Transcript URBAN STORMWATER DRAINAGE On completion of this …

Welcome to ENV4203 Public Health Engineering
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ENV4203 PUBLIC HEALTH ENGINEERING
What are the roles of the practitioner?
• Urban drainage & flood protection (module 1)
• Source, storage & transmission of water (module 2)
• Water treatment to meet standards (modules 4, 5)
• Distribution of water to consumers (module 3)
• Collection of wastewater, treatment & disposal (modules
6 - 9)
• Collection & disposal of MSW (module 10)
• Air & noise pollution abatement
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URBAN STORMWATER DRAINAGE
On completion of this module you should be
able to:
• Discuss the concepts of minor and major drainage
design
• Use the Rational Method
• Plan and design an urban stormwater drainage
system
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STORMWATER DRAINAGE PLAN
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URBAN STORMWATER DRAINAGE
Some important features
• Drainage is fundamental to urban living
• Low individual costs but high aggregate costs
• Structures are low profile and usually out of sight
• Flooding occurs when the design fails
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URBAN STORMWATER DRAINAGE
A typical gully pit
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URBAN STORMWATER DRAINAGE
When the design fails!
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URBAN STORMWATER DRAINAGE
Some important concepts
• Minor drainage design caters for the frequent
storm events and involves structures such as kerb
gutter, inlet pits and pipe system
• Frequent storms have low intensity values
• Major drainage design caters for infrequent,
low probability storms of high intensity values and
flows are channelled into roadways, natural channels
and detention basins
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URBAN STORMWATER DRAINAGE
Minor and major drainage design
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URBAN STORMWATER DRAINAGE
Goals of urban drainage
• Collect and safely convey stormwater to receiving
waters
• To flood proof important buildings/areas (major
drainage design)
• To cater for the frequent or nuisance stormwater
flows (minor drainage design)
• To retain within each catchment as much incident rain
as possible
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URBAN STORMWATER DRAINAGE
Design of urban stormwater drainage involves
• Hydrologic calculations of catchment flow rates
• Hydraulic calculations of pit energy and friction
losses, and pipe sizes
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URBAN STORMWATER DRAINAGE
Rational Method equation (hydrologic)
• Assumes a relationship between duration of rainfall
required to produce peak flow and the longest travel
time of the catchment
• Peak flow occurs when duration of storm equals the
time of concentration
• Q = FCAI
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URBAN STORMWATER DRAINAGE
Rational Method equation (continue)
• Surface hydrologic flow is based on the longest travel
time for the catchment
• Pipe hydrologic flow is based on the longest
cumulative travel time including pipe flow time for the
corresponding cumulative upstream catchment
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URBAN STORMWATER DRAINAGE
Time of concentration, tc
• The runoff travel time from the most remote point of
the catchment to the outlet
• Or the time taken from the start of the rainfall until the
whole catchment is simultaneously contributing to
flow at the outlet
• It comprises the travel time from roof gutters, open
ground, kerb gutter, pipes and channels
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URBAN STORMWATER DRAINAGE
Components of surface & pipe travel times
• Overland/allotment travel time from kinematic wave
equation
• Gutter travel time from Izzard’s equation
• Pipe travel time i.e. length/velocity
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URBAN STORMWATER DRAINAGE
Overland/allotment travel time
• Use kinematic wave equation
• t = 6.94 (L. n*)0.6 /(S0.3 I0.4)
• t I0.4 = 6.94 (L. n*)0.6 /(S0.3)
• Select t corresponding to t I0.4 from prepared table
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URBAN STORMWATER DRAINAGE
Gutter travel time
• Use Izzard’s equation
• Q = 0.375 F.[(Zg/ng).(dg8/3 - dp8/3) + (Zp/np).(dp8/3 - dc8/3)].So1/2
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URBAN STORMWATER DRAINAGE
Average Recurrence Interval (ARI), Y
• The average period between years in which a value
(rainfall or runoff) is exceeded
• It is not the time between exceedances of a given
value
• Periods between exceedances are random
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URBAN STORMWATER DRAINAGE
Rainfall intensity, I, is dependent on
• Locality of the catchment
