1. Storm drainage systems shall be provided in order to:
A. Permit unimpeded flow of natural water course except as modified by stormwater detention facilities required by § 22-406(3) or open channels pursuant to § 22-406(2)(H);
B. Ensure adequate drainage of all low points along the line of streets;
C. Intercept stormwater runoff along streets at intervals related to the extent and grade of the area drained;
D. Provide positive drainage away from on-site sewage disposal systems;
E. Take surface water from the bottom of vertical grades, lead water from springs and avoid excessive use of cross-gutters at street intersections and elsewhere;
F. Prevent overloading of drainage systems and watercourses downstream as a result of increased rate or runoff caused by the proposed development.
2. General Requirements.
A. A site drainage plan for the proposed subdivision tract shall be prepared which illustrates the following information:
(1) Mapping of the watershed area or areas in which the proposed subdivision is located;
(2) Calculations of runoff for all points of runoff concentration within the site;
(3) Complete drainage systems for the subdivision. All existing drainage features which are to be incorporated in the design shall be so identified. If the subdivision is to be developed in stages, a general drainage plan for the entire subdivision shall be presented with the first stage and appropriate development stages for the drainage system shall be indicated.
B. The existing points of natural drainage discharge onto adjacent property shall not be altered without the written approval of the affected landowners.
C. No stormwater runoff or natural drainage shall be so diverted as to overload existing drainage systems, or create flooding or the need for additional drainage structures on other private properties or public lands, without approved provisions being made by the developer for properly handling such conditions.
D. Storm drainage systems through the subdivision/land development shall be designed to convey the peak runoff that will occur when all tributary areas upstream are developed to the extent reasonably projected during the next 20 years. The calculation of this runoff rate shall take into account the land use and development regulations including runoff controls in effect in the tributary areas.
E. Where a subdivision is traversed by water courses other than permanent streams, there shall be provided a drainage easement conforming substantially with the line of such watercourse. The width of the easement shall be adequate to provide for unimpeded flow of storm runoff based on calculations made in conformance with § 22-406(4) and to provide a freeboard allowance of 1/2 foot above the design water surface level. The terms of the easement shall prohibit excavation, the placing of fill or structures, and any alterations which may adversely affect the flow of stormwater within any portion of the easement. Also, periodic cutting of vegetation in all portions of the easement shall be required.
F. Drainage facilities that are located on State highway rights-of-way shall be approved by PennDOT and a letter indicating such approval shall be directed to the Township Planning Commission.
G. All streets shall be designed so as to provide for the discharge of surface water away from their rights-of-way.
H. When it can be shown to the satisfaction of the Township Engineer that, due to topographic conditions, natural drainage swales on the site cannot adequately provide for drainage, open channels may be constructed conforming substantially to the line and grade of such natural drainage swales. Capacities of open channels shall be calculated using the Manning equation as explained in Appendix A.
Storm drainage facilities and appurtenances shall be so designed and provided as to minimize erosion in watercourse channels and at all points of discharge.
3. Stormwater Detention.
A. Stormwater detention facilities shall be used whenever increased runoff from the land development would overload drainage systems or cause significant increases in flood levels in any watercourses downstream. This will be determined by comparing the increase in runoff caused by the land development with the existing runoff rates and capacity of downstream drainage systems and watercourse.
B. Whenever detention facilities are required under § 22-406(3)(A), facilities will be designed so that the peak runoff rate at all points of discharge from the site, when developed, will not exceed the peak runoff rate at each of those points prior to development.
C. Where detention facilities are included as part of the storm drainage system, the following provisions will apply:
(1) Detention ponds shall be designed so that they return to normal conditions within approximately 12 hours after the termination of the storm, unless the Township Engineer finds that down stream conditions may warrant other design criteria for stormwater release;
(2) The developer shall demonstrate that such ponds are designed, protected and located to assure that public safety is maximized and health problems are prevented;
(3) The developer shall verify that the operation of the detention facilities will not aggravate potential downstream peaking conditions;
(4) Emergency overflow facilities shall be provided for detention facilities to handle runoff in excess of design flows;
(5) If the lands of the proposed land development will remain in common ownership, the developer shall provide written assurances to the Township that the detention ponds will be properly maintained.
