ABSTRACT
A canal drop structure is primarily meant to negotiate the fall in water levels in the canal from upstream to downstream of the drop, without endangering the safety of the structure. Normally, no gate control arrangement is provided at these drops, unless cross regulators are clubbed with the drops, which is not of common occurrence. A drop structure without gate control arrangement is designed for safety of the floor against uplift forces due to unbalanced head when the canal is flowing full or when there is no flow in the canal. With the subsequent installation of a hydro-power plant at the canal drop, the drop structure itself needs to be provided with gate control arrangement to regulate the canal flows over the drop and divert them partially or fully through the newly excavated by-pass channel on which the power plant is installed, thereby converting a mere canal drop into a drop-cum-regulator.
In this new setup, the worst condition for the safety of a drop structure aganist uplift is when water stands upto top of closed gates on upstream with or without flow in the bypass channel and, in any case, water not backing up to the rear of the strdcture. In such a situation, the unbalanced head causing uplift will be enormously increased to a value equal to the difference between upstream full supply level and downstream bed level of the canal. Strengthening the floor and deepening the end cutoff of the existing drop structure for safety against such unduly high head, will be neither feasible nor desirable, necessitating dismantlement of the entire downstream floor with end cutoff and reconstruction of.a new floor to the required length and the required thickness with a new end cutoff taken to the required depth, which adds to the cost of civil works of the mini-hydel project.
If the drop structure were initially designed and constructed as a. drop-cum-regualtor, the overall cost of the combined structure would have been much less, with the added advantage of there being no necessity for a costly by-pass channel, but for a widened channel section for a short length at the drop site to accommodate the open regulator vents and power units side by side.
As regards the returns from such mini-hydel plants at canal drops, the total annual power generation possible at a canal drop is very much less, compared to the installed capacity. While the installed capacity corresponds to the full supply discharge in the canal and the corresponding fall in water levels in the canal at the drop, the actual power generation is for partial flows in the canal during greater part of the crop periods with no generation at all during the canal closure periods. All these aspects need to be borne in mind, while assessing the cost of installation including civil works and the power generation potential at canal drops and working out the returns.
