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Project Part 1 Urban Drainage Design

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RMIT Universityyyyy School/Department/Area 2

Kerb and channel

Shoulder of a road

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Project - Example

a = 75 mb = 80 m

c = 40 m

d = 50 m10 I1 = 373. mm/hr

ARI = 1:5 years (minor)


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Rational Method

where:

Q y peak flow rate (m 3/s) for ARI of y years

Cy a dimensionless runoff coefficient for ARI of y years

A is area of catchment (km 2 ) average rainfall intensity (mm/hr) for design duration of t c hours and

ARI of y years

tc time of concentration; the longest time taken for a water particle to travelfrom a point of the catchment to the catchment outlet. This time is used todetermine the intensity of rainfall to be used for design.

F conversion factors to balance the units used; if area is in km 2 F = 0.278(or 1/3.6);

if area is in hectares F = 0.00278 (or 1/360)

,c y y y t Q FC I A

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Surface runoff from the parkland

C=0.15L = 75 m;

n*=0.3 (say Lawn)

Slope S = 2%

t = 22 (min)

I = 125 mm/hr


,c y y y t Q FC I A

125mm/hr I min3.22t 1.15102.0

)3.0*75( 94.6

tI 3.0

6 .04.0

31.3L/s6000 )60601000(

12515.0Q

Time (min) I (mm/hr) tI0.41 254.98 9.1749252 236.03 17.791715 197.92 41.45385

10 162.52 76.6237715 141.51 108.744930 108.34 195.450560 80.79 347.6077

120 59.33 614.4488240 43.21 1082.538

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Gutter diameter (parkland)

Friction factor (f) = 0.018Channel slope 1%

Assume diameter = 225 mm

V=1.57 m/s

31.3~31.1L/s Diameter okay

Surface runoff to Pit B

tc = 22.3+0.8=23.1min I = 124 mm/hr

A = 75*80m 2

C=0.15

Flow to Pit B = 30.9 L/sec


g 2V

D L

f h2

f

s / L12.312 / 4 / 225.057 .1VAQ 2

min8.075.175

V L

t

30.9L/s6000 )60601000(

12415.0Q

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Pipe AB

Pipe diameter 300mm

From Moodys diagram f=0.018

V = 1.14 m/sec

Q A=66 L/sec

Q AB > Q A

Pipe diameter is ok. D AB=300mm


s / L8.804 / 3.014.1VAQ 2 AB

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Pipes BE

Tc=23.1 min

I = 124 mm/sec

When D = 300mm Q = 80.8 L/sec

Pipe diameter is okay.

V = 1.81 m/sec

PIPE EF

tc=23.1+0.2 = 23.3 min from the parkland (longest time)

I= 123 mm/hr

Q EF =I(Aparkland Cparkland +Acarpark C carpark +AroadA C roadA +AroadE C roadE )

Q EF = 195 L/sec


72.1L/s )15008.0600015.0( )60601000(

124Q

min2.081.1

70V L

t

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11/20RMIT Universityyyyy School/Department/Area 11

Pipe System Design Hydraulic Models, Pipe Size & Limitations

1. There is steady flow in each pipe or link.2. Peak flow rates are derived from a hydrological model - Rational Model may be used

3. HGL runs along obvert of the pipe, pipe is flowing full but not under pressure

4. Depth of pipe - Depends on type of pipe, often set by defining a minimum cover for trafficload (900 mm is common)

5. Pipe sizes are determined by a pipe friction formula; Darcy-Weisbach formula is used with atrial diameter to find slope needed to convey flow rate already computed

Energy losses in pits: Pits generate turbulence that causes an energy loss and a drop in theHGL

Limiting Slopes, Velocities and Diameters: to prevent excessive sedimentdeposition and erosion, V min = 0.6 m/s and V max = 3 to 8 m/s are used; d min = 150to 225 mm; d/s pipes should never be smaller than upstream pipes

Design of Urban Drainage System

2

2 f

f

h f V S

L D g

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Pipe System Design Hydraulic Models, Pipe Size & Limitations


The normal method of calculation is to proceed from the upper reaches of thesystem to the receiving water, if the outlet pipe is then too low or too high itbecomes necessary to backtrack up the system and carry out alterations .

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Pipe System Design Hydraulic Models, Pipe Size

& Limitations

2

2

V g

1 Pipe A-B

2 Length L (m)

3 Q (L/s) 4 Trial Pipe Diameter

5 Pipe Velocity (m/s)

6 Velocity Head

7 U/S Surface Level

8 U/S Pit WL (mAHD)

9 Pit Loss Coefficient

10 Pit Head Loss

11 U/S Pit HGL

12 HGL Slope S f

13 Pipe Friction Loss

14 D/S Pit HGL

15 D/S Pit Surface Level

16 D/S Pit WL Limit

Row 2: these lengths are scaled off the plan of thearea.

Row 3: the flow rates for the pipes are those fromHydrological Design Sheet 2.

