Erosion of cohesionless sediment by groundwater seepage

Surface grains of noncohesive sediment eroded by emerging groundwater are acted upon by three forces, the tractive force of the cumulative surface flow contributed by upslope seepage, the local seepage force, and gravity. The balance of the force moments determines the mode and rate of transport. Seepage forces are strong in a narrow “sapping zone” at the upstream end of the emerging flow, where erosion occurs by mass movement and the surface gradient is determined by the balance of the seepage and gravity moments. Most of the erosion occurs in this zone, and the resultant backcutting triggers intermittent failure of overlying slopes in a “undermining zone” maintained at the angle of repose of the dry or damp sediment. In the “fluvial zone” downstream from the sapping zone the seepage force is small compared to the tractive force, and transport occurs by normal fluvial traction. The overall rate of sapping erosion in noncohesive sediments is determined by the capacity of fluvial transport to remove sediment eroded in the sapping zone. Prediction of sapping rates is complicated by the interaction between the geometry of the fluvial and sapping zones and the quantity and spatial distribution of seepage. A simulation model incorporating a groundwater flow model and sediment transport relationships closely replicates the observed evolution of sapping erosion in a two‐dimensional tank filled with noncohesive sand subjected to lateral groundwater flow.