Source: https://ecode360.com/7026261
Timestamp: 2019-03-21 12:14:25
Document Index: 477431133

Matched Legal Cases: ['§ 142', '§ 142', '§ 142', '§ 142', '§ 142', '§ 142', '§ 142', '§ 142', '§ 142', '§ 142', 'art 2', '§ 142', 'art 2', 'art 2', 'art 2', '§ 102']

Borough of Baldwin, PA Stormwater Management Performance Standards
Ch 142 Pt 2 Art XV Stormwater Management Performance Standards
§ 142-61 Stormwater zones: Monongahela River Watershed.
§ 142-62 General performance standards.
§ 142-63 Technical performance standards.
§ 142-64 Storm frequencies.
§ 142-65 Calculation methods.
§ 142-66 Release rate percentage.
§ 142-67 No-harm evaluation.
§ 142-68 Design criteria for stormwater management controls.
§ 142-69 Erosion and sedimentation controls.
§ 142-70 As-built drawings.
Article XV Stormwater Management Performance Standards
Chapter 142 Subdivision and Land Development Part 2 Stormwater Management Article XV Stormwater Management Performance Standards
For purposes of stormwater management, the municipality of Baldwin Borough is divided into stormwater management release rate zones. These zones are designated in the Monongahela River Watershed Plan, and a map of these zones may be found in Appendix D[1] of this chapter.
The following release rate zones are located within Baldwin Borough:
The location and boundaries of the watershed(s) and subareas are shown on the Municipal Stormwater Management District Zone Map which is hereby adopted as part of this section. A copy of this District Map is available through the Borough Office.
The following provisions shall be considered the overriding performance standards against which all proposed stormwater control measures shall be evaluated:
Any landowner and any person engaged in the alteration or development of land which may affect stormwater runoff characteristics shall implement such measures as are reasonably necessary to prevent injury to health, safety or other property. Such measures shall include actions as required and shall be evaluated according to the following standards:
No discharge of toxic materials into any stormwater management system shall be permitted. Where required by federal and state regulation, the landowner or developer shall be responsible for obtaining a NPDES permit for stormwater discharges.
For any development that is to be constructed in stages, the applicant must demonstrate that stormwater facilities shall be installed to manage stormwater runoff safely during each stage of development.
Where existing storm sewers are reasonably accessible, proposed developments shall be required to connect with the storm sewer system unless insufficient capacity or other reasons can be demonstrated to prevent the connection. The Borough Engineer shall determine what is reasonably accessible.
The stormwater performance standards contained in this article are intended to implement the standards and criteria contained in the Monongahela River Stormwater Management plan, adopted and approved in accordance with the Pennsylvania Storm Water Management Act.[1] If there is any discrepancy between the provisions of this section and the standards and criteria of the plan, or if the watershed plan is subsequently amended, then the standards/criteria of the current watershed plan shall govern.
Stormwater management facilities on all development sites shall control the peak stormwater discharge for the one-, two-, ten-, twenty-five-, fifty-, and one-hundred-year storm frequencies. The SCS twenty-four-hour Type II Rainfall Distribution shall be used for analyzing stormwater runoff for both pre- and post-development conditions. The twenty-four-hour total rainfall for these storm frequencies in the watershed are:
The Modified Rational Method shall be used for analyzing the stormwater runoff for small watersheds as specified in this article. The rainfall intensities for the design storms are as specified. (For additional information or data on other return periods, consult the Rainfall Duration Frequency Tables for Pennsylvania, produced by PaDEP, Office of Resource Management, Division of Stormwater Management, Harrisburg, February 1983, or in its most recent update.)
Development sites. For the purposes of computing peak discharges and runoff hydrographs from development sites and drainage areas larger than three acres, calculations shall be performed using the methodologies presented in SCS Publication, Technical Release 55 (TR 55) or TR 20, HEC 1, Penn State Runoff Method (PSRM), Modified Rational Method, or an approved method for analyzing these types of watersheds. For development sites less than three acres, the Rational Method may be utilized using the design storm criteria shown in § 142-63 or an approved method for analyzing these types of watersheds. The Borough may approve the use of simulation computer programs for the stormwater analysis and design. The appropriate calculations and worksheets or acceptable computer printouts as approved by the Borough must be submitted regardless of the methodology used for these calculations. An approved simulation of the modified puls (Haested Method Pond 2, HEC 1) routing methodology shall be used in analysis for routing the design storm hydrographs through the detention/retention facility. The proper stage/storage and stage/discharge and routed outflow hydrographs must be submitted for approval.
