The Township shall comply with the applicable provisions of
the Little Conestoga Creek Watershed Act 167 Stormwater Management
Plan, approved by the Lancaster County Commissioners on April 29,
1998, and by the DEP on June 28, 1998, and the Conestoga River Watershed
Act 167 Stormwater Management Plan, approved by the Lancaster County
Commissioners on June 8, 2005, and by the DEP on August 29, 2005.
In Manor Township, all lands within the Township are required to comply
with the provisions of these plans, unless there is a subsequent plan
adopted with more restrictive provisions. In that case, the more restrictive
provisions will apply within the applicable watershed(s).
The methods of computation used to determine peak discharge
and runoff shall be:
A. All stormwater management predevelopment versus postdevelopment calculations
shall use one of the following:
(1) The Soil-Cover-Complex Method (as set forth in the latest edition
of Urban Hydrology for Small Watersheds, Technical Release No. 55,
as published by NRCS) is the recommended and preferred method for
all facilities with a drainage area greater than or equal to 100 acres
or a time of concentration greater than 60 minutes.
(2) The Rational Method is the recommended and preferred method for all
facilities with drainage areas less than 100 acres.
(3) The design storm volumes to be used in the analysis of peak rates
of discharge should be obtained from the Precipitation-Frequency Atlas
of the United States, Atlas 14, Volume 2, Version 3.0, U.S. Department
of Commerce, National Oceanic and Atmospheric Administration (NOAA),
National Weather Service, Hydrometeorological Design Studies Center,
Silver Spring, Maryland. NOAA's Atlas 14 can be accessed at: http://hdsc.nws.noaa.gov/hdsc/pfds/.
If the Rational Method is used, the NOAA Atlas 14 data shall be used
to determine the rainfall intensity in inches per hour based on the
information for the five- through sixty-minute duration storm events.
(4) Runoff coefficients "C" and curve numbers "CN" shall be based on
the charts contained in the Appendix.
(5) For the purpose of calculating peak discharges, all agricultural
lands that contribute storm drainage to or from the project site shall
be considered cultivated lands with conservation measures in good
hydrologic condition.
(6) Hydrographs may be obtained from NRCS methods such as TR-55, TR-20,
or from use of the "modified" or "unit hydrograph" Rational Methods.
If "modified" or "unit hydrograph" Rational Methods are used, the
ascending leg of the hydrograph shall have a time of three times the
time of concentration (3 x Tc) and the descending leg shall have a
time of seven times the time of concentration (7 x Tc) to approximate
an NRCS type II hydrograph.
B. Design of on-site conveyance systems calculations may use the Rational
Method of Q=CIA, where Q is the peak discharge of the watershed in
cubic feet per second, C is the coefficient of runoff, I is the intensity
of rainfall in inches per hour, and A is the area of the watershed
in acres; or any other method approved by the Township. When the Rational
Method is used, the NOAA Atlas 14 data shall be used to determine
the rainfall intensity in inches per hour based on the information
for the five- through sixty-minute duration storm events.
C. Runoff calculations shall include a hydrologic and hydraulic analysis
indicating volume and velocities of flow and the grades, sizes, and
capacities of water-carrying structures, sediment basins, retention
and detention structures and sufficient design information to construct
such facilities. Runoff calculations shall also indicate both predevelopment
and postdevelopment rates for peak discharge of stormwater runoff
from the project site.
D. Flow calculations for water carrying structures shall be presented
in tabular form using the flow tabulation form attached (or equal)
hereto. See the Appendix.
E. Permanent detention basins shall be designed with a primary outlet discharge that is less than or equal to the requirements for postdevelopment peak rate of runoff established by §
373-15H of this chapter.
F. Runoff calculations will also be made to ensure that the runoff from
the upstream watershed area can be accommodated by the pipes, drainage
easements, watercourses, etc., on the site.
G. For the NOAA Atlas 14 rainfall, provide the following:
(1) Provide the rainfall used for the two-, ten-, twenty-five-, fifty-,
and one-hundred-year, twenty-four-hour storm events. Rainfall values
vary throughout the county depending on location.
