|
*Note: In subdivisions with no through streets,
a fifty-five (55) foot pavement radius and a sixty-seven (67) foot
right-of-way radius will be required on at least one (1) cul-de-sac
in order to facilitate school bus circulation. For individual cul-de-sacs
the fifty-five (55) foot pavement radius and sixty-seven (67) foot
right-of-way radius shall only be required if the cul-de-sac exceeds
one thousand three hundred (1,300) feet in length.
|
|
Where the topography will not allow for an overland flow path:
|
|
Q = API
| |||
|
where:
| |||
|
Q
|
=
|
Runoff in cubic feet per second
| |
|
A
|
=
|
Tributary area in acres
| |
|
I
|
=
|
Average intensity of rainfall (inches per hour) for a given
period and a given frequency
| |
|
P
|
=
|
Runoff factor based on runoff from previous and impervious surfaces
| |
|
P (runoff factors) for various impervious conditions are shown
in Tables 4-1.
| |||
|
P.I. values for various impervious conditions are shown in Table
4-2 to 4-4.
|
20-Minute Duration
| |||
---|---|---|---|
Percent Imperviousness
|
15-Year
|
20-Year
| |
5
|
1.7
|
1.8
| |
10
|
1.8
|
1.9
| |
20
|
2.0
|
2.1
| |
30
|
2.2
|
2.3
| |
40
|
2.4
|
2.5
| |
50
|
2.6
|
2.7
| |
90
|
3.4
|
3.5
| |
100
|
3.5
|
3.7
| |
Roofs for direct connection to sewer
|
4.2
|
6.0
|
Area (Abscissas)
|
"Ka" (Ordinates)
| |
---|---|---|
300 to 449 acres
|
1.00
| |
450 to 549 acres
|
.99
| |
550 to 749 acres
|
.98
| |
750 to 999 acres
|
.97
| |
1,000 to 1,280 acres
|
.96
| |
1,281 to 1,600 acres
|
.95
| |
1,601 to 1,920 acres
|
.92
| |
1,921 to 2,240 acres
|
.91
|
|
hf
= L x Sh
| ||
|
Where:
| ||
|
hf
|
=
|
Difference in water surface elevation or head in feet in length
L
|
|
L
|
=
|
Length in feet of pipe or channel
|
|
Sh
|
=
|
Hydraulic slope required for a pipe of given diameter or channel
of given cross section and for a given roughness "n", expressed as
feet of slope per foot of length
|
|
From Manning's formula: Sh = [V n / (1.486
R0.667)]2
| ||
|
Where:
| ||
|
R
|
=
|
Hydraulic radius of pipe, conduit or channel (feet) (ratio of
flow area/wetted perimeter)
|
|
V
|
=
|
Velocity of flow in feet per second (fps)
|
|
n
|
=
|
Manning's value for coefficient of roughness
|
|
Use:
| ||
|
n
|
=
|
.013 for pipes of concrete, vitrified clay and PVC pipe
|
|
n
|
=
|
.012 for formed monolithic concrete, i.e., vertical wall channels,
box culverts and for R.C.P. over 48 inches in diameter
|
|
n
|
=
|
.015 for concrete lining in ditch or channel inverts and trapezoidal
channels
|
|
n
|
=
|
.020 for grouted riprap lining on ditch or channel side slopes
|
|
n
|
=
|
.033 for gallon walled channels
|
|
Hti = (V2/2g)
| ||
|
Where:
| ||
|
V
|
=
|
Velocity in flow of outgoing pipe
|
|
g
|
=
|
Acceleration of gravity (32.2 Ft/Sec/Sec)
|
Change In Direction Of Flow (A)
|
Multiplier Of Velocity Head Of Water Being Turned (K)
| |
---|---|---|
90 degree