Stormwater collection/conveyance facilities. For the purpose of designing storm sewers, open swales and other stormwater runoff collection and conveyance facilities, the Rational Method shall be applied. Rainfall intensities for a twenty-five-year design storm with a minimum five-minute duration must be used and should be obtained from the Pennsylvania Department of Transportation's Part II Manual, Chapter 10, Rainfall Intensity Charts. However, if the existing Tc is larger than five minutes, it should be used as the rainfall duration.
Detention/retention basin. The inflow hydrograph shall be routed through the detention/retention facility by using the Modified Puls method or other recognized routing method subject to the approval of the Borough and the County. The routing and outflow hydrographs for each design storm must be submitted.
Pre-development conditions. Pre-development conditions shall be assumed to be those which exist on any site at the time of adoption of the Monongahela River Stormwater Management Plan. Hydrologic conditions for all areas with pervious cover (i.e., fields, woods, lawn areas, pastures, cropland, etc.) shall be assumed to be in good condition, and the lowest recommended SCS runoff curve number (CN) shall be applied for all pervious land uses within the respective range for each land use and hydrologic soil group.
Definition. The release rate percentage defines the percentage of the pre-development peak rate of runoff that can be discharged from an outfall on the site after development. It applies uniformly to all land development or alterations within a subarea.
Identify the specific subarea in which the development site is located from the watershed map located in Appendix D of this Part 2[1] and obtain the subarea release rate percentage from the following corresponding zone release rate percentages:
Assigned Release Rate Area
All other areas not located within an assigned zone by the Monongahela River Watershed Plan are assigned the release rate of 100%.
Compute the pre- and post-development runoff hydrographs for each stormwater outfall on the development site using an acceptable calculation method for the one-, two-, ten-, twenty-five-, fifty- and one-hundred-year storms. If the post-development peak runoff rate and the runoff volume is less than or equal to the pre-development peak runoff rate and volume, then additional stormwater control shall not be required at the outfall. If the post-development peak runoff rate and volume are greater than the pre-development peak runoff rate and volume, then stormwater detention shall be required. The maximum allowable release rate form the detention facility shall be calculated by multiplying the subarea release rate percentage by the pre-development rate of runoff from the development site for each of the four design storms.
An applicant may seek to exceed the otherwise applicable subarea release rate percentage by performing the "no-harm evaluation." This evaluation requires an independent engineering analysis to demonstrate that other reasonable options exist to prevent the occurrence of increased stormwater runoff discharge rates and/or velocities, or that measures can be provided to prevent increased stormwater discharge rates and/or velocities from increasing flood elevations and accelerating erosion at all downstream points in the watershed.
In the Monongahela River Watershed: only in instances where the discharge to a stream channel from the development site occurs directly to the Monongahela River, to an adequately sized storm or combined sewer which discharges directly into the Monongahela River, or through a properly sized and designed regional stormwater detention facility.
The analysis for the no-harm evaluation shall be submitted to the Municipal Engineer and the Allegheny County Planning Department for review and approval.
The no-harm evaluation shall be prepared by a registered engineer who is experienced in hydrology and hydraulics. The no-harm evaluation analysis shall be completed using the following procedure.
The analysis shall be completed using an approved calculation methodology in the following manner:
Develop the runoff hydrograph(s) for the design storms of the site and areas tributary to it using the approved methodology for pre-development conditions using the land use characterizations contained in the Monongahela River Watershed Stormwater Management Plan.
Develop the post-development discharge hydrograph from the proposed site using the approved methodology.
Subtract the runoff hydrograph ordinates under pre-development conditions (Step 1) from the discharge hydrograph ordinates (Step 2), maintaining the time scales of both hydrographs for one-to-one correspondence.
Obtain the hydraulic model for the existing conditions for the Monongahela River Watershed from the county.
Route the adjusted runoff hydrograph through the Monongahela River Watershed Model and note any increase in peak flows which would occur in downstream subbasins. If no increase is noted, then the no-harm has been demonstrated. If no increase is observed in peak flows, the increased potential for erosion and/or sedimentation in downstream channels resulting from any change in the flood hydrograph predicted by the model shall be evaluated. If no increased potential can be demonstrated by appropriated technical means, then the "No-Harm" exemption may be requested.
Applicants may select runoff control techniques, or combinations of techniques, as provided in this Part 2, Stormwater Management Ordinance, which are most suitable to the level of stormwater runoff control required, the type of development, and the natural features of the site. Cost of maintenance shall be one of the considerations in the designs selected. All controls are subject to the approval of the Borough Engineer. The Engineer may request specific information on design and/or operating features of the proposed stormwater controls in order to determine their suitability and adequacy in terms of the standards of this Part 2.
Urban Hydrology for Small Watersheds, Technical Release No. 55, USDA, Soil Conservation Service, June 1986 (or most recent edition).
Part II Design Manual, The Pennsylvania Department of Transportation, 1990 Edition (or most recent update).
