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<br />Mesa County, Colorado <br /> <br />(. dwg) <br /> <br />To provide a national standard without regional discrimination, the 1 percent annual chance (100-year) <br />flood has been adopted by FEMA as the base flood for floodplain management purposes. For Upper <br />Kannah Creek, the 100-year floodplain boundary has been delineated using the flood elevations <br />determined at each cross section. The floodplain boundaries were interpolated between the modeled <br />cross sections using the topographic contours with an interval of 2 feet that were obtained from the GIS <br />mapping data provided by Mesa County <br /> <br />The 100-year floodplain boundary is shown on the attached Floodplain Maps (Sheets 1 through 9). On <br />these maps, the 100-year floodplain boundary corresponds to the boundary of the areas of special flood <br />hazards (Zones AE and AO) <br /> <br />New mapping was developed for this floodplain study. Merrick was contracted to map the study reach <br />using LIDAR. The following outlines the mapping specifications <br /> <br />Central Zone, North <br /> <br />format <br />DTM meet NSSDA vertical accuracy for 2' contours (Accuracyz = 1.2' / RMSEz = 0.6') <br />One and one-half foot (1.5') pixel resolution color digital orthophotography (.tif / .tfw) <br />Digital orthophotography and topography meets NMAS for 1"=100' scale mapping <br />(horizontally) <br /> <br />Deliverables <br /> <br />delivered in AutoCAD drawing <br /> <br />4 <br /> <br />survey of ground control checkpoints <br /> <br />88) <br /> <br />(SPCS), <br /> <br />(NA VD <br /> <br />1.5' pixel resolution color digital orthophotography (.tif / .tfw) <br />AutoCAD drawing file (.dwg) of topographic basemap <br />AutoCAD drawing file (.dwg) of the DTM data <br /> <br />Datums / Units <br />Horizontal Datum - Colorado State Plane Coordinate System <br />American Datum of 1983, adjusted 1992 (NAD 83 /92) <br />Vertical Datum - North American Vertical Datum of 1988 <br />Units U. S. Survey Feet <br /> <br />1"=600' scale color stereo photography <br />Perform photo-identifiable (photo-id) GPS <br />Twelve (12) photo-id control checkpoints <br />Digital Aerial Triangulation (DA T) and report <br />Photogrammetrically compiled digital terrain model (DTM) <br />NO planimetric feature collection <br />DTM accurate to support two-foot (2') contour mapping <br />Interpolate 2' contours from DTM <br />Final DTM and topographic mapping (i.e <br /> <br />contours) <br /> <br />2 <br /> <br />Floodplain Boundaries <br /> <br />Topography <br /> <br />Mapping <br /> <br />Pro1 ect <br /> <br />5.0 <br /> <br />6.0 <br /> <br />The hydraulic analysis of the study area was based on subcritical depth computations and information <br />obtained from available GIS topographic data and field reconnaissance. The model was prepared <br />assuming that the flow is steady, gradually-varied, and can be modeled in one-dimension. The results of <br />the HEC-RAS hydraulic computer model can be seen in Table 5 - Flood Frequency - Elevation and <br />Discharge Data. <br /> <br />Locations of the cross sections used in the hydraulic analyses are shown on the Flood Profiles and on the <br />Floodplain Maps (Sheets 1 through 9). Floodway elevations and stations were also calculated as part of <br />this detailed study. The floodway extents are shown on the Floodplain Maps (Sheets 1 through 9). <br /> <br />The model was started with a known water surface elevation where the Lower Kannah Creek Floodplain <br />Information Report ended. <br /> <br />It was assumed that all culverts and channels were free of silt and debris. The hydraulic analysis for this <br />study was based on unobstructed flow at all structures. The flood elevations shown on the profiles are <br />thus considered valid only if hydraulic structures remain unobstructed, operate properly, and do not fail <br /> <br />The National Flood Insurance Program (NFIP), encourages State and local governments to adopt sound <br />Floodplain Management Programs. Therefore, each Flood Insurance Study provides 100-year flood <br />elevations, delineation of the 100-year floodplain boundary to assist communities in developing <br />measures <br /> <br />A Limited Detailed floodplain analysis of the hydraulic characteristics of Upper Kannah Creek was <br />performed to provide estimates of the water surface elevations in the drainageway during the 100-year <br />storm event. Water-surface elevations were computed using the U.S. Army Corps of Engineers HEC- <br />RAS computer modeling program, Version 3.1.3 (Reference 5) <br /> <br />Cross section data for the backwater analysis of Upper Kannah Creek was obtained from a new lidar <br />aerial flight by Merrick with 2-foot contour intervals. The contour information was imported into GIS <br />and cross-sections were cut electronically. These prepared cross-sections were imported directly into <br />HEC-GeoRAS for use as the base model. Information regarding structures that cross or pass under <br />Kannah Creek was compiled from a combination of drawings, structural reports, conversations with <br />local officials, and field measurements <br /> <br />were chosen by engineering <br />The ranges of roughness <br /> <br />Overbank <br />0.03-0.12 <br /> <br />Manning's "n" roughness coefficients used in the hydraulic computations <br />judgment and based on field observations of the wash and floodplain areas <br />coefficient values for Upper Kannah Creek are as follows <br /> <br />Channel <br />0.03-0.07 <br /> <br />Applications <br /> <br />Floodplain Management <br /> <br />Hydraulic Analysis <br /> <br />Kannah Creek <br />Floodplain Information Report <br /> <br />Stream <br />Kannah Creek <br /> <br />floodplain management <br /> <br />4.1 <br /> <br />4.0 <br />