Township of Upper Providence, PA Stormwater Management for Runoff Control

Any stormwater runoff calculation involving a project with 1.0 acre or greater of earth disturbance, including on- and off-site areas, that will require an NPDES permit shall use an accepted calculation technique that is based on the NRCS soil cover complex method. For project with less than 1.0 acre of earth disturbance, the rational method with a minimum ascending limb factor of three and a minimum receding limb multiplier of seven, universal rational method or other accepted similar method selected by the design professional that is based on the limitations and suitability of the method for a particular site. All stormwater runoff calculation methods are subject to authorization by the Township Engineer. All runoff hydrographs shall take into consideration the duration of the storm event to compute the runoff volume and routing.

[Amended 3-15-2021 by Ord. No. 588]

All calculations consistent with this chapter using the soil cover complex method shall use the appropriate design rainfall depths for the various return period storms.

For purposes of pre-development flow rate determination, undeveloped land shall be considered as "meadow, in good condition", unless the natural ground cover generates a lower curve number or rational "c" value.

All calculations using the rational method or derivation of the rational method shall use rainfall intensities consistent with appropriate times of concentration for overland flow and return periods from NRCS methodology as published in the NOAA Atlas 14. The time of concentration for sheet flow and shallow concentrated flow shall both be calculated using NRCS methodology. Times of concentration for channel and pipe flow shall be computed using Manning's equation or NRCS methodology. The predevelopment maximum flow length for sheet flow shall be 300 feet and the post development maximum flow length for sheet flow shall be 150 feet for disturbed area.

[Amended 3-15-2021 by Ord. No. 588]

All calculations using the rational method or derivation of the rational method shall use the runoff coefficients for the soil hydrologic classification and slope in Runoff Coefficients for the Rational Equation table in the PADEP E and S Manual.

[Amended 3-15-2021 by Ord. No. 588]

(6)

The design of any stormwater detention facilities intended to meet the performance standards of this chapter shall be verified by routing the design storm hydrograph through these facilities using accepted methods of practice. The Township Engineer may approve the use of any generally accepted reservoir routing technique, which shall use a total runoff volume that is consistent with the volume from a method that produces a full hydrograph. The computer routing program used must take into account the tail water effect of the discharge pipe on the orifice design as well as the submergence of the discharge pipe outlet.

[Amended 3-15-2021 by Ord. No. 588]

(7)

Outlet structures for stormwater BMPs shall be designed to meet the performance standards of this chapter using any generally accepted hydraulic analysis technique or method approved by the Township Engineer.

[Added 3-15-2021 by Ord. No. 588]

Stormwater detention and retention facilities. Stormwater detention and retention facilities shall meet the following minimum design/construction standards:

Detention basin shall be designed to facilitate regular maintenance, mowing and periodic silt removal and reseeding. Shallow broad basins are preferred to steep sided basins.

The maximum slope of the earth and detention basin embankment shall be three to one (3:1) with the exception that any slope to be maintained by the Township shall be 4:1. The top or toe of any slope shall be located a minimum of five feet from a property line. Whenever possible, the side slope and basin shape shall conform to the natural topography.

Unless permitted by the conditions of § 300-110B, detention basins shall not be located within floodplains nor within areas of floodplain or alluvial soils.

Detention basins shall be designed so they return to normal conditions within approximately 24 hours after termination of the storm, unless the Township Engineer finds that downstream conditions may warrant other design criteria for stormwater release.

If retention basins are used, the applicant shall demonstrate that such ponds are designed to protect public health, safety and welfare.

(6)

Fences may be required for any detention or retention basins where there is a permanent water surface or conditions warrant.

(7)

The minimum top width of the detention basin berm shall be 10 feet. A cutoff trench (keyway) of relative impervious material shall be provided beneath all embankments requiring fill material. The keyway shall be a minimum eight feet wide, minimum three feet deep and have 1:1 side slopes. Infiltration basins and rain gardens berms with impoundment depths less than 3.0 feet may be reduced to a minimum top width of 5.0 feet and are not required to have cutoff trench (keyway).

[Amended 3-15-2021 by Ord. No. 588]

(8)

In order to ensure proper drainage on the basin bottom, a minimum grade of 2% shall be maintained for sheet flow. Where a 2% slope cannot be maintained, low flow channels at a minimum grade of 1% constructed of concrete or other materials approved by the Township Engineer, shall be constructed between all basin inlets and the basin outlet. The bottom slope of detention facilities that are designed to also provide infiltration may be reduced to 0.0% provided that the proposed facility meets PADEP BMP design criteria.

[Amended 3-15-2021 by Ord. No. 588]

(9)

All detention and retention basin embankments shall be placed in eight-inch maximum lifts to a minimum ninety-five-percent dry density. Prior to proceeding to the next lift, compaction shall be checked by the Township Engineer or an approved soils engineer who shall provide the Township Engineer with a written report. Compaction tests shall be performed using the modified proctor method in accordance with ASTM D-1577. Compaction tests shall be run on the leading and trailing edge as well as the top of the berm.

