Applications of Remote Sensing to the Study of Estuarine Physics: Suspended Sediment Dynamics in the Columbia River Estuary

摘要

Estuarine circulation and its associated transport processes drive the environmental integrity of many near-shore habitats (the coastal ocean, rivers, estuaries and emergent wetlands). A thorough understanding and consideration of this circulation is, therefore, vital in the proper management of these habitats. The aim of this study is to bring together theory and new satellite observations in the Columbia River Estuary to increase our understanding of estuarine circulation and transport. Surface reflectance measurements gathered by the Moderate Imaging Spectroradiometer (MODIS) are first compared to in situ observations to develop an empirical model for remotely derived surface turbidity. Results indicate that MODIS data significantly correlate with in situ measurements of turbidity throughout the CRE (R2 = 0.96). Remote estimates of turbidity are then used to explore the physical processes that drive their spatial distribution. Although the response to different hydrodynamic conditions varies throughout the system, global levels of turbidity are most sensitive to fluvial and tidal inputs and increase during spring tides and high river flow. As a result, the turbidity field has temporal cycles that are consistent with the frequency of these processes. The location of the estuarine turbidity maximum (ETM) is highly dynamic and typically migrates downstream as the tidal velocity or river flow increases. The ETM becomes trapped near the Megler Bridge (river kilometer 20), however, and the presence of strong topography in this region suggests there exists an interaction between bottom topography and sediment transport.A 2-D semi-analytical model, developed herein from the simplified Navier-Stokes equations, confirms that topographic features exhibit substantial influence on longitudinal turbidity distributions. The model considers the coupled, tidally-averaged velocity (composed of gravitational circulation, internal tidal asymmetry, and river flow) and salinity fields and assumes a condition of morphodynamic equilibrium to estimate the distribution of sediment for arbitrary channel configurations. Model simulations demonstrate that topographic highs tend to increase local seaward sediment fluxes, and that topographic lows increase local landward sediment fluxes. Sediment flux convergence near topographic highs compresses the local turbidity distribution, whereas flux divergence near topographic lows dilates the distribution and, under appropriate conditions, produces multiple ETMs.In summary a combination of the model and satellite data has given valuable new insights into the sediment dynamics of estuarine environments; in particular, both show that turbidity distribution and ETM location vary considerably with tidal and river flow conditions, fluctuating on a variety of timescales, and are heavily influenced by bottom topography.