Source: https://ecode360.com/14243812
Timestamp: 2019-05-27 07:21:51
Document Index: 744537712

Matched Legal Cases: ['§ 260', '§ 260', '§ 260', '§ 260', '§ 260', '§ 260', '§ 260', '§ 260', '§ 260', '§ 260', '§ 260', '§ 260', '§ 260', '§ 260', '§ 260', '§ 260', '§ 260', '§ 260', '§ 260', '§ 260']

Township of East Hempfield, PA Stormwater Management Standards
Ch 260 Art III Stormwater Management Standards
§ 260-13 General requirements.
§ 260-14 Volume controls.
§ 260-15 Rate controls.
§ 260-16 Stormwater management performance standards.
§ 260-17 Calculation methodology.
§ 260-18 Riparian corridors.
§ 260-19 Aboveground storage facility design criteria.
§ 260-20 Subsurface storage facility design criteria.
§ 260-21 Conveyance facility design criteria.
§ 260-22 (Reserved)
Chapter 260 Stormwater Management Article III Stormwater Management Standards
Preparation of a SWM site plan is required for all regulated activities, unless preparation and submission of the SWM site plan is specifically exempted according to § 260-28 or the activity qualifies as a small project.
No regulated activities shall commence until the Township issues unconditional written approval of a SWM site plan.
The Township may, after consultation with DEP, approve measures for meeting the state water quality requirements other than those in this chapter, provided that they meet the minimum requirements of, and do not conflict with, state law including, but not limited to, the Clean Streams Law.[1] The Township shall maintain a record of consultations with DEP pursuant to this subsection. Where an NPDES permit for stormwater discharges associated with construction activities is required, issuance of an NPDES permit shall constitute satisfaction of consultation with DEP.
For all regulated activities, erosion and sediment control and stormwater management BMPs shall be designed, implemented, operated, and maintained to meet the purposes and requirements of this chapter and to meet all requirements under Title 25 of the Pennsylvania Code and the Clean Streams Law. Various BMPs and their design standards are listed in the Erosion and Sediment Pollution Control Program Manual (E8S Manual), No. 363-2134-008 (March 2012), as amended and updated (NOTE: See § 260-53I.), and the BMP Manual.
Developers have the option to propose a regional stormwater management plan or participate in a regional stormwater management plan developed by others. A regional stormwater management plan may include off-site volume and rate control, as appropriate and supported by a detailed design approved by the Township in accordance with § 260-13C. A regional stormwater management plan must meet all of the volume and rate control standards required by this chapter for the area defined by the regional stormwater management plan, but not necessarily for each individual development site. Appropriate agreements must be established to ensure the requirements of this chapter and the requirements of the regional stormwater management plan are met.
Impervious areas;
Any areas designed to initially be gravel or crushed stone shall be assumed to be impervious.
Protect health, safety, and property;
Protect and/or improve the function of floodplains, wetlands, and wooded areas.
Protect and/or improve native plant communities including those within the riparian corridor.
To the maximum extent practicable, the techniques for low-impact development (LID) practices described in the BMP Manual shall be incorporated. The proposed LID practices shall be noted on the Stormwater management site plan.
The design of all SWM facilities over karst shall include an evaluation of measures to minimize adverse effects.
A minimum depth of 24 inches between the bottom of the facility and the limiting zone, unless it is demonstrated to the satisfaction of the Township that the selected BMP has design criteria which allow for a smaller separation.
The calculation methodology to be used in the analysis of volume and peak rates of discharge shall be as required in § 260-17, calculation methodology.
Invasive vegetation may not be included in any planting schedule.
Areas proposed for infiltration BMPs shall be protected from sedimentation and compaction during the construction phase to maintain maximum infiltration capacity. Staging of earthmoving activities and selection of construction equipment should consider this protection.
A minimum ten-foot-wide access easement shall be provided for all stormwater serving multiple properties and not located within a public right-of-way. Easements shall provide for ingress and egress to a public right-of-way.
