(A) General criteria.
(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 must be subject to approval of the Borough Engineer. The Borough 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 Borough 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) Criteria for infiltration systems.
(1) Infiltration systems shall be sized and designed based upon local soil and ground water conditions.
(2) Infiltration systems shall be greater than three feet deep and shall be located at least ten feet from basement walls.
(3) Infiltration systems shall not be used to handle runoff from commercial or industrial working or parking areas. This prohibition does not extent to roof areas which are demonstrated to be suitably protected from the effects of the commercial/industrial activities.
(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) Criteria for 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 rate percentage for the two-, ten-, 25- 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 safely pass, at minimum, the post-development 100-year storm runoff without breaching or otherwise 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 maybe 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 space uses are encouraged wherever feasible, subject to the approval of the borough and 25 Pa. Code Chapter 105 regulations.
(5) Other considerations which should be incorporated into the 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) Control and removal of debris both in the storage structure and in all inlet or outlet devices shall be a design consideration;
(c) Inflow and outflow structures, pumping stations and other structures shall be designed and protected to minimize safety hazards;
(d) 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 and the like) may be necessary depending on the location of the facility;
(e) Side slope of storage ponds shall not exceed a ratio of two and one-half to one horizontal to vertical dimension;
(f) Landscaping shall be provided for the facility which harmonizes with the surrounding area; and
(g) Facility shall be located to facilitate maintenance, considering the frequency and type of equipment that will be required.
(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 sites shall be graded to provide drainage away from and around the structure in order to prevent any potential flooding damage.
(b) 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 in accordance with the approved stormwater management plan for the development site.
(c) 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.
(d) All collection/conveyance facilities shall be designed to convey the 25-year storm peak flow rate from the contributing drainage area and to carry it to the nearest suitable outlet such as a stormwater control facility, curbed street, storm sewer or natural watercourse.
(e) 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) Where practical, 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;
(b) Preferably installed after excavating and filling in the area to be traversed is completed, unless the drain is installed in the original ground with a minimum of three feet cover and/or adequate protection during the fill construction;
(c) Designed:
1. With cradle when traversing fill areas of indeterminate stability;
2. With anchors when gradient exceeds 20%; and
3. With encasement or special backfill requirements when traversing under a paved area.
(d) Designed to adequately handle the anticipated stormwater flow and be economical to construct and maintain. The minimum pipe size shall be 15 inches in diameter;
(e) Drain pipe, trenching, bedding and backfilling requirements shall conform to the requirements of the borough and/or applicable PennDOT Specifications, Form 408;
(f) 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 15 inches;
(g) Storm inlets and structures shall be designed to be adequate, safe, self-cleaning and unobtrusive and consistent with municipal standards;
(h) Appropriate grates shall be designed for all catch basins, stormwater inlets and other entrance appurtenances;
(i) Manholes shall be designed so that the top shall beat finished grade and sloped to conform 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”;
(j) Where proposed sewer connects with an existing storm sewer system, the applicant shall demonstrate that sufficient capacity exists in the downstream system to handle the additional flow; and
(k) Storm sewer outfalls shall be equipped with energy dissipation devices to prevent erosion and conform with applicable requirements of the Pennsylvania DEP for stream encroachments (25 Pa. Code Chapter 105).
(Ord. 980, passed 3-16-1993; Ord. 991, passed 2-21-1995)