[Ord. 334 (92-1), 2/5/1992, § 301; as amended by Ord. 490 (2017-05), 10/18/2017]
1. 
Maintenance of Natural Drainageways. All natural streams, channels, swales, drainage systems and/or areas of surface water concentration shall be maintained in their existing condition unless an alteration is approved by the Borough. All encroachment activities shall comply with the requirements of 25 Pa. Code, Chapter 105, "Water Obstructions and Encroachments," Rules and Regulations of the Pennsylvania Department of Environmental Protection.
2. 
The existing points of concentrated drainage discharge onto adjacent property shall not be altered.
3. 
Areas of existing diffused drainage discharge onto adjacent property shall be managed such that, at minimum, the peak diffused flow does not increase in the general direction of discharge, except as otherwise provided in this Chapter. If diffused flow is proposed to be concentrated and discharged onto adjacent property, the developer must document that there are adequate downstream conveyance facilities to safely transport the concentrated discharge or otherwise demonstrate that no harm will result from the concentrated discharge. Areas of existing diffused drainage discharge shall be subject to any applicable release rate criteria in the general direction of existing discharge whether they are proposed to be concentrated or maintained as diffused drainage areas.
4. 
Where a subdivision or development is traversed by a watercourse, drainageway, channel or stream, there shall be provided a drainage easement conforming substantially with the high water line of such watercourse attributable to a flood of one-hundred-year frequency, in order to provide for future possible widening, deepening, relocating, improving or protecting of such drainage facilities. Any changes in the existing drainageway shall be subject to the approval of the Borough Engineer and the Pennsylvania Department of Environmental Protection.
5. 
Any drainage facilities required by this Chapter that are located on state highway rights-of-way shall be subject to approval by the Pennsylvania Department of Transportation.
6. 
When it can be shown 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.
7. 
Storm drainage facilities and appurtenances shall be so designed and provided as to minimize erosion in watercourse channels and at all points of discharge.
8. 
Consideration should be given to the design and use of volume controls for stormwater management, where geology permits.
[Ord. 334 (92-1), 2/5/1992, § 302]
1. 
Mapping of Stormwater Management Districts. In order to implement the provisions of the South Branch Codorus Creek Stormwater Management Plan the Borough is hereby divided into Stormwater Management Districts consistent with the South Branch Codorus Creek Release Rate Map presented in the plan. The boundaries of the Stormwater Management Districts are shown on an Official Map which is available for inspection at the Borough office. A copy of the Official Map at a reduced scale is included in Appendix 23-3-A for general reference.
2. 
Description of Stormwater Management Districts. Three types of Stormwater Management Districts may be applicable to the Borough, namely Release Rate Districts, Provisional No-Detention Districts and Provisional 100% Release Rate Districts as described below:
A. 
Release Rate Districts. There are four Release Rate Districts which differ in the extent to which post-development runoff must be controlled. The release rates, and districts are 50%, 70%, 80% and 100%. Within a given District, the post-development peak rate of storm runoff must be controlled to the stated percentage of the predevelopment peak rate of storm runoff in order to protect downstream watershed areas.
B. 
Provisional No-Detention Districts. These watershed areas may discharge postdevelopment peak runoff without detention without adversely affecting the total watershed peak flow. In certain instances, however, the "local" runoff conveyance facilities, which transport runoff from the site to the main channel, may not have adequate capacity to safety transport runoff from the site to the main channel, may not have adequate capacity to safely transport increased peak flow associated with no detention for a proposed development. In those instances, the developer shall either use a 100% release rate control or provide increased capacity of downstream drainage elements to convey increased peak flows consistent with § 23-303, Subsection 8. In determining if adequate capacity exists in the local watershed drainage network, the developer must assume that the entire local watershed is developed per current zoning and that all new development would use the runoff controls specified by this Chapter. Similarly, any capacity improvements must be designed to convey runoff from development of all areas tributary to the improvement consistent with the capacity criteria specified in § 23-303, Subsection 3.
C. 
