HomeMy WebLinkAboutApplied Coastal 2013_11_8_201404011027315543 1
Applied Coastal Research and Engineering, Inc.
766 Falmouth Road
Suite A-1
Mashpee, MA 02649
MEMORANDUM
Date: November 8, 2013
To: Emily MacKinnon and Cormac Collier, Nantucket Land Council
From: John Ramsey, P.E. and Trey Ruthven
Subject: 2nd Response Regarding the Baxter Road Temporary Stabilization Application
We have completed a brief review of the supplemental information provided by Milone &
MacBroom (letter signed by Nicolle Burnham, P.E. dated November 1, 2013 with attachments)
regarding “Issues raised at Conservation Commission Meeting of October 30, 2013” relative to the
Baxter Road Temporary Stabilization Application. The latest information provided a design that is
substantially the same as presented during the latest Conservation Commission Hearing, without
any further analysis of other potential stabilization techniques that could provide short-term stability
to the bank with fewer adverse impacts.
Overall, there is a concern that the analysis provided by Milone & MacBroom to support the
design and mitigation for the project is highly dependent on the previous (and/or ongoing) work of
SBPF and their consultants. One primary area of scientific and engineering disagreement is related
to the calculation of minimum annual nourishment requirements for coastal armoring project of the
scale proposed previously by SBPF and now by the Town of Nantucket. In addition, the application
remains unclear regarding the actual volume that will be placed on the beach for mitigation, as
opposed to other material placed inside the geotubes, placed above the 100-year flood levels (i.e.
above the toe of the existing bluff elevation), excavated from the beach to place the geotubes, or
utilized to level the area for the coastal engineering structure placement. As discussed during
numerous hearings regarding ‘hard armoring’ along the Sconset Bluff, it is critical that mitigation be
performed in a proactive manner to ensure stability of adjacent bluffs. Reactive mitigation will not
maintain bluff stability, since failure of adjacent bluff shorelines cannot be reconstructed through
sand mitigation.
Mitigation
Similar to the past two SBPF armoring applications, the Town of Nantucket project would
cause a complete loss of the sediment supply along the armored section; however, the proposed
beach nourishment volume computed to mitigate for this loss is not based on the best available
information (e.g. long-term data compiled by both MCZM and SBPF consultants over more than 20
years). A thorough analysis of appropriate mitigation quantities should be based upon all available
information and not focused on time periods that are strictly beneficial to the applicant, at the
expense of downdrift property owners. The specific comments below address the shortcomings
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and/or incorrect analysis contained in the updated coastal bank retreat calculations provided by
Epsilon Associates:
The information contained in Table 1 implies that past SBPF calculations regarding
the loss of sediment supply caused by armoring of the Sconset Bluff have generally been
consistent with the Town of Nantucket proposal presently under review. However, it should
be made clear that the gabion project was denied by the Conservation Commission. During
the numerous public hearings it became clear that inadequate mitigation and likely adverse
impacts to downdrift properties remained concerns for a majority of the Commission.
Specifically for the gabion project, Epsilon Associates, Inc. calculated the appropriate
mitigation volume to be 19.1 to 19.5 CY/lf/year (these values excluded 13% of the total
volume eroding from the bank due to fines, with the inclusion of fines the erosion rate is
20.8 to 22.2 CY/lf/year). In their presentation, there was never any mention that this
calculation included any “overfill allowance” or extra material, as erroneously claimed in
Epsilon’s November 1, 2013 memorandum. Therefore, the computed mitigation
requirement for this previous project proposed by Epsilon Associates was more than 33%
larger than the mitigation currently proposed, and more accurately more than 50% more
than currently proposed.
