A.
For any of the activities regulated by this chapter, unless preparation of a stormwater management (SWM) site plan is specifically exempted, the preliminary or final approval of subdivision and/or land development plans, the issuance of any building or occupancy permit, the commencement of any earth disturbance activity may not proceed until the property owner or applicant or his/her agent has received written approval from the municipality of a SWM site plan that demonstrates compliance with the requirements of this chapter, and a written approval of an adequate erosion and sediment (E&S) control plan from the municipality or County Conservation District when required.
B.
SWM site plan approved by the municipality shall be on site throughout the duration of the regulated activity.
C.
The municipality may, after consultation with the Department of Environmental Protection (DEP), approve measures for meeting the state water quality requirements other than those in this chapter, provided that they meet the minimum requirements of, and do not conflict with, state law, including but not limited to the Clean Streams Law.[1]
D.
For all regulated earth disturbance activities, E&S control best management practices (BMPs) shall be designed, implemented, operated and maintained during the regulated earth disturbance activities (e.g., during construction) to meet the purposes and requirements of this chapter and to meet all requirements under Title 25 of the Pennsylvania Code and the Clean Streams Law. DEP regulations 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). In addition, under 25 Pa. Code Chapter 92, a DEP NPDES construction activities permit is required for regulated earth disturbance activities. A copy of the erosion and sediment control plan and any required permit, as required by DEP regulations, shall be available on the project site at all times. Various BMPs and their design standards are listed in the Erosion and Sediment Pollution Control Program Manual (E&S Manual), No. 363-2134-008 (April 15, 2000), as amended and updated.
E.
For all regulated activities, implementation of the water volume controls in § 190-23 is required.
F.
Impervious areas.
G.
Stormwater discharges onto adjacent property shall not be increased without written notification of the adjacent property owner(s). Such stormwater discharges shall be subject to the requirements of this chapter.
H.
All regulated activities shall include such measures as necessary to:
(1)
Protect health, safety and property;
(2)
Meet the water quality goals of this chapter by implementing measures to:
(a)
Minimize disturbance to floodplains, wetlands, and wooded areas.
(b)
Maintain or extend riparian buffers.
(c)
Avoid erosive flow conditions in natural flow pathways.
(d)
Minimize thermal impacts to waters of this commonwealth.
(e)
Disconnect impervious surfaces by directing runoff to pervious areas, wherever possible.
(3)
To the maximum extent practicable, incorporate the techniques for low-impact development practices described in the Pennsylvania Stormwater Best Management Practices Manual (BMP Manual).
I.
The design of all facilities over karst shall include an evaluation of measures to minimize adverse effects.
J.
Infiltration BMPs should be dispersed on site, made as shallow as practicable, and located to maximize use of natural on-site infiltration features while still meeting the other requirements of this chapter.
K.
Storage facilities should completely drain both the volume control and rate control capacities over a period of time not less than 24 and not more than 72 hours from the end of the design storm.
L.
The design storm precipitation amounts to be used in the analysis of peak rates of discharge shall be those from the upper limits of the 90% confidence intervals for the twenty-four-hour precipitation events in the Precipitation-Frequency Atlas of the United States, Atlas 14, Volume 2, Version 3.0, U.S. Department of Commerce, National Oceanic and Atmospheric Administration (NOAA), National Weather Service (NWS), Hydrometeorological Design Studies Center (HDSC), Silver Spring, Maryland. NOAA's Atlas 14 can be accessed at http://hdsc.nws.noaa.gov/hdsc/pfds/.
M.
N.
Various BMPs and their design standards are listed in the BMP Manual.
O.
Any stormwater facility located on state highway rights-of-way shall be subject to approval by the Pennsylvania Department of Transportation (PennDOT).
P.
Stormwater facilities shall not only handle the anticipated peak discharge from the property being subdivided or developed in accordance with the applicable release rate as set forth in this chapter, but also the existing runoff being contributed from all land at a higher elevation in the same watershed. The future use of undeveloped areas upstream shall be taken into account in the calculation of pipe sizes for storm sewer system designs. The capacity and maximum anticipated present flow of the body or system receiving the proposed system(s) discharge shall be calculated to verify its capability of receiving any additional flow caused by the development or subdivision. The runoff from any proposed development shall be subject to an evaluation that includes the anticipated runoff from other existing or proposed developments with the same watershed. Stormwater management facilities designed to serve more than one property or development in the same watershed are encouraged; consultation with the Township is required prior to design of such facilities.
Q.
Requirements for watercourses, streams, or intermittent streams.
(1)
Whenever a watercourse, stream, or intermittent stream is located within a development site, it shall remain open in its natural state and location and shall not be piped. It shall be the responsibility of the developer to stabilize eroded or erodible stream banks that can cause sedimentation in downstream waterways.