(10)

Emergency overflow facilities shall be provided for detention facilities to accommodate runoff in excess of design flows. Whenever possible, emergency spillway for the detention basins shall be constructed on undisturbed ground. Emergency spillways shall be constructed of concrete pavers, gabions or other similar materials approved by the Township Engineer. All emergency spillways shall be constructed so that the detention basin berm is protected against erosion. The minimum capacity of all emergency spillway shall be the flow rate of the 100-year design storm after development. The construction material of the emergency spillway shall extend along the upstream and downstream berm embankment slopes. The upstream edge of the emergency spillway shall be a minimum of three feet below the spillway crest elevation. The downstream slope of the spillway shall, as a minimum, extend to the top of the berm embankment. The emergency spillway shall not discharge over earthen fill or easily erodible material.

(11)

The minimum freeboard from the spillway flow depth to the top of berm shall be one foot.

[Amended 3-15-2021 by Ord. No. 588]

(12)

Antiseep collars shall be installed around the pipe barrel within the normal saturation zone of the detention basin berms. The antiseep collars and their connections to the pipe barrels shall be watertight. The antiseep collars shall extend a minimum of two feet beyond the outside of the principle pipe barrel. The maximum spacing between collars shall be 14 times the minimum projection of the collar measured perpendicular to the pipe. A minimum of two antiseep collar shall be installed on each outlet pipe.

(13)

All outlet pipes through the basin berm shall be reinforced concrete pipe, designed to withstand the loading caused by a fully saturated berm and shall be watertight joints using O-ring joint pipe. Outlet pipe shall be backfilled with material similar to the core material (semi-impervious).

(14)

The invert of the inlet pipe(s) into a basin shall be six inches above the basin floor or lining so that it can adequately drain after rainstorms. Inlet pipe(s) shall discharge to areas of the basin that slope toward the outlet structure.

(15)

Energy dissipaters and/or level spreaders shall be installed at points where pipes or drainageways drain to or from the basin. Energy dissipaters shall comply with criteria in Hydraulic Engineering Circular No. 15 - Design of Stable Channels with Flexible Linings published by the Federal Highway Administration of the United States Department of Transportation of the Engineering Field Manual for Conservation Practices. NCRS energy dissipating device calculations shall be submitted for Township review and approval.

(16)

Inlet and outlet structures shall be located at a maximum distance from one another in order to promote water quality benefits. The Township Engineer may require a rock filter or rock-filled gabion for entrapping sediments carried in stormwater if sufficient separation of inlet and outlet structures cannot be achieved.

(17)

A perforated riser, or similar sediment control device, shall be provided at each outlet of all detention basins during construction for sediment control. The riser shall be constructed of metal or concrete. The riser shall extend to a maximum elevation of two feet below the crest elevation of the emergency spillway. The perforated riser shall be designed so that the rate of outflow is controlled by the pipe barrel through the basin berm, when the depth of water within the basin exceeds the height of the riser. Circular perforations with a maximum diameter of one inch shall be spaced 12 inches vertically. The horizontal spacing shall be in accordance to DEP soil erosion and sedimentation control manual specifications. The perforations shall be cleanly cut and shall not be susceptible to enlargement. All metal risers shall be suitably coated to prevent corrosion. A trash rack or similar appurtenance shall be provided to prevent debris from entering the pipe. All risers shall have concrete base attached with a watertight connect. The base shall be of sufficient weight to prevent flotation of the riser. An antivortex device consisting of a thin vertical plate normal to the base and berm shall be provided at the top of the riser. Unless this structure is part of the permanent outlet control, it shall be removed from the site when it has been adequately stabilized as determined by the Township Engineer.

(18)

All drainage channels shall be designed to prevent erosion of the bed and banks. The maximum permissible flow velocity shall not exceed the design requirements outlined in the current Soil Erosion and Sedimentation Control Manual, published by the Pennsylvania Depart of Environmental Protection. Suitable stabilization shall be provided where required to prevent erosion of the drainage channels.

(19)

Any vegetated drainage channel requiring mowing of the vegetation shall have a maximum of three horizontal to one vertical on those areas to be mowed.

(20)

Because of the critical nature of vegetated drainage channels, the design of all vegetated channels shall, as a minimum, conform to the design requirements outlined in the current Soil Erosion and Sedimentation Control Manual, published by the Department of Environmental Protection.

§ 258-12 Stormwater conveyance system.

General.

Storm sewers, culverts, bridges and related installations shall be provided:

(a)

To permit unimpeded flow of natural watercourses and in such a manner as to protect the natural character of the watercourses and to provide regulated discharge;

(b)

To insure adequate drainage of all low points along the line of streets; and

(c)

To intercept stormwater runoff along streets at intervals reasonably related to the extent and grade of the area drained and to prevent substantial flow of water across intersections.

