The design of all regulated activities shall conform to the following minimum standards:
A.
Post-development peak discharge rates for the one-, two-, five-, ten-, twenty-five-, fifty-, and 100-year recurrence interval design storms from project sites that are not within the Susquehanna River Tributaries Watershed (as delineated on Plates B-1 and B-2 in Appendix B of this chapter[1]) shall not exceed the respective pre-development peak discharge rates. Post-development peak discharge rates for the one-, two-, five-, ten-, twenty-five-, fifty-, and 100-year recurrence interval design storms from project sites (or portion of project sites) that are within the Susquehanna River Tributaries Watershed (as delineated on Plates B-1 and B-2 in Appendix B of this chapter) shall be subject to the allowable release rates that are tabulated in Tables C-1 and C-2 in Appendix C of this chapter.[2] Stormwater runoff shall be managed so that no downstream increases in flood damages or impairment of streets and other public facilities occur. The municipal engineer may require that downstream impacts be evaluated at critical locations such as dams, tributaries, existing developments, undersized culverts, and flood-prone areas. The municipality and its engineer shall make the final determination with respect to the degree of management required for any site. The applicant/developer shall evaluate the effects of the proposed stormwater management plan on such critical locations by providing computed water surface elevations (WSEL) for the ten- and 100-year recurrence interval design storms. Methods of computation shall have prior approval of the municipal engineer. At such downstream critical locations, stormwater management may be exercised by:
(1)
Providing off-site improvements to downstream conveyances in order to contain flow increases.
(2)
Providing downstream drainage easements with sufficient widths to contain the flood limits.
[1]
Editor's Note: Appendix B is included as an attachment to this chapter.
[2]
Editor's Note: Appendix C is included as an attachment to this chapter.
B.
Stormwater management plans for all regulated activities must include infiltration and stormwater quality BMPs. For regulated activities that do not require an NPDES construction permit, the design of infiltration and stormwater quality BMPs shall be in accordance with Appendix A, Section IV of this chapter. Otherwise, the design of infiltration and stormwater quality BMPs shall be in accordance with the NPDES construction permit. The municipality and its engineer may impose stormwater quality measures in accordance with the Pennsylvania Stormwater Best Management Practices Manual (PA DEP) to protect against ground or surface water pollution where the type of business or the nature of the stormwater runoff and soils underlying stormwater management facilities would constitute a substantial risk of contamination.
C.
In establishing the watershed conditions for calculating stormwater runoff prior to development, the following assumptions shall apply:
(1)
Woodland (in good condition) or meadow shall be used for all undeveloped areas.
(2)
Average antecedent moisture conditions as defined by the Natural Resource Conservation Service shall be used for all areas.
(3)
Drainage area reductions equal to the area of undrained depressions or pond factor adjustments in accordance with the Urban Hydrology for Small Watersheds, Technical Release No. 55 (TR-55, USDA, NRCS) procedure shall be applied in determining pre-development peak discharges from karst geologic areas.
D.
Hydrologic methods. All stormwater management plans shall be reviewed by the municipal engineer. Stormwater peak discharges and stormwater runoff volumes for all stormwater management facilities shall be computed using the Soil-Cover-Complex Method as set forth in Technical Release No. 55 (TR-55, USDA, NRCS) and Technical Release No. 20 (TR-20, USDA, NRCS), with specific attention given to antecedent moisture conditions, flood routing, and peak discharge specifications included therein, and in the National Engineering Handbook, Part 630 - Hydrology (USDA, NRCS, current edition).
(1)
Permissible runoff curve numbers for use in TR-55 and TR-20 and are identified in Table A-1 of Appendix A of this chapter. Composite runoff curve numbers shall be rounded to the nearest whole number.
(2)
The use of the Rational Method is strictly prohibited.
(3)
Any approved use of regression-based hydrologic methods shall be limited to computing design flows for bridges and other stream crossings.
