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SIASCONSET COASTAL BANK
STABILIZATION AND BEACH
PRESERVATION PROJECT
ALTERNATIVES ANALYSIS
SEPTEMBER 2010
OCC Project # 210019
Prepared for:
SIASCONSET BEACH PRESERVATION FUND, INC.
Prepared by:
Ocean and Coastal Consultants, Inc.
a COWI company
50 Resnik Road, Suite 201
Plymouth, MA 02360
Tel: (508) 830-1110/Fax: (508) 830-1202
www.ocean-coastal.com
Siasconset Coastal Bank Stabilization and Beach Preservation Project September 2010 Alternatives Analysis Page i
SIASCONSET COASTAL BANK STABILIZATION AND BEACH
PRESERVATION PROJECT
ALTERNATIVES ANALYSIS
September 2010
OCC Project # 210019.0
Prepared for:
SIASCONSET BEACH PRESERVATION FUND, INC.
Prepared by:
Ocean and Coastal Consultants, Inc.
a COWI company
50 Resnik Road, Suite 201
Plymouth, MA 02360
Tel: (508) 830-1110/Fax: (508) 830-1202
www.ocean-coastal.com
Siasconset Coastal Bank Stabilization and Beach Preservation Project September 2010 Alternatives Analysis Page ii
Executive Summary
The coastal bank at the east end of Nantucket Island has experienced continued erosion resulting
in continued decrease in the setback distance between the top of the coastal bank and numerous
homes and public infrastructure on Baxter Road. Ocean and Coastal Consultants, Inc. (OCC) has investigated design alternatives for a coastal bank stabilization and beach preservation project that provides a cost-effective and environmentally sensitive means to prevent storm
damage to these homes and public infrastructure from continued unabated erosion of the coastal
bank while preserving the coastal beach.
The purpose of this alternatives analysis is to evaluate available options and develop a preferred alternative. OCC has prepared this document to provide alternative design options and present the underlying assumptions, parameters and design variables. The scope of work is based on
available background data, including previous coastal engineering studies, site surveys and
sediment sampling. The preferred alternative will be used in the permitting process and the basis
for final design plans for construction.
A three cycle alternatives analysis has been initiated to identify the preferred design to stabilize the coastal bank at the project site while preserving the coastal beach. This document presents
the results of all three cycles. Cycle 1 identifies the potential alternatives to be developed further
in the concept design. Cycle 1 considers three alternatives including the no-action alternative, a
geotextile tube+sand alternative and a marine mattress+sand alternative. For Cycle 2, conceptual
design layouts were created using the geotextile tube+sand and the marine mattress+sand alternatives. The size and location of the new coastal engineering structures will need to co-exist
with the existing conditions to provide the required level of stability to the coastal bank while
promoting a stable functional beach. The concept design is intended to avoid a negative impact
to the adjacent shorelines. Cycle 3 took the concept design selected in Cycle 2 and then
considered design modifications required to upgrade the concept from a 50-year storm design to a design capable of withstanding the 100-year storm event.
Based on the Cycle 2 analysis results presented herein, the recommended design alternative is
the stone-filled marine mattress with gabion toe protection and periodic sand nourishment
(mattress+sand design). The purpose of this design is to stabilize the toe of the coastal bank
from erosion and prevent storm damage to homes associated with a 50-year (i.e. 2% annual probability) design storm event. Differences in the geometry of the geotextile tube+sand
alternative and mattress+sand alternative result in differences in the size of the project footprint
and the amount of sand required for periodic nourishment. The mattress+sand configuration
optimizes these parameters. Additionally, the mattress+sand design has flexibility in both the
dimension (thickness) and installation approach thereby allowing confidence that the final design will result in a system that can survive the design storm conditions. The cost of imported sand
adds to the cost of the geotextile tube+sand design when compared to the mattress+sand. It was
also determined that the construction duration of the mattress+sand alternative would likely be
shorter than the geotextile tubes+sand, and require less specialized skills. This is due to the fact
that four rows of geotextile tube are required, and each row must be filled individually by personnel having experience pumping slurry into geotextile tubes. The mattress+sand structure
can either be pre-fabricated and barged to the site for placement by a crane, or filled on site by
Siasconset Coastal Bank Stabilization and Beach Preservation Project September 2010 Alternatives Analysis Page iii
the contractor. These details will be finalized during the final design phase.
The Cycle 3 analysis takes the recommended mattress+sand concept design selected in Cycle 2
and provides additional details and a more accurate cost estimate. Based on standard coastal design methodology, it was recommended that the 50-year design storm mattress+sand concept
be upgraded to consider the effects of the 100-year design storm event (i.e. 1% annual
probability). Estimated costs for the 100-year storm design versus the 50-year storm design will
only increase marginally, however the enhanced design will provide considerably more
protection against more severe storm conditions.
Siasconset Coastal Bank Stabilization and Beach Preservation Project September 2010 Alternatives Analysis Page iv
Table of Contents
EXECUTIVE SUMMARY ........................................................................................................................................II
TABLE OF CONTENTS ......................................................................................................................................... IV
LIST OF TABLES .................................................................................................................................................... VI
1. PROJECT OVERVIEW ................................................................................................................................... 1
1.1. PLANNING OBJECTIVES ............................................................................................................................... 1
1.2. PLANNING CONSTRAINTS ............................................................................................................................ 1
2. SITE DESCRIPTION ........................................................................................................................................ 2
3. SITE CONDITIONS .......................................................................................................................................... 3
3.1. CLIMATE ..................................................................................................................................................... 3
3.2. WIND........................................................................................................................................................... 3 3.3. WAVES ........................................................................................................................................................ 4
3.4. CURRENTS ................................................................................................................................................... 6 3.5. WATER LEVEL............................................................................................................................................. 7
3.5.1. Tides....................................................................................................................................................... 7
3.5.2. High Tide Line ....................................................................................................................................... 7
3.5.3. Storm Surge ........................................................................................................................................... 8
3.5.4. Relative Sea Level Rise .......................................................................................................................... 8
3.5.5. Scour ...................................................................................................................................................... 9 3.6. GEOLOGY .................................................................................................................................................... 9
3.7. GRAIN SIZE ............................................................................................................................................... 10 3.8. SEDIMENT TRANSPORT .............................................................................................................................. 10
3.9. SHORELINE CHANGE ................................................................................................................................. 10 3.10. COASTAL BANK RETREAT ......................................................................................................................... 12
4. PRELIMINARY DESIGN ALTERNATIVES .............................................................................................. 15
4.1. DESIGN PARAMETERS ............................................................................................................................... 15
4.2. CYCLE 1 - ALTERNATIVES ......................................................................................................................... 16
4.2.1. No-Action Alternative .......................................................................................................................... 16
4.2.2. Alternative #1: Geotextile Tubes+Sand ............................................................................................... 16
4.2.3. Alternative #2: Marine Mattress+Sand ............................................................................................... 19
4.2.4. Cycle 1 Preferred Alternatives............................................................................................................. 23 4.3. CYCLE 2 ANALYSIS ................................................................................................................................... 24
4.3.1. Concept Design-1: Geotextile Tubes and Sand Cover (Tube + Sand)................................................. 24
4.3.2. Concept Design-2a: Marine Mattress with Gabion Toe (Mattress+Sand) .......................................... 28
4.3.3. Cycle 2 Alternatives Matrix ................................................................................................................. 30
5. RECOMMENDED DESIGN .......................................................................................................................... 32
6. CYCLE 3 FINAL DESIGN ANALYSIS ........................................................................................................ 33
6.1. COASTAL ENGINEERING CRITERIA ............................................................................................................ 33
6.2. FINAL CONCEPT DESIGN FOR 100-YEAR STORM ....................................................................................... 33
REFERENCES .......................................................................................................................................................... 38
Siasconset Coastal Bank Stabilization and Beach Preservation Project September 2010 Alternatives Analysis Page v
List of Figures
Figure 1 - Aerial image of Nantucket Island and the project extents with property
numbers and profile lines. ............................................................................................ 2
Figure 2 – Wind Rose for National Data Buoy Center Station NTKM3 - 8449130 -
Nantucket Island, MA. ................................................................................................. 4
Figure 3 – Relative sea level rise trend for Nantucket, MA. ......................................................... 9
