Laserfiche WebLink
<br />mates of the el evations of floods of the selected recurrence inter- <br />vals along each of the flood sources. <br /> <br />Water surface elevations for the 10-, 50-, 100-, and 500-year floods <br />for the Arkansas River and Willow Creek were computed using the U.S. <br />Army Corps of Engineers HEC-2 Water Surface Profiles Program (Refer- <br />ence 6). Cross sections for this analysis were obtained from aerial <br />photographs flown in April 1980 (Reference 7l, coupl ed with appro- <br />priate field verification. All bridges and culverts were measured in <br />the field to obtain elevation data and structural geometry. <br /> <br />Locations of selected cross sections used in the hydraulic analyses <br />are shown on the Flood Profi 1 es. For stream segments for whi ch a <br />floodway is computed (Section 4.2), selected cross section locations <br />are also shown on the Flood Boundary and Floodway Map. <br /> <br />Starting water surface elevation for the 10-, 50-, 100-, and 500-year <br />floods along the Arkansas River and Wi 11 ow Creek were cal cul ated <br />using a slope area method with estimated energy grade line slopes and <br />approximated water surface elevations. <br /> <br />Roughness factors (Mannings un" values) were determined by field in- <br />spection. Ground level photography aided in the selection of rough- <br />ness factors and aerial photography was used to determine the 1 imits <br />of each roughness zone. Roughness val ues for the main channel of <br />Willow Creek ranged from 0.024 to 0.030 with flood plain values <br />ranging from 0.024 to 0.035 for all floods. Roughness values for the <br />main channel of the Arkansas River ranged from 0.02 to 0.03 with a <br />flood plain value of .065. Flood profiles were drawn showing com- <br />puted water-surface elevations to an accuracy of 0.5 foot for floods <br />of the selected recurrence intervals. The flood profiles presented <br />in thi s study are based on the effects of flow through unobstructed <br />hydraulic structures and area, thus, considered valid only if these <br />structures do not fail and remain unobstructed. <br /> <br />Horizontal control is based on Colorado State Plane Coordinate System <br />South Zone 1929 USC, and the vertical control is based upon the <br />National Geodetic Vertical Datum of 1929 (NGVD). Elevation reference <br />marks used in this study are shown on the maps. <br /> <br />4.0 FLOOD PLAIN MANAGEMENT APPLICATIONS <br /> <br />The National Flood Insurance Program encourages state and local governments <br />to adopt sound flood pl ai n management programs. Therefore, each Flood <br />Insurance Study incl udes a flood boundary map desi gned to assi st commu- <br />nities in developing sound flood plain management measures. <br /> <br />7 <br />