Transient Bedload Transport During Flashflood Bores in a Desert Gravel‐Bed Channel

Abstract

Many methods have been developed for estimating bedload flux under conditions of quasi-steady flow, in which both turbulent velocity fluctuations and changes in bedload flux are assumed to occur over timescales much shorter than variations in flow velocity. Nonetheless, the prediction of bedload transport rate tends to break down when considering timescales over which turbulent fluctuations matter. For example, Escauriaza and Sotiropoulos (2011) conducted numerical modeling in clear water scour conditions and found that bedload transport may occur even when the average bed shear stress is far below the expected threshold value for entrainment, thereby suggesting the importance of highly transient turbulence forces. Sumer et al. (2003) found that a 20% increase in turbulence intensity in the shear layer above the bed caused a 6-fold increase in bedload flux. Videography has shown that bedload motion is temporally variable (Garcia et al., 2007;Venditti et al., 2010), a behavior largely attributed to bursting events associated with turbulent eddies (Nelson et al., 1995).Unsteady flow is characterized not only by changing discharge through time but also by rapid fluctuations in turbulent velocity which, in turn, cause variable amounts of bed material disturbance (e.g., Dey, 2014;Grass, 1971). Lee et al. (2004) observed that gradually-varying unsteady flows induced bedload fluxes as much as 1.6 times greater than those under similar steady flows. Much less is known about bedload flux in very unsteady flows, such as occur in flash floods. The hydrodynamics of flood bores and the consequences for bedload transport have been examined in flume studies (e.g.,