Man Made Catchments
Planning work for planting tea like land cleaning, planting and digging of drains etc. should be done catchment-wise. A minor natural catchment is then sub-divided into man-made catchment by constructing graded contour drains from ridges to water courses at intervals down the slope.
For making the graded contour drains on the tracing paper, first the vertical interval (V.I) should be calculated. The vertical interval between the contour drains is dependent on three factors, viz., the percentage slope, soil type and rainfall in that area.
The following formula can be used to calculate the vertical interval between graded contour drains:
To convert the result to metric units, the calculated V.I. in feet should be multiplied by 0.305 to get in meters.
These formulae apply to medium textured soils under the different rainfall conditions. As run-off will be more in fine textured soils and less in coarse textured soils, adjustments should be made for soil type. Adjustments to the calculated vertical intervals for soil type should be made as follows:
| Soil type | Adjustment |
| Coarse textured soil (Sand and loamy sands) | + 12 to + 20% |
| Moderately course(Sandy loam) | + 4 to + 12% |
| Medium texture(Loam and silt loam) | + 4 to –4% |
| Moderately fine texture(Clay loam, silty clay loam) | -4 to –12% |
| Fine texture(Silty clay and clays) | 12 to –12% |
After calculating the V.I. for different situations, the graded contour drains are marked on the plan. The positions of the drains which are always graded from ridge to water course are marked and the direction of flow is also indicated. The gradient of 1:500 is maintained to the drains when they are marked on the ground.