HEC 1 and HEC 2, U. S. Army Corps of Engineers, most recent edition.
Soil Erosion and Sedimentation Control Manual, Pennsylvania Department of Environmental Resources, March 1982.
Standards and Specifications for Soil Erosion and Sediment Control, Maryland Water Resources Administration, 1983.
Urban Stormwater Management, Special Report No. 49, American Public Works Administration, 1981.
Water Resources Protection Measures in Land Development - A Handbook, University of Delaware Water Resources Center, April 1974.
Design and Construction of Sanitary and Storm Sewers, WPCF Manual of Practice No. 9, Water Pollution Control Federation, 1970.
The applicant should consider the effect on the proposed stormwater management techniques of any special soil conditions or geological hazards which may exist on the development site. In the event such conditions are identified on the development site, the Borough may require in-depth studies by a competent geotechnical engineer.
In developing a stormwater management plan for a particular site, stormwater controls shall be selected according to the following preference:
Stormwater detention/retention facilities.
Infiltration practices shall be used to the extent practicable to reduce peak runoff and promote groundwater recharge. A combination of successive practices may be used to achieve the applicable minimum control requirements. Justification shall be provided by the applicant for rejecting each of the preferred practices based on actual site conditions.
If detention facility(ies) are utilized for the development site, the facility(ies) shall be designed such that post-development peak runoff rates from the developed site are controlled to those rates defined by the subarea release rate percentage for the one-, two-, ten-, twenty-five-, fifty-, and one-hundred-year storm frequencies.
All detention facilities shall be equipped with outlet structures to provide discharge control for the four designated storm frequencies. Provisions shall also be made to safely pass the post-development one-hundred-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 one-hundred-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.
Shared-storage facilities which provide detention of runoff for more than one development site may be considered within a single subarea. Such facilities shall meet the design criteria contained in this section. In addition, runoff from the development sites involved shall be conveyed to the facility in a manner so as to avoid adverse impacts, such as flooding or erosion, to channels and properties located between the development site and the shared-storage facilities.
Where detention facilities will be utilized, multiple-use facilities such as lakes, ballfields or similar recreational uses, shall be considered first as storage facilities wherever feasible, subject to the approval of the Borough and Pennsylvania Department of Environmental Protection Chapter 105 Regulations.
Other considerations which should be incorporated into the design of the detention facilities include the following:
Inflow and outflow structures should be designed and installed to prevent erosion, and bottoms of impoundment-type structures must be protected from soil erosion. All outlet velocities must be calculated and submitted. If riprap is used, the sizing calculations must be submitted.
Control and removal of debris both in the storage structure and in all inlet or outlet devices must be a design consideration. Trash racks on the outlet structures are required to minimize debris that enters the structure.
Inflow and outflow structures, pumping stations and other structures should be protected and designed to minimize safety hazards.
The interior slope of a detention/retention basin shall be 3:1 (horizontal to vertical). The water depth at the perimeter of a storage pond should be limited to that which is safe for children. This is especially necessary if bank slopes are steep or if ponds are full and recirculating in dry periods. Restriction of access (fences, walls, etc.) may be necessary depending on the location of the facility.
Detention/retention basins shall require a supporting report from a geotechnical engineer including design recommendations for embankment construction, interior and exterior slopes, drainage swales and infiltration areas, and shall be constructed under the supervision of the geotechnical engineer.
Landscaping must be provided for the facility which harmonizes with the surrounding area. Landscaping of the pond embankment shall not be permitted.
Facility must be accessible for maintenance purposes, considering the frequency and type of equipment that will be required. A maintenance schedule must be submitted and approved by the Borough.
Details of the facility must be shown on the plan.
A riprap channel with underdrain for low flows must be sized properly and provided within the pond.
If an underground facility (i.e., tank, sump) is proposed, a cleanout manhole with steps must be provided.
An emergency spillway must be provided for all detention/retention facilities. For aboveground basins, the spills must be sized to pass the entire one-hundred-year post-development storm with one foot of freeboard.
Outlet control structures shall be constructed of reinforced concrete (cast-in-place, precast or block) and provided with trash racks approved by the Borough.
All detention facilities shall be provided with an access road (with a legal easement) for maintenance purposes. Such roads shall be minimum of 10 feet wide, have a maximum grade of 15% and be of a stone or impervious surface.
An as-built drawing shall be required for each stormwater detention facility constructed. (The drawing shall represent the volume of the facility and the stage-storage relationship.) The drawing shall be stamped by a registered professional engineer and submitted to the Borough within 60 days of the completion of the facility. No facility will be accepted until this requirement has been fulfilled.
The exterior slope of a detention/retention pond shall not exceed three horizontal to one vertical.
