[HISTORY: Adopted by the Board of Supervisors of Manor Township 12-6-1976 by Ord. No.
4-76 (Ch. 22, Part 1, of the 1985 Code of Ordinances).
Amendments noted where applicable.]
[Amended 10-1-1979 by Ord. No. 6-79]
A.
Intent. The purpose of this manual is to establish standard procedures
for the design and approval of adequate storm drain systems for new
land subdivisions within Manor Township.
B.
Exemptions. Developers of subdivisions with land areas of five acres
or less are exempt from the design requirements of this manual, provided
that not more than 15% of the land area is made impervious by proposed
construction.
C.
Scope. The design methods outlined herein apply only to drainage
areas of less than 1 1/2 square miles. For large drainage areas,
the user is referred to the PennDOT Design Manual, Part 2, Chapter
12, Section 20, "Bridge Waterway Structures" for required analysis
of maximum expected discharge and related drainage facilities.
D.
Requirements.
(1)
The importance of properly designed drainage from an economic, safety
and public relations standpoint warrants a hydrologic analysis. Requirements
for submittals of plans and calculations of proposed facilities are
outlined in the following sections. Calculations must include an investigation
of existing drainage facilities downstream of the project to insure
that they are capable of accepting the additional runoff without causing
flooding and erosion.
(2)
The basic concept of the stormwater management policy is to insure
that downstream property owners, watercourses, channels or conducts
are not adversely affected by an increase in stormwater runoff. Therefore,
stormwater runoff from any development, during and after construction,
shall be no greater than that flow resulting from the same storm event
occurring over the site of the proposed development with the land
in its existing, undeveloped condition.
(3)
A comparison analysis of the change in runoff shall be provided in
the calculations.
(4)
All increases in stormwater runoff resulting from a proposed development
shall be detained on the development site under predetermined and
controlled conditions with the rate of drainage therefrom regulated
by appropriately installed devices. Methods of detention or flow-delay
devices may include but not be limited to the following:
(a)
Wet or dry ponds and detention basins;
(b)
Roof storage and increased roof roughness;
(c)
Parking lot retention;
(d)
Porous pavements, grassed channels and vegetated strips;
(e)
Cisterns and underground reservoirs;
(f)
Increasing the roughness coefficients on the development's
surface area;
(g)
Decrease percentage of impervious area.
(5)
The use of other detention methods which prove to meet the objectives
and intent of this chapter in accordance with the regulations and
standards set forth herein will be permitted subsequent to the approval
of the Township Engineer. Various possible combinations of methods
may be evaluated on their particular merit for the type and location
of development.
(6)
Storm sewer systems within a development shall be designed to handle
the peak rate of runoff from a fifty-year frequency storm. Wherever
the provisions of federal and state laws impose a greater design frequency,
particularly in areas where drainage systems may cross highways, they
shall prevail.
(7)
All stormwater retention/detention facilities shall be designed on
the basis of providing adequate control for all storm frequencies
up to and including the fifty-year storm. All designs shall provide
emergency overflow facilities for the one-hundred-year storm, unless
positive measures are installed to control the inflow so as not to
exceed the safe capacity of the retention/detention facility.
(8)
The retention volume required shall be that necessary to handle runoff
of a fifty-year storm from the development, less that volume discharged
at the approved release rate.
(9)
The approved peak release rate of stormwater from all retention/detention
facilities for any storm event shall be that which was experienced
prior to development for the same storm event up to and including
the fifty-year storm.
(10)
Stormwater control systems may be planned and constructed in
coordination by two or more developments, so long as they are in compliance
with the applicable provisions of this chapter.
(11)
All calculations and design parameters shall be subject to the
review and approval of the Township Engineer, whose decision in matters
involving engineering judgment shall be final.
(12)
In the design of storm drainage facilities, special consideration
must be given to adjacent developed or undeveloped properties. In
no case may a change be made in the existing topography which would:
(13)
In no case may any slope exceed the normal angle of slippage
of the material involved. All slopes must be protected against erosion.
E.
Other approvals.
(1)
Designers are cautioned that the requirements contained herein are
only minimum standards established for storm drainage approval by
Manor Township. Compliance with this manual and subsequent approval
by Manor Township does not preclude the developer's full responsibility
in meeting any and all federal, state or county regulations as may
be applicable.
