Township of Ralpho, PA Design Standards and Criteria.

[Ord. 114, 10/13/1996, Art. III, § 1; as amended by Ord. 119, 4/14/1998]

All subdivision and land development activities involving an increase in impervious cover (i.e., reduction in permeability) shall be conducted in conformance with the following standards:

After installation of impervious cover, peak discharges for the one-, ten-, and one-hundred-year frequency storms from the site shall not exceed the respective peak discharge rates before development for all drainage areas and subareas. Stormwater runoff shall be controlled so that no downstream increase in flood damage or impairment of streets and other public facilities occur. The Township Engineer may require that downstream impacts be evaluated at critical locations such as dams, tributaries, existing developments, undersized culverts and flood-prone areas. The applicant shall evaluate the effects of the proposed plan on such critical locations by providing computed water surface elevations (WSEL) for the ten-, and one-hundred-year storms. Methods of computation shall have prior approval of the Township Engineer. At such downstream critical locations, stormwater control 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. The Township and its Engineer shall make the final determination with respect to the degree of control required for any site.

Groundwater Recharge. The Township may impose water quality control measures in accordance with Appendix A, § IV, to protect against ground or surface water pollution where the type of business or the nature of the runoff and soils, underlying stormwater control facilities would constitute a substantial risk of contamination.

In establishing the watershed conditions for calculating runoff prior to development, the following assumptions shall apply:

(1)

Woodland or meadow in good condition shall be used for all undeveloped areas.

(2)

Average antecedent moisture conditions as defined by the Soil Conservation Service.

(3)

Drainage area reductions equal to the area of undrained depressions or pond factor adjustments in accordance with the SCS TR 55 procedure shall be applied in determining predevelopment peak discharges from Karst geologic areas.

All plans and designs for stormwater management facilities shall be reviewed by the Township Engineer. Plans for facilities other than storm sewers should determine stormwater peak discharge and runoff by the use of the Soil Cover Complex Method as set forth in Urban Hydrology for Small Watersheds, Technical Release No. 55 (TR55), and Technical Release No. 20 (TR20), latest revisions, with specific attention given to antecedent moisture conditions, flood routing and peak discharge specifications included therein, and National Hydrology Engineering Handbook, § 4, both by the U.S. Department of Agriculture, Soil Conservation Service. The Township Engineer may permit the use of the Modified Rational Method or other methods for calculation of the storage capacity of a stormwater control facility from drainage areas of 50 acres or less.

(1)

Acceptable runoff coefficient values for use in the rational method, and permissible curve numbers for TR 55, maximum velocities, and suggested roughness coefficients and permissible velocities for channels are identified in Tables A-2 through A-5 of Appendix A, § I.

(2)

The rational method may be used in lieu of the soil cover complex method to compute design flows for the sizing of storm sewers, inlets and swales. Methods approved by the Pennsylvania Department of Transportation and/or Environmental Protection may be used to design the waterway areas of bridges.

(3)

Rainfall amounts for the return periods specified shall be determined using the Pennsylvania Department of Transportation Intensity Duration Frequency Curves presented in Appendix A, § I, as Figure A-1. Rainfall duration for hydrograph generation shall be selected for the specified recurrence intervals on the basis of twice the computed time of concentration for the given watershed and subwatersheds. In no case shall the duration be less than one hour.

(4)

In order to reduce runoff volumes from developed areas and encourage groundwater recharge, underground basin drains, infiltration trenches and cisterns are permitted to which roof leaders may be connected. These drains consist of stone-filled basins which temporarily store and release water below ground surface. Plans for such facilities shall be submitted to the Township for approval, and the basins shall be used only in those areas where soils, geologic and water table conditions permit. Performance criteria which govern the location, design, construction and maintenance of these infiltration facilities are contained in Appendix A, § IV. Suggested guidelines are contained in "Standards and Specifications for Infiltration Practices," Maryland Department of Natural Resources, Water Resources Administration, February 1984.

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 the Department of Environmental Protection and the Township.

(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 Design Manual Part 2 (DM-2), Chapter 10, of the Pennsylvania Department of Transportation.

(4)

To ensure 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 land development projects. 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 Township Engineer, with no impact on adjoining properties, unless an area specifically designed for stormwater detention is provided.

