RIVER EROSION
Black wattles have colonised the inside of a bend (right) and diverted the flow of water into the riverbank (left)
Riverbank training and protection A bulldozed riverbed with no vegetation to hold the sediment during a flood. This has created unnaturally high flow velocities
Unnaturally high flow velocities, progressive loss of vegetation and increased sediment loads are key contributing factors that cause riverbed and riverbank erosion. The downstream effects include flooding and siltation build-up in dams and weirs. The installation of groynes serves as an effective countermeasure. By Hans King, Pr Eng*
Large blocks of peat washed out from the riverbed upstream, obstructing the flow of the river and causing fresh erosion downstream. This is an extreme case, but not uncommon
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any indigenous wetland plant species naturally minimise erosion by having root masses that bind the soil, and plant upper structures that lie flat and cover the soil surface during floods. However, when invasive alien vegetation, such as black wattle and Port Jackson willow, displace indigenous wetland flora, there’s ensuing havoc. That’s because they’re woody, so they don’t lie flat during floods. As a result, they then trap floating debris and create obstructions in the river channel, which diverts the flow into the riverbanks, promoting erosion. This scenario is compounded where unnaturally high flow velocities are experienced. This may occur where soil and rocks in rivers have been
Long and low groyne structures, designed to overtop during relatively small floods, but capable of safely supporting a sustainable, wide, slow-flowing channel
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IMIESA July 2021
