(A) Stormwater runoff from all development sites shall be calculated using either the rational method or the soil-cover-complex methodology.
(B) (1) Infiltration BMP loading rate percentages in the Recommendation Chart for Infiltration Stormwater Management BMPs in Carbonate Bedrock in Appendix D shall be calculated as follows:
Area Tributary to infiltration BMP Base area of infiltration BMP * 100% |
(2) The area tributary to the infiltration BMP shall be weighted as follows:
(a) All disturbed areas to be made impervious: weight at 100%;
(b) All disturbed areas to be made pervious: weight at 50%;
(c) All undisturbed pervious areas: weight at 0%; and
(d) All existing impervious areas: weight at 100%.
(C) (1) Soil thickness is to be measured from the bottom of any proposed infiltration system. The effective soil thickness in the Recommendation Chart for Infiltration Stormwater Management BMPs in Carbonate Bedrock in Appendix D is the measured soil thickness multiplied by the thickness factor based on soil permeability (as measured by the adapted 25 Pa. Code § 73.15) percolation test in Appendix G), as follows:
Permeability Range* | Thickness Factor |
6.0 to 12.0 inches/hour | 0.8 |
2.0 to 6.0 inches/hour | 1.0 |
1.0 to 2.0 inches/hour | 1.4 |
0.75 to 1.0 inches/hour | 1.2 |
0.5 to 0.75 inches/hour | 1.0 |
* If the permeability rate (as measured by the adapted 25 Pa. Code § 73.15 percolation test in Appendix G) falls on a break between two thickness factors, the smaller thickness factor shall be used | |
(2) Sites with soil permeability greater than 12.0 inch/hour or less than one-half inch/hour, as measured by the adapted 25 Pa. Code § 73.15, percolation test in Appendix G, are not recommended for infiltration.
(D) The design of any detention basin intended to meet the requirements of this chapter shall be verified by routing the design storm hydrograph through the proposed basin using the storage indication method or other methodology demonstrated to be more appropriate. For basins designed using the Rational Method technique, the design hydrograph for routing shall be either the Universal Rational Hydrograph or the Modified Rational Method trapezoidal hydrograph which maximizes detention volume. Use of the Modified Rational hydrograph shall be consistent with the procedure described in section “PIPE.RAT” of the Users’ Manual for the Penn State Urban Hydrology Model (1987).
(E) BMPs designed to store or infiltrate runoff and discharge to surface runoff or pipe flow shall be routed using the storage indication method.
(F) BMPs designed to store or infiltrate runoff and discharge to surface runoff or pipe flow shall provide storage volume for the full WQv below the lowest outlet invert.
(G) Wet detention ponds designed to have a permanent pool for the WQv shall assume that the permanent pool volume below the primary outlet is full at the beginning of design event routing for the purposes of evaluating peak outflows.
(H) All stormwater detention facilities shall provide a minimum one foot freeboard above the maximum pool elevation associated with the two- through 25-year runoff events. A one-half-foot freeboard shall be provided above the maximum pool elevation of the 100-year runoff event. The freeboard shall be measured from the maximum pool elevation to the invert of the emergency spillway. The two- through 100- year storm events shall be controlled by the primary outlet structure. An emergency spillway for each basin shall be designed to pass the 100-year return frequency storm peak basin inflow rate with a minimum one- half foot freeboard measured to the top of basin. The freeboard criteria shall be met considering any off-site areas tributary to the basin as developed, as applicable. If this detention facility is considered to be a dam as per DEP regulations, 25 Pa. Code Chapter 105 the design of the facility must be consistent with the Chapter 105 regulations, and may be required to pass a storm greater than the 100-year event.
(I) The minimum circular orifice diameter for controlling discharge rates from detention facilities shall be three inches. Designs where a lesser size orifice would be required to fully meet release rates shall be acceptable with a three-inch orifice provided that as much of the site runoff as practical is directed to the detention facilities. The minimum three-inch diameter does not apply to the control of the WQv.
(J) (1) Runoff calculations using the soil-cover-complex method shall use the Natural Resources Conservation Service Type II 24-hour rainfall distribution. The 24-hour rainfall depths for the various return periods to be used consistent with this chapter may be taken from NOAA Atlas 14, Volume 2 Version 2.1, 2004 or the PennDOT Intensity, Duration, Frequency Field Manual (“PDT - IDF”) (May 1986) for Region 4. The following values are taken from the PDT-IDF Field Manual:
Return Period | 24-Hour Rainfall Depth |
Return Period | 24-Hour Rainfall Depth |
1-year | 2.40 inches |
2-year | 3.00 inches |
5-year | 3.60 inches |
10-year | 4.56 inches |
25-year | 5.52 inches |
50-year | 6.48 inches |
100-year | 7.44 inches |
(2) A graphical and tabular presentation of the Type II 24 hour distribution is included in Appendix C.
(K) Runoff calculations using the Rational Method shall use rainfall intensities consistent with appropriate times of concentration and return periods and NOAA Atlas 14, Volume 2 Version 2.1, 2004 or the intensity-duration-frequency curves as presented in Appendix C.
(L) Runoff curve numbers (CNs) to be used in the soil-cover-complex method shall be based upon the matrix presented in Appendix C.
(M) Runoff coefficients for use in the Rational Method shall be based upon the table presented in Appendix C.
(N) All time of concentration calculations shall use a segmental approach which may include one or all of the flow types below.
(1) (a) Sheet flow (overland flow) calculations shall use either the NRCS average velocity chart (Figure 3-1, Technical Release-55, 1975) or the modified kinematic wave travel time equation (equation 3-3, NRCS TR55, June 1986).
(b) If using the modified kinematic wave travel time equation, the sheet flow length shall be limited to 50 feet for designs using the Rational Method and limited to 150 feet for designs using the soil-cover-complex method.
(2) Shallow concentrated flow travel times shall be determined from the watercourse slope, type of surface and the velocity from Figure 3-1 of TR-55, June 1986.
(3) (a) Open channel flow travel times shall be determined from velocities calculated by the Manning Equation. Bankfull flows shall be used for determining velocities.
(4) Pipe flow travel times shall be determined from velocities calculated using the Manning Equation assuming full flow and the Manning ‘n’ values from Appendix C.
(O) If using the Rational Method, all pre-development calculations for a given discharge direction shall be based on a common time of concentration considering both on-site and any off-site drainage areas. If using the Rational Method, all post-development calculations for a given discharge direction shall be based on a common time of concentration considering both on-site and any off-site drainage areas.
(P) The Manning Equation shall be used to calculate the capacity of watercourses. Manning ‘n’ values used in the calculations shall be consistent with the table presented in Appendix C or other appropriate standard engineering ‘n’ value resources. Pipe capacities shall be determined by methods acceptable to the borough.
(Q) (1) The Pennsylvania DEP, Chapter 105, Rules and Regulations, apply to the construction, modification, operation or maintenance of both existing and proposed dams, water obstructions and encroachments throughout the watershed.
(2) Criteria for design and construction of stormwater management facilities according to this chapter may differ from the criteria that are used in the permitting of dams under the Dam Safety Program.
(Ord. 226-2007, passed 5-7-2007)