[Ord. 550, 6/8/2011]
Applicants proposing regulated activities in Lower Southampton Township that do not fall under the exemption criteria shown in § 17-106 shall submit a stormwater management (SWM) site plan consistent with this chapter to the municipality for review. The SWM criteria of this chapter shall apply to the total proposed development even if development is to take place in stages. Preparation and implementation of an approved SWM site plan is required. No regulated activities shall commence until the municipality issues written approval of a SWM site plan which demonstrates compliance with the requirements of this chapter.
SWM site plans approved by the municipality, in accordance with Part 4, shall be on site throughout the duration of the regulated activity.
The municipality may, after consultation with the Department of Environmental Protection (PADEP), approve measures for meeting the state water quality requirements other than those in this chapter, provided that they meet the minimum requirements of and do not conflict with state law, including but not limited to the Clean Streams Law.
Editor's Note: See 35 P.S. § 691.1 et seq.
For all regulated earth disturbance activities, erosion and sediment (E&S) control best management practices (BMPs) shall be designed, implemented, operated and maintained during the regulated earth disturbance activities (e.g., during construction) to meet the purposes and requirements of this chapter and to meet all requirements under Title 25 of the Pennsylvania Code and the Clean Streams Law. Various BMPs and their design standards are listed in the Erosion and Sediment Pollution Control Program Manual, No. 363-2134-008 (April 15,2000), as amended and updated.
For all regulated activities, implementation of the volume controls in § 17-303 of this chapter is required.
The measurement of impervious areas shall include all of the impervious areas in the total proposed development even if development is to take place in stages.
For development taking place in stages, the entire development plan must be used in determining conformance with this chapter.
The total impervious area on the parcel is subject to the requirements of this chapter.
Stormwater flows onto adjacent property shall not be created, increased, decreased, relocated or otherwise altered without written notification of the adjacent property owner(s) from the developer. Such stormwater flows shall be subject to the requirements of this chapter.
All regulated activities shall include such measures as necessary to:
Protect health, safety and property;
Meet the water quality goals of this chapter by implementing measures to:
Minimize disturbance to floodplains, wetlands and wooded areas.
Create, maintain, repair or extend riparian buffers.
Avoid erosive flow conditions in natural flow pathways.
Minimize thermal impacts to waters of this commonwealth.
To the maximum extent practicable, incorporate the techniques for low-impact development practices (e.g., protecting existing trees, reducing area of impervious surface, cluster development, and protecting open space) described in the Pennsylvania Stormwater Best Management Practices Manual, Pennsylvania Department of Environmental Protection (PADEP), No. 363-0300-002 (2006). See Appendix E for a summary description.
Editor's Note: Appendix E is attached to this chapter.
Infiltration BMPs should be spread out, made as shallow as practicable, and located to maximize the use of natural on-site infiltration features while still meeting the other requirements of this chapter.
The design of all facilities over karst shall include an evaluation of measures to minimize the risk of adverse effects.
Storage facilities should completely drain both the volume control and rate control capacities over a period of time not less than 24 and not more than 72 hours from the end of the design storm.
The design storm volumes to be used in the analysis of peak rates of discharge should be obtained from the Precipitation-Frequency Atlas of the United States, Atlas 14, Volume 2, Version 3.0, U.S. Department of Commerce, National Oceanic and Atmospheric Administration (NOAA), National Weather Service, Hydrometeorological Design Studies Center, Silver Spring, Maryland. NOAA's Atlas 14 can be accessed at http://hdsc.nws.noaa.gov/hdsc/pfds/
For all regulated activities, SWM BMPs shall be designed, implemented, operated and maintained to meet the purposes and requirements of this chapter and to meet all requirements under Title 25 of the Pennsylvania Code, the Clean Streams Law, and the Storm Water Management Act.
Various BMPs and their design standards are listed in the Pennsylvania Stormwater Best Management Practices Manual (PA BMP Manual).
[Ord. 550, 6/8/2011]
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.
[Ord. 550, 6/8/2011]
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.
Design Storm Method (any regulated activity). This method requires detailed modeling based on site conditions. For modeling assumptions refer to § 17-305.1.
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.
Simplified Method (regulated activities less than or equal to one acre).