|
0.7
| |
60 degree
|
0.55
| |
45 degree
|
0.47
| |
30 degree
|
0.35
| |
15 degree
|
0.18
| |
0 degree
|
0.0
| |
Other Angles
|
By interpolation
|
|
Formula: HL = K(VL)2/2g
| ||
|
Where:
| ||
|
HL
|
=
|
Feet of head lost in manhole due to change in direction of lateral
flow
|
|
VL
|
=
|
Velocity of flow in lateral in Ft/Sec
|
|
g
|
=
|
Acceleration of gravity (32.2 Ft/Sec/Sec)
|
|
K
|
=
|
Multiplier of velocity head of water being turned
|
|
Hj = Δy + Vhl - Vh2
| ||
|
Where:
| ||
|
Hj
|
=
|
junction chamber loss (ft)
|
|
Δy
|
=
|
change in hydraulic grade line through the junction in feet
|
|
Vhl
|
=
|
upstream velocity head
|
|
Vh2
|
=
|
downstream velocity head
|
|
Where:
| ||
Δy = [(Q2V2) - ((Q1V1) + {(Q3V3Cos ϴ 3) + QnVnCos ϴn)})]
0.5 (A1+A2) g
| |||
|
Where:
| ||
|
Q2
|
=
|
Discharge in cubic feet per second (cfs) at the exiting conduit
|
|
V2
|
=
|
Velocity in feet per second (fps) at the exiting conduit
|
|
A2
|
=
|
Cross sectional area of flow in square feet for the exiting
conduit
|
|
Q1
|
=
|
Discharge in cfs for the incoming pipe (main flow)
|
|
V1
|
=
|
Velocity in fps for the incoming pipe (main flow)
|
|
A1
|
=
|
Cross sectional area of flow in square feet for the incoming
pipe (main flow)
|
|
Q3, Qn
|
=
|
Discharge(s) in cfs for the branch lateral(s)
|
|
V3, Vn
|
=
|
Velocity(ies) in fps for the branch lateral(s)
|
|
ϴ3, ϴn
|
=
|
The angle between the axes of the exiting pipe and the branch
laterals(s)
|
|
g
|
=
|
Acceleration of gravity (32.2 ft/sec/sec)
|
|
Where:
| ||
|
ϴ
|
=
|
is the angle between the axes of the outfall and the incoming
laterals
|
|
The upstream hydraulic grade line may be calculated as follows:
| ||
|
Hu = [VD2/2g] - [((Qu/QD) (1-K) (Vu2/2g))
+ ((QL1/QD) (1-K) (VL12/2g)) + ((QLN/QD) (1-K) (VLN2/2g))] + HD
| ||
|
Where:
| ||
|
Hu
|
=
|
Upstream hydraulic grade line in feet
|
|
Qu
|
=
|
Upstream main line discharge in cubic feet per second
|
|
QD
|
=
|
Downstream main line discharge in cubic feet per second
|
|
QL1-QN
|
=
|
Lateral discharges in cubic feet second
|
|
Vu
|
=
|
Upstream main line velocity in feet per second
|
|
VD
|
=
|
Downstream min line velocity in feet per second
|
|
VL1 - VLN
|
=
|
Lateral velocities in feet per second
|
|
HD
|
=
|
Downstream hydraulic grade line in feet
|
|
K
|
=
|
Multiplier of velocity of water being turned
|
|
G
|
=
|
Acceleration of gravity, 32.2 ft/sec/sec
|
|
The above equation does not apply when two (2) almost equal
and opposing flows, each perpendicular to the downstream pipe, meet
and no other flows exist in the structure. In this case the head loss
is considered as the total velocity head of the downstream discharge.
|
a)
|
Trapezoidal channels.
| ||||
Sub-critical flow:
| |||||
ΔHw = 1.15 (V2/2grc) [b+D (ZL+ZR)]
| |||||
Super-critical flow:
| |||||
ΔHw = 2.6 (V2/2grc) [b+D (ZL+ZR)]
|
b)
|
Rectangular channels.
