FLOOD09751

• Designation Number: 324
• County: Archuleta
• Community: Pagosa Springs
• Basin: San Juan/Dolores
• Title: Flood Insurance Study - Pagosa Springs and Unincorporated Areas of Archuleta County, Colorado
• Date: 5/1/1991
• Designation Date: 7/1/1992
• Floodplain - Doc Type: Floodplain Report/Masterplan

Approximate analyses were used to
4~4c18pmeat peteBtial" 9~- miBimal
methods ot study were proposed to,
Town of Pagosa Springs.

study those areas having a low
flood ba7ar~~ Th~ e~^pc an~
and agreea-up~y,-FEMA and the

The San Juan River, which flows through the Town of Pagosa Springs,
rises from the San Juan Mountains in southwestern Colorado and
flows southwesterly. A small tributary, McCabe Creek, enters the
San Juan River from the north, in the west central part of Pagosa
Springs. The drainage area of the San Juan River is approximately
298 square miles at Pagosa Springs. The McCabe Creek watershed,
which is a tributary to the San Juan River, covers approximately 20
square miles.

Hydraulic Analyses

Locations of selected cross sections used in the hydraulic analyses
are shown on the Flood Profiles (Exhibit 1). For stream segments
for which a floodway was computed, selected cross section locations
are also 'shown on the Flood Insurance Rate Map.

Roughness coefficients (Manning's "n") were established by field
inspection at each cross section and checked against photographs
and a recogn1zed' standard text (Reference 10). Roughness values
for the \main channels ranged from 0.07 to 0.20 on McCabe Creek; and
from 0.03 to 0.10 on West Fork McCabe Creek. Roughness values for
the floodplains ranged from 0.07 to 0.10 on West Fork McCabe Creek,
and from 0.07 to 0.20 on McCabe Creek.

Water-surface profiles of the selected recurrence intervals on
McCabe Creek and West Fork McCabe Creek were developed using the
COE HEC-2 step-backwater computer model (Reference 11). Starting
elevations were computed using the slope-area method.

All elevations are referenced to the National Geodetic Vertical
Datum-of-.192.9. (NGVD). -Elevationrefer.ence'-lllarksused in_.P.agoBa
Springs are shown on the maps; the descriptions of the marks are
presented in -Elevation Reference 'Marks (Exhibit 3).

Floodplain Boundaries

The 100- and 500-year floodplain boundaries are shown on the Flood
Insurance Rate Map. On this map, the 100-year floodplain boundary
corresponds to the boundary of the areas of special flood hazards
(Zones A, AE, and AO); and the 500-year floodplain boundary
corresponds to the boundary of areas of moderate flood hazards. In
cases where the 100- and 500-year floodplain boundaries are close
together, only the 100-year floodplain boundary has been shown.
Small areas within the floodplain boundaries may lie above the
flood elevations but cannot be shown due to limitations of the map
scale and/or lack of-detailed topographic data.

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