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Ponding behind culvert

Ponding behind culvert

Ponding behind culvert

(OP)
When using custom pond stage data to model a culvert crossing, is it necessary to enter an elevation higher than the top of the road to define overtopping?  For example, if the elevation of the center of the road is 103.5, is it necessary to enter a value of say, 103.6, (accompanied by a very large surface area to simulate an infinite amount of storage now that the water level has over-topped the road)?  

RE: Ponding behind culvert

In general, you need to define enough storage to cover the actual elevation range that occurs during the routing.  The software will issue a warning if you exceed the defined storage.

In your case, you would probably model the roadway as an overflow weir.  For accurate routing, you must define the storage to some point above the crest.  This storage would NOT be infinite - it would be actual amount of storage controlled by the outlet devices.  This would not include any storage downstream of the weir, since this is not part of the level pool and is not controlled by the weir.

When calcuating storage based on surface are, you would continue to enter the area at each contour line.  As you excede the weir crest, use an imaginary vertical wall above the crest when calculating the area.  Remember, your goal is to define the storage inside the pond that is subject to control by the outlet device(s).

RE: Ponding behind culvert

(OP)
Thank you, psmart!

RE: Ponding behind culvert

(OP)
I have a culvert that crosses a road on a slope, not at a low point in the road.  Once the storage behind the culvert fills, any overflow will continue down the roadside ditch to another culvert.  What's the best way to model this scenario?  Right now I am using a very small broad-crested weir, which is as wide as the roadside ditch.  I have allowed a nominal breadth of 2', to represent the edge of the storage area behind the culvert.  I have a secondary routing from the culvert to this weir.  The overflow from the weir is routed through the roadside ditch, represented as a reach, to the next culvert.  I've run into this situation a few times, and never knew quite how to handle it.

RE: Ponding behind culvert

If the storage associated with the two culverts is part of the same level pool, just use a second culvert outlet routed to secondary.

But if the water levels are different, you'll need two ponds:  The first pond will have a primary culvert outlet, plus a secondary weir for the flow along the road.  The second pond will have its own a culvert outlet going under the road.

In any case, I would not use a reach.  For details please see www.hydrocad.net/reach.htm

RE: Ponding behind culvert

(OP)
I'm not sure I follow what you mean by using a second culvert outlet if the storage is the same pool for both culverts...

Thank you for your help once again!  

RE: Ponding behind culvert

I the water surface elevation is the same behind both culverts, they can be treated as separate outlets on the same "pond".  The outlet setup would be:

Device#1 = Culvert, Routing = Primary
Device#2 = Culvert, Routing = Secondary

For details on outlet configuration please see www.hydrocad.net/pond1.htm

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