(2)
The storm drainage system design should be coordinated with the Sediment
and Erosion Control Plan for the project; however, approval of the
storm drainage plan does not eliminate any requirements for an approved
Sediment and Erosion Control Plan.
[Amended 10-1-1979 by Ord. No. 6-79; 10-2-1982 by Ord. No. 3-82]
A.
Reference materials. The reference materials to be used with this
section are as follows:
(1)
"PennDOT Design Manual," Part 2, Chapter 12.
(2)
"Design Charts for Open-Channel Flow," U.S. Department of Commerce
- Bureau of Public Roads.
(3)
"Hydraulic Charts for the Selection of Highway Culverts," Hydraulic
Engineering Circular No. 5, U.S. Department of Commerce - Bureau of
Public Roads.
(4)
"Design of Roadside Drainage Channels," Hydraulic Design Series No.
4, U.S. Department of Commerce - Bureau of Public Roads.
(5)
"Capacity Charts for the Hydraulic Design of Highway Culverts," Hydraulic
Engineering Circular No. 10, U.S. Department of Commerce - Bureau
of Public Roads.
B.
Maximum expected discharge.
(1)
The maximum expected discharge (MED) may be defined as the maximum
expected quantity of water, created by the design storm, arriving
at a particular location (inlet, ditch, etc.).
(2)
The design storm is a selected intensity of rainfall, expressed in
inches per hour, which tends to occur once during a specified period
of years.
(3)
The maximum expected discharge from drainage areas less than 1 1/2 square miles shall be determined by the use of the Rational Equation. For larger drainage areas, see § 371-1C.
(4)
The Rational Equation is as follows:
|
Q = CIA
|
|
Where:
| ||||
|
Q
|
=
|
Maximum expected discharge in cubic feet per second.
| ||
|
C
|
=
|
Runoff factor expressed as a percent of the total water falling
on an area.
| ||
|
I
|
=
|
The rate of rainfall for the time of concentration of the drainage
area in inches per hour for a given storm frequency. A fifty-year
storm frequency shall be used.
| ||
|
A
|
=
|
The drainage area expressed in acres.
| ||
|
The above equation assumes that one inch of rainfall falling
on one acre of land falls at the rate of one cubic foot per second.
Thus, the total quantity of water falling on an area is represented
by IA.
|
(a)
Runoff factor ("C").
[1]
It is necessary to adjust the total quantity of water falling
on an area (IA) because a certain percentage of the water is dissipated
by evaporation, transpiration, percolation, ponding, and physical
characteristics such as sinkholes. Therefore, the runoff factor "C"
is introduced into the Rational Equation to account for the dissipated
water.
[2]
Suggested runoff factors for various types of drainage areas
are presented in Table A at the end of this manual.
[3]
Design engineers shall determine "C" factors from field inspection
of area and consideration of type of soil and average slopes of tributary
areas. "C" factors shall represent a weighted average of the areas
covered by the classification shown in Table A.[1]
[1]
Editor's Note: Table A is included in an attachment to this
chapter.
[4]
Consideration should be given to future land use changes in
the drainage area, including possible reclamation of land in areas
where special situations, such as sinkholes, exist.
(b)
Rainfall intensity ("I").
[1]
Rainfall intensity shall be determined from Figure 1.[2] This curve indicates maximum rainfall intensities for
storm durations (times of concentration, computed as described below)
from five minutes to 24 hours, occurring with various frequencies.
[2]
Editor's Note: Figure 1 is included in an attachment to this
chapter.
[2]
Time of concentration is defined as the time required for water
to flow from the most remote part of the drainage area to the point
under consideration and is the combined time of overland flow and
flow in drains, swales, gutters and ditches.
[4]
Velocity of flow in drains, swales, gutters and ditches shall be determined by the Manning Equation as discussed under Subsection C, Capacity of drainage facilities.
[5]
The time of concentration shall be calculated by the relationship:
|
Time =
|
Distance
Velocity
|
|
and shall be considered as representing the duration of storm.
In no case shall a storm duration of less than five minutes be used.
|
(c)
Area ("A").
[2]
The highest order of information available and practical shall
be used.
[3]
Care should be taken to assure that all areas delivering runoff
to the point under consideration shall be included and that physical
obstructions, such as existing facilities with inadequate capacity
inhibiting the delivery of runoff, shall be considered.
