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Module 11 Welcome to ENV4203 Public Health Engineering.

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Presentation on theme: "Module 11 Welcome to ENV4203 Public Health Engineering."— Presentation transcript:

1 Module 11 Welcome to ENV4203 Public Health Engineering

2 Module 12 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

3 Module 13 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

4 Module 14 STORMWATER DRAINAGE PLAN

5 Module 15 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

6 Module 16 URBAN STORMWATER DRAINAGE A typical gully pit

7 Module 17 URBAN STORMWATER DRAINAGE When the design fails!

8 Module 18 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

9 Module 19 URBAN STORMWATER DRAINAGE Minor and major drainage design

10 Module 110 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

11 Module 111 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

12 Module 112 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

13 Module 113

14 Module 114 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

15 Module 115 URBAN STORMWATER DRAINAGE Time of concentration, t c 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

16 Module 116 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

17 Module 117 URBAN STORMWATER DRAINAGE Overland/allotment travel time Use kinematic wave equation t = 6.94 (L. n*) 0.6 /(S 0.3 I 0.4 ) t I 0.4 = 6.94 (L. n*) 0.6 /(S 0.3 ) Select t corresponding to t I 0.4 from prepared table

18 Module 118 URBAN STORMWATER DRAINAGE Gutter travel time Use Izzard’s equation Q = F.[(Z g /n g ).(d g 8/3 - d p 8/3 ) + (Z p /n p ).(d p 8/3 - d c 8/3 )].S o 1/2

19 Module 119 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

20 Module 120 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

21 Module 121 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

22 Module 122 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

23 Module 123 Intensity-Frequency-Duration chart

24 Module 124 Required steps for IFD preparation Step 1 Determine input data Map 12I 1 2 year ARI 1 hour duration Map 22I 12 2 year ARI 12 hour duration Map 32I 72 2 year ARI 72 hour duration Map 450I 1 50 year ARI 1 hour duration Map 550I year ARI 12 hour duration Map 650I year ARI 72 hour duration Map 7Gskewness Map 8F2geographical factor 6 min, 2 ARI Map 9F50geographical factor 6 min, 50 ARI

25 Module 125 Required steps for IFD preparation Calculate the 6 min intensities for 2 and 50 years ARI 2I 6m = F2 x (2I 1 ) 0.9 equation A(3.1) 50I 6m = F50 x (50I 1 ) 0.6 equation A(3.2) Step 2 Intensities for durations less than 1 hour

26 Module 126 Required steps for IFD preparation Calculate the mean and standard deviations of the log of the rainfall intensities for the specific durations X D = log 10 (2I D /1.13) equation A(3.3) S D = x log 10 (50I D x 1.13/2I D ) equation A(3.4) YI D = Y P [antilog 10 (X D + Y K x S D )] equation A(3.5) Y K = 2[{( Y K N – G/6) x G/6 + 1} 3 – 1]/G equation A(3.6) Step 3 LP III rainfalls for 2 & 50 years for basic durations

27 Module 127 Required steps for IFD preparation This step is optional for the algebraic method However, it is recommended as a graphical confirmation of your calculations Step 4 Plot LP III for 2 & 50 years and basic durations

28 Module 128 Required steps for IFD preparation Calculate the mean and standard deviations of the log of the rainfall intensities for the basic durations X D = log 10 (2I D /1.13) equation A(3.3) S D = x log 10 (50I D x 1.13/2I D ) equation A(3.4) YI D = Y P [antilog 10 (X D + Y K x S D )] equation A(3.5) Y K = 2[{( Y K N – G/6) x G/6 + 1} 3 – 1]/G equation A(3.6) Step 5 Determine LP III rainfalls for 5, 10, 20 & 100 years and for basic durations 6 m, 1, 12, 72 h

