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

Instability in model

Erin King, modified 3 Years ago.

Instability in model

Youngling Posts: 12 Join Date: 10/5/16 Recent Posts
I am running a flow model and I am getting what look like instabilities in the velocity/ water levels at certain points. The points seem to be distributed throughout the domain some in regions with higher flow velocities which made me think this was an issue with higher velocities causing instability. I have attached two quickplot figures showing the water level and the velocity components for one point approximately 8km offshore in the domain where I am getting this problem.

In the figures there are results from two simulations, one with a 10 minute timestep and one with a 1 minute timestep.

The model is a 356 x 241 km rectangular domain of 1km grid resolution. There are four open boundaries with water level time series forcing and one open boundary with a discharge time series. Salinity is also being used.

Any advice would be very much appreciated.

Erin
Qinghua Ye, modified 3 Years ago.

RE: Instability in model

Jedi Council Member Posts: 612 Join Date: 3/2/11 Recent Posts
Hi Erin,

Happy new year!

From the 2 figures you attached, the general trend looks fine. The strong oscillation always appears during the turning period, which might imply it might come from boundary conditions. Can you also show the flow field pattern at the turning moment?

It is not so easy to find a very stable boundary conditions set with 4 open boundaries, as you may noticed, which might need a bit more concern.

Greetings,

Qinghua
Erin King, modified 3 Years ago.

RE: Instability in model

Youngling Posts: 12 Join Date: 10/5/16 Recent Posts
Thanks for the reply Qinghua. Happy New Year!

I found that increasing the reflection parameter alfa to very high values got rid of the rest of the oscillations for the most part, although not entirely. I wondered if it may be an issue from currents intersecting the boundary at an angle?

Changing the reflection parameter introduced a new issue which is the phase of the model. It became up to 30 minutes out of phase in the water levels with respect to the tide gauges when alfa was increased to 10000. The model is ahead of the tide gauge. This issue gets less as you go east, which is the direction of the tidal wave propagation in the domain.

Changing the roughness had a small effect on the phase but affected the amplitude of the wave at a certain point. I am now looking at possible adjustments to the bathymetry as the wave speed is affected by the depth. Am I on the right lines here?

I have recently looked at OpenDA, and wonder if this sort of calibration of the model would be better done using that package. Is OpenDA suitable for calibrating coastal/small regional models?

Many thanks,
Erin