C.
Capacity of drainage facilities.
(1)
The previous section has established a criteria for determining how
much water is expected to arrive at a particular location (MED). This
section is primarily concerned with the conveyance of the water arriving
at that location.
(b)
The capacity of the drainage facilities is measured in terms
of discharge and may be determined by the equation of continuity:
|
Q = AV
|
|
Where:
| ||||
|
Q
|
=
|
Discharge of water in cubic feet per second. A drainage facility
at a particular location shall hydraulically and economically accommodate
the maximum expected discharge (MED) for the location.
| ||
|
A
|
=
|
The net effective area in square feet provided by the drainage
facility. By "net effective area" is meant that cross-sectional area
of the facility which may be used to carry water. It may not be desirable
that the entire cross-sectional area of the drainage facility be utilized
to carry water.
| ||
|
V
|
=
|
The velocity of the water in feet per second. The velocity shall
generally be determined by Manning's Equation.
| ||
(c)
Manning's Equation is as follows:
|
V =
|
1.486
n
|
R2/3
|
S1/2
|
|
Where:
| ||||
|
V
|
=
|
Velocity in feet per second.
| ||
|
R
|
=
|
Hydraulic radius in feet = the net effective area (A) divided
by the wetted perimeter (W.P.):
| ||
|
R =
|
A
W.P.
|
|
The wetted perimeter is the lineal feet of the drainage facility
cross section which is wetted by the water.
| |||
|
S
|
=
|
Slope of energy line in feet per foot. For approximation, the
invert slope may be used.
| |
|
n
|
=
|
The roughness coefficient. Acceptable roughness coefficients
are presented in Table C.[4]
| |
[4]
Editor's Note: Table C is included in an attachment to this
chapter.
(2)
Additional design criteria for specific drainage facilities are presented
as follows:
(b)
Inlets.
[1]
General.
[a]
All inlets shall be in accordance with the latest
edition of the PennDOT "Standards for Roadway Construction," except
that only reinforced precast or cast-in-place reinforced concrete
inlet boxes will be allowed. No brick masonry inlets will be accepted.
[b]
Three basic types of inlets, namely, Types C, M
and S, are included in the above standards. Each type of inlet is
suited for a particular situation. Type C inlet is designated for
installation in nonmountable curbs. Type M inlet is designated for
installation in mountable curbs and Type S inlet is designated for
installation in swale or ditch areas. Inlet capacities for each specific
type of inlet under various conditions are specified in the tables
and figures as described below.
[2]
Type C inlet or Type M inlet. The capacities of Type C inlet
or Type M inlet (mountable curb) on a continuous grade are presented
in Table D for a 100% efficiency and in Figure 2 through Figure 5
for various percents of efficiency. The efficiency of an inlet is
defined as (Q2/Q1) x
100%, where Q1 is the channel flow in cfs and
Q2 is the rate of flow, in cfs, intercepted
by the inlet gratings. The capacities for these inlets under sump
conditions are indicated on Table E.[5]
[5]
Editor's Note: Table E is included in an attachment to this
chapter.
[3]
Type S inlet.
[a]
[b]
For installation of Type S inlet on continuous
grade in swale areas, a drainage dike with side slope 8:1 or flatter
(six-inch minimum height and one-foot maximum height) shall generally
be placed below the inlet to achieve maximum capacity.
[c]
A lightweight grate may be specified for Type S
inlets, provided that the inlet is located in a non-traffic area.
The inlet that will accommodate the lightweight grate shall be designated
with the suffix "LW," such as Type S-LW inlet.
[4]
Capacity and spacing.
[a]
If the capacity of an inlet under a specific condition
is not included in the tables or figures as referred to above, it
is generally satisfactory to use the value specified for the nearest
condition.
[b]
Inlets shall be constructed in all sumps. Also,
all street intersections receiving flow from curbed sections shall
be protected by inlets.
[c]
On curbed sections immediately adjacent to structures,
inlets shall be provided on each side of all structures having spans
of 20 feet or greater for grades less than 1%. An inlet shall also
be placed at the low point of sag vertical curves on curbed sections.
