A. Prevention of stormwater runoff is key objective of Chapter 93 of the DEP regulations, because runoff can change the physical, chemical, and biological integrity of waterbodies thereby impacting water quality.
B. The project plan shall describe how water quality protection requirements will be met. Infiltration BMPs shall be evaluated and utilized to the maximum extent possible to manage the net change in stormwater runoff generated so that post construction discharges do not degrade the physical, chemical, or biological characteristics of the receiving waters. These BMPs may be used to satisfy all or part of the requirements in Section 305.
C. Post construction stormwater infiltration of runoff shall replicate preconstruction infiltration of runoff to the maximum extent possible. In High Quality and Exceptional Value Watersheds, special requirements apply (refer applicable sections in this Ordinance).
D. In calculating the volume of runoff that can be infiltrated at a site, the following methodology shall be used:
1. Methodology:
Rev = [(S)( Rv)(A)]/12, where:
Rev = Recharge Volume (acre-feet)
S.= Soil specific recharge factor (inches)
Rv= Volumetric runoff coefficient, R = 0.05 + 0.009 (I), where:
I.= percent impervious area, and
S.shall be obtained based upon hydrologic soil group based upon the table below:
If more than one hydrologic soil group (HSG) is present at a site, a composite recharge volume shall be computed based upon the proportion of total site area within each HSG.
2. In selecting the appropriate infiltration BMPs, the applicant shall consider the following:
(a) Permeability and infiltration rate of the site soils.
(b) Slope and depth to bedrock.
(c) Seasonal high water table.
(d) Proximity and elevation relative to building foundations, basements, and well heads. (Infiltration BMPs should be located downgrade of these structures).
(e) Erodibility of soils.
(f) Land availability, configuration, and topography.
(g) Peak discharge and required volume control.
(h) Streambank erosion.
(i) Efficiency of the BMPs to mitigate potential water quality problems.
(j) Volume of runoff that will be effectively treated.
(k) Nature of the pollutant being removed.
(l) Maintenance requirements.
(m) Creation/protection of aquatic and wildlife habitat.
(n) Recreational value.
(o) Enhancement of aesthetic and property value.
3. A detailed soils evaluation of the project site shall be performed to determine the suitability of infiltration BMPs (refer Section 402.A). The evaluation shall be performed by a qualified professional, and at a minimum, address soil permeability, depth to bedrock, susceptibility to sinkhole formation, and subgrade stability. The site testing shall include adequate sampling of all portions of the site not limited by one hundred (100) percent protected natural resources to determine areas of the property which are suitable for infiltration BMPs. The general process for designing the infiltration BMP shall be:
(a) Analyze hydrologic soil groups as well as natural and manmade features within the watershed to determine general areas of suitability for infiltration BMPs.
(b) Provide field testing data at the elevation of the proposed infiltration zone (bottom surface of infiltration facilities) to determine appropriate percolation rate and/or hydraulic conductivity. Field Testing guidelines are identified in Appendix B.
(c) Design infiltration BMPs for required stormwater volume based on field-determined capacity at the level of the proposed infiltration surface.
4. Soil characteristics: Subject to the specific considerations in Subsection "h" below,
(a) Infiltration BMPs are particularly appropriate in hydrologic soil groups A and B, as described in the Natural Resources Conservation Service Manual TR-55.
(b) Low-erodibility factors ("K" factors) are preferred for the construction of basins,
(c) There must be a minimum depth of eighteen (18) inches between the bottom of any facility and the seasonal high water table and/or bedrock (limiting zones), except for infiltration BMPs receiving only roof runoff which shall be placed in soils having a minimum depth of twelve (12) inches between the bottom of the facility and the limiting zone. The minimum required separation between the limiting zone may be increased if required by the Township should project specific conditions exist (such as anticipated increased contaminants) which dictate greater prevention of groundwater contamination.
(d) There must be an infiltration and/or percolation rate sufficient to accept the additional stormwater load, and to drain completely as determined by field tests.
(e) A minimum of thirty (30) feet of undisturbed fill shall separate the foundation wall of any building and an infiltration BMP.
(f) The infiltration system shall have positive overflow controls to prevent storage within one foot of the finished surface of grade.
