9467

• UCREFRP Catalog Number: 9467
• Author: Colorado Water Conservation Board.
• Title: Floodplain Information Report\
• USFW Year: 1995.
• Copyright Material: NO

4.3 Field Surveys and Cross-Section Preparation
The project budget and schedule only allowed for field surveying of four cross-sections on the
Yampa 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.
Map-derived cross-sections were utilized for the remaining reaches. Some map-derived cross-
sections were modified to more accurately represent actual channel shapes. When the USGS
maps were felt to lack sufficient detail, the map data were combined with field-surveyed data
from other locations to prepare hybrid cross-sections. Canyon reaches on the Yampa River were
not represented by cross-sections since the level of detail obtained could not be displayed on the
base maps.
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
I 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, 12 of the 15 reaches on the Yampa
River were divided into the following groups:
Type 1 Cross-sections - 4 reaches
Type 2 Cross-sections - 6 reaches
Type 3 Cross-sections - 2 reaches
1 There were 3 additional reaches which were classified as follows: 2 canyon reaches for which
no cross-sections were developed; 1 reach where a detailed study had already been performed.
Table 5 identifies the cross-sections, the group tyPa 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 - 7M.

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