PERSPECTIVE /OVERVIEW PERSPECTIVE /OVERVIEW PERSPECTIVE /OVERVIEW PERSPECTIVE /OVERVIEW EnvironmentalEngineering Institut Teknologi Bandung EnvE STORM SEWER DESIGN
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EnvironmentalEngineeringInstitut Teknologi BandungEnvE
PERSPECTIVE /OVERVIEWPERSPECTIVE /OVERVIEWPERSPECTIVE /OVERVIEWPERSPECTIVE /OVERVIEW
STORM SEWER DESIGN
PERSPECTIVE /OVERVIEWPERSPECTIVE /OVERVIEWPERSPECTIVE /OVERVIEWPERSPECTIVE /OVERVIEW
STORM SEWER DESIGN
EnvironmentalEngineeringInstitut Teknologi BandungEnvE
• Adequate and properly functioning storm water drainage system.
• Construction of houses, commercial buildings, parking lots, paved
roads, and streets increases the impervious cover in a watershed
and reduces infiltration.
• urbanization changes the spatial pattern of flow in the watershed; • urbanization changes the spatial pattern of flow in the watershed;
there is an increase in the hydraulic efficiency of flow through
artificial channels, curbing, gutters, and storm drainage and
collection systems.
• These increase the volume and velocity of runoff and produce
larger peak flood discharges from urbanized watersheds.
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STORM SEWER DESIGN
EnvironmentalEngineeringInstitut Teknologi BandungEnvE
• The urban drainage system can be considered
as consisting of two major types of elements:
LOCATION ELEMENTS and TRANSFER
ELEMENTS.
• LOCATION ELEMENTS: places where the • LOCATION ELEMENTS: places where the
water stops and undergoes changes as a result
of humanly controlled processes e.g. Water
storage, water treatment, water use, WWTP
• TRANSFER ELEMENTS: connect the location
elements e.g. Channels, pipelines, storm
sewers, sanitary sewers, and strrets.
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STORM SEWER DESIGN
EnvironmentalEngineeringInstitut Teknologi BandungEnvE
• Complex urban problems involve distributed systems requiring
analysis of spatial and temporal variations.
• Urban watersheds vary in space in that the ground surface slope
and cover, and the soil type, change from place to place in the and cover, and the soil type, change from place to place in the
watershed.
• They vary in time in that hydrologic characteristics change with
the process of urbanization.
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DESIGN PHILOSOPHY
EnvironmentalEngineeringInstitut Teknologi BandungEnvE
The design of storm sewer systems involves the determination of DIAMETERS,
SLOPES, and CROWN/INVERT elevation for each pipe in the system.
• Selection of a layout/network of pipe locations.
• Once a layout has been selected, the RATIONAL METHOD can be used to select • Once a layout has been selected, the RATIONAL METHOD can be used to select
pipe diameters.
Storm drainage design can be divided into two aspects:
• RUNOFF PREDICTION
• SYSTEM DESIGN
In recent years, rainfall-runoff modeling for urban watersheds has been a
popular activity and a variety of such rainfall-runoff models are now available.
Rational Method
EnvironmentalEngineeringInstitut Teknologi BandungEnvE
• Still probably the most widely used method for design of storm sewers
[Pilgrim, 1986; Linsley, 1986].
• The idea behind the rational method is that if a rainfall of intensity i begins
instantaneously and continues indefinitely, the rate of runoff will increase until the
time of concentration tc, when all of the watershed is contributing to flow at the time of concentration tc, when all of the watershed is contributing to flow at the
outlet.
• The product of rainfall intensity i and watershed area A is the inflow rate for the
system, iA, and the ratio of this rate to the rate of peak discharge Q (which occurs at
time tc) is termed the runoff coefficient C (0<C<1). This is expressed in the rational
formula:
Q = C i A
• the duration used for the determination of the design precipitation intensity i is the
time of concentration of the watershed.
Rational Method
EnvironmentalEngineeringInstitut Teknologi BandungEnvE
• a composite analysis for various surface characteristics is required for the
drainage area with subareas.
