 |
 |
 |
 |
 |
 |
 |
|
SWAN COASTAL PLAIN DRAINAGE PROJECT
Download the final project outputs:
The need to undertake a review of the current drainage design, implementation and management practices on the Swan Coastal Plain has been widely recognised and ultimately reflected as recommendation of the State Government in the State Water Strategy 2003 (Reference 6.3.4). This is required as part of an overall plan to address all sources of nutrients and sediment at their source and all potential avenues to improve water quality in all parts of the drainage system. Resulting from the State Water Strategy recommendation the Drainage Reform Group (DRG) was formed and a Discussion Paper, White Paper and case studies were developed. A major principle included in the DRG White Paper was the need manage drainage for multiple objectives, including water quality, an issue of critical importance in most waterways of the Swan Coastal Plain and especially in the Peel-Harvey Catchment. The efficiency of the current drainage network to mitigate flooding through the rapid transportation of water also allows for the rapid transportation of nutrients with insufficient time for utilisation and assimilation through natural processes. Ultimately these nutrient rich waters drain into and cause the degradation of natural waterways, many of which are considered to be nationally and internationally significant and are protected through several statutory mechanisms. A further recommendation of the DRG is the separation of the entities responsible for the policy development, management and implementation of drainage works consistent with the principles of the COAG reforms. The proposed management models outlined in the DRG White Paper have been well received by the Peel-Harvey Catchment Council. IssueThe capacity to use the existing constructed drainage system to improve water quality has not been explored on the Swan Coastal Plain. Present improvements in water quality caused by the constructed drainage system whilst significant enough to be required in computer simulation models are incidental and not due to design of the system. Problem and purposeThe drainage system is inadequately assessed and maintenance cannot be carried out to improve water quality without adequate information and planning. Previous attempts to improve the water treatment capacity have started with an accurate assessment of the capacity of the drainage system to convey water. This shows areas of restriction (choke points) and areas where capacity is in excess of water conveyancing needs. This enables the development of prioritisation where the design of in-drain works can then be allocated to areas with greatest capacity and areas of low capacity can be targeted for upgrading and maintenance. This would minimise disturbance to the drainage system. At the same time the riparian condition will need to be assessed to avoid the disturbance of areas which have developed significant ecological value. To enable and fund the conversion of the drainage system that provides water quality improvement there needs to be an investigation into aspects of the current drainage system that provide these functions. This would include consideration of aspects such as sufficient capacity to convey water at high flows as well as providing much greater capacity to retain sediment and nutrients. Trapezoidal drains with good hydraulic efficiency need comparison with drains which have fortuitously developed many attributes associated with good nutrient and sediment removal efficiency. This is required to assess the cost benefit of potential changes to the drainage system. As has been learnt with previous assessments that there is a substantial lag time before the disturbance associated with earthworks declines and the effectiveness of the system begins to be measured. A broader survey of sites will be required to determine general differences in water quality as a function of broad drainage characteristics, accompanied by a detailed assessment of selected sites to differentiate between specific drain characteristics that provide positive or negative water quality influences. This information will be necessary to be included into a sub-regional drainage management plan complete with an assessment of the management practices that will need to be employed. This plan will need to be incorporated into the Water Quality Improvement Plan as part of a whole of catchment approach to water quality improvement. Outputs from this project include a Drainage Management Plan for the Peel-Harvey Catchment and an assessment of the water quality effects of the various recommended best management practices. Further information including project reports will be made available in the future. For further detail please contact the PHCC Executive Officer on (08) 9550 4225 or via info@peel-harvey.org.au.
 |
|||||||||||||||||||||||
