(A) Stormwater management performance districts.
(1) For purposes of stormwater management, the borough is divided into the following stormwater management districts:
District A | 50% runoff | Turtle Creek Watershed |
District B | 100% runoff | Turtle Creek Watershed |
(2) The location and boundaries of the watershed(s) and subareas are shown on the borough stormwater management district map which is hereby adopted as a part of this section.
(B) General standards.
(1) The following provisions shall be considered the overriding performance standards against which all proposed stormwater control measures shall be evaluated and shall apply throughout the borough.
(a) Any landowner and any person engaged in the alteration or development of land which may affect stormwater runoff characteristics shall implement such measures as are reasonably necessary to prevent injury to health, safety or other property. Such measures shall include such actions as are required:
1. To assure that the maximum rate of stormwater runoff is no greater after development than prior to development activities; and
2. To manage the quantity, velocity and direction of resulting stormwater runoff in a manner which otherwise adequately protects health and property from possible injury.
(2) The stormwater management plan for the development site must consider all the stormwater runoff flowing over the site.
(3) No discharge of toxic materials shall be permitted into any stormwater management system.
(C) Watershed standards; Turtle Creek Stormwater Management District.
(1) Standards. The stormwater performance standards contained in this section are intended to implement the standards and criteria contained in the Turtle Creek stormwater management plan, adopted and approved in accordance with the Pennsylvania Stormwater Management Act (32 P.S. §§ 680.1 et seq.) If there is any discrepancy between the provisions of this section and the standards and criteria of the plan, or if the watershed plan is subsequently amended, then the standards/criteria of the current watershed plan shall govern.
(2) Storm frequencies. Stormwater management facilities on all development sites shall control the peak stormwater discharge for the two-, ten-, 25- and 100-year storm frequencies. The SCS 24-hour, Type II Rainfall Distribution shall be used for analyzing stormwater runoff for both pre- and post-development conditions. The 24-hour total rainfall for these storm frequencies in the watershed are:
Storm Frequency | Rainfall Depth (inches) |
2-year | 2.50 |
10-year | 3.61 |
25-year | 4.31 |
100-year | 5.71 |
(3) Calculation methods.
(a) Development sites. For the purpose of computing peak flow rates and runoff hydrographs from development sites, calculations shall be performed using one of the following: SCS publications; Technical Release (TR) 55 or 20; EEC I; or Penn State Runoff Model.
(b) Stormwater collection/conveyance facilities. For the purposes of designing storm sewers, open swales and other stormwater runoff collection and conveyance facilities, any of the above listed calculation methods or the Rational Method may be used. Rainfall intensities for design should be obtained from the Pennsylvania Department of Transportation rainfall charts.
(c) Pre-development conditions. Pre-development conditions shall be assumed to be those which exist on any site at the time of adoption of the Turtle Creek stormwater management plan. Hydrologic conditions for all areas with pervious cover (i.e., fields, woods, lawn areas, pastures, cropland and the like) shall be assumed to be in “good” condition, and the lowest recommended SCS runoff curve number (CN) shall be applied for all pervious land uses within the respective range for each land use and hydrologic soil group.
(d) Routing of hydrographs. Routing of hydrographs through detention/retention facilities for the purpose of design those facilities shall be accomplished using the Modified-Puls Method or recognized reservoir routing method subject to the approval of the borough and county.
(4) Release rate percentage.
(a) Definition. The RELEASE RATE PERCENTAGE defines the percentage of the pre-development peak rate of runoff that can be discharged from an outfall on the site after development. It applies uniformly to all land development or alterations within a subarea. A listing of the release rate percentage by subarea appears in Appendix A of this chapter; the subareas are delineated on the borough stormwater management district map.
(b) Procedure for use.
