Borough of Fox Chapel, PA Article V Design Criteria

§ 375-26 Calculation methodology.

A. All computations used in conjunction with the analysis and design of stormwater management facilities shall be based on one or more of the following methods, or as otherwise approved in advance by the municipality:

(1) TR-55-Soil Conservation Service (SCS) Technical Release No. 55.

(2) TR-20-Soil Conservation Service (SCS) Technical Release No. 20.

(3) Modified Rational Method.

(4) Penn State Runoff Model.

(5) Virginia Tech/Penn State Runoff Model.

B. These methods for determining peak discharge shall be used to:

(1) Determine predevelopment runoff conditions;

(2) Analyze the impact of development; and

(3) Perform calculations in the design of any detention/retention facilities used in controlling runoff.

C. The SCS, Type II Rainfall Distribution shall be used for all analyses. The design storm frequencies for the watershed are:

Design Storm	24-Hour Rainfall Depth
1-year	1.97
2-year	2.35
5-year	2.88
10-year	3.30
25-year	3.90
50-year	4.40
100-year	4.92

D. Predevelopment conditions. The cover type for all sites will be considered to be the dominant land cover during the five-year period immediately preceding a proposed regulated activity:

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

(2) For the purposes of predevelopment peak flow rate and volume determination, 20% of existing disturbed impervious area, when present, shall be considered meadow (good condition).

E. Post-development conditions. The hydrologic parameters used to develop peak flow rates shall be reflective of anticipated soil runoff characteristics following grading and development of the site.

F. Time of concentration. The minimum time of concentration for any watershed shall be six minutes. A minimum six-minute time of concentration may be assumed for any post-development watershed. Calculations must be provided for all predevelopment times of concentrations. A minimum time of concentration cannot be assumed for predevelopment conditions.

G. The use of the Basic Rational Method in estimating runoff may be employed in the design of the storm sewer conveyance system within the development. The storm sewer system shall be interpreted as the conduits, culverts, inlets and appurtenant features for the conveying of stormwater to, through or from a development site to the point of final discharge or control facility. The Rational Method shall not be used in the analysis of stormwater runoff from the development in its entirety or in conjunction with the design of any retention/detention facilities or other runoff control measures.

H. Points of interest for analysis of pre- and post-development runoff must be selected to demonstrate that compliance with the requirements of §§ 375-16 through 375-17 is achieved at all points where runoff exits the perimeter of the property.

I. Stormwater runoff shall not be transferred from one watershed to another unless the watersheds are subareas of a larger watershed that are tributary to a common point of interest within or near the perimeter of the property. Transfer of runoff from one watershed to another under any other circumstances shall only be approved at the discretion of the municipality. Documentation shall be provided that peak flow rates are not increased following development and there will be no detrimental impact in downstream areas.

J. The stormwater management plan shall consider all of the stormwater runoff flowing over the project site. Runoff calculations shall be made to ensure that runoff from the upstream watershed area can be accommodated by the pipes, drainage easements and watercourses, etc. on the site.

§ 375-27 Standards for collection and conveyance facilities.

A. All stormwater collection and conveyance facilities (pipes, swales, and structures) shall be designed for a 100-year design storm event, unless the runoff would naturally drain overland to a stormwater detention facility, in which case a twenty-five-year design storm event may be used. The hydraulic gradeline must be a minimum of two feet below the surface elevation in structures and within the pipe. Swales and channels shall provide at least one foot of freeboard above the energy gradeline. Backwater effects of pipes discharging under surcharge conditions shall be included.

B. Runoff calculations shall include complete hydrology and hydraulic analysis of all downstream swales and pipe facilities to permanent stream discharge point, to the municipal boundary or connection to a state or county owned conveyance system.

C. Manholes shall not be spaced more than 300 feet apart for pipe sizes up to 24 inches in diameter and not more than 450 feet apart for larger pipe sizes.

D. No public stormwater pipe shall be less than 15 inches in diameter that conveys surface runoff. The minimum pipe slope shall be 1.0% grade or maintain velocity of two ft/s. For public storm sewer systems, only pipes related to construction of stormwater BMPs may be less than 15 inches in diameter.

E. All workmanship and materials shall conform to the municipality's construction standards. In addition, all workmanship and materials shall conform to the latest edition of PennDOT Form 408 and be supplied by manufacturers or suppliers listed in PennDOTs Bulletin 15.

