Loading...
(A) The low impact development practices provided in the BMP Manual and in Appendix B of this chapter shall be utilized for all regulated activities to the maximum extent practicable.
(B) Stormwater runoff volume controls shall be implemented using the design storm method or the simplified method as defined below. For regulated activity areas equal or less than one acre that do not require hydrologic routing to design the stormwater facilities, this chapter establishes no preference for either method; therefore, the applicant may select either method on the basis of economic considerations, the intrinsic limitations on applicability of the analytical procedures associated with each methodology, and other factors.
(1) The design storm method (CG-I in the BMP Manual) is applicable to any sized regulated activity. This method requires detailed modeling based on site conditions.
(a) Do not increase the post-development total runoff volume for all storms equal to or less than the two-year 24-hour storm;
(b) For hydrologic modeling purposes:
1. Existing non-forested pervious areas must be considered meadow (good condition) for pre-development hydrologic calculations; and
2. Twenty percent of existing impervious area, when present within the proposed project site, shall be considered meadow (good condition) for pre-development hydrologic calculations for re-development.
(2) The simplified method (CG-2 in the BMP Manual) is independent of site conditions and should be used if the design storm method is not followed. This method is not applicable to regulated activities greater than one acre or for projects that require detailed design of stormwater storage facilities. For new impervious surfaces:
(a) Stormwater facilities shall capture at least the first two inches of runoff from all new impervious surfaces;
(b) At least the first one inch of runoff from new impervious surfaces shall be permanently removed from the runoff flow, (i.e., it shall not be released into surface waters of the state). Removal options include reuse, evaporation, transpiration and infiltration;
(c) Wherever possible, infiltration facilities should be designed to accommodate infiltration of the entire permanently removed runoff; however, in all cases at least the first half-inch of the permanently removed runoff should be infiltrated; and
(d) Actual field infiltration tests at the location of the proposed elevation of the stormwater BMPs are required. Infiltration test shall be conducted in accordance with the BMP Manual. A minimum 48-hour notification of the municipality shall be provided to allow witnessing of the testing.
(3) In cases where it is not possible or desirable to use infiltration-based best management practices to partially fulfill the requirements in either divisions (B)(1) or (B)(2) above, the following procedure shall be provided to allow witnessing of the testing.
(a) At a minimum, the following documentation shall be provided to justify the decision to reduce the use of infiltration BMPs:
1. Description of and justification for field infiltration/permeability testing with respect to the type of test and test locations);
2. An interpretive narrative describing existing site soils and their structure as these relate to the interaction between soils and water. In addition to providing soil and soil profile descriptions, this narrative shall identify depth to seasonal high water tables and depth to bedrock, and provide a description of all subsurface elements (fragipans and other restrictive layers, geology and the like) that influence the direction and rate of subsurface water movement;
3. A qualitative assessment of the site’s contribution to annual aquifer recharge shall be made, along with identification of any restrictions or limitations associated with the use of engineered infiltration facilities; and
4. The provided documentation must be signed and sealed by a professional engineer or geologist.
(b) The following water quality pollutant load reductions will be required for all disturbed areas within the proposed development.
Pollutant Load | Units | Required reduction (%) |
Total nitrate (NO3) | Pounds | 50 |
Total phosphorous (TP) | Pounds | 85 |
Total suspended solids (TSS) | Pounds | 85 |
(c) The performance criteria for water quality best management practices shall be determined from the state’s Stormwater Best Management Practices Manual most current version.
(C) The applicable worksheets from the BMP Manual must be used in calculations to establish volume control.
(Ord. 1508, passed 12-20-2010)
(A) Lands contained within the county that have not had release rates established under an approved Stormwater Management Act, 32 P.S. §§ 680.1 et seq. Post-development discharge rates shall not exceed the pre-development discharge rates for the one-year, two-year, ten-year, 25-year, 50-year and 100-year storms.
(B) Lands contained within the county that have had release rates established under an approved Stormwater Management Act, 32 P.S. §§ 680.1 et seq. The post-development peak discharge rates shall be in accordance with the approved release rate map for the following watersheds (see appendices for release rate map): Lake Erie watersheds.
(Ord. 1508, passed 12-20-2010)
E&S STANDARDS
(A) The applicant shall meet requirements as contained in 25 Pa. Code, Chapters 92a and 102 as required and applicable as follows:
(1) The implementation and maintenance of erosion and sediment control BMPs;
(2) Development of written plans;
(3) Submission of plans for approval;
(4) Obtaining erosion and sediment control and NPDES permits; and
(5) Maintaining plans and permits on site.
