Impact of reach geometry on stream channel sensitivity to extreme floods

Predicting spatial and temporal variations in bank erosion due to extreme floods presents a long-standing challenge in geomorphology. We develop two methodologies for rapid, regional-scale assessments of stream reaches susceptible to channel widening. The first proposes that channel widening occurs when unit stream power exceeds a critical threshold (300W/m(2)). The second is motivated by the observation that widening often occurs at channel bends. We introduce a new metric, the bend stress parameter, which is proportional to the centripetal force exerted on a concave bank. We propose that high centripetal forces generate locally high bank shear forces and enhance channel bank erosion. We test both metrics using the geomorphic signature of Tropical Storm Irene (2011) on the White and the Saxtons Rivers, Vermont. Specifically, we test if reaches where significant channel widening occurred during Irene required one or both metrics to exceed threshold values. We observe two distinct styles of channel widening. Where unit stream power and bend stress parameter are high, widening is usually due to bank retreat. Elsewhere widening is usually due to the stripping of the upstream end of mid-channel islands. Excluding widening associated with the stripping of the heads of mid-channel islands, almost all the widening (> 98%) occurred along reaches identified as susceptible to widening. The combined metrics identify up to one-quarter of the reaches lacking susceptibility to channel widening. Copyright (c) 2014 John Wiley & Sons, Ltd.