FLOOD07698

• County: Statewide
• Community: Rock Rapids, Iowa
• Basin: Statewide
• Title: Problem Identification Report Rock River at Rock Rapids Reconnaissance Study Section 205
• Date: 4/1/1995
• Prepared For: US Army Corps of Engineers
• Prepared By: Greenhorne & O'Mara, Inc.
• Floodplain - Doc Type: Project

1
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I

HYDRAULIC ANALYSIS

EXISTING CONDITIONS
Rock Rapids is located in Lyon County, Iowa, on the right and left
banks of the Rock River. Vater surfaces profiles for the hydraulic
analysis were computed using the Corps of Engineers HEC2 Federal Highway
Commission (formerly the Bureau of Public Roads (BPRll backwater computer
model. The HEC-2 input file used in the May 1991 FIS for Rock Rapids
was provided by the Corps for use on this study. The Omaha District's
HEC2BPR computer model was required to run the FIS input file. Kinor
modifications were made to the existing conditions HEC-2 input file
including revised discharges and elimination of comment records.
Appendix B contains details of the hydraulic analysis, a flood plain
map, and a copy of the HEC-2 output file used for this portion of the
study.

Generally, the Kannings "n" values used in the FIS were 0.033 for the

channel and 0.07S for the left and right overbanks. Contraction and

expansion coefficients for most cross-sections were set at 0.3 and O.S,

respectively, while the coefficients for some areas were assumed to be

0.4 and 0.6, respectively. A discussion of these HEC-2 parameters is

included in appendix B. The starting water surface elevations at the

most downstream cross-section were taken from the existing FIS and

entered as known water surface elevations. (The FIS report did not

specify how the known water surfaces were determined). The peak discharge

values published in the FIS report and found in the FIS HEC-2 input file

were revised in the study hydraulic model. The existing flow split near

the south end of the city was not studied in detail; however, a discussion

regarding its impacts is discussed in appendix B.

The existing FIS model was rerun to verify that the computed output

data matched the water surfaces published in the Rock Rapids FIS.

Existing conditions water surface elevations were then computed for the

revised 10-, SO-, 100-, and SOO-year flows and are shown in table 3.

20