Stormwater Maintenance Design Mistakes – Underground Detention

Being responsible for the care and feeding (ie, inspection/maintenance) of well over 500 existing stormwater management facilities in 6 states, we do see quite a few designs that did not appropriately consider maintenance.  What follows is an example we recently observed.

This is a very large underground detention facility that is a concrete deck constructed over a 250′ x 125′ concrete vault.

Surface slab of underground detention facility

Concrete deck from below

The 250' x 125' vault

With dimensions that large, one would hope the designers would have paid close attention to maintenance.  Especially considering the enormous cost of construction for such a structure.  Unfortunately, the ball was dropped on one of the most important elements of this facility – the control structure.

Control structures are located on the downstream end of underground detention facilities.  The control structure typically has a weir wall with an orifice and overflow to constrict flows, causing water to inundate the storage area.  Usually being small, the orifice is also the point where sediment and debris will accumulate.  This accumulation will tend to cause clogging if not removed.  This removal is usually with a vac truck, which must access the area being cleaned directly from above, as shown in the following photo:

Vac truck requires vertical access over structure being cleaned.

Unfortunately, the designers of this particular control structure located the vertical access directly above the downstream side of the weir wall – and provided very little access to the upstream side.

View from only access - from the DOWNSTREAM side of the weir wall

View into the upstream side over the weir wall - note there is no vertical access on the other side of the weir wall

To access the upstream side of the weir wall, it would take a very small person outfitted in confined space entry gear to squeeze through the weir wall opening – not a practical solution.  Alternatively, manhole access to the larger detention “room” on the upstream side is provided – but 75 feet away from the weir wall.  Such access will do little good when the low flow orifice is clogged and the structure is full to the weir with water… and useless for the vac truck to perform routine orifice maintenance.

Conclusion:  there is virtually no access provided to the exact point that needs it the most in this million dollar marvel of underground detention stormwater engineering, rendering it a difficult maintenance problem for its entire lifespan.

Not exactly value engineering.  Definitely not sustainable.


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