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yr*` WATERSHED <br />Memorandum <br />To: Kurt Addicott, PE, Century West Engineering <br />From: Colin Butler, EIT & Chris Frei, PE, Watershed Science & Engineering <br />Date: March 12, 2019 <br />Re: DRAFT— Bowers Field EA Flood Reduction Alternatives Analysis <br />INTRODUCTION <br />Watershed Science & Engineering (WSE) completed hydraulic modeling and analysis to support the design <br />of planned improvements at Bowers Field airport near Ellensburg, WA (see Figure 1). The airport is located <br />within the 100 -year floodplain of both Whiskey and Mercer Creeks, and planned airport improvements <br />have the potential to impact flood hazards on and downstream of the airport property. WSE updated an <br />existing HEC -RAS 2D hydraulic model of Whiskey and Mercer Creeks to evaluate several alternatives for <br />reducing flood risk to airport facilities without negatively impacting downstream properties. Results from <br />this analysis will support the Bowers Field Environmental Assessment (EA) being prepared by Century <br />West Engineering (Century West). <br />BACKGROUND <br />The Bowers Field EA currently being prepared by Century West is based on planned <br />implementation of the 2018-2023 portion of the Bowers Field Airport Master Plan Capital Improvement <br />Program. Short term projects covered under the EA include rehabilitation of runway 7-25, runway 11-29, <br />runway 11-29 parallel taxiway, and AC hold areas, in addition to other site grading, pavement removal, <br />and taxiway development. WSE was contracted by Century West to complete a hydraulic analysis for <br />selecting a preferred set of actions to reduce or eliminate problematic flooding at the airport and to <br />demonstrate that proposed actions covered by the EA will not worsen 100 -year flood conditions <br />downstream from the airport. <br />Problematic flooding on the airport runway under existing conditions is the result of overland flow <br />escaping Mercer Creek northeast of the airport both upstream and downstream of Hungry Junction Road. <br />Most of this flow concentrates in a taxiway ditch parallel to runway 11-29 before ponding and flowing <br />across the runway near the intersection of runway 11-29 and runway 7-25. Flow overtopping the runways <br />ultimately impacts airport facilities and private property downstream of the runway intersection. <br />HYDRAULIC MODELING <br />WSE completed hydraulic modeling based on an existing unsteady HEC -RAS 2D hydraulic model of <br />Whiskey and Mercer Creeks that was constructed fora study to update FEMA flood maps in Kittitas County <br />(WSE, 2018). The model was modified to run in the latest version of HEC -RAS (version 5.0.6). <br />Peak flow hydrology for Whiskey and Mercer Creeks was developed previously for the 2018 FEMA <br />mapping study (WSE, 2018). 24-hour inflow hydrographs applied to the model simulate a rain -on -snow <br />event representing typical historical flooding in the region. Table 1 lists 2-, 10-, and 100 -year annual <br />V"A I E F:S H i U,!(I, I':{F&ENGI"E E RI NI BOG.: ' ,, Suit« i cll;'r .illi ":/,9 iCi ""-1, 1 -W'00 <br />