110204 Lecture on Drainage Design Rational Formula

8/7/2019 110204 Lecture on Drainage Design Rational Formula

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Drainage Design and Analysis

Using the Rational Method

Hands-On Training

9 February 2011


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The Rational Formula

The Rational Method was introduced in 1889

in England

Its genesis was based on pure reasoning, thusthe name

The Rational Method is used to compute the

peak runoff after a rainfall event

The Rational Method is applicable to small

drainage areas (2,000 hectares)


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The Rational Formula is given by:

Q = C*I*A/360

Where Q = Discharge (cubic meters per second) C = Coefficient of Runoff

I = Rainfall Intensity (millimeters per hour)

A = Drainage Area (hectares)

The Rainfall Intensity will depend on the Time ofConcentration and on the Rainfall IntensityFrequency Curve


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Time of Concentration

Tc = (L1.15)/(51*H0.385)

Where Tc = Time of Concentration (minutes)

L = Length of watershed along the mainstream(meters)

H = Difference in elevation between the mostdistant ridge in the watershed and point ofinterest under review (meters)


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Road Alignment


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Principal

Waterways


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Roadway

Crossings


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Direction

of

Waterflow


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Ridge

Lines

Point of Interest


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Point of InterestElev.

1180

Elev

1540


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Rational Formula

Time of Concentration

Tc = (L1.15)/(51*H0.385)

L = 2,572 meters H = 1540 - 1180 = 360 meters

Tc = 33.708 minutes


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Use 10 year frequency for pipe culvert


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Use C = 0.3 (gravel surface road and shoulder)


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Rational Formula

Discharge

Q = (A*C*I)/360

A = 134.73 hectares C = 0.3

I = 250 millimeters per hour

Q = 28.29 cubic meters per second


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Install:

4-1520 mm RCPC

Capacity = 4*8

= 32 m3/sec


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Exercise

Which of the following most nearly gives the DrainageArea, in hectaresa. 54 b. 41

Which of the following most nearly gives the length ofwatershed along the mainstream, in metersa. 750 b. 2,400 c.

Which of the following most nearly gives the highestelevation of the most distant ridge, in meters

a. 1,130 b. 1,020 c. 1,26 Which of the following most nearly gives the lowest

elevation at the point of interest, in metersa. 1,130 b. 1,020 c. 1,2


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Exercise

Which of the following most nearly gives theTime of concentration, in minutes

a. 7.5 b. 24.8

Which of the following most nearly gives theRainfall intensity (using frequency of 10 years), inmillimeters per hour

a. 250 b. 63

Which of the following most nearly gives theDischarge, in cubic meters per second

a. 2.8 b. 23.6

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110204 Lecture on Drainage Design Rational Formula