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Author: Andrew Rella, Ph.D. Researcher: Erin Hopson Project PI:
Jon Miller, Ph.D. Davidson Laboratory Stevens Institute of
Technology [email protected] [email protected]
The Hudson River Sustainable Shorelines Project is a multi-year
effort lead by the New York State Department of Environmental
Conservation Hudson River Nation-al Estuarine Research Reserve, in
cooperation with the Greenway Conservancy for the Hudson River
Valley. The Project is supported by NOAA through the National
Estuarine Research Reserve System Science Collaborative. Hudson
River Sustainable Shorelines Project Norrie Point Environmental
Center P O Box 315 Staatsburg, NY 12580 http:\\www.hrnerr.org (845)
889-4745 [email protected] July 2015
P R O J E C T B A C K G R O U N D Oak Point is one of six
locations included in a study called What Made Shorelines
Resili-ent: A Forensic Analysis of Shoreline Structures on the
Hudson River Following Three Historic Storms. The sites had either
traditional or non-traditional nature-based shoreline
stabili-zation techniques and were impacted by Tropical Storms
Irene and Lee in 2011 and Post-Tropical Storm Sandy in 2012.
Separate case studies describing each site and the impact of the
three storms have been prepared. Two additional reports describe
the methodology used and the common project performance factors.
All eight documents can be found at
http://www.hrnerr.org/shorelinesforensicanalysis. Each Forensic
Analysis included the review of historic photographs and design
drawings, interviews with project managers and designers, field
data collection, and modeling of the hydro-dynamic conditions
during each of the three storms. Collectively, this information was
used to create a holistic picture of each site, from which the
critical project performance factors could be determined. Impacts
from debris, undersized stones, improper slopes, as well as
monitoring and maintenance protocols, adaptive management, and
maturity of vegetation were all considered. During Sandy, the newly
constructed marsh at Oak Point was severely damaged as heavy debris
scoured the steep slopes at the project site.
S I T E B A C K G R O U N D Oak Point has had a diverse
industrial history over the past century. Once home to colonial
estates and a Cuban sugar importer, the site eventually became a
public beach, a railroad float yard, and eventually a city landfill
that served as an illegal dumping ground con-nected to the mob. It
took seven years to re-move and remediate the 50-foot-high piles of
trash dumped at the site. Currently, a $60 million warehouse that
supplies food to local bodegas and restaurants occupies the upland
portion of the site. Through an agreement with the New York State
Department of Environmental Conservation, the present own-er of the
site was given the authority to develop the interior of the
property in exchange
Figure 1 Historic photograph of Oak Point (1954).
F O R E N S I C A N A L Y S I S : O A K P O I N T , T H E B R O
N X , N Y
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for building wetlands along the waters edge. The wetlands were
intended to beautify the shoreline, create a haven for day-tripping
birds coming from the North and South Brother Islands nearby, and
stop silt and contam-inants from entering the East River. Unlike
the majority of the Forensic Analysis sites, the main objective of
the Oak Point project was to create habitat along the waters edge,
not to stabilize the shoreline or address an ero-sional problem.
The two main components of the project were the removal of the
decaying wooden bulkheads and debris from the former float yard,
and the creation and expansion of an intertidal marsh/wetland
complex, which required adding 6,000 cubic yards of sand. In the
process three beach areas were created. The project was completed
in the summer of 2012, just several months prior to Superstorm
Sandy.
S H O R E L I N E S T A B I L I Z A T I O N H I S T O R Y To
create a history of the shoreline evolution at Oak Point, we used
Google Earth for aerial photographs and www.historicaerials.com for
both aerial photographs and topographic maps. A time-lapse video of
the changes was created and is archived at
https://www.hrnerr.org/hudson-river-sustainable-shorelines/shorelines-engineering/.
The photographs show that the piers, which were constructed when
Oak Point served as a railroad float yard, became worn down and
dilapidated over the last half-century, but much of the industrial
shoreline has remained unchanged over that same period. To the west
of the project site, the shoreline consists of a rock- and
rubble-armored slope, while the shoreline to the east is stabilized
by a combination of a deteriorating bulkhead and
revetment/riprap-covered slope. Prior to redevelopment, the Oak
Point shoreline was in a similar state of disrepair and consisted
of a combination of a dilapidated wooden bulkhead and rock/rubble
revetment. From left to right, the aerial photographs in Figure 2
show the site shortly after restoration (left), immediately after
Sandy (center), and in 2014 after some of the vegetation had been
replanted (right). The entire shoreline was covered with vegetation
prior to Sandy, but during the storm over 70% of the plantings were
lost.
