March 31, 2017
Is there a Superflood in your future? Based on geologic records, scientists in the field of Paleo-Hydrology have uncovered evidence that weather events significantly outside the norm may not actually be the exception to the rule, given long enough timelines -- and the implications are concerning.
Modern dams and other infrastructures that are intended to harness mother nature may be seriously under-engineered to accommodate off-the-charts weather events that are now being shown to be much more common than has been assumed. The design of almost all major dams is based on having the ability to withstand the potential maximum rainfall or snowmelt contributions in a given topographical catch basin within the expected life span of the structure, The problem is that available data to this point -- rainfall records and historical estimations of flood impacts -- only reached back to the mid-1800's. Geologic timelines are much longer, but may be accelerating.
As reported recently by the High Country news, paleo-hydrologist Victor Baker of the University of Arizona has uncovered evidence that superfloods may not be all that uncommon. "By combing the Colorado River, the Green River and others in the Southwest for sediment deposits and other flood evidence and then carbon-dating the results, Baker has concluded the short-term record severely underestimates the size and frequency of large floods. On the Upper Colorado near Moab, Utah, Baker and his team estimated the average 500-year flood at roughly 246,000 cubic feet per second, more than double the 112,000 cfs that scientists had estimated drawing on the stream gage record alone. Baker's calculations put the 100-year flood at 171,000 cfs, also much greater than the previous estimate of 96,000 cfs. In comparison, legendary flooding in 1983 and 1984 that nearly overwhelmed Arizona's Glen Canyon Dam, just downstream, peaked at just 125,000 cfs. (The dam has been bolstered since then, and today engineers say it can handle flows up to 220,000 cfs.)"
Oroville Dam in California is the nations tallest (i.e.: holding back the deepest reservoir), and was nearly overwhelmed earlier this year when an unprecedented winter storm threatened to fill the lake over capacity, forcing engineers to dump so much excess water down an emergency spillway that it began to erode the channel and compromise the integrity of the dam.
The High Country News continues, "In California, too, super-floods may be more common than previously thought. United States Geological Survey hydrologist Michael Dettinger and UC Berkeley paleoclimatologist B. Lynn Ingram have studied the paleo-flood record across a broad swath of California and discovered that such floods happen at least every 200 years, and maybe more frequently."
I personally was witness to a storm event in Colorado in 2013 that was termed by paleo-hydrologists to have been "a 1,000 year flood." Here, the mountains of the Front Range north of Denver received over 13" of rainfall in a 72 hour period, completely filling reservoirs and saturating the soil so that no additional moisture would be absorbed. In the small town of Estes Park near Rocky Mountain National Park, the strain on the Olympus Dam required engineers to open every available floodgate and spillway to dump water down the Big Thompson river at a rate of almost 6,000 cfs, when a normally expected high flow for June snow runoff is in the neighborhood of under 2,000cfs.
The results for residents and communities downstream were devastating. Most bridges in the canyon were rated to withstand flows of not more than 3,000cfs, and had abutments washed away. Long stretches of state highway 34 were undercut and collapsed into the river channel, and hundreds of homes were destroyed. Several lives were lost as well, despite evacuation warnings from the Corp of Engineers.
A popular tailwater trout fishery, the biology of the Big Thompson was heavily affected. Large stretches of the river were scoured down to bedrock, leaving no rocks and gravel for insects to repopulate. Other corner sections were buried with large amounts of newly fractured pink granite gravels that rerouted stream flows. In the two years following the event, I noted gradual recovery of the etymology of the river, but with altered species distribution. Where previously the insect life of the Thompson showed a general orientation towards darker mayflies, midges and case builder caddis such as Brachycentrus, newly predominant hatches of lighter colored stone builder Caddis and Pale Morning Dun mayflies became more obvious. I assumed that this was a function of insects that were more adapted to the lighter colored gravel conditions in the river were being given a reproductive and behavioral advantage. While the fishery is on the mend, anglers who have not visited since the flood still won't recognize the river's runs and access points.
While new paleoflood data is being considered by the Corp of Engineers, there are reservations as to the applicability of it, given channel and runoff alterations in the built environment installed since undeveloped times in antiquity. There are undoubtedly financial considerations as well, as hydrologic dams are some of the largest public works projects that exist, and funding for retrofitting or complete overhaul of existing dams is probably not feasible in the current political climate.
While new data on old geologic events may be open to interpretation as to level of threat, there is an additional factor that needs to be considered. While historical timelines for superflood events are showing that there may have been an underestimation in frequency, the current scientific consensus on climate change may increase that variable, both in period and intensity. The 200 or 1,000 year events of the past may now be forecast to happen within considerably shorter timeframes. We may do well to make preparations accordingly.