Measuring streambank erosion due to ground water seepage: correlation to bank pore water pressure, precipitation and stream stage

Limited information exists on one of the mechanisms governing sediment input to streams: streambank erosion by ground water seepage. The objective of this research was to demonstrate the importance of streambank composition and stratigraphy in controlling seepage flow and to quantify correlation of seepage flow/erosion with precipitation, stream stage and soil pore water pressure. The streambank site was located in Northern Mississippi in the Goodwin Creek watershed. Soil samples from layers on the streambank face suggested less than an order of magnitude difference in vertical hydraulic conductivity (K.) with depth, but differences between lateral K. of a concretion layer and the vertical K-s of the underlying layers contributed to the propensity for lateral How. Goodwin Creek seeps were not similar to other seeps reported in the literature, in that eroded sediment originated from layers underneath the primary seepage layer. Subsurface flow and sediment load, quantified using 50 cm wide collection pans, were dependent on the type of seep: intermittent low-flow (LF) seeps (flow rates typically less than 0 center dot 05 L min(-1)), persistent high-flow (HF) seeps (average flow rate of 0 center dot 39 L min(-1)) and buried seeps, which eroded unconsolidated bank material from previous bank failures. The timing of LF seeps correlated to river stage and precipitation. The HF seeps at Goodwin Creek began after rainfall events resulted in the adjacent streambank reaching near saturation (i.e. soil pore water pressures greater than -5 kPa). Seep discharge from HF seeps reached a maximum of 1 center dot 0 L min(-1) and sediment concentrations commonly approached 100 g L-1. Buried seeps were intermittent but exhibited the most significant erosion rates (738 g min(-1)) and sediment concentrations (989 g L-1). In cases where perched water table conditions exist and persistent HF seeps occur, seepage erosion and bank collapse of streambank sediment may be significant. Copyright (c) 2007 John Wiley & Sons, Ltd.