A Study of Spatially Variable Bed Forms Exposed to Time-Varying Flow Conditions in a Complex Estuarine Environment, the Mouth of the Columbia River, USA

A comprehensive survey of bathymetry and sediment grain size was performed in the Mouth of the Columbia River (MCR) as part of the 2013 ONR-funded RIVET II Inlet Experiment. The MCR is a 5-km wide, dynamic estuary entrance along the Pacific NW coast of the U.S., characterized by strong tidal currents up to 3 m/s, an extreme wave climate, and mean river discharge of 5,000 m3/s. The swath survey resulted in a high-resolution bathymetric map of the river inlet, spanning 15.5 by 7.5 km of the estuary entrance. The resulting digital elevation model (DEM) has a grid spacing of 1 m and vertical position accuracy of 20 cm. Digital images of the in-situ sediments were taken to quantify spatial variability of grain size distributions of the bed sediments. The MCR seafloor is sand dominated, with mean grain size ranging between 0.16 and 0.32 mm, and covered with bed forms of various sizes. Transects were drawn on the DEM on average every 10 m, orthogonal to bed form crests. The transects were analyzed for bed form height, wavelength, orientation, and asymmetry, resulting in the identification of 60,000 bed forms with heights ranging from 10 cm to 2.7 m and wavelengths of 5 m to 152 m. Bed form heights were compared to the local water depth; the vast majority of the bed forms did not approach heights of 1/6 water depth and were assessed to be independent of depth. One region of the estuary was an exception, with highly energetic flows and large bed forms that may have been depth limited. Larger bed forms over 65 m in length are three times more likely to be ebb-flow directed. Bed forms of wavelengths less than 65 m do not statistically favor flood- or ebb-dominance. Analysis of asymmetry can be used to predict regions of ebb- and flood-dominated bed-load transport in the river mouth. Predictive models of bed form height and wavelength, such as van Rijn (1984) and Bartholdy (2005), were developed based on the grain size, depth, and modeled flow to compare to the observational data. A hydrodynamic and sediment transport model (Delft3D) was used to explore the relationship between flow dynamics and spatial variability of bed forms within the estuary entrance. Comparing the observational data with bed form and flow models will allow for increased quality of predicted bed forms in complex estuarine environments.