A direct hit by a hurricane of Harvey’s magnitude would cause catastrophic flooding that would flow over the Olmos Dam and emergency spillway by four feet and inundate portions of U.S. Highway 281 for approximately 12 days, according to models released by the San Antonio River Authority on Wednesday.
“Preliminary model results show that the Olmos Basin would receive staggering amounts of rainfall dropping enough water to fill the Alamodome 41 times, about 23.34 billion gallons of water,” stated Steven Schauer, the river authority’s director of government and public affairs, in an email to the Rivard Report. The Olmos Basin is one of several basins in the Upper San Antonio River Watershed.
But experts noted that specific issues surrounding urban flooding – which could have dire impacts on thousands of San Antonio residents – were not assessed in this model.
Harvey struck the Texas Gulf Coast in late August and caused widespread destruction and disastrous flooding in the Houston area. Harvey’s stormwater, about 40 inches, would have behaved differently in San Antonio because of climate, soil, and topography.
“Since the topography of our region is steeper than that of the Gulf Coast Region of Texas, local riverine systems would allow for flood waters to move downstream through the watershed instead of accumulating and ponding in place as was seen in Houston and other areas near the coast,” Schauer said in the email.
Harris County, which includes Houston, has a 300-foot variation in elevation, Nefi Garza, assistant director of the City’s Transportation and Capital Improvements department, said. Bexar County has 1,520-foot variance.
Garza presented the flood risk information to San Antonio City Council as part of an update on citywide emergency preparedness Wednesday, which included a look at the river authority’s modeling.
“The difference is we’re going to see higher velocities” compared to Houston floods, he said.
If a Harvey-like storm hit downtown San Antonio, Garza said, the model shows water rushing into the Witte Museum, Olmos Park neighborhood, San Antonio Zoo, River Road neighborhood, the Pearl development, and more.
Water would rise and recede rapidly “within hours, not days,” Schauer added. This means stormwater moves faster – 11 feet per second during a storm like Harvey – and has a stronger impact on erosion patterns in San Antonio.
In 2013, the river authority hired consulting firm HDR Engineering to help develop a flood warning system that now is in the process of being updated. By tapping into this high-tech network that has been installed in the Upper San Antonio River Watershed, the River Authority was able to model the effect 40 inches of rain would have on San Antonio.
The Harvey video model represents only a portion of the entire San Antonio River Watershed, the area south of Olmos Dam, north of U.S. Highway 281 and Broadway Street. Models for other areas are being developed and will be discussed at the river authority’s Nov. 15 board meeting.
While this data is helpful, it’s important to note that this analysis doesn’t include “localized flooding due to undersized storm sewer systems,” Schauer explained in his email.
FEMA only analyzes floodplains of rivers and creeks – not infrastructure failures.
“A storm of this magnitude would undoubtedly include localized street and neighborhood flooding; however, the limitations of the model do not allow for those types predictions,” he said.
SARA developed a flood risk map available online here, for residents to inspect their home or property’s threat level should a 30- or 100-year flood occur.
Schauer said it would be wise for property owners to get flood insurance regardless of their proximity to river flooding. A broken water line or clogged street drain could cause flooding unrelated to river or creek levels. “More than 20% of all flood damage claims occur from properties outside of the regulated floodplain.”
That’s what these models don’t show, said Steve Graham, assistant general manager of the river authority: Urban flooding.
“Unfortunately the model may not show blue [to indicate flooding] all over downtown, but that’s exactly what it may be,” Graham told the Rivard Report after the council meeting. Demonstrating that flooding would take more time, data, and money.
“FEMA is actually looking at this issue as well,” he said, “should they regulate urban flooding, not just riverine flooding?”
But with regulation comes insurance. Flood insurance is required in FEMA-regulated flood plains. If that’s extended to urban streets, then some property owners are worried that property values will plummet, Graham explained. “There’s a lot of push and pull as to whether you should map [and regulate] urban flooding … and that’s a national debate right now.”
There are thousands of structures in San Antonio that are still in the 100-year floodplain, Garza told council. It would cost $1.3 billion to move everything out.
Through budgets and bond programs, about 3,000 structures have been moved out of the 100-year floodplain. City ordinance prohibits further development in those areas and in the “flood fringe.” Houston has no such ordinance.
It’s cost prohibitive to update infrastructure like bridges, drainage projects, and other infrastructure to withstand a 1,000-year flood like Harvey, Garza said.
The recently-constructed Binz-Engleman Bridge, for example, cost $8.7 million and was 600 feet long to accommodate for a 100-year flood. If the city built it with Harvey in mind, it would need to be 2,560 feet long and would cost $37.1 million.
“I get it, we can’t build bridges for 1,000 year flooding events,” Mayor Ron Nirenberg said, suggesting that a change to development pattern and building codes could at least help mitigate loss of life and property during such events.
Soil is another obstacle of water management in San Antonio. He estimated that about 60-70% of San Antonio lies on top of clay, which does not absorb water as easily as other soil.
That’s another “push and pull” issue, Graham said, with developers.
“Our analysis has shown that there is an upward trend of impervious cover and an upward trend in the peak flows, so we believe there is a correlation,” he said.
Asked if impervious cover – surfaces that prevent water from reaching the ground – or clay has a larger impact on flooding, Graham said the science is out.
“We’re going through that sensitivity analysis right now.”
If we can’t build ourselves out of flooding, then we can at least have the tools – like the risk map – to help people understand and mitigate risk, said Suzanne Scott, general manager of the river authority. But she hopes another takeaway from the Harvey model and the discussion Wednesday is a new look at applying policy and regulations.
“We want to be more granular about how we address these issues,” Scott said. When policy makers start talking about sweeping changes to citywide devleopment regulations, “that’s where people get concerned.”
Scott suggests addressing issues in more targeted areas that need it most.
“It doesn’t have to be a one-size-fits-all” for the city.