A. Mapping of Stormwater Runoff Peak Rate Districts - In order to implement the provisions of this Ordinance, Tohickon Creek Watershed Stormwater Management Plan and Delaware River (North) Watershed Stormwater Management Plan, Springfield Township is hereby divided into Stormwater Runoff Peak Rate Districts consistent with the plans. The boundaries of the districts are indicated on the runoff peak rate district map that is available for inspection at the Township building. A large-scale boundary map is included as Appendix B for reference.
B. 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 (2) feet or five (5) feet topographic contours provided as part of the stormwater management plan developed for the site in accordance with the 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. Conditional No Detention Districts (Direct Discharge). Subareas identified on the official Sub-Basin Map available for inspection at the Township Office. Included in this district are 2, 3, 8-10, 18, 20, 21, 42, 43, 52, 54, 56, 57, 59, 61, 62, 67, 70-73, 76, 77, 81-83. These sub-areas may discharge post-development runoff without detention facilities without adversely affecting the total watershed peak flow. These areas are located adjacent to the Delaware River. However, where the conveyance capabilities of the local receiving facilities are not adequate to safely transport the increased peak flows from undetained runoff a 100% release rate control is applied.
2. 100 Percent Release Rate District. Subareas included in this district are 4-7, 1-17, 19, 22-28, 31-34, 41, 44-51, 53, 55, 58, 60, 63, 64, 66, 68, 69, 74, 75, 84-98, 101, 109, 111 - 114, 116, 118, 119, 121-126. 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 in the watershed of these sub-areas is of minor importance in supporting the overall watershed level runoff control. Therefore, these areas are allowed to release post-development runoff at a rate that does not exceed the existing rates of runoff.
D. Description of Delaware River (North) Watershed Stormwater Runoff Hydrologic Peak Rate Districts.
1. Conditional No Detention Districts (Direct Discharge). Subareas included in this district are 1, 2, 6-11, 20, 61-68, 70-72, 89-91, 93. These subareas may discharge post-development runoff without detention facilities without adversely affecting the total watershed peak flow. These areas are located adjacent to the Delaware River, which is 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 100 percent 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.
2. 100 Percent Release Rate District. Subareas included in this district are 3-5, 12-19, 21-60, 69, 73-88, 94-103, and 112-130. 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 in the watershed of these subareas is of minor importance in supporting the overall watershed level runoff control. Therefore, these areas are allowed to release post-development runoff at a rate that does not exceed the existing rates of runoff.
3. 75 Percent Release Rate District. Subareas included in this district are 104-111. Certain subareas require the control of stormwater runoff to a portion of the existing runoff equal to 75 percent. These areas are located in upper reaches of the watershed, specifically, areas within Springfield and Lower Saucon Townships. 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 sub-watershed.
E. Areas of the Township which are not included within either the Tohickon Creek or Delaware River (North) Watershed are located within either Saucon Creek Watershed or Perkiomen Creek Watershed, which are 100 percent release rate districts.
F. Sites Located in More Than One District – For a proposed development site located within two or more release category subareas, 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 grading plan changes the drainage areas by subarea.
G. 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, on-site 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.
H. 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 Stormwater Management District criteria.
I. “Downstream Hydraulic Capacity Analysis” – Any downstream capacity hydraulic analysis conducted in accordance with this ordinance shall use the following criteria for determining adequacy for accepting increased peak flow rates.
1. Natural or man-made channels or swales must be able to convey the increased runoff associated with a 2-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 DEP Erosion and Sediment Pollution Control Program Manual.
2. Natural or man-made channels or swales must be able to convey the increased 25-year return period runoff without creating any hazard to persons or property, or wildlife and aquatic habitat. Habitat impact should 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. 186, passed 9-12-2017)