIVE SITE 8 - Condong
Location Condong - opposite the downstream public boatramp.
Dive Time 1044 - 1105 (21 minutes), 28/10/94
Water Level (WL) 0.95 metres ISLW (RL 0.02 metres AHD)
Maximum Dive Depth 4.5 metres
Distance along tape(m)
Bed Level
(m below WL)
Notes / Remarks
RL
(m AHD)
0.4
0.1
single layer of 100-200mm diam rock in intertidal zone overlying black estuarine clay with brown mottles and lots of detritus - dry, soft-firm and tubules 0.8m rod penetration into clay
-0.08
3.4
0.5
cohesive silt which has been burrowed, and contains 1/2" bivalves 1.35m rod penetration into clay
-0.48
5.5
2.5
stiff blue estuarine clay 0.1 to 0.2 metres below surface, bivalved and burrowed, softer surface material 0.5m rod penetration
-2.48
7.5
3.2
matted detrital litter on stiff blue estuarine clay 0.1 metres below surface 1.3m penetration to stiff clay refusal
-3.18
10.2
4.2
estuarine clay 0.2m below fluffy surface containing bivalves - loose black organic sandy silt below surface 1.3m penetration
-4.18
12.9
4.2
Slightly cohesive grey silt with shelly/sandy layer 0.1m below surface - fluvial sands -4.18
15.6
4.5
coarse sandy-gravel matrix 0.1 m below surface 1m penetration with sandy refusal
-4.48
20
4
Interlayering of sand lenses amongst estuarine clays 1m penetration into coarse grey fluvial sands - no shells
-3.98
General Comments
Firm to stiff estuarine clays dominate the site, the brown mottles, relative dryness and stiffness are indicative of Pleistocene / early Holocene Age. At depth, fluvial sands and black estuarine clays interfinger, indicating that increasing strength of fluvial processes. The outcropping stiff clays rule out deep seated slip failures. It is considered that erosion is not progressive and is related to fretting and bank undercutting at water level by wave action.
DIVE SITE 9 - Condong
Location Condong Sugar Mill
Dive Time 1152 - 1207 (15 minutes), 28/10/94
Water Level (WL) 0.99 metres ISLW (RL 0.06 metres AHD)
Maximum Dive Depth 5.5 metres
Distance along tape(m)
Bed Level
(m below WL)
Notes / Remarks
RL
(m AHD)
0.0
0.0
Loose, random rocks, 200-300mm diam. ie not a competent revetment 0.06
2.4
1.0
50% coverage in 1 layer with rocks less than 100mm diam. - black estuarine basal silt 0.2m rod penetration through loose surface to covered rock rip-rap
-0.94
4.0
2.0
60% coverage of 200-400mm diam rock 0.25m rod penetration to rock rip-rap through loose fluffy mud
-1.94
5.6
3.0
200mm layer of loose silty material with some 300mm diam rock under the surface 0.45m rod penetration to covered rock rip-rap
-2.94
6.0
NR
Material under surface fluff is black, soft-firm and dry estuarine clay with some detritus, tubules and bivalves NR
8.3
4.0
Loose black sandy silt layer over more cohesive burrowed clay, shelly silt - no rock 0.3m rod penetration to firm clay refusal
-3.94
10.8
5.0
Loose surface, 100mm thick, which is black loose, weakly consolidated mud, with some detrital matter 0.3m rod penetration to firm clay
-4.94
12.9
5.5
Loose fluffy material on surface - material under is black, very dry and very firm estuarine clay with minor amounts of shell 0.3m rod penetration to firm clay
-5.44
NR: Not Recorded
General Comments
Rip rap protection extends only 2 to 3 metres below the water surface. Firm estuarine clay outcrops in the bank which is not highly erosive. There should not be a significant erosion problem at this site.
