oa Civil Engineer in South Africa - Sediment transport and a reservoir siltation formula for Zimbabwe-Rhodesia
Field measurements of the suspended sediment load of rivers draining five large catchments ranging from the driest to the wettest parts of the country have enabled a reservoir siltation formula to be developed for Zimbabwe-Rhodesia. A few below-average annual runoff events and several above average events were sampled, including a 40-year return period runoff for one catchment. Almost every significant discharge during a three-year period was measured on the rivers concerned. The laboratory analysis was facilitated by use of a turbidity meter for measuring suspended sediment - a method particularly suited to rivers carrying large proportions of clay and silt. Data for bed load transport were obtained by bed material sampling and survey of channel dimensions, carried out during the dry season, followed by the use of Einstein's calculation. The transport of dissolved solids was measured so that total losses from catchments could be compared with soil loss and soil production data.
An order of magnitude amount of 40 t / km2 / year is suggested as an upper bracket sediment transport value for design purposes, provided the catchment concerned is not undergoing larger than average soil erosion. This figure is shown to be in agreement with previously published data, based on reservoir siltation rates, for parts of the Limpopo catchment, but several times smaller than data for small catchments under heavy human occupation in Tanzania. A reservoir siltation formula allowing for the low trap efficiencies of small reservoirs is suggested.
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