Sediment transport for desert watersheds in erodible materials

Abstract

SUMMARYSediment transport for five small watersheds in arid southern Israel, eroded into shales, chalks, anhydrites and alluvium is studied. Analysis includes data from three large watersheds in the southern United States. Results indicate that floodwater heavily laden with suspended load (10-60 per cent, by weight) is characteristic of watersheds cut in erodible materials. No correlation was found between environmental characteristics such as lithology, catchment area, angle of slope or rainfall intensity and suspended-sediment concentration or texture. The carrying capacity for sands, derived both from hillslopes and channel alluvium is greatly increased by high suspended silt concentratlon, the trend being R cc C;2'5 (where R = ratio, % fines/% sand; C, = suspended load concentration, per cent by weight). The high concentration of fine sediment effectively causes more sand transport than would normally occur. This is also true for transport of coarser clastics; higher velocities, typical of highly concentrated silt suspensions, increased competence for transporting pebbles and boulders. The action of a large variety of processes (splash, wash, rilling, gullying, sliding, mudflow) combine with low infiltration and high erodibility to render every rainstorm erosionally effective, and high concentrations of washload increase both the capacity and competence of coarse clastic transport. Whole basins are thus subjected to high rates of denudation (2-10 mm/yr).

Suspended solids concentrations and river water chemistry

“…It is almost certain that as a result of the high clay concentrations the viscosity of the water is increased considerably. A relationship between percentage <50 pm and the suspended solids concentration occurs that is similar to that reported by Gerson (1977). Such relationships have to some extent been attributed by Beverage and Culbertson (1964) to the effect of high clay concentrations on the fall velocities of coarser material (Simons .…”

Reconstructing an extreme flood from boulder transport and rainfall–runoff modelling: Wadi Isla, South Sinai, Egypt

“…Depth estimates derived from the selective entrainment method (Kehew and Teller, 1993) resulted in lower, more realistic values, probably because of the projection of large clasts above the bed and the lower critical shear stress necessary to entrain them. Critical shear stress values in ephemeral desert streams are likely to be lower than humid perennial streams because of projecting clasts as well as a lack of bed armor (Gerson, 1977;Laronne et al, 1994;Bergman, 2007).…”

Stream‐driven, high‐density gravelly traction carpets: possible deposits in the Trabeg Conglomerate Formation, SW Ireland and some theoretical considerations of their origin

“…Thus, fluid density also controls the magnitude of the shear stress actually exerted on the stream bed. This highlights the significant effect that high sediment concentration, notably of suspended finergrained fractions, has on the entrainment and transport of gravel in stream flows (see also Hampton, 1975;Gerson, 1977;Lord & Kehew, 1987;Bradley & McCutcheon, 1987).…”