Sediment Concentration below Free Overfall

Abstract

Time‐varying sediment concentration exiting a plunge pool is analyzed by considering the effect of free‐overfall jet diffusion on sediment detachment and scour. The analysis results in dimensionless relationships between scour volume, sediment concentration, maximum scour depth, and time. Sediment concentration is at a maximum near the initiation of scour, when the eroding bed is at the tip of the impinging jet's potential core. Thereafter, sediment concentration decreases as a function of time and scour‐hole geometry, expressed as maximum scour depth. Dimensionless geometric relationships show that the scour volume per unit width can be expressed as twice the square of the maximum scour depth at the corresponding time for the cohesive and noncohesive soils tested. Analytical relationships between sediment concentration and time are verified by laboratory experiments of scour below a free overfall on two sand‐bed materials (one coarse and one fine) and on one cohesive soil, each replicated several times. The agreement between measured and predicted values is best for the noncohesive sands.