WAVES AND SANDBAR EROSION IN THE GRAND-CANYON - APPLYING COASTAL THEORY TO A FLUVIAL SYSTEM

Progressive erosion of channel-bank sandbars in Grand Canyon has long been thought to be associated with emplacement and operation of Glen Canyon Dam, although the specific physical mechanisms causing local erosion are poorly understood. A short-term study (order of days) of detailed flow patterns and morphologic adjustments at Stone Creek and Fern Glen sandbars during constant discharge demonstrates that surface-gravity waves and other quasi-periodic flow oscillations are important to the stability characteristics of these alluvial deposits. The primary role of waves is to agitate bottom sediments, entraining them on an intermittent basis. Mean currents associated with recirculating eddies (ordinarily of insufficient strength to entrain sediments) act, in the presence of waves, as a net background drift able to transport sediments away from the bar face and into the main channel. Thus combined wave-current interactions provide for sediment transport possibilities that might not occur in the absence of waves. Simple models of beach-foreshore equilibria developed for coastal environments show that the faces of Grand Canyon sandbars behave very much like coastal, wave-dominated features. But the wave-dependent equilibria predicted by coastal models are never attained fully because mean eddy-recirculation currents associated with the river play an important role in the fluid-sediment interactions observed on nearshore terraces. Unlike a coastal system where mean alongshore currents owe their existence to wave motion, eddy-recirculation currents in a fluvial system are completely independent of waves, vis-a-vis their hydrodynamic origin. Thus neither a purely fluvial approach nor a purely coastal approach will be completely successful in describing sandbar stability in Grand Canyon, and a hybrid model should be adopted.