CSDMS 2011 annual meeting poster Caroline Le Bouteiller

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Presentation provided during CSDMS annual meeting 2011

A velocity-dependant friction law for flexible vegetation in a 2D hydrodynamic model

Caroline Le Bouteiller, Northwest Hydraulic Consultants Vancouver, B.C. NO STATE, Canada. caroline.le-bouteiller@m4x.org


The tidal flats of Roberts Bank in British Columbia, Canada contain large areas of the intertidal zone that are vegetated with eelgrass (Zostera Marina and Zostera Japonica). This vegetation has a variable influence on the flow of tidal waters passing over the tidal flats, which we aim to describe in a large-scale 2D hydrodynamic model.

Vegetation on the surface of the tidal flats causes an increase in the roughness that modifies the flow properties. For submerged vegetation, this roughness is most strongly related to the height of the plants in the water; however, for very flexible plants such as eelgrass, the plant height changes with flow velocity since the plants bend with the currents. The roughness is therefore dependent both on flow depth and flow velocity.

Existing studies concerning the effect of flexible vegetation on flow are mostly focused on the small-scale properties of the velocity and turbulence profiles. Such results cannot be directly incorporated into 2D hydrodynamic models. 3D hydrodynamic modeling is computationally demanding and is therefore less appropriate for large-scale studies and engineering applications over large areas. In order to resolve this computational challenge we developed an integrated formulation of the effects of flexible vegetation on the flow, with the following approach: The roughness is represented through an equivalent Manning’s coefficient, which depends on both the water depth and the flow velocity.

Simulations are performed with the Telemac2d model, which has been modified to incorporate the velocity-dependent friction law. Preliminary results show that the proposed law is able to account for qualitative modifications in the tidal flow. In particular, the simulation provides an asymmetric flow pattern that correctly predicts the slower ebb velocities as compared to flood velocities, as observed in the field.

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