DIVE SITE 10 – East Murwillumbah
Location Just upstream of Condong Bridge adjacent to failing concrete retaining wall
Dive Time 1425 - 1434 (9 minutes), 28/10/94
Water Level (WL) 1.20 metres ISLW (RL 0.27 metres)
Maximum Dive Depth 4.5 - 5 metres
Distance along tape(m)
Bed Level
(m below WL)
Notes / Remarks
RL
(m AHD)
0.0
0.0
Corner of concrete wall and rubble slope 0.27
1.9
0.5
Surface Rubble, bricks and hard debris material 1.0m rod penetration into soft estuarine clay
-0.23
3.7
NR
End of the rubble debris 0.9m rod penetration into burrowed, stiff estuarine clay
NR
6.0
NR
Soft to firm black estuarine clay with numerous bivalves to 1 cm. Surface is current swept NR
6.8
3.5
Softer muddy veneer overlying stiff estuarine clay 0.9m rod penetration into clay
-3.23
9.0
NR
Firm, black estuarine clay with bivalves NR
10.0
4.5
Softer estuarine clay becoming stiff with depth 1.5m rod penetration
-4.23
13.4
4.5 - 5
soft surface - black sandy silt full of organic detritus, typical of recent mud deposits >1.8m penetration into underlying estuarine clay
-4.73 to
-4.23
NR: Not Recorded
General Comments
Relatively stiff (Pleistocene) estuarine clays are exposed throughout the bank, mantled in places with contemporary silt deposits on the bed and lower banks. The stretch to Murwillumbah comprises ad-hoc bank protection with a variety of construction techniques. The fact that they appear to be ‘protecting’ the bank can be attributed more to the inherent strength of the relative stiff Pleistocene estuarine clay than the efficiency of the various informal revetment designs.
The walls serve to protect against wave fretting. Because there seems to be no mechanisms for deep seated failure, and the outcropping clay is resistant to river scour, it is considered that this area does not have a significant long term erosion problem.
DIVE SITE 11 – Bartlets Creek
Location Outside of bend half way between Bartlets Creek and Dinseys Creek
Dive Time 1025 - 1050 (25 minutes), 24/2/98
Water Level (WL) 1.44 metres ISLW (RL 0.51 metres AHD)
Maximum Dive Depth 5.3 metres
Distance along tape(m)
Bed Level
(m below WL)
Notes / Remarks
RL
(m AHD)
0.0
0.0
Rock rubble with D50 ~ 0.3 metres overlying soil 0.51
1.0
0.4
Competent revetment with well graded rubble of D50 ~ 0.05 to 0.2 metres 0.11
2.0
0.6
Largely construction rubble, such as bricks and stones with D50 ~ 0.3 metres – 2 layers of rubble – competent revetment -0.09
3.0
NR
D50 ~ 0.3 metres random single stones on gravelly mud underlayer – reasonable competent protection 4.0
2.0
2 layers of D50 ~ 0.15 to 0.3 metres with soft mud overlying and in voids – still reasonably competent -1.49
5.0
2.5
Well graded D50 ~ 0.1 metre rubble with loose mud on top – still competent -1.99
6.0
NR
Scattered 0.1 metre stones on stiff estuarine, early Holocene mud 7.0
3.0
10cm of fluffy mud overlying odd scattered stones on a stiff mud base -2.49
9.0
3.5
Heavily burrowed stiff clay – no stones, no fluffy layer -2.99
11 - 14
4
5 – 10 cm of fluffy mud overlying burrowed stiff clay -3.49
NR: Not Recorded
General Comments
The relatively steep underwater slope (1v:2h to 1:3) is competently protected with rubble for the top half of the riverbank, including most of the intertidal zone. Some overtopping at high tide is causing isolated bank fretting and slumping.
The steep underwater slope is typical of the outside of a river bend, indicating that geomorphic processes are active, however, the stiff estuarine clay significantly limits the meandering process.