While the November 1, 2013 Epsilon review is extremely critical of the long-term
MCZM shoreline change analysis and the “purpose” of the CP&E sediment budget, neither
criticism appears based upon sound scientific or engineering principles.
o The primary criticism of the MCZM analysis is focused upon the claim that
SBPF monitoring data “has consistently shown that shoreline erosion rates in
areas where coastal banks are fronted by dunes are significantly higher than
shoreline [change] rates in areas with an eroding coastal bank.” There is no
quantitative analysis provided to support this conclusion and data from the
monitoring certainly demonstrates that many dune areas (e.g. Codfish Park) have
experienced significantly less shoreline retreat than the area along the Sconset
Bluff.
o According to Epsilon Associates, MCZM shorelines indicate shoreline
change rates within the project area are between 4.0 and 9.7 feet per year, which
would indicate that the proposed “bluff crest” erosion rate of 4.6 feet per year is
well below the average for this shoreline and not applicable to utilize as a rate for
mitigation calculations. This is further supported by the Ocean and Coastal
Consults, Inc. (SBPF engineering consultant) analysis that indicated a shoreline
erosion rate of ~8 feet per year or about 20.7 CY/lf/year (from the September
2010 Siasconset Coastal Bank Stabilization and Beach Preservation Project
Alternatives Analysis submittal on behalf of SBPF to Nantucket Conservation
Commission).
o Epsilon also indicates that the MCZM analysis “is subject to uncertainty”;
however, they never describe or attempt to quantify the uncertainty of their own
analysis. Based on sound scientific principles, the MCZM analysis typically has
an uncertainty on the order of 0.4 feet per year (an order of magnitude below the
observed shoreline recession rate). The Epsilon analysis also has inherent
uncertainties and based on utilizing “top of bank” as their baseline, these
uncertainties are magnified due to interpretation problems associated with aerial
photography (as well as all of the other uncertainties related to the typical MCZM
shoreline change analysis). As presented, the 1994 top of bank was delineated
from an aerial photograph – an analysis technique that is scientifically invalid for
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determining coastal change. As stated during many previous Conservation
Commission meetings regarding other similar SBPF filings, a lower rate of bluff
erosion relative to shoreline erosion is not possible, as this initially causes an
over-steepening of the coastal bank and eventually leads to the crest of the
coastal bank being seaward of the beach, which of course is not possible.
o Figure 1 provided in the Epsilon memorandum provides some of the best
evidence of how use of “coastal bank crest” data misrepresents ongoing
processes and appropriate shoreline change rates. Specifically, a cursory review
of the figure indicates that erosion rates for Lots 91-107A between 1994 and 2003
were relatively modest over this 9-year period, but certainly accelerated over the
2003-2013 time period. However, Epsilon chose to utilize the 1994-2013 time
period which clearly yields a lower erosion rate that is not representative. Other
data (e.g, the Woods Hole Group, Inc. surveys of bluff position) demonstrate a
recent steepening of the coastal bank in the project area, which is clearly evident
and likely the reason for the Town’s involvement and desire to stabilize the bluff.
However, the analysis of the bluff crest by Epsilon does not incorporate this
ongoing over-steepening followed by episodic collapse mechanism in the
analysis. The episodic nature of the bluff failure mechanism is the primary reason
why coastal scientists/engineers do not use the coastal bank crest position as a
valid proxy for shoreline retreat rates. The subjective data analysis provided by
Epsilon does not provide confidence that the conclusions are robust and
conservative relative to Town of Nantucket concerns for neighboring and
downdrift properties.
o Criticisms of the 2006 CP&E sediment budget (another consultant report
produced for SBPF) are completely unfounded, as this effort represents the only
significant effort by SBPF to use ‘best available measures’ to quantify sediment
transport along the Sconset Bluff region. The methodology is identical to the type
of analysis presented by Epsilon; however, it also is informed by coastal
processes data and modeling. This analysis indicated the bluff/beach system in
the project region provides approximately 24.2 CY/lf/year from the sediment
budget developed for the period from 1995 to 2005.
o Due to the inter-annual variability in shoreline change rates within the project
area, it is clear that the substantial accretion observed in 2013 is not typical for
this region. In situations similar to this, coastal scientists/engineers typically
employ a least-squares fit to all of the long-term shoreline change data to
determine shoreline change. The method currently presented by Epsilon and
incorporated into the Town application is misleading and underestimates the
actual impact to downdrift beaches that will be caused by this project. Available
shoreline positions for every Quarterly Survey should be provided as the basis for
this analysis. Use of bluff crest position data should be discontinued, as it is
misleading and is not considered sound scientific practice.
o Epsilon has never incorporated any of the sediment placement by SBPF (i.e.
bank and beach material) into the bank erosion computations. This leads to an
additional (although likely small) underestimation of coastal bank and/or beach
erosion rates.