Drainage easements shall be provided where the conveyance, treatment, or storage of stormwater, either existing or proposed, is identified on the SWM site plan. Drainage easements shall be provided to contain and convey the one-hundred-year frequency flood.
Stormwater facilities not located within a public right-of-way shall be contained in and centered within a minimum twenty-foot-wide stormwater management easement. All easements shall have adequate information to be located in the field.
Stormwater management easements are required for all on-site areas used to convey stormwater of two cfs or greater for a one-hundred-year storm. Roof drains do not require stormwater management easements.
Unless a concentrated discharge of stormwater to an adjacent property is within an existing watercourse, an easement burdening the adjacent property shall be required.
Where a development site is traversed by watercourses other than permanent streams, a drainage easement shall be provided conforming substantially to the line of such watercourses. The terms of the easement shall prohibit excavation, the placing of fill or structures, and any alterations that may affect adversely the flow of stormwater within any portion of the easement.
Nothing shall be placed, planted, set, or put within the area of an easement that would adversely affect the function of the easement or conflict with the easement agreement.
The Township may require additional stormwater control measures for stormwater discharges to special management areas including but not limited to:
Critical areas with sensitive resources (e.g., state-designated special protection waters, cold-water fisheries, carbonate or other groundwater recharge areas highly vulnerable to contamination, drainage areas to water supply reservoirs, source water protection zones, etc.)
Non-roof drains and sump pumps shall be tributary to infiltration or vegetative BMPs. Use of catchment facilities for the purpose of reuse is also permitted.
Unless specifically approved by the Township in light of circumstances unique to the site, roof drains shall not be connected to streets, sanitary or storm sewers or to roadside ditches and instead shall discharge to infiltration areas or vegetative BMPs.
Do not increase the post-development total runoff volume for all storms equal to or less than the two-year twenty-four-hour storm event.
Existing (predevelopment) non-forested previous areas must be considered meadow in good condition.
The maximum loading ratio for volume control facilities in karst areas shall be 3:1 impervious drainage area to infiltration area and 5:1 total drainage area to infiltration area. The maximum loading ratio for volume control facilities in non-karst areas shall be 5:1 impervious drainage area to infiltration area and 8:1 total drainage area to infiltration area. A higher ratio may be approved by the Township if justification is provided. Hydraulic depth may be used as an alternative to an area based loading ratio if the design hydraulic depth is shown to be less than the depth that could result from the maximum area loading ratio.
Volume control for small projects.
At least the first one inch of runoff from new impervious surfaces or an equivalent volume shall be permanently removed from the runoff flow, i.e., it shall not be released into the surface waters of this commonwealth. Removal options include reuse, evaporation, transpiration and infiltration.
A detailed geologic evaluation of the development site shall be performed in areas of carbonate geology to determine the design parameters of recharge facilities. A report shall be prepared in accordance with § 260-27A of this chapter.
If the developer can prove through analysis that the development site is in an area underlain by carbonate geology, and such geologic conditions may result in sinkhole formations, then the development site is exempt from volume control requirements as described in this chapter. However, the development site shall still be subject to NPDES and E&S requirements.
Storage facilities, including normally dry, open-top facilities, shall completely drain the volume control storage over a period of time not less than 24 hours and not more than 72 hours from the end of the design storm. Any designed infiltration at such facilities is exempt from the minimum twenty-four-hour standard, i.e., may infiltrate in a shorter period of time, provided that none of this water will be discharged into waters of this commonwealth.
Any portion of the volume control storage that meets the following criteria may also be used as rate control storage;
Volume control storage that depends on infiltration is designed according to the infiltration standards in § 260-13.
The volume control storage which will be used for rate control is that storage which is available within 24 hours from the end of the design storm based on the stabilized infiltration rate and/or the evapo-transpiration rate.
Applicable worksheets from of the BMP Manual shall be used when establishing volume controls.
Match predevelopment hydrograph.