(2)
The Township may require a developer to provide a permanent easement along any watercourse located within or along the boundary of any property being subdivided or developed. The purpose of any such watercourse easement shall be for maintenance of the channel of any watercourse; the terms of the easement shall prohibit excavation, the placing of fill or structures, and any alterations that may adversely affect the watercourse. The easement shall conform to the line of the watercourse and shall be of sufficient width to protect the natural drainage system which shall be calculated using a one-hundred-year storm event; however, in no case shall the easement be less than 30 feet in width. The developer will retain the easement until such time as one of the following is accomplished:
(a)
If an easement acceptable to the municipality is established, the maintenance shall then be the responsibility of the individual lot owners over whose property the easement passes. For land developments, the maintenance shall then be the responsibility of the owner. The record plan shall contain description of such easement(s) and notation indicating the maintenance responsibility.
(b)
A homeowners' association or other approved legal entity, approved by the Township, assumes responsibility for the maintenance of the development, including the watercourse easement.
(3)
Horsham Township will not accept dedication of watercourse easements; however, it shall, upon satisfactory installation of improvements as specified in a land development agreement with Horsham Township, maintain and repair only the structural improvements within the easement, if any, such as piping; inlets; headwalls/endwalls; and energy dissipation structures or facilities. The Township will not be responsible for any other repair/maintenance within the easement such as lawn/ground cover maintenance; vermin control; removal of obstructions and accumulated sediment or vegetation; removal of litter or garbage; repair of erosion; and restoration of vegetation. The record plan and development agreement for the approved land development shall contain a provision permitting access to such easement(s), at any reasonable time, for inspection and/or emergency repair/maintenance by Horsham Township or its designee, of all facilities deemed critical to the public's welfare. In the event the lot owner or homeowners' association responsible for maintenance fails to honor the responsibility set forth herein, in any manner, Horsham Township shall have the right of entry upon and within the area of the easement to undertake any required corrective or maintenance effort. The total cost of such, including administrative, engineering, and legal costs for enforcement, may be imposed upon the responsible party as determined by Horsham Township. Failure to remedy all associated costs described above may be subject to the imposition of a lien by the Township against the owner(s) in question, in the same manner as the Township might otherwise be empowered by law to assess or impose a lien against a property for municipal improvements.
R.
The existing points/patterns and character of natural drainage discharge onto and off of adjacent property shall not be altered without the approval of the affected adjoining property owners. Verification of such approval must be obtained, in writing, and a copy filed with Horsham Township. Approval of plans by the Township does not authorize or sanction drainage affecting adjoining properties.
S.
When directed by the Township Engineer and/or where the finished roadway grade is within one foot of the seasonal high water table as designated for the site-specific soils type as designated by the Soil Survey for Montgomery County, storm sewer shall be designed as a combination storm sewer and underdrain. An acceptable alternative to the combination of storm sewer and underdrain would be a six-inch perforated corrugated polyethylene pipe. As much as practical, all storm sewers, as required, shall be placed in the right-of-way, parallel to the roadway. When located in undedicated land, storm sewers shall be placed within an easement not less than 30 feet in width or as otherwise approved by the Township Engineer.
T.
Where applicable, stormwater management facilities or programs shall comply with the requirements of Pa. Code Title 25, Rules and Regulations, Chapter 102 (Erosion Control), Chapter 105 (Dam Safety and Waterway Management), and Chapter 106 (Floodplain Management) of the Department of Environmental Protection.
U.
The developer may be required to participate in off-site storm drainage improvements in the drainage basin within which the proposed development is located. The specific contribution/improvements required shall be as specified by the Township Council and as specified in Article VIII of the Subdivision and Land Development Ordinance.[4]
V.
Stormwater roof drains from nonresidential and multiresidential buildings (where applicable) shall not discharge water directly onto a sidewalk or a street and shall be constructed to discharge overland prior to eventual discharge into a storm sewer system and/or detention basin.
W.
Water originating from other than natural sources, such as air-conditioning units, sump pumps, or other dry weather flow, wherever practical and possible, shall be discharged overland towards natural watercourses on the property. These facilities shall not discharge water under the sidewalk, through the curb, into the gutter.
X.
Where applicable, stormwater management plans/reports shall document downstream effects of detention and delayed or altered storm peaks, including necessary steps to avoid downstream combination of peak flows.
Y.
All stormwater runoff and floodplain calculations and stormwater management facilities design shall be prepared by a professional engineer licensed in the State of Pennsylvania.
Z.
Flood protection. No stormwater runoff or natural drainage shall be so diverted as to overload existing drainage systems, or create flooding or the need for additional drainage structures on other private properties or public lands, without proper and approved provisions being made for resolving flooding conditions.
AA.
Low-point drainage. The developer shall grade and install all necessary drainage facilities to insure drainage of all low points on subdivided or developed land areas (except in areas of protected wetlands).
BB.
No person, corporation, or other such entity shall block, impede the flow of, alter, construct any structure, or deposit any material or thing, or perform any work that will affect the normal runoff onto or off of an adjacent parcel.