Provisional 100% Rates Districts. These areas should use a 100% release rate control to protect the areas upstream of the mainstream. Direct discharge or postdevelopment flows with no detention could be appropriate for these areas if the developer could prove that adequate downstream capacity exists to convey the increased peak flows generated with development site is located, without detention controls, to the mainstream. Evaluation of downstream capacity must assume development of all areas tributary to the local drainage network consistent with current zoning and that all new development would use the runoff controls specified by this Chapter, except as otherwise specified above.
[Ord. 334 (92-1), 2/5/1992, § 303; as amended by Ord. 490 (2017-05), 10/18/2017]
1. 
Any stormwater management controls required by this Chapter are subject to release rate criteria (50% through 100%) shall meet the applicable release rate criteria for each of the two-, five-, ten-, twenty-five-, fifty-, and one-hundred-year return period runoff events consistent with the calculation methodology specified in § 23-304.
2. 
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 shown 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 (road, culvert, bridge, etc.) to the topographic divide consistent with topography.
3. 
Any downstream capacity analysis conducted in accordance with this Chapter shall use the following criteria for determining adequacy for accepting increased peak flow rates:
A. 
Natural or man-made channels or swales must be able to convey increased runoff associated with a two-year return period event within their banks at velocities consistent with protection of the channels from erosion. Acceptable velocities shall be based upon criteria included in the DEP Soil Erosion and Sedimentation Control Manual (February, 1985) and presented in Appendix 23-3-B of this Chapter.
B. 
Natural or man-made channels or swales must be able to convey the increased twenty-five-year return period runoff peak within their banks or otherwise not create any hazard to persons or property.
C. 
Culverts, bridges, storm sewers or any other facilities which must pass or convey flows from the tributary area must have sufficient capacity to pass or convey the increased flows associated with the twenty-five-year return period runoff event, except for facilities located within a designated floodplain area which must be capable of passing or conveying the one-hundred-year return period runoff. Any facilities which constitute stream enclosures per DEP's 25 Pa. Code, Chapter 105, regulations shall be designed to convey the one-hundred-year return period runoff.
4. 
For a proposed development site located within a single release rate category area, the total runoff from the site shall meet the applicable release rate criteria. For development sites with multiple points of concentrated runoff discharge, individual drainage points 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.
5. 
For a proposed development site located within two or more release rate category areas, the maximum peak rate of runoff that may be discharged at any point is limited to the predevelopment peak rate of runoff at that point multiplied by the applicable release rate. The control rates shall apply regardless of any grading modifications which may change the drainage area which discharges at a given point.
6. 
For proposed development sites located partially within a release rate category area and partially within a provisional no detention area, in no event shall a significant portion of the site area subject to the release rate control be drained to the discharge point(s) located in the no detention area.
7. 
"No Harm" Option. For any proposed development site not located in a 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 "no harm" would be caused by discharging at a higher runoff rate than that specified by the plan. 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 South Branch Codorus Creek with the East Branch Codorus Creek. Proof of "no harm" must be shown using the capacity criteria specified in Subsection 3. If downstream capacity analysis is a part of the "no harm" justification. 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-, twenty-five-, and one-hundred-year) shall be the values from the calibrated Penn State Runoff Model for the South Branch Codorus Creek Watershed. These flow values would be supplied to the developer by the Borough Engineer 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 development site acreage relative to the total upstream undeveloped acreage from the identified capacity (i.e., if his site is 10% of the upstream undeveloped acreage, he may use up to 10% of the document downstream available capacity).
C. 
Developer proposed runoff controls which would generate increased peak flow rates at documented storm drainage problem areas would, by definition, be precluded from successful attempts to prove "no harm," except in conjunction with the proposed capacity improvements for the problem areas consistent with Subsection 9.
Any "no harm" justification shall be submitted by the developer as part of the drainage plan submission per Part 4.
8. 
Regional or Subregional Detention Alternatives. For certain areas within the watershed, it may be more cost-effective to provide one control facility for an entire subarea, group of subareas or portion of a subarea incorporating more than one development site than to provide an individual control facility for each development site. The initiative and funding for any regional or subregional 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 on a case-by-case basis using the hydrologic model of the watershed consistent with protection of the downstream watershed areas. "Hydrologic model" refers to the South Branch Codorus Creek version of the Penn State Runoff Model as developed for the stormwater management plan.
