The importance of properly designed drainage from an economic,
safety and public relations standpoint warrants a hydraulic analysis.
Good drainage design anticipates where surface water will accumulate
and makes provisions for the removal of excess water as rapidly as
possible in order to prevent unusual damage to private property, undue
interference with the operation of motor vehicles or traffic flow
and excessive maintenance burden.
A.
The accumulation of specific information by office and field investigations
is required for proper drainage analysis. It is necessary that plans
be prepared indicating topography, preliminary alignment and profile
information.
B.
In addition to the above, plans shall show:
(1)
All proposed curb and superelevation data.
(2)
The station of low and high points.
(3)
Existing drainage facilities.
(4)
Ditch and stream slopes.
(5)
The drainage area from the proposed topography.
(6)
Areas of sharp grades in excess of four to one (4:1) slopes.
(7)
Drainage areas from United States Geological Survey (USGS) maps or
other sources.
(8)
The preliminary location of proposed drainage facilities.
(9)
The known high water marks.
A.
The maximum expected discharge may be defined as the maximum expected
quantity of water, created by the design storm, arriving at a particular
location such as an inlet or ditch. The MED from drainage areas shall
be determined by the Rational Equation or "Q = CIA," where "Q" equals
the maximum expected discharge in cubic feet per second; "C" equals
the runoff factor expressed as a percent of the total water falling
on an area; "I" equals the rate of rainfall expressed in inches per
hour for a fifty-year storm frequency; and "A" equals the drainage
area expressed in acres.
B.
The above equation assumes that one inch of rainfall falling 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 "1A."
It is necessary to adjust the 1A because a certain percentage of the
water is dissipated by evaporation, transpiration, percolation, ponding
and physical features. Therefore C, the runoff factor, is introduced
in the Rational Equation to account for the dissipated water. The
runoff factors for various types of drainage areas are found in Table
1:
|
Table 1
Runoff Factors for the Rational Equation
| ||
|---|---|---|
|
Runoff Factor "C"
| ||
|
Type of Drainage Area or Surface
|
Minimum
|
Maximum
|
|
Roofs
|
0.90
|
1.00
|
|
Pavements, concrete or bituminous concrete
|
0.75
|
0.95
|
|
Pavements, bituminous macadam or surface-treated gravel
|
0.65
|
0.80
|
|
Pavements, gravel, macadam, etc.
|
0.25
|
0.60
|
|
Sandy soil, cultivated or light growth
|
0.10
|
0.30
|
|
Sandy soil, woods or heavy brush
|
0.10
|
0.30
|
|
Gravel, bare or light growth
|
0.20
|
0.40
|
|
Gravel, woods or heavy brush
|
0.10
|
0.35
|
|
Clay soil, bare or light growth
|
0.25
|
0.75
|
|
Clay soil, woods or heavy growth
|
0.15
|
0.60
|
|
Borough business sections
|
0.60
|
0.80
|
|
Dense residential sections
|
0.50
|
0.70
|
|
Suburban, normal residential areas
|
0.35
|
0.60
|
|
Rural areas, parks and golf courses
|
0.10
|
0.30
|
|
NOTE: In selecting the C factor, high values shall be applied
to denser soils and steep slopes; consideration shall be given to
the future land uses in the drainage area; and a weighted value of
C shall be given to a drainage area which contains several different
types of ground cover.
|
C.
The rainfall intensity "I" curve for Atlantic County shall be used
which is approximated by the equation "I = 190/25 + t," where "I"
equals the rate of rainfall expressed in inches per hour for a fifty-year
storm and "t" equals the time of concentration.
D.
The fifty-year storm frequency shall be used for all systems.
E.
To determine storm duration, the time of concentration approach shall
be used. "Time of concentration" may be defined as the interval of
time required for water from the most remote portion of the drainage
area to reach the point in question. The time of concentration may
be influenced by the type of terrain over which the water must flow
and stream velocities; prior to reaching the point in question, the
water may flow over land and subsequently flow into a stream.
F.
Stream velocities shall be calculated from Manning's Equation. The
average velocities of runoff flow for time of concentration area shall
be as follows:
|
Table 2
Recommended Average Velocities of Runoff Flow for Determining
Time of Concentration
| ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|
|
Description of Course of Runoff Water
|
Velocities in Feet Per Second (slope in percent)
| |||||||||
|
0.5
|
1.0
|
1.5
|
2.0
|
2.5
|
3.0
|
3.5
|
4.0
|
4.5
|
5.0
| |
|
Paved
|
.79
|
.93
|
1.00
|
1.05
|
1.10
|
1.13
|
1.14
|
1.19
|
1.22
|
1.26
|
|
Bare shaley soil
|
.61
|
.70
|
.75
|
.80
|
.83
|
.85
|
.89
|
.90
|
.93
|
.96
|
|
Bare soil
|
.51
|
.55
|
.58
|
.63
|
.65
|
.66
|
.68
|
.70
|
.72
|
.74
|
|
Wild grass
|
.34
|
.39
|
.42
|
.46
|
.47
|
.48
|
.49
|
.51
|
.52
|
.54
|
|
Average grass
|
.26
|
.30
|
.32
|
.34
|
.35
|
.37
|
.38
|
.40
|
.41
|
.42
|
|
Dense grass
|
.20
|
.23
|
.25
|
.27
|
.28
|
.29
|
.30
|
.30
|
.31
|
.32
|
G.
The time of concentration shall be determined and considered as representing
the duration of the storm.
H.
