Service Solicited - 2007 - Floodplain Task Force - Final Report Floodplain Task Force
March 2007
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Final Report of the Floodplain Task Force
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A Floodplain Task Force was approved by the City Council following the 2006 Spring
water runoff that resulted in flooding in various locations within the City of Eagle.
The purpose of the Task Force was to review what happened during the flooding and to
evaluate the City of Eagle's response to the events. They were also asked to research and
communicate with the regulatory agencies as to what can and should be done to help
mitigate flooding along the Boise River as well as Dry Creek.
The Task Force was comprised of water experts and representatives from impacted
subdivisions along the Boise River and Dry Creek waterways. The Task Force was
chaired by Steve Sweet from Flood Control District 10 and co-chaired by Jim Acevez
from the Brookwood subdivision. The 18 member Task Force began meeting in
September 2006 and held regular bi-weekly meetings until March 26,2007.
Members on the Task Force
Nancy Merrill, Mayor
Steve Sweet, Chair, Flood Control 1 0
Jim Acevez, Co-Chair, Brookwood
Brayton Willis, US Army Corps of Eng.
Scott Van Hoff, IDWR
Don Stockton, Eagle Pathway Chair
Bill Rode, resident
Ron Rippey, Board Member Hart Davis
Don Knichrehm, Westover Lakes
Aeencv Experts Involved in the Review
Kirk Walton, Island Woods
David Shaw, ERO Resource Group
Lisa Bowman, Cottonwood Creek
Laurie Warren, Streamside
Shannon Cook, Laguna Point
Chuck Ferguson, Flood Control 10
Mike Mongelli, City of Eagle
Kevin Howell, Rivers End
Vem Brewer, Holliday Eng.
U.S. Army Corps of Engineers, Brayton Willis
Idaho Department of Water Resources, Scott Van Hoff
Flood Control District 10, Steve Sweet
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Analvsis
During the past 6 months the Floodplain Task Force reviewed the high water problems
that occurred last spring and developed a list of short and long-term strategies to lessen
the risk for a flood occurring and to minimize the impact of a flood. During this review
process, the Task Force discussed a wide range of topics including levees and berms in
Eagle which were created long ago but today are not considered structurally sound or
certified by Corps of Engineers standards; and the accuracy of currently used FEMA
maps. It was the goal of the Task Force to have a clear understanding about the
difference between the Floodplain and Floodway, the meaning of No Adverse Impact and
how it relates to development, Green LiDAR technology and how it might hold the
potential to better map the topography and terrain features in and around the floodplain;
gravel accumulation and the natural narrowing of the Boise River resulting in reduced
river capacity; constraints to clean out the river due to regulatory requirements, the State
Lands control of regulating the removal of gravel, and the lack of funds to clean out the
river and to study options to improve flow capacity in the channel.
The Task Force also discussed the importance of education, not only to residents but also
to the development community; analysis the river's weak spots; creating proper flow
channels to convey water; the Corps of Engineers "rule curve" to determine release of
water from the 3 dams; working with the Bureau of Reclamation and other stakeholders
to allow floodwater in the NY canal; the need of a State Flood Czar; and adopting a long-
term commitment to maintenance of Boise River.
In addition to these discussion topics, Mayor Merrill made a request to the U.S. Army
Corps of Engineers in a letter dated July 18,2006 to conduct an After Action Review of
the 2006 high water event in Eagle. We were presented the report March 7, 2007, which
includes a summary of the conditions and factors considered in the real-time decision
making with the spring of 2006 Boise River Reservoirs operations, the flow split at the
head of Eagle Island, a review of the accuracy of the Federal Emergency Management
Agency maps, identified flood susceptible problem area (particularly bridges), and
suggestions for the community to consider for future flood risk reduction along the Boise
River. The intent of the After Action Review was not to lead the City of Eagle to any
particular conclusion for flood risk reduction but rather to provide information and ideas
to best assist in the decision making process.
Summarv
The discussion and conclusions of the Floodplain Task Force parallels the final analysis
of the U.s. Army Corps of Engineers. In our combined suggested solutions there are
some items that can be physically done right now to create a better flow and others are
more long term and would require the participation of other cites, agencies, and
stakeholders to develop a common vision and plan for implementation.
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While the Floodplain Task Force was not asked to conduct a benefit/cost analysis on any
of these alternatives, the following recommendations have been prioritized in respect to
potential for beneficial affect -- highlighted and numbered in red. Each recommendation
faces obstacles to achievement. Each recommendation requires further analysis in terms
of achievability and feasibility, but ultimately the Committee concluded that each
recommendation is possible to effect.
They are broken down into Near-Term Solutions with Limited Duration Benefits, Near
Term solutions with Long-Term Benefits for Localized Areas, and Structural
Alternatives.
I. NON-STRUCTURAL ALTERNATIVES
Near-Term Solution with Long-Term Benefits for Entire Valley
1. Improve the Channel Capacity bv Modifying the Reservoir Operations #1
and #7 Changing Water control is a method used to prevent increasing flood risk.
