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

unrealistic rip-channel morphology

JH
Jeff Hansen, modified 8 Years ago.

unrealistic rip-channel morphology

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

I am trying to reproduce observed nearshore morphologic changes over a week long period in a 2DH model (resolution ~5 m). I am using the van Rijn 2004 formulations and the model is doing a good job at reproducing the larger scale patterns but is predicting a lot of unrealistically deep rip channels. I have tried running the model with a variety of diffusivities (1-10 m^2/s), viscosities (0.1-1 m^2/s) and sediment parameters (bedw, susw, factor for erosion of adjacent dry cells) and can’t seem to get rid of the rip channels (which get progressively deeper because of feedback with flow). Any suggestions to smooth these out? Attached is a plot showing the modeled (left) and observed (right) bathy change as well as my mor and sed files. Also below are the keywords I am using with the van Riijn 2004 formulations.

Thanks,

Jeff


Commnt =
Roller = #Y#
Gamdis = -1.0000000e+000
Betaro = 5.0000000e-002
CstBnd = #Y#
MomTrm = #YES#
SMVelo = #GLM#
TraFrm = #vrijn04.frm#
BdfRpC = 0.5
BdfRpR = 0.5
BdfMrC = 0.5
BdfMrR = 0.5
BdfDnC = 0
BdfDnR = 0
SdfD50 = 0.000225
Commnt =
GR
Gabriel Ruiz, modified 8 Years ago.

RE: unrealistic rip-channel morphology

Padawan Posts: 59 Join Date: 9/30/11 Recent Posts
Hi Jeff,

Recently, I did a couple of tests in order to know which formulation would be the better for my morphodynamic simulation. I found that if I use Van Rijn's formulation the results are unrealistic, an experience similar as you. I have get a kind of mega ripples at the surf zone, it is unreal for my case. For this reason, I tried with Bijker's formulation and I also varied the Nikuradse values spatially. With these changes the morphodynamic simulations, the results seem more real in the region that I'm studying. Maybe, you should try with Bijker's formulation.
I'm using a nested grid of 30 m (overall grid) and 3 m (detail grid).

Regards.

Gabriel
Ben Williams, modified 8 Years ago.

RE: unrealistic rip-channel morphology

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

I have a couple of suggestions that may or may not work for you.

1) I have found that the Transpor2004 algorithm is particualy prone to feedback the moment any kind of bed slope is imposed. I have found that the wave-related bedload vectors are mostly repsonsible, as unless you have quite strong currents, the wave related bedload dominates and is very sensitive to fsusw in particular. This controls wave assymetry effects on bedload vectors. Van Rijn 2007 suggests values between 0.5 and 1.5, but I have in the past found that I can get more realistic patterns if i set this to 0 or some tiny number (0.1), depending on sediment size, bed slope, wave energy etc. In general, I have found that under purely wave forcing, suspended load is generally directed along and offshore whilst and bedload is strongly directed onshore. Therefore you can try and run a simple test model (say a 1d cross shore profile or a small section of your overall model) and try to balance these relatve forces out. This will minimise the tendancy for feedback to occur.

2) Another option is to try to scale down your over all tranasport by using fsus and fbed. Again, I have found TRANPOR2004 tends to greatly overestimate transport using default settings so try values of around 0.1 or 0.2. Obviously these comments are quite site specific and depend on the relative contrubution of waves vs currents in your sediemnt transport.

3) Maybe you need to investigate your processes in a bit more detail. Have you tried plotting maps of Hm0 and direction, wave dissipation, shear stress (magnitude), current vectors, suspended load, bed load and total transport for calm, mean and storm wave conditions? This can help with fine-tuning the suggestions in 1).

4) Altering eddy diffusivity and viscosity will help to smooth out the current stucture and the suspended load contribution, but this will help only if suspnded load is the dominant contributor to the total load.

5) You can try the Bjiker formula but you need to provide a lot more calibration parameters (which require a lot of sensitivity tests). Having said that, it is a popular transport algorithm for the coastal zone. Id be interested to see if, using published values for the calibration parameters, you get a better morphological result. Theoreticaly the TRANSPOR2004 algorithm has a better basis in physics and you can investigate it in more detail, but I am also a big believer in simplicity. An advandage of using Bjiker is that you can scale the wave-related bedload - but this is in effect similar to scaling fsusw and fbedw in TRANSPOR2004.

6) Another fudge, but try running with a multitude of grain sizes coarse and finer than your present d50, turn book keeping turned on and set morfac to 0. Then at the end of the week, see what the sediment distribution looks like. It may be that a uniform sediment size is a bit overly simple and you'd expect a slight variation over your domain. It would be tempting in this case to apply that sediemnt map as a starting condition to your morphological simulation.

Cheers,

Ben
Ben Williams, modified 8 Years ago.

RE: unrealistic rip-channel morphology

Jedi Knight Posts: 114 Join Date: 3/23/11 Recent Posts
Hi again Jeff - I just realised taht you and I have discussed these issues before and so my comments will probably be of little use to you. I have however left them on the forum in case they serve as reasonable startng suggestsions for anyone else doing morphological simulations.

Cheers,

Ben