Township of Londonderry, PA
Dauphin County
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Table of Contents
Table of Contents

§ 122 Design Criteria for Stormwater Management and Drainage Facilities.

[Ord. 2010-5, 12/6/2010, § 701]
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 predevelopment 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 preapplication 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 two feet above the seasonal high groundwater table or other soil limiting zone. The invert of stormwater facilities may be lowered if adequate subsurface 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 four-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 four feet and refilled with a suitable material mix. Suitable backfill material is subject to the approval of the Municipal Engineer.
I. 
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 than three 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 one-hundred-year post-development flow, with a blocked primary outlet structure. The height of embankment must be set as to provide a minimum one foot of freeboard through the spillway, above the maximum water surface elevation, computed when the spillway functions for the one-hundred-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 one-hundred-year event) without discharging through the emergency spillway. The maximum water depth within any stormwater management facility shall be no greater than eight feet when functioning through the primary outlet structure.
B. 
Emergency spillways shall be armored to prevent erosion during the one-hundred-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 section must be at least five feet wide at the top, and eight feet wide through the emergency spillway. For fill embankments, the side slopes shall be no steeper then 3:1 on the inside of the facility and 2:1 on the outside of the facility. For cut slopes, the side slopes shall be no steeper than 2:1.
D. 
A cutoff and key trench of impervious material shall be provided under all embankments four 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% 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% 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 facility's vicinity.
K. 
Basin outflow culverts discharging into floodplains must account for tailwater. Tailwater corresponding to the one-hundred-year floodplain elevation may 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 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 ten-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 2 1/2 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.
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 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. The maximum distance between inlets in curbed roadway sections shall be no more than 600 feet, however access to underground pipes shall be provided every 300 feet.
D. 
Standard Type "C" inlets with eight-inch hoods shall be used along vertical concrete curb roadway networks. Type "C" inlets with ten-inch hoods that provide a two-inch sump condition may be used with approval of the Municipal Engineer when roadway longitudinal slopes are 1.0% 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 two-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 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 foot 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 center lines.
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 subgrade prior to placing the base and surface course on roadways.
M. 
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.
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 systems 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°, and at all points of convergence of two or more storm sewer pipes.
Q. 
All storm drainage piping (equal to or greater than 12 inches) 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 than 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 ten-year design storm with six 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% 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:
1.75
When swale bend is 30° to 60°
2.00
When swale bend is 60° to 90°
2.50
When swale bend is 90° or greater
F. 
Swales must be designed for both temporary and permanent conditions in accordance with the latest E&S Manual.

§ 123 Calculation Methodology.

