Subsurface drainage

classification of subsurface drainage

1. welcome

2. Kelappaji College of  Agricultural Engineering & Technology Kerala Agricultural University, Tavanur-679 573 Chethan B J MTech(soil and water engineering)

3. Classification of subsurface drainage

4. Relief drainage When the system in a waterlogged area ,removal of the excess water and gives relief to the land, this system is called relief drainage. It may be open or closed(buried) type. These drains are used where the groundwater does not flow appreciable and where it is impossible to intercept the flow effectively. This is system used where land having flat with low gradient. Relief drains are usually planned in series in a gridiron or herring bone

5. Open ditches in relief drainage system They are used for large areas which have mild slope or completely slope. Open ditches can carry both surface and subsurface water. It is constructed with higher depth to serve and therefore are capable of carrying wide range of flow.

6. Buried drains in relief drainage system Any conduit buried in the soil and allowing collection of drainage water perforations or open joints and disposal of the same through the outlet is called buried drain. The drain may be clay tiles, concrete, metal, PVC pipes.

7. Interceptor drainage If the system intercepts the water coming from other areas and divert it suitably to save a certain part of the area getting from waterlogged ,then it is called interceptor drainage. The drains are placed perpendicular to the direction of ground water flow. Interceptor drains are very effective in controlling the water table for a considerable distance below or downslope from the drain, but do not have any appreciable influence on the upslope region. Extensive ground water studies are required before deciding about location and depth of ditches and drains. These system are recommended where there is substantial ground water flow from a source like canal,reservoir,or irrigated areas at higher elevations. It may be closed or open type

8. Open ditches in interception drainage system This system is same as that of open ditches in relief drains where function is to carry both surface and subsurface water. It is constructed at the base of the slope to intercept the surface and groundwater flow.

9. Buried drains in interception drainage system The high water table occur locally due to unusual subsurface formations or ground water conditions or changes in topographic features. This condition is occur in the large stream valleys or in the gradient is more. In such area seepage from uplands is more. If open ditches are constructed to intercept the flow ,it may be damaged by flood flows, causing erosion or channel change.

10. Subsurface drainage methods Tile drains Mole drains Drainage wells Deep open drains Combination drainage system

11. 1.Tile drains Tile drainage are a subsurface drainage method consist of short length pipes (30-90cm)installed at particular depth from land surface. The pipes are made of concrete or burnt clay. After digging the trench to the desired depth the pipes are held end to end without any jointing. They are covered with an envelope material in certain cases and soil is backfilled. Water enters the tile drains through the opening available between the pipes. A network of tile line is laid with a grade will remove the subsurface water.

12. 1.1Elements of tile drain system Consists of : Layout of the system Depth and spacing of the drain Size and grade of the tile lines Material of tiles Envelope materials and accessory structures Installation of tile drains

13. 1.11 Considerations for the selection of a layout Matching of layout of (subsurface drainage) with irrigation and road layout. The layout has to "fit" into the layout of the existing infrastructure. The level of the outlet of the subsurface drainage system (determined by the drain depth, lengths and slopes) has to be at or above the highest water level in the main drain system. Pumping of the system. Available drainage materials. Installation equipment. Subsoil conditions.

14. Different layouts of tile drainage system: Natural system/Random system Parallel system Herringbone system Grid iron system

15. 1.11 Layout of tile drainage system Drainage system comprises of laterals, main drains and an outlet Laterals Main drains Outlet

16. Random system This type used where there are scattered wet areas in a field somewhat isolate from each other Tile line are laid more or less at random to drain these wet areas In most cases the tile main follows the largest natural depression in the field, and submains and laterals extend to the individual wet areas.

17. Herringbone system Consist of parallel laterals that enter the main at an angle ,usually from both side. This system is used where main and submain lines in a narrow depression. This system is used where lateral are long and area requires thorough drainage.

18. Grid iron system Similar to herringbone except that laterals enter the main from only one side It is used on flat regularly shaped fields and on uniform soils It is more economical than herringbone because the number of junction and double drained areas are reduced

19. b) Parallel system: In this system the laterals are located perpendicular to main drain. Flat land with regular shape and with soils of uniform permeability can be drained by this method.

20. 1.12 . Depth and spacing of tile drains Depth and spacing are closely inter related Depends on: Texture of soil Hydraulic conductivity Types of crops grown Extend of surface drainage

21. Outlet conditions Topography of land Salt content of soil Agronomic practices Drain depth: From ground surface to the bottom of the tile, usually 1-1.5m Tiles are placed above impermeable layer

22. Deeper the drain, wider the spacing, lesser the number of drains required. Drain spacing- ’ Hooghouts drain spacing formula’ S 2 = 4K/R [H 2 -2hd+2Hd-h 2 ] where, d- Depth to the impermeable layer from the drain bottom h- Height of water in the drain H-Height of water in midway between 2 drains S- Drain spacing D-Distance from the impermeable layer to the maximum height of water between the drains K- Hydraulic conductivity R- Replenishment rate

23. 1.13 Size and grade of the tile drains Drains are designed based on Manning's formula Drains are laid on longitudinal slope: 0.05-3% Working grade: 0.2% Size of tiles pipes: 30-90cm long,10-15cm diameter