(4)
Design rainfall amounts for the specified recurrence intervals shall be determined using the Precipitation-Frequency Atlas of the United States (NOAA Atlas 14, Volume 2). Rainfall duration for the design of all stormwater management facilities shall not be less than 24 hours, unless waived in writing by the municipal engineer. All hydrologic computations utilizing the Soil-Cover-Complex Method shall use a rainfall duration of not less than 24 hours, unless waived in writing by the municipal engineer. Rainfall distribution shall be SCS Type II, unless waived in writing by the municipal engineer.
(5)
Time of concentration shall be determined in accordance with methods presented in Chapter 15 of the National Engineering Handbook, Part 630 - Hydrology (USDA, NRCS, current edition). When using the Velocity Method to compute time of concentration, the maximum length for the sheet flow segment shall not exceed 100 feet.
(6)
Designs for stormwater management facilities that will be used to reduce stormwater runoff volumes shall be submitted to the municipal engineer for review. Infiltration facilities shall only be used in areas where soils, geologic, and water table conditions permit. Performance criteria pertaining to the location, design, construction, operation, and maintenance of volume reduction facilities are contained in Article III, § 118-28, and in Appendix A, Section IV of this chapter.
(7)
Infiltration/exfiltration shall not be included in any modified Puls routing computations.
E.
Stormwater management facilities and related installations shall be provided:
(1)
To permit unimpeded flow of natural watercourses. Such flow may be redirected as required, subject to the approval of PA DEP and the Municipality.
(2)
To insure adequate drainage of all low points along the curbline of streets.
(3)
To intercept stormwater runoff along streets at intervals reasonably related to the extent and grade of the area drained, and to prevent substantial flow of water across intersections or flooded intersections during storms, in accordance with the procedures contained in the Design Manual Part 2 - Highway Design, Publication 13M (PennDOT), Chapter 10.
(4)
To insure adequate and unimpeded flow of stormwater under driveways in, near, or across natural watercourses or drainage swales. Suitable pipes or other waterways shall be provided as necessary.
(5)
To properly drain stormwater runoff from all project sites. All lot and open areas shall be designed to drain to the nearest practical street or drainage system, existing or proposed, as defined by the municipal engineer, with no impact on adjoining properties, unless an area specifically designed for stormwater detention is provided.
F.
Storm sewers and related installations:
(1)
Storm sewers, where required by zoning and land use densities, shall be placed under or immediately adjacent to the edge of the roadway, or as directed by the Municipality and its engineer, when parallel to the street within the right-of-way. When located in undedicated land, they shall be placed within a drainage easement not less than 20 feet wide, as approved by the municipal engineer. The use of properly designed, graded, and turfed drainage swales is encouraged in lieu of storm sewers in commercial and industrial areas, and where approved by the municipal engineer, in residential areas. Such swales shall be designed not only to 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 contained in Appendix A, Section II of this chapter.
(2)
The design capacity of storm sewers shall be determined in accordance with Urban Drainage Design Manual, Hydraulic Engineering Circular Number 22 (US DOT, FHA). Any computer software used to compute the hydraulic grade line (HGL) within storm sewers shall be approved in advance by the municipal engineer. Storm sewers shall be designed to convey stormwater runoff to a stormwater detention/retention/infiltration facility without surcharging inlets. To avoid surcharging inlets, and to ensure that inlets will receive stormwater runoff, the HGL at any inlet(s) shall be at least six inches below the elevation of the top of the inlet grate. Where site grading will direct stormwater all runoff to a detention/retention/infiltration facility, then the stormwater conveyance system may be designed for the ten-year recurrence interval design storm. Where site grading will not direct stormwater runoff to a detention/retention/infiltration facility, then the stormwater conveyance system shall be designed for the 100-year recurrence interval design storm; and the hydrologic method used for the design of said detention/retention/infiltration facility shall also be used for the design of the stormwater conveyance system. Conveyance of storms to a detention/retention/infiltration facility, up to and including the 100-year recurrence interval design storm, shall be provided so as not to endanger life or damage property.
