Township of Butler, PA Design Criteria

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

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

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

Easements from the affected landowner(s) are provided.

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.

Innovative stormwater BMPs and recharge facilities are encouraged (e.g., rooftop storage, drywells, cisterns, recreation area ponding, diversion structures, porous pavements, holding tanks, infiltration systems, 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 Municipal 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)

The design of all stormwater management facilities shall incorporate sound engineering principles and practices. The municipality shall reserve the right to disapprove any design that would result in the continuation or exacerbation of a documented adverse hydrologic or hydraulic condition within the watershed, as identified in the plan.

(6)

The design and construction of multiple use stormwater detention facilities are strongly encouraged. In addition to stormwater management, facilities should, where appropriate, allow for recreational uses, including ball fields, play areas, picnic grounds, etc. Consultation with the municipality and prior approval are required before design. Provision for permanent wet ponds with stormwater management capabilities may also be appropriate.

Multiple use basins should be constructed so that potentially dangerous conditions are not created.

Water quality basins or recharge basins that are designed for a slow release of water or other extended detention ponds are not permitted for recreational uses, unless the ponded areas are clearly separated and secure.

(7)

Should any stormwater management facility require a dam safety permit under PADEP 25 Pa. Code Chapter 105, the facility shall be designed in accordance with Chapter 105 and meet the regulations of Chapter 105 concerning dam safety.

Stormwater management facility design considerations. All stormwater management facilities shall meet the following design requirements:

All areas of a subdivision, land development plan or mobile home park to be developed with streets, residential lots or sites, recreation areas or other uses shall be graded to assure positive drainage.

Storm sewers, culverts and related installations shall be provided to ensure the controlled flow of natural watercourses and to guarantee the drainage of all low points along the curb or gutter lines, as well as at intervals related to slope, of all streets in or adjacent to the plan. Where driveways cross gutter, line or passage via a pipe below the driveway entrance of the type and size required by the Butler Township Engineer.

In designing the storm drainage system, the developer shall use as his guide the publication "Urban Hydrology for Small Watersheds," Technical Release No. 55, U.S. Department of Agriculture, Soil Conservation Service, June 1986, and as amended.

The predevelopment condition of the parcel to be developed, as determined by the Township Engineer, shall be considered the state of the land surface on the date that the developer entered into an agreement to purchase the property. If the property has been partially or completely developed at the time of purchase, the Board of Commissioners may, at its option, require the new owner, if he proposes to redevelop or further develop the property, to take remedial action for compliance with the requirements of this subsection. If remedial action is required, regardless of the condition of the surface, the design runoff shall not exceed curve number 75 applied to class C "good meadow" soils (see Tables 2-1 and 2-2 of TR 55). The developer may pass through stormwater originating upstream of his property, but if he elects to control it, bypass devices he installs shall be capable of handling the one-hundred-year twenty-four-hour peak flow, and he may trade off, as compensation for such installations, control measures that would normally be required on his property, provided that the total control system results in equal or better protection for downstream properties. If a property is to be laid out in lots, no reduction in lot size shall be permitted to accommodate storm drainage detention structures.

The developer shall consult with the Butler County office of the Soil Conservation Service before developing his storm drainage plans. The Township and others reviewing a stormwater control plan will use discretion in considering unusual conditions, such as drainage from several basins on the same property under development.

(4)

The stormwater drainage plan of a subdivision, land development plan or mobile home park shall be designed to assure that, after development, stormwater does not leave the property at a greater velocity or volume per second than before development commenced. The five-year twenty-four-hour peak flow of 3.3 inches, creating slightly out-of-bank conditions in downstream drainageways, shall be the maximum release rate conditions for water leaving the property.

The maximum storm of record over a twenty-four-hour period during the preceding 100 years (5.03 inches of precipitation in 24 hours or as may be subsequently revised) shall be the design condition to be used in developing the storm drainage system.

The difference between the one-hundred-year storm and the five-year storm shall be stored on the property for gradual release at the five-year rate.

Outlet works shall be designed so as to store water and control runoff for all storms of frequency up to and including the one-hundred-year storm.

(d)

Vertical riser pipes with trash racks and antivortex devices shall be provided in detention basins, unless an alternate design is approved by the Township.

(e)

Detention basins shall be designed to drain completely in a period not less than six hours.

(f)

No outlet structure from a stormwater management facility, or swale, shall discharge directly onto a municipal or state roadway without approval from the municipality or PennDOT.

(5)

Detention areas, where required to impede runoff, shall be designed to meet or exceed the following standards:

Except where a permanent pond is approved by the Township Commissioners, detention areas shall be designed to drain completely.

The height of the impoundment dam shall be not more than 10 feet measured between the top of the dam and the toe of the slope on the downstream side of the dam. The level of water within the impoundment shall not exceed 10 feet measured between the lowest point in the impoundment area behind the dam and the top of the emergency spillway.