• The peak runoff:
Q = i ΣCjAj
Assumption associated with the rational method:
• The computed peak rate of runoff at the outlet point is a function of the
average rainfall rate during the time of concentration, that is, the peak
discharge does not result from a more intense storm of shorter duration,
during which only a portion of the watershed is contributing to runoff at the
outlet.
• The time of concentration employed is the time for the runoff to become
established and flow from the most remote part of the drainage area to the
inflow point of the sewer being designed.
• Rainfall intensity is constant throughout the storm duration
Runoff Coefficient
EnvironmentalEngineeringInstitut Teknologi BandungEnvE
• The proportion of the total rainfall that will reach the storm drains depends
on the percent imperviousness, slope, and ponding character of the surface.
• impervious surfaces, such as asphalt pavements and roofs of buildings, will
produce nearly 100% runoff after the surface has become thoroughly wet,
regardless of the slope.regardless of the slope.
• Field inspection and aerial photographs are useful in estimating the nature of
the surface within the drainage area.
• The runoff coefficient is also dependent on the character and condition of
the soil. The infiltration rate decreases as rainfall continues, and is also
influenced by the antecedent moisture condition of the soil.
Runoff Coefficient for use in the rational method [Chow et al., 1988]
EnvironmentalEngineeringInstitut Teknologi BandungEnvE
Character of surface Return Period (years)
5 25
DEVELOPED
Asphaltic 0.77 0.86
Concrete/roof 0.80 0.88
Grass area (lawns, parks)
- Poor condition (grass cover less than 50% of the area)
- Flat (0-2%)
- Steep (over 7%)
0.34
0.43
0.40
0.49
- Good condition (grass cover larger than 75% of the area)
- Flat
- Steep
0.23
0.37
0.29
0.44
UNDEVELOPED
Cultivated land
- Flat
- Steep
0.34
0.42
0.40
0.48
Forest/Woodlands
- Flat
- Sttep
0.25
0.39
0.31
0.45
Rainfall Intensity
EnvironmentalEngineeringInstitut Teknologi BandungEnvE
•The rainfall intensity i is the average rainfall rate (in/hr) for a particular drainage basin.
• The intensity is selected on the basis of the design rainfall duration and return period.
• The design duration is equal to the time of concentration for the drainage area under
consideration.
• The return period is established by design standards.• The return period is established by design standards.
• the time of concentration to any point in a storm drainage system is the sum of the
inlet time to (the time it takes for flow from the remotest point to reach the sewer inlet),
and the flow time tf in the upstream sewers connected to the outer point:
tc = to + tf
tf = ΣLi/ViLi is the length of the ith pipe along the flow path, Vi is the flow velocity in the pipe.
• inlet time decreases as the slope and imperviousness of the surface increases.
EnvironmentalEngineeringInstitut Teknologi BandungEnvE
Time of Concentration Summary [Source Chow et al., 1988][Source Chow et al., 1988][Source Chow et al., 1988][Source Chow et al., 1988]
EnvironmentalEngineeringInstitut Teknologi BandungEnvE
Time of Concentration Summary
Waktu Konsentrasi Travel Time
EnvironmentalEngineeringInstitut Teknologi BandungEnvE
Table Approximate average velocities in ft/s of runoff flow for calculating time of concentration
Description of water course Slope
Calculate the time of concentration of a watershed in which the longest flow path covers
100 feet of pastures at a 5% slope, then enters a 1000-foot-long rectangular channel having
width 2 ft, roughness n = 0.015, and slope 2.5%, and receiving a lateral flow of 0.00926
cfs/ft.
Description of water course Slope
0-3 4-7 8-11 12-...
Woodlands
Pastures
Cultivated
Pavements
0-1.5
0-2.5
0-3.0
0-8.5
1.5-2.5
2.5-3.5
3.0-4.5
8.5-13.5
2.5-3.25
3.5-4.25
4.5-5.5
13.5-17
3.25-
4.25-
5.5-
17-
Outlet channel (Natural channel not well-defined) 0-2 2-4 4-7 7-
Distance along channel, l (ft) 0 200 400 600 800 1000
Δl 200 200 200 200 200
Calculated velocity (ft/s) 0 4.63 5.97 6.86 7.56 8.02
Average velocity , V (ft/s)
Travel time t= l/V (s)
Waktu Konsentrasi
EnvironmentalEngineeringInstitut Teknologi BandungEnvE
Table Approximate average velocities in ft/s of runoff flow for calculating time of concentration
Description of water course Slope (%)
0-3 4-7 8-11 12-...
