FLOOD04329

• Designation Number: 534
• County: San Miguel
• Community: Unincorporated San Miguel County
• Basin: San Juan/Dolores
• Title: Flood Insurance Study - San Miguel County and Incorporated Areas
• Date: 9/1/1992
• Designation Date: 11/1/2000
• Floodplain - Doc Type: Floodplain Report/Masterplan

On Cornet Creek, ae evidenced in 1914 and 1969, the floodflows
carried with them a great deal of debris and mud, and shallow
flooding prevailed in Telluride whenever major floods occurred.
For this reason, mudflow was considered in the hydraulic analysis.
A 20 percent water content in the mudflow was assumed, which in
turn yielded a lOa-year mudflow discharge of 1,500 cubic feet per
second (cfs). Due to inadequate channel capacity, this discharge
was routed uniformly across the alluvial fan. Because of highly
unpredictable flow direction, all possible flood routes were
considered. Three cross sections perpendicular to the !1lajor axis
of the fan were utilized in hydraulic computations for shallow
flooding to determine average - flood 'depths. Average depths were
computed using Sribniy formula (Reference 14), a modified Manning's
equation, as follows:

v = 1.486 H2/3 Sl/2
(0.194S*)

where

v = the mean velocity in feet per second
H = the average depth in feet
S = the slope i~ feet/feet

The average depths and velocities at the three cross sections as
shown on the Flood Insurance Rate Map (Exhibit 2) are shown in the
following tabulation.

Cross Section

Average
Depth
(Feet>

Mean
Velocity
(feet per second)

1
2
3

0.9
2.0
2.7

2.6
6.8
10.9

For Cornet Creek, no flood profiles were developed and the SOO-year
flooding was not studied. The la-year flooding was not studled
because the average depth of lOa-year flooding was no greater than
three feet in this area.

The slope-area method was used for determination of starting Nater-
surface elevations for all other stream studied by detailed
methods. Water-surface profi les of selected recurrence interval s
were developed using the COE HEC-2 step-backwater computer program
(Reference 15). These water-surface profiles were computed to a
0.5 foot degree of accuracy. The locations of selected cross
sections are shown on both the Flood Insurance Rate Map (Exhibit 2)
and the water-surface profiles (Exhibit 1).

Approximate methods were used to study portions of the San Miguel
River, a portion of Leopard Creek, South Fork San Miguel River, and
Gurley Ditch. Since there is not any previous study in these
areas, an area prone to flooding for the recurrence interval of

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