SOIL PREPARATION

CHAPTER 6

6.4 Subsurface Drainage Excess water drains naturally in soil in two ways, namely through: i) Deep percolation: Water moves downwards beyond the root zone. ii) Lateral down-slope drainage: Water that drains from the upper slopes will eventually reach low lying level ground or soil that is less permeable, causing a high water table or even seepage at the surface. When natural drainage is good, water tables will not develop, but the opposite is also true, and then artificial drainage will be necessary. Areas with a high water table that can either be localised or extensive often occur due to their position in the landscape, e.g. it can be low-lying or next to a river. In such cases, the lateral movement of free water is too slow for the use of cut-off drains and a network of pipe drains must be installed at calculated depths and spacings. Soils shallower than 800 mm are normally not drained. The maximum depth of the drainage system will be determined by the height of the water level in the river or other waterways where the drainage water is discharged. If the water level of the river or ditch that collects the drainage water is high, excavation of these waterways can be considered to increase the gradient from the drainage pipes to the outlets. Examples of various types of subsurface drainage systems appear in Figure 6.2.

Contours

Wet Patches

Local drains

Collection drains

Collection drains

Collection drains

FIGURE 6.2: Examples of layouts used for sub-surface drainage (redrawn after Reinders et al ., 2016).

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