Model Dam with Riser in HydroCAD?

[ktw99]

I am trying to do a dam failure analysis for a small impoundment that has an earthen dam with a riser and conduit. The structure has a 48" inlet pipe, one weir in the riser about 10' above the pipe invert, and a 42" pipe outlet from the riser structure at the same invert as the inlet. I read in a previous post that this cannot be modeled in HEC-RAS but can be modeled in HydroCAD and then put into HEC-RAS to model the rest of the site downstream. How do I model this riser structure in HydroCAD? My only idea right now is to model a 48" culvert going to a "pond" (the riser) with a 42" outlet and a weir designated as Device 1.

 

[psmart]

Although you could model the riser as a separate "pond", it's somewhat easier to model everything as a compound outlet on the reservoir (using just one pond). Like any compound outlet, start with the final device and work upstream into the pond:

Device#1 = 42" culvert, Routing=Primary
Device#2 = Weir, Routing=Device#1
Device#3 = 48" culvert, Routing=Device#1

For illustrated examples please see

http://www.hydrocad.net/pond1.htm

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[ktw99]

Thank you so much for your quick response, Peter! I tried modeling it two different ways and got very different results. The way you suggested worked well. I modeled the lake as a pond with two primary outlets (the earthen embankment and the outlet of the riser), a 6' long sharp-crested weir for the riser (the riser is a 6' diameter CMP with a weir inside) as Device#2, and the inlet to the riser as Device#3.

Here is the other way I tried modeling it. I modeled the lake as one node with two outlets: a weir (the earthen embankment) and the inlet pipe to the riser. That node goes to another node for the riser as a "pond" and entered the volume of the riser as the "pond" volume. The outlets for that node are a sharp-crested weir (Device#1) and the riser outlet pipe (Primary). The result is much different and looks really bad.

What is the difference between these two approaches? Why are the results so different? Would it be more accurate to model it using the second approach I described here because then I can enter a volume for the riser?

 

[psmart]

I may have misunderstood the nature of your "weir". I took it to be the top of the riser, when it may actually be a baffle inside the riser?

For the later case you could use:

Device#1 = 42" outlet culvert, Routing=Primary
Device#2 = Weir/baffle, Routing=Device#1
Device#3 = 48" inlet culvert, Routing=Device#2 (CHANGED routing)
Device#4 = Orifice/riser-top, Routing=Device#1 (NEW device)

This will be analogous to your double-pond scenario. As for the riser volume, the storage is too small to effect the routing, so I would use the zero-storage option for better stability. Of course, the two-pond solution will also require a tailwater-sensitive routing procedure in order for the riser "pond" to influence (reduce) the reservoir discharge.


Peter Smart
HydroCAD Software

http://www.hydrocad.net