FLOOD01307

• County: Logan
• Community: Sterling
• Title: Flood Impact Study Pawnee Creek Overflows
• Date: 11/23/1997
• Prepared For: Design Technics L.L.C.
• Prepared By: ICON Engineering, Inc.
• Floodplain - Doc Type: Flood Documentation Report

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Mr. Wayne Bethunun
Page 2

Our second analysis is based on field surveyed cross-sections supplied to ICON by Larry
McGonegal, dated October 25, 1997. The survey data was used to create cross-sections (e.g.
station and elevation data) which were formatted for use with the U.S. Army Corps of Engineers
HEC-2 water surface profile software. This software calculates the water surface elevation
(WSEL), depth and extent of flows through a given area. ICON noted that in some places the
survey data may have been too sparse to provide an accurate depiction of the terrain relief, but that
it was sufficient for the intended purpose. It was further noted that the terrain near Iris Drive and
US Highway 138 does not contain the flood flows and that they will overtop the highway. The
extent and depth of the overtopping is uncertain. Detailed data may have been collected by the
CWCB about the depth of flows over the highway during the July 1997 flood. However, as of the
date of this report, the data has not been released for public use.

The CWBC estimated that the July 31, 1997 flood produced peak overflows from Pawnee Creek
of2,800 cfs. At the request of the CWCB, peak discharges of2,8oo cfs and 3,800 cfs were used
as the potential storm flow in our second analysis. It should be noted that the July 31, 1997 storm
has been estimated by the CWCB to have had a rainfall greater than the 100-year storm event. It
should also be noted that the Current Flood Insurance Study (FIS) indicates that overflows caused
by a 100-year storm in the Pawnee Creek Watershed are equal to 5000 cfs.

Based on conversations with you and the CWCB, the following assumptions were made for this
analysis:

I. Your property was completely undeveloped for the "existing conditions" model and a
sunflower crop existed on your property which was assumed to have a roughness coefficient
of 0.045.

2, Developed areas adjacent to your property were assumed to have a roughness coefficient of
0.080.

3. All storm flows (2800 cfs or 3800 cfs) enter the study area through the most upstream
surveyed cross section number 8.

4. The starting water surface elevation at Iris Drive and Highway 138 was calculated using the
"slope-area" method, assuming an energy slope of 0.2%. This last assumption also assumes
that the full flows (2,800 cfs or 3,800 cfs) occur at the intersection of Iris Drive and US
Highway 138. This is most likely not the case because flood flows were documented to have
overtopped Highway 138 at this location. However, pending the release of more detailed
data, this assumption will provide a conservative estimate of the depth of flooding upstream of
this location.

Since the survey data was completed after the 1997 flood and after substantial earthwork had been
completed on your property, it was not considered to be useful for the baseline model. To create a
valid baseline model, existing elevation data from the Design Technics L.L.C. Subdivision plans
was used to replace the survey data within the limits of the construction. The elevation data from
the Design Technics L.L.c. Subdivision plans was provided by Liebert McAtee and represents the
most current existing conditions on your property prior to any development.