Nantasket Beach, Hull, MA Seawall Impacts on Beach Erosion

Transcription

Slide 1
© 2003 By Default!
Nantasket Beach, Hull, MA
Seawall Impacts on Beach
Erosion
February 9, 2012
John H. Winkelman
U.S. Army Corps of Engineers - New England District
696 Virginia Road, Concord, MA 01742
PH: (978) 318-8615
[email protected]
A Free sample background from www.powerpointbackgrounds.com
Slide 2
© 2003 By Default!
Presentation Overview
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Project Background
- Location/Layout
- Coastal Structures
Brief history of Corps involvement
Recent projects
Beach survey results and implications
Summary/Conclusions
Slide 3
© 2003 By Default!
Location Map
Project Location
Slide 4
© 2003 By Default!
Allerton Hill
Headland
Beach System
N
Strawberry Hill
Hingham Bay
Atlantic Ocean
Phipps Street
Northern Bathhouse
Rip Rap
Rip
Revetment
Sagamore Hill
RapRevetment
Pavilion
Atlantic Hill
DCR Reservation
Photo taken in 2001
Slide 5
© 2003 By Default!
Seawall Construction History
 1826, development along Nantasket Beach involved public and private structures supported
on timber piles with porches that projected over the high water line. Tidal flow under the
main parts of the structures was limited by timber bulkheads, which were ultimately replaced
by concrete walls and bulkheads.
 Construction approximately 1915
 Length 5,400
 Narrow structure designed to be partially buried - the sand volume in front of the wall to
provide stability.
 Previous seawall failures close to existing failure location during early 1940’s and 1960’s.
 1980 - a rehabilitation project added a 12-inch thick reinforced concrete facing to the wall.
 Existing seawall:
- 15 feet tall from top to bottom (base at +2.0’ NGVD29)
- 4 feet high by 5.5 feet wide base
- Crest of the wall (Elev. +17 ft NGVD) is 28-inches wide
- Sidewalk on the landward side of the wall is located 2 feet below the crest
- Weight of the structure is approximately 8,200 pounds per linear foot of wall
Slide 6
© 2003 By Default!
History of USACE Involvement With
Nantasket Beach
1949
Beach Erosion Control Report on Cooperative Study of Metropolitan District
Commission Beaches, MA, Part D Nantasket Beach
- Concluded the beach was stable and that maintenance for recreation be funded by
locals (this included covering or removal of cobble material).
1968
Beach Erosion Control Report on Cooperative Study of Revere and Nantasket
Beaches, MA
- Recommended placement of a 75’ wide sand berm at elevation 12.0’ NGVD.
Project never completed due to lack of local cooperation/funds.
1993
Reconnaissance Report, Nantasket Beach Shore Protection Study, Hull, MA
- Recommended the initiation of Feasibility Study
1998
Corps study and design for repairs-in-kind to the Nantasket Beach seawall. The
Corps completed the study report in August 2000. Due to review comments received
as part of the ENF process, the DCR directed the Corps on August 19, 2002 to
perform an Alternatives/Analysis Study (AAS).
2000
Draft Feasibility Report – Nantasket Beach Hull, MA Section 103
- The report is currently being revised and updated to included new study data and
information
Slide 7
© 2003 By Default!
Current/Recent Projects
Nantasket Beach USACE - Section 103 Feasibility Study
 Center revetment feasibility report near completion (01/2012)
 Feasibility Report is being revised (middle section revetment proposed)
 Grain Size/beach profile investigation (completed in 2005)
Seawall Replacement at Northern End with Revetment and TSF
 Revetments designed by Corps of Engineers
 Revetment has been constructed and accepted
DCR Beach Committee Study
 Woods Hole Group worked for DCR looked at all possible alternatives
Slide 8
© 2003 By Default!
TSF
Slide 9
© 2003 By Default!
New Northern Revetment
Slide 10
