1. Welcome to ENV4203 Public Health Engineering
Module 1

2. 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
Module 1

3. 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
Module 1

4. STORMWATER DRAINAGE PLAN
Module 1

5. 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
Module 1

6. URBAN STORMWATER DRAINAGE A typical gully pit
Module 1

7. URBAN STORMWATER DRAINAGE When the design fails!
Module 1

8. 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
Module 1

9. URBAN STORMWATER DRAINAGE Minor and major drainage design
Module 1

10. 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
Module 1

11. 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
Module 1

12. 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 = F C A I
Module 1

13. Module 1

14. 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
Module 1

15. 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
Module 1

16. 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
Module 1

17. URBAN STORMWATER DRAINAGE Overland/allotment travel time
Use kinematic wave equation
t = (L. n*)0.6 /(S0.3 I0.4)
t I0.4 = (L. n*)0.6 /(S0.3)
Select t corresponding to t I0.4 from prepared table
Module 1

18. URBAN STORMWATER DRAINAGE Gutter travel time
Use Izzard’s equation
Q = F.[(Zg/ng).(dg8/3 - dp8/3) + (Zp/np).(dp8/3 - dc8/3)].So1/2
Module 1

19. 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
Module 1

20. 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
Module 1

21. 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
Module 1

22. 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
Module 1

23. Intensity-Frequency-Duration chart
Module 1

24. 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
Module 1

25. 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) equation A(3.1)
50I6m = F50 x (50I1) equation A(3.2)
Module 1

26. 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 = 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 equation A(3.6)
Module 1

27. 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
Module 1

28. 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 = 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 equation A(3.6)
Module 1

29. 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 = x 2ID/[ log10(1.13 x 50ID/2ID)]) eqn A(3.7)
Module 1

30. 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
Module 1

31. 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
Module 1

32. URBAN STORMWATER DRAINAGE Runoff Coefficient, C
Based on 10I1 rainfall intensity
C’10 = (10I1 - 25)
C10 = 0.9 f + C’10 (1 - f)
CY = FY C10
Module 1

33. Runoff coefficients C’10 = 0.1 + 0.0133(10I1 - 25)
C10 = 0.9f + C’10 (1 – f)
Runoff coefficients
fraction impervious f
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
0.67
0.72
0.77
C100
0.43
0.50
0.56
0.69
0.76
0.82
0.89
0.95
1.00
Module 1

34. 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 i.e. 1/360
Module 1

35. 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
Module 1

36. Rational Method
Using the Rational Method equation for each rainfall event
The 4 contributing flows are
Q(1) = 0.1 x 25/ = L/s
Q(2) = 0.1 x 42/ = L/s
Q(3) = 0.1 x 48/ = L/s
Q(4) = 0.1 x (15/20) x 55/0.360 = L/s
Module 1

37. Rational Method (non-homogenous catchment)
The longest time of concentration is 60 min, and corresponding 10I60 = 25 mm/h
SCA = 1 x x = ha
Q = F CA I = x 25/0.36 = L/s
Note the anomaly that the runoff is less than for the single impervious 0.1 ha catchment of L/s
Module 1

38. 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
SCA = 1 x x 0.20 x 20/60 = ha
Q = F CA I = x 48/0.36 = L/s
Note the partial area effect resulted in a higher flow than the full area design
Module 1

39. 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
Module 1

40. URBAN STORMWATER DRAINAGE Typical Inlet Pit
Module 1

41. 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
Module 1

42. 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
Module 1

43. URBAN STORMWATER DRAINAGE Hydraulic Design
Module 1

44. URBAN STORMWATER DRAINAGE Hydraulic Design
Module 1

45. 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
Module 1

46. 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
Module 1

47. Hydraulic Design Proforma
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
Pipe L m
Q L/s
Trial pipe dia. m
V m/s
V2/
(2g)
US pit SL US
WL *
Pit coeff kw
kwV2 /(2g)
HGL US pit
[8–9]
Frict slope Sf hf
[12x2]
HGl DS
Pit
[11-13]
DS pit
WL **
1 - 3 54
0.304
0.744
0.0282
26.23
26.080
4.0
0.1128
25.967
0.0023
0.1224
25.845
24.88
24.73
2 – 3 20 47
0.6475
0.0214
24.91
24.760
0.0855
24.675
0.0017
0.0345
24.640
3 – 4
161
0.381
1.4122
0.1016
1.5
0.1525
24.488
0.006
0.3254
24.162
24.38
4 - 0 25
231
2.0262
0.2092
2.0
0.4185
23.744
0.0123
0.3080
23.436 -
23.331
0.457
1.4083
0.1011
0.2022
23.960
0.0048
0.1193
23.841
23.407
* Lower of [7] – freeboard or lowest HGL level in [16] for pipes entering US pit
** Lower of [14] or {[15] – freeboard}
Module 1

48. Hydraulic Design Proforma (cont.)
[1]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
Pipe
US invert levels, m
DS invert levels, m
Pipe slope So
[20-23/2]
Remarks
Hydraulic
[11 – 4]
Cover*
[7 – cover]
US pipe
[23 – drop]
Adopted
Lowest of
[17, 18, 19]
[16 – 4]
Cover
[15 – cover]
Adopt lowest of [21, 22]
1 – 3
25.663
25.497 -
24.426
24.147
0.0250
Low velocity
2 – 3
24.371
24.177
24.336
0.0015
3 – 4
24.107
24.067
24.117
23.781
23.567
0.0093
4 - 0
23.363
23.537
22.950
0.0165
Try lower velocity
23.503
23.485
0.0214
* Cover in this column includes manufacturer’s pipe cover, pipe diameter and pipe wall thickness
Module 1

49. Hydraulic Checking Sheet
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
Pipe L m Q
L/s
Pipe dia, m V
m/s
V2 /(2g)
DS HGL
Frict head hf
HGL just below US pit
[7 + 8]
Obvert L at upper end of pipe
Pit coeff kw
kwV2 /(2g)
Adopted US pit HGL * US SL
4 – outfall 25
231
0.457
1.4083
0.1011
23.50
0.199
23.619
23.942
2.0
0.202
24.144
24.38
3 – 4 54
161
0.381
1.4122
0.1016
0.325
24.469
24.448
1.5
0.152
24.621
24.88
2 – 3 20 47
0.304
0.6475
0.0214
0.035
24.656
24.481
4.0
0.086
24.742
24.91
1 - 2
0.7440
0.0282
0.122
24.743
25.801
0.113
25.914
26.23
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
Module 1

50. END OF MODULE 1
Module 1