Flow Resistance in Cobble and Boulder Riverbeds

Field and experimental evidence are presented to demonstrate the importance of the inflow of sand and gravel size sediments on resistance to flow in channels whose beds are primarily cobbles, rocks, and boulders. The released sediments fill the spaces between the larger roughness elements, forcing the channel to behave as a sand bed channel at a much reduced resistance to flow coefficient. Resistance to flow in these channels decreases, resulting in underestimations of:(1)Water discharge by a factor of two; (2)sediment discharge by a factor ranging between 8 and 64; and (3)velocity of flow by a factor of two. Failure to estimate these quantities with a reasonable degree of accuracy results in the following: (1)Underestimation of the actual quantity of available water; (2)improper selection of bank protection material; (3)overestimation of reservoir life; (4)unsafe design of scour depths at hydraulic structures; (5)improper design of location; location and (6)others relating to river control and development.