Figure 4 – Shoreline data for project area between 1846 and 1994 (MA Office of
Geographic and Environmental Information). ........................................................... 11
Figure 5 – Coastal Bank Retreat at Siasconset has been approximately 3 ft/yr since 1994,
from GIS analysis by Epsilon Associates. ................................................................. 14
Figure 6 – Aerial view of geotextile tubes in Sea Isle City, NJ, 1998. ....................................... 18
Figure 7 – Geotextile tubes at the base of the Ashkelon Cliffs, Israel. ....................................... 18
Figure 8 – Typical configuration of a Tensar Marine Mattress................................................... 19
Figure 9 – Completed example of marine mattresses used for riverbank stabilization. .............. 20
Figure 10 – View of a marine mattress with gabion toe coastal engineering structure permitted and installed at 28 Hinckley Lane, Nantucket (a) during construction,
(b) after construction, (c) following coastal storm erosion, and (d) after annual
nourishment to re-cover the marine mattress and gabion toe structure. .................... 21
Figure 11 – Marine mattresses installed along the shoreline in Cape May, NJ. ........................... 22
Figure 12 – Google Earth image of marine mattresses installed along the shoreline in Cape May, NJ. ..................................................................................................................... 22
Figure 13 – Google Earth image of marine mattresses installed along the shoreline in Cape
May, NJ after a 2006 USACE beachfill. .................................................................... 23
Figure 14 – Schematic cross section of Concept Design 1. .......................................................... 25
Figure 15 – Schematic cross section of Concept Design 2. .......................................................... 28
Figure 16 - Schematic cross section of Final Design Concept for 100-Year Storm Event .......... 35
Figure 17 - Schematic of Return Walls for Final Design Concept for 100-Year Storm
Event. ......................................................................................................................... 36
Siasconset Coastal Bank Stabilization and Beach Preservation Project September 2010 Alternatives Analysis Page vi
List of Tables
Table 1 - Hindcast wave data for onshore waves (0-180°) at WIS Station 73. ............................. 5
Table 2 - Return period for offshore storm conditions. ................................................................. 6
Table 3 - Tidal Datum data for NOAA Station 8449130. ............................................................. 7
Table 4 - High Tide Line for time period of May 2009 to April 2010. ......................................... 8
Table 5 - Still water storm stage elevations (from FEMA 1996 and Epsilon 2006). .................... 8
Table 6 - Grain size summary. ..................................................................................................... 10
Table 7 - Shoreline change data summary. .................................................................................. 12
Table 8 - Coastal Bank Retreat Statistics at Siasconset from 1994 to 2009 (based on a
linear regression analysis from Epsilon Associates). ................................................... 13
Table 9 - Cycle 1 alternatives matrix. .......................................................................................... 23
Table 10 - Preliminary cost estimate for Design Concept 1. ......................................................... 27
Table 11 - Preliminary cost estimate for Design Concept 2. ......................................................... 30
Table 12 - Alternatives Comparison Matrix .................................................................................. 31
Table 13 - Summary of OPC for Final Design Concept for 100-Year Storm Event. .................... 37
Siasconset Coastal Bank Stabilization and Beach Preservation Project September 2010 Alternatives Analysis Page 1
ALTERNATIVES ANALYSIS
1. Project Overview
The coastal bank at the east end of Nantucket Island has experienced continued erosion resulting
in minimal setback from the top of the coastal bank for numerous homes and public
infrastructure on Baxter Road. The Siasconset Beach Preservation Fund (SBPF) has retained
Ocean and Coastal Consultants, Inc. (OCC) to investigate design alternatives for a coastal bank stabilization project that would preserve the coastal beach while providing a cost-effective and environmentally sensitive means to prevent storm damage to these residential buildings (homes)
from continued unabated erosion of the coastal bank.
The purpose of this alternatives analysis is to evaluate available options and develop a preferred
alternative. OCC has prepared this document to provide alternative design options and present
the underlying assumptions, parameters and design variables. The scope of work is based on available background data including previous coastal engineering studies, site surveys and
sediment sampling. The preferred alternative will be used in the permitting process and the basis
for final design plans for construction.
1.1. Planning Objectives
OCC has investigated a number of approaches to stabilize the eroding coastal bank within the
project area while also preserving the adjacent coastal beach. Additional factors to consider are
the safety of homeowners, initial and maintenance costs, and potential environmental impacts.
For example, if a proposed alternative is likely to create a situation hazardous to the safety of
homeowners or is likely to have high annual maintenance costs, that alternative will not be recommended. SBPF has previously investigated numerous alternatives to prevent or minimize
the continued erosion of the coastal bank. Some of the alternatives that have been investigated
including groins, seawalls, beach nourishment and other coastal engineering approaches were
found to be not acceptable, while others were attempted and were not acceptable as they did not
provide the desired storm damage prevention. This alternatives analysis investigates two available options to meet the project goals without compromising the stated planning objectives.
1.2. Planning Constraints
The purpose of the proposed project is to provide an engineered solution to stabilize the toe of
the coastal bank within the project area, thus preventing storm damage associated with up to a 50-year (2% annual probability) design storm event for Cycle 2; and a 100-year (1% annual
probability) design storm event for Cycle 3, while preserving the coastal beach. The proposed
solution should be designed to conform to the regulatory standards of the Nantucket
Conservation Commission, the Massachusetts Department of Environmental Protection and the
US Army Corps of Engineers. An additional project constraint that will be addressed throughout the design process is the regulatory limitation that coastal engineering structures are not
permissible except to protect pre-1978 constructed residential buildings that have not been
substantially improved after August 1978.
Siasconset Coastal Bank Stabilization and Beach Preservation Project September 2010 Alternatives Analysis Page 2
2. Site Description
The project site is located on the eastern shore of Nantucket Island, Massachusetts. The
proposed project is for coastal bank stabilization along an east facing portion of coast adjacent to
fourteen (14) privately owned residential properties. Currently, the project plan is to stabilize the bottom of the coastal bank using either a geotextile tube alternative or marine mattresses with gabion toe protection alternative. The coastal area to be protected is bound on the north by Lot
#49-34 and on the south by Lot #49-18 and is approximately 1,700 linear feet in length. There
are a total of 14 lots in the project area.
Figure 1 - Aerial image of Nantucket Island and the project extents with property numbers and profile lines.
Siasconset Coastal Bank Stabilization and Beach Preservation Project September 2010 Alternatives Analysis Page 3
3. Site Conditions
The local conditions at the project site are unique and critical in determining the best alternative
to achieve project goals. Through the analysis of available data and publications, historical
trends can be identified and used as a basis for future predictions.
3.1. Climate
Nantucket Island is located in what is called a humid continental climate, which is a climate
found over large areas of landmasses in the temperate regions of the mid-latitudes where there is
a zone of conflict between polar and tropical air masses. The humid continental climate is
marked by variable weather patterns and a large seasonal temperature variance. Summers are often warm and humid with frequent thunderstorms and winters can be very cold with frequent
snowfall and persistent snow cover.
Nantucket, as is the entire coast of Massachusetts, is prone to nor'easters and to severe winter
storms. Summers can bring thunderstorms, averaging around 30 days of thunderstorm activity
per year. Nantucket Island, like the entire United States eastern seaboard, is vulnerable to hurricanes. Because its location is farther east in the Atlantic Ocean than states farther south, Massachusetts has suffered a direct hit from a major hurricane only three times since 1851.
More often, hurricanes weaken to tropical storms as they pass near Massachusetts.
Because of the influence of the Atlantic Ocean, temperatures are typically a few degrees cooler
in the summer and a few degrees warmer in the winter. A common misconception is that the climate is influenced largely by the warm Gulf Stream current, however that current turns eastward off the coast of Virginia and the local waters are more influenced by the cold Canadian
Labrador Current. As a result, the ocean temperature rarely gets above 65 °F (18 °C).
Nantucket's climate is also known for a delayed spring season, being surrounded by an ocean
which is still cold from the winter. However, it is also known for an exceptionally mild fall season due to the ocean remaining warm from the summer.
3.2. Wind
Wind data for the project site was obtained from the National Data Buoy Center Station NTKM3
- 8449130 - Nantucket Island, MA. Hourly data between January 1, 2009 and December 31, 2009 were analyzed. The results of the analysis are presented in the wind rose below. The primary wind directions and frequencies are northeast, 25%; northwest, 16%; southwest, 36%;
southeast, 24%. The majority (75%) of winds are between 1 and 11 mph and originate from the
southwest. Wind speeds greater than 7 mph generally originate from the southwest and, to a
lesser extent, the northwest and northeast.
For determining wind loads, OCC used wind data provided in ASCE Standard ASCE/SEI 7-05
Minimum Design Loads for Buildings and Other Structures. For Nantucket Island, the one-hour
duration design wind speeds for the 50-year and 100-year storm events are 79 mph (i.e. 120 mph,
3-second gust) and 85 mph (i.e. 128 mph, 3-second gust), respectively.
Siasconset Coastal Bank Stabilization and Beach Preservation Project September 2010 Alternatives Analysis Page 4
Figure 2 – Wind Rose for National Data Buoy Center Station NTKM3 - 8449130 - Nantucket Island, MA.
3.3. Waves
The project site is located along the open coast of the Atlantic Ocean and is exposed to varying daily swell as well as extreme storm waves caused by hurricanes and Nor'easters. In this
location waves are the main component of sediment transport. Typical wave data is used to
predict long term shoreline response. Extreme wave data is used to predict extreme erosion
events and for developing design parameters for coastal structures.
Wave data for the project area is based on USACE WIS data from WIS Station 73, located approximately 15 miles east of the project site (41.25 N, 69.67 W). The station is located in a
water depth of 89 feet. The average wave height is 4.2 feet with a period of 5.1 seconds and a
Siasconset Coastal Bank Stabilization and Beach Preservation Project September 2010 Alternatives Analysis Page 5
direction of 208° (SSE). Approximately half of the waves propagate from the offshore direction.
