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Mesa County, Colorado Kannah Creek Floodplain Information Report This floodplain mapping report is prepared for the 34 miles of Kannah Creek beginning on the top of the Grand Mesa and ending at the Gunnison River on the other side of Highway 50. The basin originates on top of the Grand Mesa with the majority of the top of the Mesa being included in the basin. The highpoint elevation is 10,880 feet. The major basin can be divided into two portions (an upper and lower) characterized differently by vegetation, topography, and development. This division occurs where the heavily wooded, steep upper basin flattens into the arid land of the Grand Valley. The upper portion lacks major subdivision development, and a few scattered structures exist along the river corridor. The lower potion consists of a mixture of low density subdivisions and irrigated farms. This lower portion is characterized by somewhat flatter slopes and contains smaller tributary channels. Soils in the whole basin primarily belong to the 'B' hydrologic classification, as defined by the Natural Resource Conservation Service. These soils are generally characterized by relatively high permeability and low runoff. Small. scattered pockets of 'c' and 'D' classifications also exist. Study Area 2.0 Introduction Kannah Creek is a natural drainageway located on the Grand Mesa in Mesa County, Colorado, southeast of the City of Grand Junction. Kannah Creek is a right bank tributary of the Gunnison River, a major tributary to the Colorado River. The Kannah Creek drainage basin is 133 square miles and begins at elevation 10,880 feet at the top of the Grand Mesa and joins the Gunnison River at elevation 4,660 feet. The perennial channel conveys flows east to west and crossings Highway 50 near the town of Whi tewater. 1.0 Two reports 14.01 miles of Kannah Creek floodplain have been mapped as part of this project. issued for this A total of have been Indian Creek (Station 0+00) through a distance of 6.50 miles. This reach includes 50, Private Drive Bridge and Kannah Creek drainageway 'Y Lower Kannah Creek - from the confluence with Kannah Creek Road Bridge (Station 343+32), three channel crossing structures of Highway The average longitudinal slope along the study reach is 0.025 feet per foot. The slope ranges from 0.0136 along Kannah Creek Road to 0.123 on the Grand Mesa. The channel characteristics range from a canal-like channel clean of vegetation and debris with an average Manning's roughness coefficient of 0.030 to a deep broad channel heavily vegetated and full of debris with an average Manning's roughness coeffi ci ent of 0.100 Road Bridge 'Y U er Kannah Creek - from the Kannah Creek Road Bridge (Station 0+00) at the through an upper Kannah Creek Road Bridge (Station 396+49), a distance of 7.51 miles. This reach includes four channel crossing structures of a Private Drive Bridge, Divide Road Bridge, Pedestrian Bridge and the upper Kannah Creek Road Bridge Hydrologic Analysis As a starting point, two regression equations were used for the entire basin, followed by a study of gage records for model calibration, and finally a detailed model using HEC-HMS which produced all values used in the hydraulic model. The many of lakes in the upper basin reach makes the hydrologic analysis more complicated, requiring several methods of analysis 3.0 1.1 Purpose Prior to this study, the Kannah Creek floodplain was neither published nor delineated as a FEMA Flood Zone. The two FIRM panels for this drainageway are 0801150700B and 0801150675B. This floodplain analysis was requested because it is an area of active development along the drainageway, requiring accurate hazard mapping and floodplain elevations Colora uation The Colorado Water Conservation Board's (CWCB) Guidelinesfor Determining lOO-Year Flood Flows for Approximate Floodplains in Colorado is intended to provide an approximate flow rate for "rural or largely undeveloped watersheds". These flow rates are determined through regression equations varying by subregion. Kannah Creek basin lies within the Lower Gunnison River Subregion. Applying the regression equation: 0=180.8(A).578 where A equals the basin area of 133.22 mi2 at the mouth, yields a This report was prepared to provide information relative to the occurrence of floods and to guide local officials in planning the use and regulation of the floodplain areas so that flood hazards and future flood damages are minimized gical Survey also provides subregional regression equations applicable to unregula streams. The map provided by the Analysis of the Magnitude and Frequency of Floods in Colorado bisects the Kannah Creek basin into the Northwest and Mountain regions, yielding two peak flow rates. The more conservative is the Northwest region using the equation 0=104.7(A).624 A equals the basin area of 133.22 mi2 1 100-year peak flow of3.056 cfs 100-year peak flow of 2217 cfs mouth, yields a at the where This is a "limited detailed" study as defined by FEMA, and the report data includes flooded area maps delineating the 100-year flood boundaries, flood profiles and floodwater surface elevations for the 100- year floods. No floodway analysis has been completed for this reach by this study 1.2 Related Studies Rhino Engineering also prepared two Floodplain Elevation Certificates for properties located near the Divide Road crossing on Upper Kannah Creek. The certificates were dated November 18, 2002 for 146 Divide Road, and June 9,2003 for Dalton Subdivision at Divide Road. A 3,000 cfs 100-year flow rate was used for the latter study, with a note that "There is a potential risk to any structure ifbreachment of any upstream dam occurs. If failure occurs at any dam, it may be possible that flows in excess of the 100-year design storm are possible"