All storm sewer system components shall conform to current PennDOT standards.

Drainage structures, which drain watershed areas in excess of one-half square mile (320 acres) or which have a span of eight feet or more, shall be designed for a maximum expected runoff as calculated using the Natural Resources Conservation Service Technical release 55 Urban Hydrology for Small Watersheds (less than 2,000 acres).

The design storm for the above structures shall be a 100-year storm. A water obstruction permit shall be obtained from the Pennsylvania Department of Environmental Protection for the waterway opening before final design is undertaken.

The cartway over the culvert or bridge shall be as wide as the ultimate width of the roadway approaches. Additional width may be required to provide sidewalk on one or both sides of the cartway.

Storm sewer design and construction requirements.

[Amended 6-2-2008 by Ord. No. 490]

Minimum pipe size shall be 15 inches in diameter. All storm sewer piping located under a driveway or a street, whether public or private, shall be reinforced concrete pipe, unless otherwise approved by the Township Engineer. All other storm sewer piping shall be either reinforced concrete pipe or high-density polyethylene (HDPE), unless otherwise approved by the Township Engineer.

[Amended 1-5-2009 by Ord. No. 499; 3-15-2021 by Ord. No. 588]

Minimum pipe slope shall be 0.005 ft./ft.

Minimum drop across junctions shall be two inches. At changes in pipe diameter, pipe crowns shall be matched at junctions (manhole, inlet or junction box).

Maximum distance between junctions shall be 300 feet.

Runoff to proposed storm sewers and inlets shall be calculated using the rational method.

(6)

Runoff coefficients.

Character of Land	Runoff Coefficient
Woods	0.18
Meadow	0.22
Pasture	0.30
Cultivated ground	0.35
Roofs and paving	0.95
Steep grass slopes (15% and <)	0.35
Gravel paving	0.85
Lawns (mowed)	0.26

These values should be weighted for actual design conditions by combining the various land use components, when approved by the Township

(7)

The time of concentration shall be assumed five minutes for pipes under 30 inches. For pipes 30 inches or greater, the calculated time of concentration can be utilized.

(8)

The time of concentration to inlets for grate capacity calculations shall be assumed five minutes.

(9)

All storm sewer pipes shall be designed at a minimum to accommodate a minimum of a ten-year storm. Twenty-five-year storms shall be used as required by the Township Engineer.

(10)

All storm sewer pipes at inlets in sump condition shall be designed to accommodate the fifty-year storm; floodplain and other critical areas shall be designed to accommodate the 100-year storm.

(11)

The rainfall intensity shall be obtained from the Storm Intensity-Duration-Frequency Curves for Region 5 in the Pennsylvania Department of Transportation Design Manual-Part 2.

(12)

All storm sewer pipes and inlets intended to drain to detention facilities shall be designed to accommodate the 100-year storm if the bypass or overflow runoff will not reach the basin by overland flow. In cases where the bypass or overflow runoff will flow over land, a stable swale shall be constructed to accommodate the excess runoff.

(13)

All inlets in sump condition shall be six-foot inlets or dual four-foot inlets, as needed.

(14)

All storm sewer systems shall be analyzed for both inlet and outlet control (including tailwater effects) by using the equations and nomographs as shown in the ERA's Hydraulic Design Services No. 5. In lieu of this, computer programs that calculate the actual hydraulic grade line for the storm sewer system can be used, provided that all losses (friction, bend, junction, etc.) are taken into account. Documentation for the program must be submitted for approval.

(15)

Minimum cover over pipes is two feet from finished grade to outside of pipe bell.

(16)

Inlet capacities shall be calculated using PennDOT or manufacturer's nomographs. Documentation for manufacturer's nomograph must be provided to the Township Engineer.

(17)

A minimum of one foot of freeboard must be provided between the inlet grate elevation or stormwater manhole rim and the hydraulic grade line elevation.

Shoulders in cut areas (without swales).

Water flowing in the shoulder shall not encroach more than two-thirds the shoulder width during a twenty-five-year frequency storm of five-minute duration.

The maximum velocity as determined by Manning's equation shall not exceed the allowable velocities for the specific type of shoulder material.

Inlets shall be provided to control the shoulder encroachment and water velocity.

Swales adjacent to shoulders.

Swales in cut areas shall be designed to prevent the passage of water on the cartway during a twenty-five-year frequency storm of five-minute duration.

The maximum velocity as determined by Manning's equation shall not exceed the allowable velocities for the specific type of shoulder material.

Curb sections.

The maximum encroachment of water on the roadway pavement shall not exceed three inches in depth at the curb during a twenty-five-year frequency storm of five minute duration.

Inlets shall be provided to control the encroachment of water on the pavement.

Inlets, general.

At street intersections, inlets shall be placed in the tangent portion, rather than the curved portion, of the curbing.

If the capacity of the shoulder, swale, curb section, or depressed median section exceeds the assumed inlet capacities, the inlet capacities shall govern the spacing of inlets.