Seepage erosion: A significant mechanism of stream bank failure

The role of seepage undercutting on mass failure of stream banks is not well defined relative to other streambank failure mechanisms. Seepage flow initiates undercutting, similar to development and headward migration of internal gullies, by liquefaction of soil particles, followed by mass wasting of the stream bank. The objectives of this research were to quantify the impact of seepage undercutting on streambank stability based upon seepage flow and erosion measurements from two deeply incised streams in northern Mississippi: Little Topashaw Creek (LTC) and Goodwin Creek (GC). Stream bank soil properties characterized from disturbed and undisturbed soil cores suggested that the type of seepage erosion (seepage erosion of a conductive layer and undercutting of layers underneath the seepage layer) at LTC and GC was a function of the bank stratigraphy. At LTC, seepage eroded sediment from the conductive, relatively noncohesive loamy sand layer. The GC seeps originated from the conductive concretion layer, but the flow eroded a less cohesive loamy sand layer below the seepage layer. Subsurface flow and sediment concentrations commonly exceeded 0.1 m³ d^-1 and 10 g L^-1, respectively, as quantified using collection flumes installed into the stream banks. A bank stability model, SLOPE/W, was used to simulate the bank profiles of both LTC and GC based on field measured streambank stratigraphy, soil hydraulic properties and soil strength parameters. Model simulations included numerous bank angles, ground water table elevations, and distances of seepage undercutting. SLOPE/W simulations suggested a 200-mm critical distance of seepage undercutting at which the error in neglecting seepage undercutting became greater than the error in neglecting soil-water pressure effects.