Drainage

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The objective of drainage planning is to remove the excess moisture in the root zone within a specified time so that the soil returns to field capacity without any harm to the tea plants. Quick and safe disposal of water from the catchments is the basis of drainage design. Developing outlet will augment the rate of outflow and thereby reduce water logging. Without a proper outlet, drainage planning can not be successful and thus for an efficient drainage layout, development of outlet should be taken up first with the ultimate objective of lowering the water table below the root zone depth. Studies at Tocklai indicate that improved drainage can increase tea yields to the extent of 30-35% over a period of time. The water table midway between two drains should be maintained at 90 cm below the ground level and it should not rise above this level for more than two days after heavy rains.

Drainage investigation

A drainage index map of the area showing all topographical features and outlet conditions are essential for drainage planning. Fluctuation of water table during the rainy season should be recorded using observation wells and seepage pressure with piezometer. This information together with soil properties like texture and hydraulic conductivity as well as records of highest flood level are important in designing an effective drainage system.

Open ditch drainage system

Open drainage system consist of a net work of field drains, collector drains and the main drain which discharges into natural drainage channels such as hullahs, streams or rivers. They are advantageous for removing large volumes of water from tea fields with minimum cost.

Pipe drainage system

A pipe drainage system is a drainage network in which slotted PVC, corrugated plastic and cement-asbestos pipes, etc., are laid at 1.2 to 1.5 m depth to remove excess water from the root zone. Around the pipes, a layer of graded gravels is used as
filter material to prevent clogging of the slots or joints. The advantages of the pipe drains are (1) there is no loss of planting area, (2) maintenance cost is almost nil if properly laid, (3) effective in unstable sandy soils where open drains can not be maintained and (4) does not interfere with farming operations. However, the initial cost of pipe drainage is very high.

Specification of drains

The depth, shape, and bed gradients of drains suitable for tea plantations of North East India are given in the following table.

Drain
Depth (cm)
Bottom width (cm)

Min bed gradient (%)
Field drain
105
20 – 25
0.20
Collector drain
120
30 – 50
0.15
Main drain
150
50 – 100
0.10
       
  The drains should have sufficient batter (side slope) to ensure that their sides do not collapse. The suggested tentative spacing of drains for soils with different texture are shown below:  
Region
Loamy sand
Sandy loam
Loam
Silt loam
K=3.4m/d
K=1m/d
K=0.35m/d
K=0.35m/d
Assam
25 m
15 m
12 m
9 m
Dooars & Terai
18 m
12 m
9 m
6 m
         
* K is hydraulic conductivity (m/day)  

Contour or subsidiary drains on sloping lands

Safe disposal of excess rainfall should be coupled with good soil conservation measures which include construction of graded contour drains, use of grassed water ways, retaining walls etc. It is essential that adequate vertical intervals are provided between graded contour drains depending up on the % slope, soil type and rainfall as indicated below:

 
Rainfall

Vertical interval

Rainfall below 2500 mm [(Percentage slope/4) + 3] x 0.3048
Rainfall between 2500-4400 cm [(Percentage slope/4) + 2] x 0.3048
Rainfall above 4400 cm [(Percentage slope/4) + 1.75] x 0.3048
   
 

The above is applicable to medium texture soils (loams and silt loam). For other soil types suitable adjustments (-20% to +12%) are necessary.

Pump drainage

When outfall is inadequate, disposal rate of excess water by gravity is slow and sometimes may have even backflow during peak monsoon months. Pump drainage becomes necessary when the difference between the elevation of tea fields and the highest/average water level in the outfall is negligible.