nantasket seawall stabilization project

Nantasket Seawall Stabilization Project

DCR/USACE Public Meeting

December 15, 2016 – 7:00pm to 8:30pm

Nantasket Beach Resort

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

DCR Mission Statement

To protect, promote and enhance our

common wealth of natural, cultural

and recreational resources

for the well-being of all.

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

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

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.

Enhance Safety:

Damage Caused by the Lowering Beach

Protect Future Investment:

DCR Master Plan for Water Street to Bay Street

Cross Section of DCR Master Plan at

USACE Project Area

Existing 10’ promenade shown with red line

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.

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

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

File Name

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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

File Name

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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

File Name

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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

File Name

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RECOMMENDED PLAN

USACE recognizes the

recreational usage of

beach and the concern

with impacting the limited

amount of high tide beach

DESIGN MEASURES

File Name

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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

Next Steps: Nantasket Beach Reservation

• USACE Seawall Stabilization

• Scope for design contract: master plan components

• Review options for nourishment

Q & A

Q & A

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

Mass.Parks@state.ma.us.

Additional Information