Land Management Extension Officer Clem Sturmfels
Following the devastating bushfires earlier this month across Victoria, fire-affected landholders, especially those in hilly and mountainous areas should prepare for potential flash flooding, soil erosion and landslides in the event of a thunderstorm.
After a severe bushfire, flooding, soil erosion and sediment is inevitable in these terrains, which can affect a range of farm assets including tracks, fences, stream crossings, dams, water supplies, buildings and livestock.
The combination of bare ground, water repellent soils, steep slopes and high intensity rainfall can result in the movement of large quantities of ash, sand, gravel and woody debris.
Woody debris can block dam spillways and stream crossings; flood water can wash away tracks and fences while sediment can pollute or fill dams.
Previous fires in mountainous terrain have shown that floodwater levels can be significantly higher than in unburnt landscapes, so landholders should consider implementing measures to protect their farm assets and water supply.
Constructing sediment traps and levee banks can help protect farm assets from flood debris and work by reducing the velocity of water flow and prevent sediment from being swept into farm dams or streams.
Sediment traps can be installed using wire netting, straw bales, or silt fencing material and should be set up as soon as possible after a fire to prepare properties ahead of potential thunderstorms.
Proper design and placement are essential for successful results.
Small levee banks can sometimes help protect critical areas from flooding. Levee banks can be made from earth, sandbags, weed free straw bales or corrugated iron sheets and should be well compacted, watertight and have plenty of fall so they don't become blocked with sediment.
Straw bales and corrugated iron need to be trenched into the ground, tightly butted together and be well supported with steel posts.
A string of closely spaced steel posts in the drainage line above culverts and stream crossings can also be used to trap debris and reduce potential damage.
Wherever possible, livestock should be moved away from low lying and flood prone areas.
Landholders should also divert their down pipes and cover tank inlets to protect drinking water supplies from roofs contaminated with ash, soil, manure, leaves or firefighting chemicals.
