SOIL PREPARATION

CHAPTER 6

Interceptor drain

Wick of filter material to improve flow of groundwater to drain

1.8 m

Water table before drainage

1.8 m

Water table after drainage

Seepage area or spring

Groundwater flow

Impervious layer

FIGURE 6.3: Illustration of a cut-off drain and its effect on the water table both up-slope and down-slope of the drain pipe (redrawn after Webber & Jones, 1992).

Cut-off drains must be placed at least 300 mm deep into the impermeable layer over its entire length. It is advisable to construct a cut-off drain as an open drain initially, in order to get an indication of the flow of water that has to be removed. Based on the water flow rate, pipe diameters can then be calculated if permanent pipe drains are considered. Sometimes drainage (so-called ‘pull out’ drainage) is necessary at the lower end of a vineyard due to the barrier created between loose soil and the unprepared dense soil when the plough is raised at the end of a traverse (Saayman, 2018). Rain or irrigation water that moves laterally blocks up against the unploughed soil and the last vineyard row then gradually deteriorates due to waterlogging and/or salt accumulation. A cut-off drain or other forms of subsurface drainage can solve such a problem. 6.4.2 Open ditches Open ditches are a cheap and simple method of subsurface drainage (Figure 6.4). They can also serve the purpose of removing run-off during rain and irrigation as well as receiving the discharge from pipe drains. Open ditches have a high outflow capacity on condition that they are well maintained and that they do have the correct gradient. Although simple to construct, the side walls of

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