Applicants shall provide infiltration facilities or utilize other techniques which will allow the post-development one-hundred-year hydrograph to match the predevelopment one-hundred-year hydrograph, along all parts of the hydrograph, for the development site. To match the predevelopment hydrograph, the post-development peak rate must be less than or equal to the predevelopment peak rate, and the post-development runoff volume must be less than or equal to the predevelopment volume for the same storm event. A shift in hydrograph peak time of up to five minutes and a rate variation of up to 5% at a given time may be allowable to account for the timing affect of BMPs used to manage the peak rate and runoff volume. "Volume control" volumes as given in § 260-14 above may be used as part of this option.
Where the predevelopment hydrograph cannot be matched, the post-development rates of runoff from any regulated activity shall not exceed 50% of the peak rates of runoff for the new impervious surface area prior to development for the two-, ten-, twenty-five-, fifty-, and one-hundred-year storm events. [NOTE: A twenty-four-hour SCS type II storm or an IDF Curve Rational Method storm. See Table 2 in § 260-17 or (NOAA) Atlas 14 data for the specific project site.]
All basins not including groundwater recharge and/or water quality storage shall include an outlet structure to allow for draining the basin to a completely dry position within 24 hours following the end of the design rainfall. All basins that include groundwater recharge and/or water quality storage shall include an outlet structure to allow draining the basin to the level of the groundwater recharge and/or water quality storage within 24 hours following the end of the design rainfall.
Bio-retention areas (rain gardens).
Runoff from impervious areas shall be drained to pervious areas within the development site to the maximum extent practicable.
Stormwater runoff from a development site to an adjacent property shall flow directly into a natural drainageway, watercourse, or into an existing storm sewer system, or onto adjacent properties in a manner similar to the runoff characteristics of the predevelopment flow.
Stormwater flows onto adjacent property shall not be created, increased, decreased, relocated, or otherwise altered without written notification of the adjacent property owner(s) by the developer. Such stormwater flows shall be subject to the requirements of this chapter, including the establishment of a drainage easement. Copies of all such notifications shall be included in SWM site plan submissions.
Stormwater runoff shall not be transferred from one sub-watershed to another unless they are sub-watersheds of a common watershed that join together within the perimeter of the development site and the effect of the transfer does not alter the peak discharge onto adjacent lands.
All stormwater conveyance facilities (excluding detention, retention, and wetland basin outfall structures) shall be designed to convey a twenty-five-year storm event. NOTE: a twenty-four-hour SCS Type II storm or an IDF Curve Rational Method storm. All stormwater conveyance facilities (excluding detention, retention, and wetland basin outfall structures) conveying water originating from off site shall be designed to convey a fifty-year storm event. NOTE: a twenty-four-hour SCS Type II storm or an IDF Curve Rational Method storm. Safe conveyance of the one-hundred-year runoff event NOTE: a twenty-four-hour SCS Type II storm or an IDF Curve Rational Method storm to appropriate peak rate control BMPs and throughout the site must be demonstrated in the design.
Erosion protection shall be provided along all open channels, and at all points of discharge. Flow velocities from any storm sewer may not result in erosion of the receiving channel.
Any stormwater runoff calculations involving drainage areas greater than 200 acres and time of concentration (Tc) greater than 60 minutes, including on- and off-site areas, shall use generally accepted calculation techniques based on the NRCS Soil Cover Complex Method with the rainfall depths provided in Table 2, or other method acceptable to the Township Engineer.
Stormwater runoff from all development sites shall be calculated using either the modified Rational Method, a Soil-Cover-Complex methodology, or other method acceptable to the Township. Table 1 summarizes acceptable computation methods. It is assumed that all methods will be selected by the design professional based on the individual limitations and suitability of each method for a particular development site.
For development sites less than 200 acres, Tc<60 min. or as approved by the Township
Other methodologies approved by the Township
Twenty-Four-Hour Storm Event
If the Rational Method is used, the National Oceanic and Atmospheric Administration (NOAA) Atlas 14 data (See Subsection B above.) or PennDOT Publication 584 "PennDOT Drainage Manual," 2010 Edition, or latest, or the PDT-IDF chart included in Appendix B[1] shall be used to determine the rainfall intensity in inches per hour based on the information for the five-through-sixty-minute duration storm events.
