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 C anyon Dam, although the specific physical mechanisms causing local ero sion are poorly understood. A short-term study (order of days) of deta iled flow patterns and morphologic adjustments at Stone Creek and Fern Glen sandbars during constant discharge demonstrates that surface-gra vity 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 i ntermittent 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 sedime nts away from the bar face and into the main channel. Thus combined wa ve-current interactions provide for sediment transport possibilities t hat might not occur in the absence of waves. Simple models of beach-fo reshore equilibria developed for coastal environments show that the fa ces of Grand Canyon sandbars behave very much like coastal, wave-domin ated features. But the wave-dependent equilibria predicted by coastal models are never attained fully because mean eddy-recirculation curren ts associated with the river play an important role in the fluid-sedim ent interactions observed on nearshore terraces. Unlike a coastal syst em where mean alongshore currents owe their existence to wave motion, eddy-recirculation currents in a fluvial system are completely indepen dent of waves, vis-a-vis their hydrodynamic origin. Thus neither a pur ely fluvial approach nor a purely coastal approach will be completely successful in describing sandbar stability in Grand Canyon, and a hybr id model should be adopted.