Division of catchments is based on the very simple principle that water always flows downhill at right angles to the contours. On the survey map, it is seen that in some areas contour lines are convex while in other areas contour lines are concave to the fall. Where the contour lines are convex to the fall, then the movement of water over such area will be away from the apex of the curve; on the other hand, where the lines are concave to the fall, water will collect in the apex of the curve and flow downhill in a line connecting each apex. Similarly, a line connecting the apices of the convex curves is a crest or ridge. The area between two ridges, therefore, forms a catchment because water tends to flow inwards away from the two ridges until it meets at the apices of the concave curve of the contours. Thus, a line drawn connecting the apices of the concave curves is the natural water channel for the area between two ridges and forms the main drainage channel for this catchment.
Identification of catchment
Based on this principle, major and minor natural catchments within the boundaries of the proposed tea plantation can be identified.
After receiving the survey map, planning on a catchment basis can be commenced. It is advisable to avoid drawing on the contour map and, therefore, it is suggested that the plan be made on the tracing paper which is fixed on the contour map.
First, the permanent features of the area, boundaries, building, public road, river etc. should be traced. Thereafter, the major and minor catchments should be identified marking the crest and natural water courses using the contour lines as the guide. In topographical planning the first factor to consider is access or road. The roads should follow the ridges/crests because if roads are constructed along the ridges, they have easy and direct access and pose no problem for water disposal.
After preparing the plan it should be examined carefully to see if complete road access has been achieved. If more roads are considered necessary then these should be aligned from the main ridges to main water disposal channels at right angles to the contours.
For plucking access, paths will have to be made that run down the slope across the contour planted tea and drains. These paths could become uncontrolled water channels, which though undesirable, are necessary. In order to reduce leaf damage in plucking baskets/bags, it is essential that the leaves should be emptied into large baskets. When planning, it is therefore, necessary to provide such plucking paths after every 75-110 meter or hedge.
On flat land, the topographical features of minor crests and water channels are either not obvious or non-existent. In such cases, whenever roads are considered necessary these are marked from the main ridge to water channel at right angle to the contours and in between each pair, a water channel at right angle to the contours should be marked.
With the completion of these exercises, a plan showing topographical access and the complete break down into catchment has to be made, and this becomes the Master Plan for all future development.