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


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Very low values for depth averaged velocity

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Very low values for depth averaged velocity
tidal boundary wrong flow model boundary condition depth avereraged velocity depth file
Answer (Unmark)
11/14/17 10:03 AM
I am running a flow model over a gulf. The gulf is surrounded entirely by land on three sides and there is only the sea boundary towards the south. I set this as my open boundary and input water level from astronomical tidal conditions. For the bathymetry I used the GEBCO dataset. The depth file varies from 2000m near the open boundary to about 10-30 meters inside a narrow channel between an island within the gulf and the main land.

My problem is that I have been getting very low values for the depth averaged velocities (in the order of < 0.1 m/s) when I open the com file in Quickplot. And I get these low velocities even within the channel. I know these values are wrong because the field measurements are significantly higher than this. The water level at the boundary conditions is about ~0.9 meters from the astronomical constituents, my grid sizes are about 4 km x 2km near the entrance of the gulf and come down to 250m x 250m within the channel. I set the roughness to a Manning coeffcient of 0.02 and used an eddy viscosity of 100 m^2/s. My timestep is 1 minute and I also set a smoothing time of 600 minutes.

Any ideas on what might be wrong with my model?
RE: Very low values for depth averaged velocity
Answer (Unmark)
11/27/17 12:24 PMas a reply to A D.
Hi. GEBCO is very coarse ..... 0.5 nautical miles I think. Is your channel being sufficiently resolved by this dataset?
RE: Very low values for depth averaged velocity
Answer (Unmark)
11/28/17 2:31 AMas a reply to Phil Shepperd.
Hey, thank you.

Yes I know GEBCO is pretty coarse. I used GEBCO for the entire gulf but within the channel I used some in situ data that I already have. Besides, I manually checked the depth after interpolation and it seemed okay to me.
RE: Very low values for depth averaged velocity
Answer (Unmark)
11/28/17 3:33 AMas a reply to Phil Shepperd.
My model works perfectly fine when I use some uniform depth like 20 meters or something. So it appears to be an issue with the bathymetry. But I don't see a way around this as it is wrong to set a uniform depth of 20 m for the entire gulf.
RE: Very low values for depth averaged velocity
Answer (Unmark)
11/28/17 10:38 AMas a reply to A D.

I would agree with you that it is due to the big bathymetry in the mouth. The velocity is proportional to the ratio of tidal prism volume/ the steady water volume. Thus with 20m uniform water depth it will give reasonable results.


RE: Very low values for depth averaged velocity
Answer (Unmark)
11/29/17 2:45 AMas a reply to Qinghua Ye.
Hi Qinghua,

Thank you. Yes that is the case. However, I am trying to tweak some parameters to get my values up by as much as possible.
Horizontal eddy viscosity seems to be having a decent effect so I am trying to create a space-varying file for that. The manual suggests creating a .edy file. But when I do that I get the following error.

forrtl: severe (66): output statement overflows record, unit -5, file Internal Formatted Write

The model works fine if I just set a uniform value for eddy viscosity. So, I am wondering if the format for my .edy file is wrong.
The way I created it is I just opened my bathymetry file (.dep file) and replaced the depth values with eddy viscosity values that I wanted and saved it with a .edy extension. Is the format wrong or am I missing something?

Thank you
RE: Very low values for depth averaged velocity
Answer (Unmark)
11/29/17 2:58 AMas a reply to A D.
Okay. Never mind this. It was because I had NaN values in the .edy file.