(B) Evidence of any necessary plan or permit approval for earth disturbance activities from PADEP or the County Conservation District must be provided to Wayne Township.
(C) A copy of the approved Erosion and Sediment Control Plan and any other permit, as required by PADEP or the County Conservation District, shall be available at the project site at all times if required under Chapter 102.
(D) Construction of temporary roadways (e.g., for utility construction, timber harvesting and the like) shall comply with all applicable standards for erosion and sedimentation control and stream crossing regulations under 25 Pa. Code, Chapters 102 and 105. The Erosion and Sedimentation Control Plan shall be submitted to the County Conservation District for approval and shall address the following, as applicable:
(1) Design of the roadway system, including haul roads, skid roads, landing areas, trails and storage and staging areas;
(2) Runoff control structures (e.g., diversions, culverts, detention ponds and the like);
(3) Stream crossings for both perennial and intermittent streams;
(4) Access to public roadways, including design of rock construction entrance for mud and debris control; and
(5) A remediation plan for restoring the disturbed area through re-grading, topsoil placement, reseeding and other stabilization techniques, as required.
(E) Additional erosion and sedimentation control design standards and criteria that must be applied where infiltration BMPs are proposed include the following.
(1) Areas proposed for infiltration BMPs shall be protected from sedimentation and compaction during the construction phase, as to maintain their maximum infiltration capacity.
(2) Infiltration BMPs shall be protected from receiving sediment-laden runoff.
(3) The source of protection for infiltration BMPs shall be identified (i.e., orange construction fence surrounding the perimeter of the BMP).
(Ord. 1508, passed 12-20-2010)
DESIGN CRITERIA
(A) General design guidelines.
(1) Stormwater shall not be transferred from one watershed to another, unless:
(a) The watersheds are sub-watersheds of a common watershed which join together within the perimeter of the property;
(b) The effect of the transfer does not alter the peak rate discharge onto adjacent lands; or
(c) Easements from the affected landowner(s) are provided.
(2) Consideration shall be given to the relationship of the subject property to the drainage pattern of the watershed. A concentrated discharge of stormwater to an adjacent property shall be within an existing watercourse or confined in an easement or returned to a pre-development flow type condition.
(3) Innovative stormwater BMPs and recharge facilities are encouraged (e.g., rooftop storage, drywells, cisterns, recreation area ponding, diversion structures, porous pavements, holding tanks, infiltration systems, in-line storage in storm sewers and grading patterns). They shall be located, designed and constructed in accordance with the latest technical guidance published by PADEP, provided they are accompanied by detailed engineering plans and performance capabilities and supporting site specific soils, geology, runoff and groundwater and infiltration rate data to verify proposed designs. Additional guidance from other sources may be accepted at the discretion of the city, a pre-application meeting is suggested.
(4) All existing and natural watercourses, channels, drainage systems and areas of surface water concentration shall be maintained in their existing condition unless an alteration is approved by the appropriate regulatory agency.
(5) The design of all stormwater management facilities shall incorporate sound engineering principles and practices. The municipality shall reserve the right to disapprove any design that would result in the continuation or exacerbation of a documented adverse hydrologic or hydraulic condition within the watershed, as identified in the Plan.
(6) The design and construction of multiple use stormwater detention facilities are strongly encouraged. In addition to stormwater management, facilities should, where appropriate, allow for recreational uses including ball fields, play areas, picnic grounds and the like. Consultation with the city and prior approval are required before design. Provision for permanent wet ponds with stormwater management capabilities may also be appropriate.
(a) Multiple use basins should be constructed so that potentially dangerous conditions are not created.
(b) Water quality basins or recharge basins that are designed for a slow release of water or other extended detention ponds are not permitted for recreational uses, unless the ponded areas are clearly separated and secure.
(7) Should any stormwater management facility require a dam safety permit under PADEP Chapter 105, the facility shall be designed in accordance with Chapter 105 and meet the regulations of Chapter 105 concerning dam safety.
(B) Stormwater management facility design considerations. All stormwater management facilities shall meet the following design requirements.
(1) No outlet structure from a stormwater management facility, or swale, shall discharge directly onto a municipal or state roadway without approval from the city or PennDOT.
(2) The top, or toe, of any slope of stormwater management facilities shall be located a minimum of ten feet from any property line.
(3) The minimum horizontal distance between any stormwater holding facility and a building/structure shall be 25 feet. The lowest floor elevation of any structure constructed immediately adjacent to a detention basin or other stormwater facility shall be a minimum of two feet above the 100-year water surface elevation.
