Downstream hydraulic geometry and channel adjustment during a flood along an ephemeral, arid-region drainage

Abstract

In September 1997, a dissipating tropical storm caused a flood with an estimated maximum discharge of 240 m 3/s along Yuma Wash, an ephemeral braided system draining 186 km 2 in southwest Arizona. Older high-water marks that record a flood peak of 1280 m 3/s provide a reasonable estimate for the probable maximum flood along the wash. Detailed channel cross-sectional surveys during 1995 and again in 1998, <6 months after the 1997 flood, facilitated examination of downstream hydraulic geometry and channel adjustment during the flood. Channel width increased substantially downstream (exponent of 0.78), presumably because of low bank resistance, whereas hydraulic depth and velocity had modest downstream increases (0.15 and 0.14, respectively). Channel aggradation generally occurred along wider, braided reaches; moreover, degradation occurred in narrow reaches with fewer channels. Aggradation and degradation also appeared to be governed by a threshold relationship between flow depth and vegetated bars. Degradation occurred where flow was confined within a channel or subchannel. At discharges sufficiently high to overtop vegetated bars, greater roughness facilitated sediment deposition and channels aggraded. A discriminant function correctly classified nearly 90% of the cross-sections as scoured or filled using a single hydraulic variable, maximum depth of flow during the dissipating tropical storm.