As mentioned in previous meetings, the 2013 shoreline position is aberrant relative
to recent historic trends dating back to the inception of SBPF (circa 1994). For example, the
Woods Hole Group, Inc. survey data indicates that the 2013 shoreline in the project area
has accreted since 2011. If this were the long-term trend, there certainly would be no need
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for the project, since natural forces would be re-building the beach. Of course, this is not
truly the case and this one-time accretion should be viewed as an outlier and the data
associated with this time period should not be utilized without a thorough review of historical
trends from all time periods monitored. This point is highlighted by the following quote from
the most recent Woods Hole Group monitoring report:
In the project area the shoreline along all profiles, except 89.2, advanced likely
due to a portion of sediment eroded from the bluffs remaining on the beach
Therefore, utilization of the 2013 shoreline position for mitigation calculations is misleading
and incorrect. Instead, SBPF and the Town should provide the data and a more complete
analysis (as described above) to develop an accurate long-term shoreline trend should be
utilized as the basis for the minimum amount of mitigation nourishment required.
According to the plans, as well as the presentation at the last Conservation
Commission meeting, the project design team has opted for placing the proposed armoring
seaward of the coastal bank. Based on the design, it appears that the proposed structure
will extend approximately 40 feet onto the beach. Therefore, the Town should also
consider mitigation for the loss of sediment supply associated with the beach, since the
proposed structure is effectively preventing a substantial portion of the beach sediments
from remaining a part of the active littoral system.
Based on the project plans, the properties likely to suffer increased erosion at the
north and south ends of the geotube structure are Lots 109, 113, 115 (to the north), and 83
(to the south). The impacts of the structure on properties immediately adjacent to the shore
protection structure will experience increased erosion as a result of wave energy focusing
and exacerbated wave reflection. This increase on local erosion rates is often referred to as
coastal structure “end effects”. A stand-alone mitigation strategy to proactively address
these “end effects” should also become part of the Town’s overall mitigation strategy.
Similar to the mitigation for the overall bank erosion, the volume of material should be
placed annually, regardless of monitoring results. The volume of sediment associated with
the “end effects” should not be considered part of the overall mitigation volume related to
typical bank erosion, as the “end effects” represent a local acceleration in erosion rates
directly caused by the structure.
Numerous discussions of shoreline and/or coastal bank monitoring have been
debated for nearly 20 years at Sconset. Certainly, closely spaced transects should be
considered directly adjacent to the proposed structure to ensure that the “end effects” are
effectively monitored. As mentioned above, there is a significant concern that near-term
end effects could immediately jeapordize the structures to the immediate north and south of
the project. According to the July 2013 coastal armoring NOI submitted by SBPF, dwellings
on Lots 109 and 113 are within 13 and 18 feet of the coastal bank crest, respectively.
Temporary Structure Alternatives
At the October 30, 2013 Conservation Commission hearing there was a discussion about
reexamination of design alternatives to ensure the least impactive solution was brought forward for
the temporary protection of Baxter Road to allow the Town time to secure alternative means of
access. Reviewing the additional information submitted by Milone & MacBroom on
November 1, 2013, it does not appear any serious consideration was given to alternative designs
that could minimize impacts to adjacent properties. As we have pointed out previously, Geotube
Alternative 1 – Jute Fiber Logs have been shown to work over a number of years at 79 Baxter
Road. The Jute Fiber Logs approach does require regular maintenance, however that is a direct
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result of the way the system was designed to function. The Jute Fiber Logs were designed to
release sediment to the nearshore system thereby causing minimal adverse impacts to the ability of
the coastal bank to act as a sediment source for downdrift portions of the shoreline. Epsilon
Associates, Inc. characterizes the jute design as follows in a June 13, 2008 letter to Conservation
Commission in support of an extension request for the Jute system;
when a portion of the jute bag is ruptured by wave action resulting in a rapid contribution
of the contained sediment. Both of these mechanisms of sediment contribution have
often been mischaracterized as a "failure" of the terraces. This is an inappropriate
characterization since the terraces were specifically designed by the proponent and
subsequently conditioned by the Commission to contribute sediment to the nearshore
system by these two mechanisms while minimizing project related debris in the
nearshore system. Therefore sediment release to the nearshore system during storm
events is in fact a successful result of the terrace design.
