[env-trinity] Trinity Journal: Study finds Trinity, Lewiston among NorCal dams at greatest risk from climate change

[Tom Stokely]

http://www.trinityjournal.com/news/environment/article_0771e5be-3a31-11e9-9689-971fd35ed960.html


Study finds Trinity, Lewiston among NorCal dams at greatest risk from climate change
   
   - By AMY GITTELSOHN The Trinity Journal
    
   - Feb 27, 2019
    
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Trinity Dam is among several major dams a new study identifies as being at particular risk from flooding due to climate change.

This study looks at how California’s dams will be tested in the future when more precipitation is anticipated to fall as rain rather than snow, and the snow we do get melts faster.

The Trinity Reservoir held back by Trinity Dam historically has been filled mostly by snowmelt, and the snowpack is well above average so far this year. However, the study cites literature of a trend toward rain.

The study looked at 13 major dams in northern and central California.

In a research letter on their study titled, “Climate-induced Changes in the Risk of Hydrological Failure of Major Dams in California,” authors Iman Mallakpour and Amir AghaKouchak from U.C. Irvine and Mojtaba Sadegh from Boise State University say, “Noticeably, the New Don Pedro, Shasta, Lewiston, and Trinity Dams are associated with highest potential changes in flood hazard.”

Author AghaKouchak has stressed that the researchers’ analysis is based on hydrologic failure probability, and hydrologic failure doesn’t necessarily lead to physical failure of a dam.

“However, the likelihood of failure of a water infrastructure is generally expected to increase because of more frequent exposure to extreme events,” according to the authors.

“Our results can be used as input into a structural model for future analysis,” AghaKouchak said in an email to the Journal.

The study used various models to see how the dams would be challenged if climate change continues on its current path and if steps are taken to curb greenhouse gasses.

They projected failure probability to increase for most California dams in a warming climate. Of the 13 studied, they found that only one dam, Folsom, would not have increased likelihood of failure from climate change.

The authors note that major reservoirs in California have an average age of over 50 years and were built in the previous century with limited data records and flood hazard assessment.

Looking at the frequency in flooding, the researchers found the highest change in frequency to be in the North State where Trinity, Lewiston and Shasta Dam are located. For these dams, the historical “100-year flood” is projected to become a 30-year flood under the reduced greenhouse gasses scenario and a 20-year flood under the status quo.

In other words, what used to be a flood with a 1 percent chance of occurrence in any given year will increase to up to a 5 percent chance in those northern dams.

The 2017 Oroville Dam spillway failure — which caused the evacuation of almost 200,000 residents of Yuba, Sutter and Butte counties — comes up in the report as an event caused by severe flooding and the dam’s poor quality of spillway concrete. The dam’s main spillway and emergency spillway were both damaged in the incident.

The Oroville Dam event raised concerns about Trinity Dam which doesn’t have an emergency spillway. Dam managers have said they operate Trinity Dam safely, leaving room for storm events.

This new study indicates in the coming decades there will be less room for error.

Based on the probability of flooding, the researchers found that several dams have an even greater chance of failure than Oroville, regardless of structural integrity. At the top of that list are several earthen dams, New Don Pedro on the Tuolumne River, Lewiston and Trinity, as well as concrete Shasta Dam.

The authors say the work highlights the importance of developing and modifying adaptation strategies against climate change for these aged dams’ operation and management, alongside adequate and timely maintenance.
   
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