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

Flow instability at shoreline

JH
Jeff Hansen, modified 8 Years ago.

Flow instability at shoreline

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

I have a model in which I am trying to resolves surfzone dynamics. Despite rather high resolution at the shoreline (~4 m cross-shore by ~8 in alongshore) I am getting an instability in the forcing and flow near the shoreline that then impacts results in much of the rest of the surfzone. During the incoming tide the alongshore pressure gradient in shallow water (<0.25-0.5 m) becomes unrealistically large and fluctuates wildly around 0 moving along the beach. The erratic pressure gradient results in a corresponding unrealistic local acceleration (dv/dt) and often results in the direction of the alongshore flows changing near the shoreline. As there is no physical reason for this to occur (and is also inconsistent with an abundance of observational data from the site) my only conclusion is that the model is having trouble resolving the transition from a dry cell to a wet cell. For what ever reason as the tide drops there is not a problem, only on the rising tide. I suspect the large alongshore pressure gradient is resulting from the stair-stepping of adjacent wet and dry cells, or from very small differences in water level when depths are near the threshold depth (10 cm)? I could increase the threshold depth but given my resolution this seems like a reasonable value.

Has anyone seen this before or have suggestions? I will try a simulation with ~2 m cross-shore cell size to better resolve the shoreline but I suspect this is a numerical issue that may only be alleviated by activating some type of limiter (maybe one is already active when going from a wet to a dry cell as the problem isn't evident during the decreasing tide).

Any suggestions or insights would be appreciated.

Thanks,

Jeff
SH
Shawn Harrison, modified 8 Years ago.

RE: Flow instability at shoreline

Padawan Posts: 70 Join Date: 6/25/12 Recent Posts
Is the instability happening on the first incoming tide? It helps the wetting/drying scheme to start with a high tide to avoid running an incoming tide over dry bathy..

What is your horizontal eddy viscosity?
JH
Jeff Hansen, modified 8 Years ago.

RE: Flow instability at shoreline

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

The instability occurs at every incoming tide, over multi-day simulations, even during the lower high tides (mixed semi-diurnal). I have tried eddy viscosities from 0.1 to 1 m^2/s with the same results. I suppose I could start the model at a high tide or with an increased water level as an initial condition but suspect this will not fix things as the problem occurs repeatedly.

Thanks,

Jeff
SH
Shawn Harrison, modified 8 Years ago.

RE: Flow instability at shoreline

Padawan Posts: 70 Join Date: 6/25/12 Recent Posts
Hi Jeff,
Yeah.. if it happens on successive incoming tides, then starting it on a high tide wouldn't help.. hmm...
WR
Wim Ridderinkhof, modified 8 Years ago.

RE: Flow instability at shoreline

Youngling Posts: 5 Join Date: 1/17/12 Recent Posts
Jeff, what is your time step? considering the high spatial resolution of your grid it should be around 5 sec.
JH
Jeff Hansen, modified 8 Years ago.

RE: Flow instability at shoreline

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

Thanks for the suggestion, my time step is actually 1.5 s. This is required because there is a submarine canyon just outside the surf zone (with depths>100 m), unrealistic flows develop over the canyon if my time step is larger than ~ 2 s.

Actually after further investigation the problem isn't isolated to the shoreline but actually extends outside the surf zone as well. The attached figure shows the alongshore pressure gradient (dP/dy) during an incoming and outgoing tide (contours indicate 0, 5 and 10 m isobaths). The pattern seen during the outgoing is as I would expect given alongshore gradients in wave set up (i.e. realistic). The pattern during the incoming tide is extremely noisy and unrealistic. Interestingly, the noisy pattern is extremely transient and moves around during the incoming tide, as a result the flows outside the surf zone remain small (<10 cm/s). Thus, the flows appear stable in time despite the unrealistic pattern in dP/dy (which is balanced by dv/dt). However, near the shoreline the unrealistic dP/dy can sometimes result in a change in direction of the alongshore flows.

As the focus of my current project is looking into surf zone momentum balances, I need to find and remove the source of this instability that occurs during incoming tides.

Any suggestions are welcome. I have tried increasing the depth limiter and changing "drying and flooding check" as well as a few other options in the "numerical parameters".

Thanks

Jeff
U
Michael Garvey, modified 8 Years ago.

RE: Flow instability at shoreline

Jeff,

This problem seems similar to one I've observed myself. I'm not saying this will fix your problem, but it might be worth consideration. I have found when I have flat regions (groups of adjacent cells with the same bathymetry value) that flood, this generates shock waves from the flooded region. That is, when the model floods a group of cells that have the same bathymetry value, within the one timestep, this seems to generate a wave that travels away from the flooded region.
So, it might be worth checking to see if you have any 'flat' spots in the intertidal zone. The way I've found to get around the issue is to introduce some gradient in the bathymetry in these zones, but I am sure there are other ways around it.

Michael
JH
Jeff Hansen, modified 8 Years ago.