The Boise River's channel capacity becomes restricted over several years of
drought because gravel bars form within the channel and brush grows in the
channel and on the banks. Channel capacity has declined with the regulation of
large flow events in the channel. However, high releases near the 6,500 cfs bank
full capacity at the Glenwood Bridge appear to recover channel capacity. For
example, from 1995-1996 the Boise Basin experienced high flows of 5,000 cfs to
7,000 cfs at the Glenwood Bridge, that evidently flushed out and reduced
vegetation within the Boise River's channel so that the 1997, 1998 and 1999 flood
flows of7,020 cfs, 8,350 cfs and 6,650 cfs, respectively, had less flood impacts
than the 6,500 cfs flow in 1993. However, localized flooding problems can still
occur at flows below the bank full capacity of 6,500 cfs at the Glenwood Bridge,
when a tree blocks a channel, a dike fails, or a dike is constructed or altered.
Without a comprehensive and continuing maintenance program, it is expected that
gravel bar, tree, and brush accumulation within the channel will continue.
Annualjlushing of the channel with high releases near bank full capacity of
6,500 cfs at the Glenwood Bridge may slow and somewhat reduce this type of
accumulation. (Would require interagency cooperation and support from fish
and Game as well as Homeowners)
Near-Term Solutions with Long-Term Benefits for Localized Areas:
2. Acquirinl!: emere:encv access easements: The City is doing this now and will
strengthen the codes to further ensure access for emergency flood fight activities,
and after event repairs.
3. More restrictive Floodplain Codes and Ordinances: Actions are being taken
by the Treasure Valley Partnership to coordinate a standard for, No Adverse
Impact, a 100 foot setback (to include a greenbelt easement in this area), two feet
above the base flood elevation, and a flood mitigation plan.
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4. Redefine, restore, and maintain the South to North floodwav #2 as
established by FEMA mapping on the property near the head of Eagle Island.
(Would require financial support coordination with property owners and Flood
District # 10)
5. Enhance flood education and preparedness #6 Maintain and update
contacts and stay in contact with local, state, and federal government when flows
have a potential of exceeding 7,000cfs, (flood stage). Regular checks would be
made to assure that flood response easements are maintained accessible.
Informational brochures should be prepared and handed out to all potential
homeowners in a flood risk area. Education of the Realtors Association will help
inform all future home buyers. Regular contact with the Homeowner Association
with information about future flooding will prepare the public.
6. Flow Split Control #4 A regular maintenance program consisting of
monitoring and removal of accumulation of gravel at the head of the Island will
help regulate flows in both the North and South channel of the Boise River.
II. STRUCTURAL ALTERNATIVES
Near-Term Solutions with Limited Duration Benefits for Localized Areas
1. Soft Armorinl!: Bank Protection #5 There are numerous ways to repair and
stabilize eroding river banks with the use of living and non living vegetation and
other bioengineering materials. If properly designed, these types of solutions
work well under certain conditions. Where flow conditions can cause scour, a
mix of hard and soft solutions can be a more environmentally friendly alternative
then hard armoring.
Long-Term Solutions with Long-Term Benefits for Localized Areas
2. Bridl!:e Replacement: The Eagle Road Bridge across the South channel has a
limited capacity as observed this year. The City Of Eagle should consider
evaluating the actual capacity of this bridge as well as others along the Boise
River including the Linder Bridge that appears to have similar problem. If these
bridges are determined to have limited capacity to pass high water, the local
government would have valuable information to prioritize the replacements or
upgrades necessary to reduce the risk of backwater and debris plugging. Along
with this the City of Eagle might consider the installation of reinforced concrete
pads on the upstream side of the bridges to facilitate debris and blockage removal.
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Long-Term Solutions with Long-Term Benefits Undeveloped Area (Eagle Island
State Park)
3. Set-back levees with terraced peak flow channels: Compared to the non-
structural conservation of the morphological floodplain alternative mentioned
above, the design of this type of flood control works typically requires less land for
construction. One of the notable benefits from this type of design comes from
allowing the river to meander "somewhat" naturally between levees. Many of the
existing environmental benefits can be also preserved and protected as well.
(Requires cooperation with numerous governmental agencies.)
Long-Term Solutions with Long-Term Benefits for more Frequent high flow
events (ie:l0-percent flood)
4. Utilization of New York Canal as a multi-obiective irrigation canal/flood
control works #3 The communities along the Boise River, in conjunction with the
USACE, USBR, and the support of the irrigators could enlarge and rejuvenate the
New York Canal, such that it could provide greater flood water carrying capacity.
This would mean improving the outlet works on Lake Lowell as well to bypass
flood waters that would exceed their maximum pool. This proposal could benefit
both irrigators who could get a significant rehabilitation to their canal and flood
control reduction to the lower Boise River residents who live along the river. This
alternative would require the development of cost sharing and operational
agreements that would allow for joint usage of that facility.
5. Flow Split Control with a Structure #4 The flow split at the head of Eagle
Island is constantly changing. If a constantly maintained flow split at the head of
Eagle Island is desired a structure could be constructed to provide that flow split.
You have asked the Floodplain Task Force to assess the situation and provide solutions,
this we have accomplished and now we look to the Eagle City Council to move forward
with our recommendations.
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