CC.
All plans wherein a sump pump or basement drain is proposed to be installed shall provide for discharge of the sump pump/drain overland to a natural watercourse, drainage swale, stormwater easement, or storm sewer system.
DD.
Where stormwater or surface water will be gathered within the subdivision or land development and discharged or drained in volume over lands within or beyond the boundaries of the subdivision or land development, the subdivider, developer, or builder shall reserve or obtain easements over all lands affected. The easements shall be adequate for such discharge of drainage and for carrying off of such water and for the maintenance, repair, and reconstruction of the same, including the right of passage over, including vehicles, machinery and other equipment for such purposes, and which shall be sufficient width for such passage and work. If requested by the Township, the subdivider, developer, or builder shall convey, at no cost, the easements to the Township. Otherwise, all such easements shall be owned and maintained by individual lot owner(s) or another approved entity. Ownership and maintenance responsibilities shall be noted on the record plan and agreements for such shall be as approved by the Township Solicitor.
EE.
Storm sewers and drainageways.
(1)
General provisions. All storm drains and drainage facilities such as gutters, catch basins, bridges, inlets, and culverts shall be installed and the land graded for adequate drainage as shown on the grading plan submitted and approved with the final plan. Construction of these facilities shall generally conform with PennDOT Specifications Publication 408, latest version.
(2)
When required. Storm drains and appurtenances shall be required to be constructed by the developer to take surface water from the bottom of vertical grades to lead water away from springs, and to avoid excessive use of cross gutters at street intersections and elsewhere.
(a)
Open watercourses will be permitted where they exist naturally and where, in the opinion of the Township Engineer, they will not interfere with public convenience or safety, but in fact will provide comparable or superior drainage capabilities of piped drainage. Where necessary, stream corridor improvements, including rock stabilization and gabions, shall be performed.
(b)
When submitting a plan for approval involving the construction of storm drainage facilities, the designer's computations shall be submitted in duplicate to facilitate the checking of design.
(c)
Design of storm drainage facilities shall be completed in accordance with accepted engineering practices subject to approval by the Township Engineer.
(d)
Where practical, inlets shall be situated in grassed swales or depressions.
(3)
Unnatural drainage. Wherever construction stops or concentrates the natural flow of storm drainage in such a way to affect adjoining properties, approval of the owners shall be obtained, in writing, and a copy filed with the Township. Approval of plans by the Horsham Township does not authorize or sanction drainage affecting adjoining properties.
(4)
Design criteria.
(a)
Open channels should have a parabolic or trapezoidal cross section in compliance with accepted engineering practices.
(b)
Permissible channel velocities, slopes, and cover shall be in accordance with the Natural Resource Conservation Service Engineering Field Manual, Chapter 7, Grassed Waterways and Outlets."
(c)
Existing stream channels shall be maintained in their natural state. Only under unusual circumstances will it be permitted to line, straighten, or relocate an existing stream, with approval of the Department of Environmental Protection (DEP) and the Montgomery County Conservation District.
(d)
Acceptable energy dissipation devices shall be installed to bring discharge velocities down to limits specified in the DEP Erosion and Sediment Pollution Control Program Manual, latest edition.
(e)
Rip-rapping and/or gabions may be required by the Township Engineer where erosion potential is great.
FF.
Storm sewer design.
(1)
Design flow rate.
(a)
The storm sewer system shall be designed to carry the one hundred-year peak flow rate. The design one-hundred-year peak flow rate into each inlet shall be indicated in storm sewer calculations, and the drainage area to each inlet shall be indicated on the stormwater management plan. The one-hundred-year flow rate shall be determined by the Rational Method formula Q = CIA, where:
Q | = | Peak runoff rate measured in cubic feet per second (cfs). | |
C | = | Runoff coefficient. The coefficient of stormwater runoff includes many variables, such as ground slope, ground cover, shape of drainage area, etc. | |
I | = | Intensity. average rainfall intensity in inches per hour for a time equal to the time of concentration. | |
A | = | Area. drainage area in acres. |
(b)
Appropriate values for the rainfall intensity can be found in the Pennsylvania Department of Transportation Design Manual, Part 2. Appropriate values for the runoff coefficient shall be as presented below:
Rational Method Runoff Coefficients ("C") for the Design of Storm Sewer | ||
|---|---|---|
Description of Area | Runoff Coefficient ("C") | |
Residential | ||
Two-acre lots | 0.4 | |
One-acre lots | 0.45 | |
One-half-acre lots | 0.5 | |
One-fourth-acre lots | 0.6 | |
Townhouses, multifamily | 0.7 | |
Commercial | 0.75 | |
Industrial | 0.8 | |
Parks, cemeteries | 0.35 | |
Meadow | 0.3 | |
Unimproved (forest, brush, meadow, mixed) | 0.25 | |
(c)
Where the above table is not applicable, the following factors may be used for smaller drainage areas:
Rational Method Runoff Coefficients ("C") for the Design of Storm Sewer (Smaller Watersheds) | ||
|---|---|---|
Description of Area | Runoff Coefficient ('C') | |
Impervious | 0.95 | |
Grass | 0.35 | |
Forest | 0.25 | |
(d)
More specific runoff coefficients, as stated in the Pennsylvania Department of Transportation Design Manual, Part 2, or as stated in the Pennsylvania Department of Environmental Protection publication, "Recommended Hydrologic Procedures for Computing Urban Runoff from Small Watersheds in Pennsylvania," may be used upon consultation with the Township Engineer.