(4) Stormwater management facility bottom, or surface of permanent pool, elevations must be greater than adjacent floodplain elevations (FEMA or HEC-RAS analysis). If no floodplain is defined, bottom elevations must be greater than existing ground elevations 50 feet from top of stream bank in the facility’s vicinity.
(5) Basin outflow culverts discharging into floodplains must account for tailwater. Tailwater corresponding to the 100-year floodplain elevation must be used for all design storms, or the applicant may elect to determine flood elevations of the adjacent watercourse for each design storm. The floodplain is assumed to be 50 feet from top of stream bank in areas where a floodway is not designated or no other evidence is provided.
(6) The invert of all stormwater management facilities and underground infiltration/storage facilities shall be located a minimum of two feet above the seasonal high groundwater table. The invert of stormwater facilities may be lowered if adequate sub-surface drainage is provided. Flows from underdrains need not be accounted for in volume or rate control calculations.
(7) Whenever possible, the side slopes and basin shape shall be amenable to the natural topography. Vertical side slopes and rectangular basins shall be avoided whenever possible.
(8) Exterior slopes of compacted soil shall not exceed a three to one ratio, and may be further reduced if the soil has unstable characteristics.
(9) Interior slopes of the basin shall not exceed three to one ratio.
(10) Unless specifically designed as a volume control facility, all stormwater management facilities shall have a minimum slope of 2% extending radially out from the principal outlet structure. Facilities designed as water quality/infiltration BMPs may have a bottom slope of zero.
(11) Impervious low-flow channels are not permitted within stormwater management facilities.
(12) Unless specifically designed as a volume control or water quality facility, all stormwater management facilities must empty over a period of time not less than 24 hours and not more than 72 hours from the end of the facility’s inflow hydrograph. Infiltration tests performed at the facility locations and proposed basin bottom depths, in accordance with the BMP Manual, must support time-to-empty calculations if infiltration is a factor.
(13) Energy dissipators and/or level spreaders shall be installed at points where pipes or drainageways discharge to or from basins. Discharges to drainage swales shall be dissipated or piped to an acceptable point.
(14) (a) Landscaping and planting specifications must be provided for all stormwater management basins and be specific for each type of basin.
(b) Minimal maintenance, saturation tolerant vegetation must be provided in basins designed as water quality/infiltration BMPs.
(15) A safety fence may be required, at the discretion of the municipality, for any stormwater management facility. The fence shall be a minimum of four feet high, and of a material acceptable to the municipality. A gate with a minimum opening of ten feet shall be provided for maintenance access.
(16) The primary outlet structure shall be designed to pass all design storms, up to and including the 100-year event, without discharging through the emergency spillway. All principal outlet structures shall:
(a) Be constructed of reinforced concrete or an alternative material approved by the city. When approved for use, all metal risers shall:
1. Be suitably coated to prevent corrosion;
2. Have a concrete base attached with a water-tight connection. The base shall be sufficient weight to prevent flotation of the riser;
3. Provide a trash rack or similar appurtenance to prevent debris from entering the riser; and
4. Provide an anti-vortex device, consisting of a thin vertical plate normal to the basin berm.
(b) Provide trash racks to prevent clogging of primary outflow structure stages for all orifices; and
(c) Provide outlet aprons and shall extend to the toe of the basin slope at a minimum.
(17) Any stormwater management facility designed to store runoff shall provide an emergency spillway designed to convey the 100-year post-development peak rate flow with a blocked primary outlet structure. The emergency spillway shall be designed per the following requirements:
(a) The top of embankment elevation shall provide a minimum one foot of freeboard above the maximum water surface elevation. This is to be calculated when the spillway functions for the 100-year post-development inflow, with a blocked outlet structure;
(b) Avoid locating on fill areas, whenever possible;
(c) The spillway shall be armored to prevent erosion during the 100-year post-development flow, with a blocked primary outlet structure. Synthetic liners or riprap may be used, and calculations sufficient to support proposed armor must be provided. An earthen plug must be used to accurately control the spillway invert if riprap is the proposed armoring material. Emergency spillway armor must extend up the sides of the spillway, and continue at full width to a minimum of ten feet past the toe of slope;
(d) The city may require the use of additional protection when slopes exceed a four to one ratio and spillway velocities might exceed NRCS standards for the particular soils involved; and
(e) Any underground stormwater management facility, pipe storage systems, must have a method to bypass flows higher than the required design, up to a 100-year post-development inflow, without structural failure or causing downstream harm or safety risks.
(18) Design of stormwater management facilities having three feet or more of water depth, measured vertically from the lowest elevation in the facility to the crest of the emergency spillway, shall meet the following additional requirements.
(a) The maximum water depth within any stormwater management facility shall be no greater than eight feet when functioning through the primary outlet structure.