| ||||
Sub-critical flow:
| |||||
ΔHw = (V2b/2grc)
| |||||
Super-critical flow:
| |||||
ΔHw = (V2b/grc)
|
Where:
| ||||
ΔHw
|
=
|
Change in water height above the centerline water surface
| ||
V
|
=
|
Average velocity of design flow in fps
| ||
g
|
=
|
Acceleration of gravity (32.2 Ft/Sec/Sec)
| ||
rc
|
=
|
Radius of curve on horizontal alignment in feet
| ||
b
|
=
|
Base width of channel in feet
| ||
D
|
=
|
Depth of flow in straight channel
| ||
ZL
|
=
|
Left side slope (ft/ft)
| ||
ZR
|
=
|
Right side slope (ft/ft)
|
a)
|
The optimum maximum angle between the channel axis and a line
connecting the channel sides between the entrance and exit sections
is 12.5 degrees.
| |
b)
|
Sharp angles in the structure should be avoided.
|
|
Ht = Kt ΔHH
| |||
|
Where:
| |||
|
Ht
|
=
|
Conversion loss
| |
|
Kt
|
=
|
Coefficient of head loss in transition
| |
|
ΔHH
|
=
|
Absolute change in velocity head
| |
|
Average design values for Kt are presented in the table below:
|
Type Of Transition
|
Contracting Section
|
Expanding Section
|
---|---|---|
Warped
|
0.10
|
0.20
|
Wedge
|
0.20
|
0.50
|
Cylinder-quadrant
|
0.15
|
0.25
|
Straight line
|
0.30
|
0.50
|
Square end
|
0.40
|
0.75
|
|
See Figure 4-4 for sketches of each type of transition.
|
|
HLe = Ke (ΔV2/2g)
| |||
|
Where:
| |||
|
Ke
|
=
|
Coefficient of head loss for enlargements = 1
| |
|
ΔV
|
=
|
Change in velocities between incoming and outgoing sections
| |
|
g
|
=
|
Acceleration of gravity (32.2 Ft/Sec/Sec)
|
|
The flow in a sudden contraction is first contracted and then
expanded resulting in high losses as compared to a sudden enlargement.
Thus the head loss at a sudden contraction, HLc, is:
|
HLc = Kc (ΔV2/2g)
| ||||
Where:
| ||||
Kc
|
=
|
Coefficient of head loss for contractions = 0.5
| ||
ΔV
|
=
|
Change in velocities between incoming and outgoing sections
| ||
g
|
=
|
Acceleration of gravity, Ft/Sec/Sec
|
P (RUNOFF FACTORS) FOR VARIOUS IMPERVIOUS CONDITIONS
|
Table 4-1
|
P FACTOR FOR RUNOFF
| ||||||
---|---|---|---|---|---|---|
% IMPERVIOUS
|
Duration Of Rain In Minutes
| |||||
15
|
20
|
30
|
60
|
90
|
120
| |
0
|
0.30
|
0.35
|
0.41
|
0.51
|
0.56
|
0.60
|
5
|
0.32
|
0.37
|
0.43
|
0.53
|
0.58
|
0.62
|
10
|
0.34
|
0.39
|
0.46