(g) Infiltration rates shall not be used in computing the storage volume of the infiltration system.
(h) Surface inflows shall be designed to prevent direct discharge of sediment into the infiltration system.
5. The recharge volume provided at the site shall be directed to the most permeable HSG available, except where other considerations apply such as in limestone geology.
6. Any infiltration BMP shall be capable of completely infiltrating the impounded water within forty-eight (48) hours from the peak of the storm.
7. Special attention shall be paid to proper installation of infiltration oriented stormwater management systems during the construction and to careful avoidance of soil compaction during site development.
8. Infiltration is not permitted in geologically susceptible limestone areas including the Carbonate Geology Overlay Zoning District. All stormwater facilities in this overlay district must be constructed with an impermeable liner approved by the Township to prevent seepage of water into the ground. All proposed channels or swales receiving or conveying concentrated stormwater runoff must be protected with an impermeable liner approved by the Township.
9. Caution shall be exercised where salt or chloride would be a pollutant since soils do little to filter this pollutant and it may contaminate the groundwater. Extreme Caution shall be exercised where infiltration is proposed in source water protection areas. The qualified design professional shall evaluate the possibility of groundwater contamination from the proposed infiltration/recharge facility and perform a hydrogeologic justification study if necessary. The infiltration requirement in High Quality/Exceptional Value waters shall he subject to DEP's Title 25: Chapter 93 Antidegradation Regulations.
10. The plan must include safeguards against groundwater contamination for uses which may cause groundwater contamination, should there be a mishap or spill. The Township may require the installation of a mitigative layer or an impermeable liner in BMP and/or detention basins where the possibility of groundwater contamination exists. A detailed hydrogeologic investigation may be required by the Township.
11. During the period of land disturbance, runoff shall be controlled prior to entering any proposed infiltration area. Areas proposed for infiltration BMPs shall be protected from sedimentation and compaction during the construction phase, so as to maintain their maximum infiltration capacity.
12. Infiltration BMPs shall not be constructed nor receive runoff until the entire contributory drainage area to the infiltration BMP has achieved final stabilization.
13. The requirements for volume control and infiltration are applied to all disturbed areas, even if they are ultimately to be a pervious or permeable land use given the extent to which development-related disturbance leads to compaction of the soils and reduces their infiltrative capacity.
In addition to the water quality volume, to minimize the impact of stormwater runoff on downstream stream bank erosion, a BMP must be designed to detain the proposed conditions two (2) year, twenty-four (24) hour design storm to the existing conditions one (1) year flow using the SCS Type II distribution. Additionally, provisions shall be made (such as adding a small orifice at the bottom of the outlet structure) so that the proposed conditions one (1) year storm takes a minimum of twenty-four (24) hours to drain from the facility from a point where the maximum volume of water from the one (1) year storm is captured (i.e. the maximum water surface elevation is achieved in the facility).
Release of water may begin at the start of the storm (i.e. the invert of the water quality orifice is at the invert of the facility). The design of the facility shall minimize clogging and sedimentation. Orifices smaller than four (4) inches in diameter are not' recommended. However, if the design engineer can verify that the smaller orifice is protected from clogging by use of trash racks, etc., smaller orifices may be permitted. Trash racks are required for any primary orifice.
A. Stormwater runoff which may result from Regulated Activities listed in Section 104 shall be controlled by permanent stormwater runoff BMPs that will provide the required standards within Article III. The methods of stormwater control or Best Management Practices (BMPs) which may be used to meet the required standards are described in this Ordinance and "Pennsylvania Stormwater Best Management Practice Manual", December 30, 2006, as amended. The choice of BMPs is not limited to the ones appearing in this Ordinance and the manual, however, any selected BMP must meet or exceed the runoff peak rate requirements of this Ordinance for the applicable Hydrologic District.
B. Any stormwater facility located on state highway rights-of-way shall be subject to approval by the Pennsylvania Department of Transportation.
C. Collection System Standards
1. Curb Inlets Curb inlets shall be located at curb tangents on the uphill side of street intersections, and at intervals along the curb line to control the maximum amount of encroachment of runoff on the roadway pavement so that same does not exceed a width of four feet during the design storm event. Design and location of curb inlets shall be approved by the Township.
