Storm Water Management

PETER GISOLFI ASSOCIATES

Storm Water Management of New Darien Library

The issue of storm water management for the site was of paramount concern to the Library Board and the design team during the design stage for the new Darien Library site. Everyone involved in the design process was very familiar with the pre-existing storm water conditions for Boston Post Road and the surrounding neighborhood. The proposed drainage design has been prepared to minimize the release of storm water into the municipal and state storm water systems and has been designed with the capacity to retain the majority of the onsite storm water runoff generated by rain events for a one-hundred year storm. The design will not contribute to the storm water management problems that have been experienced along Boston Post Road as less storm water will be released into the existing storm water drainage system in the post development condition as compared to the predevelopment condition. The Net Site Runoff Rates for the Proposed Conditions will be 6.97 cubic feet per second less than the predevelopment condition for a 50 year storm event. Approximately 90% of the storm water runoff will be collected by roof drains and catch basins and routed via a network of 15-inch drain pipes to on-site storm water treatment and retention facilities. These facilities consist of water quality trenches, a biofiltration basin, underground retention systems and biofilter swale with connections to the existing drainage systems along Hecker Avenue and Route 1. The biofiltration basin and main underground stormwater retention system are located in the southern portion of the parking lot at the rear of the property and has a capacity to store and infiltrate the total runoff volume from the contributing drainage area for up to a 100-year storm event. Because of the low groundwater elevation (8 to 9 feet below grade) in the area of the biofiltration basin and underground retention system, in conjunction with the very permeable coarse sandy soils condition, the site has a significant ability to infiltrate collected runoff into the ground. A biofilter basin will consist of a constructed wetland for treatment, storage, and infiltration of runoff with a piped outlet to an underground retention system. The retention system will consist of vehicle load rated Rainstore 3® units by Invisible Structures, Inc. and installed to the manufacturers specifications. Cleanout ports will be installed to provide access to the Rainstore3® system for regular maintenance. An emergency overflow catch basin structure in the biofilter basin will allow stormwater volumes in excess of the design capacity to enter the existing drainage system along Hecker Avenue and will also serve as a check dam structure to prevent the Town drainage system from backing up onto the property in this location. Parking lot catch basins will be connected directly to the biofilter basin or the underground retention system and roof drains for the rear portions of the building will discharge to nearby catch basins. Runoff from lawn and sidewalk areas in front of the building will be collected by a biofilter swale, which will allow for infiltration then discharge to new onsite catch basins prior to entering the existing Department of Transportation (DOT) drainage system along Route 1. Runoff from a portion of the north access driveway will be routed into underground infiltration units then discharge to a new onsite catch basin along with roof leaders serving the Boston Post Road side of the building, prior to entering the DOT drainage system. Permits and review from the Town Public Works Department and DOT are required for connection to the existing storm drains along Hecker Avenue and Boston Post Road, respectively. In fact, DOT is well aware of the inadequate condition of the storm system underneath Boston Post Road and has imposed very strict criteria for permitting the storm water management design for the library site. The proposed drainage design exceeds the current DOT requirements for a site at this location. The proposed storm water management plan for Darien Library minimizes the release of storm water off-site, a condition that can promote flooding in adjacent and down stream properties. The proposed design incorporates innovative stormwater management systems which are at the forefront of Best Management Practices (BMPs) for site development. The designed systems also provide means for the cleaning, biofiltration and recharging of water on-site as well as promoting optimal function of these systems through ease of maintenance.

566 Warburton Avenue Hastings-on-Hudson NY 10706 Phone: 914-478-3677 • Fax: 914-478-1600

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