4. Calculations of Runoff and Design Storm Frequency.
A. Storm drainage systems required by this Chapter shall be designed to provide protection from a 10 to 100 year storm as determined by the Township Engineer. A 10 year design storm would be appropriate where a storm in excess of the design storm would have minor impact such as inconvenience to traffic on local streets. A 25 year design storm would be appropriate where a storm in excess of the design storm would cause major inconvenience to people and traffic in high use areas such as business districts and major highways. A 100 year design storm would be appropriate where a storm in excess of the design storm would cause damage to existing or future structures or their contents.
B. Stormwater runoff from watersheds of 200 or less acres shall be calculated by the rational method as described in Manual No. 37 of the American Society of Civil Engineers. The rational method of runoff calculation is explained in Appendix A.
C. Stormwater runoff from watersheds of more than 200 acres shall be calculated using the soil cover complex method developed by the Soil Conservation Service or other appropriate method acceptable to the Township Engineer.
D. The Manning equation explained in Appendix A shall be used in calculating capacities of watercourses and storm sewers, except culverts which shall be designed using methods acceptable to the Township Engineer.
E. Complete detailed drainage calculations certified by the design engineer shall be submitted to the Township Engineer.
F. Drainage facilities that are located on State Highway rights-of-way shall be approved by PennDOT and a letter indicating such approval shall be directed to the Township Planning Commission. Whenever storm water control regulations of an "Act 167 - Storm Water Management Plan" that has been adopted by the 'Township are more restrictive, the more restrictive provisions of such plan shall apply. [Ord. 239]
5. Improvement Specifications.
A. Inlets shall be designed and located to prevent hazardous conditions for vehicles, bicycles or pedestrians.
B. The Township Engineer shall add additional specifications which may be necessary for spacing and type of inlets and manholes, minimum pipe sizes, and materials and construction methods.
6. Perpetual Maintenance Responsibilities.
A. General. All privately owned stormwater management facilities shall be maintained in the manner in which they were originally approved, and such facilities shall be continuously operable and able to meet the purpose for which they were originally designed.
B. Types of Facilities. All private detention basins, whether above or in ground, as well as any piping, outlets, energy dissipaters or other structures required to be constructed as part of the overall development and to serve the purpose of detaining or channeling stormwater from the development into the overall Township or public system shall be regulated by this Part.
C. Vegetation. All vegetation shall be kept at a level not to exceed 6 inches in height, and where vegetation is required within the basin it shall not be allowed to be overrun with weeds and other vegetation not conducive to the operation of the system. This shall include trees, bushes and other growth that would hamper the operation of the system.
D. Fencing. All fencing required by the approved plans of the design of the detention basin shall be maintained in good repair so as to preclude access to the interior, except that provisions shall be made for authorized access for necessary and required maintenance. Any damaged, falling or otherwise deteriorated fencing shall be replaced.
E. Interior Liner. The required interior lining of all detention basins must be maintained pursuant to original impervious design to provide maximum effectiveness of the operation of the system.
F. Inspections. All stormwater management facilities will be subject to an annual inspection by the Township and any violation noted shall be required to be corrected within 10 days of written notification of same. If corrections are necessary, the Township reserves the right of final approval of the proposed corrections. The cost for these annual inspections and the review and inspection of required corrections, if necessary, shall be borne fully by the facility owner and shall be in accordance with the adopted Township fee schedule in effect at that time.
G. Application. The maintenance requirements of this Part shall apply to all privately owned stormwater management facilities regardless of approval and installation date.
[Ord. 390]
(Ord. 52, Art. 5-460, 3/8/78; as amended by Ord. 239, 2/7/1990, § 1; and by Ord. 390, 11/17/1999, § 1)