• Recurrence interval used in the design
• Time of concentration or duration of storm
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URBAN STORMWATER DRAINAGE
Preparation of the Intensity-Frequency-Duration
(IFD) data for any location in Australia from
• 6 master charts of log normal design rainfall isopleths
• 1 regionalised skewness map
• 2 charts of geographical factors to determine short
duration intensities
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URBAN STORMWATER DRAINAGE
Preparation of the Intensity-Frequency-Duration
(IFD) data for any location in Australia to
produce
• Standard ARIs of 1, 2, 5, 10, 20, 50 and 100 years
• Standard durations of 5, 6, 10, 20, 30 minutes, 1, 2,
3, 6, 12, 24, 48, and 72 hours
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Intensity-Frequency-Duration chart
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Required steps for IFD preparation
Step 1 Determine input data
Map 1
2I1
2 year ARI 1 hour duration
Map 2
2I12
2 year ARI 12 hour duration
Map 3
2I72
2 year ARI 72 hour duration
Map 4
50I1
50 year ARI 1 hour duration
Map 5
50I12 50 year ARI 12 hour duration
Map 6
50I72 50 year ARI 72 hour duration
Map 7
G
skewness
Map 8
F2
geographical factor 6 min, 2 ARI
Map 9
F50
geographical factor 6 min, 50 ARI
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Required steps for IFD preparation
Step 2 Intensities for durations less than 1 hour
• Calculate the 6 min intensities for 2 and 50 years ARI
• 2I6m = F2 x (2I1)0.9
equation A(3.1)
• 50I6m = F50 x (50I1)0.6
equation A(3.2)
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Required steps for IFD preparation
Step 3 LP III rainfalls for 2 & 50 years for basic durations
•
Calculate the mean and standard deviations of the log of the
rainfall intensities for the specific durations
•
XD = log10 (2ID/1.13)
equation A(3.3)
•
SD = 0.4869 x log10(50ID x 1.13/2ID)
equation A(3.4)
•
YID = YP [antilog10(XD + YK x SD)]
equation A(3.5)
•
YK
= 2[{(YKN – G/6) x G/6 + 1}3 – 1]/G
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equation A(3.6)
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Required steps for IFD preparation
Step 4 Plot LP III for 2 & 50 years and basic durations
•
This step is optional for the algebraic method
•
However, it is recommended as a graphical confirmation of
your calculations
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Required steps for IFD preparation
Step 5 Determine LP III rainfalls for 5, 10, 20 & 100
years and for basic durations 6 m, 1, 12, 72 h
•
Calculate the mean and standard deviations of the log of the
rainfall intensities for the basic durations
•
XD = log10 (2ID/1.13)
equation A(3.3)
•
SD = 0.4869 x log10(50ID x 1.13/2ID)
equation A(3.4)
•
YID = YP [antilog10(XD + YK x SD)]
equation A(3.5)
•
YK
= 2[{(YKN – G/6) x G/6 + 1}3 – 1]/G
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equation A(3.6)
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Required steps for IFD preparation
Step 6 Calculate 1 ARI intensities for basic durations
•
Calculate the 1 year ARI intensities for basic durations D = 6 min, 1, 12,
and 72 h
•
1ID = 0.885 x 2ID/[1 + 0.4046 log10(1.13 x 50ID/2ID)])
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eqn A(3.7)
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Required steps for IFD preparation
Step 7 Interpolate from basic durations to all
other durations for all ARIs
•
Use equations A(3.8), A(3.9) and A(3.10)
•
Refer also to Table A1 in the appendix of Reading 1.1
•
Step 8 smoothing of the IFD curves via a 6th degree polynomial
is not required
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URBAN STORMWATER DRAINAGE
Runoff Coefficient, C
• Ratio of runoff to rainfall frequency curves
• Based on the 10 I1 storm intensity
• Runoff coefficient is related to the fraction impervious
of the catchment
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URBAN STORMWATER DRAINAGE
Runoff Coefficient, C
• Based on 10I1 rainfall intensity
• C’10 = 0.1 + 0.0133 (10I1 - 25)
• C10 = 0.9 f + C’10 (1 - f)
• CY = FY C10
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Runoff coefficients
C’10 = 0.1 + 0.0133(10I1 - 25)
C10 = 0.9f + C’10 (1 – f)
Runoff
coefficients
fraction impervious f
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
C10
0.36
0.41
0.47
0.53
0.58
0.63
0.68
0.74
0.79
0.85
0.90
C2
0.30
0.35
0.40
0.44
0.49
0.54
0.58
0.63
0.67
0.72
0.77
C100
0.43
0.50
0.56
0.63
0.69
0.76
0.82
0.89
0.95
1.00
1.00
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URBAN STORMWATER DRAINAGE
Hydrologic Flow
• Q = F (C A) I
m3/s
• C = runoff coefficient
• A = catchment area, ha
•
I = rainfall intensity, mm/h
• F = proportionality constant
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i.e. 1/360
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Rational Method
The time of concentration is 20 min
When does peak flow occur?
