Approvals issued and actions taken under this chapter do not relieve the applicant of the responsibility to secure required permits or approvals for activities regulated by any other code, law, regulation or ordinance.
Volume controls will mitigate increased runoff impacts, protect stream channel morphology, maintain groundwater recharge, and contribute to water quality improvements. Stormwater runoff volume control methods are based on the net change in runoff volume for the two-year storm event. Volume controls shall be implemented using the Design Storm Method in Subsection A or the Simplified Method in Subsection B below. For regulated activities equal to or less than one acre, this chapter establishes no preference for either methodology; therefore, the applicant may select either methodology on the basis of economic considerations, the intrinsic limitations of the procedures associated with each methodology, and other factors. All regulated activities greater than one acre must use the Design Storm Method.
A. Design Storm Method (any regulated activity). This method requires detailed modeling based on site conditions. For modeling assumptions, refer to §
157-14A.
(1) Post-development total runoff should not be increased from predevelopment total runoff for all storms equal to or less than the two-year, twenty-four-hour-duration precipitation.
(2) The following applies in order to estimate the increased volume of runoff for the two-year, twenty-four-hour-duration precipitation event: To calculate the runoff volume (cubic feet) for existing site conditions (predevelopment) and for the proposed developed site conditions (post-development), it is recommended to use the Soil Cover Complex Method as shown as follows. The calculated volume shall be either reused, evapotranspired, or infiltrated through structural or nonstructural means. Other means of calculating the runoff volume may be used if approved in advance by the Township Engineer.
| Soil Cover Complex Method |
| Step 1: Runoff (in) = Q = (P - 0.2S)2/(P + 0.8S) |
| Where: |
| | P | = | 2-year rainfall (in) |
| | S | = | (1,000/CN) -10 |
| Step 2: Runoff Volume (cubic feet) = Q x Area x 1/12 |
| Where: |
| | Q | = | Runoff (in) (from Step 1) |
| | Area | = | SWM area (square feet), i.e., the square-foot area of the regulated activity |
| Step 3: Required Volume = Post-development Runoff Volume – Predevelopment Runoff Volume |
| Once runoff volume is calculated for the site under both existing and proposed developed conditions, the difference between the two site conditions is the increase in runoff volume from predevelopment conditions to post-development conditions for the two-year, twenty-four-hour precipitation event. |
| The calculated volume shall be captured, reused, evapotranspired, or infiltrated through nonstructural or structural means. |
B. Simplified Method (regulated activities less than or equal to one acre).
(1) Stormwater facilities shall capture the runoff volume from at least the first two inches of runoff from all new impervious surfaces.
| Volume (cubic feet) = (2-inch runoff/12 inches) * Impervious Surface (square feet) |
(2) At least the first inch of runoff volume from the new impervious surfaces shall be permanently removed from the runoff flow, i.e., it shall not be released into the surface waters of the commonwealth. The calculated volume shall be either reused, evapotranspired or infiltrated through structural or nonstructural means.
| Volume (cubic feet) = (1-inch runoff/12 inches) * Impervious Surface (square feet) |
(3) Infiltration facilities should be designed to accommodate the first 0.5 inch of the permanently removed runoff.
(4) No more than one inch of runoff volume from impervious surfaces shall be released from the site. The release time must be over 24 to 72 hours.
C. Stormwater control measures. The applicant must demonstrate how the required volume is controlled through stormwater best management practices (BMPs), which shall provide the means necessary to capture, reuse, evaporate, transpire or infiltrate the total runoff volume.
(1) If natural resources exist on the site, the plan shall indicate the total acreage of protected area where no disturbance is proposed. The acreage of the protected area should be subtracted from the total site area and not included in the stormwater management site area acreage used in determining the volume controls.
| Stormwater Management Site Area = Total Site Area (for both pre- and post-development conditions) – Protected Area |
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| Natural resource areas should be calculated based upon the municipality's own natural resource protection ordinance. If no ordinance exists, see Table B-2 in Appendix B for guidance to assess the total protected area. For additional reference, see Chapter 5, Section 5.4.1, of the Pennsylvania BMP Manual. |
(2) Calculate the volume controls provided through nonstructural BMPs. Table B-5 in Appendix B is recommended as guidance.
(3) Volume controls provided through nonstructural BMPs should be subtracted from the required volume to determine the necessary structural BMPs.
| Required Volume Control (feet3) | – | Nonstructural Volume Control (feet3) | = | Structural Volume Requirement (feet3) |
(4) Calculate the volume controls provided through structural BMPs. Table B-6 in Appendix B is recommended as guidance. See the Pennsylvania BMP Manual, Chapter 6, for a description of the BMPs.