2. Pipe Materials All storm sewer piping shall be Class Ill reinforced concrete pipe, except when pipe class and strength is required to be increased in accordance with PennDOT Specifications. Piping shall be saw-cut at ends, as needed, and not hammered or broken. All pipe joints and lift holes must be mortared except where designed for infiltration.
3. Minimum Pipe Size Minimum pipe diameter shall be eighteen (18) inches (or an equivalent flow area of 1.76 square feet).
4. Inlet and Manhole Construction Inlet and manhole castings and concrete construction shall be equivalent to PennDOT Design Standards. Manhole castings and covers shall have the word "STORM" cast in two (2) inch high letters on the top of the cover. All inlet grates shall be "bicycle safe" structural steel
5. Open end pipes must be fitted with concrete endwalls or wing walls in accordance with PennDOT Standards.
6. Flow velocity Stormwater collection systems shall be designed to produce a minimum velocity of three (3) feet per second when flowing full. The maximum permissible velocity shall be fifteen (15) feet per second. Pipe slopes shall not be less than one half of one (0.05) percent.
7. Inlets and manholes shall be spaced at intervals not exceeding three hundred (300) feet, and shall be located wherever branches are connected or sizes are changed, and wherever there is a change in alignment or grade. For drainage lines of at least thirty-six (36) inches diameter, inlets and manholes may be spaced at intervals of four hundred (400) feet. Manholes shall be equipped with open grate lids.
8. Storm sewer bedding/backfill requirements shall conform to the construction details in Appendix H.
9. Inlets shall be located to intercept concentrated runoff prior to discharge over public/private rights-of-way, sidewalks, streets, and driveways.
10. The capacity of all Type 'C' inlets shall be based on a maximum surface flow to the inlets of four (4) cfs, calculated based on the 100-year frequency design storm event. The maximum flow to Type 'C' inlets located in low points (such as sag vertical curves) shall include the overland flow directed to the inlet as well as all bypass runoff from upstream inlets. The bypass flow from upstream inlets shall be calculated using inlet efficiency curves included in PennDOT Design Manual Part 2, latest edition. If the surface flow to an inlet exceeds four (4) cfs, additional inlets shall be provided upstream of the inlet to intercept the excessive surface flow. A Type inlet at a low point of a paved area may be designed to accept a maximum of six (6) cubic feet per second (CFS). Type 'M' inlets shall be designed to accept a maximum surface flow of six (6) CFS based on the one hundred (100) year frequency design storm event, unless otherwise approved by the Township. Double inlets will not be permitted where additional pipe and inlets can be placed upstream to intercept excessive surface flow. A maximum of twelve (12) cfs shall be permitted to be collected by 'a Type 'M' inlet located in an isolated pervious area provided the designer can verify that such an inlet would not cause stormwater to accumulate on any adjacent public or private property, outside of an associated storm sewer easement, and that the depth of the accumulated stormwater would not exceed twelve (12) inches.
11. A minimum drop of two (2) inches shall be provided between the inlet and outlet pipe invert elevations within all inlets and manholes. When varying pipe sizes enter an inlet or manhole, the elevation of crown of all pipes shall be matched.
12. Stormwater pipes shall have a minimum depth of cover of twelve (12) inches (including over the bell) or as designated by the American Concrete Pipe Association (whichever is greater), and in no case shall any part of the pipe project into the road subbase or curb. Where cover is restricted, equivalent pipe arches may be specified in lieu of circular pipe.
13. The capacity of all stormwater pipes shall be calculated utilizing the Manning Equation for open channel flow as applied to closed conduit flow. The Manning's roughness coefficient shall be 0.13 for all concrete pipe. In cases where pressure flow may occur, the hydraulic grade line shall be calculated throughout the storm sewer system to verify that at least one foot of freeboard will be provided in all inlets and manholes for the design storm event.
14. Culverts shall be designed based on procedures contained in Hydraulic Design of Highway Culverts, HDS #5, U.S. Department of Transportation, Federal Highway Administration. 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.
