Laserfiche WebLink
<br />, <br /> <br />Cross sections surveyed in 1979-80 at 2- to 3-mile intervals provided data <br />for computation of actual flood-boundary points, which then were shown on topo- <br />graphic maps prepared 10-25 years earl ier. Figure 1 illustrates the conditions at <br />cross-section 89. Although the 10-ft contour locations on the 1969 topographic <br />map agree fairly closely with the 1980 survey data, the channel had filled on the <br />left bank and eroded on the right bank. The width of the 100-year flood-prone <br />area in 1980 was about 500 ft and was located between cross-section distances of. <br />1,205 and 1,708 ft; the flood depth was 9 ft (10.0 ft gage datum minus 0.9 ft, the. <br />lowest surveyed point in the cross section). Thus flood elevations of 5,439 ft <br />above mean sea level were accurately represented on the left and right bank at~ <br />cross section 89. Flood depths for 1980 hydraulic conditions were sketched <br />between cross sections on the older topographic map, as described in the paragraph <br />below. . <br /> <br />, <br /> <br />~ <br /> <br />The idealized contour map shown in figure 2 illustrates the. method used to <br />draw flood-prone area boundaries. Generally, in places near the cross sections <br />where the flood level was calculated, lines perpendicular to the streamflow were <br />sketched at stream-contour. crossings. Points on each bank were located outward <br />0.9 (9-ft depth divided by a 10-ft contour interval) of the dista.nce to the next <br />higher contour. Similarly, another line perpendicular to the streamflow (dashed <br />I ine in fig. 2) was placed 0.1 (l-ft depth divided by. a 10-ft contour interval) <br />upstream towards the next contour crossing the stream, which was ass~~ed to be <br />1 ft higher. Thus pointsB and B' were plotted at elevation 11+9 ft of flood <br />water=20 ft, at the intersection of the dashed I ine and the 20-ft contour line. <br /> <br />" <br /> <br />Many upstream drainage channels have slopes in excess of 150 ft/mi; hydraulic <br />tonditions indicate that supercritical flows probably occur in the incised sandy <br />" channels. For. cross sections in those areas, the 100-year discharge was assumed <br />to be at critical~flow levels (minimum specific energy), and the corresponding <br />ground elevation was used for the conservative 100-year flood-plain level. <br />Computations for cross-section 70 on West Bijou Creek at the Elbert County-El Paso <br />County line are in the sections, "Computer Printouts of Cross-Section Properties" <br />and "Sample Computations of Flow" (at two cross sections on West Bijou Creek), to <br />illustrate this condition. <br /> <br />As the larger creeks cross the county boundary on the north and east, bed <br />slopes flatten to less than 50 ft/mi, and higher level subcritical flows occur at <br />100-year flood-discharge values. For example, cross section 89, West Bijou Creek <br />at the Arapahoe County-Elbert County line (See sections on "Computer Printouts of <br />Cross-Sect ion Propert i es" and "Sample Computat ions of Flow" at t\~O cross sect ions <br />on West Bijou Creek) had a stream slope of 32 ft/mi. It will pass 29,500 ft3/s at <br />a survey height of 10.0 ft. <br /> <br />. <br /> <br />Four computed values were printed on the base maps adjacent to each cross <br />section. The values include 100-year discharge, in cubic feet per second; 100- <br />year depth at the cross section, in feet; width of the 100-year flood, in feet; , <br />and velocity (discharge divided by cross-sectional area), in feet per second. <br /> <br />4 <br />