Stormwater facilities shall capture the runoff volume from at least the first two inches of runoff from all new impervious surfaces.
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.
Infiltration facilities should be designed to accommodate the first 1/2 inch of the permanently removed runoff.
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.
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.
If natural resources exist on the site, the applicant is required to submit a SWM site plan and determine the total acreage of protected natural resource 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.
Volume controls provided through nonstructural BMPs should be subtracted from the required volume to determine the necessary structural BMPs.
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 minimum soil depth of 24 inches between the bottom of the infiltration BMPs and the top of bedrock or seasonally high-water table.
An infiltration rate sufficient to accept the additional stormwater load and dewater completely as determined by field tests. A minimum of 0.2 inches/hour (inches/hour) should be utilized, and for acceptable rates, a safety factor of 50% shall be applied for design purposes (e.g., for soil which measured 0.4 inch/hour, the BMP design should use 0.2 inch/hour to ensure safe infiltration rates after construction).
All infiltration facilities shall be designed to completely infiltrate runoff volume within three days (72 hours) from the start of the design storm.
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 minimum addresses soil permeability, depth to bedrock, and subgrade stability. The general process for designing the infiltration BMP shall be:
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.
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.
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%.
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.
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.
[Ord. 550, 6/8/2011]
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 established.
Standards for managing runoff from each district in Lower Southampton Township for the two-, five-, ten-, twenty-five-, fifty- and one-hundred-year design storms are shown in Table 304.1. Development sites located in each of the management districts must control proposed development conditions runoff rates to existing conditions runoff rates for the design storms in accordance with Table 304.1.
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 the chapter.
Editor's Note: Appendix D is attached to this chapter.
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 is 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.
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.
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.
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.
[Ord. 550, 6/8/2011]
The following criteria shall be used for runoff calculations:
For development sites, the ground cover used to determine the existing conditions runoff volume and flow rate shall be as follows:
Wooded sites shall use a ground cover of "woods in good condition." A site is classified as wooded if a continuous canopy of trees exists over 1/4 acre.
All other portions of the site, including agriculture, bare earth and fallow ground, shall be considered as "meadow in good condition," unless the natural ground cover generates a lower curve number (CN) as listed in Table B-4 in Appendix B of this chapter.
Stormwater runoff peak discharges from all development sites with a drainage area equal to or greater than 200 acres shall be calculated using a generally accepted calculation technique that is based on the NRCS Soil-Cover-Complex Method. Table 305.1 summarizes acceptable computation methods. The method selected by the design professional shall be based on the individual limitations and suitability of each method for a particular site.
All calculations consistent with this chapter using the Soil-Cover-Complex Method shall use the appropriate design rainfall depths for the various return period storms according to the National Oceanic and Atmospheric Administration (NOAA) Atlas 14 rain data corresponding to the Doylestown rain gage, seen in Table B-1 in Appendix B. The SCS Type II rainfall curve from NOAA is found on Figure B-1 in Appendix B. This data may also be directly retrieved from the NOAA Atlas 14 website: hdsc.nws.noaa.gov/hdsc/pfds/orb/pa_pfds.html. If a hydrologic computer model such as PSRM or HEC-1/HEC-HMS is used for stormwater runoff calculations, then the duration of rainfall shall be 24 hours.
Runoff from proposed sites graded to the subsoil will not have the same runoff conditions as the site under existing conditions because of soil compaction even after topsoiling or seeding. The proposed condition "CN" or "C" shall increase by 5% to better reflect proposed soil conditions.
The Manning equation is preferred for one-dimensional, gradually varied, open-channel flow. In other cases, appropriate applicable methods should be applied; however, early coordination with the municipality is necessary.
Outlet structures for stormwater management facilities shall be designed to meet the performance standards of this chapter using the generally accepted hydraulic analysis technique or method of the municipality.
The design of any stormwater detention facilities intended to meet the performance standards of this chapter shall be verified by routing the design storm hydrograph through these facilities using the Storage-Indication Method. The design storm hydrograph shall be computed using a calculation method that produces a full hydrograph.
[Ord. 550, 6/8/2011]
Stormwater runoff from hot spot land uses shall be pretreated. In no case may the same BMP be employed consecutively to meet this requirement. Guidance regarding acceptable methods of pretreatment is located in Appendix G.