(A) Runoff rates. There shall be no increase in the peak rate of stormwater runoff discharge from any activity covered by the ordinance following the completion of the activity (post-development conditions) over the rate that would have occurred from the land prior to the activity (pre-development conditions); this criteria shall apply to the total activity even if the activity is to take place in stages.
(1) Off site areas which drain through a proposed development site are not subject to the control standard when determining allowable peak runoff rates; however, on-site drainage facilities shall be designed to safely convey off-site flows through the development site.
(2) Where the site area to be impacted through a proposed development activity differs significantly from the total site area, only the proposed impact area shall be subject to the runoff criteria.
(B) 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 USDA NRCS 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 | 2.6 |
10-year | 3.8 |
25-year | 4.6 |
100-year | 6.1 |
(C) Calculation methods.
(1) 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: USDA NRCS publications, Technical Release (TR) 55 or 20, HEC I or Penn State Runoff Model (PSRM) or Modified Rational Method. Under special circumstances other computation methods may be used subject to the approval of the municipality.
(2) Stormwater collection/conveyance facilities. For the purposes of designing storm sewers, open swales and other stormwater runoff collection and conveyance facilities, the Rational Method or other method as approved by the municipality may be applied. Rainfall intensities for design should be obtained from the State Department of Transportation rainfall charts.
(3) Routing of hydrographs through detention/retention facilities. Routing of hydrographs through detention/retention facilities for the purpose of designing those facilities shall be accomplished using the Modified-Puls Method or other recognized reservoir routing method subject to the approval of the municipality.
(4) Predevelopment conditions. Predevelopment conditions shall be assumed to be those which exist on any site at the time prior to the commencement of development activities. SCS runoff curve numbers selected for use in the calculations shall accurately reflect existing conditions subject to the approval of the municipality. At its discretion, the municipality may direct that 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. Impervious cover shall include, but not be limited to, any roof, parking or driveway areas, and any new streets and sidewalks. For post-development conditions, any areas designed to initially be gravel or crushed stone shall be assumed to be impervious for the purposes of these criteria. The municipality has the authority to require that computed existing runoff rates be reconciled with field observations and conditions. If the designer can substantiate through actual physical calibration that more appropriate runoff values should be used at a particular site, then appropriate variations may be made upon review and recommendations of the Municipal Engineer. Calibration shall require detailed gauge and rainfall data for the particular site in question.
(D) Post-development rates of runoff. Post-development rates of runoff shall not exceed the peak rates of runoff prior to development for the two-, ten-, 25 and 100-year design storms.
(Ord. 4-2011, passed 10-8-2011)