Effects of estuarine and fluvial processes on sediment transport over deltaic tidal flats

Abstract

Tidal flats at a river mouth feature estuarine and fluvial processes that distinguish them from tidal flats without river discharge. We combine field observations and a numerical model to investigate hydrodynamics and sediment transport on deltaic tidal flats at the mouth of the Skagit River, in Puget Sound, WA during the spring freshet. River discharge over tidal flats supplies a mean volume flux, freshwater buoyancy, and suspended sediment. Despite the shallow water depths, strong horizontal density fronts and stratification develop, resulting in a baroclinic pressure gradient and tidal variability in stratification that favor flood-directed bottom stresses. In addition to these estuarine processes, the river discharge during periods of low tide drains through a network of distributary channels on the exposed tidal flats, with strongly ebb-directed stresses. The net sediment transport depends on the balance between estuarine and fluvial processes, and is modulated on a spring-neap time scale by the tides of Puget Sound.We find that the baroclinic pressure gradient and periodic stratification enhance trapping of sediment delivered by the river on the tidal flats, particularly during neap tides, and that sediment trapping also depends on settling and scour lags, particularly for finer particles. The primary means of moving sediment off of the tidal flats are the high velocities and stresses in the distributary channels during late stages of ebbs and around low tides, with sediment export predominantly occurring during spring low tides that expose a greater portion of the flats. The 3-d finite volume numerical model was evaluated against observations and had good skill overall, particularly for velocity and salinity. The model performed poorly at simulating the shallow flows around low tides as the flats drained and river discharge was confined to distributary channels, due in part to limitations in grid resolution, seabed sediment and bathymetric data, and the wetting-and-drying scheme. Consequently, the model predicted greater sediment retention on the flats than was observed.3

Sediment Transport into the Swinomish Navigation Channel, Puget Sound—Habitat Restoration versus Navigation Maintenance Needs

“…Oceanographic data was collected at several locations in Skagit Bay and the Swinomish Channel by U.S. Geological Survey (USGS) from 1 May 2006 to 31 May 2006 [8]. Additional data was also collected in 2009, as part of an Office of Naval Research sponsored effort to study sediment transport over tidal flats when Woods Hole Oceanographic Institute (WHOI) [11] conducted an extensive field campaign with fixed and shipboard observations. This study was designed for high-resolution measurements of current profiles, salinity, temperature, and acoustic and optical back scatter.…”

“…Simulated hydrodynamic and sediment properties such as velocity, salinity and TSS were compared with measured data from the 2009 WHOI study [11]. ADCP data were available at stations 3c, 2f and 1c for bottom velocity comparison ( Figure 5).…”

“…An improved version of the hydrodynamic model of the Skagit River estuary by Yang and Khangaonkar [9,11] was used in this study. The model uses finite volume community ocean model (FVCOM) code [12] and solves the three-dimensional momentum, continuity, temperature, salinity, and density equations in an integral form by computing fluxes between non-overlapping, horizontal, and triangular control volumes.…”

“…Figure 2b (green diamond). As described in Ralston et al [11], instrument frames for high-resolution sampling within about 1 m of the bed were deployed at five locations in the intertidal zone. Near-bed instruments included pulse-coherent acoustic Doppler profilers (pcADPs), acoustic backscatter sensors (ABSs), acoustic Doppler veloci-meters (ADVs), conductivity-temperature (CT) sensors, and optical backscatter sensors (OBSs), as well as upward-looking acoustic Doppler current profilers (ADCPs) and pressure sensors.…”

Stratification on the Skagit Bay tidal flats

“…Water levels, velocities, and densities in estuaries (Ralston et al, 2010a,b) and on tidal flats (Yang and Khangaonkar, 2009;Kim and Cho, 2011;Ralston et al, 2012), have been predicted with the Finite Volume Coastal Ocean Model (FVCOM), a three-dimensional finite volume, unstructured grid, primitive equation model (Chen et al, 2003) that was developed to simulate coastal regions. Model comparisons have shown that tidal amplitudes and phases in regions with irregular coastlines or bathymetry are predicted significantly better by models with unstructured grids than by models with rectangular mesh elements (Chen et al, 2003).…”