Some Notes Drainage Design Procedure

In these notes on drainage design procedure, three drain spacing formulae are considered suitable for designing sub-surface drainage systems. The Hooghoudt and the Kirkham-Toksoz formulae have been developed for steady state conditions and the Glover formula is applicable to the transient state condition. The steady state condition implies that the surface inflow is equal to the outflow through the drains and the water table remains static, whereas in the transient state the water table level changes with time. These formulae are mainly applicable to the design of tube drains, but Hooghoudt's formula can be used for designing open drains. The various formulae can be solved directly, but graphs have been included to simplify the solution of the different equations. Sub-surface Drainage This short article will deal mainly with the use of design formulae for planning sub-surface drainage. This form of drainage is frequently used to control the level of the ground water table in cultivated lands; although ground water control is applicable to other purposes as well, this article is confined to its agricultural application. The three main functions of sub-surface drainage are(a) removal of excess ground water in the root zone of the crop plants during the growing season, (b) maintaining the water table below ground level throughout the year, particularly in reclamation areas, (c) reducing the levels of salinity and maintaining salinity levels in certain circumstances. Some Misconceptions Regarding Drainage Drainage is often thought of as a problem rather than as an integral part of irrigation design. In the past irrigation layouts were casually designed without any attention being given to the need for drainage. 'This was due to a number of factors, but mainly to an inadequate understanding of the basic procedures involved in drainage design. Economic factors accounted for many of the present problems. The cost of including drainage facilities in an irrigation scheme would increase the capital costs, while reference to drainage works would by implication suggest the existence of a problem-a feature that might put paid to the scheme. In fact, however, the cost of excluding drainage facilities in an irrigation project is courting disaster, however sound the design of the irrigation project. Furthermore, it will cost a great deal more to provide drainage at a later date, if in fact it is still possible to install drains. A second misconception appears to be associated with applying drainage design functions. The various design functions have been developed from exact mathematical analysis, electric analog techniques (Vimoke and Taylor, 1962)14 and various model analyses (Harding and Wood, 1942)2 and they are universally applicable. Naturally, the variables have to be determined for local conditions, but this is not a difficult problem, provided the basic principles from which the formulae have been derived are fully understood. A Review of Drainage Formulae A great deal of research work has been carried out in developing the various drain-spacing formulae. Soil-water relationships have been extensively studied in the U.S.A. by Kirkham, van Schilfgaarde and Luthin; in the Netherlands by Hooghoudt, Ernst, Visser, Wind and others, and in Denmark by Engelund. Drainage of large areas for various irrigation projects in the U.S.A. and in Australia, and the perennial drainage problems encountered in Holland, have made it necessary for economic design factors to be developed for drainage work. The relationship of the depth of the drain to the spacing naturally must have economic implications, as has the depth at which the water table must be maintained for successful plant growth and for the prevention of surface accumulation of salts. Both these aspects are of concern in relation to crop production in this country. Some Basic Concepts of Groundwater Movement into Drains A ilumber of basic concepts have to be considered before the actual formulations are discussed. Firstly, the two basic ideas of water movement into drains need to be stated. These are: (a) Steady state conditions In the steady state condition, it is assumed that the hydraulic head does not vary with time. I t is also assumed that the height of the water table is static when water is applied to the surface and that flow into the drains is equal to the amount of irrigation or rain water flowing through the soil surface. (b) Transient state conditions In this state, it is assumed that the hydraulic head varies with time. It is assumed that the water that has saturated the soil profile will continue to flow into the drains at a constant rate as the water table falls from one level to the other. Hooghoudt (1940)3 has developed formulations for the steady state condition based on the DupuitForchheimer assumption, which is used to simplify ' 1'90 Proceedings o f The Sout .h African Sugar Technologists' Association April 1968 the mathematical analysis in the development of the spacing equations. Kirkham (1949)4 has also developed similar equations for steady state conditions. Glover (as reported by Dumm, 1954)' developed an equation for the transient state condition. Secondly, there are two soil conditions to be considered: (a) Isotropic soils A soil may be isotropic if the texture does not vary to any great extent throughout the profile. Furthermore the hydraulic conductivity does not vary throughout the profile, that is the vertical hydraulic conductivity is equal to the horizontal conductivity. ( h ) Anisotropic soils Anisotropic soils are generally considered to be "micro-layered", but for drainage work the feature of importance is that the vertical conductivity is not equal to the horizontal conductivity. (c) Deplh to impervious layer Soils are further classed as "infinitely" deep or of "finite" depth. The limits may be an impervious rock layer or a layer of clay. If a clay layer is present. it will be defined as impervious, provided the hydraulic conductivity is less than onetenth of the conductivity of the layers above it. The depth to an impervious layer is particularly important as it has a very definite bearing on the spacing and performance of a drainage system. Topographic Consideratio~is Drainage is usually employed on fairly level land, but in certain circumstances it may be necessary to drain sloping land. When this happens one can run into difficulty with drain-spacing formulae. These formulae cannot be applied to determine the correct depth or spacing when drains are run along the contour. They apply only to spaced drains which run down the slope, at a gradient sufficient to con,vey the water at design velocities. Work is therefore being carried out in the U.S.A. to develop suitable formulations based on model studies applicable to sloping land. Luthin and Schmidt (1967)' have developed. a formula, but it is accurate only for slopes up to about 10% and further model studies are being carried out to develop a suitable formula for sloping lands greater than 10%.