Drainage field (field drainage) is a system that receives more direct water from agricultural land and distribute it to the main drainage systems that remove water from the area of agricultural land. The main drainage system should provide an outlet that is free and reliable for water discharge from the drainage field. In an underground drainage system can distinguish three categories namely lateral drainage, collector and main drainage.
Lateral also called drainage field or farm drains or suction drains that serve to control the depth of the groundwater fluctuations in agricultural land, it also serves as a collecting runoff. Of lateral water flows to the collector who transport it to the main drainage channels. The drainage system field can consist of (a) the open drainage ditches, (b) drainage mole, which is a hole underground, (c) drainage pipes, both made of clay, concrete or plastic pipes embedded underground.
If the pipes lateral ends on the collector channel, then the system is called a singular pipe drainage system. If the collector pipe also made of pipe, then the system is called a composite pipe drainage system.
a. drainage Ditch
Compared with pipe drainage system, drainage ditch has several advantages and disadvantages. Some of the benefits are (1) to dispose of ground water, (2) remove the water surface, (3) the slope of the channel to drain the water is usually smaller than the slope required on the drainage pipe. Generally for the trench slope is about 0.01%, while the pipeline of around 0.1%, (4) facilitate the control and maintenance.
Some disadvantages drainage ditch is (1) a reduction in the area of land cultivated for agriculture for their trenches, (2) the growth of weeds and precipitation causing high cost of maintenance, (3) land that is separated by the trenches will lead to the difficulty of the operation of the tools mechanical. Generally, in a flat area, the drainage system using pipes as lateral and trenches as a collector.
While in the area throughout the sloping field drainage system both laterally and collectors are made from a pipe known as a composite pipe drainage system. However, in the following situations usually ditch more suitable for use as laterally, namely:
1) If the ground water level can be controlled with a lateral distance is quite wide, so the plot of land that formed wide enough not reduce the efficiency of the use of a mechanical device. This situation can occur on soils with high infiltration.
2) If the drainage system should also be able to transport the water surface, for example on soils with low infiltration rate or in areas with high rainfall intensity. If desired acceleration of the process of maturation on alluvial newly reclaimed land.
3) If only the desired shallow ground water level, eg for pasture or peat.
b. Size Drainage Ditch
If the drainage system used is the lateral trench, then the calculation of distance and depth should be considered. For collectors, trench spacing is determined by the size of the land or the maximum length of the drainage pipe. On flat land with a singular system of drainage pipes, trench spacing is usually between 200-500 m. Water level in the trench collectors must be maintained at a depth below the drain holes of the drainage pipe (lateral).
Calculating the dimensions of the trenches following the channel design is not plated by knowing parameters such as, (1) the desired water level, (2) the discharge capacity, and (3) the type of soil. Sometimes calculating the dimensions of the trench produces a dimension that is too small, so in terms of construction and maintenance difficult to do.