Woodlands
Pastures
Cultivated
Pavements
0-1.5
0-2.5
0-3.0
0-8.5
1.5-2.5
2.5-3.5
3.0-4.5
8.5-13.5
2.5-3.25
3.5-4.25
4.5-5.5
13.5-17
3.25-
4.25-
5.5-
17-Pavements 0-8.5 8.5-13.5 13.5-17 17-
Outlet channel (Natural channel not well-defined) 0-2 2-4 4-7 7-
Average velocity: 3.0 ft/s
Distance along channel, l (ft) 0 200 400 600 800 1000
Δl 200 200 200 200 200
Calculated velocity (ft/s) 0 4.63 5.97 6.86 7.56 8.02
Average velocity , V (ft/s) 2,315 5,3 6,415 7,21 7,79
Travel time t= l/V (s) 86,4 37,7 31,2 27,7 25,7
Pipe Capacity
EnvironmentalEngineeringInstitut Teknologi BandungEnvE
•Once the design discharge Q entering the sewer pipe has been calculated by the
rational formula, the diameter of pipe D required to carry this discharge is determined.
• it is usually assumed that the pipe is flowing full under gravity but is not pressurized, so
the pipe capacity can be calculated by the Manning or Darcy-Weisbach equations for
open-channel flow. open-channel flow.
D = (2.16 Qn/√So)3/8
where Q in cfs, D in ft.
Rational Method Assessment
EnvironmentalEngineeringInstitut Teknologi BandungEnvE
Example A hypothetical drainage basin
comprising seven subcatchments is shown.
Determine the required capacity of the storm
sewer EB draining subarea III for a five-year
return period storm. This subcatchment has
an area of 4 acres, a runoff coefficient of 0.6,
and an inlet time of 10 minutes.
A
I II
III IV V and an inlet time of 10 minutes.
The design precipitation intensity for this
location is given by i = 120 T 0.175/(Td+27),
where i is the intensity in in/hr, T is the return
period, and Td is the duration in minutes.
The ground elevations at points E and B are
498.43 and 495.55 ft above mean sea level,
respectively, and the length of pipe EB is 450
ft. Assume Manning’s n is 0.015. Calculate
the flow time in the pipe.
BE
C
D
III IV V
VI VII
Rational Method Assessment
EnvironmentalEngineeringInstitut Teknologi BandungEnvE
i = 120 T 0.175/(Td+27)
= 120 (5) 0.175/(10+27)
= 4.30 in/h
The design discharge:
Q = C i A
= 0.6 x 4.30 x 4
A
BE
C
I II
III IV V
VI VII
= 0.6 x 4.30 x 4
= 10.3 cfs
The slope of the pipe EB is the difference between the
ground elevations at points E and B divided by the length of
the pipe: So = (498.43 – 495.55)/450 = 0.0064
The required pipe diameter is:
D = (2.16 Qn/√So)3/8
D = (2.16 x 10.3 x 0.015/√0.0064)3/8
= 1.71 ft
The diameter is rounded up to the next commercially
available pipe size, 1.75 ft or 21 in.
D
Rational Method Assessment
EnvironmentalEngineeringInstitut Teknologi BandungEnvE
i = 120 T 0.175/(Td+27)
= 120 (5) 0.175/(10+27)
= 4.30 in/h
The design discharge:
Q = C i A
= 0.6 x 4.30 x 4
A
BE
C
I II
III IV V
VI VII
= 0.6 x 4.30 x 4
= 10.3 cfs
The slope of the pipe EB is the difference between the
ground elevations at points E and B divided by the length of
the pipe: So = (498.43 – 495.55)/450 = 0.0064
The required pipe diameter is:
D = (2.16 Qn/√So)3/8
D = (2.16 x 10.3 x 0.015/√0.0064)3/8
= 1.71 ft
The diameter is rounded up to the next commercially
available pipe size, 1.75 ft or 21 in.
