Belle View/New Alexandria/River Towers-Flood Risk Management Status Update
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Transcript of Belle View/New Alexandria/River Towers-Flood Risk Management Status Update
![Page 1: Belle View/New Alexandria/River Towers-Flood Risk Management Status Update](https://reader034.fdocuments.us/reader034/viewer/2022052623/559b30d31a28ab440a8b47a7/html5/thumbnails/1.jpg)
Public Meeting
Belle View/New Alexandria/River Towers
Flood Risk Management Study Status Update
October 25, 2012
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Purposes of Meeting
• Review the Belle View/New
Alexandria/River Towers flood risk
management study
• Initiate a discussion with residents about
the alignment of a flood wall/levee
• Start to create consensus among
residents about the best location of the
proposed floodwall/levee
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Agenda
• Welcome
• Presentation by the U.S. Army Corps of
Engineers (Corps)
• Question/Answers and Feedback
• Final Steps
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Communities are at Risk of Flooding
*1% annual chance flood is the flood that has a 1% chance of occurring in any
given year, sometimes referred to as the 100-year flood
1% Annual
Chance
Floodplain*
Based on
Storm Surge
Elevation 11.2
ft (NGVD29)
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Flood Inundation Areas to Elev. 14 Ft.
• Sea Level Rise
will increase
chance of
flooding
• Floods higher
than the 1%
chance flood
DO occur
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Elevation Data
• Many low openings/first floors: elevation
6-11 feet
• Majority of ground elevation is 8-9 feet in
Belle View, 4-9 feet in New Alexandria
• 2% annual chance (50 year/Hurricane
Isabel) storm surge: 9.6 feet
• 1% annual chance (100 year) storm
surge: 11.2 feet
• 0.2% annual chance (500 year) storm
surge: 16.2 feet
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Previous Study
• Completed Feb 2008 – Flood Damage Reduction
Analysis for Belle Haven Watershed (by Corps)
– Fairfax County requested that Corps conduct a study to
evaluate various flood damage reduction alternatives to
determine if they are technically feasible
– Conducted under technical services program; was not a
Corps project study authorized by Congress
– County asked Corps to conduct preliminary economic
analysis to determine if a project might meet the federal
economic justification requirements
– Evaluated alternatives to protect entire study area, and
New Alexandria/Belle View independently
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Project Goals and Objectives
• Goal
– Provide adequate flood risk management
measures that are technically feasible and
financially prudent for the safety of communities
• Objectives
– Examine various alternatives for reducing flood
risk and damages – primarily from storm surge
– Minimize risk to the community
– Minimize environmental impacts
– Incorporate needs/desires of the communities to
the degree possible
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2008 Study - Most Cost Effective, Plan 1b
• Preliminary concept plan assumed top of
levee/floodwall at elevation 12.0’ (0.8’
above 1% annual chance elevation; no risk
and uncertainty analysis was conducted;
cannot say will provide “100-year
protection” and no sea level rise
considered)
• Approx. 6600 feet long; 4-6 feet above
ground
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Plan 1b (2008 Study) - Levee/Floodwall Alternative
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Plan 1b (cont.)
• Requires 2 closure structures – along Belle
Haven Road and Belle View Blvd
• Minimizes impacts to wetlands
• Impacts many trees
• Is partially located on NPS and River Towers
property
• Requires 1 pumping station
• Estimated costs Plan 1b - Levee/Wall 12’
– Construction Cost = $12.7M
– Benefit to Cost Ratio (BCR) = 2.5
– Current cost and BCR: $13.7M and 2.2
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NPS Concerns Regarding Plan 1b
• Impact to GW Memorial Parkway
• Impact to trees
• Impact to view/aesthetics
• Would prefer project to be off their
property (if on federal property, need NEPA
process, Congressional authorization, etc.)
