Application of an analytical model of critically stratified gravity-driven sediment transport and deposition to observations from the Eel River continental shelf, Northern California

Abstract

An analytical model of down-slope sediment transport and deposition by wave-supported gravity-driven flows is applied to examine the formation of the mid-shelf flood deposit on the continental margin off of the Eel River in northern California. The model reproduces observed time series of near-bed velocity and deposition following flood events when sufficient fine sediment is available to critically stratify the wave boundary layer. Comparative estimates of down-slope gravity-driven flux and along-shelf sediment delivery suggest that critical stratification will dominate the near-bed dynamics when greater amounts of sediment are delivered to the inner shelf by river floods than can be removed by gravity-driven processes. If insufficient sediment is delivered to cause critical stratification in the wave boundary layer, energetic waves can enhance drag, retard down-slope transport, and limit deposition. Analytic predictions of deposition suggest that the magnitude of wave energy is more important than the magnitude of the flood event in controlling the local thickness of mid-shelf gravity-driven deposition following floods of the Eel River. Higher wave energy increases the capacity for critically stratified gravity flows to transport sediment to the mid-shelf and results in greater gradients in flux and hence deposition. Flood magnitude determines how close to shore the flood deposit begins and how far along-shelf it extends. The bathymetry of the Eel margin plays a critical role controlling wave-supported gravity flows. Analytic predictions indicate that gravity-driven deposition on the mid-shelf begins roughly 7–8 km north of the river mouth. Closer to the river mouth, the seaward increasing mid-shelf slope associated with the concave downward subaqueous delta causes gravity-driven flux divergence, preventing significant mid-shelf deposition by wave-supported gravity flows and favoring sediment bypassing. A seaward decrease in shelf slope in the vicinity of the observed flood depo-center leads to greater flux convergence by gravity-driven flows, and hence greater deposition. Farther north, the supply of sediment diminishes sufficiently to prevent significant gravity-driven deposition. The analytic predictions of mid-shelf mud deposition are spatially and temporally consistent with field observations and provide strong evidence that wave-supported gravity flows control the emplacement and location of the Eel margin flood deposit.