Of this portion of wave data, the average wave height is 4.4 feet with a period of 5.5 seconds.
Other information pertaining to waves:
The largest onshore waves under typical conditions occur in December and January with
average wave heights of 6.3 feet and maximum wave heights of 24.3 and 26.2 feet,
respectively.
The lowest onshore waves occur in July and August with average wave heights of 2.5 and 2.8 feet, respectively, and maximum wave heights of 15.4 feet.
The largest and longest waves under typical conditions come from the northeast direction.
The 50-year storm event has a maximum significant wave height of 27.7 feet and a period
of 15.2 seconds. The 100-year storm event has a maximum significant wave height of 28.8 feet and a period of 15.2 seconds.
Table 1 - Hindcast wave data for onshore waves (0-180°) at WIS Station 73.
Period Tp Dir
(sec) (deg.)
mean max
January 6.3 26.2 6.1 93.0
February 6.1 24.3 6.2 84.0
March 5.5 24.6 5.9 95.0
April 4.3 21.0 5.4 110.0
May 3.4 15.7 5.0 119.0
June 3.0 11.2 4.7 126.0
July 2.5 15.4 4.7 145.0
August 2.8 15.4 4.9 128.0
September 3.3 21.3 5.5 115.0
October 4.3 25.3 5.3 94.0
November 5.5 20.0 5.7 93.0
December 6.3 24.3 6.0 75.0
Mean 4.4 20.4 5.5 107.0
Height (Hmo)
(feet)Month
Siasconset Coastal Bank Stabilization and Beach Preservation Project September 2010 Alternatives Analysis Page 6
Table 2 - Return period for offshore storm conditions.
Height (Hmo) Period Tp
(feet) (sec)
1 18.8 11.1
2 21.4 12.3
3 22.4 12.8
4 23.1 13.1
5 23.5 13.3
6 23.9 13.5
7 24.2 13.6
8 24.5 13.7
9 24.7 13.8
10 24.9 13.9
15 25.7 14.3
20 26.2 14.5
25 26.6 14.7
30 26.9 14.8
35 27.1 14.9
40 27.4 15.0
45 27.5 15.1
50 27.7 15.2
100 28.8*15.2*
Return
Period
(years)
* Based on Interpolated Values
3.4. Currents
Currents along the beach within the project site are generated by a combination of tidal, wave, and wind forcing. The direction and magnitude of the currents will vary based on changing
weather patterns.
Data for water level and current velocity measurements were obtained from the document
entitled Final Environmental Impact Report, Sconset Beach Nourishment Project, Nantucket, MA and dated November 30, 2006. The above referenced report summarizes data collected by the Woods Hole Group between October and November, 2005. The report states:
Sontek current meters and RDI ADCP's were used to measure waves and currents.
The ADCP was located in a water depth of approximately 24 feet.
During flood tides, the (positive) current direction is from south to north (~360°) and that during the ebb tide, the (negative) current direction is from north to south (175°).
Tides propagate from the south.
Siasconset Coastal Bank Stabilization and Beach Preservation Project September 2010 Alternatives Analysis Page 7
Two peaks in the current speed occurred every tidal cycle, one during the flood and one
during the ebb. Typical current speeds during the peak flood ranged from 2.5 to 3.0 feet per second. Typical current speeds during peak ebb ranged from 2.5 to 4.3 feet per second.
The ebb-dominated current speeds were higher during spring tide and lower during neap
tide.
There was no direct correlation between measured current velocities and observed waves.
3.5. Water Level
3.5.1. Tides
Tidal datum information for the project area was gathered from NOAA Station 8449130, located on Nantucket Island, Nantucket Sound, Massachusetts. This area is subject to mixed semi-
diurnal tides. Information for this station is in the table below.
Table 3 - Tidal Datum data for NOAA Station 8449130.
Datum Value Description
MHHW 3.37 Mean Higher High Water
MHW 3.04 Mean High Water
MSL 1.57 Mean Sea Level
MTL 1.52 Mean Tide Level
MLW 0 Mean Low Water
MLLW -0.2 Mean Lower Low Water
MN 3.03 Mean Range of Tide
Maximum 7.67 Highest Water Level on Station Datum
Max Date 10/30/1991 Date of Highest Water Level
Max Time 17:30 Time of Highest Water Level
Minimum -2.34 Lowest Water Level on Station Datum
Min Date 2/12/1981 Date of Lowest Water Level
Min Time 12:30 Time of Lowest Water Level
3.5.2. High Tide Line
The high tide line is defined as the highest high tide of each month for the previous 12 months.
The High Tide Line for the period of May 2009 to April 2010 is +4.97 feet MLW. The tidal station used to create this table was 8449130, Nantucket Island, MA.
Siasconset Coastal Bank Stabilization and Beach Preservation Project September 2010 Alternatives Analysis Page 8
Table 4 - High Tide Line for time period of May 2009 to April 2010.
Highest High
(MLW)
May-09 4.31
Jun-09 5.44
Jul-09 4.69
Aug-09 4.49
Sep-09 4.52
Oct-09 5.43
Nov-09 4.7
Dec-09 4.95
Jan-10 5.67
Feb-10 5.24
Mar-10 5.26
Apr-10 4.95
HTL: 4.97
Month
3.5.3. Storm Surge
Storm surge elevations were obtained from the FEMA Flood Insurance Study for Nantucket
(FEMA, 1996). Elevations in the table below are referenced to the Half Tide Line (HTL) and
MLW. For the eastern side of Nantucket Island, HTL was defined as +3.0 feet MLW by FEMA
in 1996. In the previous section, our analysis indicates that the High Tide Line is +4.97 feet MLW. This discrepancy is due to different definitions and uses for the term HTL and does not
affect the predicted storm surge level.
Table 5 - Still water storm stage elevations (from FEMA 1996 and Epsilon 2006).
10-year 4.8 7.8
50-year 6.2 9.2
100-year 7.2 10.2
500-year 9.5 12.5
Storm
Event
FEMA Elevations (feet
above HTL)MLW
3.5.4. Relative Sea Level Rise
Relative sea level rise data was obtained via the NOAA Tides and Currents website. Relative
sea level rise (RSLS) is the apparent change in sea level compared to a fixed vertical datum.
RSLS consists of two independent components, eustatic sea level change (the global change in
ocean water level compared to a fixed vertical datum) and subsidence (the local change in land
elevation compared to a fixed vertical datum). The mean sea level trend is 2.95 millimeters/year
Siasconset Coastal Bank Stabilization and Beach Preservation Project September 2010 Alternatives Analysis Page 9
with a 95% confidence interval of +/- 0.46 mm/yr based on monthly mean sea level data from
1965 to 2006 which is equivalent to a change of 0.97 feet in 100 years.
Figure 3 – Relative sea level rise trend for Nantucket, MA.
The plot shows the monthly mean sea level. The long-term linear trend is also shown, including its 95% confidence interval. The plotted values are relative to the most recent Mean Sea Level datum established by CO-OPS.
3.5.5. Scour
Waves impacting the beach and coastal bank during a storm can cause scour, particularly at the
toe of a coastal engineering structure. Therefore, toe protection is required. Toe protection is especially important if the seaward end of the coastal engineering structure is in water depth less
than 1.5 times the incident wave height. Waves at the coastal engineering structure will be
limited by the depth at the structure, which is determined by combination of storm surge,
additional setup due to the breaking wave and the depth at the structure’s toe. The scour design
goal is to determine the potential scour depth and width, and design the structure's toe accordingly. As a rule of thumb, the scour depth can be estimated as equal to the incident wave
height, while the apron length can be up to 3.5 times the incident wave height.
3.6. Geology
Nantucket was formed by the outermost reach of the Laurentide Ice Sheet during the recent Wisconsin Glaciation, and shaped by the subsequent rise in sea level. The island's low ridge
across the northern section was deposited as glacial moraine during a period of glacial standstill,
a period during which till continued to arrive, but melted at a stationary front. The southern part
of the island is an outwash plain, sloping away from the arc of moraine and shaped at its margins
Siasconset Coastal Bank Stabilization and Beach Preservation Project September 2010 Alternatives Analysis Page 10
by the sorting actions and transport of alongshore drift. Nantucket became an island when rising
sea levels re-flooded Nantucket Sound about 5,000–6,000 years ago.
3.7. Grain Size
A thorough sediment sampling effort was undertaken for an area on Nantucket approximately 3
miles north of the project site. Full results can be found in Final Environmental Impact Report,
Sconset Beach Nourishment Project, Nantucket, MA (Epsilon, 2006). A total of 248 samples
were collected at various cross-shore positions along each transect (including the coastal bank, coastal dune, beach, wave breaking zone and the offshore area). The composite mean grain size
for all 248 samples (weighted by elevation) is 0.86 mm with an average silt content of 2.8%. A
summary of grain size results for the coastal bank, coastal dune and coastal beach are presented
in the table below.