1. Identify the specific subarea in which the development site is located from the watershed map and obtain the subarea release rate percentage from Appendix A;
2. Compute the pre- and post-development runoff hydrographs for each stormwater outfall on the development site using an acceptable calculation method for the two-, ten-, 25- and 100-year storms. Apply no on-site detention for stormwater management but include any techniques to minimize impervious surfaces and/or increase the time of concentration for stormwater runoff flowing from the development site. If the post-development peak runoff rate and the runoff volume are less than or equal to the predevelopment peak runoff rate and volume, then additional stormwater control shall not be required at that outfall. If the post-development peak runoff rate and volume are greater than the pre-development peak runoff rate and volume, then stormwater detention will be required and the capacity of the detention facility must be calculated in the manner prescribed below;
3. Multiply the subarea release rate percentage by the pre-development rate of runoff from the development site to determine the maximum allowable release rate from any detention facility for the four prescribed storm events; and
4. Design the outlet control facilities and size the volume of the detention facility using the calculated post development hydrograph and accepted hydrograph routing procedures in consideration of the maximum allowable release rate.
(5) No harm evaluation.
(a) An applicant may seek to exceed the otherwise applicable subarea release rate percentage by performing the “no harm evaluation”. This evaluation requires an independent engineering analysis to demonstrate that other reasonable options exist to prevent the occurrence of increased stormwater runoff discharge rates and/or velocities or that measures can be provided to prevent increased stormwater discharge rates and/or velocities from increasing flood elevations and accelerating erosion at all downstream points in the watershed.
(b) A “no harm evaluation” will be considered only in instances where the discharge to a stream channel from the development occurs directly to the Monongahela River, channelized portion of Turtle Creek, or through a properly sized and designed regional stormwater detention facility.
(c) The analysis for the “no harm evaluation” shall be submitted to the Borough Engineer and County Planning Department (Westmoreland Conservation District) for review and approval.
(d) The “no harm evaluation” shall be prepared by a registered engineer who is experienced in hydrology and hydraulics. The “no harm evaluation” shall be completed using the following procedure: the Penn State Runoff Model (PSRM) is the hydrologic model required in this procedure. Use of this model would produce results from a “no harm evaluation” analysis that could be compared to the results of the watershed study.
1. Develop the runoff hydrograph(s) for the design storms of the site and areas tributary to it using the PSRM and the Turtle Creek Watershed Stormwater Management Plan land use of the development for both pre-development and post-development conditions;
2. Develop the runoff hydrograph(s) for the proposed site using the PSRM. If no management or controls are proposed, this would be equivalent to the runoff hydrograph under post-development conditions. If some management or controls are proposed, then the runoff hydrograph under post-development conditions would be modified to reflect their effect on the rate, volume and timing of discharges;
3. Subtract the runoff hydrograph ordinates under pre-development conditions (division (C)(5)(d)1. above) from the discharge hydrograph ordinates (division (C)(5)(d)2. above), maintaining the time scales of both hydrographs for one-to-one correspondence;
4. Obtain the PSRM for existing conditions for the Monongahela Watershed from the county;
5. Locate the subbasin(s) in which the proposed development is located and into which the discharge hydrograph enters. If more than one subbasin receives this incremental flow, divide the flow accordingly;
6. Add the incremental increase computed in division (C)(5)(d)3. above to the runoff hydrograph for the subbasin(s) identified in division (C)(5)(d)5. above;
7. Route the adjusted runoff hydrograph through the Monongahela Watershed PSRM and note any increase in peak flows which would occur in downstream subbasins. If no increase is noted, then the “no harm evaluation” has been demonstrated. If no increase is observed in peak flows, the increased potential for erosion and/or sedimentation in downstream channels resulting from any change in the flood hydrograph predicted by the model shall be evaluated. If no increased potential can be demonstrated by appropriate technical means, then the “no harm evaluation” exemption may be requested; and
8. If an increase if peak flow is observed in any of the downstream subbasins or increased potential for erosion and/or sedimentation is indicated, the “no harm evaluation” exemption shall not be granted.
(Ord. 980, passed 3-16-1993; Ord. 991, passed 2-21-1995)