Subsurface drainage systems are commonly used in areas where soil will not drain quickly on its own. Many agricultural fields require such systems to prevent crop flooding and allow work to continue. Highways and other paved areas often require subsurface drainage as well, in order to remove water that can accumulate under the surface after heavy rains.
Although plants require water to grow, they can be negatively effected when soil remains wet for too long. This is because plant roots require oxygen, and a soil that is saturated with water cannot hold as much air. Some types of agricultural soil, especially those that are rich in clay, can hold water for several days. A subsurface drainage system that removes excess water from a field within a day or two of a big storm can help to provide plants with an adequate amount of both water and oxygen.
An agricultural drainage system is usually composed of pipes or open channels that carry extra water out of the field to a drainage ditch or a natural waterway, such as a stream. Pipes placed under the soil surface have small holes in their walls that allow water from the soil to seep in and be carried away from the field. Drainage systems in agriculture are usually designed to work with the contours of the field, so that the force of gravity carries water away from areas that could become flooded. Systems can be designed to cover an entire field or to only drain water from problem areas.
On the surface, highways and pavements appear to dry quickly after a rain, but in actuality a good deal of water can be absorbed by these materials. Too much moisture in pavement can lead to defects, such as large cracks or potholes. Pavement sealants can be used to prevent water from being absorbed from the surface of a road, but moisture can also seep into pavements from wet soil underneath, a problem which subsurface drainage can alleviate.
Typically, there are a few layers under the asphalt or pavement that form a road's subsurface drainage system. Directly underneath the asphalt is a permeable base layer, which is made of a granular material that allows water to drain. Under this is a filter layer, which is often made of a geotextile — a type of fabric designed for use in soil. This filter prevents soil particles from clogging the base layer. Road drainage systems may also have edgedrains, or small pipes in the edge of the paved area that allow water to drain away from the surface.