stormwater retention facility

[cvg]

I am designing a stormwater retention facility. All indications are that the city allows underground detention. They also allow dewatering by dry well.
Due to site constraints, I am consideing recommending placement of underground storage pipes under an existing surface retention basin. We would directly tie the bottom of the surface basin to the underground pipes so that the pipes would fill at the same time or slightly before the surface basin starts to get any significant depth. Does anyone see any major issues with this? All I can see is that at worst case, the underground pipes may settle more than usual due to saturation of surface basin. However, when the underground pipes are under an existing surface basin, the only property damage due to settling will be in landscaping (which is a good thing considering the other option is to place underground pipes under the streets).

 

[gbam]

CVG,
The only thing I can think of would be sediment in the pipe network. If the run-off is clear then your concept should be ok. If the run-off is sediment laden then you may have a clean-out problem. Is there a sed trap in the surface basin?

I am surprised to hear that drywells are still an option. We typically have a 24 or 72 hour draining constraint so that mesquitos cannot breed. Please let us know if your concept is accepted.

 

[Xbxaddict]

How are the connections from the bottom of the basin to the pipe network made? Settlement of the filled pipes on saturated soil may cause additional stresses on the connections.

 

[tpetzold]

If you have the room for underground storage in the basin, why not just dig the basin deeper? Seems this would be less costly and easier to maintain. Here in NJ we now have the statewide BMP that we must follow and the pretreatment facilities are really expensive.

Hope this helps.