Effect of sediment on resistance to flow in cobble and boulder bed rivers

Abstract

Field and experimental evidence are presented to demonstrate the importance of the inflow of sand and gravel size sediments, released under extreme floods from watersheds and banks of streams, on resistance to flow in channels whose beds are formed of large size roughness elements such as cobbles, rocks and boulders. The released sediments fill the spacings between the large size roughness elements, and may inundate them completely, forcing the channel to behave as a sand bed channel at a much reduced resistance to flow coefficient. Under extreme conditions resistance to flow in these channels decrease to more than one-third its original value resulting in an underestimations of the following quantities: water discharge by a factor of two, sediment discharge by a factor ranging between 8 and 64, velocity of flow by a factor of two. Furthermore, an overestimation of flow depth by a factor of two can result. Impacts of failure to estimate the previous quantities with a reasonable degree of accuracy are: underestimation of the actual quantity of available water, improper selection of bank protection material, overestimation of reservoir life, unsafe design of scour depths at hydraulic structures, improper design of highway location as well as others relating to river control and development.