(b) The top of embankment width shall be:
1. For embankments up to four feet, width shall be at least six feet;
2. For embankments between four and six feet, width shall be at least eight feet; and
3. For embankments over six feet, width shall be at least ten feet.
(c) A ten-foot wide access to the basin bottom must be provided with a maximum longitudinal slope of 10%.
(d) Berms shall be constructed using soils that conform to the unified soil classification of CH, MH, CL or ML. Soils used shall be tested to determine its density analysis per ASTM 698. The embankments will be constructed in a maximum of six-inch lifts. The lifts will each be compacted to a density at least 98% of it’s the maximum dry density. Each layer of compacted fill shall be tested to determine its density per ASTM 2922 or ASTM 3017. One test per 50 cubic yards of material placed, at least one per layer, shall be performed by an independent testing agency.
(e) A cutoff and key trench of impervious material shall be provided under all embankments four feet or greater in height. The cutoff trench shall run the entire length of the embankment and tie into undisturbed natural ground.
(f) Anti-seep collars, or a PADEP approved alternative, must be provided on all outflow culverts in accordance with the methodology contained in the latest edition of the PADEP E&S Manual. An increase in seepage length of 15% must be used in accordance with the requirements for permanent anti-seep collars.
(19) Construction of stormwater management facilities:
(a) Basins used for rate control only shall be installed prior to or concurrent with any earthmoving or land disturbances which they will serve. The phasing of their construction shall be noted in the narrative and on the plan.
(b) Basins that include water quality or recharge components shall have those components installed in such a manner as to not disturb or diminish their effectiveness.
(c) Compaction test reports shall be kept on file at the site and be subject to review at all times with copies being forwarded to the city upon request.
(d) Temporary and permanent grasses or stabilization measures shall be established on the sides and base of all earthen basins within 15 days of construction.
(20) Exceptions to these requirements may be made at the discretion of the municipality for BMPs that retain or detain water, but are of a much smaller scale than traditional stormwater management facilities.
(C) Stormwater carrying facilities.
(1) All storm sewer pipes, grass waterways, open channels, swales and other stormwater carrying facilities that service drainage areas within the site must be able to convey post-development runoff from the ten-year design storm.
(2) Stormwater management facilities that convey off-site water through the site shall be designed to convey the 25-year storm event, or larger events, as determined by the City Engineer.
(3) All developments shall include provisions that allow for the overland conveyance and flow of the post-development 100-year storm event without damage to public or private property.
(4) Storm sewers:
(a) Storm sewers must be able to convey post-development runoff without surcharging inlets for the ten-year storm event.
(b) When connecting to an existing storm sewer system, the applicant must demonstrate that the proposed system will not exacerbate any existing stormwater problems and that adequate downstream capacity exists.
(c) Inlets, manholes, pipes and culverts shall be constructed in accordance with the specifications set forth in PennDOT’s Publication 408, and as detailed in the PennDOT’s Publication 72M - Standards for Roadway Construction (RC) or other detail approved by the municipality. All material and construction details, inlets, manholes, pipe trenches and the like, must be shown on the SWM Site Plan, and a note added that all construction must be in accordance with PennDOT’s Publication 408 and PennDOT’s Publication 72M, latest edition. A note shall be added to the plan stating that all frames, concrete top units and grade adjustment rings shall be set in a bed of full mortar according to Publication 408.
(d) A minimum pipe size of 18 inches in diameter shall be used in all roadway systems, public or private, proposed for construction in the municipality. Pipes shall be designed to provide a minimum velocity of two and one-half feet per second when flowing full, but in all cases, the slope shall be no less then 0.5%. Arch pipe of equivalent cross-sectional area may be substituted in lieu of circular pipe where cover or utility conflict conditions exist.
(e) All storm sewer pipes shall be laid to a minimum depth of one foot from subgrade to the crown of pipe.
(f) In curbed roadway sections, the maximum encroachment of water on the roadway pavement shall not exceed half of a through travel lane or one inch less than the depth of curb during the ten year design storm of five-minute duration. Gutter depth shall be verified by inlet capture/capacity calculations that account for road slope and opening area.
1. Inlets shall be placed at a maximum of 400 feet apart.
2. Inlets shall be placed so drainage cannot cross intersections or street centerlines.
(g) Standard type “C” inlets with eight-inch hoods shall be used along curbed roadway networks. Type “C” inlets with ten-inch hoods that provide a two-inch sump condition may be used with approval of the city when roadway longitudinal slopes are 1.0% or less.