Hydrographs may be obtained from NRCS methods such as TR-55, TR20, or from use of the "modified" or "unit hydrograph" Rational Methods. If "modified" or "unit hydrograph" Rational Methods are used, the ascending leg of the hydrograph shall have a length equal to three times the time of concentration (3xTc) and the descending leg shall have a length equal to seven times the time of concentration (7xTc) to approximate an SCS Type II hydrograph. (NOTE: See § 260-53K.)
Runoff calculations shall include a hydrologic and hydraulic analysis indicating volume and velocities of flow and the grades, sizes, and capacities of water carrying structures, sediment basins, retention and detention structures and sufficient design information to construct such facilities. Runoff calculations shall also indicate both predevelopment and post-development rates for peak discharge of stormwater runoff from all discharge points.
For the purpose of calculating predevelopment peak discharges, all runoff coefficients, both on site and off site, shall be based on actual land use assuming summer or good land conditions. Post-development runoff coefficients for off-site discharges used to design conveyance facilities shall be based on actual land use assuming winter or poor land conditions.
Runoff coefficients shall be based on the information contained in Appendix C and Appendix D[2] if the actual land use is listed in those appendixes. If the actual land use is not listed in these appendixes runoff coefficients shall be chosen from other published documentation, and a copy of said documentation shall be submitted with the SWM site plan.
Editor's Note: Said appendixes are on file in the Township offices.
A sample worksheet for calculating Tc is provided in Appendix H.[3] Times of concentration (Tc) shall be based on the following design parameters:
Sheet flow: The maximum length for each reach of sheet or overland flow before shallow concentrated or open channel flow develops is 150 feet. Flow lengths greater than 100 feet shall be justified based on the actual conditions at each development site. Sheet flow may be determined using the nomograph in Appendix F,[4] or the Manning's kinematic solution shown in the sheet flow section of Worksheet No. 1 in Appendix H.[5]
Shallow concentrated flow: Travel time for shallow concentrated flow shall be determined using Figure 3-1 from TR-55, Urban Hydrology for small watersheds, as shown in Appendix G.[6]
Open channel flows: At points where sheet and shallow concentrated flows concentrate in field depressions, swales, gutters, curbs, or pipe collection systems, the travel times to downstream end of the development site between these design points shall be based upon Manning's Equation and/or acceptable engineering design standards as determined by the Township Engineer.
The developer may use stormwater credits for nonstructural BMPs in accordance with the BMP Manual. The allowable reduction will be determined by the Township Engineer.
Times of concentration shall be calculated based on the methodology recommended in the respective model used. Times of concentration for channel and pipe flow shall be computed using Manning's Equation. Supporting documentation and calculations must be submitted for review and approval.
In order to protect and improve water quality, a riparian corridor easement shall be created and recorded as part of any land development that encompasses a riparian corridor.
Except as otherwise required by Chapter 102, the riparian corridor easement shall be measured 35 feet from the top of stream bank (on each side).
Any un-vegetated areas within the corridor shall be established with permanent vegetation.
The riparian corridor easement shall be enforceable by the Township and shall be recorded in the Lancaster County Recorder of Deeds office, so that it shall run with the land and shall limit the use of the property located therein. The easement shall allow for the continued private ownership and shall not be deemed a public right-of-way nor imply public rights of access.
Any permitted use within the riparian corridor easement shall be conducted in a manner that will improve or maintain the stream stability, and preserve and protect the ecological function of the floodplain.
Aboveground storage facilities consist of all stormwater facilities which store, infiltrate/evaporate/transpire, clean or otherwise affect stormwater runoff and the top of which is exposed to the natural environment. Aboveground storage facilities are located above the finished ground elevation. Aboveground storage facilities do not include SWM facilities designed for conveyance or cisterns.
