SUSPENDED SEDIMENT TRANSPORT IN FLASH FLOODS: TURBULENCE AND BOUNDARY SHEAR STRESS IN FLUME EXPERIMENTS

Sediment transport in flash flood dominated channels has received relatively little study compared to perennial streams, despite arid and semi-arid landscapes covering one-third of the earth’s surface. Precipitation events in arid regions can result in high surface runoff into ephemeral channels, resulting in flash floods that can be highly erosive and destructive. A flash flood hydrograph is composed of three major parts. First is a flash flood bore or “wall of water,” a front of water typically characterized by increased turbulence, followed by a rising limb with increasing discharge, and lastly a decreasing falling limb. Field observations show that sediment transport rates are elevated in the flood bore and decrease in the rising limb contrary to decreasing discharge. Two experimental facilities are used to test the hypothesis that elevated turbulence in the bore results in higher sediment transport rates; the first is a large channel-like flash flood flume and the second is a rotating drum flume. Data from these experiments show that boundary shear stresses are highest at the front of the bore and dissipate as the bore passes. Throughout the water column, turbulence is highest just above the bed, also dissipating as the bore passes. Both results have important implications for the suspension of sediment in a flash flood bore.