Investigating the sedimentation pattern at Ghazi barrage reservoir and impact of 2010 flood upon it

Ghazi Barotha Hydro Power Project is a run of river project meant for generation of 1450 MW electricity. It is Pakistan's second biggest power generating project of vital importance. Ghazi Barrage, one of the key components of the project and situated just downstream of Tarbela Dam project, diverts the Tarbela releases towards 52 KM long Power Channel, another key component, which in turn conveys it to village Barotha for power generation through Power House, the final key component of the project. Two factors which necessitated this study were heavy sediment accumulation in Tarbela dam thereby increasing the sediment inflow into the Ghazi Pond and the devastating flood of 2010 which brought huge sediments from the area upstream of Tarbela. It was important to find out what was happening to the Ghazi pond and changes it had undergone since its operation in 2005. The strategy of this study was to find out incoming and outgoing sediment load from the available data thereby evaluating load retained in the pond along-with its relationship with flows. Next Hydrographic or Bathometric survey of the pond was to be performed to obtain necessary data showing reservoir bed position with respect to deposition and time. The outcome was to be compared with consultant's a joint venture's working through mathematical modeling for sedimentation while designing the barrage and its allied structures. Flood data of 2010 was also to be compared with previous and post flood data to find out its impact on sedimentation of the reservoir.

After performing all these hectic efforts, it was observed that the increase or decrease in the outgoing sediments from the pond was proportional to incoming flows. As the incoming flow increased thereby increasing the incoming sediments, the outgoing sediments also increased. In some cases of high inflows like the one in 2010 flood case, the sediment outflows were more than the sediment inflows causing erosion of the already deposited sediments. This does not happen in deep and large reservoirs. While comparing our study results, the consultant's working did not match with our outcomes. The sediment deposition was much less than expected by the consultants which meant that the reservoir life would be much more than expected.