The money, which was awarded in the wake of the two historic floods in 2015, must be entirely spent by 2022.
“We have to come up with a near-term solution even though we can’t fund the full long-term solution,” said Eric Ratzman, a senior project manager at Halff Associates, when he presented potential projects to community members Sept. 12 at the San Marcos Activity Center.
Ratzman, accompanied by officials from the city of San Marcos, told an assembled crowd of more than 50 people about several near-term solutions and one long-term solution, all of which will be presented to the San Marcos City Council at its Oct. 16 meeting.
Some of the HUD money will be used for smaller drainage projects, but a major infrastructure project—the Blanco Riverine Flood Mitigation Project—is intended to protect above all the Blanco Gardens neighborhood, which was devastated by both 2015 floods and by several major storms in previous years.
Record-breaking floods
The 2015 Memorial Day Flood remains the flood of record in San Marcos and surrounding areas; the flood that occurred Oct. 30, 2015—referred to as the All Saints Day Flood—was nearly as severe.
According to reports prepared by the city of San Marcos, 1,558 housing units in the city sustained damage in those two floods; nearly 80 percent, or 1,246, of those homes were located in the Blanco Gardens neighborhood.
Blanco Gardens is currently prepared to withstand only a 25-year flood event, meaning that in any given year there is a 4 percent chance that it will flood. The Memorial Day flood was nearly a 200-year flood event, according to Ratzman, while the October 2015 flood was about a 50-year flood.
The Blanco Riverine Project
HUD announced in February 2016 that it would give San Marcos $25 million in Community Development Block Grant funds for disaster recovery.
In April 2018 the city held its first community meeting about the projects; it was well-attended, Ratzman said, and more than 50 comments—written, verbal and drawn on maps—were collected.
After the April meeting the city commissioned a team from Baylor University lead by Peter Allan, a professor in the department of geology, to assess the Blanco River hydrology. Working groups, both technical and in policy, were assembled with staff from San Marcos, Hays County, Caldwell County and the Guadalupe Blanco River Authority, among other entities.
“I think there is a consensus that we need to look regionally for some solutions,” said Laurie Moyer, director of engineering and capital improvements for the city of San Marcos.
The result of the work was the options presented in September, which will also be what will go in front of the San Marcos City Council.
The near-term projects
All of the options presented include a berm—basically a short wall made of earth—which Ratzman described at the community meeting as “waist-high at the at the maximum and probably 2 feet on average.” The berm would likely be planted with native prairie grasses.
Even though it requires acquiring land along River Road, the berm is an inexpensive solution, and engineers from Halff ran a simulation to see if it would be effective on its own. While it would provide protection for over 100 structures Blanco Gardens, it also takes away one of the river’s flow paths, raising the water level elsewhere along the Blanco River and adversely affecting more than 1,000 structures.
“What we needed to do was give it another flow path,” Ratzman said in the presentation.
Diversion 2 provides that alternate flow path by commandeering a plot of city-owned property south of Blanco Gardens, adjacent to the river, where a significant amount of water could be channeled.
“That’s what happens now in the flood stage,” Ratzman said, “so we’re not changing the flow path; we’re just giving it a more channelized, controlled flow path.”
The berm accompanied by Diversion 2 is the least expensive of the projects, costing $14 million and protecting 315 structures in a 100-year-flood event.
Another project considered was the berm and Diversion 5, which would direct water east via a different channel. But the cost of excavation for the longer channel, as well as the cost of acquiring land, makes it unlikely that the city or its consultants would pursue it.
A third project would be the first short-term project—the berm and Diversion 2—as well as a portion of bypass channel proposed for the long-term project. But simulations showed that while the full bypass channel will be effective, the partial channel is not effective enough to justify spending an additional $30 million.
The long-term project
The most effective, but more costly and currently unfunded, project developed by the team is a 300-foot-wide, 25-foot-deep bypass channel that would divert water from the Blanco River at a spot just past where it crosses under I-35 to a confluence with the San Marcos River south of the city.
It is a project that would require a great deal more in terms of funding and regional cooperation; it is also the part of the presentation to the community that drew the most controversy.
Of the more than 50 people who attended the Sept. 12 meeting, nearly half were from Martindale, and Ratzman said the small town had a similarly large presence at the meeting in April. Those who spoke expressed concerns that Martindale's interests would not be represented and that any flood projects for San Marcos would be at its expense.
Ratzman said the possible effect of the project on Martindale were taken into account during both design and modeling, and that there will be much more discussion before the bypass channel is built.
“That’s in the future,” he said. “There will be more opportunity to weigh in on that solution before it gets finalized.”
Next steps
Even the near-term projects are far from completion.
“We’re just now really starting,” Ratzman told those assembled at the San Marcos Activity Center. “This is the stepping stone, the starting point for looking at the project.”
The plans will go to City Council before more work is done but after the city’s Oct. 12 and Oct. 15 community meetings at the municipal building.
“We just want to make sure that they agree with what we’ve looked at,” Moyer said. “We still haven’t gotten into really strong design yet.”
If City Council approves moving forward with the berm and Diversion 2 project, more detailed field studies will be conducted over the winter and followed by another public meeting before the environmental assessments begin.
The long-term project will also be discussed by City Council. San Marcos alone will not be able to fund it, but the idea is that the long-term project will be able to reinforce and improve the near-term project.
“They build on each other,” Moyer said. “With a short-term project we’re doing something that gets us closer; we reduce the risk in Blanco Gardens.”
There have been no formal agreements with Hays County, but Debbie Ingalsbe, commissioner of Precinct 1, thinks it is worth considering.
“Bottom line,” she wrote in an email. “This is such an important project/issue to help a large area of our population, both inside the city and out, that I advocate for the county’s participation.”