|
0.56
|
0.60
|
0.64
|
15
|
0.36
|
0.41
|
0.48
|
0.58
|
0.62
|
0.66
|
20
|
0.38
|
0.44
|
0.50
|
0.60
|
0.64
|
0.67
|
25
|
0.40
|
0.46
|
0.52
|
0.62
|
0.66
|
0.69
|
30
|
0.42
|
0.48
|
0.54
|
0.64
|
0.68
|
0.71
|
35
|
0.44
|
0.50
|
0.57
|
0.66
|
0.70
|
0.73
|
40
|
0.46
|
0.52
|
0.59
|
0.68
|
0.72
|
0.74
|
45
|
0.49
|
0.54
|
0.61
|
0.71
|
0.74
|
0.76
|
50
|
0.50
|
0.56
|
0.63
|
0.73
|
0.75
|
0.78
|
55
|
0.52
|
0.58
|
0.65
|
0.75
|
0.77
|
0.80
|
60
|
0.54
|
0.60
|
0.68
|
0.77
|
0.79
|
0.81
|
65
|
0.56
|
0.63
|
0.70
|
0.79
|
0.81
|
0.83
|
70
|
0.58
|
0.65
|
0.72
|
0.81
|
0.83
|
0.85
|
75
|
0.60
|
0.67
|
0.74
|
0.84
|
0.85
|
0.87
|
80
|
0.62
|
0.69
|
0.76
|
0.86
|
0.87
|
0.88
|
85
|
0.64
|
0.71
|
0.79
|
0.88
|
0.89
|
0.90
|
90
|
0.66
|
0.73
|
0.81
|
0.90
|
0.91
|
0.92
|
95
|
0.68
|
0.75
|
0.83
|
0.92
|
0.93
|
0.94
|
100
|
0.70
|
0.77
|
0.85
|
0.94
|
0.95
|
0.95
|
VALUES OF P FOR 0% AND 100% ARE THOSE USED FOR ST. LOUIS MODIFIED
9-1939
| ||||||
| ||||||
RAINFALL INTENSITY OF 1 INCH PER HOUR ON 1 ACRE
= 1.008 CU. FT. PER SECOND ON ACRE
= 1 CU. FT. PER SECOND ON 1 ACRE (APPROXIMATELY)
| ||||||
| ||||||
P x I = Q = RUNOFF IN CU. FT. PER SEC. PER ACRE FOR GIVEN %
IMPERVIOUSNESS OF CONTRIBUTING AREA DURING A RAINFALL OF GIVEN INTENSITY
CORRESPONDING TO THE GIVEN DURATION AND A SELECTED FREQUENCY.
| ||||||
| ||||||
I = INTENSITY OF RAINFALL IN INCHES PER HOUR FOR GIVEN DURATION
AND GIVEN FREQUENCY.
| ||||||
| ||||||
RUNOFF = P = RATIO OF RUNOFF CONTRIBUTED BY AN AREA OF GIVEN
% IMPERVIOUSNESS FOR A GIVEN DURATION PERIOD TO THE RAINFALL OF A
GIVEN INTENSITY CORRESPONDING TO THE SAME DURATION PERIOD AND A SELECTED
FREQUENCY.
TABLE 4-1
| ||||||
| ||||||
P (RUNOFF FACTORS) FOR VARIOUS IMPERVIOUS CONDITIONS
|
P.I. VALUES FOR VARIOUS IMPERVIOUS CONDITIONS
(15-YEAR AND 20-YEAR RAINFALL FREQUENCIES)
Table 4-2
| |||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
P.I.FACTOR IN CUBIC FEET PER SECOND PER ACRE
| |||||||||||||
Duration Of Rain In Minutes
|
% Impervious
|
15-Year Rainfall Frequency
|
20-Year Rainfall Frequency
| ||||||||||
15
|
20
|
30
|
60
|
90
|
120
|
15
|
20
|
30
|
60
|
90
|
120
| ||
0
|
1.59
|
1.61
|
1.52
|
1.22
|
1.04
|
0.92
|
1.65
|
1.68
|
1.60
|
1.29
|
1.08
|
0.96
| |
5