On-site retention conditions. The runoff from 4.5 inches of rainfall,
less dissipation, for a twenty-four-hour period must be stored. The
storage volume shall not be less than that anticipated by the runoff
from 1.5 inches of rainfall. Dissipation will not affect the minimum
storage capacity since this amount of rainfall is anticipated to fall
in a very short time. In calculating dissipation, only the bottom
area of the trench or basin shall be utilized. The bottom surface
of the leaching area shall be at least one foot above the seasonal
high groundwater table. Soil logs and percolation tests shall be made
within the area of the proposed leaching system.
I.
Retention basins, if permitted, shall be protected from public intrusion
by both fencing and screening and shall be maintained by the developer.
The basin shall have side slopes of one to four (1:4) or shallower
to prevent erosion, to make maintenance easier and to increase the
safety factor should anyone become trapped inside. The retention basin
will also be required to meet the current soil erosion and sediment
control standards to obtain a permit from the New Jersey Soil Conservation
District Office.
J.
Drainage area. The extent of the drainage area which is independent
of the development itself shall be determined from photogrammetric
plans, roadway design plans, field observations and USGS topographic
maps.
A.
Section 150-81 established the criteria for determining the maximum expected quantity of water; this section deals with the removal of the water arriving at a particular location. The drainage facilities must have adequate capacity.
B.
Some facilities that can be utilized are shoulders, curbed sections,
inlets, storm pipes, ditches and grassed waterways.
C.
The capacity of drainage facilities is measured in terms of discharge
and may be determined by the equation of continuity: "Q = AV," where
"Q" equals the discharge of water in cubic feet per second; "A" equals
the net effective area in square feet provided by the drainage facility;
and "V" equals the velocity of water in feet per second. NOTE: The
discharge capacity for a drainage facility at a particular location
shall be at least equal to the MED for that location.
D.
Additional design criteria for specific drainage facilities:
(1)
Shoulders. Water flowing in the shoulder shall not encroach more
than 2/3 of the shoulder width. Inlets shall be provided to control
encroachment and velocity.
(2)
Curbed sections. Inlets shall be provided to prevent encroachment
on roadway pavement.
(3)
Inlets. The assumed inlet capacity shall be six cubic feet per second.
If the capacity of the shoulder or curbed section exceeds the assumed
inlet capacity, the inlet capacity shall govern the spacing of inlets.
If the capacity of the shoulder or curbed section is less than the
inlet capacity, the shoulder, grassed waterway, curbed section or
depressed section capacity shall govern the spacing of inlets. On
shoulder and curbed sections, the maximum spacing of inlets shall
not exceed 500 feet. Sufficient inlets shall be installed at street
intersections to avoid gutter overflow. Inlets shall be placed at
the low point on sag vertical curves.
(4)
Storm pipes. Where headroom is restricted, equivalent pipe arches
may be used in lieu of circular pipe. The minimum diameter of storm
pipe shall be 18 inches. Abrupt changes in the direction or slope
of pipe shall be avoided, and, if required, an inlet or manhole shall
be placed at the point of change. The minimum slope in a pipe shall
maintain a two-feet-per-second velocity in the pipe. The top of the
pipe shall not be less than one foot below subgrade or as recommended
by the manufacturer, whichever is greater. Longitudinal pipes may
serve as combination storm sewer and foundation underdrain pipe. A
typical computation table for storm sewer design shall be submitted.
(5)
Ditches and grassed waterways. Ditches and grassed waterways are
open channels which carry runoff. Transverse ditches shall not intersect
parallel ditches at right angles but shall join them at an angle of
30° in order to minimize scour and sedimentation.
A.
The need for erosion prevention extends throughout the development
and shall be considered an essential feature of good drainage design.
B.
Erosion and maintenance are minimized by the use of flat side slopes
blended with natural terrain drainage channels designed with due regard
to location, width, depth, slopes, alignment and protective treatment;
proper facilities for groundwater interception; and dikes, berms and
protective ground cover.
C.
Erosion control of culvert outlets. Since culverts create serious
erosion problems in unprotected channels, it is necessary to provide
a channel treatment which will control erosion and dissipate the excess
energy of the discharge. For culvert outlets, channel treatment based
on outlet velocities and alignment shall be provided.
D.
Erosion control of drainage channels. Within practical limits, the
possibility of erosion should be eliminated. Where channel scour is
indicated, a means for reducing velocity or for protecting the channel
should be provided.
E.
Erosion control devices for channel protection.
(1)
Grass-lined channels. Permissible velocity within channels depends
upon the type and condition of the grass cover and the texture of
the soil comprising the bed. Capacity should be computed for taller
grass than is expected to be maintained and velocity computed for
a lower grass height which is likely to be maintained.
(2)
Concrete-lined channels. The use of concrete paved gutters to control
erosion is essential when the developed velocity exceeds that which
sod can withstand. A paved channel should be located after consideration
of aesthetics. Concrete linings can be used on very flat slopes to
increase the velocity of flow to a nonsilting velocity, to more efficiently
remove water from ponded areas or to reduce the size of channel needed
to carry the design discharge.
(3)
Channels lined with stone. Such linings can be constructed of dumped,
hand-placed or grouted stone. The channel bed and slope can be lined,
or stone can be used in combination with grass or concrete. A dumped
stone lining is the most flexible since it readily adjusts itself
to uneven bank settlement.
(4)
Riprap for bank protection. Riprap slope protection shall be used
if there is any indication that erosion may occur along the bank of
the channel. Types of riprap include dumped, hand-placed, wire-enclosed,
grouted, concrete-slab or concrete riprap in bags.
F.
Compliance with the construction standards shall be required for all site improvements. See Article XII.
G.
Compliance with soil erosion and sediment control standards as set
by the United States Conservation District Office may be required.
