Hydro power dam dredging by DOP350

In the Enguri River, a hydropower dam was built during the 1970s and 1980s. Due to the natural circumstances in the area (North Western Georgia, near Abkhazia) the concrete dam could have vast dimensions. Its 271.5 m of height makes the concrete arch dam the second highest in the world; moreover it is 728 m wide. The reservoir contains some 1,100,000,000 m3 of water, and the dam supplies about half of Georgia’s power demand. The immense dam boasts 20 turbines, ensuring a total capacity of 1,300 MW.

Hydropower dam sedimentation

The Enguri River, as any natural river, will transport sediment from upstream into the reservoir. As this is a common occurrence, all reservoirs are designed with a ‘dead storage volume’. This planned volume in the reservoir will be gradually filled up with slurry over the life time of the dam due to this natural sediment flow. Once the sediment level exceeds the planned storage volume, the water storage capacity of the hydro power dam reservoir is reduced, and more importantly, the gate outlets in the dam are blocked by the sedimentation.

At this point, sediment removal at the hydropower dam bottom outlets is priority; maintenance dredging activities are to be performed in the vicinity of all the dam’s outlets. At the Enguri Hydropower Dam this maintenance dredging activity proved a challenge: the depth of the reservoir and the shear volume of the job were daunting. The material to be removed consisted of silt, sludge and slurry – with a lot of floating debris adding to the challenge.

DOP dredge pump package

For the Eguri Hydropower dredge project DCN Diving created a dedicated floating structure. The main purpose of the floating structure was to facilitate the dredging activities; the grab action for removing large debris always being secondary. This large working platform handled the DOP350 submersible dredge pump which was brought in to tackle the sedimentation, plus its power supply, booster pump station and all required auxiliaries and drives. The DOP submersible dredge pump is powered by a electric-hydraulic power pack located on the platform.

Impressive dredging depth

The main challenge of the dredge job was – and is – the dredging depth. Having started at -150 m, it is now working at an incredible dredging depth of -170 m [the final dredging depth being -190 m]. For this, the standard DOP dredge pump was fitted out with pressure compensation on the bearing housing. Moreover, the suction pipe has been lengthened, and the pressurised jet water to the mining head has been adapted to the large amounts of debris expected. The DOP350 has removed 125,000 m3 of sludge up to now, pumping the sediment from the bottom outlet gates to the reservoir’s surface and then leaving the reservoir through the intake channels of the turbines. In spite of the remarkable dredging depth, the DOP pump performs perfectly and the reservoir gradually regains its ‘dead storage volume’- a necessity in any hydropower plant.

Reservoir dredging challenges

The major dredging depth of -190 m being one challenge, reservoir dredging combines a number of demanding factors requiring brain-racking by dredging contractors. The Enguri River not only transports sediments, but also driftwood. The reservoir being enormous, the massive inflow of driftwood is as impressive: at present at least 6 football pitches full of driftwood have accumulated before the dam, hindering dredging activities.

Another challenging factor is the water line. The difference in height between the top of the dam and its surrounding service areas versus the water line of the reservoir transforms any transfer into an action which is to be well contemplated and planned before execution. Moreover, the water line is not stable. Due the seasonal changes, the water line of the Enguri dam drops some 80 m in 6 months’ time. All these factors combined have resulted in the fact that many hydro power dam reservoirs have been gradually filled up with sediment. The Enguri dam dredge project has shown that reservoir dredging is a challenge which can be brought to a good end by creative dredging contractors & the right dredging equipment.