what is minimum gradient in transportation engineering

 Minimum gradient

This is important only at locations where surface drainage is important. Camber will take care of the lateral drainage .but the longitudinal drainage along the side drains require some slope for smooth flow of water .therefore minimum gradient is provided for drainage purpose and it depends on rain fall, type of soil and other site conditions. A minimum of 1 in 500 may be sufficient for concrete drain and 1 in 200 for open soil drains are found to give satisfactory performance.

Terrain	Ruling gradient	Limiting gradient	Experimental gradient
Plain or rolling	3.3 per cent (1 in 30)	5 per cent (1 in 20)	6.7 per cent (1 in 15)
Mountainous terrain, and steep terrain having elevation more than 3,000 m above the mean sea level	5 per cent (1 in 20)	6 per cent (1 in 16.7)	7 per cent (1 in 14.3)
Steep terrain upto 3,000 m height above mean sea level	6 per cent (1 in 16.7 )	7 Percent ( 1 in 14.3 )	8 Percent ( 1 in 2.5 )

Suppose the road is with zero gradient passing through level land and open side drains are provided with a gradient of say 1 in 300. It may then be necessary to deepen the downstream end of the drain by about 3.3 m for one kilometre length of road. This course is not possible from practical considerations. Hence it is desirable to have a certain minimum gradient on roads from drainage point of view, provided topography favours this. The minimum gradient would depend on rain-fall, run-off , type of soil, topography  and site conditions.

A minimum gradient of about 1 in 500 may be sufficient to drain water in concrete drains and gutter , but on inferior surfaces of drains a slope of 1 in 200 or 0.5 percent may be needed where as on kutcha open drains ( soil drains ) steeper slopes up to 1 in 100 or 1.0 percent may be needed.