FLOOD06925

• County: Larimer
• Community: Greeley
• Stream Name: Cache La Poudre River
• Basin: South Platte
• Title: Master Floodway Planning
• Date: 3/11/1983
• Prepared For: Greeley
• Prepared By: Simons Li & Associates Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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B.

Collect and Review of Existing Information

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All existing information pertinent to this study will be collected

including topographic mapping, CaE's 1974 and 1981 reports, 1979 FEMA report,

cross-section data used for the CaE 1974 report and FEMA report, computer

printouts of the previous hydraulic computation of the study, plans of deve-

loped area since 1972 and the proposed development areas, legal descriptions

of the properties inside the established floodplain, design drawings and/or

as-built drawings of the bridges, volume of dredging if available, and any

other pertinent data. All existing information will be reviewed and collated

~or later use in the study. Topographic mapping and plans of development will

be used to update the mapping. CaE and FEMA reports, cross-section data, and

hydraulic computation computer printouts will be used to reprogram the

hydraulic computation into the HEC-2 program. Plans of development and legal

descriptions of properties within the floodplain will be used to develop a

most practical floodway plan for the study reach. Bridge plans and volume of

dredging will be used for conceptual design of the improvement plans.

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C. Field Investigation

Field reconnaissance will be performed to verify the topographic mapping

and cross-section data, measure sizes of existing bridge structures, and to

determine "n" roughness coefficients. Photographs will be taken at cross-

section locations and a brief "n" value report will be prepared discussing the

appropriate range of "n" values. The field investigation will also include a

qualitative assessment of channel stability and aggradation/degradation

tendencies. The qualitative assessment will include bank sloughing or insta-

bilities, lateral migration tendencies, evidence of scouring or deposition,

and presence of channel bed controls such as rock outcrops or man-made struc-

tures. During the stage of formulation and evaluation of alternate plans, bed

and bank samples will be collected for laboratory analysis to determine the

sediment size distribution for designing a stable channel.

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D.

Hydraulic Analysis

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A comprehensive hydraulic analysis will be performed utilizing the cross-

section information from the CaE report, the peak discharges determined by the

CaE, and the HEC-2 water surface profile computer program. Since the water

surface elevations determined by the CaE were not from the HEC-2 computer

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