Table 6 - Grain size summary.
mean % silt mean % silt mean % silt
Min.0.31 2.9 0.6 0.1 0.6 0.2
Max.0.51 28.9 0.8 1.3 0.9 0.4
Mean 0.4 13.6 0.7 0.3 0.7 0.2
Coastal Bank Coastal Dune Coastal Beach
3.8. Sediment Transport
Alongshore (or littoral) sediment transport is the movement of sand parallel to the coast
primarily driven by waves that obliquely approach the shoreline. The amount of sand transport
is a function of the size of the waves and the increasing angle that the waves make with the beach as they come in to break. This angle means that the water runs up the beach following the
angle offset from perpendicular until the up-rush stops. Gravity then pulls the back swash
directly down the beach slope so that this repetitive saw tooth pattern of water movement slowly
moves the sand on the beach in the direction that the waves make the open angle to the beach.
Net sediment transport rates determine the balance of sediment loss or gain within a system over a defined period of time. Net sediment transport for the specific project area within Sconset is
from south to north. Since the project area is located at a nodal zone where the net transport
shifts, transport at any particular time can be in either direction. Net sediment transport may
vary within a given system or timeframe.
3.9. Shoreline Change
Shoreline change has been investigated from both a long term and short term perspective. Long
term shoreline data was available from the Massachusetts Office of Geographic and
Siasconset Coastal Bank Stabilization and Beach Preservation Project September 2010 Alternatives Analysis Page 11
Environmental Information as a GIS layer. While the accuracy of this data layer is unknown,
from the time period of 1846 to 1955, the shoreline advanced approximately 300 feet
(approximately 2.75 feet per year). The current trend of erosion is evident in the shoreline data from 1955 to 1994. During this time period, the shoreline retreated at a rate of approximately 4.1
feet per year (see Figure 4).
Figure 4 – Shoreline data for project area between 1846 and 1994 (MA Office of Geographic and Environmental
Information).
Siasconset Coastal Bank Stabilization and Beach Preservation Project September 2010 Alternatives Analysis Page 12
More recent erosion rates were determined from a review of the Lighthouse Beach Dewatering
Project, October 2009, 54th Quarterly Report with Comparisons to Baseline Erosion Rates
prepared by the Woods Hole Group. Surveys have been performed within the project area quarterly since 1994. The report compared shoreline changes for three timeframes including
November 1994 to October 2009, December 2001 to October 2009 and April 2009 to October
2009. For the time periods between November 1994 to October 2009 and December 2001 to
October 2009, all profiles within the project area eroded at an average annual shoreline change
rate of 8.2 feet per year and 3.0 feet per year, respectively. During the short 6 month period from April to October 2009, all profiles accreted with an average rate of 6.44 feet per year. This could
be attributed to the large seasonal fluctuations of sediment during the summer months when
beaches in the project area tend to accrete. A summary of the shoreline change data is presented
in the table below.
Table 7 - Shoreline change data summary.
Nov-94 to
Oct 09
Shoreline
Change per
Year
Dec-01 to
Oct-09
Shoreline
Change
per Year
Apr-09 to
Oct-09
Shoreline
Change
per Year
89.2 -124.98 -8.33 -26.68 -3.34 0.35 0.7
89.5 -124.22 -8.28 -24.82 -3.10 0.03 0.06
89.8 -128.01 -8.53 -20.91 -2.61 4.37 8.74
90 -128.25 -8.55 -20.45 -2.56 6.64 13.28
90.6 -109.92 -7.33 -28.02 -3.50 4.72 9.44
Avg (per
year)-123.08 -8.21 -24.18 -3.02 3.22 6.444
Shoreline Change
(feet)Profile
The coastal beach at the base of the coastal bank within the project site is subject to large
seasonal changes in berm elevation. OCC reviewed cross sections of surveys from 1994 to 2009
from the above referenced Woods Hole Group report in order to estimate the lowest expected
beach berm elevation and determine the ideal bottom elevation of the bank stabilization structure. Based on our review of the data, the lowest expected elevation, accounting for long
term erosion and accretion as well as seasonal variations, is approximately +8.0 feet MLW. In
addition to this seasonal fluctuation, OCC estimated the potential for up to 3.5 feet of scour
during a 50-year storm event, and up to 8.0 feet of scour for a 100-year storm event.
3.10. Coastal Bank Retreat
It is important to understand that the coastal processes influencing the shoreline are different than
those affecting the coastal bank. Beach erosion is primarily driven by waves and tidal currents
within the surf zone. Bank erosion, however, is caused by slope instability. In the case of
Siasconset Coastal Bank Stabilization and Beach Preservation Project September 2010 Alternatives Analysis Page 13
Siasconset, this slope instability is caused by wave run-up during storms, which erodes the bank
toe. Because the coastal erosion processes are different, the beach and the bank do not recede at
the same rate.
The average annual retreat of the coastal bank is required to estimate the volume of sediment that
will no longer be available to the coastal system as a result of the proposed bank stabilization.
This volume is the most appropriate measure required to determine the amount of sand required
to nourish the beach. This approach is also the method which has previously been accepted by
the Massachusetts DEP and other agencies on other projects to determine the requirements for sand nourishment.
Epsilon Associates has performed an analysis of the coastal bank retreat along the Siasconset
project area utilizing aerial photographs taken over the last 15 years; specifically 1994, 2003, and
2009. The photographs were obtained from the Massachusetts Office of Geographic and
Environmental Information (i.e. MassGIS), and are geo-referenced to permit direct comparison of visible changes over time. Figure 5 provides a view of the 2009 aerial photograph, overlaid with the top of bank lines from 1994, 2003, and 2009. As summarized in Table 8, Epsilon's
linear regression analysis of GIS transects at 10-foot spacing along the study area indicates that
the coastal bank has retreated approximately 3 feet/year on average over the last 15 years.
Assuming an average bank height of 68 feet, the annual volumetric loss of the bank per linear foot is approximately (68 ft) x (3 ft) x (1 cy/ 27 ft) = 7.6 cubic yards. Accounting for the fact that approximately 13% of the sediment in the bank is fines, the annual volumetric loss should be
reduced to 6.6 cubic yards per linear foot.
Therefore, while the 1,700 linear foot beach system is losing a yearly average of 7,333 cubic
yards of material, the bank is supplying approximately 11,200 cubic yards. This implies that the material in the bank is indeed supplying sand to not only the beaches in the project area, but to down drift beaches as well.
In order for the bank and beach system to remain relatively stable in the project area, and to
replace the amount of sand that would ordinarily be contributed by the bank through erosion 6.6
cubic yards per year per linear foot of project length should be used as a nourishment target. This assumes that no material is supplied to the beach from the bank after stabilization of the entire 1,700 linear foot project length.
Table 8 - Coastal Bank Retreat Statistics at Siasconset from 1994 to 2009 (based on a linear regression analysis from Epsilon Associates).
TOP OF COASTAL BANK
RECESSION RATE (FT/YR)
MAXIMUM -5.05
MINIMUM -0.03
AVERAGE -2.96
Siasconset Coastal Bank Stabilization and Beach Preservation Project September 2010 Alternatives Analysis Page 14
Figure 5 –Coastal Bank Retreat at Siasconset has been approximately 3 ft/yr since 1994, from GIS analysis by Epsilon Associates.
Siasconset Coastal Bank Stabilization and Beach Preservation Project September 2010 Alternatives Analysis Page 15
4. Preliminary Design Alternatives
A three cycle alternatives analysis can be used to identify the preferred design to stabilize the
coastal bank at the project site. This document presents the results of Cycles 1 and 2. Cycle 1
identifies potential alternatives to be used in the concept design. The project team evaluated each alternative individually based on its ability to work within the project specific coastal system and to meet the project's design parameters. Concepts that meet most of these criteria
were then evaluated in the Cycle 2 analysis.
In Cycle 2, concept design layouts were developed using multiple combinations of the selected
Cycle 1 alternatives. These layouts were evaluated by the team with more detail using engineering and scientific judgment and experience to identify a preferred conceptual design.
In Cycle 3, the performance of the preferred conceptual design will be evaluated in more detail
and modified as necessary. The design that has the best performance and meets the project's
design parameters was selected as the preferred design layout for future evaluation in Cycle 3.
Final design details of the structures will be determined later to create a final design plan.
4.1. Design Parameters
The following is a list of the design parameters identified by OCC for the Siasconset Coastal
Bank Stabilization and Beach Preservation Project:
Project Objectives:
Protect residential buildings from coastal bank erosion and storm damage.
Provide a cost-effective, "softer" coastal engineering structure to protect the toe of the
coastal bank from further erosion due to wave attack from the 50-year design storm (2%
annual probability) conditions, or back-to-back 25 year events.
Maintain an appropriate supply of sand to the coastal beach and nearshore environment based on an application of historical coastal bank retreat rates.
Project Constraints:
Permanent or "hard" coastal engineering structures (i.e. seawalls, bulkheads or revetments) have been previously evaluated and have been found to be unacceptable.
