Provided below is a summary of information taken directly from the Williams River erosion study report, titled 2015 Riverbank Vulnerability Assessment using a Decision Support System: Seaham Weir Pool and Lower Williams River. This report was prepared by the University of New South Wales' Water Research Laboratory.
You can also download the full report (PDF, 18.4Mb).
The Williams River erosion study report details a riverbank vulnerability assessment of a 38 km section of the Williams River for two management areas on the Williams River:
- Seaham Weir Pool from Seaham Weir to upstream of Clarence Town
- Lower Williams River from Seaham Weir downstream to the Hunter River confluence at Raymond Terrace.
The key summary points outlined in the report include:
- Findings suggest that a range of factors, including boating activities, poor land management practices, and flooding, have degraded the condition of the riverbanks along the Williams River study area, rendering the riverbanks more susceptible to erosion.
- The riverbank conditions were shown to deteriorate as a direct result of the largest flood on record on the Williams River in April 2015. Typical flood impacts on the riverbanks included:
- Damage observed on the outside banks compared to the inside banks;
- Loss of sub-aquatic vegetation, reeds, and phragmites in the wave zone;
- Reduction in upper riverbank ground-cover vegetation, including exotic weeds and vines;
- Riverbank erosion and slumping; and
- Tree up-rooting.
- Land and water-based management options, including a Management Action Plan have been outlined at sites with the highest vulnerability to erosion.
- Combined land and water-based management interventions are recommended to ensure successful, long-term outcomes.
- The implementation of temporary boating restrictions that prevent further damage to riverbanks (i.e. minimal wash zones) are recommended in conjunction with remedial riverbank works.
- A staged approach is recommended for implementing the Management Action Plan. Successful implementation of the Management Action Plan will require detailed site investigation and costing, and inclusive community and stakeholder consultation.
- The management actions for the Seaham Weir Pool should be prioritised over the Lower Williams River, due to the public health concerns associated with the long-term decline in water quality.
- Following implementation and evaluation of the riverbank remediation activities, boating restrictions should be reassessed once riverbank vegetation is re-established.
Seaham Weir Pool
Riverbank conditions along the study area generally deteriorated over the assessment period from March 2012 to August 2015.
The riverbanks became more erosive over the assessment period from March 2012 to August 2015, with the majority of the erosive sites located in the wakeboard zone of the Seaham Weir Pool as indicated in the table below.
The maintenance of a quasi-static operational water level upstream of Seaham Weir appears to exacerbate erosion. Although raising the existing operational water level of Seaham Weir by approximately 300 mm was shown to modestly improve the ability of the riverbanks upstream of the weir to resist erosion, it was acknowledged that the response of riverbank vegetation to an ongoing raised operational water level was likely to be detrimental. On this basis, WRL does not recommend altering the Seaham Weir operational water level to combat riverbank erosion.
Riverbank cross-sectional survey profiles (including undercutting) were repeated at 17 locations along a straight stretch of the Seaham Weir Pool to quantify the impact of the April 2015 flood event. The results of the land survey showed that there has been a net loss (erosion) of between 250 to 750m3 of riverbank soil into the Seaham Weir Pool across the 17 monitoring sites (a 1 km stretch on one side of the river) between December 2012 and February 2016.
Monitoring of the 17 riverbank survey locations is considered to be an important part of ongoing management of water quality in the Seaham Weir Pool.
Lower Williams River
Riverbank conditions along the study area generally deteriorated over the assessment period from March 2015 to August 2015.
The riverbanks in the study area became more erosive over the assessment period from March 2015 to August 2015, with the majority of the erosive sites located in the wakeboard and waterski zones of the Lower Williams River, as indicated in the table below.
At high tide on the Lower Williams River, wave action was slightly less likely to cause riverbank erosion than at mid – low tide, but the difference was not considered sufficient to develop water-based management actions linked to the tidal stage of the river.
Land and water-based management options have been outlined to improve the DSS management outcomes (i.e. from ‘Manage’ or ‘Monitor*’ to ‘Monitor’) at sites with the highest vulnerability.
1. Stage 1 Management Action Plan involves riverbank remediation, including weed removal, native vegetation regeneration, and stock exclusion, combined with temporary boating restrictions to prevent further damage to riverbanks supported by ecoengineering structures (such as rock fillets) and battering to further reduce erosion at sites which require additional remediation.
2. Stage 2 Management Action Plan (or Riverbank Management Program) involves ongoing monitoring and evaluation of the areas for a period of up to 36 months from the completion of the riverbank remediation activities. These activities would include:
- Enforcing temporary boating restrictions that prevent further damage to riverbanks (i.e. minimal wash zones) until riverbank vegetation is re-established;
- Monitoring of revegetation and structural works post flood to identify maintenance needs;
- Maintenance of fencing installed for the control of stock movements and access to the river;
- Maintenance of revegetation works
- Ongoing monitoring of the 17 riverbank locations on the Seaham Weir Pool, with these sites re-surveyed every 24 months after the Stage 1 Management Action Plan is implemented.
If it is necessary to manage boating numbers across the study region, and recognising the potential cost of implementing the Riverbank Management Program, it is recommended that the Stage 1 Management Action Plan is implemented via a staged-approach. If this approach is undertaken, the Management Action Plan for the Seaham Weir Pool should be prioritised over the Lower Williams River, due to the public health concerns associated with the long-term decline in water quality within a critical drinking water source for the Lower Hunter region, and the exacerbated erosion issues associated with having a static water level on the Seaham Weir Pool.
In the interim on the Lower Williams River, alternative bioengineering techniques, such as fish balls, coir log walls, silt fences or other geotextile products (e.g. Flow Net), or brushing (i.e. logs of various sizes and other debris secured to the riverbank or wave zone), may be implemented to reduce wave action reaching the riverbank, hence encouraging vegetation regrowth and sediment deposition. It is acknowledged that the risk of failure for these ‘soft’, but more economical (initial installation costs only), structures is greater than rock fillets, as they can be severely damaged by high magnitude floods, and may require regular maintenance.