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 Con-servation Hudson River National 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

PROJECT BACKGROUND Matthiessen and Scenic Hudson Park in Irvington, NY, together make up one of six lo-cations included in a study called What Made Shorelines Resilient: A Forensic Analysis of Shoreline Structures on the Hudson River Following Three Historic Storms. The sites had ei-ther traditional or non-traditional nature-based shoreline stabilization 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. The six case studies plus reports on the methodology used and the common project performance factors can be found at http://www.hrnerr.org/shorelinesforensicanalysis. Each Forensic Analysis included the review of historic photographs and design drawings, interviews with project man-agers and designers, field data collection, and modeling of the hydrodynamic condi-tions 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 moni-toring and maintenance protocols, adaptive management, and maturity of vegetation were all considered. Overall, both of the Irvington park locations fared well during the three extreme weather events, each site being completely submerged but only sustain-ing minimal damage.

SITE BACKGROUND Before the area of Irvington was settled by Europeans, it was inhabited by the Wickquasgeck Indians. During the late 19th and early 20th centuries, Irvington grew into a small community surrounded by the large estates and mansions of the local wealthy businessmen. The Village of Irvington, locat-ed on the east bank of the Hudson River, has two parks that have been included in the Forensic Analysis. The southernmost of the two sites, Scenic Hudson Park, is co-owned by the Village of Irvington and The Scenic Hudson Land Trust, Inc. The park was con-structed in 2002 on a 7.5-acre former industrial site between the Hudson River and the

Figure 1 The Irvington waterfront 1859-1889.

F O R E N S I C A N A L Y S I S : M A T T H I E S S E N P A R K & S C E N I C H U D S O N P A R K , I R V I N G T O N , N Y

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Metro-North railroad tracks. The completed park includes two regulation baseball fields, two playground areas, a boat launch for non-motorized crafts, and nearly a mile of pathways that weave around the approximately 4.5 acres of open lawns used for passive recreation. Matthiessen Park is located to the north of Scenic Hudson Park, along the east bank of the Hudson River. A mixed-use redevelopment, Bridge Street Properties, spans the area between the two parks. Matthiessen Park was originally filled with sediment left over from the construction of the original Yankee Stadium in the Bronx. The shoreline of the park has been stabilized over the years with recycled materials. The present-day shoreline of Matthiessen Park is composed of an un-engineered mix of concrete debris and mismatched and unsorted stone.

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 the Irvington parks, 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 earliest aerial photographs available were from the 1950s and show a smaller Matthiessen Park bordered by the former industrial site. Between 1965 and 1994, Matthiessen Park nearly doubled in size. The aerial photograph on the left in Figure 2 shows the finished extent of Matthiessen Park to the north and the industrial area prior to redevelopment for Scenic Hudson Park, shown in the center image. The image on the right from 2011 shows little change in the two parks over the intervening decade. At Matthiessen Park, the concrete rubble and mismatched stone placed along the shoreline over the years appears to have been effective in limiting erosion (Figure 6). While both sites experienced some damage during Sandy, overall the impacts were relatively minor. The armoring and footprint of the parks remained unchanged with minimal damage to upland fixtures and utilities.

Figure 2 From left to right: Scenic Hudson Park prior to construction (1995), after construction (2002), and recent aerial photograph (2011). Matthiessen is in the upper center area of the photographs.

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 Several different types of structures were utilized along the Scenic Hudson Park shoreline, including a stone revetment, a riprap-lined slope, and a steel bulkhead. A boat ramp and smaller stone riprap-lined slope are located along the southern portion of the shoreline, while a bulkhead and larger stone revetment protects the more exposed northern shoreline, which experiences more intense forces from currents, wakes, and ice. Engineer-ing drawings showing the cross-sections of the bulkhead, riprap slope, and stone revetment are shown below.

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Figure 3 is a plan view of the engineering drawings for the site. The riprap slope (left) and stone revetment (right) cross-sections are shown in Figure 4. The stone sizes are much smaller in the riprap section, and the shoreline slope is much milder. No attempt was made to incorporate ecological principles in the design of the shoreline. Several photographs of the Scenic Hudson Park shoreline sections are presented in Figure 5. The left photograph shows the railing, river walk, benches, walking path, and nearshore vegetation in the northern section. The center photograph shows the section junction of the bulkhead and revetment. The right photograph is of the southern section and shows the stone revetment and decorative wooden cap that borders the entire length of the revet-ment.

