Question remain about whether an apartment complex near the San Marcos River contributed to flooding during the Memorial Day weekend floods, and a solid answer may not come until October.
Mike Moya, vice president with Halff & Associates, the firm conducting a study of flooding in the area of San Marcos east of I-35 between the Blanco and San Marcos rivers including the Blanco Gardens neighborhood, said the apartments did not have an adverse effect on the neighborhood compared to what would have happened had the apartments not been in place.
Moya later clarified that a more detailed study is coming. He said the detailed study should give a better idea of whether the same level of flooding that occurred in May would have occurred before the apartments were built—assuming the same amount of rainfall.
Dozens of homes in Blanco Gardens were flooded during Memorial Day weekend. Many residents said they believe the apartments acted as a dam during the event and prevented water from flowing from the Blanco River through the neighborhood and into the San Marcos River.
“This model was done utilizing standard procedures that are utilized to build the [the Federal Emergency Management Agency] maps,” Moya said. “The result is really that these areas are all in the floodplain and given this event, would have flooded regardless of whether [the Woods] was there or not.”
Moya said the amount of flooding that was reported indicates flows on the river were so large that the apartments likely had a negligible effect on the neighborhood.
Halff & Associates is working on a more in-depth, two-dimensional model of what happened during the Memorial Day weekend floods. The one-dimensional study only took into account one direction of flow. The two-dimensional study will take into account water flow to the area from multiple directions and will account for varying elevations throughout the neighborhood.
"Does it flood a little longer [because of the apartments]?," Moya said. "Maybe. Does it have an impact? Maybe. It could have some impact, but the bottom line is those homes that had two or three feet of water, this didn’t cause that level of flooding. It may have caused an extra inch."
Council Member John Thomaides said the residents in that area deserve to know if the apartments contributed to flooding, and he emphasized that no one in that area believes the apartments caused the flooding.
Council Member Lisa Prewitt said she wants to see a more detailed analysis taking into account the relatively small slope of River Road toward I-35, which runs between the apartments and Blanco Gardens. That small slope may not have allowed water to drain quickly away from the neighborhood, she said.
“I think this neighborhood deserves to have a clear analysis and only facts given to them,” Council Member Lisa Prewitt said. “I don’t want us to hear—because we’re trying to not create blame on an apartment complex when hundreds of people’s homes got devastated—that we’re giving them clear analysis.”
Moya said much of the rainfall May 24 and 25 occurred over the Blanco River upstream of Hays County. That contributed to the many reports of a “wall” of water that swept through Western Hays County, tearing homes from their foundations and uprooting massive trees.
According to the Halff study, the peak flow on the Blanco River—which has a drainage area of about 435 square miles—was equivalent to the flow on the Mississippi River, which has a drainage area of about 530,000 square miles. The flow on the Blanco River was about 1.3 million gallons per second during the event.
New elevation advisory maps are expected from FEMA in mid-September, said Laurie Moyer, director of capital improvements and engineering for the city. If San Marcos adopts those maps, they will help the city advise residents dealing with flood damage on how high they may need to elevate their homes.