[d]
If the capacity of the previously defined allowable
waterway portion of a curbed section exceeds the inlet capacities,
the inlet capacities shall govern the spacing of inlets. If the capacity
of the allowable waterway portion of a curbed section is less than
the inlet capacities, then the capacity of this portion of the curbed
section shall govern the spacing of inlets.
[e]
For curbed sections on a continuous grade, if analysis
of inlet capacities based on a 100% efficiency has resulted in a spacing
of less than 100 feet, then consideration shall be given to re-spacing
the inlets by allowing channel flow to bypass the inlets to achieve
economical effects provided that the maximum encroachment of water
on the roadway pavement shall not exceed half a through traffic lane
or one inch less than the depth of curb. Inlet spacing on curbed sections
shall not exceed 450 feet.
[f]
Inlet spacing may generally be determined from
the following formula giving due consideration to the percentage of
water bypassing the inlet and, on curbed sections, the permissible
encroachment of water on the roadway pavement.
|
L =
|
L = 43,560 Q
CIW
|
|
Where:
| ||||
|
L
|
=
|
Inlet spacing in feet.
| ||
|
Q
|
=
|
Discharge capacity of the drainage facility (inlet, swale, curb
sections, etc.) with the least capacity.
| ||
|
C
|
=
|
Runoff factor.
| ||
|
I
|
=
|
Rainfall intensity, inches per hour.
| ||
|
W
|
=
|
Average width of contributing area.
| ||
[g]
When there is a change in pipe size in an inlet,
the elevation for the top of pipes should be the same or the smaller
pipe higher. A minimum drop of two inches should be provided in the
inlet between the lowest inlet pipe invert elevation and the outlet
pipe invert elevation. Inlet boxes may have to be modified to accommodate
large pipe sizes. (Modified Type I and Type II as detailed in the
PennDOT "Standards for Roadway Construction.")
(c)
Storm pipes.
[1]
In new subdivisions, storm drains shall normally be installed
at the center line of the street. Storm drains shall be installed
within the pavement area (no less than six feet from the curb) wherever
possible.
[2]
To facilitate the solution of Manning's Equation as applied
to storm pipes, charts are presented in "Design Charts for Open-Channel
Flow" prepared by the U.S. Department of Commerce, which permit a
direct determination of the capacity of circular pipes.
[3]
Where headroom is restricted, equivalent pipe arches may be
used in lieu of circular pipe. An acceptable procedure for selecting
a pipe arch is to determine the required circular pipe size from the
charts and, subsequently, to select the equivalent pipe arch.
[4]
The minimum diameter of storm pipe shall be 18 inches, except
pipes under a twenty-five foot or greater fill shall not be less than
24 inches. The top of storm pipes shall be at least six inches below
subgrade elevation, except cast-iron pipe, which may be within three
inches. The size of a downstream storm pipe shall not be smaller than
that of the upstream storm pipe. Longitudinal pipes may serve as combination
storm sewer and foundation underdrain pipe.
[5]
Abrupt changes in direction or slope of pipe shall be avoided.
Where such abrupt changes are required, an inlet or manhole shall
be placed at the point of change. The minimum slope in a pipe shall
not be less than 0.35%.
[6]
All pipe materials and types utilized at any particular location
shall be in accordance with the PennDOT Design Manual, Part 2, Chapter
12, except that no alternate types need be specified.
(d)
Culverts.
[1]
In all cases where drainage is picked up by means of a head
wall, and inlet or outlet conditions control, the pipe shall be designed
as a culvert. When a pipe is part of a storm sewer system and crosses
the roadway, it shall be designed as a storm sewer with the same design
storm as the remainder of the system.
[2]
The minimum diameter of culvert shall be 18 inches, except pipes
under a twenty-five foot or greater fill shall not be less than 24
inches.
[3]
The procedure contained in Hydraulic Engineering Circulars No.
5 and No. 10 as prepared by the U.S. Department of Transportation,
Federal Highway Administration, Washington, D.C. shall be used for
the design of culverts. The allowable headwater should be determined
by the specific entrance conditions and good engineering judgment.
[4]
Velocities at culvert outlets shall not exceed acceptable limits
as defined below without providing erosion protection. The design
is not complete until the possibility of accelerated erosion is eliminated.
The first step is to check actual velocity against the allowable maximum
water velocities as specified in Table H. Where channel scour is indicated,
appropriate means for reducing velocity to safe levels or for protecting
the channel shall be incorporated.
