2019_KittitasCountyHMP_Volume 1_For Adoption

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CHAPTER 8. DAM FAILURE 8-10 8.4. CLIMATE CHANGE IMPACTS Potential changes to the hydrographs used to design dams due to the impacts of climate change are a growing concern for the safety of our nation’s dams. Dams are designed partly based on assumptions about a river’s flow behavior, expressed as hydrographs. As this is based on historical data, dams are not designed to be forward looking. Climate change is expected to create more variable temperatures and precipitation events, and dams are not prepared to handle the expected fluctuations in the water supply. Changes in weather patterns can have significant effects on the hydrograph used for the design of a dam. If the hygrograph changes, it is conceivable that the dam can lose some or all of its designed margin of safety, also known as freeboard. If freeboard is reduced, dam operators may be forced to release increased volumes earlier in a storm cycle in order to maintain the required margins of safety. Such early releases of increased volumes can increase flood potential downstream. Throughout the west, communities downstream of dams are already experiencing increases in stream flows from earlier releases from dams. Dams are constructed with safety features known as “spillways.” Spillways are put in place on dams as a safety measure in the event of the reservoir filling too quickly. Spillway overflow events, often referred to as “design failures,” result in increased discharges downstream and increased flooding potential. Although climate change will not increase the probability of catastrophic dam failure, it may increase the probability of design failures. 8.5. EXPOSURE An exposure analysis for dam failure was performed using dam inundation maps provided by Kittitas County, and building data from the Kittitas County Assessor. Structures and their respective total values (building and contents) are identified as at-risk if they are exposed to at least one of the inundation areas. Structures may be exposed to more than one dam failure inundation area but are not identified as part of this assessment. Data utilized for this assessment included the following datasets: – Cle Elum Dam Failure Inundation Area; – Easton Dam Failure Inundation Area; – Kachees Dam Failure Inundation Area; and – Keechelus Dam Failure Inundation Area. 8.5.1 Population All populations in a dam failure inundation zone are exposed to the risk of a dam failure. Using GIS, residential structures that intersect the combined dam inundation area were identified, and an estimate of population was calculated by multiplying the number of residential structures by the Kittitas County average of 2.32 persons per household. Using this approach, the estimated population living in the mapped inundation areas is 4,417, or 12 percent of the county’s population. Table 8-3 summarizes the at-risk population in the planning area by city. Table 8-3. Population At Risk From Dam Failure Jurisdiction Affected Population % of City Population Cle Elum 1,111 62.2% Ellensburg 722 7.7% Kittitas 0 0% Roslyn 0 0% South Cle Elum 522 94.5%