For a symmetrical trapezoidal open drain in a landscape

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For a symmetrical trapezoidal open drain in a landscape with grass and loose rock, surface, the velocity of flow of water is ___ m/sec, (round off to two decimal places), given the following data.
Water edge width at the top = 750 mm
Water edge width at the bottom = 45000 mm
Water depth = 600 mm
Manning’s coefficient of roughness = 0.05
Slope along the drain = 1 in 250

1. 4.5
2. 0.45
3. 45
4. 1.5

Correct Option: B

Velocity of water (in m/sec) =	1	× r^{2 / 3} × S^{1 / 2}
	n

Here, n = 0.05

Hydraulic radius , r =	Wet area
	Wet perimeter

=	0.36	= 0.2134
	1.6869

S =	1	= 0.004 m
	250

Velocity =	1	× (0.2134)^{2 / 3} × (0.004)^{1 / 2} = 0.45 m / sec
	0.05