I am reviewing a detention pond (Sk

[SMIAH]

I am reviewing a detention pond (Sketch attached).

The designer determined 2 WSE : 1 for the detention pond maximum water level and 1 for the manhole with control structure maximum water level.
Thing is, there's almost 1' between the 2 water levels.
I'm guessing intuitively that there is, between the pond and the manhole, another control(pipe).
This is increasing the volume of the pond of almost 30%.

Is this usual to your opinion? Should the control structure be closer to the pond or the pipe diameter increased to reduce this water difference and decrease the basin volume ?

 

[msquared48]

The only way I can see for the water level in the manhole to be lower than the pond is for orifice to be in between the pond and manhole.

I would expect any orifice to be either in or downstream of the manhole though, and the water level in the manhole and pond to be the same.

Mike McCann, PE, SE (WA)

 

[SMIAH]

What if the pipe between the pond and the manhole is undersized? (this would act as an orifice as you mention).

 

[msquared48]

Yes, but the level should equate with time.

Mike McCann, PE, SE (WA)

 

[mdrehansk]

I think, if the inlet and outlet pipe sizes of the manhole are same then water levels shall not rise in the manhole.
However do note for the provision of overflow pipe connecting to the manholes.

 

[beej67]

I don't see anything wrong with the sketch if the math is correct.

It's quite common for HGLs to drop steadily in structures as you go downstream in a network. The pond is just a big structure. The HGL in the manhole is dictated by inlet control or barrel control of the culvert downstream from it. The HGL in the pond is dictated by inlet or barrel control of the culvert between it and the manhole. There's no reason the HGLs of the pond and manhole should ever be the same. In fact, I think you could make a pretty good case that the only possible time they'd be the same is if the discharge location of the whole system was flooded and there was no flow between any structure in the sketch.

Now, whether the guy did the math right is a different issue. There's no way to tell that one way or another from the sketch and info given.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[SMIAH]

Thanks beej67.

This was done with PC-SWMM so the math seems to be alright.
I just don't think this is "optimal" as the water level in the pond is "too high" for no reason (increasing the overall volume).

 

[HydrologyStudio]

BeeJ67 is correct. What you're seeing is simply the energy loss (entrance, exit and friction) between the Manhole and the Pond.


Terry Stringer
Better Hydraulics & Hydrology Software

http://www.HydrologyStudio.com

 

[cvg]

I just don't think this is "optimal" as the water level in the pond is "too high" for no reason (increasing the overall volume).

you have an inlet loss, trash rack loss and friction loss. 1 foot of head loss does not seem unreasonable. unless you are reviewing for the owner, why would you complain about a pond with too much volume?

 

[msquared48]

That depends if the water is flowing or static. They should be the same level if static.

Mike McCann, PE, SE (WA)

 

[cvg]

Mike
of course, however there is no way for the water to be "static" without closing a valve or a gate. I don't see any indication of valves. Detention ponds rarely have gates installed, especially at the downstream end of the pipe.

 

[SMIAH]

@cvg : I am reviewing for the owner.

 

[HydrologyStudio]

Here is a profile of that same scenario I ran in Stormwater Studio. Of course there is flow exiting the pond but it illustrates how quickly energy losses can add up. The pipe exiting the pond is under HDS-5 Inlet Control.

Hope this helps.


Terry Stringer
Hydraulics & Hydrology Software

http://www.HydrologyStudio.com

 

[cvg]

so if you want to optimize the basin size, then you would have to reduce the head losses in the outlet works. that means larger trash rack, larger pipes. that may or may not reduce (optimize) the cost. also, don't forget that your basin will tend to silt up over time. that will reduce the storage volume in the pond.

 

[msquared48]

cvg:

Yes, no gates, but they do have orifices and overflows.

Also, the orifice control MH is usually very close to the detention pond with a minimum of 12" diameter outflow line.

Unless the flow is very high, to me, the head loss would not be that great, especially with the low flows I've encountered in small residential and small commercial developments - under 1 cfs even at 100 year floods.

Could be the case for larger scenarios though, I will admit. I guess I would have to see it for myself...

Mike McCann, PE, SE (WA)

 

[cvg]

well, without any sort of details provided in the OP, just wild speculation I guess. In my neck of the woods, detention basins do not have 1 cfs bleed off lines, only some retention basins do. They also do not allow 12 inch pipes in most jurisdictions for any sort of storm drain.
I wonder why the designer did not document all of this in a design report?