1. General Design Guidelines.
A. Stormwater shall not be transferred from one watershed to another, unless (1) the watersheds are sub-watersheds of a common watershed which join together within the perimeter of the property; (2) the effect of the transfer does not alter the peak rate discharge onto adjacent lands; or (3) easements from the affected landowner(s) are provided.
B. 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.
C. Stormwater BMPs and recharge facilities are encouraged (e.g., rooftop storage, drywells, cisterns, recreation area ponding, diversion structures, porous pavements, holding tanks, infiltration systems, stream channel storage, 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 Municipal Engineer (a pre-application meeting is suggested).
D. 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.
E. No outlet structure from a stormwater management facility, or swale, shall discharge directly onto a Municipal or State roadway.
F. The invert of all stormwater management facilities and underground infiltration/storage facilities shall be located a minimum of 2 feet above the seasonal high groundwater table or other soil limiting zone. The invert of stormwater facilities may be lowered if adequate sub-surface drainage, which does not alter the existing water table level, is provided.
G. Any stormwater management facility may be required to be fenced with a minimum 4-foot high fence of material acceptable to the Municipality. Gates with a minimum opening of 10 feet shall be provided for access.
H. Stormwater management facilities excavated to carbonate rock must either be fitted with an impervious clay liner, or over-excavated 4 feet and refilled with a suitable material mix. Suitable backfill material is subject to the approval of the Municipal Engineer.
9. The type, location, and number of landscaping and planting specification shall be provided for all stormwater management facilities and be specific for each type of facility.
2. Stormwater management facilities (with a depth of water equal to or greater then 3 feet measured from the lowest point inside a facility to the crest of the emergency spillway):
A Any stormwater management facility designed to store runoff and requiring a berm or earthen embankment, shall be designed to provide an emergency spillway to handle peak rate of stormwater runoff up to and including the 100-year post-development flow, with a blocked primary outlet structure. The height of embankment must be set as to provide a minimum 1 foot of freeboard through the spillway, above the maximum water surface elevation, computed when the spillway functions for the 100-year post-development inflow, with a blocked outlet structure. The primary outflow structure must be designed to pass all design storms (up to and including the 100-year event) without discharging through the emergency spillway. The maximum water depth within any stormwater management facility shall be no greater than 8 feet when functioning through the primary outlet structure.
B. Emergency spillways shall be armored to prevent erosion during the 100- year post-development flow, with blocked primary outlet structure. Synthetic liners or rip-rap 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 rip-rap 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 10 feet past the toe of slope.
C. A stormwater management facility berm cross sections must be at least 5 feet wide at the top, and 8 feet wide through the emergency spillway. Embankment side slopes shall be no steeper than 3:1 (horizontal:vertical).
D. A cutoff and key trench of impervious material shall be provided under all embankments 4 feet or greater in height.
E. Soils used for the construction of stormwater management facilities shall have low-erodibility factors (“K” factors) (refer to E&S Manual) and be identified on the SWM site plan.
F. Trash racks must be provided to prevent clogging of primary outflow structure stages for all orifices equivalent to 12 inches or smaller in diameter.
G. Anti-seep collars must be provided on all outflow culverts in accordance with the methodology contained in the latest edition of the E&S Manual. An increase in seepage length of 15 percent must be used in accordance with the requirements for permanent anti-seep collars.
H. Conventional, non-BMP stormwater management facilities (i.e., dry detention basins) 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 is the sizing of the stormwater management facility.
I. Impervious low-flow channels are not permitted within stormwater management facilities to promote water quality and groundwater recharge for frequent storm events. Facilities designed as water quality / infiltration BMPs may have a bottom slope of zero. Minimal maintenance, saturation tolerant vegetation must be provided in basins designed as water quality/infiltration BMPs. Conventional, non-BMP stormwater management facilities must have a minimum slope of 1 percent extending radially out from the primary outlet structure. Water storage below the lowest outlet structure stage (i.e., dead storage) is permitted in stormwater management facilities designed as water quality / infiltration BMPs.
J. Stormwater management facilities bottom elevations must be greater than adjacent floodplain elevations (FEMA or HEC-RAS analysis). If no floodplain is defined, bottom elevations must be higher than existing ground elevations 50 feet from top of stream bank in the facilities vicinity.