D
Rational Method Assessment
EnvironmentalEngineeringInstitut Teknologi BandungEnvE
Determine The diameter for pipes AB, BC,
and CD in the 27-acre drainage basin shown
in the figure. The area, runoff coefficients,
and inlet time for each subcatchment are
shown in Table, and the length and slope for
each pipe are in columns 2 and 3 of Table .
Use the same rainfall intensity equation and
A
I II
III IV V
Use the same rainfall intensity equation and
assume the pipes have Manning’s n = 0.015. BE
C
D
VI VIICatchment Area, A
(acres)
Runoff
coefficient,
C
Inlet time,
ti (min)
I
II
III
IV
V
VI
VII
2
3
4
4
5
4.5
4.5
0.7
0.7
0.6
0.6
0.5
0.5
0.5
5
7
10
10
15
15
15
Sewer pipe Length , L (ft) Slope, So
EB
AB
BC
CD
450
550
400
450
0.0064
0.0081
0.0064
0.0064
EnvironmentalEngineeringInstitut Teknologi BandungEnvE
The same method is used for each pipe, except that now the
time of concentration must include both inlet time and flow
time through upstream sewers. The results obtained for pipe
EB the previous case is shown in table.
A
BE
C
D
I II
III IV V
VI VII
Sewer
pipe
L (ft) So Total
area
ΣCA tc (min) i (in/hr) Design
dischar
Comput
ed
Pipe
size
Flow
velocity
Flow
time
Rational Method Assessment
pipe area
drained
(acres)
dischar
ge, Q
(cfs)
ed
sewer
diamet
er (ft)
size
used
(ft)
velocity
Q/A
(ft/s)
time
L/V
(min)
EB 450 0.0064 4 2.4 10.0 4.30 10.3 1.71 1.75 4.28 1.75
AB 550 0.0081 5 3.5 7.0 4.68 16.4 1.94 2.00 5.21 1.76
This pipe drains subcatchments I and II. From Table, A1=2 acres, C1=0.7, and the inlet time is
t1=5min, while AII=3 acres, CII=0.7, and tII=7 min. Hence, the toal area drained by pipe AB is
5 acres and ΣCA=CIAI + CIIAII = 0.7x2 + 0.7x3 = 3.5. The time concentration used is 7 min, the
larger of the two inlet times. The calculations for the required diameter are carried out in
the same way; the results are shown in the second row of the table. The calculated
diameter, 1.94 ft, is rounded up to a commercial size of 2.0 ft (24 in) for pipe AB.
IDF Curve Intensity-Duration-Frequency
EnvironmentalEngineeringInstitut Teknologi BandungEnvE
Periode
Inte
nsita
s h
ujan
( in
/jam
)
Periode
Ulang
100 tahun
50
25
10
5
2
Inte
nsita
s h
ujan
( in
/jam
)
Durasi (menit)
IDF Curve Intensity-Duration-Frequency
EnvironmentalEngineeringInstitut Teknologi BandungEnvE
Determine the design precipitation intensity and depth for a 20-minute duration
storm with a 5-year return period.
Answer:
From the IDF curves, the design intensity for a 5-year, 20-minute storm is
i = 3.5 in/h.
The corresponding precipitation depth is
i = P/Td with Td = 20 min = 0.333 h.
P = i Td = 3.50 x 0.333 = 1.17 in.
Design STORM
Determine the design rainfall depth for a 25-year 30-minute storm in City X.
P 10-min = 0.51 P5-min + 0.49 P15-min
P 30-min = 0.51 P15-min + 0.49 P60-min
For T=2 years, P2,30= 0.51x1.02 + 0.45x1.85 = 1.43 in
For T=100 years, P100,30= 0.51 x 1.86 +0.49x3.80 = 2,81 in
Coefficient a and b are 0.293 and 0.669, respectively (from Table)Coefficient a and b are 0.293 and 0.669, respectively (from Table)
P25,30 = aP2,30 + bP100,30
= 0.293 x 1.43 + 0.669x2.81
= 2.30 in
Return period T,
years
a b
5
10
25
50
0.674
0.496
0.293
0.146
0.278
0.449
0.669
0.835
Hydroeconomic Analysis
Use the maps and equations to plot IDF curves for City-O for return periods of 2, 5, 10,
25, 50, and 100 years. Consider rainfall durations ranging from 5 minutes to 1 hour.
