FLOOD06611

• County: Elbert
• Basin: South Platte
• Title: Flood Prone Areas in Elbert County, Colorado - Technical Addendum
• Date: 1/1/1982
• Prepared For: Elbert County
• Prepared By: USGS
• Floodplain - Doc Type: Floodplain Report/Masterplan

COMPUTATION METHODS

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Once cross-section hydraulic properties have been determined by computer pro-
gram E431, they can be used in conjunction with the 100-year flood-discharge value
to determine the height of the corresponding flood level at that section. Methods
shown in the "Manual for estimating flood 'characteristics of natural-flow streams
in Colorado" (McCain and Jarrett, 1976) were used to compute values for natural
stream 100-year flood discharges in Elbert County. Check calculations were made
using the methods given in "Urban hydrology for small watersheds" (U.S. Soil Con-
servation Service, 1975) to assure that 100-year discharge values were reasonable.

;

High-flow discharges and subsequently, the depths of flows in a channel, were
estimated by use of the Manning formula: '

Q=1.486AR2/3Sl/2
n

where:
Q=discharge, in cubic feet per second;
n=Manning roughness coefficient;
A=area of the cross section, in square feet;
R=hydraulic radius, in feet; and
S=slope, in feet per foot, a pure number.

The right-hand terms in the equation can be grouped into the S1/2 term times the
1.~86AR2/'3 terms; this latt~r grouping is called channel conveyance (X) and has
units of cubic feet per second. The product of conveyance (X) and (S1/2) also has
the. units of cubic feet per second (Benson and Dalrymple, 1967). Simply stated,
conveyance is a combination of several factors descriptive of channel characteris-
tics.

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Conveyance values were printed for each elevation listed in the cross-section
hydraulic properties. Bed slope for each cross section was calculated from the
.contour crossings shown on U.S. Geological Survey topographic maps. The distance
between. selected contours above and below the cross section, measured along the
streamline in combination with the fall between the contours, was assumed to pro-
vide a better representation of slope than the distance which might be measured in
the field. For example, a 20-ft drop, divided by a 4,400-ft streamline distance
between contours at cross-section 89, showed a slope, S, of 0.0045 (Sl/'2=0.067).
Then, a conveyance of 459,649 ft3/s times 0.067 equals 30,800 ft3/s, which is
4 percent greater than the 100-year flood-discharge estimate of 29,500 ft3/s for
that cross section. This value is within acceptable limits of accuracy, consider-
ing that map-contour intervals are 10 ft, and an erro~ of 0.2 ft in determining
fall also equals about 4 percent in terms of computed discharge. A water surface
of 10.0 ft provided the conveyance of 459,649 ft3/s and was used as the 100-year
flood level. (See section on "Computer Printouts of Cross-Section Properties and
Sample Computations of Flow at Two Cross Sections on West Bijou Creek.")

;

The flood level established at 10.0 ft, based on the survey datum, was trans-
ferred to the topographic map. By these means, cross-section depths and contour
crossings can be used to establish the areas inundated by the 100-year flood.

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