Horizontal/Subsurface Infrastructure & Transportation

We are currently designing a single family development with a connected detention basin system where the individual storm outfalls are all submerged (below static water surface elevation). One of the sections currently under design is experiencing an underground utility conflict that can be resolved by having several pipe lengths upstream of the outfall submerged. Personally, this is something I have been taught to avoid but I have no actual basis (practice or reference) for why or why not this situation should be avoided.

I was hoping for some insight as to whether anyone has experienced issues with this type of situation. Obviously, this results in greater potential for sediment in the system but any thoughts other than that would be appreciated.

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Garret Duhon,P.E.
Houston, TX
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Hi Garret,

Detention basins as a source of BMPs are typically used for the following two purposes: a) to temporarily detain stormwater by slowing down the release of captured runoff at a designed rate and b) to act as a slow groundwater recharge facility assuming there is a moderate infiltration rate available based on the existing soils. The amount of hydrological water detained in the detention basin is a function of the soil runoff coefficient, tributary drainage area and rainfall intensity of the region. Depending on the depth of the detention basin, all underground infrastructure is designed around the basins. Underground infrastructure utility conflicts such as sanitary sewer mainlines crossing potable water supply lines and storm drains are mitigated by utilizing the local Department of Public Health guidelines for placement of such utilities.  Any utility line below the detention basin and flowing under gravity may be designed with siphon parameters and with local pumping design methods for safe crossing through conflict zones. Sediment accumulation deposited in the bottom of the basin may be cleaned up after the temporary storm to minimize future sediment transfer through the outlet pipes of the detention basin. Hope this helps.

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Spiridon (Spiros) Lazaris, PE
MSCE, MPA, mASCE, ENV-SP

Spiros Lazaris Engineered Solutions, LLC
www.congorevival.com
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Garret,
I believe that the heart of the question was "Why do we avoid submerged pipes".  As a Public Works official of a locality with some of these submerged pipes, I can attest that there is greater sedimentation both within the pipe, and at the outfall. As a result, vegetation and debris buildup occurs at the outfall, causing additional flow restrictions and upstream performance issues during storm events. Lastly, maintenance is far costlier, as we need to use bladders to seal the pipe, dewater and then maintain the submerged section.

I hope that helps.

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LJ Hansen, P.E.
Acting Director
Dept. of Public Works
Suffolk VA
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In California, we are required to intercept and infiltrate storm water as part of the Water Quality BMP.
When it comes to tracts, we have choice of taking up a lot(s) ($) or placing a BMP in each lot.

An underground system is an "inspection concern" and a "maintenance headache".   At least one City has changed their policy, to no more individual underground BMPs in single family lots---there needs to be an HOA created for maintenance of the BMP in a central location, etc.

Several cities have required only above ground BMPs  or (Basins, surface bmps).

What ever you decide, whether for storm water or water quality, keep it simple and there should be fewer complications.

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Wendell Iwatsuru
Madole & Associates, Inc.
Rancho Cucamonga CA
(909)230-8798
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