Document Type : Research Paper
Author
Assistant Professor, Soil Conservation and Watershed Management Research Institute
Abstract
River bed load is usually estimated as a ratio of suspended load due to difficulty in direct measurement. The ratio of bed load to suspended load can be determined more accurately using reservoir sediment bathymetric information. In this research, two successive top bathymetric maps from reservoirs of Dez dam (1983 and 1997), and Minab dam (1985 and 1998) as well as suspended sediment data measured at the inflow were analyzed. After determining the position of delta crest and dividing deltaic and lacustrine parts in each reservoir, the volume of sediment were estimated followed by weight calculation based on sediment bulk density. Then, the influence of factors such as the temporal compactness of deposits, the dam sediment trap efficiency, and the contribution of upstream basin and the sub basins adjacent to the reservoir in sediment production was determined. On the other hand, the suspended sediment yield was estimated in Telehzang and Barantyn gauging stations (at reservoirs inflow) using USBR method. In the next step, the ratio of bed load to suspended load was estimated with two scenarios A) by considering the deltaic and lacustrine sediments as bed load and suspended load respectively and B) calculating the bed load by subtracting suspended sediment estimated in reservoir inflow gauge from total deposited sediment. The BLSLR was overestimated as much as 251 and 348 percent in Dez and Minab dams respectively under the Scenario "B" due to underestimation error of suspended load. Under the Scenario "A", the BLSLRs of Dez and Minab were 45 and 56 percent respectively. Two very important uncertainty factors in this method are the dam trap efficiency and bulk density of deposits particularly the deltaic coarse sediment. By changing the trap efficiency from approximately, 95 percent to 60 percent the BLSLRs were decreased from 42 percent to 23 percent in Dez River and from 53 percent to 27 percent in Minab River. A 14 percent cut in bulk density of deltaic deposit in Minab reservoir causes a reduction of seven percent in the BLSLR. Although, large differences are observed among the results of different scenarios, even the minimum ratios are much higher than 15-20 percent BLSLR that are commonly used in water resources studies.
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