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. WATERSHED <br />T- SCIENCE & ENGINEERING <br />MEMORANDUM <br />To: Mark Cook, Kittitas County Public Works <br />From: Chris Frei, P.E., Marissa Karpack, E.I.T., Watershed Science & Engineering <br />Date: December 21, 2020 <br />Re: DRAFT - Crystal Creek and Cedar Gulch Creek Hydraulic Analysis <br />1.0 INTRODUCTION <br />Watershed Science & Engineering (WSE) was hired by the Kittitas County Department of Public Works to <br />complete hydraulic modeling and analysis to evaluate flood risk along Crystal Creek and Cedar Gulch Creek <br />in Kittitas County, WA. Model results were used to prepare flood plain, mapping and supporting technical <br />documentation necessary to update effective FEMA Flood Insurance,Rate Maps (FIRMS) as part of a Letter <br />of Map Revision (LOMR). This work was partially funded t.htough a FEMA Cooperating Technical <br />Partnership (CTP) grant. <br />2.0 LOCATION <br />Crystal Creek and Cedar Gulch Creek are located in Kittitas County, WA. This current study covers 4.5 miles <br />of Crystal Creek from northeast of the city of Rioslyo (Roslyn) 'to the confluence with the Yakima River at <br />the City of Cle Elum (Cie Elum, see Figure 1), and,,2.5 miles.s f Cedar Gulch Creek from the foothills north <br />of Roslyn to where the creek enters the city's piped stormwater system near W Nevada Ave. <br />3.0 EFFECTIVE FEMA MAPPINE., <br />Effective mapping for Crystal Creek ai)d Cedar Gulch Creek are based on a 1981 Flood Insurance Study <br />(FIS). The lower 9900 few of Crystal,Creek within Cle Elum was studied by detailed methods. Flooding <br />along the channel cor iclor is mappdd as Zone AE and has a coincident floodway (no encroachments). <br />Overbank flooding within Cle Elum is mapped as Shaded Zone X (shallow 100 -year flooding). All other <br />reaches of Crystal Creek and daf Gulch were studied by approximate methods and are mapped using <br />the Zone A designation. <br />A 1991 LOMB (91-10-22P) removed Zone A flooding within Roslyn by demonstrating that 100 -year <br />flooding on Cedar Gulch Creek would be contained within Roslyn's piped stormwater system. The current <br />study does not update mapping within the region covered by the LOMR. Roslyn is in the process of <br />upgrading their stormwater system and will assess the need for updated flood mapping after those <br />improvements have been implemented. <br />For the current study it was assumed that Roslyn's stormwater system has minimal impact to flood routing <br />through the city. This is because the piped stormwater system outfalls to Crystal Creek at the same <br />location that any overland flooding would return to Crystal Creek. <br />Watershed Science & Engineering • 506 2nd Ave, Suite 2700, Seattle, WA 98104. 206-521-3000 <br />