(a) Storm water runoff from all project sites shall be calculated using a generally accepted calculation technique that is based on the NRCS soil cover complex method. Table 306-1 summarizes acceptable computation methods and the method selected by the qualified professional shall be based on the individual limitations and suitability of each method for a particular site.
Table 306-1
ACCEPTABLE COMPUTATION METHODOLOGIES FOR STORM WATER MANAGEMENT PLANS
Method | Method Developed By | Applicability |
TR-20/WINTR20 (or commercial computer package based on TR-20) | USDA NRCS | Applicable where use of full hydrology computer model is desirable or necessary. |
TR-55/WINTR55 (or commercial computer package based on TR-55) | USDA NRCS | Applicable for land development plans within limitations described in TR-55. |
HEC-HMS | U.S. Army Corps of Engineers | Applicable where use of full hydrologic computer model is desirable or necessary. |
Rational Formula (or commercial computer package based on Rational Formula) | Emil Kuichling (1889) | For sites less than 50 acres and with time of concentration less than 60 minutes (TC < 60 min), or as approved by the Municipality |
Other methods such as SWMM, WMS, etc. | Varies | Other computation methodologies approved by the Municipality |
Note: Successors to the above methods are also acceptable. | ||
(b) All calculations consistent with this chapter using the soil cover complex method shall use the appropriate design rainfall depths and intensities for the various return period storms according to the approximate center of the proposed development site, in accordance with the values obtained from the National Oceanic and Atmospheric Administration's (NOAA) Hydrometeorological Design Studies Center Precipitation Frequency Data Server (PFDS) at the following location for the Commonwealth of Pennsylvania:
Applicant shall provide documentation of PFDS data location (latitude and longitude in degrees/minutes/seconds).
(c) All calculations using the Rational Formula shall use rainfall intensities consistent with appropriate times-of-concentration for overland flow and return periods from the NOAA, PFDS website, the design storm curves from PennDOT Publication 13M, Design Manual Part 2, Chapter 10, and NOAA Atlas 14.
(d) Times-of-concentration for overland flow shall be calculated using the methodology presented in Chapter 3 of Urban Hydrology for Small Watersheds, NRCS, TR-55 (as amended or replaced from time to time by NRCS). Times-of-concentration for channel and pipe flow shall be computed using Manning's equation. NRCS lag equation divided by 0.6 as acceptable method for Tc in undeveloped areas.
(e) In order to reduce storm water runoff volumes from developed areas and encourage ground water recharge, underground basin drains, infiltration trenches, dry wells, and cisterns are permitted to which roof leaders may be connected. These drains consist of stone-filled basins that temporarily store and release water below ground surface. Plans for such facilities shall be submitted to the Municipality for approval, and the basins shall be used only in those areas where soils, geologic, and water table conditions permit.
(f) Runoff curve numbers (CN) for both existing and proposed conditions to be used in the soil cover complex method shall be obtained from Table 2-2 of the TR-55 manual.
(g) Runoff coefficients (C) for both existing and proposed conditions for use in the rational formula are provided in the Appendices attached to Ordinance 2458.
(h) All flow assumptions and source of supporting data shall be provided as part of the overall plan. The Municipality reserves the right to reject any submitted values, despite the source, and to provide a substitute source for use by the applicant.
(i) Where uniform flow is anticipated, the Manning equation shall be used for hydraulic computations, and to determine the capacity of open channels, pipes, and storm sewers. Values for Manning's roughness coefficient (n) shall be consistent with generally accepted values from a legitimate and verifiable source. All flow assumptions and source of supporting data shall be provided as part of the overall plan. The Municipality reserves the right to reject any submitted values, despite the source, and to provide a substitute source for use by the applicant. Full flow capacity shall be assumed for closed conduits. Storm sewer systems consisting of more than three pipe junctions shall be designed using hydraulic grade line computations.
(j) (1) Outlet structures for storm water management facilities shall be designed to meet the performance standards of this chapter using any generally accepted hydraulic analysis technique or method.
