The proposed Akhori dam project will store about 8.6 billion cubic metres (seven million acres feet) of surplus Indus River water that is spilled after filling Tarbela reservoir during the monsoon season.
The stored water will be released for mitigating irrigation shortages during the dry season. The project is quite simple and includes a gated intake structure, a 37-km long water conveyance channel, a reservoir, a hydropower station, a spillway, dams and ancillary works.
The reservoir, called Akhori reservoir, will be developed by constructing dams across a valley near Akhori village. The valley is situated between Attock and Fatehjang towns, on the left bank of the Haro River at an approximate distance of 40 kilometres west of Islamabad. Detailed feasibility studies of the project have confirmed its technical and economic viability.
The intake structure will be designed for supplying the water from Tarbela to Akhori reservoir and it will be constructed on the southern periphery of Tarbela reservoir. The intake will function as an additional spillway of Tarbela that will release the water into the conveyance channel for delivering to and storing in Akhori reservoir.
The invert or sill of the intake structure will be at the same level as the crest of two existing spillways of Tarbela. The hydropower station will be designed for harnessing the hydro energy of the stored water before it is released from Akhori reservoir into the Haro River. The released water will join the Indus River downstream of Ghazi Barotha hydropower station.
It is believed that the raised intake sill can divert to Akhori reservoir sediment free water that is near the top surface of the full Tarbela reservoir. This can prevent sedimentation of Akhori reservoir and thus ensure its sustainability. But ultimately the sedimentation of Tarbela is expected to put an end to the availability the sediment free water which can jeopardise the said sustainability.
It is expected that Tarbela reservoir will stop the supply of the sediment free water to Akhori as soon as the accumulated sediment depletes the storage below Tarbela spillways. The annual sediment inflow (the sediment entering Tarbela reservoir) will become equal to sediment outflow (the sediment leaving the reservoir), that is the sediment equilibrium will be established, sometime after the storage below the spillways is depleted.
That eventuality will make Tarbela a run of river hydropower project and it will commence releasing the sediment laden water from all of its outlets, including the future intake structure meant for supplying water to Akhori dam project.
That is how the sedimentation of Tarbela will ultimately affect the sustainability of Akhori reservoir. That eventuality can be avoided only by preventing the accumulation of the sediment in Tarbela reservoir or by establishing the sediment equilibrium before the storage below Tarbela spillways is depleted.
Fortunately, the earlier sediment equilibrium can be established by returning to and releasing from Tarbela during the dry season the water stored in Akhori reservoir.
The release of the returned water during the dry season from Tarbela can scour and flush out increasingly larger portions of new sediment entering the reservoir every year and consequently it can establish the earlier sediment equilibrium. In principle lower the water level in Tarbela more the quantity of the sediment flushed out by the returned water.
Therefore, by rigorously monitoring the sediment and by prudently managing the release of water, a substantial remaining storage capability of Tarbela below its spillways can be saved permanently.
Apparently, there is no immitigable disadvantage of storing the Indus water in Akhori and releasing that water from Tarbela during the dry season. Even the previous sediment management studies indicated that the sedimentation of Tarbela can be mitigated by releasing additional water when Tarbela is at its lowest level.
Therefore, the idea or concept of returning the water from Akhori to Tarbela deserves a detailed investigation because it offers major benefits without affecting the anticipated benefits of Akhori dam project.