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. 
 

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

 

 

Message Boards

Boundary Numerical Instability

Fernando Barreto, modified 7 Years ago.

Boundary Numerical Instability

Padawan Posts: 74 Join Date: 6/21/12 Recent Posts
I'm doing a simulation of a meso scale region to understand the baroclinic circulation of this place. My model contains 32 Z-layers and 3 bounds (north, east, south). To the boundary configuration I chose to impose current in the top and east boundary, and elevation in the south boundary. At this configuration I got bizarre instabilities in the boundary. It was generated a velocity jet in the interface between boundary types (elevation and current)
In another configuration I imposed current in north bound., and in the half of the east boundary; the remaning sections was elevation type (south and the rest of east). In this configuration was also generated a jet in the interface between bound types (fig). The jet can be seen in the east boundary.
Someone knows how to solve this. Can this be related with the z-model? if not, any advice?
SA
Steven Ayres, modified 6 Years ago.

RE: Boundary Numerical Instability

Padawan Posts: 33 Join Date: 4/21/11 Recent Posts
Fernando,

Were you able to resolve the boundary problems in your model? I have something similar going on in my 18-level Z-model with salinity. The model is creating velocity jets in the middle of an open stage boundary. I assume this has something to do with the salinity constituent but I'm not sure how to go about fixing the problem.

Thanks,
Steve
Fernando Barreto, modified 6 Years ago.

RE: Boundary Numerical Instability (Answer)

Padawan Posts: 74 Join Date: 6/21/12 Recent Posts
Dear Ayres,
my problem was the input of different boundary conditions (velocity and wl) in adjacent boundaries. This was resolved when I used riemann boundary conditions. What's your model configuration? boundary type, dimension?

regards,
Fernando
SA
Steven Ayres, modified 6 Years ago.

RE: Boundary Numerical Instability

Padawan Posts: 33 Join Date: 4/21/11 Recent Posts
I have water level boundaries for the tide boundaries and an inflowing river total discharge boundary. I think the problem is short wave reflections in the water level boundaries. I raised the alpha coefficient to 1000 and this helped extend the simulation time to a few days, but I still seem to get re-circulation around the open boundary. I again raised the alpha coefficient to 10,000 and am testing now.

A formula is given in the manual to determine the alpha coefficient -

Equation 9.76, Water level boundary: alpha = Td * sqrt(H/g), where Td is the time it takes for a free surface wave to travel from the left boundary to the right boundary of the model area.

Since the model is around 30 miles (48 km) across the shortest open tide boundary, and assuming the tidal wave travels around 30 miles/hour or 13.4 m/s, Td would equal 3600 seconds. Using a typical depth of 200 meters at the open boundary, alpha would be on the order of 16,000. Does that sound right?
Fernando Barreto, modified 6 Years ago.

RE: Boundary Numerical Instability

Padawan Posts: 74 Join Date: 6/21/12 Recent Posts
Hi Steven,

When I use WL boundaries I usually put alpha as 1000. I didn't know how to calculate alpha, it's really to know. In Riemman we didn't use alpha, Riemann is a weaky reflective boundary condition, so I don't know a lot about alpha.
There are many possibilities to cause your error. Could you attach your model ?, I think it's easier to help.

Regards,

Fernando Barreto
SA
Steven Ayres, modified 6 Years ago.

RE: Boundary Numerical Instability

Padawan Posts: 33 Join Date: 4/21/11 Recent Posts
The problem was definitely related to boundary reflections. When I set alpha to 10000, the boundary velocity jets disappear.