9. 
Capacity Improvements. In certain instances, primarily within the provisional no-detention and provisional 100% release category 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. Any capacity improvement would be designed based upon development of all areas tributary to the proposed improvement and the capacity criteria specified in Subsection 3. In addition, all new development upstream of a proposed capacity improvement shall be assumed to implement the applicable runoff controls consistent with this Chapter except that all new development within 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. Capacity improvements may also be provided as necessary to implement any regional or subregional detention alternatives or to implement a modified "no harm" option which proposed specific capacity improvements to document the validity of a less stringent discharge control which would not create any harm downstream.
10. 
Waiver of Runoff Control Based on Minimum Impervious Cover.
A. 
Any proposed regulated activity, except those defined in § 23-104, Subsections 4E and F, which would create 5,000 square feet or less of additional impervious cover and 25% or less total lot coverage would be exempt from meeting the runoff control provisions of this Chapter. For developments which are to take place in stages, the entire development plan must be used in determining conformance to this criteria. Additional impervious cover shall include, but not be limited to, any roof or driveway areas and any new streets and sidewalks constructed as part of or for the proposed development. Any areas which may be designed to initially be semi-pervious (e.g., gravel, crushed stone, porous payment, etc.) shall be considered impervious areas for the purpose of waiver evaluation.
B. 
Any existing developed residential lot for which a minor extension or improvement of a regulated activity pursuant to § 23-104, Subsections 4C or D is proposed shall be exempt from meeting the runoff control provisions of this Chapter; provided, however, that such extension or improvement shall not exceed 5,000 square feet of additional impervious cover and not be more than 25% of the total lot coverage.
C. 
No waiver shall be provided for any regulated activities as defined in § 23-104, Subsections 4E and F.
[Ord. 334 (92-1), 2/5/1992, § 304]
1. 
Stormwater runoff from all development sites shall be calculated using either the rational method or a SCS Technical Release Number 55 (TR 55). The rational method shall not be used for tributary area greater than 320 acres.
2. 
The design of any detention facility intended to meet the requirements of this Chapter shall be verified by routing the design storm hydrograph through the proposed basin. For basins designed using the modified rational method technique, the detention volume shall, at minimum, equal the volume derived from the approximate routing process as contained in TR55. Actual volume required will be that determined from the routing, but in no case less than the minimum from TR-55.
3. 
All stormwater detention facilities shall provide a minimum one-foot freeboard above the maximum pool elevation associated with the two- through fifty-year runoff events. An emergency spillway shall be designed to pass the one-hundred-year runoff event with a minimum one-half-foot freeboard.
4. 
All calculations using the soil cover complex method shall use the Soil Conservation Service Type II twenty-four-hour rainfall distribution. The twenty-four-hour rainfall depths for the various return periods to be used consistent with this Chapter are as follows:
Return Period
24-Hour Rainfall Depth
(inches)
2-year
3.1
5-year
3.9
10-year
4.8
25-year
5.3
50-year
6.1
one-hundred-year
6.8
A graphical and tubular presentation of the Type II-24 hour distribution as included in Appendix 23-3-C.
5. 
All calculations using the rational method shall use rainfall intensities consistent with appropriate times of concentration and return periods and the Intensity-Duration-Frequency Curves as presented in Appendix 23-3-C.
6. 
Runoff Curve Numbers (CNs) to be used in the soil cover complex method shall be based upon the matrix presented in Appendix 23-3-C.
7. 
Runoff coefficients for use in the rational method shall be based upon the table presented in Appendix 23-3-C or the nomograph for Rossmiller's formula in Appendix 23-3-C.
8. 
The Manning equation shall be used to calculate the capacity of watercourses. Manning "n" values used in the calculations shall be consistent with table presented in Appendix 23-3-C. Pipe capacities shall be determined by methods acceptable to the Borough Engineer.