FLOOD08689

• County: Montrose
• Community: Naturita
• Basin: San Juan/Dolores
• Title: FIS - Naturita
• Date: 7/6/1981
• Prepared For: Naturita
• Prepared By: FEMA
• Floodplain - Doc Type: Current FEMA Regulatory Floodplain Information

for 12 gaging stations in hydrologically similar watersheds.
The station data were separated into rainfall and snowmelt peaks
in a previous study (Reference 7). Frequency analyses were conducted
on the rainfall and snowmelt data (Reference 8). The results
of the snowmelt and rainfall-frequency analyses were combined
statistically to give a maximum flow for both events. Regional
curves of drainage area versus discharge were computed for the
four frequency floods.

Peak discharge-drainage area relationships for San Miguel River
are shown in Table 1.

Table 1. Summary of Discharges

Flooding Source
and Loca t ion

Drainage Square
(Square Miles)

Peak Discharges
(Cubic Feet per Second)
10-Year 50-Year 100-Year 500-Year

San Miguel River
At Naturita

1,080

6,350

9,300

11,100

15,200

3.2 Hydraulic Analyses

Analyses of the hydraulic characteristics of the stream in the
community were carried out to provide estimates of the elevations
of floods of the selected recurrence intervals along the stream
studied in the community.

For San Miguel River, the water-surface elevations of floods of
the selected recurrence intervals were computed through use of
the U.S. Army Corps of Engineers HEC-2 step-backwater computer
program (Reference 9).

Cross sections for the backwater analyses of San Miguel River
were obtained from a topographic map at a scale of 1:2400, which
was derived from aerial photographs (Reference 10). The below-
water sections were obtained by field measurement. All bridges,
dams, and culverts were field surveyed to obtain elevation data
and structural geometry.

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 is computed (Section 4.2), selected cross
section locations are also shown on the Flood Boundary and Flood-
way Map (Exhibit 2).

Roughness factors (Manning's "n") used in the hydraulic computations
for San Miguel River were chosen by engineering judgment and based
on field observations of the flooding source and flood plain area.
Roughness values for the main channel of San Miguel River ranged
from 0.035 to 0.040, and flood plain roughness values ranged

7