(2)
Consideration shall be given to future land use changes in the drainage area in selecting the "C" coefficient. For drainage areas containing several different types of ground cover, a weighted value of "C" shall be used. In no case shall a weighted value of "C" be less than 0.45 for an area to be changed from its natural state.
(3)
In determining the peak flow rate to individual storm sewer inlets (or other collection structures) the time of concentration method shall be used for inlet drainage areas in excess of 1.25 acres, unless otherwise specified by the Township Engineer, and a five-minute time of concentration shall be used for all other drainage areas, unless otherwise approved by the Township Engineer.
(4)
In determining the required design flow rate through a storm sewer piping system, a five-minute time of concentration (storm duration) shall be used if the five-minute time of concentration does not result in a maximum expected discharge that exceeds the capacity of a thirty-inch-diameter pipe (or equivalent flow area of 4.9 square feet).
(5)
In determining the required design flow rate through a storm sewer piping system, if a five-minute time of concentration (storm duration) results in a pipe size exceeding a thirty-inch-diameter pipe (or equivalent flow area of 4.9 square feet), the time of concentration approach shall be used in determining storm duration.
(6)
In determining the required design flow rate through a storm sewer piping system, if a five-minute time of concentration results in a pipe size exceeding 30 inches, within any run of pipe, the time of concentration approach may be used for sizing of pipes from that point on by adjusting the time of concentration.
GG.
Required stormwater management facilities.
(1)
Storm sewer easements.
(a)
Easements shall be a minimum of 30 feet in width and shall be provided in undedicated land to accommodate required storm sewer facilities and drainage swales. Such easements shall be offered for dedication to Horsham Township and shall be provided where storm sewer, drainage swales, culverts, or other structures traverse, enter or discharge onto private property. On private property, the property owner shall maintain ground cover and landscaping within the easement. The Township shall not maintain and/or repair any improvements within that easement unless stormwater runoff from public roads or public park land crosses through the easement. If stormwater runoff from public roads or public lands crosses through the easement, the Township shall, upon satisfactory installation of improvements as specified in a land development agreement with Horsham Township, maintain and repair only the structural improvements within the easement, if any, such as piping; inlets; stormwater manholes; headwalls/endwalls; and energy dissipation structures or facilities. Horsham Township will not be responsible for any other repair/maintenance within the easement, such as lawn/ground cover maintenance; vermin control; removal of obstructions from drainage swales; removal of accumulated sediment or vegetation from drainage swales; removal of litter or garbage; repair of erosion; and restoration of vegetation/ground cover.
(b)
The record plan and development agreement for the approved land development shall define easement ownership, maintenance responsibilities, and access. Specifically, the record plan shall contain a provision permitting access to such easement(s), at any reasonable time, for inspection and/or emergency repair/maintenance, by Horsham Township or its designee, of all facilities deemed critical to public welfare. In the event the lot owner or homeowners' association fails to honor their maintenance responsibilities set forth herein, in any manner, Horsham Township shall have the right of entry upon and within the area of the easement to undertake any required corrective or maintenance effort. The total cost of such, including administrative, engineering, and legal costs for enforcement, may be imposed upon the responsible party as determined by Horsham Township. Failure to remedy all associated costs described above may be subject of the imposition of a lien by the Township against the owner(s) in question in the same manner as the Township might otherwise be empowered by law to assess or impose a lien against a property for municipal improvements.
(2)
Piping.
(a)
The capacity of all pipes shall, as a minimum, provide the required carrying capacity as determined by the U.S. Department of Commerce, Hydraulic Design Series No. 5, Hydraulic Design of Highway Culverts. Where pressure flow is anticipated in storm sewer pipes (non-open-channel flow), the applicant's designer shall be required to calculate the elevation of the hydraulic grade line through the storm sewer system. Wherever the hydraulic grade line elevation exceeds the pipe crown elevation for the design flow, pipes with watertight joints must be specified.
(b)
The top of storm sewer pipes shall be at least six inches below the subgrade elevation of bituminous pavements, except for reinforced concrete pipe which may be at subgrade elevation, provided the applicant verifies the pipe will withstand current PennDOT HS-25 loading standards. For pipes in pervious areas, the top of pipe elevation shall be a minimum of one foot below the finished surface.
