Download - Nantasket Seawall Stabilization Project
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Nantasket Seawall Stabilization Project
DCR/USACE Public Meeting
December 15, 2016 – 7:00pm to 8:30pm
Nantasket Beach Resort
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Commonwealth of Massachusetts
Governor
Charles D. Baker
Lieutenant Governor
Karyn E. Polito
Energy and Environmental Secretary
Matthew A. Beaton
Department of Conservation and Recreation Commissioner
Leo P. Roy
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DCR Mission Statement
To protect, promote and enhance our
common wealth of natural, cultural
and recreational resources
for the well-being of all.
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Purpose of Meeting
• Project Goals
• Project Partnership – DCR and US Army Corps
of Engineers (USACE)
• USACE Findings Regarding Risk of Wall Failure
• USACE Recommended Plan and Design
• Project Schedule and Next Steps
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Project Goals
Seawall Stabilization
• Enhance Safety
• Protect Public and Private Infrastructure and Future Investment in
DCR Master Plan work
• Set the stage for potential beach nourishment
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Source: 2010 Coastal Engineering and Shore Protection Alternatives
Assessment (Woods Hole Group, Inc. and Louis Berger)
Enhance Safety: Protection Provided by Seawall
These lines represent where the shore line would be. Any wave action from
storms would impact areas landward of these lines.
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Enhance Safety:
Damage Caused by the Lowering Beach
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Protect Future Investment:
DCR Master Plan for Water Street to Bay Street
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Cross Section of DCR Master Plan at
USACE Project Area
Existing 10’ promenade shown with red line
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Set the Stage for Potential Nourishment
Not all Beach Nourishment
Material is the same:
Need to find the right balance
between shore protection…
…and recreation. Having a
stable seawall allows greater
emphasis on recreation.
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Project Partnership-
DCR and the US Army Corps Engineers 1. The USACE has a history of designing coastal
solutions. USACE has been working with DCR at
Nantasket Beach for decades
a) Designed southern seawall stabilization
b) Designed northern seawall replacement and
revetment
2. The Army Corps will pay for two-thirds of this project
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Prepared By Michael Riccio
December 15, 2016
NANTASKET BEACH
COASTAL STORM DAMAGE REDUCTION PROJECT
HULL, MASSACHUSETTS
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13 PROJECT PURPOSE
Prevent failure of seawall and subsequent damages to public infrastructure
• Failure experienced in 1992
• Mode of future failure evident within unprotected section of the beach
Currently Exposed Seawall Footing
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File Name
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Northern Revetment (2006)
TSF
(2004)
Exposed Seawall
(Study Area)
2,200 Linear Feet
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IDENTIFYING THE PROBLEM – RISK OF WALL FAILURE
Conceptual Instability (Coastal Processes)
Seawalls increase scour/erosion
Calculated Instability
Stability Analysis (USACE)
Coastal Modeling (USACE)
Survey Data (DCR, current)
Demonstrated Instability
Currently Exposed Footings
Previous Undermining of Wall
Previous Failure of North Section
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CONCEPTUAL INSTABILITY (COASTAL PROCESSES)
In a natural system wave energy dissipates upward
and gradually along the beach berm
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CONCEPTUAL INSTABILITY (COASTAL PROCESSES)
When a seawall is in place, such as at Nantasket Beach,
wave energy meets an impasse and can no longer
dissipate gradually…
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CONCEPTUAL INSTABILITY (COASTAL PROCESSES)
…instead, that wave energy is reflected violently both
upward and downward
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CONCEPTUAL INSTABILITY (COASTAL PROCESSES)
Energy reflected downward, creates a scouring effect
eroding beach material along the face of the seawall.
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CONCEPTUAL INSTABILITY (COASTAL PROCESSES)
Erosion continues downward and outward and begins
undermining the toe of the seawall
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CONCEPTUAL INSTABILITY (COASTAL PROCESSES)
Eventually scouring undermines the toe of the seawall
and the seawall collapses.
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CONCEPTUAL INSTABILITY (COASTAL PROCESSES)
Conceptual wave energy depicted in real
time at Nantasket Beach.
February
2008
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CALCULATED INSTABILITY
Stability Analysis (USACE)
- Concluded that “instability of the seawall is caused by the lack of
passive resistance resulting from the continued loss of sand.”
- Confirmed that the condition of the middle section was “serious and
should be addressed as soon as possible.”
- Concluded that the “sand elevation should be at or above the footing of
the seawall to maintain its stability.”
Coastal Modeling (USACE)
- Modeled performance of the existing beach for 84 different storm events
(using existing data from previous storm events).
- Identified that even 2 year storm events could expose the footing of the
seawall
Surveys Conducted Between 2006 - 2016 (DCR)
- Surveyed sand elevations along the seawall quarterly for the past 10 years.
- Demonstrated steady erosion of the material in front of the seawall.
- Does not show signs of improving conditions.
