HOW THINGS WORK
WHY DOESN’T THE CITY JUST DREDGE THE RIVER?
M
any long-time Cedar Rapids residents
best practices, and examples from other
remember watching a dredging
jurisdictions across the country.
barge work its way down the banks
In some waterways it becomes necessary to remove accumulated sediment. Dredging can preserve a river’s health and availability for commercial use. We sometimes receive questions on whether the lack of dredging has caused the Cedar River level to rise over time. Along with the National Weather Service and the Iowa Flood Center, the City of Cedar Rapids continuously monitors the depth and flow volume of the Cedar River. Our data indicates that dredging is unnecessary at this time.
98% (EAST)
a feasibility study. Thanks to the City’s public
make navigation by watercraft difficult. In
3-6B
COST ESTIMATE (2008)
outreach and technical analysis efforts,
Cedar Rapids, a private company would use
$
the USACE approved the Cedar River Flood
a dredger to take up accumulated sediment
Control System feasibility study in only 18
at the Cedar River's bottom and sell the small
months. This process has taken as many as
stones and sand for construction fill and
A belief that dredging would significantly lower the crest heights of the Cedar River during flood events is scientifically inaccurate. River levels have risen over time, but not because we stopped dredging the river. The 2008 Flood crested 19 feet above flood level — more than 11 feet above any previous flood. No amount of dredging could abate our changing climate’s impact on river levels.
INSTALL TIME
66% (WEST)
(USACE) utilized the City’s findings to complete
accumulation of silt, sand, and other debris
DREDGING THE RIVER
The USACE selected a diversion channel as the best option for part of the Fargo, N.D. flood control system. In that case, a diversion channel, combined with flood protection at the river's edge, was an effective tactic. Similar diversion channels in Cedar Rapids would cost between $3–6 billion and take more than 30 years for land acquisition and construction. By contrast, we estimate the cost of floodwalls and levees (in addition to pump stations, detention basins, and underground infrastructure) in the approved Flood Control System to be $550 million. FLOOD REDUCTION
The United States Army Corps of Engineers
of the Cedar River. In some waterways the
DIVERSION CHANNELS
five years for completion in other jurisdictions.
cement mix. In light of recent historic flood events, many wonder — why did the river
Included below are the results of the technical
dredging ever stop?
analysis on flood mitigation tactics. The
UPSTREAM RESERVOIR Reservoirs upstream would require 520,000 acres of land, take 50 years to build, and cost as much as $950 million. Permitting for an upstream reservoir would also be extremely challenging. Such protection measures would also negatively affect the Duane Arnold power plant, as many as six towns, and various roads and bridges.
USACE evaluated this matrix and approved a
This is not the only flood mitigation tactic the
combination of the second and third tactics
City of Cedar Rapids has considered. In the
below — protection at the river’s edge and
wake of the 2008 Flood, City staff took the critical issue of flood protection to the experts. With help from experienced consultants, the
protection offset from the river. The final Cedar River Flood Control System alignment has been refined over time. To see the most
City performed a technical analysis of more
FLOOD REDUCTION
current alignment, features, and timeline, visit
than 20 flood mitigation tactics. These tactics
98%
www.CityofCR.com/FloodControl.
were comprised of citizen suggestions, known
INSTALL TIME
COST ESTIMATE (2008)
ANALYSIS OF FLOOD MITIGATION TACTICS
(Showing 5 of the most and 5 of the least effective tactics studied) COST ESTIMATE (2008)
650M
$
INSTALL TIME
Dry reservoir upstream
98%
$600-650M
50 years
5-IN-1 DAM
Flood protection at river's edge
98%
$250 - 275M
10 - 15 years
Flood protection offset from river
98%
$175 - 200M
10 - 15 years
Diversion channel around Cedar Rapids (East)
98%
$5.6B
20 - 30 years
Diversion channel around Cedar Rapids (West)
66%
$2.8B
20 - 30 years
Replacing the 5-in-1 dam with a rubber dam was one of the least effective flood management tactics we studied. The current dam design does not significantly impede river flow during flood events. The dam allows full flow when its underwater gates are completely lowered and slide gates fully raised. With a flood reduction impact of only three percent, dam adjustments were not included in the final Flood Control System plan.
$10 - 20M
5 - 10 years
Dredge Cedar River
2%
$26M+
Ongoing
Remove Mays Island
2%
$175 - 200M
10 - 15 years
Flood storage at Cedar Lake
1%
$40 - 50M
10 - 20 years
Flood storage at Jones Golf Course
0%
N/A
10 - 15 years
FLOOD REDUCTION
3%
10-20M
COST ESTIMATE (2008)
4
$ OUR CR
|
AUGUST 2018
INSTALL TIME: ONGOING
2%
FLOOD REDUCTION
3%
When we evaluated the technical aspects of flood management practices, dredging the Cedar River showed only a two percent effect on flood reduction. This is because dredging would insignificantly lower the crest height of a flooded Cedar River. Dredging would also require continual contracted operation and maintenance. Disposal of dredged materials would also come at a significant cost. Overall, dredging would not be an effective flood control solution. FLOOD REDUCTION
TACTIC
Replace 5-in-1 Dam with a rubber dam
In the ten years since the 2008 Flood, we have learned a lot about our watershed. Meanwhile, significant flood events have become more intense and more common. Over the years, river volumes have increased measurably. These increases are largely due to changes in the nature of precipitation events across Iowa and the Midwest, and changes in the landscape upstream. Excess precipitation becomes runoff and contributes to the increased flow of streams leading to our river.
INSTALL TIME
26M+
COST ESTIMATE (2008)
$
FLOOD CONTROL SYSTEM
The system of floodwalls and levees known today as the Cedar River Flood Control System is a combination of two tactics — flood protection at the river's edge and protection offset from the river. These tactics were chosen for their optimal mix of effective flood suppression techniques, relative low cost, fewest overall negative impacts, and ability to integrate community amenities such as walking trails. W W W. C E D A R - R A P I D S . O R G
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