(3)
Overflow system. An overflow system shall be provided to carry all bypass flow and/or flow in excess of storm sewer design capacity to the detention basin (or other approved outlet point) when the capacity of the storm sewer system is exceeded. Stormwater runoff will not be permitted to surcharge from storm sewer structures.
(4)
Manholes.
(a)
Storm sewer manholes shall be spaced as necessitated by the proposed piping alignment; however, in no case shall manhole spacing exceed 300 feet, except between manholes connected by pipe with a diameter of 48 inches or greater, where spacing shall then not exceed 450 feet.
(b)
Manhole castings and covers shall conform to requirements of Pennsylvania Department of Transportation Publications 72 and 408, latest revisions. Manhole covers shall have the word "STORM" cast in two-inch-high letters on the top of the cover.
(5)
Inlets.
(a)
Storm sewer inlets shall be spaced in accordance with this chapter; however, in no case shall inlet spacing exceed 250 feet, except between inlets connected by pipe with a diameter of 48 inches or greater, where spacing shall then not exceed 450 feet.
(b)
Any inlet to be utilized in a storm sewer system shall conform to the design standards of Pennsylvania Department of Transportation (PennDOT) Publications 408 and 72 (Road Construction Standards). Specifically, inlets to be utilized within a public right-of-way shall be Type "C" with the "Alternate" inlet concrete top unit (which includes a Type "C" structural steel frame) and Type "M" with standard concrete top unit. All other inlets shall be Type "C" and "M" with standard concrete top units. All inlet grates shall be bicycle-safe structural steel.
(c)
At street intersections, inlets shall be placed in the tangent and not in the curved portion of the curbing. The gutter adjacent to and immediately upgrade from the inlet shall be so warped as to direct the water into the inlet. Inlets shall be required on the upslope side of all approaches to an intersection.
(d)
The crowns of all pipes tying into an inlet or manhole shall be set at equal elevations. A minimum of two inches shall be provided between the inlet pipe invert elevation and the invert elevation of the outlet pipe.
(e)
The capacity of inlets shall be based on a maximum surface flow to the inlet of four CFS (cubic feet per second). The maximum flow to inlets located in low points of paved areas (such as sag vertical curves) shall include the overland flow directed to the inlet, as well as the cumulative bypass surface runoff from inlets upstream. The bypass runoff from each upstream inlet shall be calculated using inlet efficiency curves (or other analysis method approved by the Township Engineer) found in the Pennsylvania Department of Transportation Design Manual, Part 2, latest revision. An inlet at a low point of a paved area may be designed to accept a maximum of six cubic feet per second (CFS). Type "M" inlets shall be designed to accept a maximum surface flow of six CFS, unless otherwise approved by the Township Engineer. Double inlets will not be permitted where additional pipe and inlets can be placed upstream to intercept the excessive surface flow. Under special circumstances, a maximum of 12 CFS shall be permitted to be collected by an inlet located in an isolated pervious area, provided the designer can prove such an inlet would not cause stormwater to accumulate on any adjacent public or private property, outside of the associated proposed storm sewer easement, and that the depth of the accumulated stormwater would not exceed 12 inches. Double four-foot or six-foot inlets separated by 20 linear feet of pipe shall be required if adequate efficiency is not realized with four-foot inlets.
(f)
Inlet spacing in paved areas shall be arranged so that a minimum of 80% of the gutter flow tributary to the inlet will be captured. The designer shall be required to verify that bypass surface runoff from the one-hundred-year design storm will enter the storm sewer piping system at some point, prior to discharge into a detention basin or other approved outlet point. Inlets shall be spaced so that the accumulation of surface bypass runoff and surface runoff tributary to an individual inlet will not exceed other design requirement specified in this chapter. Inlet capacity shall be based on inlet efficiency curves provided in Pennsylvania Department of Transportation, Design Manual, Part 2.
(g)
A minimum of one foot of freeboard, between the inlet grate and the design flow elevation, shall be provided in all storm sewer systems (inlets and manholes).
(h)
Inlet castings, together with their covers or gratings, shall conform to PennDOT or Horsham Township standards, as may be in effect at the time the design of the sewer is submitted.
(i)
All inlets shall be stenciled by the developer to indicate that "No Dumping Allowed — This inlet flows to . . . Creek," or other approved wording by Horsham Township. Specifications for stenciling are to be provided by Horsham Township.
(j)
Inlets in paved areas shall incorporate grit chambers.
(6)
Headwalls and endwalls.
(a)
An endwall shall be provided at the end of all pipe runs. Headwalls shall be provided at the beginning of all pipe runs. A headwall shall also be required (instead of an inlet) where calculated surface flows exceed the requirements for inlets in pervious areas. Headwalls/endwalls with a height greater than three feet may be required to have a safety fence or guiderail if so determined by the Township Engineer. Rock apron (rip-rap) or other approved velocity dissipation device shall be placed at all pipe outlets (endwalls) to reduce flow velocity and prevent erosion.