DIVE SITE 12 – Bartlets Creek
Location Outside of bend 400 metres downstream of Bartlets Creek
Dive Time 1130 - 1141 (11 minutes), 24/2/98
Water Level (WL) 1.28 metres ISLW (RL 0.35 metres AHD)
Maximum Dive Depth 3.2 metres
Distance along tape(m)
Bed Level
(m below WL)
Notes / Remarks
RL
(m AHD)
0.0
0.5
Incompetent revetment with random rubble scattered on a sub-vertical earth embankment -0.15
2.0
0.4
Scattered stones up to D50 ~0.2 meters on a friable material base with mangrove roots and stiff estuarine clay -0.05
3.0
0.6
2 layers of D50 ~0.2 metres rubble – competent revetment -0.25
4.0
1.1
2 layers of D50 ~0.15 to 0.25 metres rubble – competent revetment -0.75
5.0
1.7
D50 ~0.15 to 0.25 metres rubble – marginally competent revetment -1.35
6.0
2.2
Single layer of 0.15 to 0.2 metres rubble on stiff estuarine burrowed clay. Rubble is not interlocked and revetment is not competent -1.85
7.0
2.5
Scattered stones up to D50 0.4 metres on a stiff estuarine clay base -2.15
8.0
2.9
Isolated stones up to D50 ~0.5 metres on a stiff estuarine clay -2.55
9.0
3.0
Gravelly silty material on heavily burrowed stiff estuarine clay -2.65
15 - 20
6 – 6.5
Drop-off measured from depth-sounder ~ -6.0
NR: Not Recorded
General Comments
Historic rubble has slumped to the middle section of the bank due to a steep underwater slope, leaving the upper and lower portions of the bank relatively unprotected.
The steep underwater slope indicated meandering processes are still active, however, rates are slow due to the stiff estuarine clay.\
DIVE SITE 13 – Rous River
Location Outside of bend adjacent to Dinseys Dip corner
Dive Time 1330 - 1337 (7 minutes), 24/2/98
Water Level (WL) 0.93 metres ISLW (RL 0.0 metres AHD)
Maximum Dive Depth 3.5 metres
Distance along tape(m)
Bed Level
(m below WL)
Notes / Remarks
RL
(m AHD)
0.0
0
Bank undercut a depth of 2 metres over a height of 0.4 metres. 0.1 metres soft mottled clay (topsoil fallen in) over stiff to very stiff grey sandy estuarine clay. 0.0
2.0
1.0
0.1 metres of soft mud on top of stiff grey sandy estuarine clay -1.0
3.0
1.5
0.05 metres of loose mud on stiff estuarine clay (not sandy) -1.5
4.0
2.0
0.05 metres of fluffy mud on stiff estuarine clay -2.0
7.0
3.2
stiff estuarine clay -3.2
NR: Not Recorded
General Comments
The very deep undercutting of the bank has been caused by boat waves – the bank is inherently unstable, and is only being held together by vegetation and tree root systems. The steep underwater slope (1 in 2) is typical of a meandering channel, however, the stiff estuarine clay significantly limits the rate of bank recession.
DIVE SITE 14 – Rous River
Location Outside of bend approximately 3 kilometres upstream from Tumbulgum
Dive Time 1400 - 1407 (7 minutes), 24/2/98
Water Level (WL) 0.84 metres ISLW (RL –0.09 metres AHD)
Maximum Dive Depth 2.0 metres
Distance along tape(m)
Bed Level
(m below WL)
Notes / Remarks
RL
(m AHD)
0.0
0
Waters edge – easily erodable, burrowed, friable soft brown with blue mottled clay (classic alluvium strata) which is receding on a vertical face -0.09
1.0
0.2
0.1 metres loose brown alluvial material (from bank) on top of stiff grey, slightly mottled estuarine clay (with some oxidation) -0.29
2.0
0.3
grey stiff estuarine clay (not mottled) -0.39
3.0
0.4
0.2 metres loose mud on top of grey stiff estuarine clay -0.49
4.0
0.8
0.4 metres of loose mud containing occasional sticks and leaves on top of stiff estuarine sandy clay -0.89
5.0
1.2
0.2 metres of loose mud on top of clean medium sized sand -1.29
6.0
1.4
0.2 metres of loose mud on top of clean medium sized sand -1.49
8.0
1.9
0.3 metres loose mud with occasional sticks on a light grey silty sand (recent fluvial deposit) with some basalt gravel deposits) -1.99
~12.0
~2.5
-2.59
NR: Not Recorded
General Comments
Site is closer to downstream end of Rous River and is located in more of a straight section, hence it is susceptible to fluvial deposits, as typified by the organic material, sandy material, and basalt gravels.
The base stratum is stiff a stiff estuarine clay, which is typical of the whole upper estuary floodplain area.
