Circulation and Transport Timescales in Tidally Dominated Estuaries

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

Flash Flood Modeling on Single and Multiple Structures and Wave Impact Mitigation Using SPH

Flash flooding caused by dam breaking and tsunamis commonly results in destruction of coastal and offshore structures due to wave-structure interaction. The destructive potential of the wave impact can be assessed by determining wave height, flow velocity and the impact force/pressure imposed on the structures. Accurate evaluation of such quantities is, therefore, critical for the safety and design of both coastal and offshore structures. Yet, it is challenging for mesh-based numerical methods (e.g. finite element) to precisely model the extremely violent fluid flow that results in the case of wave-structure interaction. Weakly Compressible particle-based Smooth Particle Hydrodynamics (WCSPH) method is adopted in this work due to its robustness in simulating wave-structure interaction. In particular, SPH has the capability to simulate dam breaking and tsunamis interacting with single and multiple structures. There are three primary objectives of this thesis. First, is to present a validated and robust method for predicting local pressure on solid structures due to wave impact using SPH. The second is to assess the performance of the WCSPH method in quantifying flow characteristics (e.g. impact force, overturning moment) when the wave interacts with structures. Finally, the third objective is to implement the validated SPH methods to evaluate flow characteristics such as flow velocity and impact force to study the flood behavior on two different “city” layouts to mimic an urban flood. With these goals, five dam breaking (three dam braking on single structure and two dam breaking on multiple structures) and two tsunami (tsunami on single structure and tsunami on multiple structures) models were developed using the SPH method. The results indicate that the newly developed local impact pressure evaluation technique is valid and accurate when compared with laboratory experiment data. The results highlight the ability of SPH to predict flood characteristics like impact force, local pressure and overturning moment compared with finite volume or finite element modeling methods. It has been found from the dam breaking on multiple structures model that the flash flood could cause more destruction if the flood impacted a city oriented at an angle to the oncoming flood waters rather than a city aligned in the flood direction. Another result of this work included the implementation of a new tsunami wave maker into SPH. It was found that the tsunami wave was generated perfectly by the new tsunami wave maker adapted in this study. In addition, other wave characteristics, such as wave height, wave velocity, impact force, and overturning moment were more accurately simulated by SPH than finite element numerical methods, when compared to laboratory experiment data. In conclusion, SPH can address the needs of modeling the complex interactions between fluid flow and solid structures. Further, SPH can robustly cope with intricate and multipart geometry problems. Thus, the SPH model can be used as a superior and cost effective alternative for physical (laboratory) models replicating solitary wave propagation and is more precise than finite element numerical modeling. Further, this work has shown that city development and planning in flash flood prone areas should design streets to be aligned with oncoming flood waters for damage mitigation. Overall, this study can be used to advance coastal and civil engineering studies focused on better understanding and modeling solitary wave propagation and wave-structure interaction in offshore and coastal environment

The role of advection and density gradients in driving the residual circulation along a macrotidal and convergent estuary with non-idealized geometry

Due to variations in channel depth, width, and lateral bottom profile, estuarine residual flows can exhibit significant variation in magnitude and transverse structure along macrotidal and convergent estuaries. This article explores the along-channel residual flow (magnitude and transverse structure), forcing mechanisms and their variations along the Gironde estuary in France. With emphasis on the role of density gradient and the advective accelerations in the along channel momentum balance, the study outlines the along-channel residual flows and forcing mechanisms over the neap-spring tidal cycle during high and low river discharge conditions. The results demonstrate that the density-driven flow contribution to total residual flows is, approximately, 75% (along-channel averaged) during neap tide and 18% during spring tide for both high and low river discharge scenarios. Owing to the complex lateral variation in the channel depth and the constriction near the mouth, advective accelerations play a major role in altering the residual flow lateral structure. However, the relative importance of advection reduces in the main body of the estuary where the channel is widened with poor lateral variation in the bottom depth. The results suggest advection and the baroclinic pressure gradient produce a laterally sheared along-channel residual flow with inflow in the channel and outflow over the shoals during neap tide. During spring tide, this lateral structure is produced due to advection. The results show that even in a homogenous system, advection can induce a flow with a structure that mimics the density-driven flow. The article shows that along macrotidal estuaries the presence of complex morphological features can affect the residual flow dynamics. As such, the residual flow in these systems should be schematized not only by considering the lateral variation of bathymetry, but also the along channel complexity