The area to be occupied by the dam shall be cleared of all topsoil and organic materials prior to construction. The dam shall be built up in layers not to exceed six inches in depth with equipment providing ninety-five-percent compaction at optimum moisture conditions. The top of the dam shall be constructed 10% higher than its designed height to allow for settlement.

(d)

The core trench stabilizing the dam shall extend the full length under the dam and full height each side of the dam and shall be designed after a soil investigation at the dam site carried to at least 10 feet below the level of the toe of the dam or to bedrock. The core trench shall be not less than three feet in depth, not less than three feet in width at its bottom, and its sides shall slope not less than one-foot vertical rise to each one foot of horizontal run (1 to 1).

(e)

The settled elevation of the top of the dam at its lowest point shall be not less than one foot above the maximum water level to be impounded behind the dam and not less than two feet above the elevation of the emergency spillway.

(f)

The surfaces of the dam shall be planted in a mixture of perennial quick-catching grasses and crown vetch.

(g)

The faces of the dam shall not slope on either face less than one-foot vertical rise for each two feet of horizontal run and not more than a total on both faces of one-foot vertical rise for each five feet of horizontal run.

(h)

The horizontal drain pipe passing through the dam shall be smooth steel, schedule 40 or heavier, or corrugated steel or reinforced concrete pipe with equivalent strength and hydraulic characteristics. Antiseepage collars shall be placed not less than 28 feet apart along the length of the pipe. Each collar shall be fabricated of not less than one-fourth-inch steel plate, shall extend not less than two feet in all directions at right angles from the outer face of the pipe, shall be welded in place all around and shall be completely enclosed within the dam fill.

(i)

The top of the riser pipe to drain the pond shall be not less than three feet in height above the invert elevation of the horizontal pipe, shall be not less than four inches in diameter and shall be fitted with an antivortex device and a trash rack.

(j)

The emergency spillway shall be capable of passing the flow created by a storm producing 10 1/2 inches of rainfall over a six-hour period. Whether probable maximum precipitation will be required in the design of any detention structure will depend on the location and size of the individual structure, existing and potential development downstream, the maximum volume of water to be retained and the cross section design of the spillway. The Township and the reviewing agencies will use discretion in evaluating detention structure designs.

(k)

The low edge of a parking area, curbed or not, may serve the purpose of an emergency spillway to pass the overflow from a stormwater detention area, provided that the embankment below the edge is well stabilized with planting materials and the angle of the slope will not encourage erosion, in the opinion of the Township and other reviewing agencies. Parking areas shall not be used to store stormwater.

(l)

Stone low flow channels with underdrains shall be installed in all aboveground earthen detention facilities. Side slopes and channel slopes within the basin shall be 2% minimum.

(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. The invert of stormwater facilities may be lowered if adequate subsurface drainage is provided. Flows from underdrains need not be accounted for in volume or rate control calculations.

(7)

Exterior slopes of compacted soil shall not exceed 3:1, and may be further reduced if the soil has unstable characteristics.

(8)

Interior slopes of the basin shall not exceed 3:1.

(9)

Unless specifically designed as a volume control facility, all stormwater management facilities shall have a minimum slope of 2% extending radially out from the principal outlet structure. Facilities designed as water quality/infiltration BMPs may have a bottom slope of zero.

(10)

Landscaping and planting specifications must be provided for all stormwater management basins and be specific for each type of basin.

Minimal maintenance, saturation tolerant vegetation must be provided in basins designed as water quality/infiltration BMPs.

(11)

Written design reports and calculations shall be submitted for review and approval by the Township. Routing calculations using modified PULS methodology shall be included to assure outflow rates are in conformance with the requirements of this chapter. Calculations for storm pipe sizing shall also be included using the rational method and Manning's formula for a twenty-five-year storm using PennDOT intensity duration frequency curves from the Engineering Design Manual (latest edition).

(12)

The owner or developer shall enter into a legal agreement approved by the Township Solicitor which shall hold the Township harmless from any and all liability relating to storm drainage collection and its discharge during construction of the system and thereafter. The owner or developer shall secure, where necessary, off-site easements for storm drainage.

In all residential subdivisions and land developments where a homeowners' association is not proposed for the maintenance of common facilities, the developer shall dedicate stormwater management facilities to the Township, provided that a minimum twenty-foot right-of-way for access and easement around the entire perimeter of the facility is provided. All facilities which are dedicated to the Township shall be subject to payment of a stormwater maintenance fee as a prerequisite to acceptance of the dedication, said fee to be established by resolution of the Board of Township Commissioners based on the cubic capacity of the detention facility.

(13)

The costs of review and inspection by the Township Engineer of the stormwater management system as proposed by the developer shall be borne by the developer, whether or not the plan is ultimately approved. The Engineer shall review such changes as the developer may make in his plan for compliance with the Engineer's recommendations and shall advise the Board of Commissioners whether the revised plan is in compliance or not. The Board shall direct the Engineer to inspect an approved dam during its various stages of construction and to point out to the contractor, the developer and Board any deviations from the design as approved.

