(a) Division into districts. Each watershed has been divided into either stormwater management districts or release rate districts as shown on the respective management district or release rate maps in Appendix A.
(1) In addition to the watershed-specific requirements specified in Table 2 and 3 for each of the watersheds below, the erosion and sedimentation control (§ 1043.14), the nonstructural project design (§ 1043.15), the infiltration (§ 1043.16), the water quality (§ 1043.17) and the stream bank erosion (§ 1043.18) requirements shall be implemented.
(2) Standards for managing runoff from each subarea in a watershed for the two-, five-, ten-, 25-, 50- and 100-year design storms are shown in Tables 2 and 3. Development sites located in each of the management/release rate districts must control proposed conditions runoff rates to existing conditions runoff rates for the design storms in accordance with the tables.
(b) General. Proposed conditions rates of runoff from any regulated activity shall not exceed the peak release rates of runoff from existing conditions for the design storms specified on the stormwater management district watershed map (Appendix A) and this section.
(c) District boundaries. The boundaries of the stormwater management districts are shown on an official map that is available for inspection at the municipal and County Planning offices. A copy of the official map at a reduced scale is included in Appendix A. The exact location of the stormwater management district boundaries as they apply to a given development site shall be determined by mapping the boundaries using the two-foot topographic contours (or most accurate data required) provided as part of the SWM site plan.
(d) Sites located in more than one district or watershed. For a proposed development site located within two or more stormwater management district subareas, the peak discharge rate from any subarea shall meet the management district criteria for which the discharge is located. The natural hydrology of each respective subarea shall be maintained, and drainage shall not be redirected from one subarea to another. Under circumstances where the applicant shows this cannot be accomplished, a waiver is required by the municipality.
Table 2: Control Criteria for Chester Creek Watershed Stormwater Management Districts | |
District | Control Criteria |
100% | Post-development peak discharge for all design storms must be no greater than pre-development peak discharges |
75% | Post-development peak discharge for all design storms must be no greater than 75% of the pre-development peak discharges |
50% | Post-development peak discharge for all design storms must be no greater than 50% of the pre-development peak discharges |
Note for Chester Creek Watershed: watershed subareas with a 100% should confirm that the peak rate control requirement is at least as stringent as the Marcus Hook Creek requirement (below) | |
Table 3: Peak Rate Control Standards in the Marcus Hook Creek Watershed | ||
Proposed Condition Design Storm | Reduce to | Existing Condition Design Storm |
Table 3: Peak Rate Control Standards in the Marcus Hook Creek Watershed | ||
Proposed Condition Design Storm | Reduce to | Existing Condition Design Storm |
2-year | 1-year | |
5-year | 5-year | |
10-year | 10-year | |
25-year | 25-year | |
50-year | 50-year | |
100-year | 100-year | |
(e) Off-site areas. Off-site areas that drain through a proposed development site are not subject to release rate criteria when determining allowable peak runoff rates. On-site drainage facilities shall be designed to safely convey off-site flows through the development site.
(f) Site areas. Where the site area to be impacted by a proposed development activity differs significantly from the total site area, only the proposed impact area utilizing stormwater management measures shall be subject to the peak rate control standards noted above. Unimpacted areas for which the discharge point has not changed are not subject to the peak rate control standards.
(g) Downstream hydraulic capacity analysis. Any downstream capacity hydraulic analysis conducted in accordance with this chapter shall use the following criteria for determining adequacy for accepting increased peak flow rates:
(1) Natural or human-made channels or swales must be able to convey the increased runoff associated with a two-year storm event within their banks at velocities consistent with protection of the channels from erosion. Velocities shall be based upon criteria and methodologies acceptable to the municipality.
(2) Natural or human-made channels or swales must be able to convey increased 25-year storm event runoff without creating any increased hazard to persons or property.
(3) Culverts, bridges, storm sewers or any other hydraulic facilities which must pass or convey flows from the tributary area must be designed in accordance with 25 Pa. Code Ch. 105 (if applicable) and, at a minimum, pass the increased 25-year storm event runoff.
(4) Water quality requirements defined in § 1043.17 must be met.
(5) Post construction peak rates shall not exceed the existing peak rates for the respective subarea.
(h) Alternate criteria for redevelopment sites. For redevelopment sites, one of the following minimum design parameters shall be accomplished, whichever is most appropriate for the given site conditions as determined by the municipality:
(1) Meet the full requirements specified by Tables 2 and 3 and subsections (a) through (g) above; or
(2) Reduce the total impervious surface on the site by at least 20%, based upon a comparison of existing impervious surface to proposed impervious surface. In this case, calculations must be provided that show the peak rate has not increased.
(Ord. 928, passed 6-18-2014)