FLOOD11630

• County: Mesa
• Community: Grand Junction
• Stream Name: Colorado River
• Basin: Colorado Mainstem
• Title: Floodplaind Information Report - Red Canyon
• Date: 1/16/2007
• Prepared For: Mesa County, Grand Junction, CWCB, FEMA
• Prepared By: Matrix Design Group, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

Mesa County, Colorado

Red Canyon
Floodplain Information Report

The Red Canyon watershed basin was subdivided into a number of smaller subbasins, and the computer
aided drafting software was used to develop subbasin boundaries, areas, lengths, and slopes. Using this
information, a model was prepared in HEC-HMS. This process begins with the development of a routing
schematic, which is a simplified graphical depiction of the hydrologic network. For each of the subbasins, a
tributary area, SCS Curve Number, % impervious, and lag time are entered into the model. Information is
also entered for each of the routing reaches. The length, slope, Manning's roughness coefficient, and
physical dimensions of each reach are entered.

100-year, 24-hour design rainfall depth = 2.60 inches

Hydrologic Analysis

3.0

events

Re!!ression EquatIOns
The regression equations developed by the CWCB and USGS are not applicable to this basin because the
equations only consider one variable, which is tributary area. The Red Canyon hydrology is unique due to
the many rock outcrops and cliffs that comprise the basin. We expect the peak runoff for this basin to be
much higher than what would be predicted by the regional regression equations. Therefore, our engineering
judgment is that the equations described below predict too Iowa peak hydrology:

available to document the hydrology of past storm

No gage exists on Red Canyon. No records were
Therefore, this study must rely upon model results

The HEC-HMS model performed a rainfall-runoff simulation based on the geometric parameters developed
for the various subbasins and reaches. The model took the design rainfall and determined the quantity and
timing of the resulting runoff. Through subbasin hydrograph creation and combination, peak design flows
were determined at key design points throughout the Red Canyon watershed. Subbasins were delineated
according to key design points as well as watershed features such as topography, soil type, and land use. In
the upper portion of the major basin, subbasins were delineated according to ridgelines between the major
tributary waterways. For the lower watershed, subbasin delineation was more a function of man-made
features and topographic characteristics. In this portion of the basin, flow patterns are often altered by
embankments associated with roads and canals and can be influenced by the locations and capacities of
constructed drainage ditches.

Colorado Re!!r' uation
The Colorado Water Conservation Board's (CWCB) Guidelines for Determining 100- Year Flood Flows
for Approximate Floodplains in Colorado is intended to provide an approximate flow rate for "rural or
largely undeveloped watersheds". These flow rates are determined through regression equations varying
by subregion. Gold Star Canyon basin lies within the Lower Gunnison River Subregion. Applying the
regression equation:

The lag time for each of the subbasins was estimated to be 0.6 times the time of concentration per the SCS
TR-55 method. The time of concentration was estimated for each subbasin by determining the travel time
for the different modes of flow. These are broken up into overland flow, shallow concentrated flow, and
channel flow. Channel flow was only used when there was a well defined natural channel. The equation for
developing a travel time for overland flow is:

1 OO-year peak flow of 445 cfs.

USGS Re!!ression Equation
The United States Geological Survey also provides subregional regression equations applicable to
unregulated streams. The map provided by the Analysis of the Magnitude and Frequency of Floods in
Colorado shows the Red Canyon basin is within the Northwest regions with the following equation.

a

yields

Q=180.8(A)0578
where A equals the basin area of 4.75 square miles

0.007 * (7] * L)o

(p,O.5, * SOA

Q=1 04. 7(A)0624
where A equals the basin area of 4.75 square miles

tt

Where:
1) = Manning's Roughness Coefficient
L ~ Overland Flow length (ft)
P2 ~ The 2-year 24-hour rainfall (in)
S ~ Catchment's Slope (ft/ft)
t, ~ Travel Time (hr)

1 OO-year peak flow of 277 cfs.

Rainfall-Runoff Model
The local community recognizes the use of a rainfall-runoff model as more appropriate for this basin. A
100-year hydrologic analysis of the Red Canyon watershed was performed using HEC-HMS, a graphical
user interface which updates the Army Corps of Engineers' HEC-l model. The SCS Dimensionless Unit
Hydrograph option within HEC-HMS and various basin parameters simulates the hydrologic response of
multiple subbasins within the Red Canyon watershed and routes the developed hydrographs through various
reaches.

a

yields

The equation for developing travel time for shallow concentrated flow

L
3600 * V

t.

Where:
L ~ Shallow Concentrated Flow Length (ft)
V ~ Average Velocity of flow (ft/s)
t, ~ Travel Time (hr)

Where possible and appropriate, an aerial survey comprised of two~foot contours was provided by the Mesa
County GIS Department for the use in the hydrologic modeling process. Additionally, an aerial survey of
fifty-foot contours was provided. Another source of mapping used was the USGS Grand Junction 7.5'
quadrangle. All mapping sources were available electronically. These sources were combined with the use
of computer aided drafting software to determine basin characteristics.

2

100-year

The hydrologic analysis for the watershed was based on an SCS synthetic Type II storm, and the
24-hour design rainfall depth found on the NOAA Atlas 2 Volume III-Colorado's Isopluvial maps.