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Tunnel complex

The 16 tunnels and their tower intakes for the Yellow River dam form one of the most complex arrangements on any dam in the world.

First three and biggest are the diversion tunnels, each 14.5m internal diameter with an up to 2.5m thick concrete lining. Halfway down these 1.1km long tunnels is a chamber 'larger than the train crossovers for the Channel Tunnel', explains CGIC deputy project director Olivier Colin. Each is 25m high, and changes from a circular to square cross section in the centre where a dividing wall sits between two large sector gates. The square concrete entrance for the first of these is already carrying the low season river flow.

Once the dam is complete, the first 200m will be plugged and the remainder of the tunnel reconnected to a higher entrance though still the lowest in the intake. These trumpet shaped 'orifices' are designed to stir the water. Venturi constrictions inside will also speed the flow. The combination is intended to let the tunnels flush the reservoir bottom should it become silted near the intakes.

Above the orifices are three circular sediment tunnels 6.5m in internal diameter which will carry the heavily silted river flow in the summer season discharging it past the dam.

When the water is clearer the sediment tunnels will be shut and flow diverted to three pairs of power tunnels 7.8m internal diameter.

Above these tunnels are three 'free flow tunnels' with a 10m inside width and 1km lengths. They are the lowest level of overspill and run partly as tunnel and partly as cut and cover culverts.

Finally there is a small 3.5m diameter tunnel to provide irrigation water for the local farmlands.

At the highest level is a service spillway, an open channel 24m across with a gated entrance. This and the other tunnels discharge into three 200m by 120m plunge pools, 3m deep.

And finally there is around 1km of smaller drainage tunnel.

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