Row 4: normally smallest diameter allowed istried first.

Row 5: velocity is found by dividing Q, by thepipe area.

Row 6: V2/2g is computed, it is used in findingenergy loss due to turbulence in the pit andthat due to pipe friction.

Row 7: U/S surface level is read off the plan (say100m)Row 8: U/S Pit WL is set by subtracting the

freeboard of 0.15 m from surface level (100 -0.15); in most cases surface level is the gutterlevel at the pit inlet.

2

2

V g

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Pipe System Design Hydraulic Models, PipeSize & Limitations

Row 9: this is the coefficient that is multiplied byvelocity head to give head loss in the pit; it isfound from Figure 7.4.13 in RDM.

Row 10: head loss due to turbulence caused by pit,found by multiplying velocity head (Row 6)by Pit Loss Coefficient (Row 9).

Row 11: HGL at U/S end of the pipe, found bysubtracting head loss in the pit (Row 10)from the U/S pit water level (Row 8).

Row 12: this slope is required to convey Q = 48 l/sin the pipe of diameter 0.300 m calculated

from a pipe friction formula; assumingks=0.15 mm for concrete pipe, ks/D =0.15/300 and fully turbulent flow, theMoody Diagram, shows f = 0.018.

m/m01.017.0300.0018.0

g2V

Df

S2

f

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Pipe System Design Hydraulic Models,Pipe Size & Limitations

Row 13: this gives energy loss over thepipe length, S f L = 0.01 50 = 0.5 m.

Row 14: this shows HGL level at D/S pit(pit 2), it is found by subtracting

friction loss in the pipe from U/SHGL level and thus set the slope thepipe must have to convey water atthe velocity shown in Row 5 . Row11-Row 13

Row 15: the surface (or gutter) level atthe D/S pit, read off the plan.

Row 16: WL limit set by subtracting thefreeboard (0.15 m) from surface level.Lower of : (14) or(15 freeboard)

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& Limitations

Row 17: sets a value for invert level forhydraulic reasons, the pipe diameterbeing subtracted from HGL at U/S pit.

Row 18: gives a value for invert level based onrequired cover; pipes used here need 600mm above pipe crown, pipe thickness is0.03 m; Invert Level = 100 - 0.6 0.03 0.300 = 98.8 m.

Row 19: this invert level is derived bysubtracting from invert level of any U/Spipe entering the pit, the drop across thepit usually 30 mm, this drop is to prevent

sediment collecting in pit; as there is noU/S pipe entering pit 1 there is nocalculation for first pipe.

Row 20: adopted level is the lowest of thelevels found for different criteria in Rows17, 18 and Row 19.

1 Pipe A-B

4 Trial Pipe Diameter 0.300

7 U/S Surface Level 75.2

8 U/S Pit WL 75.0511 U/S Pit HGL 75.01

14 D/S Pit HGL 74.01

15 D/S Pit Surface Level 75.0

16 D/S Pit WL Limit 74.01

17 Hyd (Row 11 Row 4) 74.7

18 Cover (Row 7 cover) 74.3

19 U/S Pipe Invert Level - drop -

20 Adopt Invert Level LowestRow 17, 18, 19

74.3

21 Hyd [lower of Row 14 orRow16) Row 4] 73.7

22 Cover Row 15 - cover 74.1

23 Adopt Invert Level LowestRow 21, 22

73.7

24 Pipe Slope 0.012

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& Limitations

Row 21: an invert level at D/S end of the pipebased on the HGL consideration = D/SWL limit (Row 16) pipe diameter =99.35 0.300 = 98.4 m.

Row 22: an invert level at D/S end based oncover requirements for pipe = surfacelevel at D/S end (Row 15) cover pipethickness pipe diameter = 99.5 - 0.6 -0.03 0.300 = 98.4 m.

Row 23: adopted D/S invert level, the lower ofRows 21 & 22; note this value, with dropacross the pit floor, (usually 0.030 m) that

sets one of the U/S invert levels for thenext pipe; in this case 98.42 0.03 = 98.39m becomes Row 19 for next pipe 2-3.

Row 24: actual pipe slope used in drawingthe construction plans.

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Pipe System Design Hydraulic Models, PipeSize & Limitations

Inspection of the pipe friction losses will indicateif a pipe is too small and that a larger pipe isrequired.Example: the calculation regarding pipe 10-11shows a friction loss of 1.292 m for a 381 mm pipe;

accepting this value would lead to deeperexcavation for this and subsequent pipes thanwould be required for a larger pipe; usual practiceis to install pipes that have slope similar to naturalsurface slope.

Design process proceeds from U/S end to outlet;for checking purposes the process is reversed as

illustrated in Hydraulic Checking Sheet, Table14.16. The above calculations lend themselves to being

implemented on a spreadsheet and students areadvised to do this once they acquire goodunderstanding of the process.

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RMIT U i it S h l/D /A 20

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