DIVE SITE 15 – Cobaki Broadwater entrance
Location Outside of bend at the entrance of Cobaki Broadwater, at the failing upper riverbank
Dive Time 1550 - 1600 (10 minutes), 24/2/98
Water Level (WL) 0.81 metres ISLW (RL –0.11 metres AHD)
Maximum Dive Depth 2.3 metres
Distance along tape(m)
Bed Level
(m below WL)
Notes / Remarks
RL
(m AHD)
0.0
0
Sandy foreshore with random rubble to D50 ~0.3 metres -0.11
2.0
0.25
5 cm loose sand over grey soft sandy silt with a dense covering of shells -0.36
3.0
0.4
as above -0.51
5.0
1.0
soft silty sand overlying a matrix of shell lag of large oysters – effective resistance against current scour -1.11
6.0
1.2
as above -1.31
8.0
1.6
surface lag of bivalves, oyster shells, soft silty sand on the surface over shell layer of large oyster shells -1.71
10.0
1.8
sandy silt with 75mm diam oysters, mussels and bivalves shells – totally current resistant -1.91
12.0
2.0
as above, except silty matrix is firmer -2.11
14.0
2.2
lots of 75mm bivalves which are intact and imbricated -2.31
NR: Not Recorded
General Comments
The upper bank failure has nothing to do with tidal flows, current scour or wave action. Underwater slope is shallow, and well protected by a dense surface covering of large imbricated shells.
Bank instability is caused by piping failure of bank – ground water flows are taking material out of bank causing recession and outflanking of vegetation. Requires complete bank protection, however, amour can be small as it is only to hold granular material in place – ie is not going to be affected by current scour etc.
DIVE SITE 16 – Ukerebagh Island
Location Adjacent to Ukerebagh Island in the minor channel of the Tweed River main arm
Dive Time 1710 - 1720 (10 minutes), 24/2/98
Water Level (WL) 1.04 metres ISLW (RL 0.11 metres AHD)
Maximum Dive Depth 2.5 metres
Distance along tape(m)
Bed Level
(m below WL)
Notes / Remarks
RL
(m AHD)
0.0
0
Waters edge: Estuarine peat outcropping with old mangrove roots. 10cm of clean estuarine sand overlying old estuarine peat 0.11
2.0
0.25
Clean sand -0.14
5.0
0.60
Clean sand -0.49
6.0
0.90
Clean sand with lumps of peat on the slope -0.79
8.0
1.70
Detrital shells on surface with old timber stake, logs etc – Clean sand with occasional shell -1.59
10.0
2.20
Some carbonised timber, mostly detrital shell lag on top of clean sand with shells up to 50mm in size -2.09
12.0
2.40
Clean sand and estuarine peat and rotten timber (slightly algilated) – accumulation zone at base of slope – not very active. -2.29
NR: Not Recorded
General Comments
Shallow underwater slope which shows little sign of active erosion.
A.3.1 Lower Estuary (Entrance to Chinderah)
Recession
The photogrammetric data clearly displays the gradual recession of Ukerebagh Island’s east shore (site 57). The maximum distance of bank recession lies by 39m suggesting a current movement rate of 1.2m/year.
Additional erosion is evident on the east bank, 300m downstream of Barneys Point Bridge (site 54), located near the end of the training wall. The bank has gradually receded by around 10m.
Accretion
No significant sites of accretion were noted from the photogrammetry in the lower reaches of the Tweed River main arm.
Recession
Erosion opposite Chinderah Bay (site 52) is noted in 2 isolated locations which are connected by a stretch of stable bank. Both sites of erosion reveal loss of terrain in the order of 12m (or 38cm/year). Apparently the former lengthy scour has been repaired with 2 weak spots giving in to the continuing erosive forces at either end of the repaired section.
Accretion
The accretion labelled site 53 has continued to develop in the recent past. The bank has expanded by up to 12.5m in width resulting in a movement rate of 40cm/year.
Recession
Evaluation of the photogrammetric data reveals restricted erosion occurring upstream of Dodd’s Island at the entrance to Boyds Channel (site 46). During a period of 32 years the bank has receded by 10m (or 31cm/year).
Further erosion appears to exist on the southern bank of Boyds Channel, upstream of the major drain discharging into the channel (site 47). The erosion has affected the bank on a length exceeding 150m, causing the bank to withdraw by 5m within the abovementioned time frame.
