intro story Coast / Estuary

Coast / Estuary

Coastal systems are among the most dynamic physical systems on earth and are subject to a large variety of forces. The morphodynamic changes occurring to coastlines worldwide are of great interest and importance. These changes occur as a result of the erosion of sediments, its subsequent transport as bed load or suspended load, and eventual deposition. 
Estuaries are partly enclosed water bodies that have an open connection to the coast. Estuaries generally have one or more branching channels, intertidal mudflats and/or salt marshes. Intertidal areas are of high ecological importance and trap sediments (sands, silts, clays and organic matter).
Within the Delft3D modelling package a large variation of coastal and estuarine physical and chemical processes can be simulated. These include waves, tidal propagation, wind- or wave-induced water level setup, flow induced by salinity or temperature gradients, sand and mud transport, water quality and changing bathymetry (morphology). Delft3D can also be used operationally e.g. storm, surge and algal bloom forecasting. 
On this discussion page you can post questions, research discussions or just share your experience about modelling coastal and/or estuarine systems with Delft3D FM. 




Sub groups
D-Flow Flexible Mesh
Cohesive sediments & muddy systems



Message Boards

How to decide the Reflection parameter alpha value? (flow)

Esther Espinoza, modified 3 Years ago.

How to decide the Reflection parameter alpha value? (flow)

Padawan Posts: 25 Join Date: 4/6/17 Recent Posts
Hi, I'm modelling water level for an estuary area (1 month) and I would like to know if you can explain me a little bit about the reflection parameter alpha that it's needed in the bounday section of the flow module. I've read the manual, but I'm not sure how to decide it's value or how big is the influence of that parameter on the results of the water level simulation.

Thank you
Erin King, modified 3 Years ago.

RE: How to decide the Reflection parameter alpha value? (flow)

Youngling Posts: 12 Join Date: 10/5/16 Recent Posts
High values of the parameter can result in a phase offset in the simulated results. The parameter makes the boundary "less reflective" however large values of this can result in a phase offset at the boundary. I think for a large domain and a large alpha coefficient I had up to a 30 minute offset in my water levels. I think this is due to it applying a time derivative of the Riemann invariant to the boundary condition, therefore it may hold back the boundary condition for a period to achieve this, though I may be mistaken in that interpretation.

At the bottom of page 207 in the manual there is an equation to determine a suggested value for alpha. Although another way could be to do a sensitivity analysis, testing different values and finding one that works.

Another suggestion you could try is to use a locally high viscosity in the direct vicinity of the open boundary (a value of 100 m2/s over the first 5 cells). A space-varying viscosity field can be generated using Quickin in a way similar to the way you generate a bathymetry field. The format of the viscosity needed for the *.edy file is identical to the format of the *.dep file. The *.edy file also needs to contain a block of data containing the horizontal diffusivity values as well.
Dmitry Kushnir, modified 3 Years ago.

RE: How to decide the Reflection parameter alpha value? (flow)

Jedi Knight Posts: 107 Join Date: 11/9/12 Recent Posts