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Lesson 8 Special Drainage Situations 8. However, some agricultural areas are sloping. Hence, the question arises: can equations for flat lands be applied to sloping lands? When a hillside is drained by a series of parallel drains, the situation is as shown in Fig. The highest water table height h , above the drain level is now not midway between the drains, but is closer to the downslope drain.

Flow to parallel drains in a homogenous soil overlying a sloping impervious layer. Source: Ritzema, Schmidt and Luthin solved the hillside seepage problem of steady vertical recharge to parallel ditches penetrating to a sloping impervious layer.

This is in agreement with the results of Bouwer , who conducted a series of tests in sand-tank models, and the numerical simulations done by Fipps and Skaggs Because a vast majority of agricultural land will not have slopes more than 0. The above conclusion implies that we assume no difference in efficiency between drains laid parallel or perpendicular to the slope. Where the hydraulic conductivity of the soil is low, it could be advisable to lay the drains parallel to the contour lines, and hence perpendicular to the slope.

As the backfilled trenches have and retain a higher permeability than the original soil profile, any surface runoff may possibly be intercepted by the trenches.

Further discussion on the layout of subsurface drainage systems in sloping areas is given in Cavelaars et al. They can be of open type or buried pipe type.

In general, interceptor drains are used for two different purposes Ritzema, : a to intercept seepage water from neighbouring irrigation canals, and b to intercept foreign water that seeps down a hill. The first type of interceptor drains are often installed in irrigated areas parallel to and a short distance away from conveyance canals. To drain such areas, the interceptor drain is frequently installed on both sides of the waterway.

Typical interceptor drains in humid areas Fig. In irrigated regions, the seepage from an Fig. Common types of pipe drainage systems: a Natural or random; b Herringbone; c Gridiron; d Cutoff or Interceptor. Schwab et al. Source: Schwab et al. The flow towards such a drain is similar to the flow between drains with different water levels. If we assume that there is no recharge from precipitation, we can use the Dupuit Equation to calculate the flow per unit length. Problems of this type with special boundary conditions can be solved by using computers Schwab et al.

Flow towards an interceptor drain through a homogenous soil overlying a uniformly sloping layer. Source: Ritzema, The second type of interceptor or hill-side drainage is shown in Fig. Donnan presented a solution for this type of drainage. He assumed a homogeneous uniform soil layer on top of an impervious layer with a slope.

If an interceptor drain is constructed at the bottom of the hill at a height h0 above the impervious layer, the slope of the water table in the vicinity of the drain will no longer be parallel to the impervious layer, but will curve towards the drain. With a coordinate system as shown in Fig. Because of continuity, the flow with or without the interceptor drain must be equal.

Equation 8. The soil consists of a permeable layer, 6 m thick and has a hydraulic conductivity of 2. To control the water table in the area downhill from the irrigated area at a level of 2 m below the soil surface, an interceptor drain will be constructed. Calculate the required depth and capacity of the interceptor drain and the uphill elevation of the water table after the construction of the interceptor drain.

Calculation of an interceptor drain in a sloping area: A Before construction; B After construction. The elevation of the water table above the impervious layer before the construction of the interceptor drain can be calculated with using Eqn. Now, the elevation of the water table uphill from the interceptor drain can be calculated using Eqn.

Sometimes these wells are connected directly into a subsurface drain, thereby allowing the groundwater to flow by gravity rather than by pumping. Water table contour maps or piezometric level contour maps are often required prior to designing such a drainage system. The narrowest spacing applicable in practice is a matter of economics e. The hydraulic conductivity may be so low that no subsurface drainage with economically justifiable spacing is possible.

Under such conditions, we should use a surface drainage system of furrows and small ditches, possibly combined with bedding of the soil Ritzema, For moderate soil hydraulic conductivity, it may happen that the infiltration rate is too low for the water to enter the soil so that frequent surface ponding will occur. A suggested limit for the installation of a subsurface drainage system is that the infiltration rate of the soil must be such that the rainfall to be expected in two or three subsequent days must easily infiltrate during that time.

If not, a subsurface drainage system will not work satisfactorily and one has to resort to a surface drainage system Ritzema, Heavy clay soils of low hydraulic conductivity often have a top layer with a surprisingly high hydraulic conductivity because of the activity of plant roots or the presence of a tilled layer.

In such cases, rainfall will build up a perched water table on the layer just below the top layer Fig. Under such conditions, a subsurface drainage system can be effective because of the interflow in the permeable top layer, but it will only work as long as the backfilled trench remains more permeable than the original soil.

