Town of Wilton, NH

Stormwater Management Regulations

8.0 Stormwater Management System Design and Performance

8.1 Generally.

The SMP shall provide design information and drawings, performance standards and protection criteria for each element, temporary or permanent, structural or non-structural, of the Stormwater Management System, in each case in compliance with BMPs, the NHDES Alteration of Terrain program, to the extent applicable, and these regulations. The SMP shall identify all proposed above-ground LID mechanisms or structures and include digital files containing the coordinates in New Hampshire State Plane feet using the North American Datum 1983 (NAD83) or newer; and in North American Vertical Datum 1988 (NAVD88), of all structural elements of the SMS (e.g., catch basins, swales, detention/bioretention areas, piping). The design of the SMS shall take into account existing site hydrology, including flows originating from off-site, as well as seasonal high water tables.

8.2 Sizing.

  1. Each element of the SMS, including, without limitation, emergency overflow structures, shall be sized based on the 100-year frequency 24-hour storm discharge rate utilizing then-current precipitation data from the Northeast Region Climate Center (NRCC) or the most recent precipitation atlas published by the National Oceanic and Atmospheric Administration (NOAA). See NRCC website at [http://precip.eas.cornell.edu/] (unsecured website);
  2. Temporary (additional) detention, Infiltration and/or Sedimentation facilities may be designed to accommodate the storm most likely to occur during the anticipated duration of construction;
  3. The soil-cover complex method (TR 20) to compute runoff volume and peak rate of discharge shall be used to design detention structures that will reduce future condition peak rate of discharge, which design shall conform to applicable criteria set forth in the NH Stormwater Manual;

8.3 Setbacks and Buffers.

Except as necessary to transport Stormwater Runoff as part of a drainage system, no element of an SMS shall be located in any applicable (1) setback or (2) buffer required by the Planning Board or otherwise by the Wilton Land Use Laws and Regulations. Whether or not an NHDES Alteration of Terrain Permit is required in connection with any Construction Activity, all discharge from the SMS shall comply with the setback requirements from water supply sources established in the NHDES Alteration of Terrain Permit Program Rules (see Env-Wq 1508.02).

8.3-a Performance.

  1. The SMP shall include calculations of the change in Impervious Surface, removal rates and other relevant performance data for each element of the Stormwater Management System;
  2. Elements of the SMS and other areas that receive rainfall runoff shall be designed to drain within a maximum of 72 hours for vector control, unless specifically identified and
    3. approved for longer term water storage. The design of the stormwater drainage systems shall provide for the disposal of stormwater without flooding or functional impairment of streets, adjacent properties, downstream properties, soils, or vegetation; and
  3. Discharge from Drainage Systems shall not flood or functionally impair streets, adjacent or downstream properties, or onsite soils, structures, vegetation or any downstream Critical Areas.