URBAN STORMWATER DRAINAGE On completion of this module you should be able to:

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Welcome to ENV4203 Public Health Engineering
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ENV4203 PUBLIC HEALTH ENGINEERINGWhat 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 DRAINAGEOn 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 DRAINAGESome 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 DRAINAGEA typical gully pit
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URBAN STORMWATER DRAINAGEWhen the design fails!
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URBAN STORMWATER DRAINAGESome 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 DRAINAGEMinor and major
drainage design
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URBAN STORMWATER DRAINAGEGoals 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 DRAINAGEDesign 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 DRAINAGERational 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

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URBAN STORMWATER DRAINAGERational 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 DRAINAGETime 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 DRAINAGEComponents of surface
pipe travel times

• Overland/allotment travel time from kinematic
  wave equation
• Gutter travel time from Izzards equation
• Pipe travel time i.e. length/velocity

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URBAN STORMWATER DRAINAGEOverland/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 DRAINAGEGutter travel time

• Use Izzards 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 DRAINAGEAverage 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 DRAINAGERainfall 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 DRAINAGEPreparation 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 DRAINAGEPreparation 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
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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 13 1/G
  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 13 1/G
  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)) 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 DRAINAGERunoff 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 DRAINAGERunoff Coefficient, C

• Based on 10I1 rainfall intensity
• C10 0.1 0.0133 (10I1 - 25)
• C10 0.9 f C10 (1 - f)
• CY FY C10

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Runoff coefficients
C10 0.1 0.0133(10I1 - 25) C10 0.9f
C10 (1 f)
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URBAN STORMWATER DRAINAGEHydrologic 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

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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

• SCA 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

• SCA 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 DRAINAGEPit 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 DRAINAGETypical Inlet Pit
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URBAN STORMWATER DRAINAGERational 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 DRAINAGEHydraulic 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 DRAINAGEHydraulic Design
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URBAN STORMWATER DRAINAGEHydraulic Design
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URBAN STORMWATER DRAINAGEHydraulic 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 DRAINAGEHydraulic 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
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.)
Cover in this column includes manufacturers
pipe cover, pipe diameter and pipe wall thickness
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Hydraulic Checking Sheet
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