FLOOD06551

• County: Moffat
• Community: Craig
• Stream Name: Yampa River
• Basin: Yampa/White
• Title: Floodplain Information Report
• Date: 4/1/1996
• Prepared For: Routt County
• Prepared By: CWCB
• Floodplain - Doc Type: Floodplain Report/Masterplan

4.3

Field -Survevs 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.

2
3
4
5
6

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.

7

8
9
10

II

The cross-sections used for the computation of flood depths can be grouped into three different
types:

12
13
14
15

Type 1 -
Type 2 -

Field-surveyed cross-sections used only in the representative reach;
Field-surveyed cross-sections transferred from a reach with similar
characteristics using input parameters specific to the reach in question;
Hybrid cros3-3cctions using map-dcrived data and field observations;

.

Map-derived cross-seclion E-E' was prepared, but it was not used. It was omitted because the
field- surveyed cross-section, D-D', located in the same reach, better represents that reach.

T:ype:3 -

By using these categories to identify cross-section sources, 12 of the 15 reaches on the Yampa
River were divided into the following groups:

4.4

Computing Flood Deoths

Type 1 Cross-sections -
Type 2 Cross-sections -
Type 3 Cross-sections -

4 reaches
6 reaches
2 reaches

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 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 - 7M.

TABLE 5
YAMPA RIVER CROSS-SECTION IDENTIFICATION

....FLpwV~1JJ:S.(~)........
..~....I
JlYI!ll-6@lij!,p@)\

15800, 18300 (YI) NONE

....P!!$!y^TIQ~............
.OF ~L()ODi
pmrr!!$.. .

1$800, 18300 (YI) A-A'
16651,18791 (Y2) B-B'
17102,19052 (Y3) COC'
17102. 19052 (Y3) D-D'.
17102. 19052 (Y3) NONE
1831$, 19776 (Y4) F-F'
1831$, 19776 (Y4) G-G'
18548, 20003 (Y5) H-H'
18548.20003 (Y5) I-I'
18548. 20003 (Y5) J-1'
18548,20003 (Y5) NONE
19748,21433 (Y6) K-K'
35842. 40481 (Y7) L-L'
36497.41322 (Y8) M.M'

DETAILED STUDY FEMA STUDY
ALREADY PERFORMED

3 CALCULATED

2 (FROM F-F')
2 (FROM F-F')

CALCULATED

CALCULATED

Flood depths were calculated at each cross-section through a computer program (QANY) which
uses Manning's equation. The input parameters for the calculation of flow included: x and y
coordinates for points on the cross-section, Manning's 'n' values (ranging from 0.03 to 0.035 for
the channel and 0.045 to 0.05 for the overbank) (Reference 7), the channel slope (interpreted
from USGS quadrangle maps), and depth values. The program calculated flow and velocity
values for each depth value entered. The target flow values were those values established in
Section 3.0 and shown on Table 3. Rating curves were established from the calculated depths.
From the rating curves 50-year and 100-year flood elevations were established at each location.

CALCULATED

CANYON REACH

NIA

CALCULATED

3

CALCULATED
CALCULATED
CALCULATED

2 (FROM F-F')
2 (FROM G-G')
CANYON REACH

CALCULATED

NIA
CALCULATED

2 (FROM D-D')
2 (FROM G-Q')

CALCULATED

CALCULATED

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-year and 100-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 true flood depths shown in Table 6. The

17

18