From the Wikipedia Rabbit Hole Day Trip — Poulaphouca, Surge Tanks and Your Washing Machine

The Blue Ridge Dam and surge tank. Public domain image by Tennessee Valley Authority on Wikimedia Commons.

If you look to the left while crossing north over the River Liffey bridge on the N81 at Poulaphouca, you might have noticed a large tank in the distance on the edge of the gorge. This is a surge tank and its function is to protect the pressure tunnel from hydraulic shock or water hammer when feed to the turbines at the power station is turned off suddenly. Hydraulic shock occurs when water is flowing through a conduit (pipe, tunnel etc) and its flow is suddenly interrupted. The sudden deceleration causes a rise in pressure which can cause damage to pipes. Have you ever noticed the sudden thump as water turns off when your washing machine's intake valve closes once the drum is filled? This is also a water hammer effect. However by comparison, the pressure tunnel to the power station is over 400 m long and 4.8 m diameter. That's a huge volume of water in motion with lots of momentum and kinetic energy and when it stops suddenly, that energy has to go somewhere. A surge tank at the power station allows water to flow freely up into the tank, converting kinetic energy into potential energy, rather than as a damaging shock wave propagating back up the pressure tunnel to the dam.

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