[Ord. 2010-5, 12-6-2010, § 702]
1. 
All calculations shall be consistent with the guidelines set forth in the BMP Manual.
2. 
Stormwater runoff from all development sites shall be calculated using either the Rational Method or a Soil Cover Complex methodology. Methods shall be selected by the qualified professional based on the individual limitations and suitability of each method for a particular site.
3. 
Rainfall Values.
A. 
Rational Method. The Pennsylvania Department of Transportation Drainage Manual, Intensity-Duration-Frequency Curves, Publication 584, Chapter 7A, latest edition, shall be used in conjunction with the appropriate time of concentration and return period.
B. 
Soil Cover Complex Method. The Soil Conservation Service Type II, twenty-four-hour rainfall distribution shall be used in conjunction with rainfall depths from NOAA Atlas 14 or consistent with the following table.
Return Interval
(year)
24-Hour Rainfall Total
(inches)
1
2.40
2
2.90
10
4.36
25
5.43
50
6.38
100
7.48
4. 
Peak Flow Rates.
A. 
Rational Method: may be used for drainage areas up to 20 acres. Extreme caution should be used by the qualified 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 verses 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.
(1) 
The use of the Modified Rational Method to design stormwater management facilities must be approved by the Municipal Engineer.
B. 
Soil Cover Complex Method: may be used for drainage areas greater than 20 acres. This method is recommended for design of stormwater management facilities and where stormwater runoff volume must be taken into consideration.
C. 
For comparison of peak flow rates, flows shall be rounded to a tenth of a cubic foot per second (cfs).
5. 
Runoff Coefficients:
A. 
Rational Method: use Table C-1 (Appendix C).[1]
[1]
Editor's Note: Table C-1 is part of Appendix C, which is included at the end of this Chapter.
B. 
Soil Cover Complex Method: use Table C-2 (Appendix C).[2]
[2]
Editor's Note: Table C-2 is part of Appendix C, which is included at the end of this Chapter.
C. 
For the purposes of predevelopment peak flow rate and volume determination, existing nonforested pervious areas conditions shall be considered as meadow (good condition).
D. 
For the purposes of predevelopment peak flow rate and volume determination, 20% of existing impervious area, when present on the project site, and contained within the new proposed limit of disturbance, shall be considered meadow (good condition) for predevelopment hydrologic calculations for redevelopment.
6. 
Design Storm.
A. 
All drainage facilities (inlets, pipes, and swales) shall be designed to safely convey the ten-year storm.
B. 
All stormwater management facilities shall be verified by routing the proposed one-year, two-year, ten-year, twenty-five-year, and one-hundred-year hydrographs through the facility using the storage indication (Modified Puls) method. The design storm hydrograph shall be computed using a calculation method that produces a full hydrograph.
C. 
The stormwater management and drainage system shall be designed to safely convey the post-development one-hundred-year storm event to stormwater detention facilities, for the purpose of meeting peak rate control.
D. 
All structures (culvert or bridges) proposed to convey runoff under a municipal road shall be designed to pass the fifty-year design storm with a minimum one foot of freeboard measured below the lowest point along the top of the roadway.
E. 
All design within state or federal rights-of-way or that falls under the design criteria of any higher authority must meet the requirements of that agency in addition to meeting the minimum requirements of this Part.
7. 
Time of Concentration.
A. 
Time of concentration shall be computed using the NRCS Segmental Method as described in TR-55 (SCS 1986 or most current update). The length of sheet flow shall be limited to 100 feet. The Manning's "n" Roughness Coefficient for TR-55 sheet flow can be found in Table C-4 (Appendix C).[3] Time of concentration for channel and pipe flow shall be computed using Manning's equation.
[3]
Editor's Note: Table C-4 is part of Appendix C, which is included at the end of this Chapter.
B. 
For sites with insignificant channelized flow and less than 20% impervious coverage, the time of concentration may be computed using the NRCS equation for lag time:
26Equation.tif
C. 
Additionally, the following provisions shall apply to calculations for time of concentration:
(1) 
The post-development time of concentration shall never be greater that the predevelopment time of concentration for any watershed or subwatershed.
(2) 
The minimum time of concentration for any watershed shall be five minutes.
(3) 
The designer may choose to assume a five-minute time of concentration for any post-development watershed or subwatershed without providing any computations.
(4) 
The designer must provide computations for all predevelopment time of concentration paths. A five-minute time of concentration can not be assumed for predevelopment.
(5) 
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 predevelopment conditions for purpose of time of concentration calculations.
8. 
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 predevelopment flows. If left undisturbed during construction activities, areas draining to closed depressions may also be removed from peak runoff rates in the post-development analysis. New, additional contributing runoff shall not be directed to existing sinkholes or closed depressions.
9. 
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. The Manning's equation should not be used for analysis of pipes under pressure flow or for analysis of culverts. Manning's "n" values shall be obtained from Table C-3 (Appendix C).[4] 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.
[4]
Editor's Note: Table C-3 is part of Appendix C, which is included at the end of this Chapter.
10. 
The municipality may approve the use of any generally accepted full hydrograph approximation technique that shall use a total runoff volume that is consistent with the volume from a method that produces a full hydrograph.
11. 
The municipality has the authority to 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 approval of the municipality.

§ 124 (Reserved)