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Doctor of Philosophy (PhD)
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The susceptibility of estuaries to pollution has increased in the past few decades due to the increased anthropogenic inputs. The vulnerability of these estuaries to pollution is closely related to the circulation and transport in these estuaries. This work, therefore, aims to understand the transport of water-born materials in tidally dominated estuaries in relation to residual circulation and estuarine shape. The role of river discharge, tide, density gradient, and advection in altering the residual circulation and the transport timescales (flushing and residence time) are investigated. Three-dimensional hydrodynamic Eulerian-Lagrangian models are developed considering mesotidal (2 m 4 m) estuaries as the tidal could be significant compared to the mean flow. The Frenchman Bay (a mesotidal) in Maine, USA, and The Gironde estuary (macrotidal) in France are considered as study sites. The results show that density gradient and river discharge can be an important driver for the residual circulation and the flushing in wide estuaries with relatively simple geometry (simple bed profile and no constriction, headlands, or island). The results also demonstrate that the density gradient is more important to the transport than the river discharge in mesotidal estuaries and the river discharge is more important than the density gradient in macrotidal estuaries due to the increase of friction. The presence of complex morphological features gives arise for the advection to drive the residual circulation in estuaries and may affect the transport timescale. It is shown that advection can decrease the flushing time by 100% at the location where advection dominates the creation of residual circulation. It is shown that the residual circulation can be complex. Regardless of the complexity of the flow in estuaries, it is demonstrated that it is possible to predict what mechanism (river, tide, density gradient) drive the transport process in estuaries and whether to expect high or low flushing time based on simple metrics such as estuarine width and tidal excursion (the distance traveled by a water parcel over half tidal cycle). Such knowledge can facilitate better water-quality management as it provides a general idea about the transport and the water quality in estuarine environments.
Alahmed, Sohaib, "Circulation and Transport Timescales in Tidally Dominated Estuaries" (2021). Electronic Theses and Dissertations. 3387.
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