Influence of Estuarine Topography on Longitudinal Dispersion

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ABSTRACTOCE-0452054The longitudinal dispersion rate sets the residence time of waterborne material in estuaries, controls how far the salinity intrusion advances up-river and determines the response time of an estuary to changes in forcing conditions. The dispersion rate can vary by more than two orders of magnitude in different estuaries and can vary by more than an order of magnitude in a particular estuary depending on the forcing conditions. Furthermore, the mechanisms that set the dispersion rate can change with location within an estuary depending on the local geometry and the proximity to the mouth. In spite of its importance, the physics which govern estuarine dispersion remain poorly understood. This project will address the mechanisms of dispersion with a study of three estuaries with a wide range of forcing conditions and geometries. The research will include new measurements at two sites (the Merrimack and the North Rivers) and analysis of existing data and model results from the Hudson River. The field measurements will consist of 45 day deployments of cross-channel and longitudinal instrument arrays to quantify the salt flux and longitudinal salinity structure, and detailed shipboard measurements to provide high-resolution flux estimates and document the overall spatial structure of the current and salinity fields. Similar data sets from Hudson River, and a numerical model will be used to determine the mechanisms that control the dispersion rate over a broad range of forcing conditions and at different sections of the river with distinct channel morphology. This multi-system approach will allow the analysis to encompass some of the diversity of estuarine environments, allowing these results to be generalized to many estuaries worldwide. Broader impacts: Dispersion rate is the most important physical quantity affecting estuarine and coastal water quality. With increasing societal pressure on these waterways (e.g., nutrient loading and other non-point-source discharges) and higher expectations for environmental quality, understanding of the dispersion processes is an increasing priority. Better understanding will lead to better prediction and management.