SOIL PREPARATION

CHAPTER 6

between drain lines. Endrain is currently not in use in South Africa, but Reinders et al. (2016) foresee that this program can become a powerful tool for solving drainage problems in future. The DRAINMOD model, developed by the North Carolina State University, is a computer program that has proved its usefulness in analysing both the proposed and existing drainage scenarios. However, according to Reinders et al. (2016), its ease of use is hampered by the data requirements and by the rigorous calibration required for setting up the model. 6.4.5.6 MAINTENANCE Subsurface pipe drains have a long life. Life spans for pipe drains of more than 80 years are reported from Australia and even of 200 years from other countries (Webber & Jones, 1992). Some maintenance is, however, required occasionally. Furthermore, subsurface drainage is a costly investment and consequently a return on this investment, e.g. higher yields or sustainable use of the soil, must be obtained. Regular maintenance can safe-guard this investment. Carefully installed drainage systems normally require minimal maintenance after two years following installation (Table 6.5). The first year is the most critical period in its life. Ponding or excessively wet strips a day or two after heavy rain may indicate drain blockages while deep holes in the backfill are signs of the inrush of surface water due to broken drain pipes. Any section along a trench where the backfill has settled below the land surface should be filled in. Drain lines on sloping fields should also be checked for blowouts or washouts of the trenches. Blowouts occur when a drain pipe was broken or was separated, causing a restriction in water flow downstream of the blowout. TABLE 6.5: Recommended maintenance frequency of subsurface drainage systems (Reinders et al ., 2016). Age of drainage system (years) Maintenance frequency < 1 Once every 3 months 1-2 Once every 6 months > 2 Once every year Much can be learnt from the condition of a drainage system by inspecting the outflow. Excessive amounts of sand or silt deposits in the outflow pipe would indicate failure of the filter material, a loose connection, or the collapse of a drain. Cloudy water or an unusual colour of the water may indicate that surface water is entering the drain directly and not through the soil or through the filter material. Red or yellow sludge or heavy deposits of red crusts are

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