Flood dispersal and deposition by near‐bed gravitational sediment flows and oceanographic transport: A numerical modeling study of the Eel River shelf, northern California

Abstract

[1] A large flood of the Eel River, northern California, created a thick sediment deposit between water depths of 50 and 70 m in January 1997. The freshwater plume, however, confined sediment delivery to water depths shallower than 30 m. Mechanisms proposed to explain the apparent cross-shelf transport include dispersal by oceanographic currents, resuspension by energetic waves, and gravitationally forced transport of a thin layer of fluidized mud. Field observations indicate that these processes were all active but cannot determine their relative significance or whether these mechanisms alone explain the location, size, and timing of deposition. Approximately 30% of the sediment delivered by the Eel River is accounted for in the midshelf mud bed and inner shelf, but the fate of the remaining 70% is uncertain. A three-dimensional, hydrodynamic model was used to examine potential mechanisms of sediment transport on the Eel River shelf. The model includes suspended sediment transport and was modified to account for a thin, near-bed layer of fluidized mud. It was used to simulate flood dispersal on the Eel River shelf, to compare the relative importance of transport within the near-bed fluid mud layer to suspended sediment transport, and to evaluate sediment budgets for floods. Settling properties of fine-grained sediment, both within the flood plume and the fluid mud layer, critically impact depositional patterns. To a lesser degree, wind-driven ocean currents influence the volume of sediment that escapes the shelf, and wave magnitude affects the cross-shelf location of flood deposits. Though dilute suspension accounts for a large fraction of total flux, cross-shelf transport by gravitational forcing appears necessary to produce a midshelf mud deposit similar in volume, location, and timing to those seen offshore of the Eel River.Citation: Harris, C. K., P. A. Traykovski, and W. R. Geyer (2005), Flood dispersal and deposition by near-bed gravitational sediment flows and oceanographic transport: A numerical modeling study of the Eel River shelf, northern California,

Observations and modeling of wave-supported sediment gravity flows on the Po prodelta and comparison to prior observations from the Eel shelf

“…Thus, wave-supported turbidity flows could potentially enhance preservation of sediment on the prodelta, leading to a faster progradation rate than would occur without this mechanism. Additional modeling work that includes both transport mechanisms (such as that performed in Harris et al, 2004Harris et al, , 2005 would be necessary to investigate the role of wave-supported turbidity flows in contributing to the growth of the Po delta. .…”

The landward and seaward mechanisms of fine-sediment transport across intertidal flats in the shallow-water region—A numerical investigation

“…Li et al, 2004;Warner et al, 2005;Harris et al, 2005;Ralston et al, 2010;Ralston et al, this issue).…”

Model Behavior and Sensitivity in an Application of the Cohesive Bed Component of the Community Sediment Transport Modeling System for the York River Estuary, VA, USA

“…For cohesive sediment, τ c represents a bulk characteristic of the seabed, and may depend on grain size, porosity, organic content, and depositional history, and often increases with depth in the seabed and with time since deposition [24]. Many three-dimensional numerical models assume a constant τ c , even for cohesive sediment [7,[34][35][36], which can produce satisfactory results when applied for short time scales, but neglects feedbacks between erodibility, erosion, and deposition that develop in response to events such as flood deposition, spring freshets, storm erosion, or biogenic seasonal variations [37][38][39][40]. A model of the York River estuary in Virginia that defined a constant value of τ c [35,41] was able to represent the STM but underestimated suspended-sediment concentrations, despite using 0.05 mm/s for settling velocity, significantly smaller than the values inferred from recent observations [8,10,42].…”