Proposed alternatives must be the “Best Available Measures” defined at 310 CMR 10.04 as
“the most up-to-date technology or the best designs, measures or engineering practices that
have been developed and that are commercially available.”.
Proposed alternatives should meet the current performance standards of the Town of Nantucket Conservation Commission Wetlands Protection Regulations.
Coastal Engineering Criteria:
Top of Coastal Bank Stabilization = Elev. +23.5 feet MLW (accounts for wave run-up during 50-yr storm conditions)
Siasconset Coastal Bank Stabilization and Beach Preservation Project September 2010 Alternatives Analysis Page 16
50-Year Design Storm Still Water Level (SWL) = Elev. +9.2 feet MLW
Toe of Coastal Bank = Elev. +8.0 feet MLW
High Tide Line (HTL) = Elev. +4.97 feet MLW
Bottom of Coastal Bank Stabilization = Elev. +4.5 feet MLW
4.2. Cycle 1 - Alternatives
Three alternatives, including a no-action alternative, were identified and evaluated. The evaluation criteria focused on the ability to satisfy the project goals, constructability, required
maintenance, and relative costs. Each alternative was evaluated using available documentation,
engineering judgment, and previous experience.
4.2.1. No-Action Alternative
Under the no-action alternative, the project area would remain "as is." Without action it is
expected that the coastal bank will continue to erode. Continued erosion would further endanger
homes and public infrastructure.
With no action taken, this alternative does not require construction nor have a direct cost
associated with the project. However there may be indirect costs associated with this alternative when coastal storms cause further erosion and ultimately damage the landward building and
infrastructure. This alternative does not meet the project objective of protecting landward
buildings or infrastructure.
An advantage of this option would be that there would not be any immediate costs for construction. The disadvantage of this option is that the coastal bank will continue its erosional trend and result in storm damage losses of homes in the very near future. The protective function
of the coastal bank is likely to deteriorate further and create an inherent storm damage risk to the
residential buildings and infrastructure. Based on our evaluation, the no-action alternative was
not considered for further analysis in Cycle 2.
4.2.2. Alternative #1: Geotextile Tubes+Sand
Coastal engineering structures can be built in the coastal environment using a variety of
materials, typically stone, concrete or timber. A somewhat “softer” and relatively recent
innovation is the use of sand-filled geotextile tubes; OCC has extensive experience using
geotextiles in the coastal environment. High strength woven geotextiles are sewn into an empty tube and deployed. The tube is then hydraulically filled with sand to form a dense, elliptical
shaped structure. As flexible, gravity structures, geotextile tubes have some advantages over
traditional structures. Tubes have some inherent flexibility to accommodate foundation changes,
however if the subsurface changes too much due to scour, the tube can become unstable and can
fail.
Geotextile tubes are fabricated from high strength, woven polyester or polypropylene. Sheets of the geotextile are sewn together in the factory with fill sleeves (or ports) on top. Fill port spacing
is somewhat dependant on the grain size of the fill material. Since tube fabrics are woven in 15
foot wide sheets, typical tube sizes are 15, 30 and 45 foot circumference. Tubes are most stable
Siasconset Coastal Bank Stabilization and Beach Preservation Project September 2010 Alternatives Analysis Page 17
when the height to width ratio is 0.5 or less. This also corresponds to the natural shape the tube
assumes when properly filled. A 30 foot circumference tube will be filled to its optimum shape
when the width is 12 feet and the height is 6 feet.
Polypropylene, usually black, is the less expensive of the fabric types, but has a lower tensile
strength (400 lb/in. x 600 lb/in.) and can suffer from creep elongation during filling. Stress
during filling and the fact that the fabric is inherently weaker means that the filled tube will have
a lower factor of safety. Polyester, usually white, is more expensive but considerably stronger
(1,000 lb/in. x 1,000 lb/in.) and exhibits much less elongation. This means that seams are stronger and the resulting installed tube has a high factor of safety.
Both fabrics are susceptible to UV degradation and debris damage. For this reason and
aesthetics, an armor layer is often recommended in coastal applications. The best armor layer
fabric is a durable, woven vinyl-coated polyester sewn to the tube in the factory. Armor layers
can match the color of the sand and offer protection to the main tube.
Geotextile tubes, like any coastal engineering structure, need toe protection. This is often provided with a polypropylene scour apron. The scour apron protects the tube’s foundation to
prevent excessive movement. Scour can occur due to wave impact and currents.
Similar Installations of Geotextile Tubes
Geotextile tubes have been used extensively for shoreline protection projects. One example was the installation of three, 30-foot circumference tubes along an eroding stretch of beach in Sea Isle City, New Jersey in December of 1997 to protect an 18-unit apartment complex. The
construction sequence included a one foot deep trench onto which a scour apron was placed,
filling the tubes to a height of 5.5 to 6.0 feet (with a width of 12-13 feet) and covering with a one
to two foot layer of sand cover. The resulting dune was approximately 8 feet high. Three months after the installation, the project area was hit with a Nor'easter and Cape May County was declared a national disaster area. The condominium complex protected by the geotextile
tubes was not damaged.
Siasconset Coastal Bank Stabilization and Beach Preservation Project September 2010 Alternatives Analysis Page 18
Figure 6 – Aerial view of geotextile tubes in Sea Isle City, NJ, 1998.
Another example of geotextile tubes used for coastal protection is the installation of 40-foot
circumference tubes to protect the toe of a coastal bluff at Ashkelon Cliffs, Israel subject to the
full force of storm generated waves in the Mediterranean Sea. The tubes were installed at the
base of sand bluffs approximately 40 to 50 feet tall to protect upland infrastructure. An image of the installed tubes is below in Figure 7 .
Figure 7 –Geotextile tubes at the base of the Ashkelon Cliffs, Israel.
Siasconset Coastal Bank Stabilization and Beach Preservation Project September 2010 Alternatives Analysis Page 19
4.2.3. Alternative #2: Marine Mattress+Sand
The marine mattress+sand alternative consists of rock-filled containers constructed of high-
strength geogrid. Figure 8 illustrates a marine mattress structure called the “Triton Marine Mattress System” which was developed by the Tensar Corporation, manufacturer of the geogrid
panels used to form the marine mattress. These mattresses are approximately 6.5 feet wide and
are available in various lengths and thicknesses.
Similar Installations of Stone-filled Marine Mattress
Both marine mattresses and gabions are used to create sloped protective structures along the coast or waterways. The coastal engineering structure being considered for the Siasconset
project is designed to protect the lower portion of the coastal bank from erosion due to direct
wave attack. This alternative uses gabion baskets at the toe of the coastal bank to stabilize the
coastal bank by preventing scour while the marine mattress constructed on the slope of the
coastal bank stabilizes the coastal bank landform by preventing storm damage erosion due to breaking waves and run up.
Figure 8 – Typical configuration of a Tensar Marine Mattress.
Siasconset Coastal Bank Stabilization and Beach Preservation Project September 2010 Alternatives Analysis Page 20
Figure 9 – Completed example of marine mattresses used for riverbank stabilization.
Similar coastal engineering structures have been installed in various locations around the world.
One successful structure is located on the north shore of Nantucket Island at 28 Hinckley Lane,
which was installed in 2005 and is performing very well. This design consisted of a row of 18 foot long buried marine mattresses placed along the toe of the coastal bank to prevent erosion
from wave action. A vegetated dune was constructed over the marine mattresses to preserve the
natural look of the property. This installation has been tested by several coastal storms. As
shown in Figure 10, the storm waves erode the dune, which acts as a sacrificial sediment source
to nourish the beach. The marine mattresses prevent the erosion from progressing any further, which would undermine the toe of the bank and cause subsequent slope failure of the upper
bank. At the end of each winter season, the sacrificial dune is reconstructed over the marine
mattresses.
Siasconset Coastal Bank Stabilization and Beach Preservation Project September 2010 Alternatives Analysis Page 21
Figure 10 – View of a marine mattress with gabion toe coastal engineering structure permitted and installed at 28
Hinckley Lane, Nantucket (a) during construction, (b) after construction, (c) following coastal storm erosion, and (d)
after annual nourishment to re-cover the marine mattress and gabion toe structure.
A 450 linear foot marine mattress in a similar wave environment is located in Cape May, NJ (see Figure 11). This marine mattress was installed in 1995 by the New Jersey Department of
Environmental Protection. The 50-year design wave height for Cape May, NJ is listed as 22.6
feet by the USACE (1997). The toe of the structure is covered by 2-4 ton stone for scour
protection in contrast to the gabion baskets at Hinckley Lane. Figure 11 shows three remnant
concrete footers of a structure lost to erosion in the foreground. Also shown is the end of the structure which has been undermined due to flanking. This underscores the importance of
designing returns in a marine mattress structure.
Siasconset Coastal Bank Stabilization and Beach Preservation Project September 2010 Alternatives Analysis Page 22
Figure 11 – Marine mattresses installed along the shoreline in Cape May, NJ.