0.850.90 0.90
0.85
0.80
Design STORM
Use the National Weather Service maps to plot IDF curves for City X, for return period 2, 5, 10,
25, 50, and 100 years. Consider rainfall duration ranging from 5 minutes to 1 hour.
Answer:
The six maps presented in the figure show precipitation for 5-, 15-, and 60-minute durations and
2- and 100-year return periods. The six values for this City X are:
P2,5 = 0.48 in P100,5 0.87 in P2,15 1.02 in P100,15 1.86 in P2,60 1.85 in P100,60 3.80 in
The results are shown in the table in terms of precipitation depth, and are converted into intensity
map
The results are shown in the table in terms of precipitation depth, and are converted into intensity
by dividing by duration.
For example P25,30 = 2.30 in � 4.60 in/hr.
P 10-min = 0.51 P5-min + 0.49 P15-min
P 30-min = 0.51 P15-min + 0.49 P60-min P25,30 = aP2,30 + bP100,30
Coefficient a and b are 0.293 and 0.669, respectively (from Table)
Return period T,
years
a b
5
10
25
50
0.674
0.496
0.293
0.146
0.278
0.449
0.669
0.835
Return period
T (year)
Duration, Td (min)
5 10 15 30 60
2
5
10
25
50
100
0.48
0.57
0.63
0.72
0.80
0.87
0.80
0.94
1.05
1.21
1.33
1.45
1.02
1.20
1.34
1.54
1.70
1.86
1.43
1.74
1.97
2.30
2.56
2.81
1.85
2.30
2.62
3.08
3.44
3.80
Design precipitation
depths (in) at City X for
various Td and T
Design STORM
Return period
T (year)
Duration, Td (min)
5 10 15 30 60
2
5
10
25
50
100
0.48
0.57
0.63
0.72
0.80
0.87
0.80
0.94
1.05
1.21
1.33
1.45
1.02
1.20
1.34
1.54
1.70
1.86
1.43
1.74
1.97
2.30
2.56
2.81
1.85
2.30
2.62
3.08
3.44
3.80
Design precipitation
depths (in) at City X for
various Td and T
11.0
10.0
9.0
Inte
nsi
ty (
in/h
)
Duration (min)
9.0
8.0
7.0
6.0
5.0
4.0
3.0
2.5
2.0
1.5
1.01 5 10 15 30 60
Return Period (years)
100
50
25
10
5
2
Kurva Intensitas Hujan
EnvironmentalEngineeringInstitut Teknologi BandungEnvE
Dari data hujan yang dihasilkan oleh penakar hujan otomatis di Lokasi X dapat
dilihat besaran intensitas hujan ‘ekstrim’ seperti tampak pada tabel.
Tahun 5 10 15 30 45 60 120 360 720 1440
1985 122 138 118 122 87 70 50 20 9 71985
1986
1987
1988
1989
1990
1991
1992
1993
122
156
138
147
126
192
129
142
133
138
134
149
125
174
120
164
118
159
118
124
129
160
154
132
149
134
140
122 87 70 50 20 9 7
Kurva Intensitas Hujan
EnvironmentalEngineeringInstitut Teknologi BandungEnvE
Dari data hujan yang dihasilkan oleh penakar hujan otomatis di Lokasi X dapat
dilihat besaran intensitas hujan ‘ekstrim’ seperti tampak pada tabel.
