Attenuating peak flows
Attenuating peak flows
(OP)
hi guys,
I am trying to attenuate the peak flows for the 2, 10, 100 yrs storm to the pre-developement peak discharges.
The pond has a riser structure with 2 orifices at different elevations and also a principle spillway.
I tried modifying the elevations of the orifices but still can't get it below pre-development discharge.
The pond is 8-ft deep already.
Is there any way I can decrease the outlet discharges to the pre-development level?
I am trying to attenuate the peak flows for the 2, 10, 100 yrs storm to the pre-developement peak discharges.
The pond has a riser structure with 2 orifices at different elevations and also a principle spillway.
I tried modifying the elevations of the orifices but still can't get it below pre-development discharge.
The pond is 8-ft deep already.
Is there any way I can decrease the outlet discharges to the pre-development level?





RE: Attenuating peak flows
RE: Attenuating peak flows
or just increasing the size of the pond itself.
The pond does not have a pre-treatment forebay which was not required back then when it was designed. Would adding a forebay (which holds 10% of water quality vol) reduce the peak discharges?
I have attached a pdf of the pond for clarification.
RE: Attenuating peak flows
b. increase the size of the pond.
RE: Attenuating peak flows
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: Attenuating peak flows
I have tried manipulating the size and elevation of the devices on the riser structure.
I can not increase the size of the pond itself due to physical limitation. The pond is big already. But I am thinking about increasing the height of the riser structure by 1-ft. That will make the pond 9-ft deep and 3.448 ac-ft of storage (150000 cu-ft).
I have attached a pdf of the pond here.
RE: Attenuating peak flows
If you're not getting that much attenuation, there may be a problem with your outlet device setup.
You may want to post your HydroCAD project file.
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: Attenuating peak flows
I have posted my hdyrocad project file.
RE: Attenuating peak flows
The project file you posted is empty. It may have been locked (open in HydroCAD) when you attempted to upload.
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: Attenuating peak flows
here are the desired peak discharges:
2-yr = 3.22 cfs
10-yr = 4.45 cfs
100-yr = 15.04 cfs
I have also reposted the project file here.
RE: Attenuating peak flows
I see that the 10-year event comes 0.1' above the weir, which is probably contributing the unwanted flow. You'll probably need to raise the weir and/or provide a bit more storage below that elevation.
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: Attenuating peak flows
The other issue is incorporating a forebay and an emergency spillway to the hydrocad model. I am kinda new to hydrocad and don't know how to model it. Much help is appreciated.
RE: Attenuating peak flows
Sharp-crested is usually sufficient. Broad-crested may give greater accuracy in some cases, but the effect on the hydrograph routing and peak elevation is often insignificant. For details see www.hydrocad.net/weir1.htm
The forebay modeling will depend on the specific design, and whether it has any significant effect on the hydrograph routing. For general modeling guidance please see www.hydrocad.net/howto.htm
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: Attenuating peak flows
Now, according to MD regulation, the forebay is sized to contain 0.1" per impervious acre. Please tell me if I am missing anything.
If I have 4.284 acres of impervious.
0.1*4.284 = 0.4284 ac-in
.4284 ac-in x (43560/12) = 1555.1 cu-ft
so, this means that my forebay should be sized at 1560 cu-ft min. Actual size provided = 1600 cu-ft.
I am not sure what depth I should make the forebay? I can't find any regulation on this. And how do I model this in Hydrocad?
Any suggestion or recommendation would be appreciated.
RE: Attenuating peak flows
For larger events the forebay will presumably be innundated by the main pond. To handle this tailwater effect you will need to use a tailwater-sensitive routing procedure, such as DSI. See www.hydrocad.net/tailwater.htm
Or you could model the foreaby as part of the main pond storage. This is probably OK for the larger events (when the forebay floods-out), but it won't model the retention effects of the forebay for smaller events. In some cases you might use separate models for these two scenarios.
Perhaps others can help you with specific design guidance...
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: Attenuating peak flows
I think I would model both scenarios and compare.
I am hoping that including the forebay would attenuate the peak discharges even further. I am not sure what the effects are.
RE: Attenuating peak flows
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: Attenuating peak flows
what is the recommended percent void for a stormwater pond constructed of earth berms?
Hydrocad talked about stones and gravels, but not earth-berms or compact soils. The pond is a Micropool extended detention pond, and I am assuming that it is a earthen structure.
RE: Attenuating peak flows
In general, if you're not sure about a field, leave the default value as it is. HydroCAD will let you know if any essential information is missing.
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: Attenuating peak flows