FLOOD03676

• Designation Number: 519
• County: Weld
• Community: Greeley
• Stream Name: Cache la Poudre River
• Basin: South Platte
• Title: Floodplain Study - Cache la Poudre River, Greeley, Weld County, Colorado
• Date: 10/1/1999
• Designation Date: 5/1/2000
• Floodplain - Doc Type: Floodplain Report/Masterplan

5.0 HYDRAULIC ANALYSES METHODS

5.1 Hydraulic Models

Flood water surface profiles for this study \\'ere llt:vcloped with the Corps of Engineers'

HEC-2 computer program for the personal computer (Reference 6). New HEC-2 hydraulic

models were developed for two separate reaches of the Cacbe La Poudre River, The first reach

CReach I) was from the confluence with the South Platte River upstream to the Ash Avenue

bridge at Greeley, a stream dist;mce of approximately 4.9 miles. The reach through the City of

Greeley (Reach 2) was modeled based on the 1997 hyc.raulic model developed by thc City's

consultant and provided to the Corps by the City. Moclifi.cations were made to the consultant's

hydraulic model and will be discussed in a following section. The final reach (Reach 3) was from

the 25th Avenue bridge at Greeley to the downstream limit ofthe Windsor FIS detailed study, a

stream distance of approximately lOA miles, The hydraulic model from FEMA's current

effective study of the Cache la Poudre River near \Vindsor was obtained and uscd to reproduce

the water surface profiles given in the FEMA study.

5.2 Top'ographic Data

Cross section data for tbe new hydraulic model reacbes was provided by the City of

Greeley ;md Weld County using pbotogrammetric methods from aerial photography taken in

May, 19H7, The aerial pbotogrammetry provided channel geometry to the water surface

e[,~vation on the date of the aerial photography, Beeame the stream normally is very shallow, no

acjustments to tbe stream bed e.levations obtained fi'om photogrammetry.

Topographic mapping with 2 foot contour interval~; was provide by the City of Greeley

from their Geographic Information System (GIS). Tbe GIS was used to compute the Cache La

Poudre River channd distance from the Platte Rive]!' for each cross section. Bridge data was

obtained from infolTtlation provided by the City of Greeley, Weld County and horn field

m~asurements made in June, 1997.

At severalloeations, gravel pit operations have created deep excavations in the lIwrbank

ar~as of the flood plain. Where gravel pits were located on cross sections, modifications were

made to the cross section geometry where deep excavations were shown in the overbanks. The

ground elevations in such areas were set at the level ofth,~ uncxcavated ground along tbe

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