How the 15.5 BCM capacity of the reservoir would be filled up and what would be its consequence?
The first time it would be filled up from the empty position. Hence, it would require 15.5 BCM of water. Most likely, it would be filled up over a few years depending on flow during the rainy season. However, till the fill up is completed, the downstream flow will be lower. Dam fill up is generally done during high flow so that the effect is moderated downstream. It also needs to be mentioned that reservoir fill up is a one time process. This would have no effect over long term yearly flow of the river. Some of the experts projected (pg 21) that 15.5 BCM of reservoir would cause 491 cumec (which is equivalent of 15.5 BCM per year) reduced flow downstream. This is not true. The dam is filled up only once and the water is used dynamically to fill up in rainy season and to release in dry season (look at Maithon reservoir graph and Karnaphuli inflow-outflow statistics). Recently there was a contention between India and Pakistan regarding this first time fill up of the reservoir.
