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

 

 

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Waves distorting harmonic tides, help?

CS
Curtis Swanepoel, modified 2 Years ago.

Waves distorting harmonic tides, help?

Youngling Posts: 11 Join Date: 4/4/16 Recent Posts
hi

When only tides are implemented in the flow module. The water level plot shoes the correct increase and decrease expected with the tides.

HOWEVER

When the flow is coupled with the wave where i have a wavecon_file. The water level behaves fine at first and then goes all over the place and no longer responds to the tides as shown in the attach picture. Is this a numerical problem? The depth averaged velocity all shows very erratic behavior at this time.

How can it be fixed? I understand there should be some disturbance of the tides from the waves but not this much

Thanks
Ben Williams, modified 2 Years ago.

RE: Waves distorting harmonic tides, help?

Jedi Knight Posts: 114 Join Date: 3/23/11 Recent Posts
Hi Curtis,

This is a numerical problem.

Some first guesses as to what may be causing it:

1) Check for instabilities caused by depth-induced wave breaking at your boundary. Make sure you don't have breaking waves at your boundary, unless this is on purpose and occurs over ~5 grid cells.

2) Your spin-up interval could be insufficient, especially if you are causing a perturbation at the beginning of your simulation by not starting the water level at the exact same level as that specified at your boundary. Either improve your starting condition toy match your boundary, or wait for oscillations to die down before starting the wave coupling.

3) Either your your timestep or your grid resolution may be insufficeintly fine for fully coupled wave-current simulations.

Best,


Ben
CS
Curtis Swanepoel, modified 2 Years ago.

RE: Waves distorting harmonic tides, help?

Youngling Posts: 11 Join Date: 4/4/16 Recent Posts
Hi Ben

Thank you very much for your reply.

My model takes 16 hours to run so I will test out all three of your suggestions and let you know the outcome.

Cheers
CS
Curtis Swanepoel, modified 2 Years ago.

RE: Waves distorting harmonic tides, help?

Youngling Posts: 11 Join Date: 4/4/16 Recent Posts
when testing the spin up interval. I made it unusually long( two days long) out of the 14 day simulation and now there are missing data points with the water level? but these are just points where the bed is exposed, but there is still this erratic jump visible

Extra information

i have a 3 grid model shown in the second attachment. The flow model is 2km squared and it surrounds the Island.
The boundary conditions for the flow model are: North, South, and west are all neumann boundaries and the east is a water level boundary with the tides put in the bct file.

The second grid covers the fringing reef and extends further out so that it does not interfere with wave breaking. The third grid is very large and is where I have my wave con file being used on the North, South, and eastern boundary.
Time step=0.02 min
Flow grid cell size 25*25m
Grid 2 cell size 50*50
Grid 3 very coarse 250*250

My aim is to half the flow and second grid cell sizes but this currently just takes too long and thus Ive used larger cells
Ben Williams, modified 2 Years ago.

RE: Waves distorting harmonic tides, help?

Jedi Knight Posts: 114 Join Date: 3/23/11 Recent Posts
HI Curtis,

Maybe try simplifying your model a bit until it works and then gradually build up complexity. Your site looks quite complicated in terms of rapid changes in depth and wave/current processes occuring.

For example, remove the tidal signal from your east boundary. Close off your west boundary (make dry land) and then keep the north and south as Neumann boundaries. Then you can check to see what the water level is doing, and also check map outputs of your current magnitude and direction.

In general you need your wave model to be the same resolution or finer than your flow model. This is especially the case where you have wave breaking, which is clearly occurring at your site.

You also don't make clear which is your flow model grid - is it (2) or (3)? (1) probably won't work as your boundaries are too close to the island.

It is usually better to have your flow model aligned so that it runs parallel to some bathymetric contour. Given the geometry of your island I would be tempted to do just that, running your flow model in a curvilinear fashion from NW to SE along say the 20m contour. Admitedly this is probably not that easy. The goal is to try to locate your boundaries far enough away from the area of current generation (breaking waves in the nearshore and around the island), but whilst keeping your flow model in water shallow enough that you don't have to have a tiny time step to satisfy your courant number.

You could try a simple 'toy' model that is a simple sketch of the physical processes at your site. That is, heavily schematised and simplified. Just an island of approximate dimension and depth, but with much simplified bathymetry on the island and the surrouunding area. Then experiment with what grid resolutions work and how your boundary design affects your model stability or predicted currents. To do this it might help to output time series of water level, current speed and direction, and wave height, period, direction and orbital velocity at various locations around your island.

Then, once you think you have a reasonable idea of how things are behaving in your model, build up complexity until you have a reasonably realistic approximation of your site. Do you have validation data?

Good luck.

Ben