|
1.70
|
1.70
|
1.59
|
1.27
|
1.08
|
0.95
|
1.76
|
1.78
|
1.68
|
1.34
|
1.11
|
0.99
| |
10
|
1.80
|
1.79
|
1.68
|
1.33
|
1.12
|
0.97
|
1.87
|
1.87
|
1.77
|
1.40
|
1.15
|
1.02
| |
15
|
1.91
|
1.89
|
1.76
|
1.38
|
1.15
|
1.00
|
1.98
|
1.97
|
1.85
|
1.45
|
1.19
|
1.05
| |
20
|
2.01
|
2.00
|
1.85
|
1.43
|
1.18
|
1.03
|
2.09
|
2.09
|
1.95
|
1.50
|
1.22
|
1.07
| |
25
|
2.12
|
2.09
|
1.92
|
1.49
|
1.22
|
1.06
|
2.20
|
2.18
|
2.03
|
1.56
|
1.26
|
1.10
| |
30
|
2.23
|
2.19
|
2.00
|
1.54
|
1.26
|
1.08
|
2.31
|
2.28
|
2.11
|
1.61
|
1.30
|
1.13
| |
35
|
2.33
|
2.28
|
2.09
|
1.58
|
1.29
|
1.11
|
2.42
|
2.38
|
2.20
|
1.66
|
1.33
|
1.16
| |
40
|
2.44
|
2.39
|
2.16
|
1.63
|
1.33
|
1.13
|
2.53
|
2.50
|
2.28
|
1.71
|
1.37
|
1.18
| |
45
|
2.54
|
2.48
|
2.26
|
1.69
|
1.37
|
1.16
|
2.64
|
2.59
|
2.38
|
1.78
|
1.41
|
1.22
| |
50
|
2.65
|
2.58
|
2.33
|
1.74
|
1.40
|
1.19
|
2.75
|
2.69
|
2.46
|
1.83
|
1.44
|
1.24
| |
55
|
2.76
|
2.67
|
2.41
|
1.79
|
1.43
|
1.22
|
2.86
|
2.78
|
2.54
|
1.88
|
1.48
|
1.27
| |
60
|
2.86
|
2.76
|
2.50
|
1.85
|
1.47
|
1.24
|
2.97
|
2.88
|
2.63
|
1.94
|
1.52
|
1.30
| |
65
|
2.97
|
2.88
|
2.57
|
1.90
|
1.51
|
1.27
|
3.08
|
3.00
|
2.71
|
1.99
|
1.56
|
1.33
| |
70
|
3.07
|
2.97
|
2.66
|
1.94
|
1.54
|
1.29
|
3.19
|
3.10
|
2.81
|
2.04
|
1.59
|
1.35
| |
75
|
3.18
|
3.06
|
2.74
|
2.00
|
1.58
|
1.32
|
3.30
|
3.19
|
2.89
|
2.10
|
1.63
|
1.38
| |
80
|
3.29
|
3.15
|
2.81
|
2.05
|
1.62
|
1.35
|
3.41
|
3.29
|
2.96
|
2.15
|
1.67
|
1.41
| |
85
|
3.39
|
3.24
|
2.90
|
2.10
|
1.65
|
1.38
|
3.52
|
3.38
|
3.06
|
2.21
|
1.70
|
1.44
| |
90
|
3.50
|
3.36
|
2.98
|
2.16
|
1.68
|
1.40
|
3.63
|
3.50
|
3.14
|
2.27
|
1.74
|
1.46
| |
95
|
3.60
|
3.45
|
3.07
|
2.21
|
1.72
|
1.43
|
3.74
|
3.60
|
3.24
|
2.32
|
1.78
|
1.50
| |
100
|
3.71
|
3.54
|
3.15
|
2.26
|
1.76
|
1.45
|
3.85
|
3.70
|
3.32
|
2.37
|
1.81
|
1.52
| |
RAINFALL
|
5.30
|
4.60
|
3.70
|
2.40
|
1.86
|
1.53
|
5.50
|
4.80
|
3.90
|
2.52
|
1.92
|
1.60
|
P.I. VALUES FOR VARIOUS IMPERVIOUS CONDITIONS
(10-YEAR AND 100-YEAR RAINFALL FREQUENCIES)
Table 4-4
| |||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
P.I. FACTOR IN CUBIC FEET PER SECOND PER ACRE
| |||||||||||||
Duration Of Rain In Minutes
|
% Impervious
|
10-Year Rainfall Frequency
|
