Invited lecture: Assessing the geomorphic effectiveness of extreme floods along resistant-boundary channels

摘要

One of the challenges facing those seeking to mitigate flood hazards involves predicting the extent of change in channel boundaries during a flood. This challenge is exacerbated along channels formed in erosionally-resistant materials such as very coarse-grained or cohesive substrates. Few equations exist which accurately predict erosion thresholds for either cobble/boulder clasts or bedrock. This results in part from the fact that these substrates are commonly very heterogeneous because of a range of grain sizes in the unconsolidated material, or discontinuities such as joints in the bedrock. Precise quantification of erosional thresholds is also difficult because of the highly turbulent and fluctuating hydraulic forces characteristic of resistant-boundary channels. However, in many cases, effective flood hazard mitigation depends on being able to predict sites of erosion, deposition, and stability at the reach-scale of channel segments hundreds of meters to kilometers in length. This may be done using a generalized dataset to define a lower threshold value for substantial flood modification of channel boundaries as a function of stream power per unit area and drainage area: ω=21A~(0.36) where ω is stream power per unit area (W/m~2) and A is drainage area (km~2). Prediction of channel response to floods may also be based on a comparison of hydraulic driving forces (estimated using surveyed channel geometry and step-backwater modeling) and substrate resisting forces (estimated using grain-size distribution for cobble/boulder substrates, or rock-mass strength for bedrock). This latter approach may be used to develop a river-specific threshold based on gradient and valley width, above which erosion predominates, and below which deposition occurs. The magnitude of erosion and deposition depend on flood magnitude and duration; potential sediment supply from the valley bottom, valley walls, and tributaries; and threshold resistance of the channel boundaries. Case studies illustrating these various approaches to assessing geomorphic impacts of floods come from the Dudh Kosi in Nepal, the Big Thompson River in the United States, and the Burdekin River in Australia.