Accretion
Accretion (site 45) is discernible according to photogrammetric data. From 1962 on, the bank continually pushes the river back northwards. reaching a maximum distance of 12m in 1994, ie. gaining 38cm/year.
Recession
No recession of the riverbanks has occurred in the vicinity of Tweed Broadwater since 1962.
Accretion
The photogrammetric data does not show any evidence of accretion in the vicinity of Tweed Broadwater, however, significant lengths of the riverbank in the photogrammetry analyses have not been plotted due to foreshore vegetation.
Recession
The photogrammetric information identified erosion to the north of Stotts Island (site 38) but only to a limited extent. Most of the river’s border at the site shows no sign of erosion between 1962 and 1994, which implies that the erosion will have taken place during an earlier period of time. However, in the vicinity of the floodgate between "River Bend" and "Tweedside" the bank has been disturbed. A primary scour documented in 1971 was accompanied by a second by 1985. In 1994 the site had eroded over a length in excess of 50m, with the river bank being pushed back by 6.5m. The rate of bank movement at this specific location is 28cm/year.
Accretion
No information is provided on bank movements on Rawson Island (site 35). Findings regarding accretion along Stotts Channel (site 42) are supported by the photogrammetric evaluation. The accretion comprises a maximum width of 12m, thereby broadening and straightening the channel’s bank over more than 300m. The current rate of accretion results to 38cm/year.
Erosion of the northern bank opposite Tumbulgum (site 30) is not confirmed by the photogrammetric data. This indicates that the development of scour has come to a halt before 1961.
The data, however, does suggest the following additional sites of erosion:
- Some 200m downstream of the above named site, erosion appears to affect the northern bank (site 31).
Further erosion appears at both the upstream (site 27) and downstream (site 29) ends of the accretion on the opposite lying southern bank.
In each case the bank recession consists of 6m, completed during 32 years. Accretion
The accretion at the northern end of Tumbulgum (site 28) noticed by comparing a recent aerial photo with the historical map is also reflected in the plotted photogrammetric data. The bank movement during the recent 32 year period was, however, modest reaching a maximum of 5m (or 16cm/year).
Recession
The erosion identified above on the north-west bank of Ukerebagh Island (TI02) cannot be commented due to lack of photogrammetric data for the specific site.
The erosion labelled as site 59 has continued to take place until 1994. The maximum distance of bank recession within the observed 32 years amounts to 25m (or 80cm/year).
Accretion
Relevant areas of accretion were not discernible on the basis of available data.
Recession
The available data did not provide adequate information regarding bank recession.
Accretion
At site 70 the location of the creek’s banks in 1971 and 1985 indicate substantial accretion in the vicinity of the western bridge head.
APPENDIX B - RIPARIAN SPECIES LIST
Source: Tweed Shire Council
Appropriate Local Riparian Plants for Riverbank Stabilisation
Tweed Estuary.
Tree Species:
| Acacia bakeri | Marble Wood | |
| Acacia melanoxylon | Black Wood | |
| Acmena ingens | Red Apple | |
| Archidendron muellerianum | Veiny Lace Flower | |
| Archonophoenix cunninghamiana | Bangalow / Piccabeen Palm | |
| Araucaria cunninghamii | Hoop Pine | |
| Arytera divarcata | Coogera | |
| Arytera distylis | Twin Leaved Coogera | |
