FLOOD05038

• County: San Miguel
• Community: Norwood
• Basin: Gunnison
• Title: FloodPlain Information Report Town of Norwood
• Date: 9/1/2000
• Prepared For: Norwood
• Prepared By: CWCB
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Hvdraulic Analvsis
Hydraulic methods used for the approximate floodplain delineations shown on the Norwood
FHBM panels are not available or have not been documented by FEMA. For future studies, an
approximate loo-year flood elevation at a particular location on the river may be obtained by
using the loo-year peak flow value (as provided in this report) in conjunction with an acceptable
hydraulic analysis. Hydraulic analyses for determining Ioo-year water surface elevations for
approximate floodplains may be performed using FEMA's Quick-2 computer program, the Corps
of Engineers' HEC-2 computer program, F1owmaster, or other acceptable hydraulic methods.
Channel and floodplain cross-sections may be obtained from detailed topographic mapping or
from field surveys. Cross-sections obtained from USGS quadrangle maps are not advised.

To investigate the appropriateness of the FHBM the floodplain, the Town's topographic mapping
and F10wmaster hydraulic software was used to calculate normal depth for Ed Joe Draw/Gurley
Ditch upstream and downstream of the highway. Normal depth represents the depth at which the
channel would flow absent any downstream channel constrictions that may cause the depth to
increase. If downstream constrictions exist, the normal depth calculation will result in a
floodplain width that is too narrow. Table 2 shows estimated floodplain widths calculated
assuming normal depth and those widths shown on the maps. The approximate floodplains are
shown in Figures 3 and 4 in the Appendix.

s ma '-year 0 IplalD WI s or oe raw
Location Calculated 100-year Floodplain Width on Floodplain Width on San
FloodDlaln Width "' Norwood FHBM Mlauel Co. FIS '"
Uostream of Hiahwav 230 210 280
Downstream of Hillhwav 200 150 210

E ti ted 100

Table 2
nodI' 'dth f; EdJ D

Notes:
'"
.,

Calculated using normal depth and cross-sections from the Town's topographic mapping.
The San Miguel Co. FIRMs do not show floodplains wnhin or immediately adjacent to the Town. The
measurements are taken in Ed Joe Draw approximately l!. mile upstream of the town limits and
approximately 'A mile downstream of the town limits.

Some downstream constrictions exist which may cause a wider floodplain. Because this analysis
clearly shows a flood risk, it is advised that floodplain shown on the FHBM be used for planning
purposes along Ed Joe Draw/Gurley Ditch.

Floodplain Mappin!!:
The existing FHBM and FIS for Norwood depicts areas subject to loo-year flooding based on
approximate technical methods. These maps do not show flood profiles, base flood elevations,
or sufficient detail in order for County officials, engineers, developers, and land owners to make
floodplain determinations with any certainty. The Colorado Water Conservation Board can be
contacted to request technical assistance for approximate floodplain analyses as needed.

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