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

Wave-driven longshore currents - Freak values

JG
João Gil, modified 7 Years ago.

Wave-driven longshore currents - Freak values

Padawan Posts: 29 Join Date: 10/24/12 Recent Posts
Good afternoon,

I’m currently modeling wave-driven longshore currents and comparing the obtained results with data from LSTF (Visckburg, USA, US Army Engineer Research and Development Center’s Coastal and Hydraulics Laboratory).
I must say the results are pretty good. However, DELFT3D seems to compute freak values near the shoreline, y=27m – attachment 1. On the past days I’ve been trying to identify the origin of this problem by varying parameters such as: viscosity, roughness, time step, advection scheme for momentum and roller model parameters (Gamdis, Ndis, Alfaro, Betaro). Unfortunately, I was only able to conclude that it has something to do with the roller model, see attachment 2 (“Roller=#no#”).

Finally, how can I use Ruessink depth-varying breaking index (Gamdis)? I’ve already been looking in both FLOW and WAVE manuals, but nothing is discussed about this aspect.

Attachment 1 – Crosshore distribution of longshore currents with roller model;
Attachment 2 – Crosshore distribution of longshore currents without roller model;

I have used two different grids for both FLOW and WAVE modules:
WAVE grid -> 152x30m Grid cell: 4x0.3m
FLOW grid -> 30.4x30m Grid cell: 0.8x0.3m

I appreciate your consideration.
Best regards,
João Gil
JH
Jeff Hansen, modified 7 Years ago.

RE: Wave-driven longshore currents - Freak values

Padawan Posts: 29 Join Date: 7/12/11 Recent Posts
João,

I have seen this as well. You can try and set the maximum wave height to depth ratio (gamma) by using the keyword Gammax (in the mdf file), by default I think it is set to 2 but in your case a value of 0.6 or 0.7 is more realistic. This will minimize unrealistic wave forcing in shallow water. I was also just told that using manning roughness can sometimes help in these situations.

To answer your other question, to activate the Ruessink gamma formulation set Gammadis equal to -1.

Hope this helps.

Jeff
JG
João Gil, modified 7 Years ago.

RE: Wave-driven longshore currents - Freak values

Padawan Posts: 29 Join Date: 10/24/12 Recent Posts
Jeff Hansen:
João,

I have seen this as well. You can try and set the maximum wave height to depth ratio (gamma) by using the keyword Gammax (in the mdf file), by default I think it is set to 2 but in your case a value of 0.6 or 0.7 is more realistic. This will minimize unrealistic wave forcing in shallow water. I was also just told that using manning roughness can sometimes help in these situations.

To answer your other question, to activate the Ruessink gamma formulation set Gammadis equal to -1.

Hope this helps.

Jeff


Thanks for replying. I've already set GamMax to 0.7 but the problem still remains.

Best regards,
João Gil
JG
João Gil, modified 7 Years ago.

RE: Wave-driven longshore currents - Freak values

Padawan Posts: 29 Join Date: 10/24/12 Recent Posts
I've finally found the problem. I have set two different threshold depth in WAVE and FLOW module, which was leading to freak values.

Best regards,
João Gil