(2) The design of any storm water 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. For drainage areas greater than 200 acres in size, the design storm hydrograph shall be computed using a calculation method that produces a full hydrograph (i.e. TR-20, TR-55, and HEC-HMS).
(k) Storm water management and related facilities shall be provided:
(1) To permit unimpeded flow of natural watercourses. Such flow may be redirected as required, subject to the approval of the Pennsylvania Department of Environmental Protection and the Municipality.
(2) To ensure adequate drainage of all street low points.
(l) Storm sewers and related installations:
(1) When located in undedicated land, they shall be placed within a drainage easement not less than 20 feet wide as approved by the Municipality.
(2) The use of properly designed, graded, and vegetated drainage channels is encouraged in lieu of storm sewers in commercial and industrial areas and, where approved by the Municipality, in residential areas. Such swales shall be designed to not only carry the required discharge without excessive erosion, but also to increase the time of concentration, reduce the peak discharge and velocity, and permit the water to percolate into the soil, where appropriate. Criteria related to the use and design of drainage swales are as follows:
A. The maximum encroachment of water on the roadway pavement along roadside swales in cut areas shall not exceed half of a through traffic lane during a ten-year frequency storm of five minute duration. Frequent and/or sustained flooding of the sub-base shall be avoided.
B. The design of all vegetated channels shall, as a minimum, conform to the design procedures outlined in the Erosion and Sediment Pollution Control Program Manual (PA DEP). Inlets shall be provided to limit road shoulder encroachment and water velocity.
C. The side slope for any vegetated drainage channel requiring mowing of the vegetation shall have a maximum grade of three horizontal to one vertical on those areas to be mowed. Maximum side slopes for any vegetated drainage channel shall be two horizontal to one vertical.
D. Erosion prevention: All drainage swales shall be designed to prevent the erosion of the bed and bank areas. Suitable temporary and/or permanent stabilization during vegetative cover establishment shall be provided to prevent erosion.
E. Storm sewers or drainage swales shall discharge to a detention or retention basin to attenuate the peak rate and volume, respectively of storm water runoff, except as provided in the plan.
(3) Storm inlet types and inlet assemblies shall conform to the Pennsylvania Department of Transportation Standards for Roadway Construction as approved by the Municipality.
(4) When evidence available to the Municipality indicates that existing storm sewers have sufficient capacity as determined by hydrograph summation and are accessible, proposed storm water facilities may connect to the existing storm sewers so long as the peak rate of discharge does not exceed the amount permitted by this chapter.
(m) Downstream Analysis.
(1) Where deemed necessary by the Municipal Engineer, the applicant shall submit an analysis of the impacts of detained storm water flows on downstream areas within the watershed, established with the concurrence of the Municipal Engineer. The analysis shall include hydrologic and hydraulic calculations necessary to determine the impact of peak discharge modifications of the proposed development on critical locations such as dams, tributaries, existing developments, undersized culverts, and flood- prone areas. Review and comment of the analysis by the engineer of a downstream Municipality shall be obtained as deemed necessary.
(n) Multiple Use Basins. The design and construction of multiple use storm water detention facilities are strongly encouraged. In addition to storm water management; where appropriate, facilities allow for recreational uses including: ball fields, play areas, picnic grounds, etc. Provision for parking facilities within basins and permanent wet ponds with storm water management capabilities may also be appropriate. Prior approval and consultation with the Municipality are required before design. Multiple use basins shall be constructed so that potentially dangerous conditions are not created.
(o) Alternative Detention Facilities. Alternative storm water detention facilities including roof top, subsurface basins or tanks and in-pipe detention storage, or other approved alternative designs are permitted as determined by the Municipality.
(p) Landscaping of Storm Water Management Facilities. Facilities constructed with berms or earthen embankments shall not be landscaped along the top of the impoundment berm, embankment, nor shall other facility areas constructed from compacted fill materials be landscaped. Heavy vegetative cover root penetration can cause soil weakening and damage to facility piping.
(Ord. 2458. Passed 11-15-10.)