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River Towers Concerns Regarding Plan 1b
• Impacts to Dykes Marsh
• Impacts to River Towers property
• Impacts to view/aesthetics
• Impact of pump station on their property
• Requested that the project be moved off
their property
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Community Concerns Regarding Plan 1b
• Impacts to view/aesthetics & trees
• Impacts to Dyke Marsh
• Impacts to private property
• Level of protection & want it to fix the
problem
• FEMA certification & the need for
insurance
• Global warming/sea level rise
• Environmental impacts
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Study Progress
• 2009 – County conducted field survey and
tree survey for study
• 2010 – 2012 – Development and evaluation of
new alternatives – Team identified various new alignments
– Coordinated alignments with NPS and made revisions
– Investigated portable flood barriers
– Developed concept plans, costs and benefits for the new
plans
– Completed a preliminary sea level rise analysis
– Coordinated with community leaders
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Previous and New/Modified Floodwall/Levee Alignments
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Height of Protection Overview
• 1% annual chance flood elevation (100-yr
elevation based on storm surge) = 11.2 feet
• FEMA - for certification of project (no flood
insurance requirement), FEMA requires the
project be 3 feet higher than the 1% annual chance
elevation = 14.2 feet
• CORPS – for Corps built projects, wall must be
built higher than the design event to account for
risk and uncertainty (varies per project, but for the
1% annual chance flood design, typically 3-4 feet
higher than the 1% annual chance flood elevation)
plus sea level rise must be considered = 14.2-15.2
feet + SLR consideration
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Height of Protection (Cont.)
• Sea level rise predictions by the year 2100 – Based on historic rate = 1 foot
– Based on Intergovernmental Panel on Climate Change (IPCC) study = between 0.6 - 1.9 feet
– Based on National Research Council (NRC study) = between 1.5 – 4.7 feet
• Height of floodwall/levee has not been decided; initially top of protection at elevation 12 feet has been used for concept plan so that is can be compared with original plan. However, 3 heights of protection will eventually be evaluated and compared for the final alignment; higher protection is recommended to account for risk and uncertainty and sea level rise
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Corps’ Vegetation/Tree Setback
Requirements
• Trees can adversely impact floodwalls and levees and
cause the flood protection system to fail
• Vegetation/tree-free zone (except grass) extends 15 feet
on each side of a floodwall
• If a large tree has the potential to damage the wall if it
overturns, then it should be removed. General rule of
thumb - trees should be a minimum distance of half their
height from a floodwall
• County surveyed trees along potential alignments;
arborist preliminarily identified highest priority trees
based on species/condition
• Tree setback will be based on tree height and risk; we are
currently showing a 40-foot setback
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Alternative
Floodwall/Levee
Alignments
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Boulevard View South (South of Belle View Blvd)
• Alternative BVS1 – Wall along eastern edge
of Boulevard View (original alignment)
• Alternative BVS2 – Wall along center of
Boulevard View
• Alternative BVS3 – Wall along western
edge of Boulevard View
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Boulevard View South
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Alternative BVS1
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Boulevard View South
Looking South
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Concept View of BVS1 Looking South
Floodwall Elevation 12 Feet
Wall approximately 4 feet high
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Concept View of BVS1Looking South
Floodwall Elevation 14.5 Feet
Wall approximately 6.5 feet high
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Alternative BVS2
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Alternative BVS3
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Revised Alternative BVS3
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Concept View of BVS3 Looking South
Floodwall Elevation 12 Feet
Wall approximately 4 feet high;
Revised BVS3 wall would be constructed
further west, closer to BV buildings
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Concept View of BVS3 Looking South
Floodwall Elevation 14.5 Feet
Wall approximately 6.5 feet high;
Revised BVS3 wall would be constructed
further west, closer to BV buildings
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Boulevard View South
Looking West
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Concept View of BVS3 Looking West
Floodwall Elevation 12 Feet
Wall approximately 4 feet high;
Revised BVS3 wall would be closer to buildings
Closure structure for access
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Concept View of BVS3 Looking West
Floodwall Elevation 14.5 Feet
Wall approximately 6.5 feet high;
Revised BVS3 wall would be closer to buildings
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Sample Closure Structures
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Boulevard View Central
(From Belle View Blvd north to Old Towne Rd)
• Alternative BVC1 – Wall along eastern edge of
Boulevard View (original alignment)
• Alternative BVC2 – Move Boulevard View to the
west; wall along eastern edge of Boulevard View
• Alternative BVC3 – Wall on west side of Boulevard
View
