Township of Lower Paxton, PA Erosion and Sedimentation (E&S) Standards

Evidence of any necessary plan or permit approval for earth disturbance activities from PADEP or the Dauphin County Conservation District must be provided to the Township.

A copy of the approved erosion and sediment control plan and any other permit, as required by PADEP or the Dauphin County Conservation District, shall be available at the project site at all times if required under Chapter 102.

Construction of temporary roadways (e.g., for utility construction, timber harvesting, etc.) shall comply with all applicable standards for erosion and sedimentation control and stream crossing regulations under 25 Pa. Code, Chapters 102 and 105. The erosion and sedimentation control plan shall be submitted to the Dauphin County Conservation District for approval and shall address the following, as applicable:

(1)

Design of the roadway system, including haul roads, skid roads, landing areas, trails, and storage and staging areas.

(2)

Runoff control structures (e.g., diversions, culverts, detention ponds, etc.).

(3)

Stream crossings for both perennial and intermittent streams.

(4)

Access to public roadways, including design of rock construction entrance for mud and debris control.

(5)

A remediation plan for restoring the disturbed area through regrading, topsoil placement, reseeding, and other stabilization techniques, as required.

Additional erosion and sedimentation control design standards and criteria that must be applied where infiltration BMPs are proposed include the following:

(1)

Areas proposed for infiltration BMPs shall be protected from sedimentation and compaction during the construction phase, as to maintain their maximum infiltration capacity.

(2)

Infiltration BMPs shall be protected from receiving sediment-laden runoff.

(3)

The source of protection for infiltration BMPs shall be identified (i.e., orange construction fence surrounding the perimeter of the BMP).

§ 170-502 Design criteria statement for stormwater management and drainage facilities; stormwater management district implementation provisions.

Off-site areas. Off-site areas that 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.

"No harm" option. For any proposed development site, the applicant has the option of using a less-restrictive runoff control (including no detention), if the applicant can prove that "no harm" would be caused by discharging at a higher runoff rate than existing conditions. The "no harm" option is used when an applicant can prove that the post-development hydrographs can match predevelopment hydrographs, or if it can be proven that the post-development conditions will not cause increases in peak flows at all points downstream. Proof of "no harm" would have to be shown based upon a downstream impact evaluation, which shall include a downstream hydraulic capacity analysis consistent with § 170-502C to determine if adequate hydraulic capacity exists. The applicant shall submit this evaluation of the impacts due to increased downstream stormwater flows in the watershed to the Township.

(1)

The downstream impact evaluation shall include hydrologic and hydraulic calculations necessary to determine the impact of hydrograph timing modifications due to the proposed development upon a dam, highway, structure, natural point of restricted stream flow, or any stream channel section established with the concurrence of the Township.

(2)

Proof of "no harm" must be shown from the point of discharge through entire downstream network to the mouth of Paxton Creek or Spring Creek, as applicable, at the Susquehanna River, or from the mouth of Beaver Creek, as applicable, at the Swatara Creek. For those areas outside of the Paxton, Spring, or Beaver Creek Watersheds, proof of "no harm" must be shown through the entire downstream network to the ultimate discharge point at the Susquehanna River or Swatara Creek as applicable.

(3)

The peak flow values to be used for downstream areas for the design return period storms (two-year, ten-year, and twenty-five-year) shall be the values from the calibrated model for the Dauphin County Act 167 Plan. These flow values can be obtained from their respective watershed plans.

(4)

Applicant-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.

(5)

A financial hardship shall not constitute grounds for granting a "no harm" exemption.

(6)

Capacity improvements may be provided as necessary to implement the "no harm" option, which proposes specific capacity improvements to demonstrate that a less stringent discharge control would not create any harm downstream.

(7)

Any available capacity in the downstream conveyance system as documented by an applicant may be used by the applicant only in proportion to his/her development site acreage relative to the total upstream undeveloped acreage from the identified capacity (i.e., if the applicant's site is 10% of the upstream undeveloped acreage, he may use up to 10% of the documented downstream available capacity).

