Flow Resistance of Riprap

Flow‐resistance coefficients are essential in determining both water‐surface elevations and velocities that are used in many aspects of channel design. Channel boundaries requiring scour protection are often armored wifn riprap revetments that have roughness characteristics that differ from other channel boundaries. Resistance of flow over riprap boundaries is determined from flume data and is compared to the logarithmic and power‐law equations. Results show that the power‐law equation is applicable to intermediate scale roughness of riprapped surfaces. The logarithmic equation is applicable to small‐scale roughness with Ks on the order of 2D90. The power‐law equation can be used for most riprap problems, which covers the range d/D90 from 3 to 30 without significant error. Use of particle size D90 gives slightly better results than use of D50. Strickler's equation is derived from the power‐law equation and found to compare favorably with the results of other investigators. Results are applicable to riprap placed with a relatively smooth surface typical of projects constructed in dry conditions.