FLOOD10348

• County: Statewide
• Stream Name: Colorado River
• Title: Floodplain Information Report, Volume 4, Colorado River from Rifle to Utah Stateline
• Date: 5/1/1995
• Prepared For: Recovery Implementation Program for Endangered Fish Species in the Upper Colo
• Prepared By: CWCB
• Floodplain - Doc Type: Floodplain Report/Masterplan

I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I

In performing the depth calculations, the flow on the river the day of the survey was obtained
from the USGS and subtracted from the established 50 and l00-year flows for each particular
hydrologic point. The depth of water on the day of the survey was estimated and added to the
flood depth calculations to obtain the flood depths as shown in Table 6. The methodology used
for computing flood depths at the cross-sections is similar to the accepted methodologies used
by state and federal agencies in performing detailed analyses except that there is no balancing
of energy from one cross-section to the next.

The one field-surveyed cross-section and the two map-derived cross-section are graphically
represented in Figures 7A - 7C. An example rating curve was prepared for the field-surveyed
cross-section. It is presented in Plate 16, after the floodplain maps.

4.5 Delineating Flood Boundaries

The calculated flood depths for the unstudied stream reaches were ftrst plotted on the quadrangle
maps at the cross-section locations. The resulting floodplain widths were compared to the
floodplain widths on the cross-section plots. The floodplain boundaries between the cross-
sections were then approximated by delineating flood depth contours and interpreting the
relationship between those flood depth contours and the ground contours. Consideration was
given to the slope within each reach. For all of the previously studied reaches the detailed
delineations were transferred directly onto the base mapping without any flood depth
computations. The delineations for the previously unstudied reaches were blended with the
existing detailed delineations on the base maps. The ftnalized information was displayed on the
USGS 7.5' quadrangle maps (scale of 1" = 2,000'). A table identifying the quadrangle maps
used in each plate and the map scale is given in Figure 8. The location of each floodplain map
plate is shown in Figure 9, and the floodplain maps are shown in Plates 1-15.

19