(8)

The Township in approving a "no harm" option shall notify all downstream municipalities of such approval. The Township shall make all relevant information used to prove a "no harm" option available upon request to any Township within the Paxton, Spring, or Beaver Creek Watershed.

(9)

Any "no harm" justifications shall be submitted by the applicant as part of the drainage plan submission. The burden of proof shall rest on the applicant.

Downstream hydraulic capacity analysis. Any downstream capacity hydraulic analysis conducted in accordance with this article shall use the following criteria for determining adequacy for accepting increased peak flow rates:

(1)

Natural or man-made channels or swales must be able to convey the increased runoff associated with a twenty-five-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 Erosion and Sediment Pollution Control Program Manual.

Regional detention alternatives. For certain areas within the watershed, 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 is the responsibility of prospective applicants. 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 calibrated model as developed for the Dauphin County Act 167 Stormwater Management Plan.

Capacity improvements may be provided as necessary to permit direct discharge in a provisional no-detention district, to implement any regional or subregional detention alternatives, or to implement a "no harm" option which proposes specific capacity improvements to document the validity of less stringent discharge control which would not create any harm downstream.

General design guidelines:

(1)

Stormwater shall not be transferred from one watershed to another, unless:

(a)

The watersheds are subwatersheds of a commonwatershed which join together within the perimeter of the property;

(b)

The effect of the transfer does not alter the peak rate discharge onto adjacent lands; or

(c)

Easements from the affected landowner(s) are provided.

(2)

Consideration shall be given to the relationship of the subject property to the drainage pattern of the watershed. A concentrated discharge of stormwater to an adjacent property shall be within an existing watercourse or confined in an easement or returned to a predevelopment flow type condition.

(3)

Stormwater BMPs and recharge facilities are encouraged (e.g., rooftop storage, drywells, cisterns, recreation area ponding, diversion structures, porous pavements, holding tanks, infiltration systems, stream channel storage, in-line storage in storm sewers, and grading patterns). They shall be located, designed, and constructed in accordance with the latest technical guidance published by PADEP, provided they are accompanied by detailed engineering plans and performance capabilities and supporting site-specific soils, geology, runoff and groundwater and infiltration rate data to verify proposed designs. Additional guidance from other sources may be accepted at the discretion of the Township Engineer (a preapplication meeting is suggested).

(4)

All existing and natural watercourses, channels, drainage systems and areas of surface water concentration shall be maintained in their existing condition unless an alteration is approved by the appropriate regulatory agency.

(5)

No outlet structure from a stormwater management facility, or swale, shall discharge directly onto a municipal or state roadway.

(6)

The invert of all stormwater management facilities and underground infiltration/storage facilities shall be located a minimum of two feet above the seasonal high groundwater table or other soil limiting zone. The invert of stormwater facilities may be lowered if adequate subsurface drainage, which does not alter the existing water table level, is provided.

(7)

Any stormwater management facility may be required to be fenced with a minimum four-foot-high fence of material acceptable to the Township. Gates with a minimum opening of 10 feet shall be provided for access.

(8)

Stormwater management facilities excavated to carbonate rock must either be fitted with an impervious clay liner, or over-excavated four feet and refilled with a suitable material mix. Suitable backfill material is subject to the approval of the Municipal Engineer.

(9)

The type, location, and number of landscaping and planting specification shall be provided for all stormwater management facilities and be specific for each type of facility.

Topsoil and seeding.

(1)

All pond areas, including bottoms, side slopes, and top of berms, shall be provided with a minimum four inches of topsoil and shall be seeded and mulched with Formula B seed mixture in accordance with PENNDOT Publication 408, Section 804.

Where uniform flow is anticipated, Manning's Equation shall be used for hydraulic computations and to determine the capacity of open channels, pipes, and storm sewers. Values for Manning's roughness coefficient "n" shall be consistent with values as presented in Table C-3, Appendix C of this chapter.[1] Manning's Equation should not be used for analysis of pipes under pressure flow or for analysis of culverts.

[1]

Editor's Note: Appendix C is included as an attachment to this chapter.

The Township has the authority to require that computed existing runoff rates be reconciled with field observations and conditions and site history. If the designer can substantiate, through actual physical calibration, that more appropriate runoff and time of concentration values should be utilized at a particular site, then appropriate variations may be made upon review and recommendation of the Township Engineer.

