These valves use a screw type valve instead of a simple flap, and they have a pressure regulating mechanism that will vent water to the ground if the intake pressure exceeds a set level.

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The fatal pressure release took place at a project known as Intake No. 3, which is under way near Lake Mead.

CNN: STORY HIGHLIGHTS

To defuse the pressure, the prime minister, Lee Hsien Loong, says Singapore will slow down the intake of migrants while accentuating the privileges of citizenship.

ECONOMIST: Singapore and immigration