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.
|
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.
|
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
|
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
|
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.
|
Facility
|
Maximum Side Slope
| ||
---|---|---|---|
Channel
|
2:1
| ||
Ditch
|
4:1
| ||
Swale
|
6:1
|