100-Year Rainfall Frequency
| ||||||||||
15
|
20
|
30
|
60
|
90
|
120
|
15
|
20
|
30
|
60
|
90
|
120
| ||
0
|
1.48
|
1.51
|
1.42
|
1.15
|
0.95
|
0.83
|
2.10
|
2.17
|
2.06
|
1.68
|
1.10
|
1.18
| |
5
|
1.57
|
1.59
|
1.49
|
1.19
|
0.99
|
0.86
|
2.24
|
2.29
|
2.16
|
1.75
|
1.45
|
1.22
| |
10
|
1.67
|
1.68
|
1.57
|
1.25
|
1.02
|
0.88
|
2.38
|
2.42
|
2.28
|
1.83
|
1.50
|
1.24
| |
15
|
1.77
|
1.76
|
1.64
|
1.29
|
1.05
|
0.90
|
2.52
|
2.54
|
2.38
|
1.90
|
1.55
|
1.28
| |
20
|
1.87
|
1.87
|
1.73
|
1.34
|
1.08
|
0.92
|
2.66
|
2.70
|
2.51
|
1.96
|
1.59
|
1.31
| |
25
|
1.97
|
1.96
|
1.80
|
1.40
|
1.11
|
0.95
|
2.80
|
2.82
|
2.61
|
2.05
|
1.64
|
1.35
| |
30
|
2.07
|
2.04
|
1.87
|
1.44
|
1.15
|
0.97
|
2.94
|
2.95
|
2.71
|
2.11
|
1.69
|
1.38
| |
35
|
2.16
|
2.13
|
1.95
|
1.49
|
1.18
|
1.00
|
3.08
|
3.07
|
2.84
|
2.18
|
1.74
|
1.42
| |
40
|
2.26
|
2.24
|
2.02
|
1.53
|
1.22
|
1.02
|
3.22
|
3.22
|
2.94
|
2.24
|
1.79
|
1.45
| |
45
|
2.36
|
2.32
|
2.11
|
1.59
|
1.25
|
1.05
|
3.36
|
3.35
|
3.06
|
2.33
|
1.84
|
1.49
| |
50
|
2.46
|
2.41
|
2.18
|
1.63
|
1.28
|
1.07
|
3.50
|
3.47
|
3.16
|
2.39
|
1.88
|
1.52
| |
55
|
2.56
|
2.49
|
2.25
|
1.68
|
1.31
|
1.10
|
3.64
|
3.60
|
3.26
|
2.46
|
1.93
|
1.56
| |
60
|
2.66
|
2.58
|
2.34
|
1.73
|
1.34
|
1.12
|
3.78
|
3.72
|
3.39
|
2.54
|
1.98
|
1.59
| |
65
|
2.76
|
2.69
|
2.40
|
1.78
|
1.38
|
1.15
|
3.92
|
3.88
|
3.49
|
2.61
|
2.03
|
1.63
| |
70
|
2.85
|
2.77
|
2.49
|
1.82
|
1.41
|
1.17
|
4.06
|
4.00
|
3.61
|
2.67
|
2.07
|
1.66
| |
75
|
2.95
|
2.86
|
2.56
|
1.88
|
1.44
|
1.19
|
4.20
|
4.12
|
3.71
|
2.76
|
2.13
|
1.70
| |
80
|
3.05
|
2.95
|
2.63
|
1.92
|
1.48
|
1.21
|
4.34
|
4.25
|
3.82
|
2.82
|
2.18
|
1.72
| |
85
|
3.15
|
3.03
|
2.72
|
1.97
|
1.50
|
1.24
|
4.48
|
4.37
|
3.94
|
2.89
|
2.21
|
1.76
| |
90
|
3.25
|
3.14
|
2.79
|
2.03
|
1.54
|
1.26
|
4.62
|
4.53
|
4.04
|
2.97
|
2.26
|
1.79
| |
95
|
3.35
|
3.23
|
2.87
|
2.07
|
1.57
|
1.29
|
4.76
|
4.65
|
4.17
|
3.04
|
2.31
|
1.83
| |
100
|
3.44
|
3.31
|
2.94
|
2.11
|
1.61
|
1.31
|
4.90
|
4.77
|
4.27
|
3.10
|
2.36
|
1.86
| |
RAINFALL
|
4.92
|
4.30
|
3.46
|
2.25
|
1.70
|
1.38
|
7.00
|
6.20
|
5.02
|
3.30
|
2.50
|
1.96
|
RAINFALL INTENSITY — DURATION CURVES (Figure 4-1)
(2, 5, 10, 15, 20 and 100-Year Rainfall Frequency)
|
---|