D E S I G N A N D E C O L O G I C A L A L T E R A T I O N S The
decaying wooden bulkheads and armored slopes were removed prior to
the construction of the wetland. Of the existing piers, three were
left standing to serve as wave breaks to reduce wave and current
forces on the shoreline. Inland of the piers, 6,000 cubic yards of
sand were imported to form three beaches and expand the intertidal
marsh. The two remaining acres of the site were planted with
shrubs, trees, and grasses. A riprap revetment was used in places
to protect against strong converging currents, with boulders
ranging from 1,000
Figure 2 Left to right: recently constructed (August 2012),
after Sandy (November 2012), and after replanting June 2014).
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pounds in the most sheltered areas to 3,000 pounds in the
exposed southwest corner. A mile-long, 15-foot-high cast-concrete
wall was also installed, lining the entire property to prevent
flooding and protect the upland infrastructure. A typical
cross-section and the plan view of the project are shown in Figures
3 and 4, respectively.
Figure 5 below shows the different marsh areas during the summer
of 2012, immediately after planting. The left and center images
show the east and west flanks of the fringe marsh, respectively,
which border the ends of the project area. The right image shows an
upland area with trees and shrubs planted to further stabilize the
bank. Unfortunately, the project was completed just three months
before the Sandy, before the vegetation was firmly established.
Figure 3 Cross-section presenting engineering design (Oak Point
Property LLC).
Figure 4 Overview of Oak Points intertidal design (Oak Point
Property LLC).
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C O L L E C T I O N O F E N G I N E E R I N G D A T A Multiple
sources of data were collected and analyzed to understand the
behavior of the Oak Point restoration project. The conclusions of
the Forensic Analysis were based on the following sources/types of
information:
Historic Aerial Photographs Topographic Maps Photographs
(construction, pre- and post-storm photographs of the site) Initial
Site Visit Discussions with Developer Engineering Plans
Correspondence with Permit Staff Final Site Visit (including
topographic/bathymetric survey) Hindcast of Storm Conditions (Wave
and Water Level Climatology)
C H A R A C T E R I Z A T I O N O F S I T E C O N D I T I O N S
The Oak Point shoreline is located in a VE Zone with a base flood
elevation (BFE) of 16 ft NAVD88, according to the preliminary Flood
Insurance Rate Maps released by FEMA in 2013 (Panel 3604970092G).
The VE zone designation signifies that the expectation is that the
shoreline area surrounding Oak Point will be impacted by waves
greater than 3 ft during the 1% annual chance of occurrence storm,
while the BFE represents the expected water level during the same
storm. The BFE represents a useful baseline with which to compare
both the typical and storm conditions at the site. The Sustainable
Shorelines physical forces climatology
(http://www.hrnerr.org/hudson-river-sustainable-shorelines/shorelines-engineering/physical-forces-statistics/)
dataset was used to characterize the conditions during a typical
year. The climatology is based on a one-year numerical simulation
of conditions within the Hudson and was generated using an
ultra-high resolution version of the NYHOPS numerical model. The
clima-tology was developed based on the conditions in 2010 and
included one significant Noreaster.
Figure 5 East bank marsh (left), west bank marsh (center), and
upland plantings (right).
Table 1 Physical Forces climatology.
Parameter Climatology WL95% (ft NAVD 88) 4.65 H95% (ft) 0.54
Hmed (ft) 0.16
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Based on the modeling results, the water level that was exceeded
only 5% of the time (WL95%) in 2010 was 4.65 ft NAVD 88, while the
wave height exceeded only 5% of the time (H95%) was 0.54 ft. The
median wave height (Hmed) for the 2010 hindcast was only 0.16 ft.
