FLOOD01153

• County: Montezuma
• Community: Dolores
• Stream Name: Upper Dolores River
• Title: An Evaluation of the Dolores River at Dolores
• Date: 9/1/1992
• Prepared For: Dolores
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Floodplain Report/Masterplan

Dolores River

was no need to redefine the lOa-year flood plain boundary as
shown on the FEMA map (Attachment 2). Through most of Dolores,
lOa-year flood depths would average about 2 feet. Cross sections
of the channel and overbank areas showing the resulting lOa-year
water-surface elevations were subsequently plotted. These are
included in Appendix c.

Subsequently, a multiple-profile model was developed to
determine the 10-, 50-, and 500-year water-surface elevations for
existing conditions. The starting water-surface elevation for
the 10- and 50-year was 6,924 feet (msl), the top of the active
conservation pool (this elevation was also used for the lOa-year
run described above). The USBR Operations section Chief
responsible for McPhee Reservoir stated that the maximum water
surface is 6,928 feet, based on a 600-year rainflood event, and
that for the 500-year event, the water surface would be
approximately 6,927 feet. Therefore, 6,927 feet became the
starting water surface for the 500-year run. The results are
shown on Attachment 4 and in Appendix B as "NEWDOL10.0UT." The
la-year event would be contained within the channel. The 50-year
event would be similar in areal extent to the lOa-year event, but
the flood depths would only average about 1 foot or less. The
500-year flood plain boundary has been added to the Flood Hazard
Information Report loa-year flood plain shown on Attachment 3.

The sensitivity of the lOa-year water-surface profile was
checked by varying the Manning'S "n" values. An "n" value
determination was made for the channel and over banks using the
relatively new methodology described in the u.s. Geological
Survey publication entitled "Determination of Roughness
Coefficients for Streams in Colorado" and a technical paper by
Robert Jarrett entitled "Hydraulics of High-Gradient Streams"
published in the November 1984 Journal of Hydraulic Engineering.
This procedure resulted in slightly higher "n" values of 0.054
for the channel and 0.1 and 0.06 for the left and right
overbanks. These values were higher than the previous flood
studies and are believed the upward bound of "n" values based on
observed existing conditions. The result~ng water-surface
profile showed an increase of 1/2 to 1 foot at most cross
sections. The HEC-2 file is included in Appendix B as
"NEWDOL11. OUT. "

Lastly, the model was modified to simulate the hydraulic
conditions of a potential levee project. To determine the
required height needed to contain floodflows by raising existing
levees and constructing new levees along the right bank within
the study reach, flows were modeled so as to be confined to the
main channel and left overbank area. Landowners in the left
overbank area subject to flooding strongly opposed levees being
built on their property as part of the USBR's project, as

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