No stormwater runoff or natural drainage water
shall be so diverted as to overload existing drainage systems or create
flooding or the need for additional drainage structures on other private
properties or public lands without proper and approved provisions
being made for taking care of these conditions. Watersheds with drainage
areas over 1/2 square mile come under the jurisdiction of the New
Jersey Department of Environmental Protection. Approval of this agency
shall be secured for any culvert or bridge crossing falling into this
category. Copies of any correspondence with the Department of Environmental
Protection shall be supplied to the Township Engineer.
A.
The quantities of water for which the collection system
shall be designed shall be in accordance with the rational method
of design, using the following formula:
|
Q = CIA
| |||
|---|---|---|---|
|
Q
|
=
|
The required capacity in cubic feet per second
for the collection system at the point of design.
| |
|
A
|
=
|
The tributary drainage area in acres and shall
include areas tributary from outside sources as well as from within
the site itself.
| |
|
C
|
=
|
The coefficient of runoff applicable to the
drainage area and shall be based on consideration of soil conditions,
average slope of the drainage area and degree of ultimate development
of the area, but in no case shall be less than the following coefficients:
| |
|
Usage
|
Density
(units per acre)
|
Topography
| |||
|---|---|---|---|---|---|
|
Flat (0% to 10%)
|
Rolling (10% to 20%)
|
Hilly (over 20%)
| |||
|
Residential: High density
|
4 to 6
|
0.50 (0.40)
|
0.55 (0.45)
|
0.60 (0.55)
| |
|
Medium density
|
1 to 4
|
0.45 (0.35)
|
0.50 (0.40)
|
0.55 (0.50)
| |
|
Low density
|
Less than 1
|
0.40 (0.30)
|
0.45 (0.35)
|
0.50 (0.45)
| |
|
Impervious surfaces
|
0.90
|
0.90
|
0.90
| ||
|
Undeveloped: Woodland
|
0.30 (0.20)
|
0.35 (0.25)
|
0.50 (0.45)
| ||
|
Pasture and grassed area
|
0.30 (0.20)
|
0.36 (0.20)
|
0.42 (0.37)
| ||
|
Cultivated fields
|
0.50 (0.50)
|
0.60 (0.50)
|
0.72 (0.62)
| ||
|
The above runoff coefficients assume that clay
soils (sand gravel soils coefficients are indicated in parenthesis)
are present and that paved areas (parking) have been provided, and
any deviation from these factors would result in an adjustment in
C values. Determination of nonlisted uses shall be a subject of special
study and approved by the Township Engineer.
| ||||
|
I
|
=
|
The rainfall intensity in inches per hour and
shall be determined from the graph entitled "Rainfall Intensity Duration
for Essex and Union Counties," copies of which are on file with the
Township Clerk and which is made a part hereof. The following intensities
shall be used to determine Q:
| ||
|
(a)
|
Systems carrying roadway drainage: 10 years.
| |||
|
(b)
|
Crosspipes or culverts carrying runoff from
one side of the roadway to the other and all facilities downstream
thereof: 25 years; tributary areas over 20 acres: 50 years.
| |||
|
(c)
|
At low points and cuts from which water cannot
escape without overflowing the roadway: 25 years.
| |||
|
The minimum time of concentration shall be 10
minutes.
| ||||
B.
Pipes or conduits shall use designed using Manning's
equation for velocity:
 | |||
|---|---|---|---|
|
R
|
=
|
The hydraulic radius of the conduit flowing
full.
| |
|
n
|
=
|
Manning's roughness coefficient, which shall
be as follows:
| |
|
Concrete pipe
|
0.012
| ||
|
Corrugated metal, 2 2/3" x 1/2" corrugations:
| |||
|
Plain or fully coated
|
0.024
| ||
|
Paved invert
|
0.020
| ||
|
Fully paved
|
0.012
| ||
|
3" x 1" corrugation
|
0.027
| ||
|
6" x 2" corrugation field bolted
|
0.032
| ||
|
n for other materials shall be as recommended
by the Township Engineer.
| |||
|
S
|
=
|
The slope of the energy gradient in feet per
foot and shall generally be considered as being the same as the slope
of the invert of the pipe (So). However, such slopes shall be checked
against the available hydraulic gradient whenever the system flows
full or where the system discharges into a body of water.
|
C.
Minimum grades.
(1)
Whenever possible to ensure a self-cleansing velocity,
pipes shall be placed on grades that will provide a minimum velocity
of three feet per second when flowing 1/4 full. The following table
indicates minimum grades to accomplish this:
|
Desirable Minimum Gradients for Concrete
Pipe
| ||
|---|---|---|
|
Diameter
(inches)
|
Slope
n = 0.012
| |
|
15 (minimum permissible)
|
0.008
| |
|
18
|
0.006
| |
|
24
|
0.005
| |
|
30
|
0.004
| |
|
36
|
0.003
| |
|
42
|
0.002
| |
|
48
|
0.0016
| |
|
54
|
0.0014
| |
|
60
|
0.0012
| |
|
66
|
0.0011
| |
(2)
Where steep grades are encountered, full-flow velocities
over 15 feet per second should be avoided by the use of drop manholes
or inlets in the line.
D.
Manholes or inlets shall be provided in drain lines
not more than 350 feet apart and wherever branches are connected or
sizes are changed and wherever there is a change in alignment or grade.