| Austromyrtus bidwillii | Python Tree | |
| Banksia integrifolia | Coastal Banksia | |
| Callitrus macleayana | Stringybark Pine | |
| Castanospermum australe | Black Bean | |
| Casurina glauca | River Oak (st) | |
| Casurina distyla | Black Cockatoo Food (st) | |
| Commersonia bartramia | Brown Kurrajong (st) | |
| Cryptocaria foetida | Stinking Cryptocaria | |
| Cryptocaria obovata | Pepperberry Tree | |
| Cryptocaria triplenervis | Three Veined Cryptocaria | |
| Cupaniopsis anacardioides | Tuckeroo (st) | |
| Cupar~iopsis newmannii | Long-Leaved Tuckeroo (E) | |
| Diploglottis australis | Native Tamarind | |
| Diploglottis cambelii | Small Leaved Tamarind (E) | |
| Eleocarpus grandis | Blue Fig / Quondong | |
| Eleocarpus obovata | Hard Quondong | |
| Endiandra globosa | Black Walnut | |
| Euodia erellyana | Pink Euodia | |
| Ficus coronata | Sandpaper Fig | |
| Ficus macrophylla | Moreton Bay | |
| Ficus watkinsonia | Strangler Fig | |
| Ficus virens | White Fig | |
| Flindersia australis | Teak | |
| Flindersia schottiana | Coodgerie | |
| Cordyline congesta | (E) | |
| Cordyline petiolaris | Broad Leaved Palm Lilly | |
| Cordyline rubra | Red Fruited Palm Lilly | |
| Cordyline stricta | Narrow Leaved Palm Lilly | |
| Crinum pedunculatum | River Lilly (st) | |
| Dawsonia superba | Giant Moss (gc) | |
| Dianella congesta | Flax Lilly (gc) | |
| Elatostema reticulatum | Rainforest Spinach | |
| Helmholtzia glaberrima | Stream Lilly | |
| Histiopteris incise | Bats Wing Fern | |
| Hovea longifolia | Purple Pear Bush | |
| Hymenosporum flavum | Native Frangipani | |
| Hydrocotyle pedicellosa | Pennywort | |
| Juncus krausii | Sea Rush (gc - st) | |
| Juncus usitatus | Common Rush (gc - st) | |
| Leptospermum laevigatum | Coastal Tea Tree | |
| Linospadix monostachya | Walking Stick Palm | |
| Lobelia trigonocaulis | Forest Lobelia | |
| Lomandra hyaterix | Spiny Mat Rush (gc) | |
| Lomandra longifolia | Mat Rush (gc) | |
| Lycopodium cernuum | Coral Fern - club moss (gc) | |
| Melastoma affine | Blue Tounge | |
| Microcitrus australis | Finger Lime | |
| Myoporum insulare | Coastal Boobialla | |
| Panicum pygmaeum | Dwarf Panic | |
| Peperomia tetraphylla | Four Leaved Pepper Plant | |
| Rhodomyrtus psidiodes | Native Guava | |
| Sporobolis virginicus | Salt Couch (gc - st) | |
| Solanum sp. aff. Cinereum | Tall Nightshade | |
| Syzygium hodgkinsoniae | Red Lilly Pilly (R.) | |
| Viola hedeacea | Ivy Leaved Violet (gc) | |
Vines, Creepers. Scramblers & Epiphytes
| Aristolochia praevenosa | Richmond Birdwing Vine |
| Asplenium australasicum | Bird's Nest Fern |
| Calamus muelleri | Lawyer Vine |
| Dioscorea transverse | Native Yam |
| Eustrephus latifolius | Wombat Berry |
| Flagellaria indica | Whip Vine |
| Freycinetia excelsa | Climbing Pandanus (E) |
| Hardenbergia violacea | False Sarsparilla |
| Maclura cochinchinsis | Cockspur |
| Millettia megasperma | Native Wysteria |
| Passiflora aurantia var. aurantia | Red Passion Flower |
| Glochidion ferdinandii | Cheese Tree |
| Glochidion sumatranum | Umbrella Cheese Tree |
| Grevillea robusta | Silky Oak |
| Guoia semiglauca | Guioa |
| Harpulia pendula | Tulip Wood |
| Hibiscus tiliaceous | Cottonwood (st) |
| Jagera pseudorus | Foambark (st) |
| Lepiderema pulchella | Fine-leaved Tuckeroo (E) |
| Livistonia australis | Cabbage Tree Palm |
| Lophostemon suaveolens | Swamp Turpentine |
| Macaranga tanarius | Dinner Plate Tree (st) |
| Mallotus philipenensis | Red Kamala |
| Melaleauca quinquinervia | Paper Bark (st) |
| Melia azaderach | White Cedar |
| Neolitsea dealbata | Bollygum |
| Omalanthus populifolius | Bleeding Heart |
