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Kannah Creek Floodplain Information Report Mesa County, Colorado GIS software was used to land use type, and well as watershed features such as topography, soil determine the following subwatershed parameters ) ml Contributing Drainage Area (sq Catchment Length (mi.) Catchment Slope (ft/ft) . . . Previous Studies Two previous reports were consulted for hydrology near the Highway 50 bridge with about 100 square miles tributary: RG Consulting Engineers, Inc Floodplain Application Permitfor Bridge Maintenance Activities and Rhino Engineering, Inc Floodplain Elevation Certificate. RG engineers used approximate drainage basin values and an unknown model to predict the 100-year peak flow rate of3,710 cfs, while Rhino engineers used the assumed rate of 3,000 cfs. Both reports cited difficulty in obtaining information to determine the peak flow rates Using this information, a hydrologic model was prepared in HEC-HMS. This process begins with the development of a routing schematic, which is a simplified graphical depiction of the hydrologic network. For each of the subwatersheds, a tributary area, SCS Curve Number, % impervious, and lag time are entered into the model. Information is also entered for each of the routing reaches. The length, Manning's roughness coefficient, and physical dimensions of each reach were entered as data slope, input. HEC-HMS Model (Used for this Stud A 100-year hydrologic analysis of the Kannah Creek watershed was preformed using HEC-HMS, a graphical user interface which updates the Army Corps of Engineers' HEC-l model. SCS Dimensionless Unit Hydrograph option within HEC-HMS and various basin parameters simulates the hydrologic response of multiple subwatersheds within the Kannah Creek watershed and routes the developed hydrographs through various reaches The HEC-HMS model performed a rainfall-runoff simulation based on the geometric parameters developed for the various subwatersheds, reaches, and reservoirs. The model used the design rainfall and determined the quantity and timing of the resulting runoff. Through subwatershed hydrograph creation and combination, peak design flows were determined at key design points throughout the Kannah Creek watershed. 100-year design maps. The 24-Hr The hydrologic analysis for the watershed was based on an SCS synthetic storm and the rainfall depth that was found on the NOAA Atlas 2 Volume III-Colorado's Isopluvia rainfall depths are summarized in Table 1 - 24-Hr Rainfall below. The model was calibrated to the USGS gage location, shown as "Junction 4." The modell00-year was calculation to be 1,708 cfs at gage, which is slightly higher than predicated by the 54 years of gage data. The modeled 100-year flow further downstream at the Highway 50 was 2,843 cfs which is in reasonable agreement with previous studies by CDOT and others at 3,000 cfs to not warrant a restudy of the crossing structures Summary of Discharges Drainage Area TABLE 4 Return 24-hr Frequency Rainfall* (yr) (in) 2 1.00 5 1.40 10 1.70 25 2.05 50 2.30 100 2.60 500 3.25 TABLE 3 24-hr Rainfal Peak Discharges (cfsl 50- 100- 500- Year Year Year 10- Year (Sguare Milesl Floodinfl Source and Location Kannah Creek * The 24-hour rainfall depths for the 2-yr through 1 OO-yr storm events were found on the NOAA Atlas 2 Volume III-Colorado Isopluvial maps. 3,879 5,317 6,545 9.102 1,708 2,329 2,843 4.271 1,007 1,362 1,649 2.651 190 251 291 649 57.52 71.31 98.13 133.22 Upper Kannah Creek Road Bridge near Gage 1.9 mi. upstream Divide Road near Juniata Res Confluence with North Fork Kannah Creek Confluence with Indian Creek A linear interpolate was used to estimate the 500-year design rainfall depth. The 500-year flood hydrology may be similar a flood that could be experienced in this basin by an upstream dam break. An aerial survey comprised of two-foot contours was provided by the Mesa County GIS Department for use in the hydrologic modeling process. Another source of mapping used for the initial hydrologic and hydraulic modeling was the USGS 7Yz' quadrangles. Both of the mapping sources were available electronically. These two sources were used with computer aided drafting software to develop sub- watershed boundaries, areas and slopes 3 The Kannah Creek watershed basin was subdivided into 20 smaller subwatersheds. The subbasins were primarily delineated according to ridgelines between the major tributary waterways and the areas that drain to the various tributary waterways. Subbasins were delineated according to key design points as