A proper Surface drainage system shapes and molds the land into a watershed that can convey the runoff into an organized drainage system comprising of trench drains, catch basins, and storm sewers. If the surface drainage is below par, it will have a dangerous impact on the subsurface drainage.
Surface drainage assists with the regulated removal of surface runoff on account of the irrigation, precipitation, spring thaw, irrigation, or anything else that leads to surface water accumulation.
If the overflow is not dealt in time, it may have an adverse impact on the land and surrounding structures. Improper planning may lead to erosion and sedimentation.
An effective surface drainage system is the only way to cope with heavy volumes of surface water accumulation after bouts of intense rainfall. The lack of one may overwhelm and saturate the capacity of the soil to absorb water, which could lead to extensive flooding and changes in landscape structure.
Types of Surface Drainage Systems
Some of the major drainage systems are:
- Open drains
- Humps and Hollows
- Levees
- Grassed waterways
1. Open Drains
On the basis of dimensions, Open drains can be of three types:
Shallow
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.
Medium Depth
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
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.
2. 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.
3. Levees
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.
4. Grassed Waterways
A grassed waterway is typically quite shallow and maybe narrow to a few meters in width. These can be used to regulate drain outflows going down slopes, so as to cheaply prevent erosion.
The ratio between the vertical and horizontal sections of the waterway needs to be in 1:4 proportion. A necessary prerequisite of such a waterway is a dense expanse of grass. The quantity of water to be evacuated, as well as the steepness of the slope, will have a bearing on both the size and the shape of the waterway.
The steeper the slope, the wider should be the waterway. The bottom of the waterway should be horizontal to enable the water to spread out evenly. It is important that such waterways not be constructed in areas prone to erosion, or they will fail.
FAQs
A Surface drainage system shapes and molds the land into a watershed that can convey the runoff into an organized drainage system comprising of trench drains, catch basins, and storm sewers
Some of the major drainage systems are Open drains, Humps and Hollows, Levees, and Grassed waterways.
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.
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