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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6

program, the caE backwater computational input data needs to be reformulated

and reprogramed into the HEC-2 program. As informed by the City and CWCB, a

HEC-2 computation has been attempted using the cross-section data from the

previous caE study. The results are approximately six inches to one foot dif-

ferent from the previously determined water surface elevation. As indicated

in the RFP, the previous output data needs to be reproduced before performing

any additional hydraulic analysis. A calibration of the HEC-2 model with the

previous output is required. The calibration procedures are described as

follows:

(1) Code the existing cross-section data into the HEC-2 format.

(2) Plot the cross secitons utilizing SLA's in-house program. SLA

has developed a cross-section plotting program using HEC-2

input data. A sample plot is presented in Figure 1.

(3) Check cross-section plot against the topographic mapping for

cross-section location and accuracy. Since a reproduction of

the caE study output data is required, minor discrepancies

between cross section and topographic mapping will not be

corrected. Also, the topographic mapping has four-foot contour

intervals and major discrepancies are not expected.

(4) Make a hydraulic analysis utilizing HEC-2 and all parameters

used in the previous analysis and compare the output with pre-

vious study results.

(5) Modify HEC-2 input data to reproduce CaE study output. The

HEC-2 results can be modified by a number of acceptable

methods. The modification of HEC-2 input data will be per-

formed in the sequence of (a) adjust "n" values, (b) adjust the

channel expansion and contraction coefficient, (c) input known

water surface elevation at control structures such as bridges,

(d) modify the channel bed elevation, and (e) a combination of

the above. The adjustment of "n" values and channel expansion

and contraction coefficient may bring the HEC-2 results within

the acceptable range of the previous study. Since there are

eight bridges within the study reach, entering known water sur-

face elevations upstream of each bridge will break the entire

study reach into short reaches. It is anticipated that the

HEC~2 results should be very close to the previous study with