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Coastal Processes (Before Study)
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Headland beach system with both headlands armored
Offshore drumlins were, and maybe a source of beach material
Sediment transport from north to south
Beach comprised of bi-modal grain size distribution (sand and
cobble/gravel
 It was contended that the seawall was increasing the erosion
rate in the DCR reservation and for the entire beach system by
causing a sediment sink.
 Based upon literature, personal accounts, and various interest groups
the beach experienced vertical erosion of 5 to 10 feet between the
1940’s and 2000
 Between 1950 and 1968 more than 125,000 cubic yards (20+ yd3/ft of
beach of cobbles and gravel had been reportedly removed over an 18year period by beach maintenance crews.
Slide 11
© 2003 By Default!
Beach Surveys
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1963 Beach Survey (USACE) - 8 profiles collected to below -30 ftNAVD88
2005 Beach Characterization Study (USACE Contract)
– Ocean Survey Incorporated from Old Saybrook, CT performed work.
– Sampling done mostly with Vibracores to 4’ in depth
– Ponar (grab samples) used where bottom contained too much
cobble
– Eight profiles surveyed from Seawall/Dune to -35 ft-NAVD88
– During the previous summer the DCR and the Corps dug tests pits to
determine extent of cobble and size
– Effort designed to match USACE survey and sampling effort from
1963
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2007 LIDAR data (USACE SHOALS system)
2010 LIDAR data (USACE CHARTS system)
#8
#1
#2
#3
#4
#5
#6
#7
#8
#1
#3
#5
#
7
2010 NCMP
LIDAR Data
Nantasket Beach Profile
1963, 2005, and 2010 Comparison (2005 Profile #3)
20
1963 Profile #16 (Profile #3 in 2005 Survey)
2005 Profile #3
10
2010 Profile #3
Elevation (ft-NAVD88)
0
0.0
500.0
1,000.0
1,500.0
2,000.0
-10
-20
-30
-40
Distance from Base Point (feet)
2,500.0
3,000.0
1963 and 2005 Comparison (2005 Profile #4)
20
1963 Profile #14
1963 Profile #15
10
2005 Profile #4
2010 Profile #4
0
0.0
500.0
1,000.0
1,500.0
2,000.0
-10
-20
-30
-40
*The extrapolated seawall base point elevation is highlighted in orange
-50
2,500.0
3,000.0
20
2005 Profile #5
10
2010 Profile #5
0
0.0
500.0
1,000.0
1,500.0
2,000.0
-10
-20
-30
-40
2,500.0
3,000.0
20
1963 Profile #8
1963 Profile #10
10
2005 Profile #6
2010 Profile #6
0
0.0
500.0
1,000.0
1,500.0
2,000.0
2,500.0
-10
-20
-30
-40
3,000.0
3,500.0
20
2005 Profile #7
10
2010 Profile #7
0
0.0
500.0
1,000.0
1,500.0
2,000.0
2,500.0
-10
-20
-30
-40
3,000.0
3,500.0
Slide 21
© 2003 By Default!
Volume Calculations (1963 to 2005)
Slide 22
© 2003 By Default!
Volume Calculations (1963 to 2010)
Slide 23
© 2003 By Default!
Volume Change Summary
Slide 24
© 2003 By Default!
Study Findings
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Depth change or vertical beach change is very
small when averaged over length of profile
Very strong counter evidence to the claim that the
seawall has accelerated beach erosion
Woods Hole Group from Falmouth, MA concluded
sediment transport is from south to north
Presence of bi-modal sediment confirmed in 2005
study and similar to 1963 sampling effort
Volume change
of between 1963 and 2005 of
3
124,000 yds is very similar to reported
cobble
3
volume removed of 125,000 yds
Significant change between 2005 and 2010 (LIDAR
issue)
Slide 25
© 2003 By Default!
Summary/Conclusions
 Anecdotal evidence and personal accounts are
important but actual data cannot be equaled
 Money spent on data collection well worth it even
though it is expensive
 Once again shown that seawalls do not necessarily
increase beach erosion

Nantasket Beach, Hull, MA Seawall Impacts on Beach Erosion

Transcription

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