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

Unstable boundary using Neumann BCs

Susan Clark, modified 8 Years ago.

Unstable boundary using Neumann BCs

Hi all,

I'm trying to figure out what is causing the instabilities in water level at my seaward boundary.

My domain is appx 3km by 1km, and it is coastal with the coast on the west side of the domain. I have three boundaries. The eastern open ocean boundary is forced by astronomical water level, with constant magnitude across the boundary and a 5 degree change in phase from the northern corner to the southern corner. The northern and southern boundaries are Neumann boundary conditions with constant amplitude (on the order of 10^-6) and phase (90 degrees less than the corresponding open ocean corner) across the boundary.

After about 2 days of the run using 1min time steps, storing data every 60min, the eastern ocean boundary gets unstable, with one gridcell wide, boundary length, strips of very high (8m) sea level developing, usually alternating with another one gridcell wide strip of very low sea level. These persist for a few timesteps before the model crashes.

I have examined my boundary conditions very closely, and I would say they are set up correctly, if only they worked! I must be missing something. Does anyone have any ideas as to what is causing this instability?

Many thanks,
Katherine Cronin, modified 8 Years ago.

RE: Unstable boundary using Neumann BCs

Padawan Posts: 42 Join Date: 2/23/11 Recent Posts
Hi Jeanette,

Do you observe high velocities near the boundary before the model crashes?

best wishes,

Dr. Ahmed S. M. Ahmed, modified 8 Years ago.

RE: Unstable boundary using Neumann BCs (Answer)

Youngling Posts: 17 Join Date: 3/24/11 Recent Posts

Stelling (1984) introduced a so-called reflection coefficient, alpha, to make the boundaries less reflective for disturbances with the Eigen frequency of the model area (Riemann or weakly reflective boundaries).

This reduces the spin-up time of a model from a cold . The reflection coefficient, alpha, should be chosen sufficiently large to damp the short waves introduced at the start of the simulation. The recommended value for tidal computations is 50 or 100.

Good luck

Kind Regards
Dr. Ahmed S. M. Ahmed