Sediment levels entering the city’s water treatment plants are at 8,000 percent of normal levels, officials from Austin Water, the city’s public water utility said. The system, which typically produces 300 million gallons of clean water, has only been able to produce 105 million gallons per day due to the stress caused by the sediment.
Engineering and water treatment system experts blame a cocktail of factors, from the extreme rainfall to the geography and geology of the region. All agree that there was no realistic way to plan for this emergency.
From geography to geology
Professor Randall Charbeneau, a water environmental engineer expert and vice-chancellor of research for the University of Texas System, said in his 40 years at the University of Texas, he has never seen anything like this.
“This is a very rare event … our water quality is usually excellent, the levels [of sediment] are extremely unusual,” Charbeneau said. “[This is] not a thing you plan a water distribution system around.”
Charbeneau pointed to a several variables. For weeks, Central Texas has been hit with heavy and consistent rainfall that caused destructive flooding up stream at the Pedernales and Llano rivers—two Hill Country waterways that sit atop of highly impervious layers of limestone. Rather than absorbing into soil, the rushing flash flood waters washed up the layers of soil over the limestone and carried the sediment into the rivers’ mouth, Lake Travis.
He said there are no dams between the rivers and Lake Travis. As the rain continued, the sediment-filled floodwaters rushed into Lake Travis and carried into Austin’s waterways, resulting in the current water treatment plant issues. Emlea Chanslor, spokesperson for Austin Water, said the water’s turbidity—a measurement of sediment—is typically at a 5. For the last few days, that level has floated around 400.
Director of Austin Water Greg Meszaros said the drought conditions that preceded the extended rainfall in the region contributed to levels of sediment as well. The soil gets baked out and becomes very fine during droughts and Meszaros said when the rain came, it washed up the very fine soil with it, which he said is tough to separate through the treatment process.
“The water treatment plants are designed for water that doesn’t have the level of sediment they’ve experienced,” Charbeneau said. “[The sediment levels] are something you couldn’t have planned for."
Charbeneau said the sediment levels are so extraordinary that he doesn’t believe it will change how the city operates in the future.
A proactive approach
Desmond Lawler, a professor and chair of the civil engineering program at the University of Texas, Austin, said the city’s treatment plants have seen high turbidity levels before, but it typically lasts for a brief period of time, during which the city can dip into its reserves. This time, the sediment has continued to flow in for days and the city has depleted its reserves, resulting in consumption restrictions.
“[Treating] water quality that is dramatically different and worse than usual is a tremendous strain on the treatment process,” said Lawler, whose work focuses on water quality and treatment processes.
Lawler said he understands the city’s boil water notice to be “very proactive” but said the austere consumption mandates are crucial to helping the system come back to full operation. On Tuesday, Austin's city manager, Spencer Cronk, confirmed that assumption and said the boil water notice was "absolutely proactive."
John Keane, a water sector leader with Stantec, said the filtration system has to work overtime to both pump out treated water and backwash the sediment out of its system. A Stantec subsidiary, MWH Constructors, performed at-risk construction management for Austin’s Water Treatment Plant 4.
“The more dirt that comes in, the more water that needs to be used to clean the system,” Keane said. The water in Austin is typically very clear and the treatment system is not built for such high sediment water.
Keane, who has worked on water treatment plants across the country, including in Houston, said it would be fiscally unrealistic for the city to plan for such an extraordinary scenario. Although the system is under irregular stress, Keane said he doesn’t expect the system to incur any long-term damage.
Editor's Note: A previous version of this article misattributed John Keane.