Concrete rubble and stone armor the shoreline at Matthiessen Park, placed without engineering designs. Photographs in Figure 6 show views to the south (left) and north (center), and the rubble-lined shoreline (right).

Figure 3 Plan view of engineering plans for Scenic Hudson Park (Han-Padron Associates LLP).

Figure 4 Cross-sections of the stone riprap near the boat ramp (left) and the stone revetment (right) (Han-Padron Associ-ates LLP).

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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 Irvington shoreline. The conclusions of the Forensic Analysis at Irvington 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)

Figure 5 Photographs from Scenic Hudson Park: upland (left), end of bulkhead (center), and revetment (right).

Figure 6 Photographs from Matthiessen Park: south view (left), north view (center), and revetment (right).

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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 Irvington shoreline is located in a special flood hazard area (VE zone) with a base flood elevation (BFE) of 12 ft NAVD88, while the majority of the interior portions of the two parks are located in an AE zone with a BFE of 9 ft NAVD88 (panel 36119C0261G, preliminary firm release date December 8, 2014). The BFE represents the water elevation expected during the 1% annual chance of occurrence or 100-yr storm event. The BFE represents a useful baseline with which to compare both the typical and storm conditions at the site. The Sustain-able 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 was based on a one-year numerical simulation of conditions within the Hudson, generated using an ultra-high resolution version of the NYHOPS model. The climatology was devel-oped based on the conditions in 2010 and included one significant Noreaster. Maximum water levels (WLmax) of 5.14 and 5.17 ft above NAVD88 were simulated for Matthiessen Park and Scenic Hudson Park, respectively. The maximum (Hmax) and median (Hmed) wind-wave heights for the two sites were very similar, with those at Mat-thiessen Park found to be slightly higher (2.59 ft and 0.37 ft, respectively).

An analysis of the fetches at the site confirms that the site is moderate to high energy with respect to wind-waves. The relevant fetches are shown in Figure 7, where the average and maximum fetches were found to be 12,360 ft (2.3 mi) and 21,650 ft (4.1 mi), respectively. Ice information collected by the U.S. Coast Guard and compiled as part of the climatology (http://www.hrnerr.org/hudson-river-sustainable-shorelines/shorelines-engineering/ice-conditions/) indicates that the site is subjected to minimal icing. The median ice thickness (Ice tmed) observed during the ice season (December-March) was 1.6, with thicknesses up to 4.3 occurring 10% of the time (Ice t90%). Wake observations recorded over a 2-day period during the summer of 2012 and 2013 by summer students found average and maximum wakes (Hwake) of 3 and 12, respectively.

Topographic and bathymetric surveys of both sites (Figure 8) were conducted to obtain detailed information about upland elevations, nearshore slopes, and offshore depths. The depth contours at Matthiessen Park are slightly more irregular and indicate that there is a fairly shallow area off the northern edge of the park. As the bottom slopes toward the channel, there also appears to be a slight break in slope at an elevation of -12 ft NAVD88. Offshore of Scenic Hudson Park, the bottom slopes off consistently and rapidly to a depth of approxi-mately -50 ft NAVD88.

Table 1 - Climatology data two Irvington park sites.

Parameter Matthiessen Park

Scenic Hudson Park

WLmax (ft NAVD88) 5.14 5.17 Hmax (ft) 2.59 2.53 Hmed (ft) 0.37 0.30 Ice tmed (in) 1.6 Ice t90% (in) 4.3 Avg Hwake (in) 3 Max Hwake (in) 12

Figure 7 Fetch analysis at Irvington.

