FLOOD11626

• Floodplain - Doc Type: Floodplain Report/Masterplan

Gold Star Canyon
Floodplain Information Report

Mesa County, Colorado

II storm and the 100-year
III-Colorado's Isopluvial

for the watershed was based on an SCS synthetic Type
depth of 2.60 inches found on the NOAA Atlas 2 Volume

The hydrologic analysis
24-hour design rainfal
maps

The Gold Star 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

were available to document the hydrology of past storm
results

uations
The regression equations developed by the CWCB and USGS are not a licable to this basin because the
equations only consider one variable, which is tributary area. The Gold Star 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

Hydrologic Analysis

on Gold Star Canyon. No records
this study must rely upon model

No gage exists
events. Therefore,

3.0

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 Gold Star 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

Colorado Regression Equation
The Colorado Water Conservation Board's (CWCB) Guidelinesfor 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

0=180.8(A)o.578
where A equals the basin area of3.5 square miles, yields a

constructed drainage ditches

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

100-year peak flow of 373 cfs

USGS Regression 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 Gold Star Canyon basin is within the Northwest regions with the following

= .0.007 * (1] * L)08
(P205) * SOA

t

equation

0=104.7(A)o.624
where A equals the basin area of3.5 square miles, yields a

Where
11 = Manning's Roughness Coefficient
L = Overland Flow length (ft)
P2 = The 2-year 24-hour rainfall
S = Catchment's Slope (ft/ft)
tt = Travel Time (hr)

(in)

100-year peak flow of 229 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 Gold Star Canyon watershed was performed using HEC-HMS, a
graphical user interface which updates the Army Corps of Engineers' HEC-1 model. The SCS
Dimensionless Unit Hydrograph option within HEC-HMS and various basin parameters simulates the
hydrologic response of multiple subbasins within the Gold Star Canyon watershed and routes the developed
hydrographs through various reaches

2

IS

The equation for developing travel time for shallow concentrated flow

L
3600 * V

=

Where
L = Shallow Concentrated Flow Length (ft)
V = Average Velocity of flow (ft/s)
tt = Travel Time (hr)

tt

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 Colorado National
Monument 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