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Converting Channel into Detention Pond (help) please :-)

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gonygonygo

Civil/Environmental
Jan 14, 2011
2
Hello all Engineers,

I am a new member to this site and occasionally browse the forum topics when I have a question. However, I have been unable to find any topics specific to my question this time around.

Here's a little background about myself... I am a 5-year EIT (taking the PE in April '11)that has moderate design experience modeling storm water flow.

Here's my question:
I have been tasked with designing a "flow-control" structure for a channel on the apron of a headwall directly in front of (2) exisitng pipe arch culverts. The culverts are located at a dirt road crossing of the channel. The discharge from the culverts is flooding the area downstream. I do not have any specific goal (velocity reduction, design storm, etc.); just to see what I CAN come up with and not flood the upstream area in front of culverts. BTW, there are no grading or disturbing activities allowed with this project due to permit restrictions.

Once designed, the upstream end of the channel before the culverts will serve as a "detention pond". The design is to not change base flows for small storm events, but to restrict discharge for large storm events.

My "flow control" will be one of the two options:
1) Add a restricting plate (reducing x-sectional area) at the inlet of the existing culverts...
2) Add a riser structure in front of the existing culverts...

I am planning to use Hydraflow Hydrographs, but I have never used it before to work with Pipe-Arch culverts as my detention pond outlet. I have only worked with circular or rectangular pipe culverts. I am proficient with all other areas of setting up my hydrograph(s), reach(es), and pond.

Is there a way around this? Is there some way I can incorporate a culvert analysis using Hydraflow Express (which DOES model pipe arch culverts) into Hydrographs to perform my routing and (stage-storage) elevations?

I appreciate any input or suggestions on performing this task! Thank You in advance!!
 
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If you make your upstream control such that it's conveyance is much less than the arch culverts, then I wouldn't imagine you'd have to model the culverts in your hydraflow model. Set your hydraflow model up with whatever weir and orifice characteristics you decide, then do a separate capacity analysis on the arch culverts to accept the peak discharges from the control.

My opinion, anyway. I haven't used hydraflow hydrographs in a decade.


Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -
 
Thanks for your comment. Sorry for my delayed response. I was pulled away from this for a while.

The reason I am modeling the watershed through the dual arch culverts is to have existing flows to compare with those generated once my upstream control is in place.

Existing condition:
I have already modeled this watershed through my "existing reservoir" (ditch with dual-culvert outlet). For the pond outlet, I used the same width/height dimensions of the arch culverts, except made them rectangular in shape. I assume, I can multiply the flows by some factor to adjust for shape. The only other option allows for ciruclar culverts for the existing condition.

Proposed condition:
I will use a copy of the same model for the existing condition, and the detention pond will have the same storage capacity. The only difference will be the outlet structure with other controls upstream the culverts.

I am now going through tutorials for EPA SWMM 5.0 to repeat the entire process and check for accuracy. I believe my final outlet can be modeled as dual arch pipes using this program.
 
how about option 3, overflow weir with a notch for low flows
 
I agree with CVG for this application (low flow notches, major weir). Just need to be sure to check possible backwater effects from the culverts, which sounds like you are doing, I think.
 
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