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

RE: Berm creation in stormy condition!!

ZM
zahra moosavi, modified 2 Years ago.

Berm creation in stormy condition!!

Youngling Posts: 21 Join Date: 12/22/14 Recent Posts
Hi all,
Dose any body know what is the reason of berm creation even in stormy condition? Actually i tested different combinations of fsus, fbed,fsusw,fbedw in order to have proper values to reproduce winter bed profile, but unfortunately in all of them a berm developed at shoreline. It would be really helpful if any body can help me in this regards.

Best Regards
Ben Williams, modified 2 Years ago.

RE: Berm creation in stormy condition!!

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

Onshore transport of sediment is something that is almost guaranteed to happen in Delft3D when using 2D simulation, due to the wave the TRANSPOOR algorithm is implemented. This is because there is no mechanism for undertow, which in the 3D model is responsible for offshore transport under wave conditions.

In general the following help promote offshore transport:

* Enable 3D model (3 layers minimum), suggest logarithmic spacing from bed.
* Reduce fsusw and fbedw to something like 0.1
* Enable the roller model
* Make sure that your wave model is 'use and extend' both the water surface and the currents


You will probably still have a few problems at the shoreline, which will be a function of your resolution, how smooth and steep your bathymetry is near the shore, your grain size, and a few other things.

Once you have the 3D model enabled, plot the vertical distribution of current speed and direction (onshore/ offshore) for select locations along your beach profile to gain some additional understanding. The same goes for plotting the vertical distribution of current-related band and suspended load, and wave-related bed and suspended load.

Additional practical guidance for 'tweaking' the sediment transport model to give vaguely reasonable results can be found elsewhere on this forum.

Good luck,

Ben
ZM
zahra moosavi, modified 2 Years ago.

RE: Berm creation in stormy condition!!

Youngling Posts: 21 Join Date: 12/22/14 Recent Posts
Hi Ben,
Thank you very much for your comments. actually i have run the Flow-Wave coupled model in 3d mode with 10 layers ( 2,8,12,12,16,16,12,12,8,2 % of total depth). i also have used the 'use and extend' option for water surface, current(in depth averaged type) and bathymetry. I attached the figure of results for fsus=1, fbed=1,fsusw=0.1,fbedw=0.1.
I only have not enabled roller model and i will enable it and compare the results. i hope this could solve the problem.

Attachments:

Ben Williams, modified 2 Years ago.

RE: Berm creation in stormy condition!!

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

Very useful diagnostic plot. You do indeed have currents moving offshore at the bed but apparently onshore movement of sediment. Are you simulating fine/ medium sand (which can be transported in suspension by waves), or coarse sand/ gravel (which are almost exclusively bedload)?

If coarse sand / gravel, then you will get onshore movement because the offshore currents generated by the waves cannot move the sediment. The current-related transport is typically negligible, which means that the wave-related load is doing all the work. In this case Delft3D assumes transport is purely in the wave direction and there is little you can do about this. [Other than increasing your wave height and period, but then is this the scenario you want to considerr?].

Typically tweaking fsusw is all about trying to achieve a balance between the wave-related load, which is imposed in the wave vector at each cell, and the current-related load, which is imposed in the current vector at each cell.

If fine/ medium sand, some suggestions:

1) I see that your resolution is something like 10m, yet your profile is quite steep (1V:10H?) compared to sandy beaches. It is possible that you are not adequately capturing your wave breaking, which you want to occur over about 5 grid cells in order to get wave dissipation , currents and transport correct. Smaller waves break over a shorter distance, as do shorter period waves. As a practical guide, aim for ~5m resolution in the breaking zone as a starting place.

2) Reproduce your plot, but with wave height and dissipation instead of current vectors. This will inform if you are adequately describing wave breaking, which in turn is driving your morphology.

3) A similar plot, but showing suspended sediment concentration in each cell. On the same plot, show somehow the direction and magnitude of transport on the bed at each grid cell (or every few grid cells) along your profile.

If nothing works then this is telling you something about the physicality of your scenario, and you might try changing either the sediment diameter or beach slope until you are simulating conditions that would typically have offshore migrating bars in real life (e.g. Duck, Edgmond). Noting that getting onshore/offshore bar migration even vaguely correct in morphological models is a tricky business.

Best,

Ben
ZM
zahra moosavi, modified 2 Years ago.

RE: Berm creation in stormy condition!!

