MorphoDYNAmics of tidal inlets: 3D numerical MOdelling and impacts on pollutant circulation. – DYNAMO

A new 3D numerical modelling system, capable of simulating hydrodynamic circulation, waves, sediment transport and bed evolution will be improved and applied at the inlet of Bonne-Ans lagoon.
Field measurements will be carried out at this site to characterise hydrodynamic circulation, wav propagation at the studied site. Repetitve bathymetric surveys and a video-imagery system will allow demonstrating morphological changes at the studied site.

This project aims firstly at applying a fully-coupled 3D morphodynamic modelling system to tidal inlet environments.
The application of this modelling system together with field measurements will allow improving the understanding of the physical processes conroling the dynamics of tidal inlets. Particular emphasis will be put ont the relationships with adjacent shorelines and the circulation of pollutants in the lagoon.

The modelling system SELFE is currently being used by 6 researchers and PhD students at LIENSs: the improvements performed during DYNAMO will be useful for other projects relying on SELFE at LIENSs.
The physical processes found in DYNAMO will probabmy be valid for other tidal inlets and estuaries.

Dodet, G., Bertin, X., Bruneau, N., Fortunato, A.B., Nahon, A. and Roland, A., 2013. Wave-current interactions in a wave-dominated tidal inlet. Journal of Geophysical Research: Oceans, Volume 118, Issue 3, 1587–1605,

Tidal inlets constitute navigation routes and they control the stability of adjacent shorelines and the exchanges of water, sediment and dissolved material between the ocean and back barrier lagoons. Tidal inlets are thus of critical and growing socio-economic and environmental importance worldwide, and particularly in Europe, due to their intense use for aquaculture and tourism. Tidal inlets are often exceptionally dynamic, due to the combination of strong tidal currents, shallow channels and an energetic wave climate. This exceptional dynamics drives fast and large morphological changes, making their behavior hard to predict and their sustainable management difficult to achieve. To address this problem, the development of morphodynamic modeling systems appears as the most promising perspective. Morphodynamic modeling systems couple a set of modules to simulate hydrodynamic circulation, wave propagation, sediment transport and bottom evolution. 2DH morphodynamic modeling systems have known an intense development over the last decades, but the accuracy of their predictions was shown to decrease for energetic waves or in meandered channels. This problem is related to the limitations of the 2DH approach, which prevent the correct representation of depth varying processes, such as undertows or meander circulation. To overcome this problem, the development of 3D morphodynamic modeling systems constitutes a unique perspective.
DYNAMO aims at: (1) improving an existing 3D community morphodynamic modelling system; (2) applying this modelling system at a inlet/lagoon system associated with large socio-economic and environmental importance; (3) evaluating the benefits of the 3D approach compared to traditional 2DH simulations; (4) improving our understanding on the behaviour of tidal inlets at various time scales, including interactions with the adjacent shorelines; (5) investigating the effect of tidal inlet morphodynamics on residence times and the circulation of pollutant in the backbarrier lagoons. To reach these goals, DYNAMO will first improve the community modelling system SELFE with respect to wave-current interactions in 3D, turbulent closure scheme, bottom friction and numerical methods to solve the Exner equation. The improved modelling system will then be applied to the Bonne Anse Lagoon/Inlet, located to the mouth of the Gironde Estuary and characterised by fast morphological changes combined to large socio-economic and environmental interests. Model predictions will be calibrated and validated with field measurements of bathymetry, waves and currents as well as video-imagery data. Once validated, the modelling system will be used to investigate the physical processes controlling the relationships between the tidal inlet and its adjacent shorelines. The impact of inlet morphodynamics on residence times and pollutant circulation will then be analysed. DYNAMO will first result in a state-of the-art numerical modelling system that will be used after the project by several of its members while the numerical developments carried out in the scope of the project will be shared with the French coastal modelling community (namely MARS and TELEMAC teams). The unpublished 3D approach applied to a tidal inlet will provide new scientific knowledge at an international level. Finally, this knowledge will improve the management of the studied system at local scale, while more generic conclusions, applicable to other tidal inlets, will also be proposed.