FLOOD08415

• Designation Number: 504
• County: Weld
• Community: Severance
• Basin: South Platte
• Title: Floodplain Information Report - Severance, Weld County
• Date: 9/1/1999
• Designation Date: 9/1/1999
• Floodplain - Doc Type: Floodplain Report/Masterplan

In The Slough hydraulic model, the starting water,surface elevations were based on flood
routing studies through John Law Reservoir. Roughness coefficients (Manning's "n" values)
for this study were determined by field inspection, survey data, photographs, and engineering
judgment. Assigned values ranged from 0.035 to 0.043 in the channel and from 0.070 to
0.090 in the overbank areas. Higher roughness coefficients were assigned to areas where
structures or heavy trees exist. Expansion and contraction coefficients ranged from 0.1 to
0.3 and 0.6 to 0.8, respectively. Lower expansion and contraction rates were assigned to
open areas and higher coefficients were assigned to culverts. Near some culverts,
encroachments were set to eliminate the ineffective areas on the downstream and upstream
cross sections of the culvert to account for the proper expansion and contraction near lhe
culverts.

Locations of selected cross sections used in the hydraulic analyses are shown on lhe Flood
Profiles (Exhibit I). For stream segments for which a floodway was computed (Section 4.2),
selected cross--section locations are also shown on the Flood Insurance Rate Map (Exhibit 2).

The hydraulic analyses for this study were based on unobstructed flow. The flood elevations
shown on the profiles are thus considered valid only if hydraulic structures remain
unobstructed, operate properly, and do not fail.

All elevations are referenced to the National Geodetic Vertical Datum of 1929 (NGVD).
Elevation reference marks and their descriptions are shown on lhe maps.

4.0 FLOODPI.AIN MANAGEMENT APPI.JCATIONS

The NFIP encourages State and local governments to adopt sound floodplain management programs.
Therefore, each Flood Insurance Study provides lOO-year flood elevations and delineations of lhe 100,
and 500-year floodplain boundaries and lOO-year floodway to assist communities in developing
floodplain management measures.

4.1 Floodplain Boundaries

To provide a national standard without regional discrimination, the I-percent annual chance
(IOO,year) flood has been adopted by FEMA as the base flood for floodplain management
purposes. The 0.2-percent annual chance (500'year) flood is employed to indicate additional
areas of flood risk in the community. For each stream studied by detailed methods, lhe 100,
and 500'year floodplain boundaries have been delineated using the flood elevations
determined at each cross section. Between cross sections, the boundaries were interpolated
using topographic maps at a scale of 1:24,000, with a contour interval of 10 feet (Reference
8).

The 100- and 500-year floodplain boundaries are shown on the Flood Insurance Rate Map
(Exhibit 2). On this map, the lOO-year floodplain boundary corresponds to the boundary of
the areas of special flood hazards (Zones A and AE), and the 500-year floodplain boundary
corresponds to the boundary of areas of moderate flood hazards_ In cases where the 100- and
5OD-year floodplain boundaries are close together, only the lOO-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.

5