Youngling Posts: 21 Join Date: 12/22/14 Recent Posts
Dear Ben

Thank you very much Ben for your comments and for your time. Sorry for the delay due to the some personal problems.
Actually i am simulating fine/ medium sand(D50=0.2mm) and the slope of bed profile is 1V:150H.
I also reproduced wave height and dissipation plots(Fig1 and Fig2) but i could not find any problems.
In order to know the reason of having onshore movement of sediment in the case of offshore current i set fsusw=0 and fbedw=0 which force the movement in the direction of wave vector be zero.
But it also shows the onshore sediment movement. two things seem strange to me(attached file):
-Velocity in the near shore(370 to 470m) is offshore in all depths but the bed stress and sediment transport are in onshore direction.
-bedload transport is zero in most area except 5000-6000m.
It would be really appreciate if you have any idea about these problems.

Best Regards
Ben Williams, modified 2 Years ago.

RE: Berm creation in stormy condition!!

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

Some thoughts:

1) Your wave condition is a bit extreme and is showing on-shore directed dissipation even at the boundary. Have you tried a wave that is not breaking at the boundary? For example, a 2m wave? It's hard to work out what the depth is at your boundary but typically one would want it not to be breaking or dissipating.

2) Is your model 2DV (as in beach profile model, just one grid cell wide) or fully 3D? If 3D, plot a map of current vectors for top and bottom layers. Just to see what is going on and if there are any spurious currents being generated at your boundary. Are there any long-shore currents at the site? Superimpse on the vector map bathymetric contours of your site (suggest 1m or 2m intervals).

3) You seem to be plotting variables for different cross-sections (m =25 vs m=50).

4) What is your wave incidence angle are the lateral boundaries of your wave model much greater in spatial extent than your flow model?

It's not unusual to get behavior like this but seems typically related to the way one sets up the model (grid design, bathymetry etc) and boundary conditions.

Ben
ZM
zahra moosavi, modified 2 Years ago.

RE: Berm creation in stormy condition!!

Youngling Posts: 21 Join Date: 12/22/14 Recent Posts
Hi Ben
regarding your comments, i have to say that:

1-The depth at the boundary is 40m and i also have tried 1m wave at boundary condition and the attached files show no dissipation and breaking at boundary.(Wave.rar)

2- The model is fully 3D and there are not any spurious currents at boundary, but although the wave incidence is normal to the coast, longshore current has been generated.(Flow.rar)

3-m=25 is the central section of flow model and m=50 is the central section of wave model, so the results have plotted at the same sections.

4-As i mentioned the wave incidence angle is normal to the coast and the lateral boundaries of wave model are 2500m far from boundaries of flow model(grid.png)

Best Regards,
YJ
Yoan Jericko, modified 16 Days ago.

RE: Berm creation in stormy condition!!

Youngling Posts: 16 Join Date: 9/26/19 Recent Posts
Ben Williams:

Hi Zahra,

Onshore transport of sediment is something that is almost guaranteed to happen in Delft3D when using 2D simulation, due to the wave the TRANSPOOR algorithm is implemented. This is because there is no mechanism for undertow, which in the 3D model is responsible for offshore transport under wave conditions.

In general the following help promote offshore transport:

* Enable 3D model (3 layers minimum), suggest logarithmic spacing from bed.
* Reduce fsusw and fbedw to something like 0.1
* Enable the roller model
* Make sure that your wave model is 'use and extend' both the water surface and the currents


You will probably still have a few problems at the shoreline, which will be a function of your resolution, how smooth and steep your bathymetry is near the shore, your grain size, and a few other things.

Once you have the 3D model enabled, plot the vertical distribution of current speed and direction (onshore/ offshore) for select locations along your beach profile to gain some additional understanding. The same goes for plotting the vertical distribution of current-related band and suspended load, and wave-related bed and suspended load.

Additional practical guidance for 'tweaking' the sediment transport model to give vaguely reasonable results can be found elsewhere on this forum.

Good luck,
auto clicker word unscrambler jumble solver Ben
Thank you very much for your comments. actually i have run the Flow-Wave coupled model in 3d mode with 10 layers ( 2,8,12,12,16,16,12,12,8,2 % of total depth). i also have used the 'use and extend' option for water surface, current(in depth averaged type) and bathymetry. I attached the figure of results for fsus=1, fbed=1,fsusw=0.1,fbedw=0.1.