15. Storm sewer structures (e.g. endwalls, inlets, and sections, etc.) may not be located on top of, or within ten (10) feet of electric, communication, water, sanitary sewer, or gas services and/or mains, unless approval is received from the Township and the Authority or Utility having jurisdiction over same.
16. Stormwater pipes must be oriented at right angles to electric, water, sanitary sewer, and gas utilities when crossing above or beneath same. Crossing angles of less than ninety (90) degrees will only be permitted at the discretion of the Township. When skewed crossings are permitted, interior angles between alignment of the storm sewer pipe and utility may not be less than forty-five (45) degrees. Vertical and horizontal design of storm sewer must be linear.
17. Roadway underdrain is required along both sides of all proposed roadways, existing roadways proposed to be widened, and within existing or proposed roadside swales as directed by the Township.
18. Where a public storm sewer system is not located within a right-of-way, or dedicated public property, a twenty (20) feet wide easement shall be established to encompass the storm sewer system. For multiple pipes or utilities, the width of the easement shall be a minimum of thirty (30) feet.
19. A minimum of one (1) foot of freeboard, between the inlet grate and the design flow elevation, shall be provided in all storm sewer systems (inlets and manholes) for the one hundred (100) year frequency design storm event.
D. Open Swales and Gutters Open swales shall be designed on the basis of Manning's Formula as indicated for collection systems with the following considerations:
1. Roughness Coefficient The roughness coefficient shall be 0.040 for earth swales.
2. Bank Slopes Slopes for swale banks shall not be steeper than one (1) vertical to four (4) horizontal.
3. Flow Velocity- The maximum velocity of flow as determined by Mannings equation shall not exceed the allowable velocities as shown in the following table for the specific type of material, unless otherwise approved by the Township and the Bucks County Conservation District
Note: Source of the following design criteria is the Pennsylvania Department of Environmental Protection, Bureau of Soil and Water Conservation Publication, Erosion and Sediment Control Program Manual.
ALLOWABLE VELOCITY
4. Swales shall be stabilized with biodegradable erosion control matting to permit establishment of permanent vegetation. Swales shall be of such shape and size to effectively contain the one hundred (100) year, Rational Method design storm, or greater, and to conform to all other specifications of the Township.
5. To minimize sheet flow of stormwater across lots located on the lower side of roads or streets, and to divert flow away from building areas, the cross-section of the street as constructed shall provide for parallel ditches or swales or curb on the lower side which shall discharge only at drainage easements, unless otherwise approved by the Township.
6. Gutters and swales adjacent to road paving shall be permitted to carry a maximum flow of four (4) cubic feet per second prior to discharge away from the street surface, unless it is proven to the satisfaction of the Township by engineering calculations that the road slopes or other factors would allow higher gutter or swale capacity.
7. Flows larger than those permitted in gutters and roadside swales may be conveyed in swales outside the required road right-of-way in separate drainage easements, or may be conveyed in pipes or culverts inside or outside the required road right-of-way.
8. Existing and proposed swales shall be provided with underdrains as deemed necessary by the Township should overland seepage result in potential maintenance problems. Underdrains must discharge into a natural drainage channel or stormwater management system.
9. Where drainage swales are used to divert surface waters away from buildings, they shall be sodded, landscaped, or otherwise protected as required and shall be of a slope, shape, and size conforming with the requirements of the Township. Concentration of surface water runoff shall be permitted only in swales, watercourses, retention or detention basins, bioretention areas, or other areas designed to meet the objectives of this Ordinance.
E. Bridge and Culvert Design
Any proposed bridge or culvert to convey flow within a watercourse, perennial stream, intermittent stream or ephemeral stream shall be designed in accordance with the following principals:
1. Culverts and bridges shall be designed with an open bottom to maintain natural sediment transport and bed roughness, avoiding acceleration of water velocity above the natural (preexisting) condition. Rock (rip rap) lining (native material if possible) shall be installed within the culvert as needed to prevent erosion within the structure. Approximate top of rock lining must be at the level of the existing stream bottom so as to maintain normal water level and unimpeded movement of native animal species.
2. Bottom of opening shall be designed to match the bankfull channel condition in terms of width and depth. The cross-sectional area of the bankfull channel (measured at a reference location upstream of the structure) shall be matched with area in the crossing structure.
