Page Banner

United States Department of Agriculture

Agricultural Research Service

Title: Roughness of Loose Rock Riprap on Steep Slopes

Authors
item Rice, Charles
item Kadavy, Kem
item Robinson, Kerry

Submitted to: American Society of Civil Engineers Journal of Hydraulic Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 1, 1998
Publication Date: N/A

Interpretive Summary: A study was conducted to develop a relationship to determine the Manning roughness coefficient for loose rock riprap laid on steep slopes. Relationships are available to predict roughness coefficients for riprap laid on flat slopes, but these relationships are suspect when applied to steep slopes. Tests were conducted with riprap-lined channels with bed slopes ranging from 0.025 to 0.333 using angular riprap with median diameters ranging from 52 to 278 mm. The results showed that roughness increases with an increase in bed slope and riprap size. A relationship was developed to predict the Manning roughness coefficient as a function of the bed slope and median riprap size. The predictions with this relationship did not agree with roughness prediction equations developed from data obtained from lowland and mountain rivers. The Manning roughness relationship developed from the current study will permit the calculation of accurate values for the flow depth in riprap-lined channels on steep slopes. Accurate values of the flow depth are necessary for the efficient and safe design of riprap-lined channels on steep slopes.

Technical Abstract: A study was conducted to determine the roughness of rock riprap laid on steep slopes. Tests were conducted in a 1.07-m wide flume and two 2.74-m wide field-scale structures using angular riprap with median diameters ranging from 52 to 278 mm and bed slopes ranging from 0.025 to 0.333. The results did not agree with roughness prediction equations developed from data obtained from lowland and mountain rivers. The results showed that roughness increases with an increase in bed slope and riprap size. Empirical relationships were developed to predict the Manning roughness coefficient as a function of the median riprap diameter and bed slope. The Darcy-Weisbach friction factor was predicted as a function of ratio, flow depth/84% riprap diameter.

Last Modified: 11/28/2014
Footer Content Back to Top of Page