Water hurled down Waller Creek during the heavy storms that pounded Austin last Friday.
It was another test run for the Waller Creek Tunnel which, contrary to what the city originally expected, has been receiving water during rainy weather for months in its unfinished condition. That’s because a design flaw in the tunnel’s intake facility, which came to light last year, set back construction on that part of the project.
The city monitored the $155 million tunnel system during the recent storm and is “doing a full look at what happened with the Waller Creek Tunnel,” said Stephanie Lott, a spokeswoman for the city’s Watershed Protection Department, this week. Lott said there’s one thing the city does know for sure: Water topped the dam at the tunnel’s intake facility.
That’s higher than water rose in the Memorial Day flood, which was the tunnel’s first major test and prompted the city to assess the performance of the system that is supposed to remove more than 28 acres of downtown from the flood plain, allowing for redevelopment and the revitalization of the creek corridor.
A city staff member wrote that the “Memorial Day flood event has raised concerns about both the interim performance of the tunnel during the remainder of the construction phase and the ultimate performance of the completed tunnel facility” in a June letter to the engineering firms that designed the tunnel.
During the Memorial Day flood, water in the pond by the then-incomplete dam rose to about one foot lower than the dam’s crest. That is higher than water is projected to rise during a 100-year flood event — a severe downpour that has a 1 percent chance of occurring in any year — once the tunnel is complete, the staffer wrote.
“We did not expect the Pond level to be higher than the design 100-year event level in an event that was significantly less than a 100-year event,” Karl McArthur of the city’s Watershed Protection Department wrote. The Memorial Day flood was between a 10- and 25-year flood event, which has up to a 10 percent chance of occurring in a year, in large part because the ground was saturated from prior rainfall, creating more runoff.
The tunnel system is supposed to prevent water from flowing over the banks of Waller Creek in a 100-year flood event.
Models the city ran after Memorial Day found that in the worst-case scenario, including a 100-year event and an incomplete tunnel, the flood plain would actually expand slightly to include an additional part of Red River Street. Lott said Red River Street didn’t flood last Friday.
Waller Creek did flood 12th Street with about 6 inches of water last Friday, said Scott Prinsen, a spokesman for the city’s Watershed Protection Department. The city is still assessing the rest of the creek, he said.
But it’s not done yet
In interviews, city officials emphasized that the tunnel is still unfinished and that, even in its interim state, it is still diverting water that would otherwise flow down the creek.
“Even before the entire project is finished, there’s some level of protection already provided to Waller Creek,” Joe Pantalion, the city’s interim watershed protection director, said in an interview before last week’s deluge. “But again, just to remind everyone, the project is not complete.”
The tunnel project suffered a major setback last year, when city officials said they discovered the design of the intake facility at Waterloo Park blocked a protected sightline of the Capitol. The facility was about 16 feet too tall, so pieces of the partly constructed building were demolished and a redesign began.
The City Council added $5.6 million in June to the $149 million budget for the tunnel. City officials said the money was needed to keep the project moving while they negotiated with the contractor and engineering firms over paying for the design flaw.
Even with the system not yet complete, city officials decided this spring to remove the bulkheads preventing water from entering the tunnel, figuring the city would gain some drainage benefit.
The city doesn’t have a measurement of how much water entered the tunnel Memorial Day, but said a “majority” of creek water continued to flow downstream rather than being diverted into the tunnel because the dam was just about 20 percent complete.
The peak water level in Waller Creek at Red River Street, downstream of the intake facility, was 10.4 feet during the Memorial Day storm that rained an average of 2.8 inches in the Waller Creek basin, the city’s Watershed Protection Department said. Two weekends ago, during the rain on Oct. 24 when the dam was finished, the peak water level was 4.7 feet with an average of 4.8 inches of rainfall.
City officials said they have made small tweaks since Memorial Day to improve the performance of the tunnel in its interim condition and are taking the opportunity now to learn about the tunnel. But they said they won’t fully know whether the original design of the completed tunnel will pan out until the months-long testing and training period before contractors hand over the finished project to the city.
The intake facility should be complete in early 2016, said Roxanne Cook of the city’s Public Works Department. The last major component of the project, an inlet at Eighth Street, should be complete by early 2017 or before, she said.
Debris building up
The intake facility has 30-foot-tall screens that are meant to filter out trash and debris before stormwater flows into the tunnel. “Gripper rakes” attached to the facility’s ceiling will then whisk that material away from the screens.
But those rakes aren’t in place yet. So during the Memorial Day downpour, more trash and debris built up on the screens than designers ever expected (the city didn’t provide information on whether the same thing happened last Friday). The June letter from the city said more than half of the screens were covered with trash and debris during the Memorial Day flood. That caused the high water elevation in the pond, Pantalion said.
Part of the buildup was caused by an erosion-control device placed on the screens during construction, the letter said. Debris and trash also clogged the bar screens all the way up from the bottom of the pond, as the pond by the intake facility was dry before the flood, which shouldn’t normally be the case, Pantalion said.
If the screens were fully covered during a 100-year storm, that could cause flooding along about 32,804 square feet of Red River Street between 12th and 15th Streets not currently in the flood plain, the Watershed Protection Department said. City staffers monitoring the tunnel during storms will set up barricades, if needed, along Red River Street, Pantalion said.