RE: Flow instability at shoreline

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

Thanks for you response. The pattern you describe sounds similar to what I am seeing but mine doesn't seem to be related to bathymetry.

I have done a lot more testing since my last post and have found some interesting (not in a good way) things. I turns out my problem is entirely related to the tide and I have been able to reproduce in a variety of different models forced with differing tidal boundary conditions, including a purely synthetic model with an alongshore uniform beach profile. It appears that as the tide is increasing an instability results at the shoreline and results in offshore propagating waves. I have attached two animations from a simulation in which I wrote output at every computational timestep (1.5 s). First is a animation showing a cross-shore profile of the water level and cross-shore pressure gradient. The video shows the water level smoothly increasing with the incoming tide, then suddenly the water level drops at the shoreline by several cm resulting in an offshore propagating wave. Keep in mind this is from a simulation with no waves only tides. The second animation shows a pcolor image of water level deviation (from mean of all cells at each timestep) in my entire domain (offshore and lateral boundaries shown in black). Initially there is some minor variability that disappears, then a bit later an increasing number of instabilities occur at the shoreline. The "waves" then propagate offshore and alongshore much like edge waves. The cross-shore profile animation shows the development of the very first instability at ~y=350 seen in the pcolor animation.

If anyone has seen this before, or has suggestions as to how these water level oscillations can be damped I would sincerely appreciate it.

Thanks,

Jeff
JH
Jeff Hansen, modified 8 Years ago.

RE: Flow instability at shoreline

Padawan Posts: 29 Join Date: 7/12/11 Recent Posts
I have still have yet to resolve this problem but have better determined what the cause is. As more of the beach face floods during an incoming tide the water level drops at the wet/dry interface by several cm, rapidly floods and results in an offshore directed wave. The timing seems to be dictated by the combination of the beach face slope and incoming water level slope. If the bathymetry is alongshore variable the outgoing "wave" refracts and almost looks like edge waves.

I have tried altering all of the various parameters associated with wetting and drying (depth limiter, how depth is defined in cells) as well as tried a variety of different solvers (cyclic, flood, implicit upwind). I can get the problem to go away in some models just by setting the depth limiter to be very small (~5 mm) but once I turn waves on (in addition to tides) the problem returns.

Is anyone aware of a simple limiter (keyword?) that sets the maximum water level change in any one timestep? It seems like such a limiter would suppress the formation of the outgoing wave and basically resolve my problem.

Any suggestions would be greatly appreciated.

Jeff
BS
Bas Stengs, modified 8 Years ago.

RE: Flow instability at shoreline

Youngling Posts: 2 Join Date: 9/25/12 Recent Posts
He Jeff,

Modelling the interface of the dry/wet regions can sometimes be a difficult task. Your problem sounds familiar to a case I have been working on, only I did not include waves. I simulated the hydrodynamics induced by tides of a nearly uniform coastline. At rising tide, instabilities appeared in the velocity field.
I found a kind of similar solution as you did, mainly by reducing the depth limiter. Furthermore I changed the depth definition of the water level points and velocity points, by means of the Dpsopt and Dpuopt respectively.
In summary, my first simulation gave instabilities in the velocity profile during incoming tide, with the following settings:

Dpsopt = #MEAN#
Dpuopt = #MOR#
Dryflc = 1.0000000e-001

These instabilities disappeared after I changed these settings to:

Dpsopt = #MAX#
Dpuopt = #MIN#
Dryflc = 1.0000000e-004

Changing the Dpuopt from #MOR# to #MIN# should give similar results only the first one is applied in morphological modelling.
In the velocity plots, which I submitted, one can clearly see the difference. The first plot gives locally high velocities (0.5m/s) at the shoreline. The second plot is much smoother and does not show any numerical instabilities.

I hope that these experiences will help you with your model.

Kind regards,
Bas
JH
Jeff Hansen, modified 8 Years ago.

RE: Flow instability at shoreline

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

Thank you for the suggestion. Unfortunately your proposed solution does not work with waves in addition to tides, but does work with tides alone as you found. After looking at your plots it does appear you were seeing the same instability which further reinforces my believe that this is a common problem which ideally would be resolved.

Regards,

Jeff Hanse
BS
Bas Stengs, modified 8 Years ago.

RE: Flow instability at shoreline

Youngling Posts: 2 Join Date: 9/25/12 Recent Posts
He Jeff,

This sounds like a problem which goes behind the solutions that are discussed.
However, you mentioned that you have tried different scenarios with different settings of the 'Advection scheme for momentum' (Cyclic/Waqua/Flood).
Considering your case, applying the Flood scheme would be most suitable. As you can review in the Delft3D-FLOW manual.

All the best.

Bas
JH
Jeff Hansen, modified 8 Years ago.

RE: Flow instability at shoreline

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

I also thought the flood solver would be the solution, despite that it is designed for much more extreme cases than the simple flooding of a beach, but the flood solver did actually not change things much at all so I think the problem is actually independent of the solver.

Thanks

jeff