(b)
All headwalls and endwalls shall be stenciled by the developer to indicate that "No Dumping Allowed — This inlet flows to . . . Creek," or other approved wording by Horsham Township. Specifications for stenciling are to be provided by Horsham Township.
(7)
Curb and roadside swales.
(a)
Roadside swales adjacent to shoulders. When swales are provided in cut areas, the water shall not encroach upon the roadway area during a twenty-five-year design storm of five-minute duration. Storm sewer inlets shall be provided to control the roadway encroachment and water velocity.
(b)
Shoulder swales (in cut areas - no roadside swale). Water flowing in the shoulder shall not encroach more than 2/3 the shoulder width during a twenty-five-year design storm of five-minute duration (time-of-concentration). Storm sewer inlets shall be provided to control the shoulder encroachment and water velocity.
(c)
Curbed sections. The maximum encroachment of runoff on the roadway pavement shall not exceed 1/2 of the traveled lane during a twenty-five-year design storm of five-minute duration. Inlets shall be provided to control the encroachment of water on the traveled lane.
(8)
Bridges, culverts, and drainage channels.
(a)
Bridges and culverts shall be designed and constructed to meet current Pennsylvania Department of Transportation construction and loading standards (HS-25 loading standards). They shall be constructed to the full width of the right-of-way or to an adequate dimension to accommodate special grade conditions. Approval of the Pennsylvania Department of Environmental Protection is required for all improvements in and along all waters of the commonwealth. The applicant shall provide verification of Department of Environmental Protection approval for all such proposed bridges and culverts.
(b)
Approval of the Pennsylvania Department of Environmental Protection is required for all improvements in and along waters of the commonwealth. Where applicable, stormwater management facilities or programs shall comply with the requirements of Chapter 102 (Erosion Control), Chapter 105 (Dam Safety and Waterway Management), and Chapter 106 (Floodplain Management) of Pennsylvania Code, Title 25, Rules and Regulations of the Department of Environmental Protection.
(c)
All bridges, culverts, and drainage channels shall be designed to convey a flow rate equal to a one-hundred-year, twenty-four-hour storm as defined by the U.S. Department of Agriculture, Soil Conservation Service, Technical Release No. 55. All bridges and culverts shall be designed to pass the one-hundred-year design storm without increasing the extent and depth of the one-hundred-year floodplain.
(d)
Drainage channels shall be designed to convey the design discharge at a stable, nonerosive velocity. Channel linings that are assumed in the design must be established before the channel is utilized for its stated objective. Diversions or alternate channel linings may be necessary to provide channel conditions in the field to match design assumptions. If the design channel lining cannot be developed before the design discharge is introduced, then the designer must check the stability of the channel assuming a bare earth condition.
(e)
Maximum channel velocities for vegetated and rock-lined channels shall be as set forth in the Pennsylvania Department of Environmental Protection, Bureau of Soil and Water Conservation Publication, Erosion and Sediment Control Program Manual, latest revision.
(f)
The outfall of drainage channels shall be stable under the design discharge. Energy dissipaters (rock lining/rip rap) shall be utilized to return drainage channel discharge to a stable velocity.
(g)
Any vegetated drainage channel requiring mowing of the vegetation shall have a maximum grade (side slopes) of four horizontal to one vertical (4:1 - 25%) on those areas to be mowed.
(9)
Stormwater detention/retention basins.
(a)
Detention or retention basin easements, ownership, and maintenance. Prior to granting of final approval of any subdivision or land development plan, the developer shall provide written assurance, satisfactory to the Township, that the detention or retention basin and associated stormwater management facilities will be properly maintained. Such assurances shall be in a form to act as a covenant that will run with the land, and shall provide Township maintenance at the cost of the landowner in case of default, and further provide for assessment of costs and penalties in case of default. If all or a portion of the facilities are on property which will be conveyed to an individual, homeowners' association or any other eventual owner, the guarantees must be in such a form that they will carry through to the new owners. Horsham Township will not accept dedication or maintenance responsibilities for any detention or retention basin or similar facility located on private grounds, unless otherwise approved by Township Council.
[1]
When a private entity (such as a homeowners' association) retains ownership of any stormwater management facility, such entity shall be responsible for maintenance of the facility. In such case, approval of the stormwater management facility plans shall be conditioned upon the private entity agreeing to be responsible for all maintenance of the stormwater management facility. This agreement shall be in writing, shall be in recordable form and shall, in addition to any other terms deemed necessary by the Township, contain a provision permitting access to such facilities deemed critical to the public welfare, for inspection at any reasonable time by Horsham Township or its designee.
