Please release me, let me go: an approach to determining the effects of dam operations on riverbank erosion. (10817)
Riverbank erosion downstream of reservoirs, in the form of mass failures, have been attributed in many situations to the rate of regulated flow drawdown simply because the failures are observed after a release. There are a multitude of factors, other than how flows are managed, that can cause, or at least exacerbate bank erosion in regulated rivers. This paper outlines a framework that can be used to assess the effect of flow regulation on riverbank erosion rates and processes. The first step is to evaluate the trajectory of the stream pre reservoir by looking at how both anthropogenic and non-anthropogenic drivers have pre-conditioned the channel. This includes factors that will influence the sediment yield and hydrology of the channel including: catchment topography; geology; climate; land clearance; and, in-stream mining. Step two is to assess the geomorphic trajectory change resulting from the presence and operation of the reservoir. The position of the reservoir in the catchment, its volume relative to the upstream channel, the modes of operation and trapping efficiency will all influence the already modified channel downstream. This can: amplify incision or aggradation; reverse these processes; stall channel change; or result in a complex response of all of these. The third step is to identify the existing erosion processes operating in the channel. The final step is to assess the consequences of the dam release strategies on the system. For example, evidence of bank toe scour relative to baseflow provisions and the rate of drawdown compared to the rate of exfiltration. This framework assists in attributing the extent of bank erosion relative to flow operation strategies and provides a basis for managing flow deliveries to achieve an acceptable rate of bank erosion.