Principal outlet structures for stormwater management facilities shall be designed to meet the performance standards of this chapter using any generally accepted hydraulic analysis technique or method.

At all points of concentrated flow discharge from a site, an adequate channel must be provided downstream of the point of discharge to accommodate the flows from the facility so as to prevent damage to adjacent properties. If an adequate channel does not exist, an adequate channel shall be constructed or the point of discharge shall be piped to a point where an adequate channel exists. The approval and written consent of all affected downstream property owners must also be provided in the form of an executed drainage easement.

Stormwater management facilities (with a depth of water equal to or greater then three feet measured from the lowest point inside a facility to the crest of the emergency spillway):

(1)

Any stormwater management facility designed to store runoff and requiring a berm or earthen embankment, shall be designed to provide an emergency spillway to handle peak rate of stormwater runoff up to and including the 100-year post-development flow, with a blocked primary outlet structure. The height of embankment must be set as to provide a minimum one foot of freeboard through the spillway, above the maximum water surface elevation, computed when the spillway functions for the 100-year post-development inflow, with a blocked outlet structure. The primary outflow structure must be designed to pass all design storms (up to and including the 100-year event) without discharging through the emergency spillway. The maximum water depth within any stormwater management facility shall be no greater than eight feet when functioning through the primary outlet structure.

(2)

Emergency spillways shall be armored to prevent erosion during the 100-year post-development flow, with blocked primary outlet structure. Synthetic liners or rip-rap may be used, and calculations sufficient to support proposed armor must be provided. An earthen plug must be used to accurately control the spillway invert if rip-rap is the proposed armoring material. Emergency spillway armor must extend up the sides of the spillway, and continue at full width to a minimum of 10 feet past the toe of slope.

(3)

A stormwater management facility berm cross sections must be at least five feet wide at the top, and eight feet wide through the emergency spillway. For fill embankments, the side slopes shall be no steeper then 3:1 on the inside of the facility and 3:1 on the outside of the facility. For cut slopes, the side slopes shall be no steeper then 2:1.

(4)

A cutoff and key trench of impervious material shall be provided under all embankments four feet or greater in height.

(5)

Soils used for the construction of stormwater management facilities shall have low-erodibility factors ("K" factors) (refer to E&S Manual) and be identified on the SWM site plan.

(6)

Trash racks must be provided to prevent clogging of primary outflow structure stages for all orifices equivalent to 12 inches or smaller in diameter.

(7)

Anti-seep collars must be provided on all outflow culverts in accordance with the methodology contained in the latest edition of the E&S Manual. An increase in seepage length of 15% must be used in accordance with the requirements for permanent anti-seep collars.

(8)

Conventional, non-BMP stormwater management facilities (i.e., dry detention basins) must empty over a period of time not less than 24 hours and not more than 72 hours from the end of the facility's inflow hydrograph. Infiltration tests performed at the facility locations and proposed basin bottom depths, in accordance with the BMP Manual, must support time-to-empty calculations if infiltration is a factor is the sizing of the stormwater management facility.

(9)

Impervious low-flow channels are not permitted within stormwater management facilities to promote water quality and groundwater recharge for frequent storm events. Facilities designed as water quality/infiltration BMPs may have a bottom slope of zero. Minimal maintenance, saturation tolerant vegetation must be provided in basins designed as water quality/infiltration BMPs. Conventional, non-BMP stormwater management facilities must have a minimum slope of 1% extending radially out from the primary outlet structure. Water storage below the lowest outlet structure stage (i.e., dead storage) is permitted in stormwater management facilities designed as water quality/infiltration BMPs.

(10)

Stormwater management facilities bottom elevations must be greater than adjacent floodplain elevations (FEMA or HEC-RAS analysis). If no floodplain is defined, bottom elevations must be higher than existing ground elevations 50 feet from top of stream bank in the facilities vicinity.

