## Monday, December 29, 2008

### Modeling a spillway and culvert combination

Written by Chris Goodell, P.E., D. WRE | WEST Consultants

What's the best way to model a culvert with a flow controlled spillway stuck on the front like this for a dam breach simulation?
You could try putting a cross section just upstream of the bridge/culvert to include the spillway crest (as part of the cross section station elevation points). You’ll have to “straighten out” the spillway, but just make sure the overall length is correct. This will only work if the culvert (in the picture) is the only flow passage through this structure (i.e. it’s in the main channel), and the spillway obstructing can be modeled across the entire cross section. This may be difficult to stabilize for an unsteady flow model so here are a couple of other options to try out if needed.

For a dam breach model, you’re probably looking at conditions where the water surface is cresting the road, most likely submerging the spillway. In this case (and assuming you’ll be breaching some other part of the embankment), I would treat the spillway and culvert as one structure-an inline structure in RAS. Model the culvert as a gate. You’ll have to either use a rectangular shape to approximate the culvert, or come up with a user-defined rating curve. The rating curve could be easily determined by setting up a different geometry file with this crossing modeled as a cmp pipe-arch culvert. Then run a number of different steady flows through it. This will give you a good rating curve. Then import it as a use-defined rating curve for your gate. Perhaps you increase the entrance loss coefficient slightly to account for energy loss as flow lines contract and expand over the spillway structure before entering the culvert.

Under normal flow conditions, I would set up two geometries-one with just the culvert, and one with just the spillway. Then look at the conditions you are most concerned with and pick the geometry that controls the flow at the structure (whichever geometry produces a higher headwater elevation for a given flow amount). If you really want to see a full unsteady flow simulation (where control is switching from spillway to culvert, and back to spillway, then you’ll have to break the model up into multiple simulations, with the end of the previous simulation used as the initial conditions for the next one (using a restart file). You’ll want to split the simulations up where control switches from spillway to culvert. This will take some trial and error work.

1. RAS only has pipe arch culvert property tables for corrugated metal. How would you model an rcpa culvert? When I enter the rise for an rcpa culvert, the corresponding span is off. Should I just use the size associated with the equivalent cmpa?

2. If the spans allowed in RAS are not consistent with the true spans of a particular culvert type, I would get the rise correct (this is more important, because it affects when hydraulic transitions take place), and adjust the Discharge coefficient to account for the different flow area. For example, if the span that RAS allows you to select is 2 meters (with a computed flow area of 1.6 m^2), but your culvert is really 2.1 meters wide (with a computed flow area of 1.8 m^2), then go with the 2 meter span, and adjust your coefficient up by a factor of the ratio between resulting flow areas (1.8/1.6 = 1.13). So if your true discharge coefficient is 0.6, then for the simulation, use a discharge coefficient of 0.68. Of course, if you're running an unsteady flow model, or multiple profiles in a steady flow model, you may have a different discharge coefficient for each profile (unless you are using a rectangular culvert). It's up to you which coefficient you should choose.

3. How to model a gate in front of a circular culvert.

Did it with a sluice gate in front of a culvert but I don't get the same resultats by hand calculations.

It's the effective area under the gate I think as it flows through a circular culvert. The end section of the gate isn't circular.

Should I do it separately as you mention?

4. Keep in mind that if you use a gate in RAS, it will use the gate equations, not the culvert equations, and visa versa. Please read up on both computational procedures in the hydraulic reference manual, so you can make sure you are reproducing what RAS does, when you do your hand calculations. Personally, I would try the same technique (model separately and see which one controls) if it's a steady flow model.