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

SCS Technical Release No.-20 and involve the use of rainfall frequency

data, soils and ground cover information, and time of concentration

computations. ,Soils information was obtained from the Forest Service

for the area within the San Juan National Forest and from local SCS soil

surveys for the: balance of the watershed. Cover condi,tions in the

watershed were obtained by on-site inspection. Cross-section surveys

and on-site roughness coefficients were used for velocity and travel

time computations to determine the time of concentration at selected

locations. Hydrographs and corresponding peak elevations were obtained

from SCS TR-20 computer program. Cross-section rating tables were

developed from water surface profile computations and were part of the
input data to this program.

Through the use of these computer programs the peak discharges and flood
'crest elevations for the 10-, 50-, 100-, and 500-year floods were
determined. These floods have an average occurrence of once in the

number of years as indicated. For example, the 100-year frequency flood

occurs once in 100'years on the average, and has a one percent chance of

being equaled or exceeded in any given year.

Output data from the computer programs were used in developing: the

flood profile drawings, flood outlines on the flood hazard maps, and the

data for the flood frequency-elevation and discharge tables.

Hydraulics - The hydraulic anaiyses for both the San Juan River and

McCabe Creek segments of the study were based on the SCS Water Surface

Profile Program, WSP-2.

Flood outlines for the 100- and 500-year floods were located on flood

hazard area maps using water surface elevations, cross-section data, and

by interpolation between the cross-sections. The location of flood

lines at road crossings were computed using the normal openings of

bridges and culverts. Because of the multitude of possible events In

which sediment and debris could cause blockage of bridges and culverts,

these considerations were not included in the study.

Input data for this program includes: cross-section information, reach

lengths, contributing drainage area measurements, channel and flood

plain roughness coefficients, and discharge values. The cross-section,

reach length, and drainage area data was obtained from field surveys and

photogramrnetric maps prepared especially for this study. Typical "n"

values (roughness coefficients) range from 0.045 to 0.110 and were

determined from photographs and field inspection. Discharge values for

the San Juan River segment were determined from streamgage records.

Estimated flow rates were used on the McCabe Creek segment and required

additional analysis through the SCS TR-20 program.

Soil Surveys

Parts of the study area have been mapped with soil surveys. Soil survey

information can be applied in managing farms, ranches, and woodlands; ,in

selecting sites for roads, ponds, buildings, or other structures; and in

appraising the value of tracts of land for agriculture, industry, or

recreation. Information regarding the soil surveys can be obtained by
contacting the Soil Conservation Service, 2490 West 26th Avenue, Denver,
Colorado 80217, or the SCS Field Office, Stranger Building, in pagosa
Springs, Colorado 81147.

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