(2) Provide the location (longitude and latitude) or a description of
the location for which the rainfall applies.
(3) If rainfalls from more than one location are used, provide the methodology
by which the design rainfall was calculated.
H. Sheet flow may be determined using the nomograph found in the Appendix,
or the TR-55 Manning's kinematic solution shown in the sheet flow
section of Worksheet No. 1 in the Appendix.
For aboveground facilities which include rate control and have a facility depth less than two feet, the standards in §
373-23 may be used. All other facilities that include rate control shall meet the standards in this section. Facilities that include volume control shall also meet the requirements of §
373-22.
A. An impervious core/key trench, when required, shall consist of a
cutoff trench (below existing grade) and a core trench (above existing
grade). A key trench may not be required wherever it can be shown
that another design feature, such as the use of an impermeable liner,
accomplishes the same purpose.
B. The core should extend up both abutments to the ten-year water surface
elevation or six inches below the emergency spillway elevation, whichever
is lower.
C. All pipe collars (antiseep collars), shall be designed in accordance with Chapter
7 of the DEP E&S Manual. The material shall consist of concrete or otherwise nondegradable material around the outfall barrel and shall be watertight.
D. Embankment fill material. The embankment fill material shall be taken
from an appropriate borrow area which shall be free of roots, stumps,
wood, rubbish, stones greater than six inches, frozen or other objectionable
materials.
E. Bottom slope. The minimum bottom slope of facilities not designed
for infiltration shall be 1%. A flatter slope may be used if an equivalent
dewatering mechanism is provided.
F. Pretreatment elements. When required, pretreatment elements shall
consist of forebays, or alternate approved by the Municipal Engineer,
to keep silt to a smaller portion of the facility for ease of maintenance.
G. Infiltration basins. Within basins designed for infiltration, existing native vegetation shall be preserved, if possible. For existing unvegetated areas or for infiltration basins that require excavation, a planting plan shall be prepared in accordance with §
373-9 and the BMP Manual, which is designed to promote infiltration.
H. All discharge control devices with appurtenances shall be made of
reinforced concrete and stainless steel. Bolts/fasteners shall be
stainless steel.
I. Use of the spillway to convey flows greater than the fifty-year design
storm is permitted.
J. Emergency use. The spillway shall be designed to convey the one-hundred-year
peak inflow.
K. When required, freeboard shall be measured from the top of the water
surface elevation for emergency use.
L. All basins shall be structurally sound and shall be constructed of
sound and durable materials. The completed structure and the foundation
of all basins shall be stable under all probable conditions of operation
and shall be capable of discharging the peak discharge of a postdevelopment
one-hundred-year storm event through the primary emergency and/or
spillway facilities, in a condition that assumes the primary outlet(s)
are blocked, which will not damage the integrity of the facility or
the downstream drainage areas.
M. The effect on downstream areas if the basin embankment fails shall
be considered in the design of all basins. Where possible, the basin
shall be designed to minimize the potential damage caused by such
failure of the embankment.
N. All detention basins shall include an outlet structure to permit
draining the rate control volume in the basin to the level of a two-year
storm within 24 hours (exclusive of BMP or volume control storage).
O. All outlet structures and emergency spillways shall include a satisfactory
means of dissipating the energy of flow at its outlet to assure conveyance
of flow without endangering the safety and integrity of the basin
and the downstream drainage area.
P. A cutoff trench of relatively impervious clay material shall be provided
within all basin embankments, except for those embankments with side
slope ratios of three horizontal to one vertical or flatter. Embankments
with flatter side slopes shall have a key trench.
Q. All culverts through basin embankments shall have properly spaced
concrete cutoff collars or welded antiseep collars.
R. A minimum one-foot freeboard above the design elevation of the water
surface at the emergency spillway shall be provided.
S. No outlet structure from a detention basin or swale shall discharge
directly onto a Township road, but shall discharge into a culvert
under a Township road.
T. The minimum top width of dams up to 10 feet in height shall be equal
to 2/3 of the dam height, but in no case shall the top width be less
than five feet.