(h) For inlets containing a change in pipe size, the elevation for the crown of the pipes shall be the same or the smaller pipe’s crown shall be at a higher elevation.
(i) All inlets shall provide a minimum two-inch drop between the lowest inlet pipe invert elevation and the outlet pipe invert elevation.
(j) On curbed sections, a double inlet shall be placed at the low point of sag vertical curves, or an inlet shall be placed on each side of the low point at a distance not to exceed 100 feet, or at an elevation not to exceed 0.2 feet above the low point.
(k) At all roadway low points, swales and easements shall be provided behind the curb or swale and through adjacent properties to channelize and direct any overflow of stormwater runoff away from dwellings and structures.
(l) All inlets in paved areas shall have heavy duty bicycle safe grating. A note to this effect shall be added to the SWM Site Plan or inlet details therein.
(m) Inlets must be sized to accept the specified pipe sizes without knocking out any of the inlet corners. All pipes entering or exiting inlets shall be cut flush with the inside wall of the inlet. A note to this effect shall be added to the SWM Site Plan or inlet details therein.
(n) Inlets shall have weep holes covered with geotextile fabric placed at appropriate elevations to completely drain the sub grade prior to placing the base and surface course on roadways.
(o) Inlets, junction boxes or manholes greater then five feet in depth shall be equipped with ladder rungs and shall be detailed on the SWM Site Plan.
(p) Inlets shall not have a sump condition in the bottom unless designed as a water quality BMP or specifically approved by the municipality. Pipe shall be flush with the bottom of the box or concrete channels shall be poured.
(q) Accessible drainage structures shall be located on continuous storm sewer system at all vertical dislocations, at all locations where a transition in storm sewer pipe sizing is required, at all vertical and horizontal angle points exceeding five degrees, and at all points of convergence of two or more storm sewer pipes.
(r) All storm drainage piping shall be provided with either reinforced concrete headwalls or end sections of compatible material as the pipe involved at its entrance and discharge.
(s) 1. Outlet protection and energy dissipaters shall be provided at all surface discharge points in order to minimize erosion consistent with the E&S Manual.
2. Flow velocities and volumes from any storm sewer shall not result in a degradation of the receiving channel.
(t) Stormwater roof drains and pipes shall not be connected to storm sewers or discharge onto impervious areas without approval by the city.
(5) Swale conveyance facilities:
(a) Swales must be able to convey post-development runoff from a ten-year design storm with six inches of freeboard to top of the swale.
(b) Swales shall have side slopes no steeper than three to one ratio.
(c) All swales shall be designed, labeled on the SWM Site Plan and details provided to adequately construct and maintain the design dimension of the swales.
(d) Swales shall be designed for stability using velocity or shear criteria. Velocity criteria may be used for channels with less than 10% slope. Shear criteria may be used for all swales. Documentation must be provided to support velocity and/or shear limitations used in calculations.
(e) Where swale bends occur, the computed velocities or shear stresses shall be multiplied by the following factor for the purpose of designing swale erosion protection:
1. One and three-fourths: when swale bend is 30 to 60 degrees;
2. Two: when swale bend is 60 to 90 degrees; and
3. Two and one-half: when swale bend is 90 degrees or greater.
(f) Manning’s “n” values used for swale capacity design must reflect the permanent condition.
(Ord. 1508, passed 12-20-2010)
(A) Calculations. All calculations shall be consistent with the guidelines set forth in the BMP Manual, as amended herein.
(B) Methodology. Stormwater runoff from all development sites shall be calculated using either the rational method or the NRCS rainfall-runoff methodology. Other methods shall be selected by the design professional based on the individual limitations and suitability of each method for a particular site and approved by the city.
(C) Rainfall values. NRCS rainfall-runoff method. The soil conservation service type II, 24-hour rainfall distribution shall be used in conjunction with rainfall depths from NOAA Atlas 14 or be consistent with the following table.
Return Interval (Year) | 24-Hour Rainfall Total (Inches) |
1 | 2.13 |
2 | 2.56 |
10 | 3.71 |
25 | 4.46 |
50 | 5.09 |
100 | 5.76 |
(D) Runoff volume.
(1) Rational method. Not to be used to calculate runoff volume.
(2) NRCS rainfall-runoff method. This method shall be used to estimate the change in volume due to regulated activities. Combining curve numbers for land areas proposed for development with Curve Numbers for areas unaffected by the proposed development into a single weighted curve number is not acceptable.
(E) Peak flow rates.
(1) Rational method. This method may be used for design of conveyance facilities only. Extreme caution should be used by the design professional if the watershed has more then one main drainage channel, if the watershed is divided so that hydrologic properties are significantly different in one versus the other, if the time of concentration exceeds 60 minutes or if stormwater runoff volume is an important factor. The combination of rational method hydrographs based on timing shall be prohibited.