All basins shall be structurally sound and shall be constructed of sound and durable materials. The completed structure and the foundation of all basins shall be stable under all probable conditions of operation.
Watertight joints in piping?
*Pretreatment required for infiltration BMPs unless shown to be unnecessary
SLHDPE = Smooth lined high density polyethylene pipe
RCP = Reinforced concrete pipe
For the purposes of the design criteria, the facility depth is defined to be the depth between the bottom invert of the lowest orifice and the invert of the spillway. If there is no spillway, the top of the berm shall be used. For basins with no orifices or outlet structure at the bottom of the basin, the bottom elevation of the basin shall be used.
Facilities with a facility depth greater than six feet shall not be permitted in residential areas.
The maximum depth of water for aboveground storage facilities without restricted access shall not exceed six feet unless approved by modification or waiver by the Board of Supervisors. Access to basins with a maximum depth of water greater than six feet shall be restricted by fencing that will discourage access.
Materials used for the core shall conform to the Unified Soil Classification GC, SC, CH, or CL and must have at least 30% passing the No. 200 sieve.
The area under the embankment shall be cleared, grubbed and stripped of topsoil to a depth of two feet prior to any placement and compaction of earthfill.
The core shall be constructed concurrently with the outer shell of the embankment. Core and key trench shall be constructed to a minimum of 95% standard Proctor density.
Pipe collars. All pipe collars, when required, shall be designed in accordance with Chapter 7 of the E&S Manual. The material shall consist of concrete or otherwise nondegradable material around the outfall barrel and shall be watertight.
Embankment fill material. The embankment fill material shall be taken from an appropriate borrow area which shall be free of roots, stumps, wood, rubbish, stones greater than six inches, frozen or other objectionable materials.
Embankment compaction. When required, embankments shall be compacted by sheepsfoot or pad roller. The loose lift thickness shall be nine inches or less, depending on roller size, and the maximum particle size is six inches or less 2/3 of the lift thickness). Five passes of the compaction equipment over the entire surface of each lift is required. Embankment compaction to visible nonmovement is also required.
Dewatering features. When required, dewatering shall be provided through the use of underdrain, surface device, or alternate approved by the Township Engineer. If the facility is to be used for infiltration, the dewatering device should be capable of being disconnected and only be made operational if the basin is not dewatering within the required time frame.
Pretreatment elements. When required, pretreatment elements shall consist of forebays, or alternate approved by the Township Engineer, to keep silt to a smaller portion of the facility for ease of maintenance.
Infiltration basins. Within basins designed for infiltration, existing native vegetation shall be preserved, if possible. For existing unvegetated areas or for infiltration basins that require excavation, a planting plan shall be prepared in accordance with § 260-13N and the BMP Manual which is designed to promote infiltration.
For facilities with a depth less than two feet, the designer must specify a suitable outlet structure.
Emergency use. The spillway shall be designed to convey the one-hundred-year peak rate of runoff which enters the basin after development in a manner which will not damage the integrity of the facility and will not create a downstream hazard.
Breach analysis. The Township may require a breach analysis based on site-specific conditions and concern of threat for downstream property. When required, the breach analysis shall be conducted in accordance with the NRCS methodology, the United States Army Corps of Engineers methodology (HEC-1) or other methodologies as approved by the Township.
Subsurface storage facilities consist of all stormwater facilities which store, infiltrate/evaporate/transpire, clean or otherwise affect stormwater runoff and the top of which is not exposed to the natural environment. Subsurface facilities are located below the finished ground elevation. Subsurface facilities do not include SWM facilities designed for conveyance.
Subsurface storage facilities shall comply with the design criteria in the following table:
For facilities with the bottom five feet or more below the average grade of the ground surface, a manhole or other means acceptable to the Township shall be provided for access to and monitoring of the facility.
Stone for infiltration beds. The stone used for infiltration beds shall be clean washed, uniformly graded coarse aggregate. The void ratio for design shall be assumed to be 40%.
