Nantasket Beach, Hull, MA Seawall Impacts on Beach Erosion. February 9, 2012

Transcription

1 Slide 1 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 PH: (978) John.H.Winkelman@usace.army.mil

2 Slide 2 Presentation Overview Project Background - Location/Layout - Coastal Structures Brief history of Corps involvement Recent projects Beach survey results and implications Summary/Conclusions

3 Slide 3 Location Map Project Location

4 Slide 4 Allerton Hill Headland Beach System N Strawberry Hill Hingham Bay Atlantic Ocean Northern Bathhouse Phipps Street Rip Rap Revetment Rip RapRevetment Sagamore Hill Pavilion Atlantic Hill DCR Reservation Photo taken in 2001

5 Slide 5 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 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

6 Slide 6 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) 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 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 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) Draft Feasibility Report Nantasket Beach Hull, MA Section The report is currently being revised and updated to included new study data and information

7 Slide 7 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

8 Slide 8 TSF

9 Slide 9 New Northern Revetment

10 Slide 10 Coastal Processes (Before Study) 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+ yd 3 /ft of beach of cobbles and gravel had been reportedly removed over an 18- year period by beach maintenance crews.

11 Slide 11 Beach Surveys 1963 Beach Survey (USACE) - 8 profiles collected to below -30 ft- NAVD 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 LIDAR data (USACE SHOALS system) 2010 LIDAR data (USACE CHARTS system)

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13 #8 #1 #2 #3 #4 #5 #6 #7

14 #8 #1 #3 #5 # 7

15 2010 NCMP LIDAR Data

16 Elevation (ft-navd88) Nantasket Beach Profile 1963, 2005, and 2010 Comparison (2005 Profile #3) 1963 Profile #16 (Profile #3 in 2005 Survey) 2005 Profile # Profile # , , , , , Distance from Base Point (feet)

17 Elevation (ft-navd88) Nantasket Beach Profile 1963 and 2005 Comparison (2005 Profile #4) 1963 Profile # Profile # Profile # Profile # , , , , , *The extrapolated seawall base point elevation is highlighted in orange -50 Distance from Base Point (feet)

18 Elevation (ft-navd88) Nantasket Beach Profile 1963, 2005, and 2010 Comparison (2005 Profile #5) 1963 Profile #12 (Profile #5 in 2005 Survey) 2005 Profile # Profile # , , , , , *The extrapolated seawall base point elevation is highlighted in orange -40 Distance from Base Point (feet)

19 Elevation (ft-navd88) Nantasket Beach Profile 1963, 2005, and 2010 Comparison (2005 Profile #6) 1963 Profile # Profile # Profile # Profile # , , , , , , *The extrapolated seawall base point elevation is highlighted in orange -40 Distance from Base Point (feet)

20 Elevation (ft-navd88) 20 Nantasket Beach Profile 1963, 2005, and 2010 Comparison (2005 Profile #7) 1963 Profile #5 (Profile #7 in 2005 Survey) Profile # Profile # , , , , , , *The extrapolated seawall base point elevation is highlighted in orange -40 Distance from Base Point (feet)

21 Volume Calculations (1963 to 2005) Slide 21

22 Volume Calculations (1963 to 2010) Slide 22

23 Slide 23 Volume Change Summary

24 Slide 24 Study Findings 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 124,000 yds 3 is very similar to reported cobble volume removed of 125,000 yds 3 Significant change between 2005 and 2010 (LIDAR issue)

25 Slide 25 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