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 roadway side of the curb, or as directed by the Township, 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 Township Engineer. The use of properly designed, graded and surfed drainage swales is encouraged in lieu of storm sewers in commercial and industrial areas and, where approved by the Township 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, § II.

(2)

The design capacity of storm sewer and drainage swales shall be determined in accordance with the ten-year frequency storm of the duration equal to the time of concentration. Storm drainage systems shall be designed without surcharging inlets to provide controlled conveyance of the ten-year storm into a detention basin or similar facility utilized to control the rate of runoff. Conveyance of storms to the stormwater pond, up to and including the one-hundred-year frequency, shall be provided so as not to endanger life or seriously damage property.

(3)

Inlet types and inlet assemblies shall conform to the Pennsylvania Department of Transportation Standards for Roadway Construction as approved by the Township Engineer.

(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 which provide a natural point of pending of surface stormwater.

(b)

Where the Township deems it necessary because of special land requirements, special inlets may be approved.

(c)

The interval between inlets collecting stormwater runoff shall be determined in accordance with DM-2, Chapter 10, Section 5, "Capacity of Waterway Areas." 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 design storm of five minute duration. Inlets shall be provided to control the encroachment of water on the pavement. When inlets are used in a storm system within the right-of-way limits of a street in lieu of manholes, the spacing of such inlets shall not exceed the maximum distance of 450 feet.

(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 sewer mains. The construction locations of accessible drainage structures shall be as indicated on the subdivision drainage plan or area drainage plan approved by the Township.

(5)

When evidence available to the Township indicates that existing storm sewers have sufficient capacity as determined by hydrograph summation and are accessible, the subdivider may connect their stormwater facilities to the existing storm sewers so long as the peak rate of discharge does not exceed the amount permitted by Subpart C, § 121, Subsection A, of this Part.

Bridges and culverts shall have ample waterway to carry expected flows, based on a minimum storm frequency of 10 years for driveways; 25 years for local streets; 50 years for collector streets; and 100 years for arterials; or as required by the Township Engineer. Bridge and/or culvert construction shall be in accordance with the Pennsylvania Department of Transportation specifications.

(1)

The design criteria contained in this Subpart are intended for use in conjunction with the Chapter 105, Regulations of the Pennsylvania Department of Environmental Protection entitled "Water Obstructions and Encroachments." All information and regulations contained in Chapter 105 shall be considered to be incorporated into this Subpart as if reproduced in full. A DEP permit in accordance with Chapter 105 shall be required for any obstruction or encroachment in the regulated waters of the commonwealth, prior to the approval of the stormwater plan. All areas of the Township shall be classified as rural, suburban or urban, as determined by the Township Engineer (See DEP § 105.161) for bridge and culvert designs. In the event any question or conflict arises between this Part and the DEP Chapter 105 Regulations, the design criteria contained in the DEP regulations shall govern.

(2)

Refer to Appendix A, § II, for additional design criteria.

Detention or retention basins for the control of stormwater peak discharges shall meet the following requirements:

(1)

Basins shall be installed prior to or concurrent with any earthmoving or land disturbances which they will serve. The phasing of their construction shall be noted in the narrative and on the plan.

(2)

Energy dissipators and/or level spreaders shall be installed at points where pipes or drainageways discharge to or from basins. Generally, outlet pipes designed to carry the predevelopment, one-year storm flow will be permitted to discharge to a stream with only an energy dissipator; discharges to drainage swales shall be spread with a level spreader or piped to an acceptable point of discharge downstream.

(3)

Outlet structures within detention/retention basins shall be constructed of reinforced concrete or an approved alternate. With the exception of those openings designed to carry perennial stream flows, design openings shall have childproof, nonclogging trash racks over all openings 12 inches or larger in any dimension. Outlet aprons shall be designed and shall extend at a minimum to the toe of the basin slope. Where spillways will be used to control peak discharges in excess of a ten-year storm, the control weirs shall be constructed to withstand the pressures of impounded waters and convey flows at computed outlet velocities without erosion.

(a)

Detention facilities shall be designed to release their total volumes detained within the following maximum time periods:

	Roofs, Parking Lots	24 hours
	Detention Basins	48 hours
	Infiltration Facilities	72 hours

(4)

When the Pennsylvania Department of Environmental Protection (DEP) requires facilities to be permitted, the designer shall submit all information to the DEP and obtain all necessary approvals and permits. Such facilities shall not create health or sanitation problems. Consideration shall be given to safety standards.