The marine mattress at Cape May has been in place for 15 years. Figure 12 and Figure 13 are
Google Earth images showing the exposed condition of the structure in 2002 and post USACE
beachfill. The storm season during the winter of 1997 and 1998 was particularly energetic,
resulting in a Presidential Disaster Declaration for New Jersey's coastal counties.
Figure 12 – Google Earth image of marine mattresses installed along the shoreline in Cape May, NJ.
Siasconset Coastal Bank Stabilization and Beach Preservation Project September 2010 Alternatives Analysis Page 23
Figure 13 – Google Earth image of marine mattresses installed along the shoreline in Cape May, NJ after a 2006 USACE beachfill.
4.2.4. Cycle 1 Preferred Alternatives
A summary of the available alternatives considered at the project location are shown in the table
below.
Table 9 - Cycle 1 alternatives matrix.
Satisfy Design
Parameters
Ease of
Construction*
Required
Maintenance*Cost*Considered for
Cycle 2
1 No Action no N/A high **none no
2 Geotextile Tubes Yes moderate moderate high Yes
3
Marine Mattress with
Gabion Toe Yes simple less
moderate
to high Yes
* - relative to other alternatives
** - damage to upland infrastructure expected to be high
Alternative
Siasconset Coastal Bank Stabilization and Beach Preservation Project September 2010 Alternatives Analysis Page 24
4.3. Cycle 2 Analysis
Conceptual design layouts were created using the geotextile tube alternative (Tube+Sand) and
the marine mattress with gabion toe alternative (Mattress+Sand). The size and location of the new coastal engineering structures were analyzed on their ability to provide the required level of
stabilization of the coastal bank while preserving the coastal beach. Additionally, using “best
available measures,” the concept design must minimize adverse effects on the adjacent coastal
beaches.1 The two alternatives analyzed during this cycle (the Tube+Sand and Mattress+Sand
alternatives) are commonly used, well established techniques for providing this type and level of toe protection. Examples of previous applications of both of these design alternatives are
described in section 4.2.2 and 4.2.3. Each layout was evaluated using engineering and scientific
judgment and experience with an overall project goal of stabilizing the coastal bank in
accordance with the design parameters.
4.3.1. Concept Design-1: Geotextile Tubes and Sand Cover (Tube+Sand)
Design
Concept Design 1 consists of four, 30-foot circumference geotextiles tubes installed in a terraced
alignment, with clean sand fill cover. Construction requires excavating the existing profile to +4.5 feet MLW and installing a 3-foot circumference anchor tube and scour apron. Tubes will
then be installed and filled on the excavated terraces to approximately 5 feet tall and 11 feet
wide. After tubes have been filled, a clean sand fill will be placed to a top elevation of
approximately +23.5 feet MLW. The sand fill will be placed on a 1V:2.5H slope to meet
existing grade while maintaining a continuous one foot thick sand cover over the filled tubes. A schematic cross section is presented below in Figure 14.
Toe protection is accomplished in this option by use of a scour apron and anchor tube. The
bottom tube is installed with a base elevation of +4.5 feet MLW. This elevation is determined to
be adequate for scour resulting from a 50-year storm. The scour apron extends for ten (10) feet
seaward and ends with a three (3) foot circumference sand tube anchor. Where the seabed is subject to further erosion or scour at seaside edge of the revetment, the extended scour apron will
dynamically adjust by lowering down; under the weight of the anchor tube weight; and will
protect against further erosion and scour stabilization will ultimately be achieved. This will
account for the back-to-back 25-year storm scenario.
1 See Coastal Bank performance standard at 310 CMR 10.30 (3)(a). “Best Available Measures” is defined at 310 CMR 10.04 as “the most up-to-date technology or the best designs, measures or engineering practices that have been
developed and that are commercially available.”
Siasconset Coastal Bank Stabilization and Beach Preservation Project September 2010 Alternatives Analysis Page 25
Figure 14 – Schematic cross section of Concept Design 1.
Maintenance
Maintenance of the Tube+Sand system consists of replacing the sand covering once per year. In
the event of a storm, the system may become uncovered. An inspection of the uncovered tubes should be undertaken when visible to check for holes or any obvious signs of movement or
settling. The project is being designed such that major damage is not expected except for major
storm events or combinations of storms having exceptional duration. During a severe storm
event, the geotextile tube with scour apron and anchor tube may experience complete or partial
loss of sand covering due to wave attack and scour at the toe of the structure. This is expected and will not compromise a properly maintained system. The system was designed with a scour
apron and anchor tube to prevent undermining and destabilizing of the main geotextile tube's
foundation. The scour apron prevents the system from scouring between the main geotextile
tube and the anchor tube. Any depression that develops due to scour beneath and seaward of the
free end of the anchor tube will cause the anchor tube to adjust downward into it. This adjustment will therefore hinder the progression of future scour that could potentially undermine
and destabilize the main geotextile tube.
In an event where the geotextile tube exposed and is damaged, the damaged section should be
repaired. Holes smaller than 3 inches in diameter can be closed by the use of cable ties if there is
slack in the geotextile, a piece of nonwoven geotextile inserted in the hole, or the "sock method" which is to put a sock or other small flexible container in the hole and fill with expanding foam
to form a seal. Damage including holes larger than 3 inches in diameter but not larger than 8 feet
by 2 feet (axial length by circumferential width) should be repaired by a competent contractor
utilizing a plywood repair or a patch attached by hog rings or sewing with heavy duty polyester
thread. An alternative repair uses a neoprene gasket, backing ring and firm plastic plate in the shape of the repair placed over the hole and bolted into place.
Siasconset Coastal Bank Stabilization and Beach Preservation Project September 2010 Alternatives Analysis Page 26
For the purposes of the Alternatives Analysis level of design, the yearly maintenance costs are
estimated to be $16,750. This estimated cost includes two components: 1) the mobilization of a
contractor once every year to repair holes caused by debris, and 2) the mobilization of a contractor every ten years to replace failed tubes. Other actions such as post-storm inspections
and minor repairs (holes less than three inches) are considered within the ability of the
homeowners or SBPF to accomplish. The cost of sand replacement has been accounted for in a
separate yearly cost.
Removal of the System
The removal of the Tube+Sand system, if required by the regulatory agencies, would incur the
following costs; mobilization of a barge with crane, slicing open the sand-filled tubes, disposing
of the geotextiles in a landfill. Re-grading of the excavated sand is anticipated. For the purposes
of the Alternatives Analysis level of design, the costs of removal are expected to be 60% of the
initial construction mobilization costs (due to no pumps needed), and 15% of the costs of the geotextile tubes and scour aprons. The anticipated estimate for removal is $440,000.
Costs
It has been assumed elsewhere in the report that the average annual amount of sand lost from the
coastal beach system is 6.6 cubic yards per linear foot. At a cost of $50 per cubic yard, the annual sand nourishment costs for this project area, regardless of which design option is chosen,
is $561,000. Additionally, the face of the bank is to be vegetated above the protective structure.
This vegetation would be to counteract surface erosion, not deep-seated geotechnical
adjustments. Therefore, a cost effective way to vegetate the 139,400 square feet of exposed bank
face would be planting beach grass. The cost estimate is approximately $118,490, and is applicable to both design options.
An order of magnitude cost estimate for the initial construction of this concept is provided in the
table below.
Siasconset Coastal Bank Stabilization and Beach Preservation Project September 2010 Alternatives Analysis Page 27
Table 10 - Preliminary cost estimate for Concept Design 1.
Option 1
Geotextile tubes
Project length is 1,700 linear feet
Item Unit Cost
Geotextile tubes 6,800 l.f 1,020,000$
Scour aprons 130,000 sf 650,000$
Sand for tubes 16,000 cy 800,000$
Sand for cover 13,250 cy 662,500$
vegegtation 139,400 sf 118,490$
Subtotal 3,250,990$
Mobilization (10%)325,099$
Contingency (25%)812,748$
TOTAL 4,388,837$
$577,750Expected annual maintenance for tubes and sand
With the tube configuration, the slope of the tube structure is 1V:2.5H. This means that the toe
of the sand fill is further out from the existing toe of the bluff. Based on Concept Design 1, both
the minimum and maximum amount of sand to be placed on the tube structure is the same, and is
presently estimated to be 7.8 cubic yards per linear foot. Over the 1,700 linear foot project length, the total quantity of material required is 13,250 cubic yards. At $50 per cubic yard, the
cost is approximately $662,000.
In the case of a project smaller than the full 1,700 linear feet, the costs will be similar on a per
linear foot basis, with an increase in the mobilization percentage. For example, a 300 linear foot
tube project, with 50-foot returns, may have mobilization costs of 20%, resulting in an estimated project cost of $1,080,675.
Advantages vs. Disadvantages
An advantage to the Tube+Sand option is that, if a failure does occur or the tubes need to be removed, the sand filling the tubes us simply introduced into the littoral system and the only
disposal requirements is the fabric. On the other hand, if marine mattresses needed to be
removed, disposing of the stone fill may become an issue.
