Author(s): Muhammad Ashiq
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Abstract: The bed material fractional size distribution at the confluence of two river systems depends upon the bed formation of the receiving and incoming river systems as well as on the individual r i ver's bed load size distribution. The bed material and bed load particle sizes of the rivers reduce, substantially, close to and toward the point of confluence. These variations in the particle size distributions may become necessary to determine if and when a reservoir has to be constructed downstream of the confluence point. This study of Snake and Clearwater river systems and their confluence, therefore, examines both aspects. First, variations in the bed material fractional sizes at the confluence point of the two rivers have been examined through a grid diagram. A model (fractional based) determining the bed material size distribution at the confluence point has been developed, which performed well when tested, with a mean discrepancy ratio of 1.09 and 85.7% of the DR values in the range of 0.75 to 1.25. Secondly, effects of the extreme flow conditions (i.e. drought and flood) on the size distribution of the bed load of each individual river system have been examined qualitatively and quantitatively. Under the drought flow condition, for both river systems, variation in the trans ported bed load fractional sizes (i.e. D16, D35, D50, D65, and. D84) was independent of flow rate but dependent on the source of flow and time when a significant flow event occurred. The transported fractional sizes, generally, belonged to the fine mode (silt+ sand) of sediment. For the flood flow condition the variation in fractional sizes depended upon: the time of flow event occurrence and its duration if flow of the event is large enough to breach the armour layer; source of flow; and vegetation growth in the catchment area. Nevertheless the fractional sizes were almost independent of the flow rate. The transported fractional sizes belonged to both modes (fine and coarse) of sediment.
Year: 1995