28 Alluvial Steep Channels: Flow Resistance, Bedload Transport Prediction, and Transition to Debris Flows

摘要

Studies on flow resistance in steep streams and shallow flows have received increasing attention in recent decades. However, for mountain rivers, characterized by coarse bed materials and steep slopes, traditional flow resistance relations generally provide poor predictions, with typical errors of ±30% (Bathurst, 2002). Several studies have shown that the traditional semi-logarithmic equation used to estimate flow resistance in deeper flows in more gently sloped channels may no longer be valid for shallow flows. Instead a power-law type flow resistance relation may be more appropriate for steeper and rougher channels: √8/? = a(d/D_x)~b (28.1) wherein ? = Darcy-Weisbach flow resistance coefficient, d = flow depth, Dx = grain size of the bed material for which x% is finer, a = empirical coefficient, and b = empirical exponent. The exponent b tends to increase from about 0 or 1/6 (Chezy equation or Manning-Strickler equation) up to about 1 for steep channels, i.e. it tends to increase with increasing bed slope and decreasing relative flow depth (Ferguson, 2007; Rickenmann and Recking, 2011); the exponent may possibly also decrease with increasing uniformity of the bed material distribution (Bathurst, 2002). Power-law type flow resistance relations can be transformed into dimensionless hydraulic geometry equations (Ferguson, 2007), which have been increasingly proposed for steep channels based both on flume and field observations (Aberle and Smart, 2003; Ferguson, 2007; Comiti et al., 2007, 2009; Zimmermann, 2010; Comiti and Mao, Chapter 26, this volume). Rickenmann (1991, 1994) introduced similar approaches, and Rickenmann and Recking (2011) have confirmed the suitability of such an approach based on more than 3000 field measurements, including rough and steep streams.