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H I N D C A S T 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. Simulations were produced for each of the parks, but due to their close proximity the results were virtually identical and therefore only the results from Matthiessen Park are discussed here. The hindcast water levels during both Irene and Sandy (Figure 9) significantly exceeded the 95th percentile based on the 2010 climatology. The water levels simulated during Sandy match up well with nearby watermarks collected by the USGS at Piermont (9.7 ft NAVD88) and Hastings-on-Hudson (8.9 ft NAVD88). When compared to the surrounding land elevations, which are on the order of +5 ft NAVD 88, the hindcasts indicate that the fringes of the parks were submerged during the peak of the storms. The storm surge extent mapped by FEMA after Sandy (Figure 11) confirms the hindcast. The hindcast wave heights (Figure 10) during both Irene and Sandy significantly exceeded both the 95th percentile and maximum wave heights from the 2010 climatology. While these results indicate the relative significance of these storms, in an absolute sense, wave heights of between 1.5 and 2.5 ft are generally not considered high energy.

Figure 8 Topographic and bathymetric survey at Irvington, NY (left: Matthiessen Park, right: Scenic Hudson Park).

Figure 9 Modeled water levels (ft NAVD 88) at Irvington sites during Irene, Lee, and Sandy.

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D O C U M E N T E D P E R F O R M A N C E Scenic Hudson Park The footprint and structural armoring of Scenic Hudson Park remained virtually unaffected by Superstorm Sandy. During the storm, the entire shoreline and outer fringes of the park were inundated, which likely helped to reduce the damages. The Irvington Fire Chief was on site throughout the storm and reported water as far as the railroad tracks that run parallel to the shoreline, along the western face of the property. A watermark was found in a nearby toolshed during the first site visit to support this first-hand observation. The damage that did occur within the park (Figure 12) consisted of medium to large pieces of wooden debris that were strewn along the length of the revetment and riprap (left), uprooted trees and plantings (center), and the displacement of the timber cap beam from its original location along the crest of the stone revetment (right). The majority of the timber cap remained connected, but in some places the entire beam had been pushed as far inland as anchored obstruc-tions would allow. Although there was some displacement of riprap, the March 2014 site visit identified strong root systems embedded within the riprap slope that may have helped to stabilize the shoreline. Months after the storm a secondary impact was identified, as some of the vegetation that survived Sandy subsequently died from saltwater intrusion.

Figure 10 Wave height at Irvington sites during Irene, Lee, and Sandy.

Figure 11 FEMA Sandy storm surge extent.

Figure 12 Scenic Hudson Park: Woody debris (left), displaced vegetation (center), and displaced wooden trim (right).

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Matthiessen Park

Like the Scenic Hudson Park shoreline, the Matthiessen Park shoreline was completely inundated during Sandy and was virtually unaffected by the storm. This is in spite of the fact that the Matthiessen shoreline was un-engineered. The large size of the stones and pieces of concrete debris (Figure 6) likely contributed to the ability of the shoreline to withstand the storm. The damage that did occur (Figure 13) consisted of smaller stones being dislodged and transported inland from the revetment (left), erosion of the leeside of the structure due to overtop-ping (center), and the displacement of several decorative features such as lamp poles (right). This damage occurred even though municipal officials had constructed a 2-ft dune to protect the park prior to the storm.

Figure 13 Matthiessen Park: Small stones moved inland (left), erosion from overtopping (center), and displaced lamppost upland (right).

F I N D I N G S A significant amount of engineering was applied to Scenic Hudson Park in the design phase and the structure fared incredibly well during the storm because of it. During the storm, the entire shoreline and outer fringes of the park were inundated, which likely helped to reduce the damage. The damage that did occur within the park consisted of medium to large pieces of wooden debris that were strewn along the length of the revetment and riprap, uprooted trees and plantings, and the displacement of the timber cap beam from its original location along the crest of the stone revetment. The timber cap beam was displaced during Sandy when it was floated out of place by the rising floodwaters. Since the beam appears to have been a decorative element, little effort was made to secure it to the rest of the structure. Although there was some displacement of riprap, the March 2014 site visit identified strong root systems embedded within the riprap slope that may have helped to stabilize the shoreline.

Matthiessen Park was also completely inundated during Sandy and thus virtually unaffected by the storm. Despite the fact that the Matthiessen shoreline was not engineered, the large size of the stones used to construct the armoring served to protect the shore. Damage that did occur consisted of smaller stones being dislodged and transported inland from the revetment, erosion of the lee side of the structure due to overtopping, and the displacement of several decorative features such as lamp poles. Municipal officials constructed a 2-ft dune behind the revetment just prior to the storm, and it is likely that this feature played a significant role in the prevention of additional leeside scour.