Let us examine 4 rainfall events
• Rainfall (1) 10I60 = 25 mm/h
• Rainfall (2) 10I25 = 42 mm/h
• Rainfall (3) 10I20 = 48 mm/h
• Rainfall (4) 10I15 = 55 mm/h
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Rational Method
Using the Rational Method equation for
each rainfall event
The 4 contributing flows are
•
Q(1) = 0.1 x 25/0.360 = 6.94 L/s
•
Q(2) = 0.1 x 42/0.360 = 11.67 L/s
•
Q(3) = 0.1 x 48/0.360 = 13.33 L/s
•
Q(4) = 0.1 x (15/20) x 55/0.360 = 11.46 L/s
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Rational Method (non-homogenous catchment)
The longest time of concentration is
60 min, and corresponding 10I60 =
25 mm/h
CA = 1 x 0.1 + 0.4 x 0.20 = 0.18 ha
Q = F CA I = 0.18 x 25/0.36 = 12.5 L/s
Note the anomaly that the runoff is less than for the
single impervious 0.1 ha catchment of 13.33 L/s
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Rational Method (non-homogenous catchment)
The partial area effect uses time of
concentration of 20 min for the
impervious area, and corresponding
10I20 = 48 mm/h
CA = 1 x 0.1 + 0.4 x 0.20 x 20/60 = 0.127 ha
Q = F CA I = 0.127 x 48/0.36 = 16.93 L/s
Note the partial area effect resulted in a higher flow than
the full area design
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URBAN STORMWATER DRAINAGE
Pit Entry Capacity
• Design for performance and safety
• Grate and kerb inlets
• Use standard design based on local authority
requirements
•
On-grade and sag pits
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URBAN STORMWATER DRAINAGE
Typical Inlet Pit
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URBAN STORMWATER DRAINAGE
Rational Method has some inconsistencies
• Rainfall intensity is assumed uniform (temporal and
spatial)
• Antecedent catchment condition is not recognised
• Partial area effects may result in larger flows
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URBAN STORMWATER DRAINAGE
Hydraulic Design
• Simplest design is open channel flow
• Adopted design is full-flow under pressure or
surcharge where water rises within pits but do not
overflow on to streets
• This allows greater freedom is selecting pipe slopes,
improved prediction of hydraulic behaviour and
consistency in design
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URBAN STORMWATER DRAINAGE
Hydraulic Design
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URBAN STORMWATER DRAINAGE
Hydraulic Design
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URBAN STORMWATER DRAINAGE
Hydraulic Design (continue)
• Friction slope  Pipe slope
• Allow 150 mm freeboard for USWL & DSWL
• USWL - DSWL  Losses
• Losses = Friction + Pit energy losses
• Calculate pipe size to satisfy above condition
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URBAN STORMWATER DRAINAGE
Hydraulic Design (continue)
• Limiting downstream condition may be pipe invert
level or flood level
• Carry out hydraulic check from downstream limiting
condition and work upwards
• Ensure no overflow at gully pits
• Good practice to include horizontal profile
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Hydraulic Design Proforma
[1]
[3]
[4]
L
m
Q
L/s
Trial
pipe
dia.