Normally dry, open-top, storage BMPs should drain in 24 hours to 72 hours.
Criteria for collection/conveyance facilities. All stormwater runoff collection or conveyance facilities, whether storm sewers or other open or closed channels, shall be designed in accordance with the following basic standards:
Lots located on the high side of streets shall extend roof and french drains to the storm sewer in the street or to the gutter line of the street if no sewer exists. Low side lots shall extend roof and french drains to a stormwater collection/conveyance system or natural watercourse in accordance with the approved stormwater management plan for the development site.
Collection/conveyance facilities should not be installed parallel and close to the top or bottom of a major embankment to avoid the possibility of causing the embankment to fail.
All collection/conveyance facilities shall be designed to convey the twenty-five-year storm peak flow rate from the contributing drainage area and to carry it to the nearest suitable outlet or natural watercourse. Off site conveyance shall be provided with easements to an existing storm sewer system or natural watercourse.
Where drainage swales or open channels are used, they shall be suitably lined to prevent erosion and designed to avoid excessive velocities. Calculations must be presented for the velocities and it must be shown that the proposed surface for the swale is adequate.
Wherever storm sewers are proposed to be utilized, they must comply with the following criteria:
Designed to traverse under seeded and planted areas. If constructed within 10 feet of the road paving, walks or other surfaced areas, the drains shall have a narrow trench, and maximum compaction of backfill to prevent settlement of the superimposed surface or development.
Installed after excavating and filling in the area to be traversed is completed, unless the drain is installed in the original ground with a minimum of three feet cover and/or adequate protection during the fill construction.
Designed with cradle when traversing fill areas of indeterminate stability; designed with anchors when gradient exceeds 20%, and designed with encasement or special backfill requirements when traversing under a paved area.
Designed to adequately handle the anticipated stormwater flow and to be constructed and maintained economically. The minimum pipe size shall be 15 inches in diameter.
Drain pipe, trenching, bedding and backfilling requirements shall conform to the requirements of the Borough and/or applicable requirements of the Borough and/or applicable requirements of the Pennsylvania Department of Transportation Specifications, Publication 408, latest edition.
All pipe shall be made of either PVC, corrugated plastic pipe, corrugated plastic pipe with smooth interior walls, corrugated metal/steel or reinforced concrete. All corrugated metal pipe shall be polymer coated, with bonding and paved inverts where prone to erode. Pipe under a borough cartway shall be reinforced concrete pipe with a minimum diameter of 15 inches.
Storm inlets and structures shall be designed to be adequate, safe, self-cleaning and unobtrusive and shall be consistent with the standards of the Borough.
Appropriate grates shall be designed for all catch basins, stormwater inlets and other entrance appurtenances in accordance with borough specifications.
Manholes shall be designed so that the top shall be at finished grade and sloped to conform to slope of finished grade. Top castings of structures located in roads or parking areas shall be machined or installed to preclude rattling.
Where a proposed storm sewer connects with an existing storm sewer system, the applicant shall demonstrate that sufficient capacity exists in the downstream system to handle the additional stormwater flow.
Storm sewer outfalls shall be equipped with energy dissipation to prevent erosion and conform with applicable requirements of the PaDEP for stream encroachments (Chapter 105 of the Department's Rules and Regulations).
Proposed erosion and sedimentation pollution control measures should be submitted with the stormwater management plan. These plans shall follow the guidelines set forth in the Borough's current Erosion and Sediment Pollution Control Ordinance under a separate cover. In addition, the plan shall be prepared in accordance with the Pennsylvania Erosion/Sedimentation Regulations (25 Pa. Code, Chapter 102) and the standards and guidelines of the county conservation district.
DEP has regulations that require an erosion and sediment control plan for any earth disturbance activity of 5,000 square feet or more under 25 Pa. Code § 102.4(b).
Evidence of any necessary permit(s) for regulated earth disturbance activities from the appropriate DEP regional office or county conservation district must be provided to the municipality. The issuance of an NPDES construction permit (or permit coverage under the statewide General Permit (PAG-2) satisfies the requirements Subsection C.
A copy of the erosion and sediment control plan and any required, permit, as required by DEP regulations, shall be available at the project site at all times.
Prior to releasing the final bond moneys, the Borough shall receive an as-built drawing, signed and scaled by a registered surveyor, and these plans shall include the following:
The location of any roads or driveways constructed for the land development or single-family home.
The location, tops and flow lines of all storm sewers constructed within the land development or single-family home.
The location, depth and contours of the detention or retention facilities constructed for the land development or single-family home.
A detail of the outlet structure for the detention or retention facility as constructed.
The location and details of all post-construction stormwater management BMPs including water quality devices.
A revised stormwater management report reflecting the final contours for a detention pond as constructed.