(a) Generally.
(1) Applicants may select runoff control techniques, or a combination of techniques, which are most suitable to control stormwater runoff from the development site. All controls are subject to the approval of the Municipal Engineer. The Municipal Engineer may request specific information on design and/or operating features of the proposed stormwater controls in order to determine their suitability and adequacy in terms of the standards of this chapter.
(2) The applicant should consider the effect of the proposed stormwater management techniques on any special soil conditions or geological hazards which may exist on the development site. In the event such conditions are identified on the site, the Municipal Engineer may require in depth studies by a competent geotechnical engineer. Not all stormwater control methods may be advisable or allowable at a particular development site.
(3) The stormwater management practices to be used in developing a stormwater management plan for a particular site shall be selected according to the following order of preference:
A. Infiltration of runoff on-site;
B. Flow attenuation by use of open vegetated swales and natural depressions; and
C. Stormwater detention/retention structures.
(4) Infiltration practices shall be used to the extent practicable to reduce volume increases and promote groundwater recharge. A combination of successive practices may be used to achieve the applicable minimum control requirements. Justification shall be provided by the applicant for rejecting each of the preferred practices based on actual site conditions.
(b) Infiltration Systems.
(1) Infiltration systems shall be sized and designed based upon local soil and groundwater conditions.
(2) Infiltration systems greater than three feet deep shall be located at least ten feet from basement walls.
(3) Infiltration systems designed to handle runoff from commercial or industrial parking areas shall be located a minimum of 100 feet from any water supply well.
(4) Infiltration systems may not receive runoff until the entire drainage area to the system has received final stabilization.
(5) The stormwater infiltration facility design shall provide an overflow system with measures to provide a non-erosive velocity of flow along its length and at the outfall.
(c) Stormwater Detention Facilities.
(1) If detention facilities are utilized for the development site, the facility(ies) shall be designed such that post-development peak runoff rates from the developed site are controlled to those rates defined by the subarea release percentage for the two, ten, twenty-five and 100-year storm frequencies.
(2) All detention facilities shall be equipped with outlet structures to provide discharge control for the four designated storm frequencies. Provisions shall also be made to safety pass the postdevelopment 100-year storm runoff without damaging (i.e. impairing) the continued function of the facilities.
(3) Shared-storage facilities, which provide detention of runoff for more than one development site within a single subarea, may be considered and are encouraged. Such facilities shall meet the criteria contained in this section. In addition, runoff from the development sites involved shall be conveyed to the facility in a manner that avoids adverse impacts (such as flooding or erosion) to channels and properties located between the development site and the shared-storage facilities.
(4) 'Where detention facilities will be utilized, multiple use facilities, such as wetlands, lakes, ballfields or similar recreational/open spaces uses, are encouraged wherever feasible, subject to the approval of the Municipality.
(5) Other considerations which should be incorporated into design of the detention facilities include:
A. Inflow and outflow structures shall be designed and installed to prevent erosion, and bottoms of impoundment-type structures should be protected from soil erosion.
B. Standpipes shall be constructed of reinforced concrete or castin-place, precast or block and shall be provided with debris grates.
C. Control and removal of debris both in the storage structure and in all inlet or outlet devices shall be a design consideration.
D. Inflow and outflow structures, pumping stations, and other structures shall be designed and protected to minimize safety hazards.
E. An emergency spillway or overflow shall be provided that is capable of passing the peak flow generated by a 100-year, twenty-four hour storm following construction.
F. The water depth at the perimeter of a storage pond should be limited to that which is safe for children. This is especially necessary if bank slopes are steep or if ponds are full and recirculating in dry periods Restriction of access (fence, walls, etc.) may be necessary depending on the location of the facility. Unless otherwise approved, the maximum planned water depth shall not exceed four feet.
G. The facility shall be designed with a minimum freeboard of one foot between the peak emergency spillway design flow elevation and the top of the embankment.
H. Side slope of storage ponds shall not exceed a ratio of two to one horizontal to vertical dimension.
I. Landscaping shall be provided for the facility which harmonizes with the surrounding area.
J. The facility shall be located to facilitate maintenance, considering the frequency and type of equipment that will be required.
K. All detention facilities shall be provided with an access road for maintenance purposes. Such roads shall be a minimum of ten feet wide and have a maximum grade of fifteen percent.
(d) Criteria for Collection/Conveyance Facilities.
(1) All stormwater runoff collection or conveyance facilities, whether storm sewers or other open or closed channels, shall be designed in accordance with the following basic standards:
A. All collection/conveyance facilities shall be designed to convey the runoff from what has been calculated to be a ten-year storm for this vicinity as shown on the Pennsylvania Department of Transportation rainfall charts. Where the flow from the ten-year storm exceeds twenty c.f.s., then from that point downstream, all storm drainage systems and structures shall be designed to handle the peak flow from a twenty-five year storm. Where a storm drainage structure must handle the flow in a stream within a Federally designed floodplain, the structure shall be designed to handle the peak flow from a 100-year storm. The developer and the Planning Commission should keep in mind that more severe storms occur at less frequent intervals and, where feasible, so design subdivisions that especially heavy runoff, exceeding the capacity of the required channels or storm sewers, can be handled with the least possible damage to improvements and structures.
