FLOOD04873

• Designation Number: 359
• County: Archuleta
• Community: Pagosa Springs
• Stream Name: San Juan River and McCabe Creek
• Basin: San Juan/Dolores
• Title: Flood Hazard Analysis - San Juan River and McCabe Creek in the vicinity of Pagosa Springs, Archuleta County, Colorado
• Date: 9/1/1977
• Designation Date: 5/1/1994
• Floodplain - Doc Type: Floodplain Report/Masterplan

FLOOD POTENTIAL ~ PRESENT CONDITIONS

Any obstruction In the flood plain, natural or man-made, Influences the

depth and width of the area flooded. Trees and brush growing along the

streams retard the out of channel flood flows causing backwater and

greater depths of flooding. Retardance factors are an Important part of

the hydraul ic studies used in the flood hazard analyses. Road crossings

with inadequate culvert and/or bridge openings often cause channel

constrictions and obstruction to flood flows. Elevated roads in the

flood plain act as barriers which raise the water surface at some

locations. Other road crossings are lower than the flood plain and are

over topped during flood stage. The effects of culvert and bridge

constrictions a,re graphically displayed on the flood profile drawings,

Exhibits A~l through A-5. Trees, brush, and debris washed out during

the periods of high runoff, are carried downstream Into, bridges and

culverts. These obstructions act as dams, backing the water upstream,

until they break loose, causing a sudden surge or wall of water with

additional destructive force. Obstructions caused by debris clogging

culverts and bridges are variable, resulting In a multitude of flow

conditions which are unpredictable for any given flood event. The

potential effect of debris clogging are not included in the flood

profile determinations.

Amount of damages and hazardous conditions caused by any flood,ar-e

dependent upon: ,developments in the flood plain, topography of the

flooded area, -obstructions and debris, depths and velocities of flow,

and rates of rise and duration of flooding.

In respect to the total flood plain, the largest area subject to flood-

ing is used for agricultural purposes. Under present conditions,

potential floods of the 100- and 500-year frequency magnitude would

inundate portions of the residential and commercial developments in

pagosa Springs. In the low lying areas, floods of lesser magnitude can

cause similar problems on a smaller scale. The approximate-area of the

100-year flood is shown on the Flood Hazard Sheet Index, sheets 1 and 2.

Detailed outlines showing the approximate limits of 100~ and 500-year-
floods are located on the Flood Hazard Area Maps, sheets 1 through 9 and
Supplement 2. Flood damage along the San Juan River east of Pagosa
Springs would be primarily agricultural, however, travel on U. S. High-
way 160 and county roads would be interrupted. Losses would include
livestock, hay and pasture, deposition of sediment and debris, erosion,
and damage to buildings and irrigation systems. In Pagosa Springs

floodwaters from both the San Juan River and McCabe Creek could cause

damages to buildings and contents, lawns, gardens, temporary loss of

business, inventory replacement, cleanup costs and repair. Streets,

road and bridge crossings, water and sewer lines, communication and

power lines, and the sew~ge disposal lagoons are also subject to damage.

Flood waters flowing at high velocities creates dangerous conditions~

Velocities exceeding 3 feet per second with depths of 2 feet or more are

generally considered hazardous. Those conditions would exist at various

locations on the San Juan River and McCabe Creek during large floods.

Rapidly rising streams with associated deep flows are also hazardous to

an unsuspecting public, especially at road crossings. On McCabe Creek a

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