(11)

Basin outflow culverts discharging into floodplains must account for tailwater. Tailwater corresponding to the 100-year floodplain elevation may be used for all design storms, or the applicant may elect to determine flood elevations of the adjacent watercourse for each design storm. The floodplain is assumed to be 50 feet from top of stream bank in areas where a floodplain is not designated, or where no other evidence is provided.

(12)

Exceptions to these requirements may be made at the discretion of the Township for BMPs that retain or detain water, but are of a much smaller scale than traditional stormwater management facilities.

Storm sewer facilities:

(1)

Storm sewers must be able to convey post-development runoff from a ten-year design storm without surcharging inlets where appropriate. When connecting to an existing storm sewer system, the applicant must demonstrate that the proposed system will not exacerbate any existing stormwater problems and that adequate downstream capacity exists.

(2)

A minimum pipe size of 15 inches in diameter shall be used in all roadway systems (public or private) proposed for construction. Pipes shall be designed to provide a minimum velocity of 2 1/2 feet per second when flowing full, but in all cases, the slope shall be no less then 0.5%. Arch pipe of equivalent cross-sectional area may be substituted in lieu of circular pipe where cover or utility conflict conditions exist.

(3)

In proposed curbed roadway sections, the maximum encroachment of water on the roadway pavement shall not exceed half of a through travel lane or one inch less than the depth of curb during the ten-year design storm of five-minute duration. Gutter depth shall be verified by inlet capture/capacity calculations that account for road slope and opening area. The maximum distance between inlets in curbed roadway sections shall be no more then 600 feet, however access to underground pipes shall be provided every 300 feet.

(4)

Standard Type "C" inlets with eight-inch hoods shall be used along vertical concrete curbs roadway networks. Type "C" inlets with ten-inch hoods that provide a two-inch sump condition may be used with approval of the Municipal Engineer when roadway longitudinal slopes are 1.0% or less.

(5)

For inlets containing a change in pipe size, the elevation for the crown of the pipes shall be the same or the smaller pipe's crown shall be at a higher elevation.

(6)

All inlets shall provide a minimum two-inch drop between the lowest inlet pipe invert elevation and the outlet pipe invert elevation.

(7)

On curbed sections, a double inlet shall be placed at the low point of sag vertical curves, or an inlet shall be placed at the low point and on each side of the low point at a distance not to exceed 100 feet, or at an elevation not to exceed 0.2 foot above the low point.

(8)

At all roadway low points, swales and easements shall be provided behind the curb or swale and through adjacent properties to channelize and direct any overflow of stormwater runoff away from dwellings and structures.

(9)

Inlets shall be placed so drainage cannot cross intersections or street center lines.

(10)

All inlets in paved areas shall have heavy-duty bicycle-safe grating consistent with PennDOT Publication 72M. A note to this effect shall be added to the SWM site plan or inlet details therein.

(11)

Inlets must be sized to accept the specified pipe sizes without corner penetrations. All pipes entering or exiting inlets shall be cut flush with the inlet wall. A note to this effect shall be added to the SWM site plan or inlet details therein.

(12)

Inlets shall have weep holes covered with geotextile fabric placed at appropriate elevations to completely drain the subgrade prior to placing the base and surface course on roadways.

(13)

Inlets, junction boxes, or manholes greater then five feet in depth shall be equipped with ladder rungs and shall be detailed on the SWM site plan.

(14)

Inlets shall not have a sump condition in the bottom (unless designed as a water quality BMP). Pipes shall be flush with the bottom of the box or concrete channels shall be poured.

(15)

Inlets, manholes, pipes, and culverts shall be constructed in accordance with the specifications set forth in PennDOT's Publication 408, latest edition, and as detailed in the PennDOT's Publication 72M, Standards for Roadway Construction (RC), latest edition, or as approved by the Municipal Engineer. All material and construction details (inlets, manholes, pipe trenches, etc.), must be shown on the SWM site plan, and a note added that all construction must be in accordance with PennDOT's Publication 408 and PennDOT's Publication 72M, latest edition. A note shall be added to the plan stating that all frames, concrete top units, and grade adjustment rings shall be set in a bed of full mortar according to Publication 408.

