2D flow area connection to storage reservoir

[RasL DazL]

I have to construct a dam breach model of an offline reservoir. For starters, I have constructed a simple model that is entirely 2 dimensional and does not contain the reservoir. I realize that combinations of different 1d/2d geometries may be more appropriate, but I am just trying to get myself up to speed asap. I watched a tutorial in which the presenter showed how to perform a 2d simulation for what I am assuming was a “natural” watershed:

I intend to calibrate my model to a historic flood where flood extents were mapped, however there was no reservoir breach. The observed upstream(blue) and downstream(red) hydrographs are shown in the plot below. The hydrograph data were available at 15 minute intervals.

Following the tutorial I used
these settings:
48 hour spin up
Diffusion Wave for the 2d flow area
Upstream BC: flow hydrograph, EG slope = 0.05
Downstream BC: Normal depth, friction slope = 0.01
100 foot cell size

I wanted to use the 15 minute flow data, but when I tried to use it as input, the time duration in the input dialogue didn’t have enough rows (i.e. not enough days to cover the event). I ended up converting to hourly and running at different computational intervals. I found that 10 second interval produced the most realistic result.

Questions:

1-Is it possible to extend the number of days in the simulation such that I can use the 15 minute data?

2-The result roughly corresponded to the mapped flood extent., but if I didn’t specify Manning’s N values for the cells, how was the model able to run?

3-Is there a post explaining what the solver is doing with the normal depth assumption at downstream BC?

4-At some point I may need to specify inflow and release from the reservoir during the calibration. Is it possible to specify flow from the 2d area into the reservoir and releases from the reservoir to the 2d flow area?

Your help is greatly appreciated.

Thanks,
Chris

[Anonymous]

1. You can change the number of rows. Click on the No. Ordinates button on the flow editor.

2. When you create the 2D area, there is a default Manning’s n value. It looks like it is set to 0.06. You can change this before creating the 2D area or use the Land Cover to specify n values.

3. I don’t know. There is a hydraulic reference manual for RAS 2D, so you could look at that. Normal depth is a fairly straight forward concept, I would think.

4. If you have good terrain, doing everything in 2D might be easiest. Use a storage area connector to represent the dam and the dam breach. At the upstream end you can attach a flow BC line to have inflow into the reservoir. The SA connector can have a spillway and gates for releases.

Diffusion wave is easiest when creating a model. But you will want to switch to full momentum for the actual dam break.

You might want to look at the sample 2D data sets that come with RAS. The Bald Eagle data set show a dam break and it shows how you can do it either as a single 2D area or two 2D areas.

[RasL DazL]

Thanks for your help.

[Author]

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