Author: Andrew Rella, Ph.D. Researcher: Erin Hopson Project PI: Jon Miller, Ph.D. Davidson Laboratory Stevens Institute of Technology arellla@stevens.edu jmiller@stevens.edu

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 hrnerr@dec.ny.gov 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).