[2]
When any stormwater management facility is located on an individual lot, and when maintenance thereof is the responsibility of that landowner, a description of the facility or systems and the terms of the required maintenance shall be incorporated on a plat of the property. The plat shall be recorded with the Montgomery County Recorder of Deeds within 90 days following Township approval. The plat of the property shall also contain a provision permitting access to such facilities deemed critical to public welfare, for inspection at any reasonable time by Horsham Township or its designee. The Township may also require as a condition of approval that any deed conveying any interest in such lot contain language indicating that the conveyance is subject to an express covenant by the grantee that the grantee will maintain the stormwater management facility.
[3]
Upon presentation of proper credentials, duly authorized representatives of Horsham Township may enter at any reasonable time upon any property within the Township to investigate or to ascertain the condition of the subject property with regard to any matter regulated by this chapter.
[4]
The failure of any person, individual lot owner or private entity to properly maintain any stormwater management facility shall be construed to be a violation of this section and is declared to be a public nuisance, subjecting the violator to any and all penalties provided by law. Specifically, in the event the entity responsible for maintenance fails to honor those responsibilities set forth herein, in any manner, Horsham Township shall have the right of entry upon the area of such facilities to undertake any required corrective or maintenance effort. The total cost of such, including administrative, engineering, and legal costs for enforcement, may be imposed upon the responsible party as determined by Horsham Township. Failure to remedy all associated costs described above may be subject of the imposition of a lien by the Township against the owner(s) in question, in the same manner as the Township might otherwise be empowered by law to assess or impose a lien against a property for municipal improvements.
(b)
When basins are provided, they shall be designed to utilize the natural contours of the land whenever possible. When such design is not practical, the construction of the basin shall utilize slopes as flat as possible to blend the structure into the terrain. Detention basins shall be designed to facilitate regular maintenance, mowing and periodic de-silting and reseeding. In residential developments, shallow broad basins shall be provided for recreational use.
(c)
Except with the one-year design storm, basins shall be designed so that they return to normal conditions within approximately 12 hours after the termination of the storm, unless the Township Engineer finds that downstream conditions may warrant other design criteria for stormwater release.
(d)
Emergency overflow facilities shall be provided for basins to handle runoff in excess of design flows, or in the event the outlet structure becomes blocked and passes no flow.
(e)
If the land of the proposed subdivision or land development will be conveyed to two or more separate owners, the developer shall provide written assurance and deed restrictions to the Township that the owners will properly maintain the basin(s).
(f)
Basins shall not be located within floodplains; nor within areas of floodplain soils with the exception that areas of alluvial soils may be utilized, if approved by the Pennsylvania Department of Environmental Protection and Army Corps of Engineers (where applicable), if proof is accepted that the area is not subject to flooding.
(g)
The use of regional basins to combine and eliminate numerous smaller basins is encouraged. Consultation with the Township is required prior to design of a regional basin.
(h)
Landscaping and planting in and around the perimeter of basins shall be provided. It shall be aesthetically pleasing and compatible with surrounding land uses, and require minimum maintenance. Proposed planting shall also be in accordance with the provisions of this chapter and as recommended by the Township Landscape Architect. A plan of basin planting shall be submitted to the Township for review and approval.
(i)
If a stormwater management basin will serve as a temporary sediment control device, the temporary sediment control measures shall be shown including perforated riser pipes or standboxes, filter berms, cleanout stakes and other measures as may be required by Pennsylvania Department of Environmental Protection Chapter 102, Regulations. Plans for such facilities shall require Montgomery County Conservation District approval prior to implementation. Sedimentation basins shall be in place prior to any earthmoving activities within their tributary drainage areas. A note summarizing the above shall be required on the record plan of subdivisions and land developments as well as in land development agreement with the Township.
(j)
Stormwater management basins shall be in place before the creation of any additional impervious surfaces on the site. Runoff shall not be directed to an infiltration structure until all tributary drainage areas are stabilized.
(k)
Where permanent retention (pond) facilities are proposed, there shall be a safety ledge, three feet wide at the maximum water surface level. If the pond is to be stocked, the Pennsylvania Fish and Game Commission shall approve the stocking plan.
(l)
All basins shall have slopes of three horizontal to one vertical (3:1 - 33.3%), or less on the basin's outer berm and four horizontal to one vertical or less on the basin's inner berm. The top or toe of any slope shall be located a minimum of five feet from any property line. The maximum difference between the top of berm elevation and the invert elevation of the outlet structure shall be seven feet.
(m)
All portions of the detention basin bottom shall have a minimum slope of 2%. The Township, on the recommendation of the Township Engineer, may accept a basin bottom slope of less than 2% provided vegetative plantings consistent with current best management practices are provided.
(n)
All basin embankments shall be placed in lifts not to exceed one foot in thickness and each lift shall be compacted to a minimum of 95% of Modified Proctor Density as established by ASTM D-1557. Prior to proceeding to the next lift, the compaction shall be checked by a Soils Engineer employed by the applicant/developer. Compaction tests shall be run on the leading and trailing edge of the berm along with the top of the berm. Verification of required compaction shall be submitted to the Township Engineer prior to utilization of any basin for stormwater management.
