Plunge Pool Erosion in Cohesive Channels Below a Free Overfall

Abstract

ABSTRACT Asmall-scale model was used to evaluate factors which influence plunge pool erosion below a free overfall in cohesive material. The model represented conditions near the head of discontinuous gullies with a small catchment area. A procedure was developed to prepare cohesive samples of known vane shear strength. Ground silica was compacted to a predetermined dry bulk density and wetted under vacuum while its volume was kept constant. A relationship was established between the dry bulk density and the vane shear strength. It was assumed that the cohesiveness of the soil rather than the grain size determines the shape and size of most natural plunge pools in cohesive material. The characteristic length of the model was the fall height, which equaled the gully height minus the tailwater depth. Using similitude principles, 32 model runs were made, using virtually clear water, to determine dimensionless plunge pool sizes. These were found to vary with dimensionless terms which included the vane shear strength, discharge rate, upstream channel width, tailwater height and time. The fall height also influenced the plunge pool dimensions. The slope and roughness of the upstream channel appeared to have no influence. In cohesive material, plunge pools below a free overfall differ from those below culvert outlets. The former were initially shallow and wide and reached only gradually a more constant shape (constant depth-length ratio), while other research reported the latter to have a uniform shape, independent of scour time. Relatively deep and narrow plunge pools developed for low values of the dimensionless terms which included the discharge rate, tailwater height, and width of the upstream channel. The dimensionless plunge pool size increased with increasing values of the dimensionless upstream channel width for low values ( 0.6). Plunge pools were found to decrease the stability of gully walls. The highest reductions occurred for long scouring times, material with a low vane shear strength, and low tailwater.