(2) NRCS rainfall-runoff method.
(a) This method is recommended for design of stormwater management facilities and where stormwater runoff volume must be taken into consideration. The following provides guidance on the model applicability:
1. NRCS’s TR-20 or HEC-HMS: no size limitations; and
2. Other models as pre-approved by the municipality.
(b) The NRCS antecedent runoff condition II (ARC II, previously AMC II) must be used for all simulations. The use of continuous simulation models that vary the ARC are not permitted for stormwater management purposes.
(3) Rate comparison. For comparison of peak flow rates, flows shall be rounded to a tenth of a cubic foot per second (cfs).
(F) Runoff coefficients.
(1) Rational method. Use Table C-1 (Appendix C).
(2) NRCS rainfall-runoff method. Use table C-2 (Appendix C). Curve numbers (CN) should be rounded to tenths for use in hydrologic models as they are a design tool with statistical variability. For large sites, CNs should realistically be rounded to the nearest whole number.
(3) Pre-development peak flow rate.
(a) For the purposes of pre-development peak flow rate and volume determination, existing non-forested pervious areas conditions shall be considered as meadow (good condition).
(b) For the purposes of pre-development peak flow rate and volume determination, 20% of existing impervious area, when present, shall be considered meadow (good condition).
(G) Design storm.
(1) All stormwater management facilities shall be verified by routing the proposed one-year, two-year, ten-year, 25-year, 50-year and 100-year hydrographs through the facility using the storage indication method or modified puls method. The design storm hydrograph shall be computed using a calculation method that produces a full hydrograph.
(2) The stormwater management and drainage system shall be designed to safely convey the post development 100-year storm event to stormwater detention facilities, for the purpose of meeting peak rate control.
(H) Time of concentration.
(1) The time of concentration is to represent the average condition that best reflects the hydrologic response of the area. The following time of concentration (Tc) computational methodologies shall be used unless another method is pre-approved by the city:
(a) Pre-development - NRCS’s lag equation:
Time of concentration = Tc = [(Tlag/.6) * 60] (minutes)
Tlag = L0.8 (S+1)0.7
1900Y
Where:
Ttag = Lag time (hours)
L = Hydraulic length of watershed (feet)
Y = Average overland slope of watershed (percent)
S = Maximum retention in watershed as defined by: S = [(1000/CN) - 10]
CN = NRCS curve number for watershed
(b) Post-development; commercial, industrial or other areas with large impervious areas (>20% impervious area) - NRCS segmental method. The length of sheet flow shall be limited to 100 feet. Tc for channel and pipe flow shall be computed using Manning’s equation.
(c) Post-development; residential, cluster or other low impact designs less than or equal to 20% impervious area - NRCS lag equation or NRCS segmental method.
(2) Additionally, the following provisions shall apply to calculations for time of concentration:
(a) The post-development Tc shall never be greater than the pre-development Tc for any watershed or sub-watershed. This includes when the designer has specifically used swales to reduce flow velocities. In the event that the designer believes that the post-development Tc is greater, it will still be set by default equal to the pre-development Tc for modeling purposes.
(b) The minimum Tc for any watershed shall be five minutes.
(c) The designer must provide computations for all pre-development Tc paths. A five minute Tc can not be assumed for pre-development.
(d) Undetained fringe areas (areas that are not tributary to a stormwater facility but where a reasonable effort has been made to convey runoff from all new impervious coverage to best management practices) may be assumed to represent the pre-development conditions for purpose of Tc calculation.
(I) Drainage. Drainage areas tributary to sinkholes or closed depressions in areas underlain by limestone or carbonate geologic features shall be excluded from the modeled point of analysis defining pre-development flows. If left undisturbed during construction activities, areas draining to closed depressions may also be used to reduce peak runoff rates in the post-development analysis. New, additional contributing runoff should not be directed to existing sinkholes or closed depressions.
(J) Uniform flow. Where uniform flow is anticipated, the Manning’s equation shall be used for hydraulic computations and to determine the capacity of open channels, pipes and storm sewers. Manning’s “n” values shall be obtained from PennDOT’s Drainage Manual, Publication 584. Inlet control shall be checked at all inlet boxes to ensure the headwater depth during the ten-year design event is contained below the top of grate for each inlet box.
(K) Existing runoff rates. The municipality may require that computed existing runoff rates be reconciled with field observations, conditions and site history, if the designer can substantiate, through actual physical calibration, that more appropriate runoff and time of concentration values should be utilized at a particular site, then appropriate variations may be made upon review and recommendation of the municipality.
