Modelling upstream of an innland delta and a lake

[nilscp]

Hi,

My name is Nils and I am from Norway. I am working on a project in Norway, where three rivers (one big and two small rivers) converge to an innland delta (and then to a lake).

I have in the project to model an area that is approximately 5-6 km upstream of the delta in one of the small river. The lake downstream of the delta controls the water level upstream in the studied small river. A complete modelling of the lake+delta+small river will be costful due to required bathymetry (cross sections) for a large area.

My question is certainly trivial but I do not have lot of experiences in modelling. Is it a good way to model the project area? Can I stop the model upstream of the delta/lake even thought the water level will be controlled by the lake?

P.S: steady state will be used in this project.

Second question: A 200-year flood in the modelled area can be caused by the combination of different scenarios (high water level in the lake + 20-year flood in the small river or mean water level in the lake + 100-year flood in the small river and so on…)

What is the best way to deal with such problem?

Thank you again to have create this really good blog and forum.

Best regards

[Chris G.]

Hi Nils-

If the lake is causing a backwater into the river upstream, then you’ll have to know something about that relationship if you want to omit the lake and delta from the model. Normal Depth will not work as a downstream boundary in your case. If you are in steady flow, you’ll need a single water surface elevation for your downstream boundary. If your area of interest in your model is far enough upstream, away from your downstream boundary, you can be quite liberal with how you select your downstream boundary water surface. Any errors associated with that selection will work themselves out as the computations progress upstream (as long as the backwater from the lake does not extend up to your area of interest. Check here for an article written about this: http://hecrasmodel.blogspot.com/2010/01/downstream-boundary-normal-depth.html

For your second question, I don’t see it as a problem at all. Eeach scenario will be a different plan with its own flow file. A single geometry file will be shared by all of the plans.

Best of luck!

[Anonymous]

Thank you for your answer and thank you for your time!

[Author]

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