An analysis of the fetches at the site confirms that the site is
subjected to low-moderate energy with respect to wind waves. The
relevant fetches are shown in Figure 6, where the average and
maximum fetches were found to be 5,950 ft (1.1 mi) and 13,140 ft
(2.5 mi), respectively. No wake observations were made at Oak
Point, but significant wakes are expected due to the proximity of
the navigation channel and the number and size of the vessels
transiting this section of the East River.
Topographic and bathymetric surveys of the site were conducted
in order to obtain detailed information about upland elevations,
nearshore slopes, and offshore depths. The survey data indicate
(Figure 7) that offshore of the project site the contours are
fairly uniform, sloping rapidly between depths of -5 ft (NAVD 88)
and -30 ft, then more gently from -35 ft out to a depth of -50 ft.
The bermed beach is at an elevation of 5 ft and the farthest extent
of the wooden piers are at an elevation of 0 ft. Above the
waterline the slopes were made artificially steep in order to
maximize the buildable area.
H I N D C A S T I N G S T O R M C O N D I T I O N S Conditions
during the three historic storms were hindcast using the NYHOPS
numerical model. The water levels hindcast (Figure 9) during both
Irene and Sandy significantly exceeded the 95th percentile based on
the 2010 climatology. When compared to the surrounding upland
elevations, the hindcast suggests that flooding impacts were
limited to the restored wetland and sloping bank, an observation
that is confirmed by post-Sandy FEMA mapping (Figure 8). The
hindcast maximum water level during Sandy matches up well with a
nearby high-water mark collected by the USGS, which registered 10.6
ft NAVD88. The wave heights hindcast (Figure 10) during both Irene
and Sandy significantly exceeded the 95th percentile wave height
from the 2010 climatolo-gy. While these results indicate the
relative significance of these
Figure 7 Fetch analysis at Oak Point. Figure 7 Topographic and
bathymetric survey results.
Figure 8 FEMA Hurricane Sandy storm surge extent.
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storms, in an absolute sense, wave heights approaching 2 ft are
typically not considered high-energy conditions. Small wave heights
are surprising considering the large fetches surrounding the Bronx
shoreline.
D O C U M E N T E D P E R F O R M A N C E The Oak Point project
was completed several months prior to Sandy, which caused
significant damage, scouring the banks, uprooting vegetation, and
leaving large piles of wooden debris and trash (Figure 11). It was
estimated that Sandy removed over 70% of the vegetation at the
site, though a lone evergreen tree survived. Some of the structural
elements of the project did survive, however, including the
revetment at the corner of the peninsula and the 15-foot-high,
mile-long cast concrete wall. Since then, some of the vegetation
has recovered naturally, and some has been replanted. Several
growing seasons at minimum will be required to determine whether
these latest efforts will be successful.
Figure 10 Modeled water levels (ft NAVD 88) at Oak Point during
Irene and Sandy.
Figure 10 Modeled wave heights at Oak Point during Irene and
Sandy.
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F I N D I N G S The Forensic Analysis has identified several
factors that potentially led to the failure of the Oak Point
shoreline during Sandy and are roughly consistent with a findings
report produced after Hurricane Sandy by the landscape architect.
The findings report was generated in part based on a discussion of
the site conditions convened by the site designer, which included
representatives from NOAA, USGS, NY Sea Grant, and the Stevens
Institute. The steep, 2:1 slope, at the site was a significant
contributor to the impressive loss of vegetation experienced during
Sandy. The constraints posed by limiting fill to areas above the
mean high-water line, meeting FEMAs base flood elevation
requirements of +13 ft NAVD88, and trying to maximize the buildable
area created a significant challenge at the site. The amount of
large wooden debris and trash deposited within the site, in
particular adjacent to some of the more heavily eroded/scoured
areas, suggests that debris impact was one of the primary reasons
that such extensive damage occurred. Given the size and amount of
debris found on the site, it is ex-tremely unlikely that even
mature vegetation would have survived the storm. Even under
non-storm conditions, currents of up to 6 knots can be found just
offshore of the site, and debris build-up within the pier
structures is common. This suggests that debris impact (including
floating ice) may be a long-term problem at the site, particu-larly
if the steep side slopes are not addressed.
Figure 12 Debris flooded marsh (left) and severe shoreline
erosion (right).
Figure 12 Evidence of damaged and lost plants (left) and debris
and erosion (right).