m
V
m/s
V2/
(2g)
US
pit
SL
US
WL *
1-3
54
54
0.304
0.744
0.0282
26.23
2–3
20
47
0.304
0.6475
0.0214
3–4
54
161
0.381
1.4122
4-0
25
231
0.381
0.457
Pipe
*
**
[2]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
Pit
coeff
kw
kwV2
/(2g)
HGL
US
pit
[8–9]
26.080
4.0
0.1128
24.91
24.760
4.0
0.1016
24.88
24.640
2.0262
0.2092
24.38
1.4083
0.1011
24.38
[12]
[13]
[14]
[15]
[16]
Sf
hf
[12x
2]
HGl
DS
Pit
[1113]
DS
pit
SL
DS
WL
**
25.967
0.0023
0.1224
25.845
24.88
24.73
0.0855
24.675
0.0017
0.0345
24.640
24.88
24.640
1.5
0.1525
24.488
0.006
0.3254
24.162
24.38
24.162
24.162
2.0
0.4185
23.744
0.0123
0.3080
23.436
-
23.331
24.162
2.0
0.2022
23.960
0.0048
0.1193
23.841
-
23.407
Frict
slope
Lower of [7] – freeboard or lowest HGL level in [16] for pipes entering US pit
Lower of [14] or {[15] – freeboard}
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Hydraulic Design Proforma (cont.)
[1]
[17]
[18]
[19]
[20]
[21]
US invert levels, m
[22]
[23]
DS invert levels, m
[24]
[25]
Remarks
Hydraulic
[11 – 4]
Cover*
[7 –
cover]
US pipe
[23 –
drop]
Adopted
Lowest of
[17, 18,
19]
Hydraulic
[16 – 4]
Cover
[15 –
cover]
Adopt
lowest of
[21, 22]
Pipe
slope
So
[20-23/2]
1–3
25.663
25.497
-
25.497
24.426
24.147
24.147
0.0250
Low
velocity
2–3
24.371
24.177
-
24.177
24.336
24.147
24.147
0.0015
Low
velocity
3–4
24.107
24.067
24.117
24.067
23.781
23.567
23.567
0.0093
4-0
23.363
23.567
23.537
23.363
22.950
-
22.950
0.0165
23.503
23.485
23.537
23.485
22.950
-
22.950
0.0214
Pipe
Try lower
velocity
* Cover in this column includes manufacturer’s pipe cover, pipe diameter and pipe wall thickness
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Hydraulic Checking Sheet
[1]
[2]
[3]
L
m
Q
L/s
4–
outfall
25
3–4
[4]
[5]
[6]
[7]
[8]
Frict
head
[9]
[10]
[11]
HGL
just
below
US pit
[7 + 8]
Obvert L
at upper
end of
pipe
Pit
coeff
kw
[12]
[13]
[14
]
Adopte
d US
pit
HGL *
US
SL
Pipe
dia, m
V
m/s
/(2g)
DS
HGL
231
0.457
1.4083
0.1011
23.50
0.199
23.619
23.942
2.0
0.202
24.144
24.3
8
54
161
0.381
1.4122
0.1016
24.144
0.325
24.469
24.448
1.5
0.152
24.621
24.8
8
2–3
20
47
0.304
0.6475
0.0214
24.621
0.035
24.656
24.481
4.0
0.086
24.742
24.9
1
1-2
54
54
0.304
0.7440
0.0282
24.621
0.122
24.743
25.801
4.0
0.113
25.914
26.2
3
Pipe
V2
hf
kwV2
/(2g)
The governing DS HGL level at outfall must be the pipe obvert level for full pipe flow or the flood llevel
* (higher of 9 or 10) + 12
Column 10: US invert level + pipe diameter
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END OF MODULE 1
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