(Ord. 1508, passed 12-20-2010)
SWM SITE PLAN AND REPORT REQUIREMENTS
For any of the activities regulated by this chapter and not eligible for the exemptions provided in § 152.021, the final approval of subdivision and/or land development plans, the issuance of any building or occupancy permit or the commencement of any land disturbance activity, may not proceed until the applicant has received written approval of a SWM Site Plan from the municipality.
(Ord. 1508, passed 12-20-2010)
(A) (1) The SWM Site Plan and SWM Site Report shall consist of all applicable calculations, maps and plans. All SWM Site Plan materials shall be submitted to the municipality in a format that is clear, concise, legible, neat and well organized; otherwise, the SWM Site Plan shall be rejected.
(2) Appropriate sections from the municipal subdivision and land development ordinance, and other applicable local ordinances, shall be followed in preparing the SWM Site Plan.
(B) SWM Site Plan shall include, but not be limited to:
(1) Plans shall be of one size and in a form that meets the requirements for recording in the office of the County Recorder of Deeds:
(a) Plans for tracts of less than 20 acres shall be drawn at a scale of one inch equals no more than 50 feet;
(b) Plans for tracts of 20 acres or more, plans shall be drawn at a scale of one inch equals no more than 100 feet; and
(c) All lettering shall be drawn to a size to be legible if the plans are reduced to half size.
(2) The name of the development; name and location address of the property site; name, address and telephone number of the applicant/owner of the property; and name, address, telephone number, email address and engineering seal of the individual preparing the SWM Site Plan:
(3) The date of submission and dates of all revisions;
(4) A graphical and written scale on all drawings and maps;
(5) A north arrow on all drawings and maps;
(6) A location map at a minimum scale of one inch equals 1,000 feet and illustrates the project relative to highways, municipalities or other identifiable landmarks;
(7) Metes and bounds description of the entire tract perimeter;
(8) Existing and final contours at intervals:
(a) Slopes less than 5%: no greater than one foot;
(b) Slopes between 5% and 15%: no greater than two feet; and
(c) Steep slopes, greater than 15%: five-foot contour intervals may be used.
(9) Perimeters of existing water bodies within the project area including stream banks, lakes, ponds, springs, field delineated wetlands or other bodies of water, sinkholes, flood hazard boundaries (FEMA delineated floodplains and floodways), areas of natural vegetation to be preserved, the total extent of the upstream area draining through the site and overland drainage paths. In addition, any areas necessary to determine downstream impacts, where required for proposed stormwater management facilities must be shown;
(10) The location of all existing and proposed utilities, on-lot wastewater facilities, water supply wells, sanitary sewers and water lines on and within 50 feet of property lines, including inlets, manholes, valves, meters, poles, chambers, junction boxes and other utility system components;
(11) A key map showing all existing human-made features beyond the property boundary that may be affected by the project;
(12) Soil names and boundaries with identification of the hydraulic soil group classification including rock outcroppings;
(13) Proposed impervious surfaces, structures, roads, paved areas and buildings, including plans and profiles of roads and paved areas and floor elevations of buildings;
(14) Existing and proposed land use(s);
(15) Horizontal alignment, vertical profiles and cross-sections of all open channels, pipes, swales and other BMPs;
(16) The location and clear identification of the nature of permanent stormwater BMPs;
(17) The location of all erosion and sedimentation control facilities, shown separately from the SWM Site Plan (typically an E&S Plan);
(18) A minimum 20-foot wide access easement around all stormwater management facilities that would provide ingress to and egress from a public right-of-way. In lieu of providing an easement to the public right-of-way, a note may be added to the plan granting the municipality or their designees access to all easements via the nearest public right-of-way;
(19) Construction details for all drainage and stormwater BMPs;
(20) Construction sequence;
(21) Identification of short-term and long-term ownership, operations and maintenance responsibilities; and
(22) Notes and statements:
(a) A statement, signed by the landowner, acknowledging that the stormwater BMPs are fixtures that cannot be altered or removed without prior approval by the municipality;
(b) A statement referencing the operation and maintenance (O&M) agreement and stating that the O&M agreement is part of the SWM Site Plan;
(c) A note indicating that record drawings will be provided for all stormwater facilities prior to occupancy, or the release of the surety bond;
(d) The following signature block for the registered professional preparing the Stormwater Management Plan:
“I, , hereby certify that the Stormwater Management Plan meets all design standards and criteria of the City of Corry’s Stormwater Management Ordinance.”