(o)
Emergency overflow facilities/spillway shall be provided with basins in order to convey basin inflow in excess of design flows, out of the basin, or in the event the outlet structure becomes blocked and is unable to convey flow. Emergency spillways discharging over embankments shall be lined. Lining for emergency spillways shall incorporate native colors and materials where possible including mono slab revetments, grass pavers and native stone. Mono slab revetments and grass pavers shall be backfilled with topsoil and seeded. The lining shall extend to the toe of the embankment on the outside of the berm, and shall extend to an elevation of three feet below the spillway crest on the inside of the berm. Vegetated spillways may be utilized for spillways constructed entirely on undisturbed ground (i.e., not discharging over fill material) if the designer can demonstrate that flow velocities through the spillway will not cause erosion of the spillway. A dense cover of vegetation shall be rapidly established in such spillways by sodding or seeding with a geotextile anchor. Such a vegetated spillway must be stabilized before runoff is directed to the basin. The minimum capacity of all emergency spillways shall be equivalent to the peak flow rate of the one-hundred-year, post-development design storm (incoming to the basin) with the primary outlet structure blocked. All spillway velocities shall be based upon flows during an assumed clogged primary outlet condition.
(p)
In all cases, the discharge end of the basin shall be provided with a properly designed outlet control structure (headwall, orifice structure or other approved flow control structure), culvert pipe, and endwall. Perforated riser pipes alone, without provision for permanent outlet control structure (as stated above) and culvert pipe, will not be acceptable for permanent basins.
(q)
The minimum top of basin berm width (at the design elevation) shall be 10 feet. A cutoff trench (keyway) of impervious material shall be provided under all embankments that require fill material. The cutoff trench shall be a minimum of eight feet wide, three feet deep and have side slopes of one horizontal to one vertical.
(r)
The minimum freeboard through the emergency spillway shall be two feet. "Freeboard" is defined as the difference between the design flow elevation through the spillway and the elevation of the top of the settled basin berm.
(s)
Antiseep collars shall be installed around the pipe barrel and shall be centered within the normal saturation zone of the berm. The antiseep collars and their connections to the pipe barrel shall be watertight. The antiseep collars shall be cast-in-place and extend a minimum of two feet beyond the outside of the principal pipe barrel. Precast collars may be permitted when approved by the Township Engineer. A minimum of two collars shall be installed on each basin outlet pipe.
(t)
A perforated riser, sized in accordance with Montgomery County Conservation District requirements, shall be provided at each basin outlet structure (if more than one is to be utilized) for sediment control, Risers shall not be removed until such time the entire area tributary to the basin has been permanently stabilized and until approved by the Montgomery County Conservation District and/or the Township Engineer.
(u)
All basin outlet pipes shall be watertight reinforced concrete having "O-ring" joints. All joints shall be mortared. To minimize clogging and to facilitate cleaning, outlet pipes shall have an internal diameter of at least 18 inches and a minimum grade of 1/2%.
(v)
Energy dissipating devices (rock lining/rip-rap, or other approved means) shall be provided at all basin outlets and shall be sized in accordance with Pennsylvania Department of Environmental Protection, Bureau of Soil and Water Conservation Publication, Erosion and Sediment Control Program Manual, latest revision.
(w)
Stone gabion baskets shall not be permitted for use in construction of detention/retention basins.
(x)
Access easement and stabilized drive to stormwater detention facilities shall be provided for maintenance and operation. This access easement shall be cleared and, when possible, be at least 30 feet in width. Proximity of detention facilities to public right-of-way shall be encouraged in order to minimize the length of accessways. Multiple accesses shall be encouraged for major facilities. The developer shall provide access easements and drives of reinforced concrete checker-block (backfilled with topsoil and seeded) or other similar paver acceptable to the Township Engineer, over a six-inch bed of compacted PennDOT type 2A coarse aggregate (or approved equivalent). Accessways to basins shall be a minimum of 12 feet in width and be no steeper in slope than 12 feet horizontal to one-foot vertical. In addition, depressed curb should be provided where the accessway enters a street/driveway and the stabilized driveway shall extend from the bottom of the interior basin berm embankment to the point of access to the basin. Access easement shall be owned and maintained by the individual lot owner(s) or homeowners' association but shall be established to permit access by Horsham Township or its designee, for emergency inspection and/or maintenance, at any reasonable time.
(y)
The minimum distance between a proposed basin discharge point and a downstream property boundary shall be 50 feet.
(10)
Best management practices. The Pennsylvania Handbook of Best Management Practices for Developing Areas (1998) shall serve as a design guide for structural and nonstructural best management practices. Additional design guidance may also be obtained from other related references prepared by the U.S. Environmental Protection Agency, the Washington Metropolitan Council of Governments, the Natural Resource Conservation Service, and the Pennsylvania Department of Environmental Protection. All BMP designs are subject to approval by the Township Engineer.