FLOOD03210

• County: Otero
• Community: Otero County
• Stream Name: Arkansas River, Timpas Creek
• Basin: Arkansas
• Title: Technical Addendum for Arkansas River, Timpas Creek
• Date: 3/1/1981
• Prepared For: Otero County
• Prepared By: MSM Consultants, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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surveyed cross sections and the digitized cross sections from the
topographic maps are shown by reference points on the flooded
area maps (Plates 3 through 11) and the flood profiles (Plates 12
through 22).

The 10-, SO- 100-, and SOD-year flood events were used as the
flood frequencies for this floodplain analysis. Thus the data
developed in this report will be compatible not only for
regulation purposes, and H.B. 1041 designation but also for
Federal Insurance Administration flood insurance rate studies.
Only the 100-year floodplain is shown on the Flooded Area plates
as requested by the Colorado Water Conservation Board.

DESCRIPTION OF TYPICAL VEGETATION

o
The typical vegetation in the study area of the Arkansas River
Valley consists primarily of native and cultivated grasses, with
dense patches of willows along the channel banks. This situation
is typical of almost all of the cross-sections of this study,
with the exception of wide meanders that cut away at agricultural
fields. In these cases, there is usually no heavy growth along
the channel banks.

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Typical "n"
explained in
determination

values were assigned to the overbank
the section concerning the method of
in this Technical Addendum.

areas as
"n" value

METHOD OF "n" VALUE DETERMINATION

This report includes photographs of the study streams (at the
cross-sections) with assigned "n" values. These assigned "n"
values are intended to be typical of various cross sections along
the streams. Photographs and general data for this report were
collected during field reconnaissance to Otero County on May 21 -
24, 1980.

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Due to the type of vegetation in close proximity to the channel
banks of the study streams, five or more "n" values were assigned
to a single cross section. The "n" values indicated on the
included photographs are based on what is shown in the
photographs. The portions of the cross section denoted by
"far-left overbank" and/or "far-right overbank" which are not
shown in the photographs usually will have a range of "n" values
due to the variation in types of vegetation, terrain, etc.

The Mannings roughness coefficient "n" is a value used to
estimate the natural resistance to flow in a given channel or
overbank. In the hydraulic analysis for the related flood hazard
study, the "n" values are used by the HEC-2 Single Step-Backwater
Computer Program, (Reference 1) to describe the roughness of
terrain along the study reaches. This value is estimated for the

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