(a) General Standards. 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 City of Sharon:
(1) Any landowner and any person engaged in 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:
A. To assure that the maximum rate of stormwater runoff is no greater after development than prior to development activities; or
B. 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 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.
(b) Specific Standards.
(1) Application; conflict of laws. The stormwater performance standards contained in this section are intended to be applied in all areas of the City of Sharon where no approved Act 167 Watershed Storm Water Management Plan is in effect and to implement the standards and criteria for areas for which such Act 167 Storm Water Management Plans have been adopted and approved in accordance with the Pennsylvania Storm Water Management Act. If there is any discrepancy between provisions of this section and the standards and criteria of such approved 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, twenty-five and 100-year storm frequencies. The SCS twenty-four hour, Type II Rainfall Distribution shall be used for analyzing stormwater runoff for both pre- and post-development conditions. The twenty-four hour total rainfall for these storm frequencies in the watershed are:
Storm Frequency | Rainfall Depth (in.) |
2-year | 2.28 |
10-year | 3.84 |
25-year | 4.80 |
100-year | 6.24 |
(For additional information or data on other storm return periods, consult the Storm Intensity - Duration - Frequency Curves, as published by the Pennsylvania Department of Transportation Storm Intensity - Duration - Frequency Charts, May 1986 edition or its successor publication or standard.)
(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 the SCS Technical Release (TR) 55 Urban Hydrology for Small Watersheds.
B. Stormwater collection/conveyance facilities. For the purpose of designing storm sewers, open swales and other stormwater runoff collection and conveyance facilities, the rational method shall be applied. Rainfall intensities for design should be obtained from the Pennsylvania Department of Transportation rainfall charts.
C. Predevelopment conditions. Predevelopment conditions shall be assumed to be those which exist on any site prior to the start of land alteration and/or development. 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) shall be applied for all pervious land uses within the respective range for each land use and hydrologic soil group
(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 development. It applies uniformly to all land development or alterations within a subarea. In the absence of an approved Act 167 Stormwater Management Plan a uniform release rate percentage of 100 shall be applied. In areas for which an approved Act 167 Plan is in effect, the appropriate release rate percentage defined in the plan shall be applied.
B. Procedure for use.
1. Determine the appropriate release rate percentage.
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, twenty-five 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 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 predevelopment 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 predevelopment rate of runoff from the development site to determine the maximum allowable release rate from any detention facility for the four prescribed storm events.
(Ord. 14-91. Passed 8-28-91.)