Balancing Water Surface Elevations

[Tony O.]

I am working on a project where the proposed stormwater management device for quality control is required to be "offline". Essentially there is a manhole structure with a small diameter pipe that is set at a lower elevation than the outfall pipe. This smaller diameter pipe diverts the first flush of runoff to the facility (a submerged gravel wetland (SGW). At the same manhole, there is larger 36" outfall pipe that then directs all flows greater than quality to the outfall. The invert elevation of the 36" pipe is set at the maximum ponding elevation within the SGW 123.29. Once the maximum podning elevation is achieved the flow will then be directed out of the 36" pipe.

The question is regarding the 10yr HGL within the manhole which is approximately elevation 125. Since the flow is unrestricted to the downstream 36" pipe, the thought is that the water surface elevation within the offline SGW would stay at 123.29 and water would flow out the 36" pipe rather than pushing water into the SGW and over topping that facility.

Any thoughts are appreciated. I have attached a plan and profile of the area.


[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1644115212/tips/Profile_bcqzxb.pdf[/url]

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1644115213/tips/Plan_blotgq.pdf[/url]

 

[psmart]

I see no reason why the WSE in the SGW couldn't be as high as the WSE in the manhole. So, yes, the SGW will overtop.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[coloeng]

If the WSE in the manhole is 125, what keeps the water from flowing into the WSG? The embankment for the SGW should be above the highest WSE expected in the manhole, because the WSE in the WSG will be at or just below the WSE in the manhole until the flow subsides and the excess water in the WSG begins to flow back out.

 

[KaBone]

Can you double check my numbers?
Using FlowMaster: 36" pipe, 41.09 cfs, 8.9% slope, n=0.013; i get a normal depth of 11.10" (0.925'), and velocity of 22.18 fps. That would put the HGL at the outlet pipe at 124.2'.

I see the logic of your design. The elevation of the SGW (123.29') would be higher than the 10yr elevation of the pond (120.04'). If the SGW WSE increases it will want to flow into the pond. As the WSE in the manhole increases there will be a preference to go the pond, but i think there will still be flow to the SGW.

I suggest an overflow spillway at the SGW. The 10yr storm is for water quality; there is a 1yr event (by definition 100% chance of occurrence) that will exceed the 10yr event. Right now I don't see a means for controlling the overflow. Don't know what that open area is across the border to France (FRA), maybe there?