There are several disadvantages associated with this design concept. First, due to the amount of
sand required for filling the tubes, the costs are much higher. Also, mobilizing equipment to the beach face may present coordination issues during construction. Another disadvantage is the construction time and project duration associated with filling four 1,700 foot long geotextile
tubes. Experience has shown that marine mattresses can be installed at a much quicker rate than
filled geotextile tubes.
Siasconset Coastal Bank Stabilization and Beach Preservation Project September 2010 Alternatives Analysis Page 28
4.3.2. Concept Design-2a: Marine Mattress with Gabion Toe (Mattress+Sand)
Design
Concept Design 2 consists of stone filled marine mattress placed at the base of the coastal bank
on a 1V:1.5H slope with gabions at the toe. Sand fill will be required to create this grade and the
placement of a scour apron.
The gabion toe protection at the seaward end of the marine mattresses will consist of two rows of
6 ft x 4 ft x 6 ft geogrid baskets filled with 4- to 6-inch stone, which will provide a total width of twelve feet of scour protection in front of the marine mattresses. The scour elevation at the
seaward row will be +2.5' MLW to account for two 25-yr storms in succession; the inner row
will be placed two (2) feet higher to provide the elevation transition.
After the marine mattress and gabion structure is installed, a clean sand fill will be placed to
cover the system and provide a sacrificial berm to nourish the beach. The berm will extend 10 feet seaward of the landward edge of the Mattress+Sand structure and then be graded on a 1V:2H
slope to meet existing grade, maintaining a consistent cover of one foot over the structure.
Figure 15 – Schematic cross section of Concept Design 2.
Maintenance
Maintenance of the Mattress+Sand system consists of replacing the sand covering once per year. In the event of a storm, the system may become uncovered. An inspection of the uncovered
Siasconset Coastal Bank Stabilization and Beach Preservation Project September 2010 Alternatives Analysis Page 29
mattress should be undertaken when visible to check for broken ribs or any obvious signs of
movement or settling. The project is design accounts for the anticipated wave forces and scour,
such that the main concern would be damage to exposed geogrids from any heavy debris in the surf zone. In the event of damage, the geogrids may be repaired in place with 30 pound tensile
strength polyester or polyethylene cable ties (not nylon). If any individual compartments were to
break open, the stone should be replaced, and the section of failed geogrid patched.
Degradation of the geogrid from exposure to the sun and marine environment is not a concern for
the geogrids because all of the components are made from either HDPE or copolymer (HDPE and PP) with carbon black to prevent UV degradation. One of the oldest marine mattress
projects installed by Tensar is located at the end of the runway at Logan Airport in Boston,
which was installed in 1994. The expected life span of the geogrids is expected to be up to 40
years, unless subjected to physical damage from large debris.
For the purposes of the Alternatives Analysis level of design, the yearly maintenance costs are estimated to be $3,000. This estimated cost includes the mobilization of a contractor once every five years to replace stone and sections of geogrid. Other actions such as post-storm inspections
and minor repairs are considered within the ability of the homeowners or SBPF to accomplish.
The cost of sand replacement has been accounted for in a separate yearly cost.
Removal
The removal of the Mattress+Sand system, if required by the regulatory agencies, would incur
the following costs; mobilization of a barge with crane, removal of the stone-filled mattresses,
opening the mattress compartments, disposal of the geogrid in a landfill, recycling of the rebar
and re-use of the stone. No re-grading of the bank is anticipated. For the purposes of the Alternatives Analysis level of design, the costs of removal are expected to be 90% of the initial
construction mobilization costs, and 10% of the costs of the marine mattresses and gabion
baskets. The anticipated estimate for removal is therefore $300,000.
Costs
It has been assumed elsewhere in the report that the average annual amount of sand lost from the
coastal beach system is 6.6 cubic yards per linear foot. At a cost of $50 per cubic yard, the
annual sand nourishment costs for this project area, regardless of which design option is chosen,
is $561,000. Additionally, the face of the bank is to be vegetated above the protective structure. This vegetation would be to counteract surface erosion, not deep-seated geotechnical
adjustments. Therefore, a cost effective way to vegetate the 139,400 square feet of exposed bank
face would be to plant beach grass. The cost estimate is approximately $118,490, and is
applicable to both design options.
An order of magnitude cost estimate for the initial construction of this design concept is provided in the table below.
Siasconset Coastal Bank Stabilization and Beach Preservation Project September 2010 Alternatives Analysis Page 30
Table 11 - Preliminary cost estimate for Design Concept 2.
Option 2
Marine Mattress w/ gabion basket
Project length is 1,700 linear feet
Item Unit Cost A Cost B
gabion baskets 568 units 17,000$ 17,000$
marine mattresses 68,000 SF 1,156,000$ 1,156,000$
sand
small profile 9,450 cy 472,300$ N/A
large profile 30,700 cy N/A 1,543,800$
vegegtation 17 days 118,490$ 118,490$
stone for baskets 3,030 cy 151,500$ 151,500$
rebar 200,000$ 200,000$
Subtotal 2,115,290$ 3,186,790$
Mobilization (10%)211,529$ 318,679$
Contingency (25%)528,823$ 796,698$
TOTAL 2,855,642$ 4,302,167$
$561,300Expected annual maintenance for matresses and sand
In the case of a project smaller than the full 1,700 linear feet, the costs will be similar on a per
linear foot basis, with an increase in the mobilization percentage. For example, a 300 linear foot
marine mattress project, with 50 ft returns, may have mobilization costs of 15%, resulting in an
estimated project cost of $669,300.
Advantages vs. Disadvantages
With the Mattress+Sand configuration, the slope of the structure can be steeper at 1V:1.5H. This
means that the toe of the sand fill can be to the existing toe of the bluff allowing more usable
beach. Conversely, the sand fill can be designed to be greater, extending seaward to a similar position on the beach as the tube configuration. Therefore, there is a range of sand fill
quantities. Based on Design Concept 2, the maximum amount of sand to be placed on the
marine mattress revetment is presently estimated to be 18 cubic yards per linear foot. Over the
1,700 linear foot project length, the total quantity of material required would be 30,700 cubic
yards. At $50 per cubic yard, the cost is $1,534,800. The minimum amount of sand that could be placed and still provide adequate covering of the marine mattress structure is 5.5 cubic yards
per linear foot totaling 9,450 cubic yards at a cost of approximately $472,300.
4.3.3. Cycle 2 Alternatives Matrix
A summary of alternatives is presented in the table below:
Coastal Bank Stabilization and Beach Preservation Project September 2010
Alternatives Analysis Page 31
Table 12 - Alternatives Comparison Matrix
Alternative Benefits Disadvantages Achieve
Goals
COSTS Permitting
Issues
Expected
Periodic
Renourishment Construction Maintenance Removal
(1)
No Action
No required construction
No direct costs associated with
this alternative
The beach and bluff face will continue
to erode.
Potential for further endangerment of
landward infrastructure including
homes.
There is a potential for indirect costs
related to damage incurred during
storm events.
No, this alternative does not achieve
the project goals of stabilizing the
coastal bank and preserving the
beach.
None None None None 1 year and after a
major storm event
(2)
Geotextile
Tubes
+
Sand
Considered a "softer" solution
than stone and concrete.
Tubes have an inherent
flexibility to accommodate
foundation changes.
Less material to dispose of if
removal is required.
Although tubes have flexibility, too
much scour at the toe could
potentially lead to structural failure.
More susceptible to damage from
vandalism and debris.
Susceptible to UV degradation.
Mobilization of equipment to island
could increase project costs.
Long duration of construction time to
fill 1,700 linear feet of geotextile tube
(times four).
Achieves project goals of protecting
the toe of the bluff and preserving
the beach.
This alternative is designed for a 50-
year storm.
$4.4 Million $16,750 $440,000
Moderate to Difficult.
May be seen in a slightly
more favorable light by
regulators because of
the "soft" nature of the
tubes as well as the ease
of removal.
11,000 cubic yards
annually
(3)
Marine
Mattresses
+
Sand
Steeper design slope of bank
requires less fill material,
although more sand could be
added for a larger beach.
Marine mattresses can be
installed quicker than filling of
geotextile tubes resulting in
shorter construction timelines.
Less susceptible to vandalism
and damage from debris.
Disposal of mattresses and stone fill
could be problematic if removal is
required.
Less flexibility than geotextile tubes.
Considered a "harder" solution.
Achieves project goals of protecting
the toe of the bluff and preserving
the beach.
This alternative is designed for a 50-
year storm.
$2.8 Million
to
$4.3 Million
$3,000 $300,000
Difficult - Placing hard
structures on a beach
and bluff is usually
discouraged by
regulators.
11,000 cubic yards
annually
Coastal Bank Stabilization and Beach Preservation Project September 2010
Alternatives Analysis Page 32
5. Recommended Design
Based on the foregoing analyses, the recommended design that meets the regulatory design
standard of “best available measures” is the marine mattress+sand alternative. Both options were designed to provide similar stabilization to the coastal bank to provide storm damage prevention
for homes from a 50-year probability storm event. Therefore both options have similar crest
elevations. Differences in the geometry of the tubes and marine mattresses result in differences
in the footprint and amount of initial sand required. Additionally, the marine mattress+sand
design has flexibility in both the dimension (thickness) and installation approach thereby allowing confidence that the final design will result in a robust coastal engineering structure.