(16)

Accessible drainage structures shall be located on continuous storm sewer system at all vertical dislocations, at all locations where a transition in storm sewer pipe sizing is required, at all vertical and horizontal angle points exceeding 5° and at all points of convergence of two or more storm sewer pipes.

(17)

All storm drainage piping (equal to or greater than 12 inches) discharging to the ground surface shall be provided with either reinforced concrete headwalls and end sections or plastic and metal pipe end sections compatible with the pipe size involved in accordance with PennDOT Publication 408 and Publication 72M.

(18)

Outlet protection shall be provided at all surface discharge points with storm drainage piping (equal to or greater than 12 inches) in order to minimize erosion consistent with the E&S Manual.

(19)

Pavement base drain shall be provided at all low point in cut areas, toe of slope areas, and other areas as dictated by proven engineering principles and design judgment. All base drain shall be in accordance with PennDOT Publication 408.

Swale conveyance facilities:

(1)

Swales must be able to convey post-development runoff from a ten-year design storm with six inches of freeboard to top of the swale.

(2)

Swales shall have side slopes no steeper than 3:1.

(3)

All swales shall be designed, labeled on the SWM site plan, and details provided to adequately construct and maintain the design dimension of the swales.

(4)

Swales shall be designed for stability using velocity or shear criteria. Velocity criteria may be used for channels with less than 10% slope. Shear criteria may be used for all swales. Documentation must be provided to support velocity and/or shear limitations used in calculations.

(5)

Where swale bends occur, the computed velocities or shear stresses shall be multiplied by the following factor for the purpose of designing swale erosion protection:

(a)

When swale bend is 30° to 60°: 1.75.

(b)

When swale bend is 60° to 90°: 2.00.

(c)

When swale bend is 90° or greater: 2.50.

(6)

Swales must be designed for both temporary and permanent conditions in accordance with the latest E&S Manual.

§ 170-503 Calculation methodology.

All calculations shall be consistent with the guidelines set forth in the BMP Manual.

Stormwater runoff from all development sites shall be calculated using either the Rational Method or a Soil Cover Complex methodology. Methods shall be selected by the qualified professional based on the individual limitations and suitability of each method for a particular site.

Rainfall values:

(1)

Rational Method. The Pennsylvania Department of Transportation Drainage Manual, Intensity-Duration-Frequency Curves, Publication 584, Chapter 7A, latest edition, shall be used in conjunction with the appropriate time-of-concentration and return period.

(2)

Soil Cover Complex Method. The Soil Conservation Service Type II, twenty-four-hour rainfall distribution shall be used in conjunction with rainfall depths from NOAA Atlas 14 or consistent with the following table:

Return Interval (year)	24-hour Rainfall Total (inches)
1	2.40
2	2.90
10	4.36
25	5.43
50	6.38
100	7.48

Peak flow rates:

(1)

Rational Method: may be used for drainage areas up to 20 acres. Extreme caution should be used by the qualified professional if the watershed has more then one main drainage channel, if the watershed is divided so that hydrologic properties are significantly different in one verses the other, if the time of concentration exceeds 60 minutes, or if stormwater runoff volume is an important factor. The combination of Rational Method hydrographs based on timing shall be prohibited.

(a)

The use of the Modified Rational Method to design stormwater management facilities must be approved by the Municipal Engineer.

(2)

Soil Cover Complex Method: may be used for drainage areas greater then 20 acres. This method is recommended for design of stormwater management facilities and where stormwater runoff volume must be taken into consideration.

(3)

For comparison of peak flow rates, flows shall be rounded to a tenth of a cubic foot per second (cfs).

Runoff coefficients:

(1)

Rational Method: Use Table C-1 (Appendix C).[1]

[1]

Editor's Note: Appendix C is included as an attachment to this chapter.

(2)

Soil Cover Complex Method: Use Table C-2 (Appendix C).

(3)

For the purposes of predevelopment peak flow rate and volume determination, existing nonforested pervious areas conditions shall be considered as meadow (good condition).

(4)

For the purposes of predevelopment peak flow rate and volume determination, 20% of existing impervious area, when present on the project site, and contained within the new proposed limit of disturbance, shall be considered meadow (good condition) for predevelopment hydrologic calculations for redevelopment.