(5) Infiltration BMPs intended to receive runoff from developed areas shall be selected based on the suitability of soils and site conditions (see Table B-6 in Appendix B for a list of Infiltration BMPs). Infiltration BMPs shall be constructed on soils that have the following characteristics:
(a) A minimum soil depth of 24 inches between the bottom of the infiltration BMPs and the top of bedrock or seasonally high water table.
(b) An infiltration rate sufficient to accept the additional stormwater load and dewater completely as determined by field tests. A minimum of 0.2 inch per hour should be utilized, and for acceptable rates a safety factor of 50% should be applied for design purposes (e.g., for soil which measured 0.4 inch per hour, the BMP design should use 0.2 inch per hour to ensure safe infiltration rates after construction).
(c) All open-air infiltration facilities shall be designed to completely infiltrate runoff volume within three days (72 hours) from the start of the design storm. Subsurface systems shall be designed to infiltrate the stored volume in 48 hours or less with a minimum infiltration rate and percolation rate of greater than 0.2 inch per hour.
(6) Soils. A soils evaluation of the project site shall be required to determine the suitability of infiltration facilities. All regulated activities are required to perform a detailed soils evaluation by a qualified design professional which, at a minimum, addresses soil permeability, depth to bedrock, and subgrade stability. The general process for designing the infiltration BMP shall be:
(a) Analyze hydrologic soil groups as well as natural and man-made features within the site to determine general areas of suitability for infiltration practices. In areas where development on fill material is under consideration, conduct geotechnical investigations of subgrade stability; infiltration may not be ruled out without conducting these tests.
(b) Provide field tests such as double-ring infiltrometer or hydraulic conductivity tests (at the level of the proposed infiltration surface) to determine the appropriate hydraulic conductivity rate. Percolation tests are not recommended for design purposes. The Township Engineer shall observe all field testing of soils.
(c) Design the infiltration structure based on field-determined capacity at the level of the proposed infiltration surface and based on the safety factor of 50%.
(d) If on-lot infiltration structures are proposed, it must be demonstrated to the municipality that the soils are conducive to infiltrate on the lots identified.
(e) An impermeable liner will be required in detention basins where the possibility of groundwater contamination exists. A detailed hydrogeologic investigation may be required by the municipality.
Peak rate controls for large storms, up to the one-hundred-year event, are essential in order to protect against immediate downstream erosion and flooding. The following peak rate controls have been determined through hydrologic modeling of the Neshaminy Creek Watershed.
A. Standards for managing runoff from each subarea in the Neshaminy Creek Watershed for the two-, five-, ten-, twenty-five-, fifty-, and one-hundred-year design storms are shown in Table 157-13. Development sites located in each of the management districts must control the proposed development conditions' runoff rates to existing conditions' runoff rates for the design storms in accordance with Table 157-13.
| Table 157-13 |
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| Peak Rate Runoff Control Standards by Stormwater Management Districts in the Neshaminy Creek Watershed (includes Little Neshaminy Creek) |
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| District | Design Storm Post-Development (Proposed Conditions) | Design Storm Predevelopment (Existing Conditions) |
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| A | 2-year | 1-year |
| | 5-year | 5-year |
| | 10-year | 10-year |
| | 25-year | 25-year |
| | 50-year | 50-year |
| | 100-year | 100-year |
| B | 2-year | 1-year |
| | 5-year | 2-year |
| | 10-year | 5-year |
| | 25-year | 10-year |
| | 50-year | 25-year |
| | 100-year | 50-year |
| C | 2-year | 2-year |
| | 5-year | 5-year |
| | 10-year | 10-year |
| | 25-year | 25-year |
| | 50-year | 50-year |
| | 100-year | 100-year |
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 D) and in this section of this chapter.
C. District boundaries. The boundaries of the stormwater management districts are shown on official maps and are available for inspection at the Municipal Office and County Planning Offices. A copy of the map at a reduced scale, and four other maps with zoomed-in extents, are included in Appendix D. 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. For a proposed development site located within two or more stormwater management district category subareas, the peak discharge rate from any subarea shall meet the management district criteria for the district in which the discharge is located.
E. Off-site areas. When calculating the allowable peak runoff rates, developers do not have to account for runoff draining into the subject development site from an off-site area. On-site drainage facilities shall be designed to safely convey off-site flows through the development site.
F. Site areas. The stormwater management site area is the only area subject to the management district criteria. Nonimpacted areas or nonregulated activities bypassing the stormwater management facilities would not be subject to the management district criteria.
G. 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 Township:
(1) Meet the full requirements specified by Table 157-13 and §
157-13A through
F; 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.