The cost of imported sand adds to the cost of the geotextile tube+sand design when compared to
the marine mattress+sand design. It was also determined that the construction duration of the
marine mattresses is likely shorter than the tubes, and require less specialized skills. This is due
to the fact that four rows of geotubes are required, and each row must be filled individually by personnel having experience pumping slurry into tubes. The marine mattresses however, can be
pre-fabricated and either pre-filled with stone, or filled on site by the contractor. These details
will be finalized during the final design phase.
It has been determined that the average annual amount of sand lost from the coastal beach system
is 6.6 cubic yards per linear foot. At a cost of $50 per cubic yard for sand delivered in-place, the annual nourishment costs for the project area, regardless of which design option is chosen, is
approximately $561,000.
Additionally, the face of the bank is to be vegetated above the protective structure. The purpose
of the vegetation is to stabilize the middle and upper slopes of the coastal bank from surface
erosion, not deep-seated geotechnical adjustments. Several options exist for vegetating the bank including hydroseeding, planting beach grass or planting woody vegetation. It is recommended
that beach grass be used to vegetate the slope because this type of vegetation has a stronger root
system and has previously been permitted on Nantucket Island. The cost for this option is $0.85
per square foot, compared to woody vegetation that costs up to $10 per square foot.
The beach grass plantings include a jute erosion control net that is installed until the root system fully develops. If the coastal bank is vegetated from the top of the marine mattress to the top of
the bluff, this results in a total area of 139,400 square feet over the project area. The cost for this
is $118,490, and is applicable to both design options. If a more robust design is required to
address stormwater drainage, ground water seepage or to stop future adjustment of the existing
bank slopes, then a cost for this will need to be factored in.
Coastal Bank Stabilization and Beach Preservation Project September 2010
Alternatives Analysis Page 33
6. Cycle 3 Final Design Analysis
During the Cycle 2 Analysis, preliminary designs were developed for the comparison of the Tube+Sand concept to the Mattress+Sand concept for the 50-year storm event. Factors such as
impacts to the environment, overall functionality of the system and construction and
maintenance costs were compared, resulting in the conclusion that the most suitable alternative
for the project was the Mattress+Sand option.
The Mattress+Sand concept was further refined during the Cycle 3 analysis. Because standard coastal engineering practice generally considers a 100-year storm event as the most appropriate
level for design, OCC increased the assumed wave, wind and storm surge parameters to upgrade
the 50-year storm design to the more severe storm conditions produced by the 100-year event.
While the upgrading to a 100-year storm parameters results in a more substantial design, the
associated labor costs will only increase slightly, since the overall construction process remains unchanged. Thus, because the 100-year design offers greater protection for only a relatively
marginal cost increase, the 100-year design for the Mattress+Sand concept is the recommended
final design alternative.
6.1. Coastal Engineering Criteria
The following design criteria were used to provide protection from the anticipated 100-year
storm conditions:
High Tide Line (HTL) Elevation = +4.97 feet MLW
Beach elevation at toe of coastal bank = +8.0 feet MLW
Estimated depth of scour at the bank toe = 8.0 feet.
100-year design storm Still Water Level (SWL) Elevation = +10.2 feet MLW
To protect the lower slope from erosion due to wave run-up during 100-year
storm conditions, the top of the proposed coastal engineering structure should be
extended to Elevation +25.0 feet MLW;
6.2. Final Concept Design for 100-Year Storm
Design
As described previously for the 50-year storm design concept, the lower portions of the bank
slope will be prepared initially by grading the existing lower slope as required along a 1,700-foot-long stretch of coastal bank area to provide a maximum stable slope of 1V:1.5H prior to placement of the marine mattresses.
Following slope preparation a scour apron, consisting of three rows of 4 ft x 5 ft x 6 ft gabion
baskets will be buried along the toe of the prepared coastal bank. Each gabion basket will be
Coastal Bank Stabilization and Beach Preservation Project September 2010
Alternatives Analysis Page 34
filled with 12- to 22-inch diameter stones. These toe gabions will provide a 15-foot wide scour apron seaward of the marine mattress array. The base elevation of the seaward row of gabions will lie at +0.0 feet MLW, the base of the middle row will lie at +2.0 feet MLW and the
landward row will lie at +4.0 feet. This design will provide the intended level of scour
protection from a 100-year storm event
After the gabion baskets are constructed, an array of stone-filled marine mattresses will be installed. The fill for the mattresses will consist of angular crushed stone approximately 3- to 6- inches in diameter. The bottom of the mattress array will be located at the top of the landward
toe gabions and will then extend up the bank face to an elevation of approximately +25 feet
MLW. The mattresses will be placed for a collective dimension of 38-feet long x 6-feet wide x
18-inches thick. The end-to-end splicing of the two adjacent mattresses will be accomplished
using an HDPE bar to form a bodkin connection (as recommended by the manufacturer). Mattresses will be anchored to the existing bank slope using J-Hooks fabricated from #4 steel
reinforcing bars, spaced approximately 4 feet on center.
In addition to the Mattress+Sand system that runs parallel to the shoreline, return walls
consisting of stone filled gabions will also be constructed at either end of the system to protect
against flanking erosion. The object of the return walls will be to provide protection for exposed faces of the slope that run perpendicular to the shoreline. The return walls will consist of a total
of two layers of gabion baskets. The bottom row will consist of three adjacent 4 ft x 5 ft x 6 ft
gabion baskets filled with 3- to 6-inch diameter stone and the top row will contain two adjacent 4
ft x 4 ft x 6 ft gabion baskets filled with 3 to 6-inch diameter stone.
The total number of return walls required will be dependant on the number of residents that elect
to participate in the project. A return wall will be required each time the system is terminated.
Specifically, if the system runs continuous for multiple properties, return walls will only be
required at the start and end, however if the system is installed at every other property, return
walls will be required at each property line.
The final construction element will consists of placing a minimum 12-inch-thick layer of clean,
beach compatible sand over the marine mattress array and toe gabions such that the berm will
extend 15 feet seaward of the landward edge of the structure and will form a 1H:2V grade.
Maintenance
Maintenance requirements for the structure will the same as required for the 50-year storm design concept and will consist primarily of replenishing the sacrificial sand berm covering the
mattresses and toe gabions as necessary to maintain the contribution of beach-compatible
sediment from the Project area. Additional maintenance may be required on occasion to repair
or patch any damaged geogrid or to replace any associated lost fill stone.
Removal of the System
Costs for removal of the mattresses and gabions, if required by the regulatory agencies, are similar to those projected for removal of the 50-year design concept, as the labor required would
be similar.
Coastal Bank Stabilization and Beach Preservation Project September 2010
Alternatives Analysis Page 35
Figure 16 - Schematic cross section of Final Design Concept for 100-Year Storm Event
Coastal Bank Stabilization and Beach Preservation Project September 2010
Alternatives Analysis Page 36
Figure 17 - Schematic of Return Walls for Final Design Concept for 100-Year Storm Event.
Coastal Bank Stabilization and Beach Preservation Project September 2010
Alternatives Analysis Page 37
Costs
Based on the more comprehensive design concept obtained in the Cycle 3 Analysis, an Opinion of Probable Cost (OPC) estimate was completed for the final design concept. Compared to the
Order of Magnitude estimate performed for the Cycle 2 analyses, the OPC estimate provides a
much more detailed approximation of potential construction costs. The OPC utilizes labor rates
based on prevailing wage or Davis-Bacon rates for the geographic location of the site and equipment rates based on those of contractors in the area. Durations of tasks are developed in consideration of the type of activity and any site constraints, material quantities are taken off the
drawing or sketches available and priced out using current material costs or actual vendor quotes
whenever possible. The amount of contingency is developed as a function of the level of design
and the degree of uncertainty in portions of the work. The contingency is generally developed to
give the client a realistic budget number that will generally reflect the total construction cost due to any unknowns or potential material cost variations. A summary of the OPC developed for the
100-year storm final design concept is included below. For estimating purposes, a total of eight
(8) returns walls were assumed to be required.
Table 13 - Summary of OPC for Final Design Concept for 100-Year Storm Event.
ITEM NO. WORK ITEM DESCRIPTION
OPC PRICE
(LUMP SUM)
1 MOBILIZATION $50,000
2 MARINE MATTRESS CONSTRUCTION $6,948,000
3 DE-MOBILIZATION $37,000
4 GABION RETURN WALLS (8 ASSUMED) $120,000
TOTAL $7,155,000
OPINIONS OF PROBABLE COST INCLUDE THE FOLLOWING MARK-UPS:
GENERAL CONDITIONS: 5%
OVERHEAD: 10%
PROFIT: 10%
SALES TAX: 0%
ESCALATION: 0%
CONTINGENCY: 20%
Coastal Bank Stabilization and Beach Preservation Project September 2010
Alternatives Analysis Page 38
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