Presenter: Yan Jia (CIRCA UConn)

Description:
Predicting the water quality and the statistics of flooding in estuaries that are connected to the coastal ocean by narrow (~10m) inlets are critical in many areas. Observations show a 33% tidal attenuation through an inlet to a marsh basin in Guilford, CT. Simulations of the effect of inlet width (W) on the tides between two idealized basins are studied using four horizontally two-dimensional hydrodynamic models (ROMS, FVCOM, ADCIRC, and SCHISM) that solve the same equations with different gridding, numerical schemes, and shoreline velocity slip conditions. The along-channel pressure gradient force is primarily balanced by horizontal eddy stress divergence and the advection of momentum is feeble. For the no-slip scenario, the across-stream shear is augmented along the inlet banks and leads to a large along-channel pressure gradient. For the slip scenario, water levels drop downstream at the inlet ends where the changing of flow divergence enhances the horizontal eddy viscosity. All the no-slip model results are more consistent with the Guilford data: ROMS and FVCOM give comparable constraints, while ADICR is much less. SCHISM is more efficient and increasing the horizontal viscosity in the inlet significantly improves its simulations. The results can direct model selection and grid design in coastal modeling. A simple model for η_i the water level in a basin of area A that is connected by an inlet with length L and depth D, to a large basin in which η_o(t) the water level oscillates can be expressed as ∂η_i/∂t=α(D+ η_o)(η_o- η_i). For the no-slip scenario, α=(1/2γ-1/24)gW³/(εAL), where g is the gravity acceleration, ε is the horizontal eddy viscosity, γ=3.2 is based on the ROMS results; for the slip scenario, α=gβW²/((π-2)εA), where β=0.5 is based on the slip-FVCOM results. The simple models compare well with the numerical models. They can provide quick and useful guidance on exchange rates between basins for ecologists and engineers.