Alignment of pipes shall be in a straight line between manholes or
inlets, except that radius pipe may be used in pipes over 60 inches
in diameter, provided that a manhole or inlet is incorporated at not
more than five-hundred-foot intervals.
E.
Inlets; pipe lines.
(1)
Inlet spacing shall permit a spread of water to the
edge of the traveled way, edge of shoulder for rural roads or edge
of parking lane, eight feet maximum, for urban roads during a storm,
with an intensity of three inches per hour. Inlet spacing may vary
up to 350 feet, depending on the contributory drainage area and the
longitudinal and cross-slopes of the roadway. Inlets at intersections
shall be located to intercept water before it crosses the intersection
or crosswalk areas, and inlets shall not be placed in the crosswalk
area. Pairs of inlets shall be placed at low points.
(2)
Inlets intercepting swales or in parking areas shall
be designed so that no more than six inches, three inches in parking
areas, shall pond over an inlet during a storm with an intensity of
three inches per hour.
(3)
Pipe lines starting or ending in ditches, streams
or other bodies of water shall terminate with either a headwall or
a flared end section. Riprap or other suitable material shall be provided
to prevent erosion.
Where brooks, ditches, swales or other channel
flows are used, they shall be designed by the use of Manning's equation
as indicated for collection systems above, with the following considerations:
A.
The friction factor (n) shall be:
|
Unlined
|
Factor
| |
|---|---|---|
|
Earth, uniform section
|
0.018 to 0.027
| |
|
Earth, nonuniform section
|
0.025 to 0.040
| |
|
Earth not maintained
|
0.080 to 0.012
|
|
Lined Concrete
|
Factor
| ||
|---|---|---|---|
|
Finished
|
0.015
| ||
|
Rough
|
0.25
| ||
|
Riprap
|
0.35
| ||
B.
Design velocities should be limited to a non-erosive
velocity and shall be determined based on an analysis of the soil
conditions, but in any case shall not exceed the following:
|
Materials
|
Velocity
(feet per second)
| |
|---|---|---|
|
Fine sand to fine loam
|
2.50 to 3.50
| |
|
Stiff clay to hardpan
|
3.75 to 6
| |
|
Concrete-lined ditch
|
15
| |
|
Velocities shall be controlled by use of check
dams or baffles where necessary.
| ||
C.
A ditch or brook right-of-way of sufficient minimum
width to include a ten-foot access strip in addition to the width
of the ditch or brook from bank top to bank top shall be offered for
dedication to the Township for drainage purposes. Such right-of-way
shall shown on the drainage plan and on the final plat.
D.
Swales shall be provided in residential subdivisions
so that an area not in excess of the minimum lot area in the subdivision
shall drain without being intercepted. This requirement may be varied
for wooded sites or pervious soils.
E.
Swales shall be provided in other areas so that no
more than one impervious acre shall drain without being intercepted.
F.
Jute mesh slope protection shall be used in swales
or grassed ditches on gradients exceeding 3%. Paved ditches shall
be used where these gradients exceed 10% and significant discharge
is expected.
G.
Side slopes for a drainageway shall be as follows:
|
Facility
|
Maximum Side Slope
| ||
|---|---|---|---|
|
Channel
|
2:1
| ||
|
Ditch
|
4:1
| ||
|
Swale
|
6:1
|
A.
All pipe shall be concrete unless specific unique
design considerations exist for the use of other types of materials
and then only with the approval of the Township Engineer.
B.
Minimum cover for reinforced concrete pipe shall be
based upon a depth of cover of two feet from top of pavement to top
of pipe.
C.
Minimum cover for corrugated metal pipe shall be based
upon a depth of cover of one foot from the top of the pipe to the
top of the pavement or upon the manufacturers recommendations, whichever
is greater.
D.
Maximum cover for pipe shall be based on rational
analysis through use of Marston's formula. For reference, see the
booklet entitled "Reinforced Concrete Pipe Culverts," issued by the
United States Department of Commerce, and the manufacturer's recommendations
for flexible pipe.
E.
Standard strength reinforced concrete pipe shall conform
to Class 3, Wall B, and extra-strength reinforced concrete pipe shall
conform to Class 4, Wall B, of ASTM Specifications C-76 of AASHO Designation
M-170. Strengths for corrugated metal pipe shall be as required by
the New Jersey State Highway Department Standard Specifications, current
edition and amendments.
F.
Box culverts shall be designed in accordance with
American Association of State Highway Officials recommendations.
A.
Criteria not incorporated herein, necessary for design,
shall be based on recommendations incorporated with Hydraulic Engineering
Series circulars published by the Federal Highway Administration and
ASCE publication current edition, and as approved by the Township
Engineer.
B.
Design methods prepared by the Natural Resources Conservation
Service may be used for drainage design with the approval of the Township
Engineer.
All site plans shall incorporate adequate storm
drainage facilities. The facilities shall be designed in accordance
with the above standards, except that in addition to the use of standard
inlets as prescribed for use in Township streets, the use of inlet
Type B-3 may be permitted. These shall be as shown in the standard
details for Mansfield Township, copies of which are on file in the
Township Clerk's office.
A.
Inlets in parking areas shall be spaced and sized
to prevent a spread of water into the parking aisles during a storm
with an intensity of three inches per hour. Inlets in driveways and
roadways within parking areas shall be spaced and sized to provide
a minimum of an eight-foot wide lane for each design traveled lane
during a storm with an intensity of three inches per hour.
