(A) Applicants shall provide a comparative pre- and post-construction stormwater management hydrograph analysis for each direction of discharge and for the site overall to demonstrate compliance with the provisions of this chapter.
(B) Any stormwater management controls required by this chapter and subject to a dual release rate criteria shall meet the applicable release rate criteria for each of the two-, ten-, 25- and 100-year return period runoff events consistent with the calculation methodology specified in § 153.041.
(C) The exact location of the stormwater management district boundaries as they apply to a given development site shall be determined by mapping the boundaries using the two-foot topographic contours provided as part of the drainage plan. The district boundaries as originally drawn coincide with topographic divides or, in certain instances, are drawn from the intersection of the watercourse and a physical feature such as the confluence with another watercourse or a potential flow obstruction (e.g., road, culvert, bridge and the like). The physical feature is the downstream limit of the subarea and the subarea boundary is drawn from that point up slope to each topographic divide along the path perpendicular to the contour lines.
(D) Any downstream capacity analysis conducted in accordance with this chapter shall use the following criteria for determining adequacy for accepting increased peak flow rates.
(1) Natural or human-made channels or swales must be able to convey the increased runoff associated with a two-year return period event within their banks at velocities consistent with protection of the channels from erosion.
(2) Natural or human-made channels or swales must be able to convey the increased 25-year return period runoff without creating any hazard to persons or property.
(3) Culverts, bridges, storm sewers or any other facilities which must pass or convey flows from the tributary area must be designed in accordance with DEP regulations, 25 Pa. Code Chapter 105 (if applicable) and, at minimum, pass the increased 25-year return period runoff.
(E) For a proposed development site located within one release rate category subarea, the total runoff from the site shall meet the applicable release rate criteria. For development sites with multiple directions of runoff discharge, individual drainage directions may be designed for up to a 100% release rate so long as the total runoff from the site is controlled to the applicable release rate.
(F) For a proposed development site located within two or more release category subareas, the peak discharge rate from any subarea shall be the pre-development peak discharge for that subarea multiplied by the applicable release rate. The calculated peak discharges shall apply regardless of whether the grading plan changes the drainage area by subarea. An exception to the above may be granted if discharges from multiple subareas re-combine in proximity to the site. In this case, peak discharge in any direction may be a 100% release rate provided that the overall site discharge meets the weighted average release rate.
(G) For a proposed development site located partially within a release rate category subarea and partially within a conditional/provisional no detention subarea, the size of the pre-development drainage area on a site may not be changed post-development to create potentially adverse conditions on downstream properties except as part of a “no harm” or hardship waiver procedure.
(H) No portion of a site may be regraded between the Bushkill Creek Watershed and any adjacent watershed except as part of a “no harm” or hardship waiver procedure.
(I) (1) Within a release rate category area, for a proposed development site which has areas which drain to a closed depression(s), the design release from the site will be the lesser of:
(a) The applicable release rate flow assuming no closed depression(s); or
(b) The existing peak flow actually leaving the site.
(2) In cases where division (I)(1)(b) above would result in an unreasonably small design release, the design discharge of less than or equal to the release rate will be determined by the available downstream conveyance capacity to the main channel calculated using division (D) above and the minimum orifice criteria.
(J) Off-site areas which drain through a proposed development site are not subject to release rate criteria when determining allowable peak runoff rates. However, on-site drainage facilities shall be designed to safely convey off-site flows through the development site using the capacity criteria in division (D) above and the detention criteria in § 153.041.
(K) For development sites proposed to take place in phases, all detention ponds shall be designed to meet the applicable release rate(s) applied to all site areas tributary to the proposed pond discharge direction. All site tributary areas will be assumed as developed, regardless of whether all site tributary acres are proposed for development at that time. An exception shall be sites with multiple detention ponds in series where only the downstream pond must be designed to the stated release rate.
(L) Where the site area to be impacted by a proposed development activity differs significantly from the total site area, only the proposed impact area shall be subject to the release rate criteria. The impact area includes any proposed cover or grading changes.
(M) Development proposals which, through groundwater recharge or other means, do not increase either the rate or volume of runoff discharged from the site compared to pre-development are not subject to the release rate provisions of this chapter.
(N) “No harm” water quantity option.
(1) For any proposed development site not located in a conditional/provisional no detention district, the developer has the option of using a less restrictive runoff control (including no detention) if the developer can prove that special circumstances exist for the proposed development site and that “no harm” would be caused by discharging at a higher runoff rate than that specified by the plan. Special circumstances are defined as any hydrologic or hydraulic aspects of the development itself not specifically considered in the development of the plan runoff control strategy. Proof of “no harm” would have to be shown from the development site through the remainder of the downstream drainage network to the confluence of the creek with the Delaware or Lehigh River. Proof of “no harm” must be shown using the capacity criteria specified in division (D) above if downstream capacity analysis is a part of the “no harm” justification.
(2) Attempts to prove “no harm” based upon downstream peak flow versus capacity analysis shall be governed by the following provisions:
(a) The peak flow values to be used for downstream areas for the design return period storms (two-, ten-, 25- and 100-year) shall be the values from the calibrated PSRM model for the Bushkill Creek or as calculated by an applicant using an alternate method acceptable to the borough. The flow values from the PSRM model would be supplied to the developer by the borough upon request.
(b) Any available capacity in the downstream conveyance system as documented by a developer may be used by the developer only in proportion to his or her development site acreage relative to the total upstream undeveloped acreage from the identified capacity (i.e., if his or her site is 10% of the upstream undeveloped acreage, he or she may use up to 10% of the documented downstream available capacity).
(c) Developer-proposed runoff controls which would generate increased peak flow rates at storm drainage problem areas would, by definition, be precluded from successful attempts to prove “no harm”, except in conjunction with proposed capacity improvements for the problem areas consistent with division (P) below.
(O) Regional detention alternatives: for certain areas within the study area, it may be more cost-effective to provide one control facility for more than one development site than to provide an individual control facility for each development site. The initiative and funding for any regional runoff control alternatives are the responsibility of prospective developers. The design of any regional control basins must incorporate reasonable development of the entire upstream watershed. The peak outflow of a regional basin would be determined based on the required release rate at the point of discharge.
(P) Capacity improvements.
(1) In certain instances, primarily within the conditional/provisional no detention areas, local drainage conditions may dictate more stringent levels of runoff control than those based upon protection of the entire watershed. In these instances, if the developer could prove that it would be feasible to provide capacity improvements to relieve the capacity deficiency in the local drainage network, then the capacity improvements could be provided by the developer in lieu of runoff controls on the development site. Peak flow calculations shall be done assuming that the local watershed is in the existing condition and then assuming that the local watershed is developed per current zoning and using the specified runoff controls. Any capacity improvements would be designed using the larger of the above peak flows and the capacity criteria specified in division (D) above. All new development in the entire subarea(s) within which the proposed development site is located shall be assumed to implement the developer’s proposed discharge control, if any.
(2) Capacity improvements may also be provided as necessary to implement any regional detention alternatives or to implement a modified “no harm” option which proposes specific capacity improvements to provide that a less stringent discharge control would not create any harm downstream.
(Ord. 226-2007, passed 5-7-2007)