Tabel Perhitungan Periode Ulang dan Probabilits (untuk durasi 5 menit)
T=(n+1)/m
Tahun Intensitas Hujan
ekstrim (mm/jam)
Ranking,
m
Periode
ulang, T
Proba-
bilitas, P
1985
1986
1987
1988
1989
1990
1991
1992
1993
122
156
138
147
126
192
129
142
133
9
2
5
3
8
1
7
4
6
1.11
5
2
3.33
1.25
10
1.43
2.5
1.66
90
20
50
30
80
10
70
40
60
Kurva Intensitas Hujan
EnvironmentalEngineeringInstitut Teknologi BandungEnvE
Tabel Intensitas ekstrim (mm/jam) untuk Periode Ulang 5 dan 10 tahun
X, Durasi
(menit)
Y, Intensitas hujan ekstrim,
periode ulang 5 tahun
Y, ntensitas hujan ekstrim,
periode ulang 10 tahun
5
10
156
138
192
17410
15
30
45
60
120
360
720
1440
138
132
104
80
73
40
16
8
5
174
160
122
100
80
50
20
12
7
Kurva Intensitas Hujan
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Tabel Kurva Interpolasi Least Square
X,
Durasi
(menit)
Y XY Y2 XY2 logX logY LogX.
LogY
(Log
X)2
Y √X Y2√X
5 156 780 24336 121680 0.69 2.19 1.53 0.48 348 544165
10
15
30
45
60
120
360
720
1440
156
138
132
104
80
73
40
16
8
5
780 24336 121680 0.69 2.19 1.53 0.48 348 54416
38760 85294 187190 18.4 16.6 26.8 39.9 123 350625
Kurva Intensitas Hujan
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Tiga rumus kurva yang harus dipilih:
- Kurva No. 1: y = a/(x+b) --------� Persamaan Talbot: I = a/(t+b)
a = (Σxy)(Σy2) - (Σxy2)(Σy)
10 (Σy2) - (Σy)(Σy)
b = (Σy)(Σxy) - 10(Σxy2)
10 (Σy2) - (Σy)(Σy)
a = 1625.86 (10=jumlah tahun) b = 121.28
- Kurva No. 2: y = a/Xn -------------� Persamaan Sherman: I = a/Xn
- Kurva No. 3: y = a/((√x) + b) -----� Persamaan Ishiguro: I = a/((√t) + b)
a = 1625.86 (10=jumlah tahun) b = 121.28
log a = (Σlogx)(Σlogx)2 - (Σlogx.logy)(Σy)
10 (Σy2) - (Σlogx)(Σlogx)
n = (Σlogy)(Σlogx) - 10(Σlogx logy)
10 (Σlogx)2 - (Σlogx)(Σlogx)
a = - 8.42 n = - 4.55
a = (Σy√x)(Σy2) - (Σy2√x)(Σy)
10 (Σy2) - (Σy)(Σy)
b = (Σy)(Σy√x) - 10(Σy2√x)
10 (Σy2) - (Σy)(Σy)
a = 746.77 b = 2.732
Kurva Intensitas Hujan
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Selisih intensitas dalam angka mutlak positif dinyatakan dalam dy1, dy2, dy3. Nilai
dy terkecil menunjukkan kurva terbaik untuk dipilih.
No. x y y1 dy1 y2 dy2 y3 dy3
1
2
3
5
10
15
156
138
132
128.64
123.74
119.20
27.35
14.25
12.79
5.8E-06 156 158.57 2.57
3
4
5
6
7
8
9
10
15
30
45
60
120
360
720
1440
132
140
80
73
40
16
8
5
119.20 12.79
Jumlah 153.9 985235 99.2
Mean 15.39 98523.5 9.92
Yang dipilih adalah Kurva no.3, pada dy3 rata-rata terkecil yaitu 9.92.
Determine Areal Rainfall:
� Arithmetic P = ΣP/n
� Thiessen Polygon P = 1/A Σ AjPj
� Isohyet P = 1/A Σ AiPi
Areal RAINFALL
Isohyet P = 1/A Σ AiPi
Determine Areal Rainfall:
� Thiessen Polygon
FloodAreal RAINFALL
Determine Areal Rainfall:
� Isohyet
Areal RAINFALL
Determine Rainfall:
� Arithmetic
� Thiessen Polygon
� Isohyet
Design FLOWFlood Control Reservoir Design
P5=50 mm
P2=20 mm
P1=10 mm
IsohyetP2=20 mm
P3=30 mmP4=40 mm