(e)
Open channels.
[1]
Open channels shall be so located as to change the stream alignment
as little as possible. However, it shall generally be considered desirable
to eliminate bends, to cross streets (and future streets) normal to
the street and to eliminate stream channels running through the center
of a property where location near or on a property line is feasible.
[2]
The most common types of open channels are triangular, trapezoidal,
and rectangular. The trapezoidal shape is preferred due to its higher
hydraulic efficiency. Triangular shapes require less right-of-way
and are readily maintained with a grader. Rectangular shapes are generally
used in rock areas. The maximum bank slope on earth channels shall
be three horizontal to one vertical.
[3]
To facilitate the solution of Manning's Equation and the continuity
equations as applied to open channels, charts are presented in "Design
Charts for Open-Channel Flow."
[4]
Where the depth of design flow is slightly below critical depth,
channels shall have freeboard adequate to cope with the effect of
hydraulic jump. Open channels shall have one foot of freeboard to
accommodate silting. Where no appreciable silt deposits will occur,
this one-foot freeboard may be eliminated.
[5]
Transverse channels shall join parallel channels at an angle
of approximately 30° with the parallel channel to minimize scour
and sedimentation.
[6]
Table H lists maximum permissible velocities for various channel
lining materials.[10] Where calculated velocities exceed those permissible for
earth, channels shall be paved or riprapped. Protective linings for
channels and streams can be very expensive. Therefore, a special effort
should be made to develop the lowest-cost erosion protection, including
maintenance, for the particular location.
[10]
Editor's Note: Table H is included in an attachment to this
chapter.
(f)
Detention basins. Detention basins may be constructed to temporarily
detain the stormwater runoff. When such basins are used, the following
design principles shall be observed:
[1]
The maximum planned depth (without a permanent pool) shall not
exceed five feet.
[2]
The approach slopes of the basin will conform as closely as
possible to natural land contours. Erosion control measures shall
be provided as well as devices or measures to insure public safety.
[3]
Levee side slopes shall not exceed 3:1.
[4]
Outlet control structures shall be designed as simply as possible
and shall operate automatically; they shall be of a safe capacity
and be designed to limit the discharge to that which would have occurred
with the land in its existing condition, prior to development.
[5]
Emergency overflow facilities must be provided for the one-hundred-year
storm, unless positive measures are installed to control the inflow
so as not to exceed the safe capacity of the basin.
(g)
Wet bottom basins. Where part of a detention basin will contain
a permanent pool of water, the following requirements shall apply:
[1]
The minimum normal depth of water before the introduction of
stormwater shall be four feet.
[2]
For emergency purposes, cleaning or shoreline maintenance, facilities
shall be provided or plans prepared for the use of auxiliary equipment
to permit emptying and drainage.
[3]
Aeration facilities may be required, dependent on the quality
of the influent and detention time.
[4]
The side slopes shall be a nonerosive material with a slope
of 3:1 or flatter. There shall be a freeboard of 12 inches to 18 inches
above the high-water elevation on all wet bottom basins. Alternate
designs for side slopes may be considered under special circumstances
where good engineering practice is demonstrated.
(h)
Dry bottom basins. Detention basins which will not contain a
permanent pool of water shall comply with the following requirements:
[1]
Provisions must be incorporated to facilitate interior drainage,
to include the provision of natural grades to outlet structures, longitudinal
and transverse grades to perimeter drainage facilities, or the installation
of subsurface drains.
[2]
These basins may be designed to serve secondary purposes for
recreation, open space or other types of use which will not be adversely
affected by occasional or intermittent flooding.
(i)
Rooftop storage. Detention storage requirements may be met in
total or in part by detention on flat roofs. Details of such designs
to be included in the building permit applications shall include the
depth and volume of storage, details of outlet devices and down drains,
elevations of overflow scuppers, design loadings for the roof structure
and emergency overflow provisions. Direct connection of roof drains
to sewers or streets will be prohibited.
(j)
Parking lot storage. Paved parking lots may be designed to provide
temporary detention storage of stormwater on all or a portion of their
surfaces. Parking lot ponding should be arranged so that pedestrians
can reach their destinations without walking through ponded water.
