(A) Mapping of stormwater runoff peak rate districts. In order to implement the provisions of this chapter and the Perkiomen Creek Watershed and Tohickon Creek Watershed Stormwater Management Plans, the township is hereby divided into stormwater runoff peak rate districts consistent with the plans. The boundaries of the districts are indicated on the Stormwater Runoff Peak Rate District map that is available for inspection at the township building. A copy of this Stormwater Runoff Peak Rate District map is attached hereto and incorporated herein as Appendix C of this chapter.
(B) Stormwater Runoff Peak Rate District boundary. The exact location of the Stormwater Runoff Peak Rate District boundary as it applies to a given development site shall be determined by mapping the boundaries using the two feet or five feet topographic contours provided as part of the stormwater management plan developed for the site in accordance with the township’s subdivision and land development ordinance. The District boundaries, as originally drawn, coincide with topographic divides or, in certain instances, are drawn from the intersection of the watercourse or a potential flow obstruction to the topographic divide consistent with topography. The locations determined on the stormwater management plan shall be reviewed and verified by the Township Engineer.
(C) Description of Tohickon Creek watershed stormwater runoff hydrologic peak rate districts.
(1) One hundred Percent Release Rate District. Subareas included in this District are 11 through 16, 22 through 28, 31 through 34, 41, 44 through 48, 50, 51, 54 and 56 through 59. These subareas are not expected to incur a great deal of development growth due to location, topography, soils or a combination of all three factors. Also, the location of these subareas in the watershed is of minor importance in supporting the overall watershed level runoff control. Therefore, these subareas are allowed to release post-development runoff at a rate that does not exceed the existing rates of runoff.
(2) Ninety Percent Release Rate District. Subareas included in this District are 29, 30 and 35 through 40. Certain subareas require the control of stormwater runoff to a portion of the existing runoff equal to 90%. These subareas are located in upper reaches of the watershed. In order to ensure uniform watershed-level runoff control, the assignment of this release rate on a widespread basis will uniformly restrict the future runoff in a fashion that favors no particular subarea.
(3) Conditional No Detention District (direct discharge). Subareas included in this District are 18, 20, 21, 42, 43, 49, 52, 62, 70, 72, 76 and 77. These subareas may discharge post-development runoff without the use of detention facilities without adversely affecting the total watershed peak flow. These subareas are located adjacent to Tohickon Creek or Lake Nockamixon, which waterbodies are capable of absorbing undetained runoff without affecting the watershed level control. In certain instances, the conveyance capabilities of the local receiving facilities may not be adequate to safely transport the increased peak flows from undetained runoff. In these cases, the developer shall ensure that one 100% release rate control is applied to the particular receiving stream(s), and/or the developer may provide increased capacity of those receiving facilities in order to ensure safe passage of any undetained runoff.
(D) Description of East Branch Perkiomen Creek Watershed Stormwater Runoff Hydrologic Peak Rate District . Subareas included in the 100% Release Rate District are 1 through 10, 17 and 19. These subareas are not expected to incur a great deal of development growth due to location, topography, soils, subarea or a combination of all these factors. Also, the location of these subareas in the watershed is of minor importance in supporting the overall watershed level runoff control. Therefore, these subareas are allowed to release post-development runoff at a rate that does not exceed the existing rates of runoff.
(E) Sites located in more than one district. For a proposed development site located within two or more subareas located in more than one release rate district, the peak discharge rate from any subarea shall be the pre-development peak discharge for that subarea multiplied by the applicable release rate. The calculated peak discharges shall apply regardless of whether the proposed grading changes the drainage areas by subarea.
(F) 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 or volume reduction. However, onsite drainage facilities shall be designed to safely convey off-site flows through or around the development site to existing points of discharge from the property.
(G) Exception. Where the area of a site being impacted by a proposed development activity differs significantly from the total site area, only the proposed disturbed area utilizing stormwater management measures shall be subject to the management district criteria. Unimpacted or undisturbed areas that do not flow into, or bypass, the stormwater management facilities shall not be subject to the requirements of this section.
(H) 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 return period event within their banks at velocities consistent with protection of the channels from erosion. Acceptable velocities shall be based upon criteria included in the E&S Manual.
(2) Natural or human-made channels or swales must be able to convey the increased 25-year return period runoff without creating any hazard to persons, property or wildlife and aquatic habitat. Habitat impact shall be minimized or avoided.
(3) Culverts, bridges, storm sewers or any other facilities which must pass or convey flows from the tributary area must be designed in accordance with DEP, Chapter 105 regulations (if applicable) and, at a minimum, pass the increased 25-year return period runoff.
(Ord. 239, passed 10-14-2020)