3. Above the bankfull elevation, the width shall increase a minimum of thirty (30) percent to disperse the energy of higher flow volumes and avoid undermining of the supporting structure by secondary currents.
4. The total cross-sectional area of the structure opening must be equal to or greater than the flood prone area (cross-sectional stream area at a depth of twice the maximum bankfull depth, measured at a reference location upstream of the structure). The flood prone area is approximately equal to the area flooded by a fifty (50) year return flood.
5. All bridges, culverts, and drainage channels shall be designed to convey a flow rate equal to a one hundred (100) year, twenty-four (24) 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 convey the one hundred (100) year design storm without increasing the extent and depth of the one hundred (100) year flood plain.
F. Storm Sewer Design
1. Design flow rate The storm sewer system shall be designed to carry the one hundred (100) year frequency design storm peak flow rate. The drainage area and runoff coefficient to each inlet shall be indicated on the stormwater management plan. The one hundred (100) 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.
Appropriate values for the rainfall intensity can be found in the Pennsylvania Department of Transportation, Design Manual Part 2 (refer Appendix A).
2. Consideration shall be given to future land use changes in the drainage area in selecting the Rational ("C") coefficient. For drainage areas containing several different types of ground cover, a weighted value of "C" shall be used.
3. In determining the peak flow rate to individual storm sewer inlets (or other collection structures) the time of concentration method (as referenced in Section 309) shall be used for inlet drainage areas in excess of one (1) acre, unless otherwise approved by the Township. For inlet drainage areas less than one (1) acre, a five (5) minute time of concentration shall be used unless otherwise approved by the Township.
4. In determining the required design flow rate through a storm sewer piping system, if a five (5) minute time of concentration (storm duration) results in a pipe size exceeding a thirty (30) inch diameter pipe (or equivalent flow area of 4.9 square feet), the time of concentration approach (as defined herein) shall be used in determining storm duration.
5. In determining the required design flow rate through a storm sewer piping system, if a five (5) minute time of concentration results in a pipe size exceeding thirty (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.
6. 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.
G. Grading and Drainage
1. After completion of rough grading, a minimum of eight (8) inches of topsoil shall be returned to remaining disturbed areas prior to final grading and seeding.
2. Lots shall be graded to secure proper drainage away from buildings and to prevent the collection of storm water in pools. Minimum two (2) percent slopes shall be maintained away from and around all structures. Separation between the top of foundation wall (or slab) shall comply with Township Building Code requirements.
3. Construction - The developer shall construct and/or install such drainage structures and/or pipe which are necessary to prevent erosion damage and to satisfactorily carry off such surface waters to the nearest practical street, storm drain or natural water course.
4. Excavation - No excavation shall be made with a cut face steeper in slope than four (4) horizontal to one (1) vertical (4:1 = 25 percent), except under one or more of the following conditions:
(a) The material in which the excavation is made is sufficiently stable to sustain a slope of steeper than 4:1 and a written statement (certification) from a Professional civil engineer, licensed in the Commonwealth of Pennsylvania and experienced in erosion control, to this effect is submitted to the Township Engineer for review. This statement shall indicate the site has been inspected and that the deviation from the slope specified herein will not result in injury to persons or damage to property.
(b) A concrete, segmental block, or stone masonry wall, constructed in accordance with Township requirements, is provided to support the face of the excavation.
5. Fill - No fill shall be made which creates any exposed surface steeper in slope than four (4) horizontal to one (1) vertical (4:1 = 25 percent) except under one or more of the following conditions:
(a) The fill is located so that settlement, sliding, or erosion will not result in property damage or be a hazard to adjoining property, streets, alleys, or buildings.
(b) A written statement from a Professional civil engineer, licensed in the Commonwealth of Pennsylvania and experienced in erosion control, certifying the site has been inspected and that the proposed deviation from the slope specified above will not endanger any property or result in property damage, is submitted to and approved by the Township.
(c) A concrete, segmental block, or stone masonry wall, constructed in accordance with Township requirements, is provided to support the face of the excavation.