Over the winter of 2012/13 the Jute Fiber Logs were damaged by storms and 30 feet of bank at the
north end of the 79 Baxter Road was eroded. Examining aerial photographs suggests that offsets
along bank face resulted in focusing of wave energy at the ends of the Jute Fiber Logs. The
localized increase in wave energy resulted in end effect scour and bank erosion on neighboring
properties which led to the system being flanked. Flanking and end effect scour are the outcome of
a structure not been properly designed and then mitigated for. In past hearings Epsilon has
indicated that the volume of mitigation associated with the jute project were on the order of the
volumes currently being proposed and thus low mitigation volumes are likely a key factor in the
damage at the north end of the project. The erosion of the bank illustrates how critical mitigation
volumes are to ensure the success of a project. For any project along the Sconset Bluff to succeed,
it is critical that nourishment volumes be carefully considered and appropriate volumes be placed
on the beach; otherwise the structure will fail and in the interim, the structure will result in significant
impacts to downdrift properties. The Town of Nantucket should not be protecting Baxter Road at the
detriment of neighboring property owners whom the Town’s project is seeking to help by preserving
Baxter Road.
During the October 30th hearing the commission members also requested additional
information about hybrid geotextile/jute designs, cases where similar systems have failed and cases
where similar system have succeeded. That information was not provided at the November 6th
meeting, but rather the Town DPW indicated that they do not believe a jute system would work due
to the level of design risk. However, no information regarding some type of hybrid alternatives that
would be more appropriate for short-term bank protection have been provided and we suggest that
the Town be asked to re-visit the alternatives analysis.
Geotube Design Considerations
It is clear that scour represent a critical concern in the design of the proposed structure.
Scour in front of the structure is directly tied to the incident wave energy, wave reflection, and
volume of sediment available within the littoral system to keep the structure outside of the active
surf zone. It has been mentioned that the proposed system was optimized to minimize seaward
encroachment onto the beach. However, a quick look at the reflection coefficients for a structure of
this type reveal that wave reflection off the structure is going to be significant. Using the effective
structure slope, the reflected waves range from 70- to 90-percent of the incident wave height, on a
micro scale of each geotube lift, the reflected waves approach 100-percent of the incident wave
height. It is clear that the design of this structure is going to result in the lowering of the beach
height and reduction in beach width in front of the structure, which will allow larger waves to impact
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the structure over future storms. The details of the design need to be reconsidered to minimize
impacts to the coastal system while providing the necessary protection to Baxter Road.
There does not appear to be any design features with the proposed geotube design to
address and minimize end effect scour on neighboring properties. Immediately to the north and
south of the proposed project, the homes at 109, 113, and 115 Baxter Road are within 11 feet, 13
feet, and 18 feet of the end of the coastal bank at the end of the proposed geotube structure (Table
1 from the SBPF July 2013 NOI). The proposed geotube design and mitigation plan has not
alleviated or even minimized the potential impacts to these dwellings. If the proposed structure is
constructed, it will cut off the natural supply of bank and beach sediment from the littoral system,
starving the shoreline immediately north of and south of the structure resulting in an acceleration of
ongoing erosion. In addition the end of the structure will focus wave energy on the adjacent coastal
bank further accelerating the erosion along the adjoining properties. The project as proposed is
directly jeopardizing the adjoining properties and dwellings.
We remain concerned with the Town of Nantucket attempting to permit and construct a
coastal structure that will result in significant wave reflection due to the vertical and hard nature of
the geotextile tubes, a structure that will cut of the natural supply of sediment from the littoral
system in coastal environment where the shoreline is retreating in excess of 5 feet per year, and a
proposed mitigation plan that is not sufficient to offset the adverse project impacts.
