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CHAPTER 11. FLOOD <br />11-13 <br />• Extreme climatic events will become more frequent, necessitating improvement in flood <br />protection, drought preparedness and emergency response. <br />The amount of snow is critical for water supply and environmental needs, but so is the timing of snowmelt <br />runoff into rivers and streams. Rising snowlines caused by climate change will allow more mountain area <br />to contribute to peak storm runoff. High frequency flood event s (e.g. 10 -year floods) in particular will <br />likely increase with a changing climate. Reduced snowpack and shifts in streamflow seasonality due to <br />climate change pose an additional challenge to reservoir system managers as they strive both to minimize <br />flood risk and to satisfy warm season water demands. <br />Along with reductions in the amount of the snowpack and accelerated snowmelt, scientists project greater <br />storm intensity, resulting in more direct runoff and flooding. Changes in watershed vegetation and soil <br />moisture conditions will likewise change runoff and recharge patterns. As stream flows and velocities <br />change, erosion patterns will also change, altering channel shapes and depths, possibly increasing <br />sedimentation behind dams, and affecting habitat and water quality. Floodplain and municipal water supply <br />infrastructure are also vulnerable to projected increases in extreme precipitation and flood risk. With <br />potential increases in the frequency and intensity of wildfires due to climate change, there i s potential for <br />more floods following fire, which increase sediment loads and water quality impacts. <br />As hydrology changes, what is currently considered a 100-year flood may strike more often, leaving many <br />communities at greater risk. Planners will need to factor a new level of safety into the design, operation, <br />and regulation of flood protection facilities such as dams, floodways, bypass channels and levees, as well <br />as the design of local sewers and storm drains. <br />11.5. EXPOSURE <br />The Level 2 Hazus protocol was used to assess the risk and vulnerability to flooding in the planning <br />area. GIS building and assessor data (replacement cost values and detailed structure information) were <br />loaded into Hazus. An updated inventory was used in place of the Hazus defaults for essential facilities. <br />Preliminary Kittitas County Flood Insurance Rate Maps (FIRMs) were used to delineate flood hazard <br />areas and estimate potential losses from the 0.2-, 1-, 2-, 4-, and 10-percent annual change flood events <br />(where flood depth grids were available). Where available, flood depth grids were integrated into the <br />model and vulnerability numbers were generated in Hazus. Where flood depth grids were unavailable, <br />an exposure analysis was performed to identify structures exposed to flood risk. All data sources have <br />a level of accuracy acceptable for planning purposes <br />11.5.1 Population <br />Population counts of those living in the floodplain were generated by analyzing County assessor and parcel <br />data that intersect with the 100-year floodplain identified on FIRMs. Using GIS, residential structures that <br />intersected the floodplain were identified, and an estimate of population was calculated by multiplying the <br />residential structures by the average Kittitas County household size of 2.32 persons per household. Using <br />this approach, it was estimated that the exposed population for the entire county is 2,225 within the 100- <br />year floodplain (5.2 percent of the total county population). <br />11.5.2 Property <br />Structures in the Floodplain <br />Table 11-3 and Table 11-4 summarize the total area and number of structures in the floodplain by <br />municipality. Using GIS, it was determined that there are 959 structures within the 100-year floodplain and <br />1,869 structures within the 500-year floodplain. In the 100-year floodplain, about 71 percent of these