| Pittosporum revolutum | Hairy Pittosporum |
| Pittosporum undulatum | Sweet Pittosporum |
| Planchonella austalis | Black Apple |
| Polyscias elegens | Cellerywood |
| Psychotria loniceroides | Hairy Psychotria |
| Phodosphaera rhodanthema | Deep Yellow Wood |
| Sarcopteryx stipata | Steelwood |
| Stenocarpus sinatus | Firewheel Tree |
| Sterculia quadrifida | Peanut Tree |
| Syzlglum francisii | Giant Watergum |
| Syzigium leuhmannii | Riberry |
| Syzigium mooreii | Coolamon / Duroby (E) |
| Tristaniopsis laurina | Water Gum |
Understorey Species
| Acmena smithii | Creek Lilly Pilly |
| Acrostchia speciosum | Bungawal Fern (gc - st) |
| Acrostichum speciosum | Mangrove Fern (st) |
| Alectryon coriaceus | Beach Alectryon |
| Alocasia brisbanensis | Cunjevoi |
| Alpinea caerulea | Native Ginger |
| Austromyrtus dulcis | Midyim Berry (gc) |
| Blechnum nudum | Fishbone Water Fern (gc) |
| Blechnum cartilaginum | Gristle Fern (gc) |
| Callicarpa pedunculata | Velvet Leaf |
| Cassia brewsteri var. marksiana | Brush Cassia (E) |
| Citriobatus pauciflorus | Orange Thorn |
| Commelina cyanea | Native Wandering Jew (gc) |
| Burney Vine | |
| Pothos longipes | Pothos |
| Rubus sp. | Bramble |
| Smilax australis | Australian Sarsparilla |
Mangrove Tree Species
| Aegieerus eornieulatum | River Mangrove |
| Avleenma marma | Grey Mangrove |
| Bruguiera gymnorrhiza | Blaek Mangrove |
| Exoeearia agalloeha | Milky Mangrove |
| Myoporum aeuminatum | Mangrove Boobiala |
| Rhizophora stylosa | Red Mangrove |
Salt Marsh Shrubs
| Enehylaena tomentosa | |
| Halosaeia haloenemoides | |
| Sarcoeornia quinqueflora | |
| Sesuvium portulaeastrum | |
| Sueada arbuseuloides |
Sedges, Reeds & Rushes
| Baumea juncea | (st) |
| Bulboschoenus fluviatilis | (st) |
| Cyperus sp. | (st) |
| Juncus kraussii | Sea Rush (st) |
| Juncus usitatus | Common Rush (st) |
| Phragmites australis | Common Reed (st) |
| Sarnolus repens | (st) |
| Trigloehin striate | (st) |
| Typha sp | Bullrush |
Sea Grass Species
| Zostera caprieorn | Eel Grass |
| Halophila ovalis | Sea Wrack |
| Halodule uninervis | ? |
For further information contact Grant Periott, Riparian Project Officer Tweed Shire Council. This is only a rough guide as to the potential local species for riparian regeneration projects. More detail on plant habit and ecology is available on request.
Grant Periott Ext. 349 -Resource Room.
1 Patterson Britton and Partners (1996) Upper Tweed Estuary Management Plan Prepared for Tweed Shire Council
2 Patterson Britton and Partners (1995) Tweed River Bank Erosion Processes: Barneys Point to Murwillumbah Prepared for Tweed Shire Council, January 1995
3 WBM Oceanics (1992) River Management Plan: Upper Tweed Estuary. Hydrodynamic and Water Quality Assessments. Prepared for NSW Public Works
4 Druery BM and Curedale JW (1979) Tweed River Dynamics Study Department of Public Works NSW, Report No. 78009, ISBN 7240 2715 7
5 Druery BM, Blumberg G, Thomas CR (1996) The Role of Trees in Streambank Protection Prec. 36th Annual Floodplain Mgt Conf, Grafton
6 Druery BM Managing Riverbank Erosion Hazard Prec. 37th Annual Floodplain Mgt Conf, Maitland
7 NSW Public Works (1991) Managing the Tweed - Feasibility Study: Technical Report No 3.2.1 - Tidal Hydraulics Prepared by Oceanics Australia, Report No 3886.1, November 1989
8 Barling R and Moore I (1992) The Role of Buffer Strips in the Management of Waterway Pollution in Woodfull J, Finlayson B and McMahon T Workshop Proceedings "The Role of Buffer Strips in the Management of Water Pollution from Diffuse Urban and Rural Sources" University of Melbourne Occasional Series LWRRDC
- End Of Document -