F. Manhole and inlet castings shall conform to the Pennsylvania Department of Transportation Form 408 and PennDOT Standards for Roadway Construction. Inlet grates shall be bicycle safe. Frames and grates shall be cast iron or structural steel. Concrete frames shall not be permitted.

G. All connections to existing storm sewer pipes shall be made by construction of a suitable junction box (inlet or manhole) to provide access for cleanout. No blind connections shall be permitted.

H. The discharge of stormwater runoff shall be to a well-defined drainage course, which has a defined bed and bank. If stormwater runoff cannot be discharged to a defined drainage course, documentation of written permission from each downstream property owner shall be provided for all properties between the source of discharge and the defined drainage course. The use of level spreaders or similar BMPs that promote sheet flow may be permitted if the applicant provides documentation that the peak post-development 100-year discharge is no greater than predevelopment two-year discharge and a letter from the geotechnical engineer of record regarding slope stability of the downstream area is provided. Use of a level spreader requires a waiver to be granted by the municipality.

I. No property owner shall obstruct or alter the existing flow or location or add a new flow of stormwater runoff so as to direct it onto another property, or the carrying capacity of a stream, channel or drainage swale to the detriment of any other property owner, whether upstream or downstream. All subdivision and/or land development plans containing streams, channels, drainage swales, storm sewers or other conveyance systems that cross property boundaries, existing or proposed, or whose discharge crosses such boundaries shall contain a note stating the above.

§ 375-28 Standards for stormwater detention facilities.

A. All retention/detention facilities shall be equipped with multistage outlet structures to provide discharge control for each designated storm frequency. Provisions shall also be made to safely pass the post-development 100-year storm runoff in the event of an outlet structure failure without damaging or impairing the continued function of the facilities (i.e., impairing the continued function of the facilities). The facility(ies) must have an emergency outlet which is able to discharge the post-development 100-year peak flow. Should any stormwater management facilities qualify as a dam under PaDEP Chapter 105 criteria, the facility shall be designed in accordance with those regulations and meet the regulations concerning dam safety.

B. Any stormwater management facility designed to store runoff shall provide an emergency spillway designed to convey the unattenuated 100-year post-development peak rate flow with a blocked primary outlet structure and an inundation stormwater to the invert elevation of the spillway, with a minimum one-foot freeboard to the crest of the embankment.

C. Woody vegetation shall not be permitted on the embankments or within 25 feet of the emergency spillway.

D. The water depth of a storage facility which is not fenced shall be limited to two feet unless approved by the municipality. Otherwise, appropriate fencing at least four feet in height shall be required.

E. Side slopes of storage facilities shall not exceed a ratio of 3:1 horizontal to vertical dimension. The crest of the embankment shall have a minimum width of not less than 10 feet.

F. The facility shall be equipped with an access road at least 10 feet wide and with a maximum of grade of 15%. Access roads greater than 12% grade must be paved. Access roads 12% or less shall have a minimum six-inch depth noneroding aggregate surface. A gate with a minimum opening of 10 feet shall be provided for maintenance access. An access easement with a minimum width of 20 feet to all stormwater detention facilities shall be provided. The access easement shall include a statement on the recorded plan from the owner/operator of the facility granting access to the municipality.

G. All stormwater facility outlet structures shall have suitable gaskets to prevent leakage and piping of water through the facility embankment. All storm pipe installed through the facility embankment must be constructed of reinforced concrete pipe. Use of high-performance polypropylene pipe may be permitted with documentation of pipe manufacturer's backfill requirements.

H. A geotechnical investigation report for the construction of the stormwater detention/retention and infiltration facilities must be provided including design recommendation for embankment construction, interior and exterior slopes, drainage swales and infiltration areas.

I. Basin outlet structures shall have nonclogging trash racks overall design openings. Periodic cleaning of debris from trash racks shall be included in the operation and maintenance plan.

J. Inlet structures and outlet structures shall be separated to the greatest extent possible in order to maximize the flow path through the basin.

K. BMPs must be designed to protect and maintain existing uses (e.g., drinking water use; cold water fishery use) and maintain the level of water quality necessary to protect those uses in all streams, and to protect and maintain water quality in "Special Protection" streams, as required by statewide regulations at 25 Pa. Code, Chapter 93 (collectively referred to herein as "State Water Quality Requirements").