Houston dam begins to overspill as Harvey triggers unprecedented impact

Houston dam begins to overspill as Harvey triggers unprecedented impact

‘We have never faced this before, we have uncertainty in how the water is going to react’ official says, as water levels in reservoirs threaten monitoring equipment

A vital dam in suburban Houston that protects the central city began overspilling on Tuesday, and officials said the rainfall from Harvey is so unprecedented they do not know what the impact on surrounding communities will be.

Water levels in the Addicks reservoir have reached 108ft, said Jeff Lindner, a Harris County flood control district meteorologist.

He warned that neighbourhoods in the spillway zone would begin to see street and possibly structural flooding.

“We have never faced this before. We have uncertainty in how the water is going to react as it moves out of the spillway and into the surrounding area,” Lindner told a news conference on Tuesday. “We are trying to wrap our heads around what this water will do.”

Linder named six subdivisions that appear most at imminent risk and told residents: “If you want to leave, now is the time to leave. The reason being, once the water comes into the street you’re not going to be able to leave.”

Another major dam and reservoir nearby, Barker, is also enduring exceptionally high water levels, and some residents in streets to the west of it are under voluntary evacuation orders. They also face the possibility that their roads could be rendered impassable just as the storm’s precipitation appears to be decreasing in intensity.

“New streets will continue to flood, new homes will continue to flood … the amount of water that goes over the spillway will be increasing,” Lindner said. He added that it is unlikely to happen quickly but could leave some homes inundated for a month.

“I completely understand the uncertainty that people are dealing with. This is a complex situation,” he added. “My own house is in the spillway region of [Addicks] reservoir so I completely understand what you’re going through.”

Addicks and Barker were constructed in the 1940s by the US army corps of engineers, which maintains them.

The dams are designed to control the flow of water into the Buffalo bayou, a river that stretches for dozens of miles and goes through the heart of the city, depositing water in the ship channel to the east.

As Harvey battered the region over the weekend, the corps began “controlled releases” of water into the bayou and warned that surrounding areas were at increased risk of flooding as a result.

The corps is performing a balancing act with water levels that sees some nearby places flood, or existing floods worsen, while protecting a much larger area from potentially catastrophic effects.

The dams have been undergoing extensive repairs because several years ago the corps deemed them at “extremely high risk” of failing. The federal agency has repeatedly denied this means there is a serious chance of a disastrous breach.

Many parts to the east of the dams were already flooded by the bayou even before the corps started the controlled releases. The bayou is so high that it has submerged outlets designed to feed it to relieve the reservoir levels.

The Houston region’s dramatic population growth – about a million new residents in the past decade – has seen extensive construction of new houses, offices and apartment complexes built around the dams in what once was empty grassland.

The area is now dubbed the Energy Corridor because it is popular with oil industry workers. A number of companies are headquartered there.

“If you’re building in areas that are considered storage for the reservoir, that is an issue,” a corps spokesman said.

To add to the problems, Lindner said that a reservoir monitoring gauge in the Barker reservoir was flooded on Monday evening and no longer works, and because so many streets and highways are unusable, United States Geological Survey officials are unable to reach the site and repair it.

“We have law enforcement personnel going to pick them up in a high-water vehicle to take them to Barker and establish a temporary measuring gauge. Based on the elevation of Addicks we are concerned we may lose the Addicks gauge,” he said.