6. Slopes and Fences - The top or bottom edge of slopes shall be a minimum of five (5) feet from property or right-of-way lines of streets or alleys in order to permit the normal rounding of the edge without encroaching on the abutting property. Where walls or slopes (steeper than two (2) horizontal to one (1) vertical) are approved under the criteria in this Ordinance, and are five (5) feet or more in height, a protective fence shall be required at the top of the wall (or bank).
7. Clean up - All lots must be kept free of any debris or nuisances whatsoever during construction.
8. Design of erosion and sedimentation control facilities (particularly stormwater/sediment basins) shall incorporate Best Management Practices as defined herein.
9. Cut and fill operations shall be kept to a minimum. Wherever feasible, natural vegetation shall be retained, protected, and supplemented. Cut and fills shall not endanger or otherwise adversely impact adjoining property.
10. No grading equipment shall be permitted to be loaded and/or unloaded on a public street, and no grading equipment shall be permitted to travel on or across a public street unless licensed for operation on public thoroughfares.
11. Grading equipment shall not be permitted to cross intermittent and perennial streams. Temporary crossing shall only be permitted where application is made, and approval is received, from the Pennsylvania Department of Environmental Protection (where applicable), the Bucks County Conservation District, and Solebury Township.
12. Design of energy dissipation for high volume and/or high velocity discharge from storm sewer pipes and channels shall be in accordance with Hydraulic Engineering Circular No. 14, "Hydraulic Design of Energy Dissipaters for Culverts and Channels" as published by Department of Transportation, FHA, when deemed necessary by the Township, and as approved by the Bucks County Conservation District.
13. To control the dissemination of mud and dirt on to public roads and driveways, tire cleaning areas constructed of AASHTO #1 stone (underlain by geotextile structural fabric), at least fifty (50) feet in length shall be installed at each point of access to the site and individual lots (upon construction of internal streets in a binder condition). When deemed necessary by the Township, washing stations shall also be set-up at every construction entrance in order to wash mud and dirt from exiting vehicles. Appropriate measures must be taken to control runoff from such locations. The developer shall be responsible for the placement of appropriate signage identifying construction entrances and washing stations. Construction entrances shall be maintained by the developer during construction, as determined by the Township.
14. In the event any mud and/or debris is transported from the site onto a public roadway, the debris shall be removed immediately and the roadway swept and/or washed as deemed necessary by the Township at the owner's expense.
15. Adequate provision shall be made to prevent surface water from damaging the cut face of excavation and the sloping surfaces of fills.
H. Stormwater Detention/Retention Basins
1. If permanent ponds (retention basin) are proposed, the developer shall demonstrate that such ponds are designed to protect the public's health and safety.
2. During construction, duly authorized representatives of Solebury Township may enter at any reasonable time upon any property within the Township to investigate whether construction activity is in compliance with this Ordinance.
3. 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.
4. Except with the one (1) year design storm, basins shall be designed so that they return to normal conditions within approximately twelve (12) hours after the termination of the storm, unless the Township determines that downstream conditions may warrant other design criteria for stormwater release.
5. Landscaping and planting in and around the perimeter of basins shall be provided. Proposed planting shall also be in accordance with the provisions of this Ordinance, the Subdivision and Land Development Ordinance, and as recommended by the Township Landscape Architect/Planner.
6. 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, clean-out stakes and other measures as may be required by Pennsylvania Department of Environmental Protection, Chapter 102 Regulations. Plans for such facilities shall require Bucks 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 identifying the above criteria shall be required on the Record Plan of subdivisions and land developments as well as in the development agreement with the Township.
7. Stormwater management basins shall be in place before the creation of any new impervious surfaces on the site.
8. Runoff shall not be directed to any infiltration structure until all tributary drainage areas are stabilized.
9. Where permanent retention (pond) facilities are proposed, there shall be a safety ledge, ten (10) feet wide, no greater than fifteen (15) inches and no less than twelve (12) inches below the permanent water surface level.
10. All basins shall have slopes of four (4) horizontal to one (1) vertical (4:1 = 25 percent), or flatter on the basin's outer berm and four (5) horizontal to one (1) vertical or less on the basin's inner berm. The top or toe of any slope shall be located a minimum of five (5) 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 (7) feet.
