On Course: San Antonio's Mission Reach River Improvements Program
The San Antonio River Oversight Committee had invested a great deal of time and effort in developing a long-term improvement program for Mission Reach, a nine-mile stretch of the river that includes a trail system connecting many of the area's historic missions. The original plan called for undoing many of the flood control measures built in the twentieth century, such as straightened channels, dams, and concrete structures. Though effective in reducing the risk of flooding to homes and businesses over the years, the changes had robbed the Reach of much of its beauty and environmental quality.
By restoring the natural meanders of the San Antonio River and incorporating a system of less-obtrusive flood prevention measures, the committee hoped to restore the lost balance between the Reach's developed and natural environments. The program, which would also create new opportunities for visitors to explore the area's rich history and environmental diversity, enjoyed strong citizen support and—perhaps most importantly—a solid funding strategy through partnerships with the City of San Antonio, Bexar County, and the US Army Corps of Engineers.
A master plan commissioned by the committee pulled together many divergent stakeholder groups and developed a strong consensus for returning the neglected portions of the river to assets for San Antonio. Moving the master plan forward to implementation, however, became an immense task. As more data were gathered and an accurate hydraulic model was constructed of the river, it became apparent that some strong refinement was needed to move the project forward.
"The original master plan for Mission Reach was based on incomplete data about the area's fluvial geomorphology—the science of how the grade and shape of a river channel affects its flow," explains Kevin Conner, a landscape architect with Carter & Burgess who is leading the firm's preliminary design services for the Mission Reach improvements program. "That data indicated a river course with many tight meanders. The more we studied and modeled the river flows, it became clear that was not the case."
Indeed, expert studies commissioned by Carter & Burgess revealed a Mission Reach with fewer, more gradual meanders. Left unaddressed, those discrepancies would have had major implications on the river and adjacent flood-prone areas.
"The flow of a hundred-year flood carries 80,000 cubic feet of water per second," Conner says. "That's enough to fill 900 average-size swimming pools in an hour, and certainly enough to overwhelm anything in the floodplain."
This revelation was certainly not the news the committee wanted to hear. Not only would the revised fluvial geomorphology force a fundamental change to the design of Mission Reach, but it would also make the project more expensive.
"No doubt, we were at a crossroads," agrees Jim Boenig, director of engineering for the San Antonio River Authority. "This issue could have easily cost the project much of the support needed to carry it through."
But just as rivers ultimately find a way to the sea, the project team simply altered the course of the Mission Reach River Improvements Program. "Obviously, changing the river's fluvial geomorphology characteristics was out of the question," Conner says. "So instead of adapting the entire vision, we looked for ways to adapt the design and still fulfill as much of the vision as possible."
For example, the revised design calls for the addition of land to designated areas to achieve more meanders combined with a different riffle structure. "Instead of building several weirs in the main channel, we specified changes in the grade that would reduce the speed of the flow," Conner explains. "We also designed wider spots in the river that give the appearance of more meanders."
Boenig agrees that although the revised design is not what the oversight committee originally conceived, it may very well be the next best thing. "From the air, the river will be far less sinuous than many people imagined," he says. "But from ground level or a bridge, it will have all the appearances of the classic meandering river."
Conner notes that although the Mission Reach project still faces funding and technical hurdles, the experience has provided valuable lessons. "There's no such thing as 'overcommunicating' when multiple agencies are involved," he says. "Many of the issues that forced redesign affected the funding structure and responsibilities. You cannot assume that every agency will see these issues the same way. The biggest lesson is the importance of sticking to the vision. When you do that, the community will support you as you work out the details."
That's a lesson Boenig heartily endorses. "We learned that there was no 'right answer,' but rather a range of options that we had to consider individually and as part of the overall strategy."