FLOOD04642

• County: Mesa
• Community: Delta
• Stream Name: Gunnison
• Basin: Gunnison
• Title: Floodplain Information Report
• Date: 4/1/1996
• Prepared For: Mesa County
• Prepared By: CWCB
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

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.

TABLE 5
GUNNISON RIVER CROSS-SECTION SELECTION

.Iifl.!~~~

.,-...'_._.:...'_..:.._.:..,-..-,-.-.:.'..........-:'..',"-',""
D.~iYYATIqN.............../I
..... .... ...J!JJOO ...............................
..~~...........................i................
.-,.,-,...-,..--...'.....

4.3

Field Survevs and Cross-Section PreDaration

22000, 24700 (G2) NONE

DETAILED STUDY
ALREADY
PERFORMED

FEMA STUDY

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 cross-sections using map-derived data and field observations;

2 23093, 25893 (G3) A-A' CALCULATED
3 23093, 25893 (03) NONE CANYON REACH N/A
4 24004, 26914 (G4) B-B' 3 CALCULATED
5 24004, 26914 (G4) C-C' 3 CALCULATED
6 24004, 26914 (G4) CA-CA' 2 (FROM B-B') CALCULATED
7 24856, 27868 (05) D-D' I CALCULATED
8 24856, 27868 (G5) E-E' 3 CALCULATED
9 24856, 27868 (G5) NONE CANYON REACH N/A
10 26162, 29334 (G6) G-G" I CALCULATED
11 26162, 29334 (G6) NONE CANYON REACH N/A
12 27000, 30000 (G7) H-H' 2 (FROM E-E') CALCULATED
· - Map- enve cross-sectIOn F-F was prepare, ut It was not use . It was offiltte
cross-section, G-G', located in the same reach, better represents that reach.
4.4 Computing Flood DeDths

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 ~ere 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 1>: shown on the base maps. There was one exception where a cross-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 3 -

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 I Cross-sections -
Type 2 Cross-sections -
Type 3 Cross-sections -

3 reaches
2 reaches
3 reaches

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 (rangingfrom 0.03 to 0.032for
the channel and 0.04 to 0.055 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 1 DO-year flood elevations were established at each location.

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 5 identifies the cross-sections, the group type, and flow values used to calculate the flood
depths. The cross-sections, with 50-year and 1 DO-year flood lines illustrated, are presented after
. Section 4.4 in Figures 7A-7H.

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
methodology used for computing flood depths at the cross-sections is similar to the accepted

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