(5)

Downstream Analysis.

(a)

Where deemed necessary by the Township Engineer, the applicant shall submit an analysis of the impacts of detained stormwater flows on downstream areas within the watershed, established with the concurrence of the Township 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.

(b)

Review and comment of the analysis by the Engineer of a downstream municipality shall be obtained when stormwater management facilities are proposed within 1,000 feet of the affected downstream municipality.

(6)

Detention basins may be waived by the Township, upon recommendation of the Township Engineer, at sites in close proximity to larger receiving streams, depending on the hydrology of the watershed. This is to facilitate drainage prior to main stream flooding. It shall be incumbent upon the applicant to demonstrate that no downstream increase in stream flooding or channel erosion will result in accordance with §§ 121, Subsection H(5), above, and that no increases in peak discharge within the receiving stream will occur as outlined under § 121, Subsection A.

(7)

Multiple Use Basins. The design and construction of multiple use stormwater detention facilities are strongly encouraged. In addition to stormwater management, facilities should where appropriate allow for recreational uses including: ballfields, play areas, picnic grounds, etc. Provision for parking facilities within basins and permanent wet ponds with stormwater management capabilities may also be appropriate. Prior approval and consultation with the Township are required before design. Multiple use basins should be constructed so that potentially dangerous conditions are not created.

(8)

Multiple Development Basins. Stormwater management facilities designed to serve more than one property or development in the same watershed are encouraged. Staged construction of existing or proposed multiple-use detention facilities by several developers in conjunction with watershed development is encouraged. Each developer shall be responsible for the incremental increase in runoff generated by the respective development and incremental construction improvements necessary for the overall detention facility. Prior approval and consultation with the Township is required before design of such facilities.

(9)

Alternative Detention Facilities. Alternative stormwater 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 Township Engineer.

(10)

Specific criteria related to the design of detention basins is contained in Appendix A, § III.

All calculations shall be submitted to the Township Engineer on computation sheets for approval. If the Township Engineer determines through review and independent computation that the size of pipes or holding ponds is insufficient, the Township may require the developer to increase the size of pipes or capacity of the holding pond. If the storm drainage system design is completed on a computer installation, sufficient supporting data shall be provided to allow comprehensive review by Township officials.

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, reviewed and approved as part of the zoning permit process for the proposed structure.

Natural drainageways shall be utilized to the maximum extent possible in carrying stormwater runoff, provided such use remains consistent with the purpose of this Part specified in Subpart A, § 103.

Stormwater management facilities located outside of existing or proposed rights-of-way shall be located within and accessible by easements as follows:

(1)

Drainage Easements. Where a tract is traversed by a watercourse, drainageway, channel or stream, there shall be provided a drainage easement paralleling the line of such watercourse, drainageway, channel or stream. The width of the drainage easement will be adequate to preserve the unimpeded flow of natural drainage in the one-hundred-year floodplain, in accordance with computed top widths for water surface elevations determined under § 121, Subsection A, of this Subpart. Drainage easements shall provide for occasional maintenance for the purpose of widening, deepening, improving or protecting such drainage facilities.

(2)

Access Easements. Where proposed stormwater management facilities are not adjacent to proposed or existing public rights-of-way or are not accessible due to physical constraints, as determined by the Township Engineer, a twenty-foot wide passable access easement specifying rights of entry shall be provided. Access easements shall provide for vehicle ingress and egress on grades of less than 10% for carrying out inspection or maintenance activities.

(3)

Maintenance Easements. A maintenance easement shall be provided which encompasses the stormwater facility and appurtenances and provides for access for maintenance purposes. The maintenance easement must be located at least 20 feet outside of the one-hundred-year surface elevation and the stormwater facility and appurtenances.

(4)

Easements shall stipulate that no trees, shrubs, structures, excavation or fill be placed, and no regrading be performed within the area of the easement without written approval from the Township upon review by the Township Engineer. Upon approval of the Township Engineer, such landscaping may be placed in maintenance easements, provided it does not impede access.

(5)

Whenever practicable, easements shall be parallel with and conjunctive to property lines of the subdivision.

(6)

All easement agreements shall be recorded with a reference to the recorded easement indicated on the site plan. The format and content of the easement agreement shall be reviewed and approved by the Township Engineer and Solicitor.