Flood Model Shows River Walk Could Become a ‘Bathtub’

If a Harvey-like hurricane struck just north of downtown, the River Walk could turn into a “bathtub,” said Stephen Graham, San Antonio River Authority assistant general manager. About 12-18 inches of water would flood downtown streets, according to flood models released by the river authority Wednesday.

The models show that several San Antonio neighborhoods and commercial corridors would be inundated with flood waters, Graham said, but past investments in key drainage projects lessen the potential blow.

“I know it sounds bad, but our infrastructure is paying off,” he said.

The first flood model, which showed the impact to Olmos Dam and U.S. Highway 281, was presented to City Council two weeks ago. The three additional models for downtown, Apache Creek/Elmendorf Lake, and Concepcion Creek, were recently finalized and shown to board members Wednesday.

The Olmos Dam, San Pedro Creek tunnel, and San Antonio River tunnels trap and divert 100-year floodwaters to avoid urban areas, Graham said, but they would still be no match for the 1,000-year flood like Hurricane Harvey that struck Houston in August.

“You could not plan for that kind of event,” Graham said. “What you can plan for is how can we be resilient.”

San Antonio also has the advantage of a steeper land grade than Houston, which is “flat as a pancake,” Graham said.

But these models reflect assumed flow patterns of water – that is, how the water would flow if there weren’t streets and buildings in the way.

“The flooding would be worse than even this picture shows,” Graham said.

This kind of urban flooding is harder and more expensive to map, but emerging data and technology are improving the more accurate, “two-dimensional” mapping of potential floods, said Anthony Henry, senior hydraulic modeler with HDR. The engineering firm has worked with the river authority for many years on mapping and modeling projects. Such maps would take into account impervious surfaces, curbs, and undersized storm systems.

In the case of Concepcion Creek, for instance, a “one-dimensional” model shows water flooding along the creek. But a “two-dimensional” model that incorporates physical features shows water pooling up behind train tracks and flooding along their path.

Access to data and collection of that data are some of the challenges for two-dimensional modeling, Henry said.

“Historically, computing power was limiting the models you could make,” Henry said. “2-D has only come around in the last decade or so and [continues] to improve as it falls on the heels of computer power. It’s getting faster and cheaper to make 2-D models.”

But FEMA, for instance, uses only one-dimensional flood plain data. Only residents in flood plains are required to have flood insurance even though urban areas outside those plains routinely flood. Residents in those areas, therefore, should double-check if they have flood insurance, said Suzanne Scott, river authority general manager.

“Oftentimes homeowners believe it’s part of their base policy,” Scott said. “It is not.”