rational method (ex)

TarmiziIsmail 1 Q = Peak flow (m 3 /s); C = Runoff coefficient (Table 2.5 MSMA 2 nd Ed) i = Average rainfall intensity (mm/hr) A = Drainage area (ha). 360 A i C Q = Assumptions used in the Rational Method are as follows: • The peak flow occurs when the entire catchment is contributing to the flow; • The rainfall intensity is the uniform over the entire catchment area; and • The rainfall intensity is uniform over a time duration equal to the time of concentration, t The Rational Method is not recommended for use where: • The catchment area is greater than 80 ha (TxDOT, 2009); • Ponding of stormwater in the catchment might affect peak discharge; and • The design and operation of large and more costly drainage facilities are to be undertaken, particularly if they involve storage.

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TarmiziIsmail 1

Q = Peak flow (m3/s);C = Runoff coefficient (Table 2.5 MSMA 2nd Ed)i = Average rainfall intensity (mm/hr)A = Drainage area (ha).360

AiCQ =Assumptions used in the Rational Method

are as follows: The peak flow occurs when the entire

catchment is contributing to the flow; The rainfall intensity is the uniform over

the entire catchment area; and The rainfall intensity is uniform over a

time duration equal to the time of concentration, t

The Rational Method is not recommended for use where:

The catchment area is greater than 80 ha (TxDOT, 2009);

Ponding of stormwater in the catchment might affect peak discharge; and

The design and operation of large and more costly drainage facilities are to be undertaken, particularly if they involve storage.
TarmiziIsmail 2

D1

D2

D3

A1

A2

A3
TarmiziIsmail 3

A1 = 1.4HaC1 = 0.9td1 = 7.8minto1 = 4.3min

D1

D2

D3

tc = 12.1min

A1 = 1.4HaC1 = 0.9ttd12d12 = 9.2min= 9.2minto12 = 4.3min

A2 = 1.12HaC2 = 0.9td2 = 5.0minto2 = 4.8min

tc = 13.5min

tc = 9.8min

Q = 0.59

Q = 0.56

Q = 0.54

Q = 1.10Q = 1.10

A2 = 1.12HaC2 = 0.9ttd23d23 = 6.5min= 6.5minto23 = 4.8min

A3 = 1.04HaC3 = 0.9td3 = 5.1minto3 = 4.8min

tc = 11.3min

tc = 9.9min

Q = 0.53

Q = 0.49

Q = 1.54Q = 1.54

A1 = 1.4HaC1 = 0.9ttd13d13 = 10.6min= 10.6minto13 = 4.3min

tc = 14.9min Q = 0.51
TarmiziIsmail 4

C1A1t01 = 5mintd1 = 3minC2

A2t02 = 2mintd2 = 4min

C3A3t03 = 5mintd3 = 7min

C4A4t04 = 1mintd4 = 4min

C5A5t05 = 2mintd5 = 8min

( )( )360

2145@21

2211 AAiAAACAC

Qt +

++

=+=

( )( )360

4321745@4321

44332211 AAAAiAAAA

ACACACAC

Qt +++

++++++

=++=

( )( )360

543218745@54321

5544332211 AAAAAiAAAAA

ACACACACAC

Qt ++++

++++++++

=+++=
TarmiziIsmail 5

PROBLEMPROBLEM

Using Rational Method procedure to calculate a 20 yrar ARI peak discharge from a subcatchment area of 40.7ha in Wangsa Maju, Kuala Lumpur. Also develop the runoff hydrograph using the RHM for drain AB on 5 and 10 min durations design storms
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