Photographic Guidance for Selecting Flow Resistance Coefficients in High-Gradient Channels

Practitioners are oftentimes required to predict flow resistance coefficients, such as Manning's n, in high-gradient channels (slopes > 2 %). Measurements of actual velocity and flow resistance indicate that reach-average resistance coef-ficients are substantially higher than commonly encountered in low-gradient channels, with Manning's n typically falling between 0.1 to 0.3 for bankfull flows in step-pool and cascade channels (Yochum and others 2012) and flow re-sistance increasing with decreasing discharge (Lee and Ferguson 2002, Comiti and others 2007, Reid and Hickin 2008, David and others 2010, Yochum and others 2012). Photographic guides for visual comparison with actual channels (Barnes 1967, Aldridge and Garrett 1973, Arcement and Schneider 1989, Hicks and Mason 1998, Coon 1998, Phillips and Ingersoll 1998) provide assistance for estimating these values in lower-gradient channels, but little assistance has been available for high-gradient streams. The selection of appropriate resistance coefficients is essential for improved confidence in hydraulic modeling, stream assessments, stream restoration design, geomorphic analysis, and ecological studies.