The massive reservoirs built to protect downtown Houston are in trouble — and one is spilling over for the first time

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The record-breaking rains Harvey continues to pour onto Houston and the surrounding area have flooded thousands of people out of their homes, with tens of thousands potentially needing to take refuge in shelters.

On Sunday night, the US Army Corps of Engineers began a controlled release of water from the dams at the Addicks and Barker reservoirs for the first time ever. The reservoirs were built to protect downtown Houston from water that could otherwise come flooding though Buffalo Bayou, which is normally a small river running through the city.

The hope was that a controlled release would prevent Addicks and Barker  — which were built in 1945 and 1948 after previous catastrophic floods devastated downtown Houston — from spilling over and causing even more uncontrolled damage.

But it was not enough. 

Addicks began spilling over for the first time in history on Tuesday morning after topping the 108 ft. water-level mark. The gauge measuring water levels in Barker broke on Tuesday  — some predict it will start spilling over as well.

Officials reported that water had breached a levee in Brazoria County on Tuesday as well, and told residents to evacuate immediately to escape flash flooding.

No one knows exactly where all the water from Addicks and Barker will go, but officials have estimated that some homes will be flooded for months.

Upstream from the reservoirs, residential neighbourhoods have been built up. These communities are mostly in what’s considered a 500-year floodplain, according to Slate, which means they’re considered to have less than a .2% chance of flooding each year. But they flooded last year and are flooding again now.

Water spilling out of the dams can go upstream, flow out through emergency spillways, or get released from the dam gates.

It will all end up flowing downstream eventually, however, into Buffalo Bayou and downtown. The question is how fast that will happen and how quickly that could cause the waters there to rise.

The worst case scenario would involve the water damaging the dams themselves, which is why the Army Corps are continuously monitoring dam integrity. Fortunately, Corps officials told Texas Tribune reporter Neena Satija that they are not concerned the dams could give way in any uncontrolled manner.