• Revised Alternative BVC3 – Wall on west side of
Boulevard View south of I Street; north of I Street
– shift road west and wall on east side
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Boulevard View Central
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Alternative BVC1
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Boulevard View Central Looking
Southeast Along Road Near H Street
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Concept View of BVC1 Looking Southeast
Floodwall Elevation 12 Feet
Wall approximately 3 feet high
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Concept View of BVC1 Looking East
Floodwall Elevation 14.5 Feet
Wall approximately 5.5 feet high
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Boulevard View Central Looking West
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Concept View of BVC1 Looking West
Floodwall Elevation 12 Feet
Wall approximately 3 feet high
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Concept View of BVC1 Looking West
Floodwall Elevation 14.5 Feet
Wall approximately 5.5 feet high
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Alternative BVC2
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Alternative BVC3
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Revised Alternative BVC3
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Boulevard View North
(From Old Towne Rd to Belle Haven Rd)
• Alternative BVN1 - Wall behind houses
along NPS property (original alignment)
• Alternative BVN2 – Wall behind houses on
private property
• Alternative BVN3 – Wall runs along Old
Towne Rd and 10th Street; 7 houses
outside protection
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Boulevard View North
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Alternative BVN1
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Alternative BVN2
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Boulevard View North
BVN1 and BVN2 - Wall would run
near existing fence (existing wood
fence is approx. 4’4” high)
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Alternative BVN3
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New Alignment –Floodwall along
GWMP
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Southern Area
(near River Towers)
• Alternative S1 – Levee/wall along south
side of River Towers (original alignment)
• Alternative S1A – Levee/wall along south
side of River Towers closer to building
• Alternative S2 – Wall adjacent to Belle View
Condo north of West channel
• Alternative S3 – Wall along southern curb
of BV parking lot north of West channel
• Alternative S4 – Wall along northern curb
of parking lot south of West channel
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Southern Area
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Alternative S1
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Looking South from River Towers
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Concept View of S1 Looking South
Levee Elevation 12 Feet
Levee approximately 7 feet high
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Concept View of S1 Looking South
Levee Elevation 14.5 Feet
Levee approximately 9.5 feet high
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New Alignment - Alternative S1A
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Looking West Behind Eastern River Tower
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Looking South Behind Eastern
River Tower
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Looking West Behind Center River Tower
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Looking South Behind Center River Tower
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Alternatives S2, S3, and S4
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Alternative S2 – Western Part
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Southern Area Looking East Between Belle View Condo and West Channel
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Concept View of S2 Looking East
Floodwall Elevation 12 Feet
Wall approximately 4 feet high
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Concept View of S2 Looking East
Floodwall Elevation 14.5 Feet
Wall approximately 6.5 feet high
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Alternative S3 – Western Part
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Concept View of S3 Looking East
Floodwall Elevation 12 Feet
Wall approximately 4 feet high
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Concept View of S3 Looking East
Floodwall Elevation 14.5 Feet
Wall approximately 6.5 feet high
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Alternative S4 – Western Part
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Southern Area Looking Northwest
across West Channel
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Eastern Part of Alternative S2, S3, S4
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Southern Area (Eastern Part) Looking
North from River Towers
S2, S3 and S4 - Wall would replace existing fences
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Southern Area (Far Eastern Part)
S2, S3 and S4 - Wall would extend along this area
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Closure Structures/Increased Risk
• The more closure structures a project has,
the higher the risk of possible failure
• Original alignment (S1, BVS1, BVC1, BVN1)
= 2 road closures
• Innermost alignment (S2, BVS3, BVC3,
BVN3) = approx. 50 closures (roads,
sidewalks, and driveways)
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Evaluation of New Plans
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New Plan 1
• BVN3, BVC3, BVS3,
S1a
• No impacts to NPS
property
• River Towers within line
of protection; project on
River Towers property
• Houses near 10th St.