29 Module 129 Required steps for IFD preparation Calculate the 1 year ARI intensities for basic durations D = 6 min, 1, 12, and 72 h 1I D = x 2I D /[ log 10 (1.13 x 50I D /2I D )]) eqn A(3.7) Step 6 Calculate 1 ARI intensities for basic durations

30 Module 130 Required steps for IFD preparation 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 6 th degree polynomial is not required Step 7 Interpolate from basic durations to all other durations for all ARIs

31 Module 131 URBAN STORMWATER DRAINAGE Runoff Coefficient, C Ratio of runoff to rainfall frequency curves Based on the 10 I 1 storm intensity Runoff coefficient is related to the fraction impervious of the catchment

32 Module 132 URBAN STORMWATER DRAINAGE Runoff Coefficient, C Based on 10I 1 rainfall intensity C’ 10 = (10I ) C 10 = 0.9 f + C’ 10 (1 - f) C Y = F Y C 10

33 Module 133 Runoff coefficients fraction impervious f C C2C C C’ 10 = (10I ) C 10 = 0.9f + C’ 10 (1 – f)

34 Module 134 URBAN STORMWATER DRAINAGE Hydrologic Flow Q = F  (C A) I m 3 /s C = runoff coefficient A = catchment area, ha I = rainfall intensity, mm/h F = proportionality constant i.e. 1/360

35 Module 135 Rational Method Let us examine 4 rainfall events Rainfall (1) 10I 60 = 25 mm/h Rainfall (2) 10I 25 = 42 mm/h Rainfall (3) 10I 20 = 48 mm/h Rainfall (4) 10I 15 = 55 mm/h The time of concentration is 20 min When does peak flow occur?

36 Module 136 Rational Method 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 = L/s Q(3) = 0.1 x 48/0.360 = L/s Q(4) = 0.1 x (15/20) x 55/0.360 = L/s Using the Rational Method equation for each rainfall event

37 Module 137 Rational Method (non-homogenous catchment)  CA = 1 x 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 L/s The longest time of concentration is 60 min, and corresponding 10I 60 = 25 mm/h

38 Module 138 Rational Method (non-homogenous catchment)  CA = 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 The partial area effect uses time of concentration of 20 min for the impervious area, and corresponding 10I 20 = 48 mm/h

39 Module 139 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

40 Module 140 URBAN STORMWATER DRAINAGE Typical Inlet Pit

41 Module 141 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

42 Module 142 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

43 Module 143 URBAN STORMWATER DRAINAGE Hydraulic Design

44 Module 144 URBAN STORMWATER DRAINAGE Hydraulic Design

45 Module 145 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

46 Module 146 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

47 Module 147 Hydraulic Design Proforma [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] Pipe LmLm Q L/s Trial pipe dia. m V m/s V 2 / (2g) US pit SL US WL * Pit coeff k w k w V 2 /(2g) HGL US pit [8–9] Frict slope S f h f [12x 2] HGl DS Pit [11- 13] DS pit SL DS WL ** – – * Lower of [7] – freeboard or lowest HGL level in [16] for pipes entering US pit ** Lower of [14] or {[15] – freeboard}

48 Module 148 Hydraulic Design Proforma (cont.) [1][17][18][19][20][21][22][23][24][25] Pipe US invert levels, mDS invert levels, m Pipe slope S o [20-23/2] 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] 1 – Low velocity 2 – Low velocity 3 – Try lower velocity * Cover in this column includes manufacturer’s pipe cover, pipe diameter and pipe wall thickness

49 Module 149 Hydraulic Checking Sheet [1][2][3][4][5][6][7][8][9][10][11][12][13] [14 ] Pipe LmLm Q L/s Pipe dia, m V m/s V 2 /(2g) DS HGL Frict head h f HGL just below US pit [7 + 8] Obvert L at upper end of pipe Pit coeff k w k w V 2 /(2g) Adopte d US pit HGL * US SL 4 – outfall – – 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

50 Module 150 END OF MODULE 1


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