Attenuating peak flows

[kenn100]

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?

 

[gbam]

Have you tried increasing your storage volume?

 

[kenn100]

Do you mean increase the height of the riser structure?
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.

 

[TerryScan]

a. reduce the size of the orifice.
b. increase the size of the pond.

 

[psmart]

Since you're using HydroCAD, be sure you're modeling the riser correctly. For details see

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

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[kenn100]

I am using hydrocad and modeled the devices correctly.
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.

 

[psmart]

Are your results consistent with the peak reduction that is predicted by the pond-sizing report? This report will typically give a best-case reduction that can be obtained with an optimum design. For details see

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

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

http://www.hydrocad.net

 

[kenn100]

The results are not really consistant with the estimate from the pondsizing report. I am not sure if I understand how to use the pondsizing report following the link. it tells you the ac-ft you need to get the desired peak discharge. Is this the size you have to adjust the pond physically to get the desired peak discharge?

I have posted my hdyrocad project file.

 

[psmart]

The sizing report shows the amount of water you need to detain to achieve a specific reduction in the peak. This means you need to have this much storage available, AND you need to restrict the flow in order to fill (detain) this entire volume at the time of peak inflow.

The project file you posted is empty. It may have been locked (open in HydroCAD) when you attempted to upload.



Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[kenn100]

I was able to get the 2 yr & 100 yr storm peak discharges below the pre-developement discharge. But NOT the 10-yr storm discharge.

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.

 

[psmart]

You seem to be within 10%, so you're on the right track.

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

http://www.hydrocad.net

 

[kenn100]

I was able to get the 10-yr storm peak discharges below the required discharges by just reducing the size of the low-flow orifice from 9" to 8" and reducing the elevation.

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.

 

[psmart]

You can use a weir for the spillway, routed to secondary if you need independent routing. This will also give a separate report of primary vs. secondary 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

http://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

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

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[kenn100]

The forebay is part of the pond. its a cell within the pond itself and shares same elevation as the pool, just a berm separating the forebay and the pond pool if you will.

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.

 

[psmart]

You can model the forebay as a separate pond. The outlet control will be the berm, modeled as a weir.

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

http://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

http://www.hydrocad.net

 

[kenn100]

Thanks psmart.

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.

 

[psmart]

The forebay may help with the smaller events, but since it only holds 0.1" of runoff it's not going to have much effect (if any) as the runoff goes over an inch for your typical events.



Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[kenn100]

Hydrocad wants me to enter information about the overall percent void of my pond.
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.

 

[psmart]

The voids apply to the storage space in the pond - not the berm. They are used primarily for underground storage with stone backfill. For normal open storage, leave the voids at 100%. For details please click the help button on any storage screen and follow the links.

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

http://www.hydrocad.net

 

[kenn100]

Thanks guys. I will make the changes and hope it works out well.