B. The rational method shall ordinarily be used in computing runoff using the formula:
Q = C * I * A
Wherein
Q = water reaching the channel, culvert, bridge or storm sewer in cubic feet per second (c.f.s.)
I = rainfall rate in inches per hour
A = drainage area in acres
C = coefficient of runoff as follows:
areas primarily paved or in building (such as shopping centers) = 0.90
primarily residential areas with lots smaller than 7,500 square feet of apartment areas = 0.60
primarily residential areas with lots 7,500 square feet to 1/2 acre = 0.50
primarily residential areas with lots 20,000 square feet or over = 0.40
cemeteries, park land and other permanent areas = 0.35
In computing "I", the concentration time for each collection area shall be computed and the rainfall per hour determined by use of the Storm Intensity - Duration - Frequency Curves, as published by the Pennsylvania Department of Transportation Storm Intensity - Duration - Frequency Charts, May 1986 edition, or its successor publication or standard.
C. Required design of streets and grading in relation to storm drainage shall be such that runoff from roofs, driveways and other impervious surfaces shall be towards the street or other approved storm drainage system and will be collected in the storm sewers or street drainage system.
D. All sites shall be graded to provide drainage away from and around the structure ln order to prevent any potential flooding damage.
E. Lots located on the high side of streets shall extend roof and french drains to the curb line storm sewer (if applicable). Low side lots shall extend roof and french drains to a stormwater collection/conveyance/control system or natural watercourse ln accordance with the approved stormwater management plan for the development site.
F. Storm sewers shall be required to be installed along both sides of the streets in all subdivisions.
G. Open channel drainage between or across lots carrying stormwater from public rights-of-way or storm sewers shall not be permitted. Such drainage shall be contained in approved storm sewers. Open channel drainage shall be permitted in natural streams or stream beds provided adequate provisions for erosion control are made where necessary.
H. Subdivision and development of any area increases and concentrates the runoff of stormwater from the area. Subdividers are warned that such increase may cause flood or erosion damage to properties lying downstream. Downstream disposal of stormwater shall be directly into an existing storm sewer system or natural watercourse of adequate capacity, located on the property of the developer. Stormwater shall not be discharged onto private property, unless the developer obtains drainage easement(s) as needed from downstream property owner(s), and installs a stormwater conveyance system to carry the stormwater from the developer's property across the private property to a natural watercourse or storm sewer system.
I. Collection/conveyance facilities should not be installed parallel and close to the top or bottom of a major embankment to avoid the possibility of failing or causing the embankment to fail.
J. Where drainage swales or open channels are used, they shall be suitably lined to prevent erosion and designed to avoid excessive velocities.
(2) Wherever storm sewers are proposed to be utilized, they shall comply with the following criteria:
A. The storm sewer shall be located according to specifications of the City Engineer.
B. The design of storm sewer layout, including the handling of abrupt changes in direction, shall be as approved by the City Engineer.
C. Where practical, storm sewers should be designed to traverse under seeded and planted areas. If constructed within ten feet of road paving, walks or other surfaced areas, drains shall have a narrow trench and maximum compaction of backfill to prevent settlement of the superimposed surface or development.
D. Storm sewers preferably should be installed after excavating and filling in the area to be traversed is complete, unless the drain is installed in the original ground with a minimum of three feet cover and/or adequate protection during the fill construction.
E. Storm sewers shall be designed with cradle when traversing fill areas of indeterminate stability, with anchors when gradient exceeds twenty percent, and with encasement or special backfill requirements when traversing under a paved area.
F. Storm sewers shall be designed to adequately handle the anticipated stormwater flow and be economical to construct and maintain. The minimum pipe size shall be twelve inches in diameter and the minimum grade shall be 0.5 percent.
G. Drain pipe, trenching, bedding and backfilling requirements shall conform to the requirements of the Municipality and/or applicable PENNDOT Specifications, Form 408.
H. All corrugated metal pipe shall be polymer coated, and with asbestos bonding and paved inverts where prone to erode. Pipe within a Municipal right of way shall be reinforced concrete pipe with a minimum diameter of fifteen inches.
I. Storm inlets and structures shall be designed to be adequate, safe, self-cleaning and unobtrusive. Inlets shall be constructed of precast concrete of a design approved by the City Engineer. A two-feet by two-feet precast inlet shall be permitted where pipe diameter is fifteen inches or less and where no more than two pipes are inserted into the inlet. The size and design of larger inlets shall be as approved by the City Engineer.
J. Appropriate grates shall be designed for all catch basins, stormwater inlets and other entrance appurtenances.
K. Manholes shall not be more than 300 feet apart where pipe sizes of twenty-four inches or less are used and not more than 450 feet apart where larger sizes are installed. When approved by the City Engineer, inlets may be substituted for manholes.
L. Manholes shall be designed so that the top shall be at finished grade and sloped to confirm to the slope of the finished grade. Top castings of structures located in roads or parking areas shall be machined or installed to preclude "rattling."
M. Where a proposed sewer connects with an existing sewer system, the applicant shall demonstrate that sufficient capacity exists in the downstream system to handle the additional flow.
N. Storm sewers outfalls shall be equipped with energy dissipation devices to prevent erosion and conform with applicable requirements of the Pennsylvania DER for stream encroachments (Chapter 105 of Pennsylvania DER Rules and Regulations).
(Ord. 14-91. Passed 8-28-91.)