Impact of reach geometry on stream channel sensitivity to extreme floods

Abstract

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 (300 W/m2). 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 © 2014 John Wiley & Sons, Ltd.

Channel response to extreme floods: Insights on controlling factors from six mountain rivers in northern Apennines, Italy

“…Therefore, in terms of hazard, documenting the type and magnitude of channel response is crucial in identifying controlling factors of such response and in developing tools to enable channel dynamic predictions. Recently, Buraas et al (2014) stated that there is still a general lack in the capability to predict where major geomorphic changes take place during an extreme flood event. In this respect, the regression models and other outcomes of this study can be used to predict the degree of channel widening and to support planning along river corridors through hazard mapping.…”

“…Some works have confirmed that hydraulic forces may not be sufficient to explain geomorphic effects (e.g., Heritage et al, 2004;Nardi and Rinaldi, 2015), and consequently, attempts have been made to include other factors. For instance, human interventions and structures have been considered by Langhammer (2010); bedload supply and pre-flood channel planform by Dean and Schmidt (2013); lateral confinement by Thompson and Croke (2013); a bend stress parameter by Buraas et al (2014).…”

Quantifying River Channel Stability at the Basin Scale

“…In terms of lateral instability, Buraas et al [184] made some headway by demonstrating how widening and large scale geomorphic change relate to crossing a threshold specific stream power (for the 2-year flow) of 300 W·m −2 during flood events, which facilitated a regional assessment of reaches susceptible to bank erosion. Conceivably then, it would be possible to estimate the proportion of AGE that might be associated with large scale bank retreat and map susceptible reaches.…”

“…The employment of flow duration curves in the energy auditing methodology intrinsic to Equation (7) represents a marked difference with other approaches that use single, representative discharges derived from either regional expressions (e.g., [2,116,120,184]) or broad hydrological models (e.g., [118,123,125]). Indeed, Biron et al [126] considered the weakest area in most stream power mapping approaches to be the single, regional-based estimation of bankfull discharge.…”

Quantifying the impacts of land use change on flooding in data-poor watersheds in El Salvador with community-based model calibration

“…Urbanization can increase flood velocity and thus decrease the time to flood peak, reducing the time a community has to evacuate homes. In addition, urbanization and channel confinement have been shown to increase stream power and dangerous fluvial erosion damages that lead to bank failure (Kline and Cahoon 2010;Buraas 2014). Fluvial erosion studies required to assess potential bank failure also depend on streamflow measurements unavailable in this area of study.…”