Backfill material. Material consistency and placement depths for backfill shall be (at a minimum) per all applicable pipe manufacturer's recommendations, further providing it should be free of large (not exceeding six inches in any dimension) objectionable or detritus material. Select non-aggregate material should be indigenous to the surrounding soil material for non-vehicular areas. Backfill within vehicular areas shall comply with this section unless otherwise specified in the Township Road Ordinance[1] or by the Township Engineer. Furthermore, if the design concept includes the migration of runoff through the backfill to reach the infiltration facility, the material shall be well-drained, free of excess clay or claylike materials and generally uniform in gradation.
Editor's Note: See Ch. 222, Streets and Sidewalks.
When located under pavement, the top of the subsurface facility shall be a minimum of three inches below the bottom of pavement subbase.
Infiltration facilities shall be designed with measures to protect infiltration facilities from compaction and sedimentation during and after construction.
Conveyance facilities consist of all stormwater facilities that carry flow, which may be located either above or below the finished grade. Conveyance facilities do not include SWM facilities which store, infiltrate/evaporate/transpire, or clean stormwater runoff.
Non-Vehicular Loading
*SLHDPE, *RCP
Access/maintenance port frequency (maximum)
* Pipes crossing Township Collector Streets shall be RCP.
N/A = Not applicable or no criteria specified
Conveyance pipes, culverts, manholes, inlets and endwalls within the public street right-of-way or proposed for dedication shall conform to the requirements of PennDOT Standards for Roadway Construction, Publication No. 72M, as directed by the Township Engineer.
Conveyance pipes, culverts, manholes, inlets and endwalls outside the public street right-of-way which are subject to vehicular loading shall be designed for the HS-25 loading condition.
All material and workmanship for conveyance facilities shall conform to current PennDOT 408 specifications.
Backfill requirements. Backfill material. Material consistency and placement depths for backfill shall be (at a minimum) per all applicable pipe manufacturer's recommendations, further providing it should be free of large (not exceeding six inches in any dimension) objectionable or detritus material. Select non-aggregate material should be indigenous to the surrounding soil material for non-vehicular areas. Backfill within vehicular areas shall comply with this section unless otherwise specified in the Township Road Ordinance[1] or by the Township Engineer.
Access/maintenance ports. An access/maintenance port is required may either be an inlet or manhole.
The street crossing angle shall be measured between the pipe centerline and the street centerline.
The roughness coefficient (Manning "n" values) used for conveyance pipe capacity calculations should be determined in accordance with Appendix E,[2] or per the manufacturer's specifications.
Within the public street right-of-way, the gutter spread based on the twenty-five-year storm shall be no greater than 1/2 of the travel lane and have a maximum depth of three inches at the curbline. A parking lane shall not be considered as part of the travel lane. In the absence of pavement markings separating a travel lane from the parking lane, the parking lane shall be assumed to be seven feet wide if parking is permitted on the street.
All inlets placed in paved areas shall have heavy duty bicycle-safe grating consistent with PennDOT Publication 72M, latest edition. A note to this effect shall be added to the SWM site plan or inlet details therein.
Inlets within swales shall have PennDOT Type "M" top units or equivalent approved by the Township Engineer.
All vegetated swales shall have a minimum slope of 1% unless otherwise approved by the Township Engineer.
The "n" factors to be used for paved or riprap swales or gutters shall be based upon accepted engineering design practices, as approved by the Township Engineer.
Culverts. In addition to the material requirements in this section, culverts designed to convey waters of this commonwealth may be constructed with either a corrugated metal arch or a precast concrete culvert.
Level spreaders.
Energy dissipaters. Energy dissipaters shall be designed in accordance with the requirements in the E&S Manual.
Where the connecting pipe has a diameter 18 inches or greater, headwalls and endwalls shall be provided with a protective barrier device to prevent entry of the storm sewer pipe by unauthorized persons. Such protection devices shall be designed to be removable for cleaning.