Modeling an open channel into a piping system.

Question

Modeling an open channel into a piping system.

dhaught1 (Civil/Environmental)

(OP)

30 Aug 12 14:18

I have an area that has seen flooding and I am tying to model the existing conditions. The area is large 100 plus acres and has a number of different culverts, open channels, fields, residential areas and piping systems. I have read the threads along with some videos on line to help. The existing model i have shows flooding during a 2-yr 24 hr event which doesn't happen in real life.

What i have done is the culvert piping under a roadway it treated as a pond with a culvert outlet. The culverts discharge to an open channel for 1000+ feet and goes into a 48"x36" CMP storm drain system. How do I model the open channel. The flooding is concentrated at where the open channel goes int the CMP. I am proposing two detention ponds to slowly release to the CMP. Plese help with the copen channel model.

psmart (Civil) · Answer 1 · 2012-08-30 14:54:35

You've asked specifically about the open channel, but it sounds like the flooding is due to the restricted capacity of the CMP. The channel itself could be modeled as a reach, to allow for travel time, but the portion of the channel that floods at the entrance to the CMP would probably be treated as a pond with a culvert outlet. The "pond" storage would include whatever area is inundated as the WSE rises at the CMP entrance.

Of course, if the CMP is the entrance to a storm sewer system, further modeling of the system may be required to account for tailwater effects.

Peter Smart
HydroCAD Software
www.hydrocad.net

dhaught1 (Civil/Environmental) · Answer 2 · 2012-08-30 15:23:31

The CMP is the beinning of the system catchbasins also flow into the pipe furthur down stream and they surcharge and cause flooding also. I anm not sure if the condition of the pipe where i can't see, but knowing the authority i would assume there is debris in it they are not keen on maintenance.

What would i choose for the reach? (pipe, Rect/Vee/Trap, Parabolic)? In some areas it has vertical sides and windy in others it is sheet flow with grassy bottom. I have used box pipe with a mannings of 0.03. but the numbers do not seem realistic.

I have HydroCAD 10.00

psmart (Civil) · Answer 3 · 2012-08-30 16:08:04

A HydroCAD reach routing is based on a single cross section. If the XS varies, you will have to use a representative XS. Choose the shape option that best fits the channel. You can also define a custom cross-section.

When you say "the numbers do not seam realistic", exactly what results are you referring to? Remember that the reach routing will only account for normal open-channel flow within the channel. It will not reflect tailwater effects due to the CMP restriction, which would need to be modeled as a pond-and-culvert as noted in my previous reply. In other words, the reach routing is intended to model conveyance without tailwater effects. For details see www.hydrocad.net/reach.htm

Peter Smart
HydroCAD Software
www.hydrocad.net

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Modeling an open channel into a piping system.