Outlets will be designed so as to slowly empty the stored waters,
and depths of storage must be limited so as to prevent damage to parked
vehicles. Parking lot storage may be used in those areas where the
health, safety, and general welfare of the community will not be adversely
affected.
(k)
Detention storage. All or a portion of the detention storage
may also be provided in underground detention facilities such as cisterns
and covered ponds.
A.
General instructions.
(1)
All plans and calculations for proposed storm drain systems shall be submitted to the Manor Township Board of Supervisors for review and approval. No submittal shall be considered approved until such time as written and duly authorized notification of same is received by the applicant. The form and content of required submittals are outlined in Subsections C and D.
(2)
All submittals shall contain the professional seal, signature and
registration number of the registered engineer or surveyor responsible
for the design.
B.
Standard drafting practices:
(1)
Final contract drawings shall be prepared in ink or pencil or a combination
of both on tracing cloth or good quality Mylar reproducible sheets.
(2)
A complete title block shall be shown on all drawings. Title blocks
shall contain at least the following information:
(3)
A revision block shall be provided on each sheet with space for all
revision dates and nature of revision.
(4)
A complete description of all bench marks shall be shown, including
location and elevation.
(5)
Each drawing shall be so oriented that the North arrow points toward
the top or toward the left side of the sheet.
(6)
All drawings in the same project shall be cross-referenced.
C.
Preliminary report. Prior to the preparation of final plans, two
copies of a preliminary report shall be submitted for review. Reports
shall generally contain the following information:
(1)
General description of the project.
(2)
Discussion of proposed work, as to the location, size and other pertinent
data.
(3)
Recommendations and conclusions.
(4)
Supporting drawings, calculations and any other data necessary to
support and depict the written conclusions. Drawings shall include
a map of the entire drainage area showing the proposed storm drainage
system in conjunction with existing drains. Calculations shall include
runoff data and preliminary sizing of proposed drains, plus an investigation
of the effect of the proposed construction on existing downstream
facilities. Additional submittals of the preliminary report may be
required, at the discretion of the Board of Supervisors, incorporating
revisions and recommendations made in a previous review.
D.
Final plans and calculations.
(1)
Following approval of the preliminary report, final storm drain drawings
and calculations shall be prepared and two copies of same shall be
submitted for review. Final drawings shall include, but not be limited
to, the following information:
(a)
A location plan showing the names of streets and roads leading
to the site and the distance to nearest municipalities.
(b)
A drainage area map showing entire drainage area with each tributary
area lettered for reference to calculations and tabulation forms.
The proposed drainage system shall be indicated schematically, complete
with inlets, manholes and structures.
(c)
A plan and a profile of proposed system showing the following:
[1]
All property lines, right-of-way lines, lot numbers and front
dimensions of lots.
[2]
All roads and streets with names, curblines, pavement widths
and cross slopes, grades and curb radii.
[3]
All existing and proposed utilities completely labeled.
[4]
Elevation contour lines at five-foot vertical intervals on proposed
finished ground.
[5]
Entire proposed storm drain system including inlets, manholes,
pipes, ditches, etc., completely labeled as to type and size and with
invert elevations and slopes shown. Location of all components shall
be dimensioned from property lines.
(d)
North arrow on all plan views.
(e)
Appropriate scale shown as a bar scale.
(3)
All structural calculations and/or references to standard loading
tables shall be shown. Where information pertinent to design, such
as core borings, has been collected, this information shall also be
submitted.
(4)
Following incorporation of review comments and approval by the Board
of Supervisors, all original drawings and documents shall be presented
for signature by a duly authorized representative of the Board. Such
signature shall constitute final approval and all originals shall
become the property of Manor Township.
See Attachment 1 of this chapter.[1]
[1]
Editor's Note: Attachment 1 is included as an attachment to this chapter.
Any person who violates or permits a violation of this chapter
shall, upon conviction in a summary proceeding brought before a Magisterial
District Judge under the Pennsylvania Rules of Criminal Procedure,
be guilty of a summary offense and shall be punishable by a fine of
not more than $1,000, plus costs of prosecution. In default of payment
thereof, the defendant may be sentenced to imprisonment for a term
not exceeding 90 days. Each day or portion thereof that such violation
continues or is permitted to continue shall constitute a separate
offense, and each section of this chapter that is violated shall also
constitute a separate offense.
