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**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 Module 1

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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 Module 1

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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 Module 1

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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 Module 1

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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 Module 1

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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 Module 1

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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 = F C A I Module 1

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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 Module 1

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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 Module 1

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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 Module 1

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**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

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**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

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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 Module 1

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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 Module 1

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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 Module 1

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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 Module 1

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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 Module 1

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

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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 = 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

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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 Module 1

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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 = 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

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

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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 Module 1

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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 Module 1

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**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

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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 0.1 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

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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 i.e. 1/360 Module 1

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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 Module 1

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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/ = 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

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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 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

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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 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

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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 Module 1

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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 Module 1

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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 Module 1

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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 Module 1

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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 Module 1

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**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

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**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

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**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

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END OF MODULE 1 Module 1

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