Laying out Catchments in the field
So far all the work in connection with catchments planning has been done in the office and the time has now come to inspect the ground to explore the conversion of the estate into topographical units according to the Master Plan, and identifying the catchments on the ground.
The various stages for laying out man made catchments are detailed below:
- Preparation of sketch: Prepare a sketch of the catchment to be laid out on a scale of not less than 64 inches to one mile (approximately 1m to one kilometer). The sketch should show the catchment boundary, percentage slope, plucking paths, collector and contour drains.
- Checking the plan against topography: Walk over the catchment with the sketch to see if the plan you have made will fit in conveniently with the lie of the land. Some alternations may be necessary and where they are required they should be made and recorded on the sketch.
- Marking of plucking paths and collector drains :With the aid of the sketch, measuring tape and stakes, mark on the ground the plucking paths and the collector drains with the stakes in such a manner that it is possible to distinguish between them.
- Marking of starting point of each contour drain: For the sketch, it will be possible to determine the distance between each of the graded contour drains and their starting points. These should be carefully measured and marked on the ground. To avoid wastage of space, the distance between drain should be adjusted so that the total distance is divisible by the space that is going to be left between tea rows e.g. presuming a planting distance of 120 cm between rows then the distance between the contour drains should be exactly divided by 120 cm.
- Marking the contour drain on the ground: A constant fall of 1:500 from the ridge to the collector drain should be the gradient given to the contour drains. To mark the position of the drains on the ground a Dumpy Level is essential.
The method of marking the position of graded contour drains on the ground is to stake points 20 ft (for any distance) apart, a long the line of the drain so that the fall between stakes 1 in 500 which is a fall of 0.04 between stakes. This is done by setting up the dumpy level in a suitable position and after selecting a convenient temporary bench mark (TBM) and recording the level reading on the TBM, the level staff is held at the starting point of the topmost contour drains in the catchment and level is recorded. The staff man then moves 20 feet along contour towards the collector drain, a reading is then taken through the dumpy level and the man holding the staff is made to adjust until staff reading of 0.04 feet more than the previous reading is seen. When correct reading is obtained, the position of the staff is marked by a stake. The process is repeated until the complete length of the first contour drain has been traversed. All contour drains are marked on the ground in the same manner. After completing the staking, the drain lines are smoothened out.
