Riprap Criteria below Pipe Outlet

In this study, a general relationship for incipient motion of the sediment below a jet is first developed. Then, for the particular case of a pipe outlet, an experimental program has been conducted to test the resulting equation and to supply the necessary coefficients. For visual observation of sediments, the half-jet technique was used, which made it possible to conduct a large number of tests in a relatively short period of time. The methods of sizing riprap, for a no-scour plunge-pool energy dissipator, based on incipient motion and incipient failure, are presented. Other topics investigated and discussed in this paper include effects of gradation and thickness of riprap as well as criteria for a gravel filter. A unified relationship for various riprap gradations is proposed by using D30 for the incipient motion and D90 for the incipient failure as the characteristic particle size.