Spillway Analysis

[AJ]

I am analyzing water surface elevations within a spillway, based on inflow, to determine the spillway capacity just prior to the dam overtopping. The spillway has experienced significant changes (mostly erosion) since it was built and no longer resembles what is shown in the design drawings. I am using a storage area (volume-elevation) connected to a 2D area via a SA/2D Area connection. The lidar data and survey data show that the first ~60 ft of the spillway has an adverse slope (see image below). When the model runs, it eventually becomes unstable and crashes. Using the Boundary Condition Volume Check option it appears that the source of the instability is at the SA/2D Area connection (1D/2D flow error increases from 10 cfs to 3400 cfs withing 5 hours of simulation time, time step is 2 sec).

To solve the problem, I created an artificial weir at the location of the connection (station 0 in the image below). I minimized the weir to 2 inches in height and 6 inches in width (might be able to reduce the width more but haven’t tried yet). My thinking is that the presence of the low weir will result in a slightly lower spillway capacity than if there were no weir their, making the model slightly conservative.

Is this an acceptable work around? Or does anyone have any advice on how to handle this?

I considered placing the SA/2D Area connection at the high point in the spillway (~ station 60 in image below) but this point is just downstream of the dam and am not sure how it would affect the over topping of the dam. Thought I would get some other opinions before I put more work into it. Any advice would be greatly appreciated.

[Chad Ballard]

Have you tried forcing a WSE at the upstream? Do you need to have the SA/2D connection?

[cameron]

Have you thought of modeling the spillway in 2D? I have modeled spillway capacity before using different 2D models, but HEC-RAS should be able to do it as well if you make the grid cells small enough.

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