FLOOD03745

• County: Weld
• Community: Weld County and Unincorporated Areas
• Basin: South Platte
• Title: FIS - Weld County and Unincorporated Areas
• Date: 9/22/1999
• Prepared For: Weld County
• Prepared By: FEMA
• Floodplain - Doc Type: Current FEMA Regulatory Floodplain Information

In areas where divided flow paths were not present, the floodway was
computed using the proportional conveyance reduction from each side
of the floodplain, with adj ustments as necessary to obtain a
consistent and reasonable floodway. Refer to Table 2, "Floodway
Data."

9.2 Second Revision

This study was revised on September 22, 1999, to add flood-hazard
information for The Slough near the Town of Severance, Colorado,
which affects the unincorporated areas of Weld County.

The hydraulic analysis and floodplain and floodway boundary
delineations for the restudy were developed by the U.S. Army Corps
of Engineers (USACE), Omaha District, for FEMA, under Interagency
Agreement No. EMW-95-E-4759, Project Order No.6. This work was
completed in November 1997. The hydrologic analysis for the restudy
was taken directly from a USACE report entitled ~'Flood Plain
Information Report, The Slough, Severance, Colorado," dated May 1981
(Reference 441.

The study reach along The Slough extends from just upstream of the
John Law Reservoir to 1 mile upstream of County Road 74, a distance
of 1.6 river miles.

Little flood history is available for The Slough in the Town of
Severance. Interviews with local residents indicate the only floods
of any consequence occurred during summer 1971 or 1972, 1975, and
1977 (Reference 441.

The hydrology used for the restudy was taken directly from the
above-referenced USACE Flood Plain Information report (Reference
44) . The hydrologic analysis for The Slough consisted of
determining the magnitude of the 10~, SO~, 100-, and SOO-year
floods. Because stream-gaging data for The Slough were not
available, the Environmental Protection Agency Storm Water
Management Model (Reference 45) was used to develop a rainfall-
runoff model of the basin. Rainfall input for the hydrologic model
was derived from National Oceanic and Atmospheric Administration
NOAA Atlas 2, "Precipitation-Frequency Atlas of the Western United
states, Volume III - Colorado" (Reference 8).

Values for the SOG-year flood were extrapolated. Infiltration
losses of 1 inch per hour were used, based on results of the
Missouri River Basin Interagency Committee "Missouri River Basin
Comprehensive Framework Study" (Reference 46).

Storage-discharge relationships were developed for the Black Hollow
and Loop Lake Reservoir systems. These relationships were based on
storage and discharge capacity data for these darns and reservoirs
contained in the "Inventory of Darns of the United States" (Reference
47). These relationships showed that Black Hollow Reservoir has a
significant effect on runoff from storms centered over the entire
basin of The Slough. Because of this phenomenon, the final
hydrologic analysis assumed a storm centered over the 16-square-mile
drainage area between Black Hollow and the Town of Severance.

The effect of the John Law Reservoir on flood flows was determined
by routing flood events through the reservoir. Information on this
dam was also taken from the "Inventory of Dams of the United States"
(Reference 47).

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