What is the planning an agricultural subsurface drainage system?
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Levees are surface channels usually created on land with a gradient in such a manner that the soil removed to create these forms a levee on the downslope of these channels.
This helps the surface runoff not build so much velocity while flowing down a slope that it starts eroding the land. The banks or levees have to follow the lay of the slope and make the water flow down gently and not create make the lane below susceptible to landslips.
Ideally, such levees should have a spacing of 30 to 50 meters for slopes that possess a gradient of five to twelve degrees. Every levee can cater to an area of about 3 hectares with a total length that is less than 400 meters. Care must be taken to establish a grass cover immediately after creating a levee, or the channel could quickly erode.
Humps and Hollows
This is a system where a surface is shaped into parallel humps separated by hollows. This allows the humped shapes to shed excess moisture into the hollows which double up as shallow surface drains. This type of surface drainage is ideal for areas where tile or mole drainage is not possible on account of inadequate depth or fall of the soil.
The humps and hollows system creates a series of lateral surface drains that help discharge water into headland drains. One can use formulae to calculate the size of the drain taking into account the amount of water required to be removed.
The spacing between the humps may vary between 10 to 20 meters depending upon the speed with which the water needs to be removed. The greater the space between the humps, the slower will be the rate of discharge of water.
On the basis of dimensions, Open drains can be of three types:
These are only up to 300mm deep and can be created with the help of a hand shovel. They help remove water from shallow depressions and direct them to a larger drain or a stream. These are not suited for draining a large area of land and are more in the shape of a temporary arrangement.
Such drains are between 300mm to a meter deep and are created with the help of an excavator. These are typically V-shaped and flat at the bottom and a gradient that is steep enough to help water flow through quickly without damaging the drain walls or bottom. They are best suited for flat areas.
Large open drains can be several meters deep and wide and are created with the help of dragline excavators, bulldozers, or scrapers and are capable of evacuating large volumes of water.