Design storm:

(1)

All drainage facilities (inlets, pipes, and swales) shall be designed to safely convey the ten-year storm.

(2)

All stormwater management facilities shall be verified by routing the proposed one-year, two-year, ten-year, twenty-five-year, fifty-year, and 100-year hydrographs through the facility using the Storage Indication (Modified Puls) Method. The design storm hydrograph shall be computed using a calculation method that produces a full hydrograph.

(3)

The stormwater management and drainage system shall be designed to safely convey the post-development 100-year storm event to stormwater detention facilities, for the purpose of meeting peak rate control.

(4)

All structures (culvert or bridges) proposed to convey runoff under a municipal road shall be designed to pass the fifty-year design storm with a minimum one foot of freeboard measured below the lowest point along the top of the roadway.

(5)

All design within state or federal rights-of-way or that falls under the design criteria of any higher authority must meet the requirements of that agency in addition to meeting the minimum requirements of this chapter.

Time of concentration:

(1)

Time of concentration shall be computed using the NRCS Segmental Method as described in TR-55 (SCS 1986, or most current update). The length of sheet flow shall be limited to 100 feet. The Manning's "n" Roughness Coefficient for TR-55 sheet flow can be found in Table C-4 (Appendix C).[2] Time of concentration for channel and pipe flow shall be computed using Manning's Equation.

[2]

Editor's Note: Appendix C is included as an attachment to this chapter.

(2)

For sites with insignificant channelized flow and less than 20% imperviousness coverage, the time of concentration may be computed using the NRCS equation for lag time:

Time of Concentration = T_c = [(T_lag/.6)*60](minutes)
T_lag	=	L^0.8	(S + 1)^0.7
			1900	Y

Where:
	T_lag	=	Lag time (hours).
	L	=	Hydraulic length of watershed (feet).
	Y	=	Average overland slope of watershed (percent).
	S	=	Maximum retention in watershed as defined by: S = [(1,000/CN) - 10].
	CN	=	NRCS Curve Number for watershed as defined by the NRCS Loss Method.

(3)

Additionally, the following provisions shall apply to calculations for time of concentration:

(a)

The post-development time of concentration shall never be greater that the predevelopment time of concentration for any watershed or subwatershed.

(b)

The minimum time of concentration for any watershed shall be five minutes.

(c)

The designer may choose to assume a five-minute time of concentration for any post-development watershed or subwatershed without providing any computations.

(d)

The designer must provide computations for all predevelopment time-of-concentration paths. A five-minute time of concentration can not be assumed for predevelopment.

(e)

Undetained fringe areas (areas that are not tributary to a stormwater facility but where a reasonable effort has been made to convey runoff from all new impervious coverage to best management practices) may be assumed to represent the predevelopment conditions for purpose of time-of-concentration calculations.

Drainage areas tributary to sinkholes or closed depressions in areas underlain by limestone or carbonate geologic features shall be excluded from the modeled point of analysis defining predevelopment flows. If left undisturbed during construction activities, areas draining to closed depressions may also be removed from peak runoff rates in the post-development analysis. New, additional contributing runoff shall not be directed to existing sinkholes or closed depressions.

Where uniform flow is anticipated, Manning's Equation shall be used for hydraulic computations and to determine the capacity of open channels, pipes, and storm sewers. Manning's Equation should not be used for analysis of pipes under pressure flow or for analysis of culverts. Manning's "n" values shall be obtained from Table C-3 (Appendix C).[3] Inlet control shall be checked at all inlet boxes to ensure the headwater depth during the ten-year design event is contained below the top of grate for each inlet box.

[3]

Editor's Note: Appendix C is included as an attachment to this chapter.

The Township may approve the use of any generally accepted full hydrograph approximation technique that shall use a total runoff volume that is consistent with the volume from a method that produces a full hydrograph.

The Township has the authority to require that computed existing runoff rates be reconciled with field observations, conditions and site history. If the designer can substantiate, through actual physical calibration, that more-appropriate runoff and time-of-concentration values should be utilized at a particular site, then appropriate variations may be made upon review and approval of the Township.