IMPACT OF REACH GEOMETRY ON THE SUSCEPTIBILITY TO SIGNIFICANT CHANNEL WIDENING DURING EXTREME FLOODS

The spatial and temporal variations in geomorphic responses to extreme floods present a long-standing challenge in geomorphology. The variety of responses reflects the complexity of factors controlling the geomorphic adjustment of channels to floods. The need for a predictive understanding of the geomorphic responses to disturbances such as extreme floods has taken on greater urgency recently due to the increasing recognition of the need to restore and sustainably manage freshwater ecosystems. Studies show that unit stream power above a critical threshold (300 W/m²) is a prerequisite, but not necessarily sufficient to cause large scale channel widening. Thus, predictive models for geomorphic change must consider and quantify not only unit stream power, but also the potential impact of reach geometry. Accordingly, we introduce a new metric, bend stress, which is proportional to the centripetal force exerted on the concave bank as water flows around a bend. Thus, high centripetal forces generate locally high shear forces and increase the susceptibility of the channel bank to erosion. This study uses the geomorphic signature of recent Tropical Storm Irene on the White River (1787 km²) and the Saxtons River (186 km²), Vermont, to investigate the impact of reach geometry in controlling the susceptibility of a reach to channel widening during floods. Our objective is to develop a methodology that allows for rapid, regional-scale assessments to identify locations susceptible to channel widening. Specifically, we propose that reaches of significant channel widening during Tropical Storm Irene were dictated by requirements of one or both of these metrics exceeding a threshold value. Sixty-five percent or more of the locations where widening was greater than three standard deviations of the changes during the pre-Irene control period (1992-2010) are located along reaches where both metrics exceed threshold values, yet both these thresholds are exceeded on only ~45 % of the total length of the study reaches. The results demonstrate the importance of both unit stream power and channel geometry in controlling the spatial variation of channel widening during floods. Together the metrics represent a simplified and rapid assessment of the susceptibility of reaches to significant widening during future extreme floods.