Contamination of farm dam water supply is an immediate threat after bushfire, warns Department of Primary Industries Soils and Water Team Leader Bruce Radford. "Large amounts of ash, sediment and debris can be washed into dams after rain resulting in unusable water supplies," Mr Radford said. Farmers in fire affected areas need to act now, before significant rains, to ensure they protect their water supplies. Farmers also need to take steps to protect water supplies from runoff out of stock containment areas. Mr Radford advises that contamination of water supply by ash, sediment and organic matter does not generally pose a health risk to stock. However, stock often refuse to drink the contaminated water. High volumes of sediment entering dams will reduce the storage capacity of the dam. Large amounts of organic matter, e.g. dung, can result in algal blooms, deoxygenation of the water and water becoming unusable.
Contamination of farm dam water supply is an immediate threat after bushfire, warns Department of Primary Industries Soils and Water Team Leader Bruce Radford. "Large amounts of ash, sediment and debris can be washed into dams after rain resulting in unusable water supplies," Mr Radford said. Farmers in fire affected areas need to act now, before significant rains, to ensure they protect their water supplies. Farmers also need to take steps to protect water supplies from runoff out of stock containment areas.
Mr Radford advises that contamination of water supply by ash, sediment and organic matter does not generally pose a health risk to stock. However, stock often refuse to drink the contaminated water. High volumes of sediment entering dams will reduce the storage capacity of the dam. Large amounts of organic matter, e.g. dung, can result in algal blooms, deoxygenation of the water and water becoming unusable.
"The risk of dam contamination depends on there being enough rain to generate runoff, the amount of debris, dung and other material in the catchment area and the slope of the catchment above the water supply," Mr Radford said.
What you can do
Filter structures need to be put in place now before significant rains occur. Filter structures will slow the movement of water and reduce the amount of material entering dams. Types of filter structures include silt fences, earthen silt traps and levy banks. The type of filter structure you will need depends on the size and slope of your catchment.
Large and/or steep catchments, 30ha or greater
Large and/or steep catchments are at significant risk as they have the potential to generate a lot of runoff quickly. In these catchments earthen silt traps or levy banks can be constructed to prevent large volumes of sediment entering the dam. An alternative option is to construct a series of structures designed to filter out different sized materials. For example, the first filter might consist of pine posts and wire mesh to filter out large debris like branches, bark and leaves. The second filter might consist of steel posts and shade cloth to filter out larger sediment and the third filter would consist of specialist filter material secured with steel posts to filter out silt and dust. In the case of large and/or steep catchments, the Goulburn Broken Catchment Management Authority recommends you seek specialist advice from Bruce Radford, Department of Primary Industries on 5784 0603 or Carl Walters, Goulburn Broken Catchment Management Authority on 5820 1115.
Small catchments, 30ha or less
"For dams with small catchments of about 30ha or less, silt fences can be easily constructed using materials specifically designed and manufactured for erosion control. Other materials such as hay bales, shade-cloth, wire mesh and sand bags can also be used effectively as silt traps," said Mr Radford.
Mr Radford suggests 2-3 silt fences 20-30m apart above the dam catchment. "Two to three fences are an insurance policy in case of large flows and the first fence fails," he said.
To construct the silt fence using specialist materials, shade cloth or wire netting, position the material flat on the ground where the fence will go. Dig a trench along the base of the material to 150mm. Fold the material back in an L-shape in the direction of the flow in the bottom of the trench. Backfill the trench so that the material is buried to a depth of about 150mm. Once the material is buried, install stakes at 1-3m intervals on the opposite side to the water flow. Attach the material to the stakes using tie-wire or cable ties.
For a more sturdy fence, Carl Walters from the Goulburn Broken Catchment Management Authority suggests landholders install the trench and steel posts first with an anchor post at each end. Strain a wire at the top of the fence. Thread a bottom wire through the steel posts and fence material and then strain. Backfill the fence material in an L-shape in the bottom of the trench. Secure the fence to the top wire using ring fasteners then secure the material to the steel posts using tie-wire.
For a hay bale silt fence, line the bales up and secure them in place using steel posts.
Mr Radford recommends that any structure built to trap silt and sediment needs to be maintained and cleaned regularly to maintain the effectiveness of the structure.
For more information and assistance contact Bruce Radford, Department of Primary Industries, Broadford on 5784 0603 or Carl Walters, Goulburn Broken Catchment Management Authority, Shepparton on 5820 1115. It may be possible to bulk order materials required for constructing silt fences. Contact your local Landcare group or Bridget Clark/Janet Hagen on 5736 0105 to register your requirements.
Release Ends
For more information please contact:
Rhiannon Apted at GB CMA on 5736 0108