Evaluating Streambank Retreat Prediction Using the BANCS Model in the Valley and Ridge Physiographic Province

Abstract

Excess sediment in streams is harmful to the environment, economy, and human health. Streambanks account for an estimated 7-92% of sediment and 6-93% of total-phosphorus loads to streams depending on the watershed. Stream stabilization through stream restoration has become a common practice to satisfy the 2010 Chesapeake Bay total maximum daily load (TMDL) due its value in credits received per dollar spent. Bank erosion is most commonly credited through the Bank Assessment for Non-point source Consequences of Sediment (BANCS) framework, an empirically-derived model that predicts bankfull bank erosion rates using Bank Erodibility Hazard Index (BEHI), an indicator of bank stability, and Near-Bank Stress (NBS), an indicator of applied flow energy at bankfull discharge. This study assessed the BANCS framework in the Valley and Ridge physiographic province where it has not previously been applied. The spatial and temporal variability of erosion data was assessed to determine the impact of different erosion measurement schemes on bank erosion estimates and BANCS curves, and alternate NBS methods that capture flow energy beyond bankfull were applied. Three years of monthly erosion data on 64 streambanks were used to assess the spatial and temporal variability of erosion measurements and subsequently develop the erosion curves. Predicted erosion rates were then compared to measured erosion rates on three banks in the Valley and Ridge of Southwest Virginia. Analysis of spatial variability suggests bank retreat measurements should be made every three channel widths to reliably quantify reach-scale load estimates. Furthermore, a minimum monitoring period of 12 months is recommended to ensure seasonal patterns in bank retreat are captured. These results also bring into question the efficacy of the BANCS model as a crediting tool, as the developed statistical relationships between erosion rates, BEHI, and multiple NBS methods were not statistically significant. The limited number of significant curves had low r2 values (r2 < 0.1) indicating measures of NBS and BEHI do not adequately explain the natural variability of bank retreat in the Valley and Ridge of Southwest Virginia.