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Top of Seawall Footing
CALCULATED INSTABILITY – (CURRENT SURVEY DATA)
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September 2016
Seasonal Variability of Beach Profile
DEMONSTRATED INSTABILITY
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June 2016
Seasonal Variability of Beach Profile
DEMONSTRATED INSTABILITY
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2004 CONDITIONS
DEMONSTRATED INSTABILITY
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2004 CONDITIONS
DEMONSTRATED INSTABILITY
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2004 CONDITIONS
DEMONSTRATED INSTABILITY
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Seawall collapse at Nantasket Beach caused
by undermining during 1992 coastal storm
DEMONSTRATED INSTABILITY
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POTENTIAL SOLUTIONS
USACE Feasibility Study
Authorized to study, design, and construct small coastal storm
damage reduction projects Section 103 - Continuing Authorities
Program
Must demonstrate a public benefit and positive Cost Benefit Ratio
Cannot be used for the purpose of providing a recreational beach
Requires a 65/35 percent cost share with non-federal sponsor for
implementation
The Federal cost for planning, design, and construction of any one
project CANNOT EXCEED $5,000,000
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POTENTIAL SOLUTIONS - BEACH NOURISHMENT
Beach Nourishment
Process
- Beach Profile is rebuilt and
new material acts as
sacrificial material
protecting the toe of the wall
- Sacrificial material erodes
over time
- Beach continues to function
if re-nourished as needed.
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BEACH NOURISHMENT ALTERNATIVES
10 year level of Protection - 50’ wide beach berm at elevation 9.25’
- 246,000 cubic yards of material
- Positive Benefit Cost Ratio
- Total Project Cost w/ maintenance
$55 million
25 year level of Protection - 75’ wide beach berm at elevation 9.25’
- 378,000 cubic yards of material
- Positive Benefit Cost Ratio
- Total Project w/ maintenance
- $50 million
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Revetment Process
- Sloped Revetment creates more natural slope allowing
waves to break and run up the revetment, reducing the
downward wave energy that causes scour and erosion.
- Reduced scour reduces risk of the wall undermining and
collapsing.
POTENTIAL SOLUTIONS - STONE REVETMENT
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POTENTIAL SOLUTIONS - REVETMENT
Project Area Without Revetment Project Area With Revetment
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REVETMENT ALTERNATIVES
10 year level of Protection
25 year level of Protection
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Stone Revetment – 10 Year Storm Event Protection Level - 8.25’ Crest
- 20’ Wide Slope
- Built to elevation 10’
- 14’ wide exposed revetment (i.e. 27,000 square feet of ‘lost beach’)
- Initial Construction Cost $5.8 million
RECOMMENDED PLAN
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RECOMMENDED PLAN
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USACE recognizes the
recreational usage of
beach and the concern
with impacting the limited
amount of high tide beach
DESIGN MEASURES
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ORIGINAL DESIGN 40’ wide revetment
≈30’ of revetment exposed
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REVISED DESIGN (REDUCED FOOTPRINT)
COULD reduce the slope
(Slope reduced from 3:1 to 2:1)
COULD NOT reduce the profile
(Design criteria requires 10’ thickness)
COULD NOT lower the revetment
(excavating below the footing would
risk failure during construction)
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REVISED DESIGN (REDUCED FOOTPRINT)
COULD reduce the slope
(Slope reduced from 3:1 to 2:1)
COULD NOT reduce the profile
(Design criteria requires 10’ thickness)
COULD NOT lower the revetment
(excavating below the footing would
risk failure during construction)
Revetment width of 40’ reduced to 33’
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REVISED DESIGN (REDUCED FOOTPRINT)
60’ wide project footprint = 130,000 sf
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REVISED DESIGN (REDUCED FOOTPRINT)
≈14’ of revetment exposed
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REVISED DESIGN (REDUCED FOOTPRINT)
Remainder backfilled and covered
≈14’ of revetment exposed ≈14’ of revetment exposed
(27,000 sf of beach “lost”)
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PROJECT SCHEDULE
95% Design Complete Nov. 2016
Submit Permit Application Dec. 2016
Obtain Permits Mar. 2017
Issue Solicitation (30 Days) Apr. 2017
Open Bids May 2017
Award Contract Jun. 2017
Preconstruction Meeting Aug. 2017
Issue Notice to Proceed Aug. 2017
Mobilize Equipment Begin Work Sept. 2017
Complete Project Mar. 2018
• Begin Construction no sooner than Labor Day 2017
• Finish Construction no later than Memorial Day 2018
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Next Steps: Nantasket Beach Reservation
• USACE Seawall Stabilization
• Scope for design contract: master plan components
• Review options for nourishment
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Q & A
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Q & A
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For more information: http://www.mass.gov/eea/agencies/dcr/public-outreach/public-meetings/
If you have comments or suggestions on this project:
Submit online:
http://www.mass.gov/eea/agencies/dcr/public-outreach/submit-public-comments/
Write: Department of Conservation and Recreation
Office of Public Outreach
251 Causeway Street, Suite 600
Boston, MA 02114
Deadline – Comments accepted by DCR through Thursday, December 29, 2016
Note: Public comments submitted to DCR may be posted on the DCR website in their entirety.
If you wish to subscribe to a DCR general information or project-related
listserv: contact DCR’s Office of Community Relations at 617-626-4973 or
Additional Information