FLOOD00880

• County: Mesa
• Community: Rifle
• Stream Name: Colorado River
• Title: Floodplain Information Report - Colorado River Mainstem From Rifle to Utah-Colorado State line
• Date: 4/1/1996
• Prepared By: CWCB
• Floodplain - Doc Type: Floodplain Report/Masterplan

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4.3 Field Surveys and Cross-Section Preparation

Since most of the study reach on the Colorado River mainstem had already been studied by
detailed methods, and since the unstudied portions of the study reach fell into the "Narrow" or
"Canyon" categories, only one cross-section was actually field-surveyed. The selection of the
field-surveyed cross-section was based on the location of a series of fish ponds at the Island
Acres State Park where the Colorado Division of Wildlife had an interest in some specific
floodplain information. The availability of a benchmark, easy access and the ability to survey
the opposite bank dictated the specific location within the State Park. A field estimate was made
of the average water depth, but the channel bottom was not surveyed. Estimates of the
Mannings 'n' values were made at the surveyed cross-section location during the field survey.

Map-derived cross-sections were utilized for the remaining unstudied reaches. No adjustments
were made to those cross-sections prior to performing the depth computations. Canyon reaches
on the Colorado River mainstem generally were not represented by cross-sections since the level
of detail obtained could not be shown on the base maps. One exception is noted below.

Since the majority of the study reach consisted of existing detailed information or canyons (64
of the 93 miles are detailed, 24.5 miles are in canyons), only three cross-sections were used in
the hydraulic analysis. Two types of cross-sections were used in the unstudied reaches of the
Colorado River mainstem:

Type 1 -
Type 2 -

Field-surveyed cross-sections used only in the representative reach;
Map-derived cross-sections used only in the representative reach.

By using these categories to identify cross-section sources, 3 of the 12 reaches on the Colorado
River mainstem were divided into the following groups:

Type I Cross-section -
Type 2 Cross-sections -

I reach
2 reaches

One of the map-derived cross-sections (Type 2) was used for a canyon reach. Although that
cross-section is included in this report, the floodplain delineation was not significantly different
than it would have been simply following the canyon wall topography, as was done in the other
3 canyon reaches.

There were 9 additional reaches which were classified as follows: 3 canyon reaches for which
no cross-sections were developed (this excludes the 4th canyon reach for which a map-derived
cross-section was used); 6 reaches where detailed studies had already been performed.

Table 5 identifies the cross-sections, the group type and flow values used to calculate the flood
depths. The cross-sections, with 50-year and IOO-year flood lines illustrated, are presented after
Section 4.4 in Figures 7A - 7C.

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