24. Diameter of tile drains, d = 5.3(D c ) 0.375 A 0.375 S -0.1875 where, d- Internal diameter of tile drain, cm D c – Drainage coefficient, cm/day A-Drainage area, ha S- Hydraulic gradient or tile grade, m/m

25. 1.14 Material of tiles Clay, Concrete, PVC/Plastic pipes, Bituminous fiber or steel Pipe material should be: Resistant to weathering and freezing Have high density Have sufficient strength to withstand static and dynamic loads Have uniformity in shape and wall thickness

26. 1.5. Envelope materials and accessory structures Envelope materials: Materials that cover the drains placed in less pervious strata Gravel, coarse sand etc. Prevents inflow of soil into the drains Increases effective drain diameter

27. Coarsest material is placed immediately over the tile Minimum thickness of envelope: 7.5cm

28. Recommendations for gravel envelope (USBR): For uniform soils, D 50 of envelope/D 50 of soil = 5 to 10 For graded soils, D 50 of envelope/ D 50 of soil = 12 to 58

29. Manholes and sedimentation basins : Vertical structures installed at regular intervals along the tile lines. Constructed by concrete or brick masonry. Helps in cleaning and inspection A man can enter and work within it Placed about 60 cm below ground surface Covered by concrete block

30. Inlet to tile drains: Allows water into the drain Two types: Blind inlet- cheaper but chances of clogging Surface inlet- have provisions to prevent the trash entering into the drains but costly

31. Fig: Blind inlet Fig: Surface inlet

32. Outlet for drains: Water from tile drains are discharged into big size surface drains Either by gravity or pumping: Gravity outlet- invert level of tile drain is higher than fully supply level of surface drain Pump outlet- bed level of the outlet is higher tan the discharging tile drain

33. 1.6 Installation of tile drains: Consists of: Excavation of a trench Laying the tile at predetermined grade, depth and spacing Putting the envelope material and backfilling the soil Installation of tile should start from outlet

34. 2.Mole drainage systems A mole drain is an unlined underground drainage channel, formed by pulling a solid object, usually a solid cylinder with a wedge-shaped point at the end, through the soil at the proper slope and depth, without a trench having to be dug. Mole drains are produced in the subsoil by means of mole plough. These are pipe less drains. Mole drainage is applied only under very specific conditions, mainly in stable clayey soils. The effect of mole drainage is a rapid removal of excess water from the surface layers, rather than at controlling the water table as such.

35. The mole drains have a life span of only a few years(usually 10-15 years) and have to be renewed frequently. Depth of mole drain vary from 45-120cm depending upon moling equipment The length of mole drain is decided by length of affected area, soil type, availability of outlet etc. Safe length vary from 20-80 metre. If soil condition permits, the maximum length can be about 200 m. Smaller diameter have longer life than larger diameter. Diameter varies from 7.5 to 15 cm. Mole drains are closer spacing than tile drains. A spacing of 2 to 5 metres is usually followed.

36. Safe gradient of 0.2-3% are given. Too low a velocity prolongs saturation of the drain and high velocity causes erosion. Stagnation of water in the mole drain will weaken the walls ,leading to deterioration of the channel. Mole drain finally discharges into an open ditch and therefore, last portion of the mole should be provided with the pipe.

37. Mole drainage systems

38. Mole outlet protection comprises of a 1-2m long pipe inserted into the mole channel

39. Clean gravel of size 3-5mm are suitable. Ideal time for laying mole drain is soon after harvest of the crop. Extensively practiced in England, Western Europe and New Zealand.

40. 2.1 Factors affecting life mole drains Structural stability of subsoil. Amount and intensity of rainfall. Temperature variations. Method of forming mole drain. soil moisture content at the time of formation of mole drain. Diameter of drain.

41. 3.Drainage wells It is also called as vertical drainage. A tube well drainage system consists of a network of tube wells to lower the water table, including provisions for running the pumps, and surface drains to dispose of the excess water. Tube well drainage is used in areas with a high soil permeability and preferably fresh groundwater that can be reused for irrigation. The system is operation and maintenance intensive and requires a continuous diesel or electrical power supply.

42. 3.1 Drainage by wells is feasible only under certain condition as mentioned below Aquifer condition: the area to be drain should be underlain by an aquifer, pumping from which will adequately lower the water table. Water quality: the underground water should be of satisfactory quality. Thus the pumped water could be used for irrigation. Soil condition: there should be any intervening layers of low hydraulic conductivity impending the movement of water.

43. 3.2 Multiple well system Multiple wells are installed in a common area of influence. Cone of depressions overlap each other. Results in increased drawdown in each well. Provides more drainage effect. Wells are arranged in various patterns. Isolated groups or continuous pattern. Suction lines of each well is connected to a common pump.

44. Fig: Multiple well system

45. 4.Deep open drains It is used for subsurface drainage. It uses interceptor drains commonly as their length required is generally less than relief drains. The capacity of interception drain estimated using Darcy’s law. Land area required need constant maintenance

46. 5. Combination of surface and subsurface drains system Transport of drainage over long distances is done by open drains. A combination of open drain and tile drain has to be used for the effective drainage of an area. Tile drain are initially costly but they do not take away from the cultivation. Combination of vertical drainage and surface drainage also used for effective drainage.

47. Disadvantages of subsurface drains Require high initial cost. Requires steeper gradient. Repair works are costly and inconvenient. Only seepage water is removed. Construction is difficult.