Our Water Matters

A Meandering (River) Walk Down Flash-Flood Alley — Continued

In the last issue of Our Water Matters, readers learned about how meteorological phenomena heavily influenced by the Gulf of Mexico can collide with the Balcones Escarpment of Central Texas, giving rise to what the National Weather Service calls “Flash-Flood Alley.” While the term has become a global household phrase associated with the recent July 4^th floods along the Guadalupe River, the entire region, including San Antonio and its eponymous river, have long been subject to intensive flash flooding with heavy loss of life. In fact, according to Steven P. Ramsey, a former chief engineer with the San Antonio River Authority (SARA), flooding along the San Antonio River became a “recognized hazard” over 300 years ago when a flood resulted in the relocation of the Misión San Antonio de Valero (also known as the Alamo) to a “safer site” back in 1724.

Despite the known risks, intermittent flooding in the San Antonio River Basin would continue to claim lives for another two centuries before anything significant was ever done about it.  Ultimately, it took the deadly flood of 1921 that killed some 215 people across South-Central Texas, including at least 51 in San Antonio, to finally convince city leaders of the need for action. In a beautifully researched 2016 article for Civil Engineering Magazine titled “History Lesson: San Antonio’s River Walk,” journalist T.R. Witcher describes the 1921 flood as “perhaps the most famous storm in San Antonio’s history” when rainfall reached 20 to 25 inches in some parts of the system, and “water rushed along some stretches” with flows reaching 42,400 cubic feet per second. To provide some perspective, that volume exceeds the average flow rate over Victoria Falls in southern Africa, one of the world’s most prolific waterfalls. 

In the flood’s aftermath, the city’s engineers “proposed two solutions,” according to Witcher. “The first was to create a dry reservoir on the Olmos Creek … by building a massive concrete dam.” Located about 4 miles upstream from the city’s business district, the dam was intended to “hold off waters that would otherwise flood the river as it headed into the city and swamp downtown.” Once a storm had passed, the dam could then slowly release this retained water back into the river.  “The second fix was to bypass the … ‘great bend’ of the San Antonio River as it passed through downtown by constructing a 70-feet-wide channel.” The Olmos Dam was completed in 1927, and the river bend bypass channel was completed in 1930. According to San Antonio Express-News meteorologist Anthony Franze, “These features divert water from downtown San Antonio during major flood events, keeping the river … downtown at a constant level.”

In the midst of these projects, an architect named Robert H.H. Hugman introduced a plan in 1929 that he called “The Shops of Aragon and Romula,” which would eventually come to be known as the San Antonio River Walk. A 1989 article by Lila Knight in Texas Architect described the project as “a flood-prevention proposal that not only allowed for commercial development, but maintained the natural beauty of the river and its flora.” According to Knight, the original River Walk project involved “the construction of 17,000 ft of walkways, 31 stairways leading from 21 bridges, and the planting of more than 11,000 trees and shrubs” extending over 21 blocks along the river.

From 1939 to 1941, “the pilot channel was deepened, three dams were constructed, underground drains built, and floodgates installed at both ends of the river loop,” according to Witcher. Another flood in 1946 prompted Congress to authorize the San Antonio River Channel Improvements Project, “an effort to provide flood protection along more than 30 miles of the river system and its tributaries” that was completed in the 1960s and 1970s. 

More improvements were implemented in the 1980s and 1990s when the “SARA and the U.S. Army Corps of Engineers built two extensive underground tunnels to carry floodwater, which still posed a danger below the Olmos Dam, while keeping the river itself intact,” reports Witcher. The tunnels were designed as “inverted siphons, U-shaped structures in which water entering the tunnels upstream displaces an equal amount of water downstream without the need for pumps. Excess rainwater from the San Antonio River upstream of downtown can be diverted underground into the tunnels. It then passes beneath the downtown area and ascends to rejoin the river at a lower elevation downstream of downtown.” According to SARA Assistant General Manager Stephen Graham, “That was an extremely innovative game changer for our community … It pulled out huge amounts of our downtown from the floodplain. It provided 100-year-flood protection without moving a single bridge, a single utility.” While the cost of such projects often paralyzes decision-making, Witcher accurately points out that the tunnels “helped avert a major flood” when a huge storm hit San Antonio in 1998, and “with regard to damage avoided, the project basically paid for itself after that one event.”

The ASCE’s Historic Civil Engineering Landmark Program officially recognized the San Antonio River’s flood-control measures in 1996. The reasons for this honor are perhaps best summed up by Ramsey, the former SARA chief engineer: “Standing alone, most of the engineering solutions to control flooding and enhance the beauty of the river were not new. What is special is how various solutions were blended to produce something unique.” Hopefully, as the communities affected by the July 4^th floods begin the work of rebuilding, they can learn from the example of San Antonio, diligently apply the lessons of their own past, and formulate a sound approach to riverside redevelopment.

Trey Gerfers serves as general manager of the Presidio County Underground Water Conservation District. A San Antonio native, he has lived in Marfa since 2013 and can be reached at tgerfers@pcuwcd.org.