Township of Unity, PA
Westmoreland County
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Table of Contents
Table of Contents
The purpose of the following sections is to limit and restrict stormwater runoff peak flows from subdivisions and other land development to pre-development flow rates as indicated herein.
The following activities are included within the scope of this chapter:
Land developments, including, without limitation, all commercial, industrial and institutional development regardless of size and whether or not such development is considered new or as an addition to existing, commercial, industrial or institutional development. Development of individual residential lots that predate this chapter shall be exempt from the provisions of this chapter.
All subdivisions consisting of four or more lots, regardless of the overall size of the development. The enumeration of lots shall include the residual tract. Provided, however, that if a tract of ground or land is subdivided, containing fewer than four lots, and said tract is subdivided in the future, the original number of lots in the initial subdivision shall be considered when future subdivisions are filed, i.e., a tract of ground is subdivided into two lots and a residual tract; and then the residual tract is subdivided into another lot plus the residual tract, the total lots will consist of four lots and the provisions of this article shall apply.
Land disturbance activities involving 10,000 square feet or more of disturbed area.
Normal agricultural and farming uses shall be exempt from the operation of this chapter.
Permits and approvals issued pursuant to this chapter do not relieve the applicant of the responsibility to secure required permits or approvals for activities regulated by any other applicable code, rule, act, ordinance, or regulatory agency. If more stringent requirements concerning regulation or stormwater control are contained in the other code, rule, or ordinance, the more stringent regulations shall apply.
Stormwater retention. The post-development stormwater runoff volume for the one-year, twenty-four-hour storm event from all disturbed areas of the development site shall be 100% retained on site for retention, infiltration, extended detention or some combination of these.
Recurrence interval (storm frequency data). Stormwater management facilities on all development sites shall control the peak stormwater discharge for the one-, two-, ten-, twenty-five-, fifty- and one-hundred-year storm frequencies such that the total post-development peak discharge rate from all disturbed areas of the site does not exceed 50% of the pre-development peak runoff rate from these areas for each respective design storm. The SCS twenty-four-hour, Type II rainfall distribution shall be used for analyzing stormwater runoff for both pre- and post-development conditions. The design storm twenty-four-hour rainfall amounts shall be taken from NOAA Atlas 14 Version 3 Data, or other valid data as deemed suitable by the Township:
Recurrence Interval
Twenty-Four-Hour Storm Volume
(inches of rainfall)
Pre-development and post-development stormwater runoff rates and volumes from all sites shall be calculated using either the rational method or a soil cover complex methodology.
The following table lists acceptable computation methods for developing design hydrographs. The selection of the computation method(s) by the design professional shall be based on the individual limitations and suitability of each method for a particular site.
Acceptable Computation Methods for Stormwater Management Plans
Method Developed by
TR-20 or commercial package based on TR-20
When use of full model is desirable or necessary
TR-55 or commercial package based on TR-55
Applicable for plans within the models limitations
U.S. Army Corps of Engineering
When full model is desirable or necessary
Virginia Tech and Penn State University
When full model is desirable or necessary
Rational method or commercial package based on rational method
Emil Kuiching (1889)
For sites less than 200 acres
Other methods
As approved by the Township Engineer
Any stormwater runoff calculations involving areas greater than 200 acres, including on- and off-site areas, shall use a generally accepted calculation technique that is based on the NRCS soil cover complex method. For development of the design runoff hydrographs, the following assumptions shall be used:
Average antecedent moisture conditions.
For use with TR-55 the storm rainfall shall have a twenty-four-hour duration with a SCS Type II time distribution. For use with the Rational Formula, the rainfall duration shall equal the watershed Tc.
Meadow in good condition shall be used in pre-development runoff calculations for all areas of existing cultivation.
All areas other than cultivation shall use the land cover condition, which existed during the past 10 consecutive years.
All areas to be disturbed during construction and subsequently returned to open space will be assumed to be reduced one hydrologic group category level for post-development runoff.
Pre-development conditions. Pre-development conditions shall be assumed to be those which exist on any site at the time of adoption of this chapter. Hydrologic conditions for all areas with pervious cover (i.e., fields, woods, lawn areas, pastures, cropland, etc.) shall be assumed to be in good condition, and the lowest recommended SCS runoff curve number (CN), or runoff coefficient (C) for the Rational Method, shall be applied for all pervious land uses within the respective range for each land use and hydrologic soil group. For redevelopment sites or sites with existing impervious cover, a minimum of 20% of the existing impervious cover within the limits of disturbance of the project site shall be assumed to be meadow in good condition and the appropriate CN, or C, for this assumed cover type and for its specific underlying soil shall be used.
Areas of impervious cover and other cover types shall be measured from scaled drawings based on aerial photography or ground survey data.
The routing of hydrographs through detention/retention facilities for the purpose of designing those facilities shall be accomplished using the Modified-Puls Method or a recognized reservoir routing method subject to the approval of Unity Township or the Township's representative.
Maintenance of natural drainage ways. All natural streams, channels, swales, drainage systems and/or areas of surface water concentration shall be maintained in their existing condition unless an exception is approved by Unity Township or official. All encroachment activities shall comply with Chapter 105, Water Obstructions and Encroachments, of Title 25 of the Rules and Regulations of the Pennsylvania Department of Environmental Protection.
Methods of stormwater runoff detention and control.
The following is a list of detention and control methods suitable for use in stormwater management systems. The selection of control methods is not limited to the ones present on this list:
Detention and/or retention structures.
Rain gardens.
Infiltration basins and/or rock sumps.
Grass channels and vegetated strips.
Routed flow over grass.
Decreased impervious areas coverage.
Porous pavement and concrete lattice block surface.
Cisterns and underground reservoirs including piping systems.
Rooftop storage.
Parking lot and street ponding.
The Township, however, reserves the right to approve or deny any or all of the above control methods for or on any individual project. Other control methods not mentioned above which, in the sole discretion of the Township, meet the criteria of this subsection will be permitted when approved by the Township or an official representative thereof, in writing.
All best management practices (BMPs) used to meet the requirements of this chapter shall conform to the state water quality requirements and any more stringent requirements as determined by the Township. The PA DEP Bureau of Watershed Management document entitled "Pennsylvania Stormwater Best Management Practices Manual," latest edition, shall be used to conform to these requirements.
The stormwater management plan for the development site must consider all the stormwater runoff flowing over the site, including flow from areas up-gradient to the site.
A review and analysis of downstream impacts of stormwater discharge downstream of the site which shall include the discharge path from the point of discharge to a blue-line receiving water of the Commonwealth as indicated on the USGS 7.5-minute quadrangle topographic maps. This review shall include photographs of the discharge path to the receiving water of the commonwealth.