Much has been written on potential effects and adaptations for California’s water supply from climate warming, particularly from changes in snowpack accumulation and melting, sea level rise, and possible overall drying or wetting trends. But what about floods?
In a recent paper in the journal San Francisco Estuary and Watershed Science, we along with co-authors from the US Army Corps of Engineers review much of the literature to date and examine how Shasta, Oroville, and New Bullards Bar reservoirs might adapt to floods in a warmer climate, including a climate that is either wetter or drier.
Since no one knows exactly what future floods will look like, the nine largest floods from the historical record were hydrologically modified to be warmer and either wetter or drier, using the National Weather Service hydrologic model used for flood forecasting. These many modifications to past major floods were then run through a US Army Corps of Engineers’ model for flood operation of these reservoirs to evaluate what might happen, given the way we currently operate these reservoirs. The results were both reassuring and disturbing.
1. Warming generally worsened flood inflows into reservoirs. Even with less precipitation, warmer conditions often increased flood inflows to reservoirs. When more precipitation fell as rain, rather than snow, and more existing snowpack melted, flood volumes increased. This was particularly true for historical storms that were “cold”, where much of the precipitation was held as snowpack. Warm storms, which historically produced less snow, were less affected by warming.
2. Reservoirs with flood operating rules that respond to the wetness of their watersheds seemed to adapt well to changes in climate, even fairly severe changes in temperature and precipitation. This was true for Shasta and Oroville Reservoirs, whose existing flood operation rules vary with moisture conditions upstream. This shows that existing reservoirs may have considerable ability to accommodate flooding effects of climate warming.
3. Reservoirs with flood operating rules that do not respond to upstream conditions may perform poorly with climate warming. For example, New Bullards Bar’s flood rules do not change with upstream snowpack and wetness conditions. For many plausible climate changes, modifications of past floods overtopped this dam, a potentially catastrophic flood risk for downstream residents.
Large uncertainties are common when dealing with both the future and the weather. Nevertheless, some things can be known, or at least strongly suspected and supported, from reasoning that is organized, refined, and tested using computer modeling.
Accommodating changes in climate with changes in operating rules can often require changes in reservoir outlets (which can be costly) and changes in federal operating policies and authorizing legislation (which can be protracted and difficult). Nevertheless, it is comforting to know that existing policies for some reservoirs seem to do well with changes in climate, and that making other reservoirs more reactive to upstream wetness conditions might make them more resilient to changes in climate, even before we know what the changes are. Such changes in policies, while politically awkward and requiring some expense, appeared likely to be less expensive than major reservoir expansions or the costs of a major flood.
In terms of floods, climate warming need not mean that the sky is falling. We are likely to have considerable ability to respond effectively, but in some cases, we will likely to need to make major changes. Appropriate preparations will not be easy, but they should be possible with capable institutions at the federal (US Army Corps of Engineers), state (DWR, Central Valley Flood Protection Board), and local (counties, cities, and levee districts) levels.
Willis, Ann D; Lund, Jay R.; Townsley, Edwin S.; Faber, Beth A (2011), “Climate Change and Flood Operations in the Sacramento Basin, California,” San Francisco Estuary and Watershed Science, July, Vol. 9, No. 2, 18 pages.