11. All portions of a dry detention basin bottom shall have a minimum slope of two (2) percent.
12. All basin embankments shall be placed in lifts not to exceed one (1) foot in thickness and each lift shall be compacted to a minimum of ninety-five (95) percent of Modified Proctor Density as established by A.S.T.M. 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 prior to utilization of any basin for stormwater management.
13. Emergency overflow facilities/spillway shall be provided within 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 constructed of reinforced concrete checker-blocks to protect the berm against erosion. The checkerblocks shall be back-filled with topsoil and seeded. Checkerblock lining shall extend to the toe of the embankment on the outside of the berm, and shall extend to an elevation of three (3) 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). A dense cover of vegetation shall be rapidly established in such spillways by sodding or seeding with a geotextile anchor. The 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 (100) year, post-development design storm (entering to the basin).
14. 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 are not permitted for permanent basins.
15. The minimum top of basin berm width (at the design elevation) shall be ten (10) feet. A cut-off trench (keyway) of impervious material shall be provided under all embankments that require fill material. The cut-off trench shall be a minimum of eight (8) feet wide, three (3) feet deep and have side slopes of one (1) horizontal to one (1) vertical.
16. The minimum freeboard through the emergency spillway shall be one (1) foot. 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.
17. Anti-seep collars shall be installed around the pipe barrel and shall be centered within the normal saturation zone of the berm. The anti-seep collars and their connections to the pipe barrel shall be watertight. The anti-seep collars shall be cast-in-place and extend a minimum of two (2) feet beyond the outside of the principal pipe barrel. Precast collars shall be permitted if approved by the Township. A minimum of two (2) collars shall be installed on each basin outlet pipe.
18. A perforated riser, sized in accordance with Bucks 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 time the entire area tributary to the basin has been permanently stabilized and until approved by the Bucks County Conservation District.
19. All basin outlet pipes shall be watertight reinforced concrete having "0-Ring" joints. All joints shall be mortared.
20. A minimum of six (6) inches is required between the top of outlet structure box and the emergency spillway elevation. Six (6) inches, minimum, is also required between the one hundred (100) year water surface elevation and the top of outlet structure box.
21. Energy dissipating devices (rock lining/rip rap, or other approved materials) 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.
22. Stone gabion baskets shall not be permitted for use in construction of detention/retention basins.
23. 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 twenty (20) feet in width. Multiple accesses shall be encouraged for major facilities. The developer shall provide access easements and drives of reinforced concrete checker-block (back-filled with topsoil and seeded) or other similar paver acceptable to the Township Engineer, over a six (6) inch bed of compacted PennDOT type 3A coarse aggregate (or approved equivalent). Accessways to basins shall be a minimum of ten (10) feet wide and be no steeper in slope than ten (10) feet horizontal to one (1) feet vertical (10:1). In addition, depressed curb and concrete apron shall 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 homeowner's association but shall be established to permit access by Solebury Township or its designee, for emergency inspection and/or maintenance, at any reasonable time.
24. Split rail fence with wire mesh backing is may be required around all detention or retention basin where directed by Solebury Township. Fence shall consist of locust posts (two or three rail), four (4) feet high, with assorted hardwood rails (eight (8) feet to ten (10) feet long), and epoxy coated wire mesh (black or green in color) installed six (6) inches above finished grade. Each basin fence installation shall include two points of access with ten (10) feet wide self closing, self latch gates.
I. General Design Requirements
1. Prior to finish grading of a development site and final overlay of streets, roads, and driveways, temporary measures, acceptable to the Township, shall be taken to ensure that all runoff intended to be intercepted and collected by an inlet or other facility, will be collected. The plan shall include such details, notes, or specification including bituminous "eyebrows" at inlets, diversion berms, etc.
2. 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 connected to a storm sewer, or street drainage structure, or other approved stormwater conveyance facility that is designed as part of a stormwater management BMP.
3. All stormwater runoff and floodplain calculations and stormwater management facilities design shall be prepared by a Professional Engineer licensed in the Commonwealth of Pennsylvania.
4. When subdivisions or land developments are submitted to the Township for approval in sections, a complete storm sewer design for the proposed subdivision and land development shall be submitted. The proposed design must include the entire tract and not a portion.
