If any flood control structures or channels are damaged or overtopped in Pima County during storms this winter, it won’t be the first time.
In northern California in the 1980s and in Colorado in 2013, officials reported that concrete and rock-based flood control structures were damaged or overtopped during floods.
The structures were different than the flood control projects installed in the Tucson area. Ours are made of a soil-cement mixture, whereas the California structures are concrete, a highly compacted mixture of Portland cement, water and rock, sand or gravel.
The Colorado structures are riprap, a layer of stones used to protect soil from erosion in an area of concentrated runoff.
But in California and Colorado, experts said the channels overflowed in part due to the buildup of sediment. That’s a concern cited by University of Arizona Hydrology Prof. Victor Baker, who worries the Santa Cruz River’s soil cement lining through downtown Tucson could be overtopped if a big flood rolls in during this El Niño-era fall and winter.
In Northern California’s Marin County and Santa Cruz County in the 1980s, two concrete flood control channels were overtopped when carrying significantly less floodwater than they were designed to carry, said Phillip Williams, a retired civil engineer and flood management specialist who ran a San Francisco-based consulting firm from the 1980s until recently.
In 1982, for instance, the channel in Marin’s Corte Madera Creek overtopped carrying 4,700 cubic feet of water and debris per second. The channel was designed to carry 7,800 cubic feet per second, Williams reported in a study written during the 1980s.
The culprit for the unexpected flooding was a term he calls “roughness.” It can refer to the physical roughness of the channel material, or the presence of sediment, gravel or boulders passing over it. Analyzing the 1982 Marin County flood, Williams found that a large bar of coarse sediment had formed at the mouth of the creek.
In Colorado, the riprap channels have been overtopped at times, although the bigger issue is that floods eventually find weakness in channel banks and start carving into them, causing massive erosion, said Chris Sturm, stream restoration coordinator for the Colorado Water Conservation Board in Denver.
“Traditional flood control channels are designed for a static environment. We want to shepherd a 100-year flood, so we’re going to design this thing to pass a 100-year flood,” Sturm said. “Sometimes they do it very well, sometimes they don’t.”
Rivers have a natural tendency to meander, and having hardened banks along them is at odds with what they want to do, he said.
Pima County used riprap in the 1970s and early 1980s, but eventually replaced it with soil cement because it holds up better. If El Niño brings large floods this winter, the county’s soil cemented channels will get one of their biggest tests.