(14)

Catch basins small meet the Township standards and shall have inverts poured to allow self-cleaning. Catch basins shall be located no further apart than 250 feet, measured between catch basins on the same side of the street.

(15)

Graded areas shall be stabilized with erosion-resisting plantings placed immediately after the completion of grading. Graded slopes produced by placing fill earth over the preexisting surface shall be keyed in accordance with sound geotechnical practices.

(16)

Grades for embankments resulting from preparation of building lots or sites shall not exceed one-foot vertical rise for every two feet of horizontal run for fill slopes or one-foot vertical rise for every 1 1/2 feet horizontal run for slopes created by excavating in areas that have lain dormant for at least two years, except that steeper slopes may be permitted in areas where, in the opinion of a professional engineer, as evidenced in a written report concurred in by the Township Engineer, conditions are such as to allow slopes up to a maximum grade determined by the engineers.

(17)

No grading shall occur within five feet of any subdivision or development plan boundary, except as is needed for the entrance of streets or to grade off land immediately adjacent to a street to the street's elevation.

(18)

All persons, partnerships or corporations intending to excavate, fill or grade land in the Township shall be required to apply for a grading permit under the terms of Chapter 188, Grading and Excavating, of the Code of the Township of Butler, as amended, and shall have a valid grading permit in their possession prior to the start of any grading work. Work shall be undertaken in conformance with the requirements of this chapter and applicable regulations of Butler County.

§ 242-22 Calculation methodology.

All calculations shall be consistent with the guidelines set forth in the BMP Manual, as amended herein.

Stormwater runoff from all development sites shall be calculated using either the Rational Method or the NRCS Rainfall-Runoff Methodology. Other methods shall be selected by the design professional based on the individual limitations and suitability of each method for a particular site and approved by the Municipal Engineer.

Rainfall values.

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.

NRCS Rainfall-Runoff 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 be consistent with the following table:

	Return Interval (year)	24-hour Rainfall Total (inches)
	5	3.30
	100	5.03

Runoff volume.

Rational Method: not to be used to calculate runoff volume.

NRCS Rainfall-Runoff Method. This method shall be used to estimate the change in volume due to regulated activities. Combining curve numbers for land areas proposed for development with curve numbers for areas unaffected by the proposed development into a single weighted curve number is not acceptable.

Peak flow rates.

Rational Method. This method may be used for design of conveyance facilities only. Extreme caution should be used by the design 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 versus 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.

NRCS Rainfall-Runoff Method.

This method is recommended for design of stormwater management facilities and where stormwater runoff volume must be taken into consideration. The following provides guidance on the model applicability:

[1]

NRCS's TR-55: limited to 100 acres in size.

[2]

NRCS's TR-20, WinTR-20, WinTR-55, HEC-HMS: no watershed size limitations.

[3]

Other models as preapproved by the Municipal Engineer.

The NRCS antecedent runoff condition II (ARC II, previously AMC II) must be used for all simulations. The use of continuous simulation models that vary the ARC are not permitted for stormwater management purposes.

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

Runoff coefficients.

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

[1]

Editor's Note: Appendix C is included at the end of this chapter.

NRCS Rainfall-Runoff Method: use Table C-2 (Appendix C).[2] Curve numbers (CN) should be rounded to tenths for use in hydrologic models as they are a design tool with statistical variability. For large sites, CNs should realistically be rounded to the nearest whole number.

[2]

Editor's Note: Appendix C is included at the end of this chapter.

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, shall be considered meadow (good condition).

Design storm.

All stormwater management facilities shall be verified by routing the proposed one-, two-, ten-, twenty-five-, fifty-, and one-hundred-year hydrographs through the facility using the storage indication method or modified PULS method. The design storm hydrograph shall be computed using a calculation method that produces a full hydrograph.

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

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.

Time of concentration.

The time of concentration is to represent the average condition that best reflects the hydrologic response of the area. The following time of concentration (T_c) computational methodologies shall be used unless another method is preapproved by the Municipal Engineer:

Predevelopment: NRCS's Lag Equation:

Time of Concentration = T_c = [(T_lag/0.6)*60] (minutes)

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

Postdevelopment; commercial, industrial, or other areas with large impervious areas (greater than 20% impervious area): NRCS Segmental Method. The length of sheet flow shall be limited to 100 feet. T_c for channel and pipe flow shall be computed using Manning's equation.

Postdevelopment; residential, cluster, or other low-impact designs less than or equal to 20% impervious area: NRCS Lag Equation or NRCS Segmental Method.

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

The postdevelopment T_c shall never be greater that the predevelopment T_c for any watershed or subwatershed. This includes when the designer has specifically used swales to reduce flow velocities. In the event that the designer believes that the postdevelopment T_c is greater, it will still be set by default equal to the predevelopment T_c for modeling purposes.

The minimum T_c for any watershed shall be five minutes.