(3)
Storm inlet types and inlet assemblies shall conform to the Standards for Roadway Construction, Publication 72M (PennDOT) and the following criteria:
(a)
Inlets shall, at a minimum, be located at the lowest point of street intersections to intercept the stormwater before it reaches pedestrian crossings; or at sag points of vertical curves in the street alignment that provide a natural point of ponding of surface stormwater.
(b)
Where the Municipality and its engineer deem it necessary because of special land requirements, special inlets may be required.
(c)
The interval between inlets collecting stormwater runoff shall be determined in accordance with the Design Manual Part 2 - Highway Design, Publication 13M (PennDOT), Chapter 10, Section 5, "Capacity of Waterway Areas," or Urban Drainage Design Manual (HEC-22, US DOT, FHA).
(d)
In curbed sections, the maximum encroachment of water on the roadway pavement shall not exceed half of a through traffic lane or one inch less than the depth of curb during the ten-year recurrence interval design storm. Inlets shall be provided to limit the encroachment of water on the pavement. When inlets are used in a storm sewer system within the right-of-way limits of a roadway in lieu of manholes, the spacing of such inlets shall not exceed the maximum distance of 450 feet.
(e)
The design of storm inlets shall be in accordance with Drainage of Highway Pavements, Hydraulic Engineering Circular Number 12, (US DOT, FHA).
(f)
The designer shall evaluate the potential for clogging of storm inlets, and where the potential for clogging exists, the designer shall design storm inlets using clogging factors that have been approved in advance by the municipal engineer.
(4)
Accessible drainage structures shall be located on a continuous storm sewer system at all vertical dislocations, at all locations where a transition in storm sewer pipe sizing is required, at all vertical and horizontal angle points exceeding 5°, and at all points of convergence of two or more influent storm sewers.
(5)
When evidence available to the Municipality and its engineer indicates that existing storm sewers have sufficient capacity, as determined by hydrograph summation, and where such existing storm sewers are accessible, proposed stormwater management facilities may connect to these existing storm sewers as long as the peak discharge does not exceed the amount permitted by § 118-26A of this article.
G.
Bridges and culverts shall have ample waterway to carry expected flows, based on the following minimum design storms: ten-year recurrence interval for driveways; twenty-five-year recurrence interval for local streets; fifty-year recurrence interval for collector streets; and 100-year recurrence interval for arterials; or as otherwise required by the Municipality and its engineer. Bridge and/or culvert construction shall be in accordance with PennDOT specifications.
(1)
The design criteria contained in this article are intended for use in conjunction with the Chapter 105 Regulations of PA DEP entitled, "Water Obstructions and Encroachments." All information and regulations contained in Chapter 105 shall be considered to be incorporated into this article as if reproduced in full. A PA DEP permit in accordance with Chapter 105 shall be required for any obstruction or encroachment in regulated waters of the Commonwealth, prior to the approval of the stormwater management plan. All areas of the Municipality shall be classified as rural, suburban, or urban, as determined by the Municipality and its engineer (see PA DEP Section 105.161 for bridge and culvert designs). In the event any question or conflict arises between this article and the PA DEP Chapter 105 Regulations, the design criteria contained in the PA DEP regulations shall govern.
(2)
Refer to Appendix A, Section II of this chapter for additional design criteria.
H.
Detention, retention, or infiltration facilities for the management of stormwater peak discharges shall meet the following requirements:
(1)
Facilities shall be installed prior to or concurrent with any earth-disturbance activity. The phasing of their construction shall be noted on the stormwater management plan.
(2)
The design of all facilities over limestone formations shall include measures, designed by a professional geologist, to prevent groundwater contamination, and where required, sinkhole formation. Soils used for the construction of basins shall have moderate to low erodibility factors (i.e., "K" factors of 0.32 or less).
(3)
Energy dissipaters and/or level spreaders shall be installed at points where pipes or drainageways discharge to or from facilities.
