FLOOD10342

• County: Weld
• Community: Greeley
• Stream Name: Poudre River
• Basin: South Platte
• Title: Greeley FIS Revision - Poudre River - COE
• Floodplain - Doc Type: Correspondence

ll/V~/~VVV ~~;UI ~AA qUZZZl4~~n

CORPS OF ENG. PLANNING

~003

, .

~ I

and along the East 8th Street overflow to supplement existing information. The City of Greeley
has agreed to provide the necessary surveys.

Hydrology _ The peak discharges used in the FPMS study were from the 1988 revision to the
Cache La Poudre River hydrology. This hydrology was accepted by the CWCB and was to be
used in future studies on the Cache La Poudre River.

Hydraulics - For the FPMS study, 10-, 50-, 100-, and 500-year existing conditions water surface
profiles were developed. A one-foot rise floodway was also developed. The baseline hydraulics
from the FPMS study will be revised using new surveys of the U.S. Highway 85 bypass north of
East 8th Street in the split flow computations. The split flow computations in the FPMS study used
elevation information from 2 foot contour mapping. The Cache La Poudre main channel flood
boundaries and floodway will be revised, if necessary.

The water surface profiles for the 8th Street split flows will be recomputed using the split flow
discharges computed using the new baseline hydraulics. A revised floodway will be computed
along East 8th Street. For the floodway analysis, the East gill Street roadway and ditches will be
considered as "the stream channel". The revised floodway alignment will be adjusted, where
possible to try to meet the City's concerns. It is possible that the average depths of the gth Street
overflow for the100-year flood will average from 2 to 3 feet, which would meet the requirements
for an AH shallow flooding zone designation. FEMA may not require a floodway in an AH
Zone.

Mapping - The mapping for this study will be prepared using the City of Greeley's GIS
mapping system. The flood boundaries and floodway delineation from the Corps FPMS study
were added to the GIS by the City to produce flood boundary and floodway maps. Some
revisions to the flood boundaries and flood way delineation may be needed following the
adjustments to the hydraulic analyses. Features specific to FEMA mapping such as Base Flood
Elevatioris and Flood Insurance Zones will be annotated to hard copy maps and furnished to the
City to revise the GIS mapping. Draft maps produced from the GIS will be reviewed and revised
if necessary before the final hardcopy and electronic versions are submitted.

Scales for hardcopy maps will follow those used on the existing Flood Insurance Rate Maps
(FIRMs). The elevation datum for the revision will be the National Geodetic Vertical Datum of
1929, which was used for the City's GIS mapping

Coordination - Significant coordination activities will be required with the City and County so
ensure that the communities accept the study results. The draft study results will be furnished to
the CWCB for review and comments. The Corps, as the study contractor, will attend and
participate in intermediate and final meetings to present the preliminary and final FIRMs to the'
City and County.

2