SOIL PREPARATION

TABLE 6.3: Guideline for spacing of pipe drains according to depth and texture (Afd. Landbou-ingenieurswese, 1979). Spacing of pipe drains depending on depth (m) 0.90 1.20 1.80 2.70 Sand 40 45 60 90 Loam 22 25 33 50 Clay 13 15 20 30 6.4.5.4 PIPE SIZES AND SLOPES The capacity of a drainage pipe, i.e. the maximum volume of water it can transport, is determined by its internal diameter, slope of the land and the material from which the pipe is manufactured. According to Reinders et al. (2016), plastic piping of 70-90 mm diameter is mostly used for laterals, while 110-200 mm is most common for collectors. The required drain pipe diameter can be calculated (Reinders et al. , 2016), or it can be determined using standard diagrams (Van Woerkom & Streutker, 1971). The importance of installing the correct pipe size is self-evident. Pipes that are unnecessarily large are a waste of money since large pipes increase the cost of the drainage system. Main collector drain pipe on the other hand must be able to convey all the water from the laterals. Considering all the economic and technical implications, it is advisable to obtain expert advice in deciding on drainage pipe sizes; in fact, professional help with the design of the entire drainage system is recommended. In order to maintain a water flow speed that will prevent silting up of pipes, the following minimum slopes are recommended for drainage pipes (Table 6.4): TABLE 6.4: Recommended slopes for drainage pipes of different diameters (Western Cape Department of Agriculture, 1998). Pipe diameter (mm) Minimum slope (%) 75 0.4 110 0.2 > 160 0.1 6.4.5.5 COMPUTER SIMULATION MODELS Formulas for the calculation of drain spacing and pipe sizing can be solved using spread sheets (Reinders et al. , 2016). There are also computer programs such as EnDrain (Oosterbaan, 2016) that can calculate the drainage water discharge as well as spacing between subsurface drains and depth of the water table Depth (m) Texture

100 | SOIL DRAINAGE