(4)
Outlet structures within aboveground facilities shall incorporate childproof, nonclogging trash racks or grates over all horizontally oriented openings. All vertically oriented openings over 12 inches or larger in any dimension where entry by a child could cause injury or death should be covered with childproof, nonclogging trash racks, except where such openings carry perennial stream flows. Design openings less than six inches in any dimension should be covered with a pipe screen (e.g., Neenah R-7512 or equivalent). Measures to completely drain facilities in the event of clogging of the primary design opening(s) shall be incorporated into the design of outlet structures. Outlet pipes should have a minimum inside diameter of 15 inches (or a cross-sectional area of 176 square inches), except that pipes under 25 feet or greater fill should not be less than 24 inches (or a cross-sectional area of 453 square inches), and should be reinforced concrete pipe. Outlet aprons shall be designed and shall extend, at a minimum, to the toe of slope within an aboveground facility. Where spillways will be used to manage peak discharges in excess of the ten-year recurrence interval design storm, such spillways shall be constructed to withstand the pressures of impounded waters, and convey flows at computed outlet velocities without erosion. Stormwater management facilities shall be designed to release their total volume within the following maximum time periods:
(5)
When PA DEP requires facilities to be permitted, the designer shall submit all information to the PA DEP Regional Office, and obtain all necessary approvals and permits pursuant to Pennsylvania Code, Title 25, Chapter 105, Dam Safety and Encroachment Act.
(6)
Downstream analysis:
(a)
Where deemed necessary by the Municipality and its engineer, the applicant/developer shall submit an analysis of the impacts of detained stormwater flows on downstream areas within the watershed. The analysis shall include hydrologic and hydraulic calculations necessary to determine the impact of peak discharge modifications on critical locations such as dams, tributaries, existing developments, undersized culverts, and floodprone areas.
(b)
Review and comment of the analysis by the engineer of a downstream municipality shall be obtained as deemed necessary by the municipality.
(7)
Stormwater detention/retention/infiltration facilities may be waived by the Municipality at sites in close proximity to large receiving streams, depending on the hydrology of the watershed. It shall be incumbent upon the applicant/developer to demonstrate that no downstream increase in stream flooding or channel erosion will result, in accordance with § 118-26H(6) of this article, and that no increases in peak discharge within the receiving stream will occur, as outlined under § 118-26A of this article.
(8)
Multiple-use facilities. The design and construction of multiple-use aboveground stormwater management facilities are strongly encouraged. In addition to stormwater management, multiple-use facilities, where appropriate, allow for recreational uses including: ballfields, play areas, picnic grounds, etc. Provisions for parking facilities within aboveground stormwater management facilities basins may also be appropriate. The applicant/developer shall consult with the Municipality and its engineer prior to designing multiple-use facilities. Multiple-use facilities should be constructed so that potentially dangerous conditions are not created.
(9)
Multiple development facilities. Stormwater detention/retention/infiltration facilities designed to serve more than one property or development in the same watershed are encouraged. The applicant/developer shall consult with the Municipality and its engineer prior to designing multiple-development facilities.
(10)
Alternative facilities. Alternative stormwater detention/retention/infiltration facilities including roof top storage, subsurface basins or tanks and in-pipe detention storage, or other approved alternative designs that may be permitted by the Municipality and its engineer are encouraged.
(11)
Specific criteria related to the design of stormwater detention/retention/infiltration facilities is contained in Appendix A, Section III of this chapter.
I.
All calculations shall be submitted to the municipal engineer on computation sheets for approval. If the municipal engineer determines through review and independent computation that the size(s) of any stormwater management facility/Facilities is/are insufficient, the Municipality and its engineer may require the applicant/developer to increase the size(s) of said stormwater management facility/Facilities. If the design of the stormwater management facilities is completed using computer software, sufficient supporting data shall be provided to allow comprehensive review by the municipal engineer.
J.
When the elevation of any existing or proposed entrance to a structure, including windows, is lower than the elevation of the public cartway serving that site, a grading plan shall be submitted to the Municipality for review and approval as part of the zoning permit process for the proposed structure.