Perched water table built up in heavy clay soil, just below the top layer with a higher hydraulic conductivity. Source: Ritzema, Unless one can expect the hydraulic conductivity of the subsoil to increase with time e.

In fact, a system reaching just below the top layer should be sufficient. The system can be improved by filling the trench with coarse material or adding material like lime.

Further improvement can be sought in mole drainage, perpendicular to the subsurface lines. In none of the cases discussed above, it is possible to apply a drainage theory, because the exact flow paths of water are not known Ritzema, Also, these heavy soils often have a seasonal variation in hydraulic conductivity because of swelling and shrinking.

It is also known as a pipeless drain. Where soil conditions are favorable, mole drains function efficiently for the first few years and then gradually deteriorate. Mole drains have been successful in England, New Zealand, and several European countries.

Their maximum life is 10 to 30 years Schwab et al. Mole drains are unlined cylindrical channels of about 10 cm normal range is 5 to 10 cm in diameter, which are artificially created in the subsoil without digging a trench from the surface.

They are constructed at a depth shallower than one meter usually 40 to 60 cm to work as a subsurface drain by intercepting gravitational soil-water and conveying it towards the drain outlet. The mole drain outlets are protected by installing a rigid pipe for the last about 2 m length of the mole drain, protruding out sufficiently to permit a clear overfall of drainage effluent without disturbing the soil around the downstream end of the mole drain.

Mole drains are constructed using a mole plough Fig. The bullet is attached to the plough and creates a cylindrical channel as the mole plough is pulled forward by a tractor. In order to have stable mole drains, the soil must be cohesive, with a greater percentage of clay Jha and Koga, Schematic diagram of mole drainage: a Cross-section of a mole channel showing cracks around it; b Profile of a mole plough.

The soil moisture content at the time of moling plays an important role in crack formation, mole channel stability, and power requirement to pull the mole plough.

If the soil is too wet, cracks may not develop properly, the internal mole surface will get smeared and will not permit adequate transfer of gravitational water in the soil profile across the mole surface, and the mole channel may collapse rapidly.

On the other hand, if the soil is too dry, cracks will develop but the power requirement will be too large and the mole may also collapse due to the dislodgement of relatively drier soil particles at the inner mole surface.

The appropriate moisture content to construct mole drains is judged from experience. A general guideline in this respect is that the soil moisture content should be between the Liquid Limit and the Plastic Limit Bhattacharya and Michael, Besides the moisture content, the bulk density of the soil is also an important factor governing the power requirement and the crack formation during moling.

If the soil above the mole drain is less pervious, subsoiling helps in developing wider path for the shallow soil water to flow towards the mole drain. Some of these soils are suitable for mole drainage. An excellent review of the theory and practice of mole drainage is presented by Jha and Koga , while a case study on the performance evaluation of mole drainage in Bangkok clay soils is reported in Jha and Koga References Bhattacharya, A.

Land Drainage: Principles, Methods and Applications. Konark Publishers Pvt. Bouwer, H. Tile drainage of sloping fields. Agricultural Engineering Cavelaars, J. Subsurface Drainage Systems. In: H. Donnan, W. Drainage of agricultural land using interceptor lines. Fipps, G. Influence of slope on subsurface drainage of hillsides.

Water Resources Research, 25 7 : Jha, M. Mole drainage: Prospective drainage solution to Bangkok clay soils. Agricultural Water Management, 28 3 : Design and practice of pipeless drainage systems: A review.

Ritzema, H. Subsurface Flow to Drains. Schmidt, P. The drainage of sloping lands.



When the conditions are so created that the crop root-zone gets deprived of proper aeration due to the presence of excessive moisture or water content, the tract is said to be waterlogged. The water logging affects the land in various ways. The various after effects are the following: 1. Growth of Water Loving Wild Plants 3. Impossibility of Tillage Operations 4. Accumulation of Harmful Salts 5. Lowering of Soil Temperature 6.



Subsurface drainage in humid regions Subsurface drainage in arid regions Construction of drains Related Primary Literature Additional Reading The removal of water from the surface of the land and the control of the shallow ground water table improves the soil as a medium for plant growth. The sources of excess water may be precipitation, snowmelt, irrigation water, overland flow or underground seepage from adjacent areas, artesian flow from deep aquifers, floodwater from channels, or water applied for such special purposes as leaching salts from the soil or for achieving temperature control. The content above is only an excerpt. You may already have access to this content. Sign In Get AccessScience for your institution. Subscribe To learn more about subscribing to AccessScience, or to request a no-risk trial of this award-winning scientific reference for your institution, fill in your information and a member of our Sales Team will contact you as soon as possible. Recommend AccessScience to your librarian.


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