K. Basin outflow culverts discharging into floodplains must account for tailwater. Tailwater corresponding to the 100-year floodplain elevation may be used for all design storms, or the applicant may elect to determine flood elevations of the 50 feet from top of stream bank in areas where a floodplain is not designated, or where no other evidence is provided.
L. 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.
3. Storm Sewer Facilities.
A. Storm sewers must be able to convey post-development runoff from a 10- year design storm without surcharging inlets where appropriate. 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.
B. A minimum pipe size of 15 inches in diameter shall be used in all roadway systems (public or private) proposed for construction. Pipes shall be designed to provide a minimum velocity of 2½ feet per second when flowing full, but in all cases, the slope shall be no less then 0.5 percent. Arch pipe of equivalent cross- sectional area may be substituted in lieu of circular pipe where cover or utility conflict conditions exist.
C. In proposed curbed roadway sections, the maximum encroachment of water on the roadway pavement shall not exceed half of a through travel lane or 1 inch less than the depth of curb during the 10-year design storm of 5-minute duration. Gutter depth shall be verified by inlet capture/capacity calculations that account for road slope and opening area. The maximum distance between inlets in curbed roadway sections shall be no more then 600 feet, however access to underground pipes shall be provided every 300 feet.
D. Standard type “C” inlets with 8-inch hoods shall be used along vertical concrete curbs roadway networks. Type “C” inlets with 10-inch hoods that provide a 2-inch sump condition may be used with approval of the Municipal Engineer when roadway longitudinal slopes are 1.0 percent or less.
E. 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.
F. All inlets shall provide a minimum 2-inch drop between the lowest inlet pipe invert elevation and the outlet pipe invert elevation.
G. On curbed sections, a double inlet shall be placed at the low point of sag on vertical curves, or an inlet shall be placed at the low point and 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.
H. 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.
I. Inlets shall be placed so drainage cannot cross intersections or street centerlines.
J. All inlets in paved areas shall have heavy duty bicycle safe grating consistent with PennDOT Publication 72M. A note to this effect shall be added to the SWM site plan or inlet details therein.
K. 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 inlet wall. A note to this effect shall be added to the SWM site plan or inlet details therein.
L. 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.
M. Inlets, junction boxes, or manholes greater then 5 feet in depth shall be equipped with ladder rungs and shall be detailed on the SWM site plan.
N. Inlets shall not have a sump condition in the bottom (unless designed as a water quality BMP). Pipes shall be flush with the bottom of the box or concrete channels shall be poured.
O. Inlets, manholes, pipes, and culverts shall be constructed in accordance with the specifications set forth in PennDOT's Publication 408, latest edition, and as detailed in the PennDOT's Publication 72M, “Standards for Roadway Construction (RC),” latest edition, or as approved by the Municipal Engineer. All material and construction details (inlets, manholes, pipe trenches, etc.), 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.
P. 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 5 degrees, and at all points of convergence of two or more storm sewer pipes.
Q. All storm drainage piping (equal to or greater then 12") discharging to the ground surface shall be provided with either reinforced concrete headwalls and end sections or plastic and metal pipe end sections compatible with the pipe size involved in accordance with PennDOT Publication 408 and Publication 72M.
R. Outlet protection shall be provided at all surface discharge points with storm drainage piping (equal to or greater then 12 inches) in order to minimize erosion consistent with the E&S Manual.
S. Pavement base drain shall be provided at all low point in cut areas, toe of slope areas, and other areas as dictated by proven engineering principles and design judgment. All base drain shall be in accordance with PennDOT Publication 408.
4. Swale Conveyance Facilities:
A. Swales must be able to convey post-development runoff from a 10-year design storm with 6 inches of freeboard to top of the swale.
B. Swales shall have side slopes no steeper than 3:1.
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 percent 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) 1.75–when swale bend is 30 to 60 degrees.
(2) 2.00–when swale bend is 60 to 90 degrees.
(3) 2.50–when swale bend is 90 degrees or greater.
F. Swales must be designed for both temporary and permanent conditions in accordance with the latest E&S Manual.
(Ord. 11-01, 2/8/2011, §701)