Detention Basin Retrofits

The City of St. Peters began a comprehensive citywide stormwater management program (SMP) aimed at preserving natural habitat, flood mitigation, creek bank stabilization, and detention basin retrofits/rehabilitation, all with the goal of reducing property losses and pollution and increasing water quality. The Country Crossing Channel Improvements North & South Detention Basin retrofits are accomplished under the SMP.

The existing dry basins are located in the fully developed residential subdivision known as Country Crossing. The basins operate in series and their purpose is to provide flood protection upstream of the receiving stream. Stormwater enters the North Detention Basin at the upstream end, passes through the South Detention Basin, and discharges to the receiving stream through a single outlet works. The existing outlet works consists of a low flow rectangular orifice in a reinforced concrete standpipe that discharges to the receiving stream via a 66-inch concrete pipe with a flared end. The standpipe functions as an open-top “Morning Glory” for high volume/low-frequency rainfall events. A flat “trash” collection grate is located at the standpipe top. The detention basin was designed and constructed in the early 1990s to meet the regulatory requirements in force at that time. Those requirements provide storage volume to detain runoff from a 25-yr/24-hr duration rainfall event at the pre-development discharge rate. In addition, pond performance is also evaluated for the 10-yr/20-min, the 25-yr/20-min, and the 10-yr/24-hr duration rainfall events. Water quality and channel protection provisions were not required at that time.

Subsequently, a detention facility was constructed directly upstream of the North Detention Basin for the Olhms Farm Subdivision, diminishing the need for, and functionality of, the North & South Detention Basins. Over time the basins became a sediment trap clogged with invasive vegetation species, becoming a nuisance and causing damage to adjacent properties.

The goal of the project is to restore the existing ponds to a free discharging natural forested wetland channel within the existing basin footprint to eliminate the nuisance, mitigate backyard flooding and enhance water quality. Construction elements include pond bottom excavation and re-grading that includes a sinusoidal pilot channel, breaching of the embankment that separated the downstream end of the South basin from the receiving stream that created the pond, removal of the existing outlet works, fabric encapsulated soil lifts, bank protection/biostabilization, grade controls to create step pools and riffles, forest wetland plants/seeding and site restoration. Native species are used for the plantings.

Deliverables included a pre-design study with the development of alternate solutions. The selection of the preferred alternative was based on a constructability review, benefit/cost, and private property owner buy-in. Plans, specifications, and estimates (PS&Es) suitable for bidding and construction were prepared by HR Green. HR Green provided limited construction phase services that included consultation, advice, and occasional construction observation.