9469

• UCREFRP Catalog Number: 9469
• Author: Colorado Water Conservation Board.
• Title: Floodplain Information Report\
• USFW Year: 1995.
• Copyright Material: NO

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4.3 Field Surveys and Cross-Section Preparation
The project budget and schedule only allowed for field surveying of three cross-sections on the
Gunnison River. The selection of the field-surveyed cross-sections was based on the availability
of a nearby benchmark, the accessibility of the location, and the ability to survey the opposite
bank. Field estimates were made of the average water depth, but the channel bottoms were not
surveyed. Estimations of the Mannings 'n' value were made at each surveyed cross-section
location during the field survey.
' For some reaches, field-surveyed cross-sections from other stream reaches were transferred to the
reach in question. Appropriate adjustments were made to ground elevations. When flow depths
were computed, the appropriate slope values, Manning's 'n' values, and flow values were used.
' Ma -derived cross-sections were utilized for the remainin reaches. Some ma -derived cross-
p g P
sections were modified to more accurately represent actual channel shapes. Canyon reaches on
the Gunnison were not generally represented by cross-sections since the level of detail obtained
could not be shown on the base maps. There was one exception where across-section was used
' for a canyon reach, and it is discussed below.
' The cross-sections used for the computation of flood depths can be grouped into three different
types:
' Type 1 - Field surveyed cross-sections used only in the representative reach;
Type 2 - Field surveyed cross-sections transferred from a reach with similar
characteristics using input parameters specific to the reach in question;
Type 3 - Hybrid cross-sections using map-derived data and field observations;
By using these categories to identify cross-section sources, 8 of the 12 reaches on the Gunnison
River were divided into the following groups:
Type 1Cross-sections - 3 reaches
Type 2Cross-sections - 2 reaches
Type 3Cross-sections - 3 reaches
' There were 4 additional reaches which were classified as follows: 3 canyon reaches for which
no cross-sections were developed; 1 reaach where a detailed study had already been performed.
Table S identifies the cross-sections, the group type, and flow values used to calculate the flood
depths. The cross-sections, with 50-year and 100-year flood lines illustrated, are presented after
Section 4.4 in Figures 7A-7H.
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