outside line of protection
• Estimated cost: $26.5 M
• BCR 1.2
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New Plan 2
• BVN3, BVC3, BVS3, S2
• No impacts to NPS
property
• River Towers and houses
near 10th St. outside line
of protection
• Not on River Towers
property
• Estimated cost: $31.4 M
• BCR 1.0
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New Plan 3
• BVN3, BVC3, BVS3, S4
• No impacts to NPS
property
• River Towers and
houses near 10th St.
outside line of protection
• S4 section on River
Towers property
• Estimated cost: $28.6 M
• BCR 1.1
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New Plan 4
• BVN1, BVC3, BVS3,
Sa1
• On NPS and River
Towers property
• All structures are within
line of protection
• Estimated cost: $22.5 M
• BCR 1.4
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New Plan 5
• GWMP, Sa1
• On NPS and River
Towers property
• All structures are within
line of protection
• Estimated cost: $35.2 M
• BCR 0.9
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Challenges
• Except for original plan (that will have major impact
to NPS trees), all BCR’s are less than 1.4
• Majority of alternatives would include numerous
closure structures that County would be
responsible for maintaining and closing
• Innermost alignments would impact Boulevard View
– many large trees in front of condos would have to be removed
– roadway would have to be realigned in sections
– significant impact to utilities
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Challenges (Cont’d)
• BVN3 would not protect houses near 10th street;
many challenges with aligning wall down 10th
street
• Alignments off of River Towers property
– plans are more expensive
– cross roads and tributary, cross utilities, may need
additional pump station
– RT property would be outside line of protection and would
be flooded if floodwaters were higher than elev. 11-12 ft.
(HVAC, elevators, utilities, parking lots, etc.)
– would need to leave one closure gate open for emergency
vehicles; would have to close just before flooding
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Challenges (Cont’d)
• GWMP alignment
– has less impacts to trees and utilities and less
closures than other alignments (however closures will
be large)
– road will be partially closed during/after closure
installation
– project cost will be high
– will have significant review process due to NPS land
(Congressional approval, NEPA, etc.), and may likely
be considered an “impairment”, which would prevent
the construction of the project
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Potential Next Steps
• Based on feedback from public, County may select
a project alignment for further analysis and design
• Corps will develop, evaluate and compare concept
plans for the selected alignment for 3 different
heights of protection and County will select a final
height of protection
• If alignment impacts NPS property, formal NEPA
process will be initiated concurrently
• Corps will develop more detailed design for the final
project
• Depending on many factors, the earliest project
construction would occur would be 4-5 years after
completion of 65% design
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Minimize Your Flood Risk!
• Purchase flood insurance, even if you are not
required to
• Sign up for flood alerts through the County’s
RiverWatch Program (www.fairfaxcounty.gov/cean)
• Move critical and valuable items from the basement
or the first floor (documents, old photographs,
computer, etc.)
• Consider elevating new utilities when they are being
installed since they are expensive to replace (hot
water heater, air conditioning/heating unit)
• Have an evacuation plan for you and your family
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For More Information Contact:
Donald Demetrius, P.E., PhD., Chief
Watershed Projects Evaluation Branch, Stormwater Planning
Division
Fairfax County Department of Public Works and Environmental
Services
12000 Government Center Parkway
Fairfax, VA 22035
703-324-5500, TTy 711
Presentation will be available on-line at:
http://www.slideshare.net/fairfaxcounty
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Questions?
Thoughts?
Concerns?