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 Cedar
River, Cedar Rapids, Iowa
Frequently Asked Questions
1. How big was the June 2008
event?
The June 2008 event is the flood of
record. The peak water surface elevation was 732.50 feet North
American Vertical Datum (NAVD) from a discharge of about 140,000
cubic feet per second. Based upon the discharge frequency
analysis conducted in the Study, this flood was in excess of the
500-year (.002 exceedance probability) event. The previous flood
of record within the 107 years of U.S. Geological Survey gage
record was 721.0 feet NAVD from a discharge of 73,000 cubic feet
per second. This is approximately 12 feet lower than the June
2008 flood event.
2. What is the current damage estimate for the June 2008
Flood?
The City’s current damage estimate
is $2.4 billion not including economic damages, which the City
has estimated to be another $3.3 billion. Ten square miles of
land area were flooded by this flood and about 7,200 properties
were either damaged or destroyed.
3. Why aren’t levees and floodwalls justified on both
sides of the river?
In order for the Corps to make a
recommendation for Federal interest the damages prevented by the
project need to exceed the project costs. Study Alternative 1C
affords protection to properties on both sides of the Cedar
River; however, its BCR is less than 1.0. In order for the Corps
to recommend an FRM project, the BCR must be greater than 1.0.
4. Will water surface elevations change at Eighth Avenue
in Cedar Rapids due to the construction of the FRM project?
Hydraulic modeling for the Corps Recommended Plan (Alternative 4
- levees only on the east side of the river) and the City
Preferred Plan Alternative 1 (levees on both sides of the river)
shows an increase of water surface elevations of approximately
0.3 feet for the 1% annual chance exceedance, or 100-year, event
(94,100 cubic feet per second) at the 8th Avenue bridge. The
impacts of the 0.2% annual chance exceedance, or 500-year, event
(122,000 cubic feet per second) near the 8th Avenue Bridge are
approximately 0.7 feet for Alternative 4 and 3 feet for
Alternative 1.
The results are presented in Table A-31 and Figures A-6 and A-7
of Appendix A – Hydraulics and Hydrology report.
5. Will water surface elevations change upstream of
Cedar Rapids due to the construction of the FRM project?
Hydraulic modeling for the Corps Recommended Plan (Alternative 4
- levees only on the east side of the river) and the City
Preferred Plan Alternative 1 (levees on both sides of the river)
shows an increase of water surface elevations of approximately
0.1 feet and 0.3 feet, respectively, at Edgewood Road for the 1%
annual chance exceedance, or 100-year, event (94,100 cubic feet
per second). The State of Iowa regulates the floodplain for the
1% annual chance exceedance, results of the model show that the
impacts upstream are within the regulations for the 1% annual
chance event for both Alternatives 1 and 4.
The hydraulic model for the Cedar Rapids Feasibility Study was
extended upstream to the Linn County line to estimate the
potential upstream impacts of Alternatives 1 and 4 for larger
magnitude flooding events. The impacts of the 0.2% annual chance
exceedance, or 500-year, event (122,000 cubic feet per second)
for Alternative 4 are approximately 0.3 feet at Edgewood Road
and are negligible at Palo.
The impacts of the 0.2% annual chance exceedance event for
Alternative 1 are 2.5 feet at Edgewood Road tapering down to 0.5
feet at Palo. The model results show an increase of
approximately 1 foot in Palo for a 140,000 cubic feet per second
flow (2008 event) for Alternative 1. The model results at Palo
are estimates because Alternative 1 was determined to not be the
Corps Recommended Plan and hydraulic modeling efforts to
determine the impacts of Alternative 1 were scaled back. As a
result, the best available information was used to extend the
model from Edgewood Road to the Linn County line. Channel data
was not available and the model assumes channel geometry. For
large floods this level of detail is acceptable for planning
efforts because the majority of the flow is conveyed overbank.
The results are presented in Table A-31 and Figures A-6 and A-7
of Appendix A – Hydraulics and Hydrology report.
6. Will water surface elevations change downstream of
Cedar Rapids due to the construction of the FRM project?
The reduction of storage from the flood plain by the
construction of the Alternative 4 levees is small (0.3%) in
comparison to the total volume of water passing Cedar Rapids
during a large, infrequent flood. Alternatives 1 and 4 were
modeled for the range of flows from 94,100 cfs to 143,300 cfs
indicating no additional impacts downstream of the project. The
velocities downstream of the project are estimated to be
unaffected by Alternatives 1 or 4.
7. How will the construction of Alternative 4C impact
historical buildings and other cultural resources?
Archeological and architectural
resources will be affected by Alternative 4C however, the extent
of the impact will not be fully known until a more detailed
archeological, architectural and cultural survey is completed.
All potential impacts will be avoided to the greatest extent
possible.
8. Why was the alignment on the west side along the
river’s edge, where the current berms are located, screened out?
The West Side River’s Edge Alignment
was screened out in Planning Process Iteration Two (starting on
page 157 of the Report) based on initial economic analysis BCRs,
and Proposed Voluntary Property Acquisition FEMA Locations (page
160 of the Report).
9. Will the construction of a levee on the east side
change the flood insurance maps?
Changes in the Flood Insurance Rate
Maps (FIRM) may result from this study. If a levee(s) were
constructed that provided protection against the base (100-year)
flood, the FIRM would need to be updated to reflect this line of
protection as well as any changes in the base flood profile that
result from the project. Additionally, as part of this study
effort the flood flow frequency estimates and hydraulic model
through Cedar Rapids were updated to include recent flood
records and the current river geometry. This analysis resulted
in improved estimates of the base (100-year) flood profile for
existing conditions that the City may use to request a map
revision from FEMA regardless of whether a project is
constructed.
10. What will be the result of the Feasibility Study?
The result will be a feasibility
report which evaluates flood risk management alternatives and
makes recommendations regarding implementation of a federal
project. In order for the Corps to recommend a project it must
be environmentally acceptable, economically feasible, and
provide sound engineering alternatives.
11. How does the city’s River Corridor Redevelopment
Plan influence the Corps Feasibility Study?
Input from the city’s plan was
considered in the Corps’ planning process and was analyzed as an
alternative. The city has provided a great deal of detailed
information that was incorporated into the Feasibility Study
Report.
12. What important/significant natural resources may be
impacted from levee alignments?
No significant impacts to any
category of natural resources or important habitats are
anticipated.
13. The Corps plan separates the city from its river,
what can be done to establish a connection to the river, if the
Corps plan is built?
The Feasibility Study identified the
removable wall favored by the City resulted in higher project
costs. As a result the majority of the removable walls were
eliminated from the Corps plan. Opportunities to use removable
floodwalls, on a limited basis, will be considered during the
design phase.
14. Why didn’t the Corps plan make a greater use of non-
structural options?
Non-structural options were
considered extensively as the Corps formulated flood risk
management plans for the Cedar River at Cedar Rapids, but none
were economically justified. The city with the assistance of
other federal agencies is implementing a voluntary buyout
program which is a non structural option. Additionally, the Iowa
– Cedar River Basin study will consider the application of
non-structural measures throughout the basin.
15. How soon will construction of the Corps project
begin?
There is no simple answer to this
question as many things have to happen over which the Corps has
no control, in order for construction to begin. Key items
affecting the schedule that have not occurred are passage of
Congressional authorization and funding. After these two key
Congressional actions are passed, the earliest construction
could start is approximately 2 years.
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