Every year, in the Vietnam Mekong Delta Coastal Zone (VMDCZ), erosions cause approximately 300 ha of agricultural land loss. Therefore, measures for shoreline protection are urgently needed. This paper discusses the impacts of protection measures in the Go-Cong Coastal Zone to prevent erosion/accretion processes, predicted by two numerical models, MIKE21-FM and TELEMAC-2D. Hard and soft measures have been proposed using breakwaters and sandbars, respectively. The simulations show that the erosion/accretion trends provided by both models are similar. For breakwaters, MIKE21-FM provides less accretion than TELEMAC-2D in areas extending over 300 m and 500 m from shorelines. However, for sandbars, MIKE21-FM shows higher accretion within areas extending over 500 m but less than 300 m. Sandbars cause higher accretion in a larger area, extending over 1000 m offshore. The simulation results allow us to propose two alternative measures: (1) a row of several breakwater units will be implanted at 300 m offshore. The length of each unit is 600 m, with a gap between two neighbouring units of 70 m and a crest elevation of 2.2 m above mean sea level (MSL). (2) A row of sandbar units will be posed at 500 m offshore, with a unit length of 1000 m and a gap between the two neighbouring units of 200 m. The crest elevation is fixed at MSL.

Bacterial pollution in the water comes in particular from Escherichia coli and fecal coliforms, responsible for gastroenteritis and diarrhea, intestinal streptococci or enterococci (urinary tract infections and peritonitis), salmonella which can cause serious gastroenteritis, shigella (dysen-teritis, gastroenteritis), cholera vibrio (cholera). As 23 sites on the Seine and Marne Rivers (Parisian Region) would be identified as the natation competition sites for the Paris-2024 Olympic and Paralympic Games, the water quality at these sites should be seriously monitored. Numerical modeling can be considered one powerful tool to watch the water quality parameters. However, measurements show that the water quality is not homogeneous in a river cross-section, and one-dimensional (1D) models are not enough to accurately calculate the bacteriological concentration dispersion in the aquatic environments. Therefore, a two-dimensional model has been developed by coupling the TELEMAC-2D model and its water quality module WAQTEL for simulating bathing water quality in the Seine and Marne Rivers. The model was validated against in situ measurements and was compared against a 1D model. Results show that this model can simulate not only the longitudinal evolution but also the transverse dispersion of bacteriological pollutants. Then, a 3D multi-layer model has been developed around a bathing site using the TELEMAC-3D model. The result of the 3D model is promising and allows us to get a finer representation of the bacteriological concentration in three dimensions.

The aim of this work is to develop an original semi-coupled approach for solving the Saint-Venant and Exner equation system. The shallow-water equations are solved first to calculate the full evolution of flow fields, and then the Exner equation is solved to model the morphodynamic response. The numerical method is based on a projection method, which consists in combining the momentum and continuity equations in order to establish a Poisson-type equation for water surface levels. The present formulation is identified as a semi-coupled approach because the projection method incorporates the change in the bed evolution in its approximation. A second-order numerical scheme has been proposed using an unstructured finite-volume technique and an implicit time integration method. Several benchmarks are used to demonstrate the capabilities, accuracy and performance of the model.

A full 3D numerical model is used for studying tidal asymmetry, estuarine circulation, and saline intrusion in the Gironde estuary. The model is calibrated and verified using the data measured during two field surveys in the Gironde estuary. Harmonic analysis of numerical results is proposed to understand how the superposition of M2, M4 and M6 components generate a complex estuarine circulation and salinity intrusion in the Gironde estuary. The numerical results show that the M6 component plays a significant role as important as the M4 one in modifying the nature of tidal asymmetry, especially in the Gironde upper estuary. In this case, the use of the phase lag between M2 and M4, neglecting M6, to predict the tidal asymmetry nature could produce errors. The effect of asymmetrical tides on saline intrusion and residual circulation is specifically discussed here.