Sediment Contributions
As we have previously stated, the proposed geotubes structure is designed to act in a
similar manner to a revetment by isolating the coastal bank and beach from erosive forces. By
cutting off the supply of material from the coastal bank to the littoral system the project will shift and
magnify erosion onto adjacent Town owned beach and neighboring properties along the coastal
bank which already face significant erosional concerns. The Town of Nantucket should not put any
properties at greater risk due inadequacies in the mitigation planning and analysis.
The following table illustrates the current volumes of sand proposed as part of the geotube
project:
Placement Location Rate of Placement
(CY/LF)
Length of Placement
(Feet)
Total Volume (CY)
Inside Geotubes 4.22 (each tube) 1,500 25,320
Leveling Sand 2.3 1,500 3,450
Nourishment Sand 14.3 1,500 21,450
TOTAL VOLUME 50,220
*Reproduced from MMI’s October 25, 2013 letter to the Conservation Commission
The first and largest volume listed within the table is the 25,320 CY of sediment
contained within the geotubes. The sediment within the geotubes should not be considered
mitigation nourishment, since the sediment is isolated from the littoral system within the
geotubes and provides no mitigation value to the shorelines updrift and downdrift of the
proposed structure.
The leveling sand should not be considered mitigation nourishment; the sediment is
isolated from the littoral system behind the geotubes and the geo-textile scour apron.
The 18 CY/lf (27,000 CY) of excavated beach material associated with the
placement of the fourth geotube below the existing beach face is currently available to
downdrift beaches within the littoral system should erosion occur, if the displaced beach
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material is utilized for leveling of bank, sand cover, and/or mitigation nourishment it
represents a loss of available beach material from the littoral system and hence should be
appropriately mitigated for with additional beach nourishment.
The geotextile selected for the structure requires a two foot cover of sand over the
entire structure to prevent UV damage. The two foot sand cover has been characterized as
a portion of the annual mitigation by Milone & MacBroom. The placement of mitigation
nourishment over the top of the geotubes up to an elevation of +28 feet (16 feet above the
100-Year base flood elevation) places a significant volume of the mitigation outside of the
active littoral zone, thereby reducing the effectiveness of the mitigation to moderate the
downdrift impacts. If sand cover is required to prevent UV damage over the 3 to 5 year
design life of the structure, then the additional volume of sediment required should be
provided for independently of the mitigation. Therefore, sand placed on the geotube
structure should not be considered mitigation.
It is clear that the current volumes of sediment associated with the proposed structure (see
table above) should not be consider as part of the annual mitigation nourishment for the structure.
Mitigation should be addressed separately.
Conclusions
Reviewing the narrative presented within the NOI for the 2013 Baxter Road Temporary
Stabilization Application, as well as the follow-up documentation provided by Milone & MacBroom
(letter signed by Nicolle Burnham, P.E. dated November 1, 2013 with attachments), illustrates that
regardless of the stated temporary and minimal nature of the proposed project, the proposed
geotube structure will cut off the supply of material from the coastal bank to the littoral system.
Failure to adequately mitigate for the project will shift and magnify erosion onto adjacent Town
owned beach and neighboring properties along the coastal bank. Some of these adjacent
properties are within 20 feet of the bank crest. Analysis provided by SBPF consultants indicates that
the minimum annual mitigation nourishment should be on the order of 22 CY/lf/year or
approximately 33,000 CY per year. This value is consistent with MCZM shoreline change data for
the project region. The updated analysis provided by Epsilon Associates is technically flawed and
should not be utilized by the Town as the basis for computing mitigation volumes. The minimum
mitigation volume should provide for one-to-one mitigation of the material that is currently being
provided from the coastal bank to the littoral system. It is important to note that the one-to-one
mitigation does not account for any additional erosion which is likely to occur due to end effects,
wave reflection, and disturbance of the coastal bank and beach during construction. The goal of
mitigation is not to prevent erosion in front of the proposed structure, but to prevent the acceleration
of erosion on adjacent shorelines.
In addition to mitigation concerns, additional analysis of alternatives has not been provided,
monitoring details remain unclear, the failure criteria presented is nonspecific and not quantitative,
additionally the details regarding construction protocols also remain unclear.
.