Sediment Concentration Measurements Below Small Headcuts

by Otto R. Stein, Colorado State Univ, Ft. Collins, United States,
Pierre Y. Julien, Colorado State Univ, Ft. Collins, United States,



Document Type: Proceeding Paper

Part of: Hydraulic Engineering

Abstract:

This study is designed to determine erosion rates downstream from a small headcut. The sediment concentration from a scour hole produced by a plunging jet is measured as a function of time in a 10.4 cm wide laboratory flume at 3.7% slope. The drop height is controlled upstream by a plexiglas bed and the jet impinges downstream on a alluvial bed. A total of 24 experiments were conducted on three different saturated bed materials, two sands (mean diameters of 1.5 and 0.22 mm) and one cohesive loam soil (46% fine sand, 28% silt, 26% clay). Unit discharges vary from 0.00154 to 0.00493 m²/s and drop heights range from 1.0 to 4.0 cm. The equation for a free falling nappe is used to determine properties of the jet at tailwater impingement. In all cases, sediment concentration from the scour hole decreases logarithmically with time. All runs for a given bed material are approximated by a single curve after using the normalizing parameter T^*= T U_o/Y_o which involves the velocity U_o and thickness Y_o of the jet at tailwater impingement. However, more scatter is apparent for the cohesive soil and deviations from the logarithmic decay curve exist at both short or long times.

Subject Headings: Saturated soils | Sediment | Erosion | Bed materials | Scour | Alluvial channels | Tailwater

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