(e) The following signature block for the municipality reviewing the Stormwater Management Plan:
“I, , have reviewed this Stormwater Management Plan in accordance with the Design Standards and Criteria of the City of Corry’s Stormwater Management Ordinance.”
(C) SWM Site Report shall include (but not limited to):
(1) General data including:
(a) Project name;
(b) Project location, address of the property site;
(c) Name, address and telephone number of the applicant/owner of the property;
(d) Name, address, telephone number, email address and engineering seal of the individual preparing the SWM Site Report; and
(e) Date of submission and revisions.
(2) Project description narrative that clearly discusses the project and provides the following information, where applicable:
(a) Narrative.
1. Statement of the regulated activity describing what is being proposed. Overall stormwater management concept with description of permanent stormwater management techniques, including construction specifications and materials to be used for stormwater management facilities;
2. Expected project schedule;
3. Location map showing the project site and its location relative to release rate districts;
4. Detailed description of the existing site conditions, including a site evaluation completed for projects proposed in areas of carbonate geology or karst topography, and other environmentally sensitive areas such as brownfields;
5. Total site area, pre and post, which must be equal or have an explanation as to why it is not;
6. Total site impervious area;
7. Total off-site areas;
8. Number and description of stormwater management facilities;
9. Type of development;
10. Pre-development land use;
11. Whether site is a water quality sensitive (WQS) development;
12. Whether site is in a defined sensitive area;
13. Types of water quality and recharge systems used, if applicable;
14. Complete hydrologic, hydraulic and structural computations for all stormwater management facilities;
15. A written maintenance plan for all stormwater features including detention facilities and other stormwater management elements;
16. Identification of ownership and maintenance responsibility for all permanent stormwater management facilities; and
17. Other pertinent information, as required
(b) Summary tables.
1. Pre-development hydrologic soil group (HSG) assumptions, curve numbers (CN), computation of average slope, hydraulic length, computed time of concentration;
2. Existing conditions runoff volume and peak rate of runoff;
3. Post-development runoff volume and peak rate of runoff;
4. Undetained areas, areas to ponds;
5. Land use for each sub-area;
6. Hydrologic soil group (HSG) assumptions, curve numbers (CN);
7. Time of concentration computed for each sub-area;
8. Post-development peak rate of runoff routed to ponds and out;
9. Pond maximum return period design data, including maximum water surface elevation, berm elevation and emergency spillway elevation; and
10. Water quality depth and volume requirements.
(c) Calculations.
1. Complete hydrologic, hydraulic and structural computations, calculations, assumptions and criteria for the design of all stormwater BMPs;
2. Details of the berm embankment and outlet structure indicating the embankment top elevation, embankment side slopes, top width of embankment, emergency spillway elevation, perforated riser dimensions, pipe barrel dimensions and dimensions and spacing of antiseep collars;
3. Design computations for the control structures, (pipe barrel and riser, and the like);
4. A plot or table of the stage-storage (volume vs. elevation) and all supporting computations; and
5. Routing computations.
(d) Drawings.
1. Drainage area maps for all watersheds and inlets depicting the time of concentration path for both existing conditions and post-developed condition;
2. All stormwater management facilities must be located on a plan and described in detail including easements and buffers boundaries;
3. Reports that do not clearly indicate the above information may be rejected for review by the municipality and will be returned to the applicant;
4. Description of, justification, and actual field results for infiltration testing with respect to the type of test and test location for the design of infiltration BMPs;
5. The effect of the project, in terms of runoff volumes, water quality and peak flows, on surrounding properties and aquatic features and on any existing municipal stormwater collection system that may receive runoff from the project site;
6. Description of the proposed changes to the land surface and vegetative cover including the type and amount of impervious area to be added; and
7. Identification of short-term and long-term ownership, operation and maintenance responsibilities as well as schedules and costs for inspection and maintenance activities for each permanent stormwater or drainage BMP, including provisions for permanent access or maintenance easements.
(D) Supplemental information to be provided prior to recording of the SWM Site Plan, as applicable:
(1) Signed and executed operations and maintenance agreement (Appendix A);
(2) Signed and executed easements, as required for all on-site and off-site work;
(3) An Erosion and Sedimentation Control Plan and approval letter from the County Conservation District;
(4) A NPDES permit;
(5) Permits from PADEP and ACOE;
(6) Geologic assessment;
(7) Soils investigation report, including boring logs, compaction requirements and recommendations for construction of detention basins; and
(8) A highway occupancy permit from PennDOT when utilization of a PennDOT storm drainage system is proposed or when proposed facilities would encroach onto a PennDOT right-of-way.
(Ord. 1508, passed 12-20-2010)
Loading...