Forensic Analysis: Common Project Performance Factors

BACKGROUND

The intent of the forensic analysis was to investigate the

site conditions of six engineered shorelines that were

impacted by Tropical Storms Irene and Lee in 2011 and

Post-Tropical Storm Sandy in 2012. The selected pro-

jects included both traditional and non-traditional

shoreline stabilization, and included both projects that

were significantly damaged, as well as those with only

minimal damage. The objective was to identify the crit-

ical factors that determined the success or failure of the

projects. Project objectives included; determining pat-

terns among structures that survived (minimal to no

damage, the structures functionality was not compro-

mised) and those that failed (moderate to ultimate dam-

age, structures functionality was compromised); deter-

mining which aspects of structural maintenance lead to

the failure/survival; determining impacts from extreme waves, water levels, and increased

currents; and determining the impact of vegetation on structure failure/survival. The final

project locations were selected from a list of 20 potential sites: with ease of access, availability

of data and stakeholder input being strongly considered. The six sites that were selected were;

Coxsackie Boat Launch, Coxsackie, NY; Esopus Meadows Preserve, Esopus, NY; Oak Point,

Bronx, NY; Hunts Point Landing, Bronx, NY; Habirshaw Park, Yonkers, NY; and Matthiessen

Park/Scenic Hudson Park, Irvington, NY. The Coxsackie, Habirshaw, Esopus Meadows, and

Irvington sites experienced minimal damage from the extreme storm events. Hunts Point

Landing and Oak Point in the Bronx were more severely impacted. Tools used in the analysis

included historic aerial photographs, topographic maps, site photographs, engineering plans,

correspondence with site planners, site visits, topographic/bathymetric surveys, and detailed

Author: Jon Miller

Researcher: Andrew Rella

Project Manager: Jon Miller

Davidson Laboratory

Stevens Inst. 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 Envi-

ronmental Conservation Hud-

son River National Estuarine

Research Reserve, in coopera-

tion with the Greenway Con-

servancy for the Hudson River

Valley.

The Project is supported by

NOAA through the National Es-

tuarine Research Reserve Sys-

tem 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

FORENSIC ANALYSIS: COMMON PROJECT PERFORMANCE FACTORS

Figure 1 - Map showing site locations.

hydrodynamic modeling that was used to establish both the typical and storm conditions at each site. This

data was compiled and was used to create a holistic picture of each site including the background and storm

conditions. This evidence was used to develop conclusions based on engineering judgement as to why

each project performed the way it did. Separate reports have be generated that describe the evidence and

conclusions for each individual site. This document summarizes the common themes that were identified

through these analyses, and presents some recommendations for improving regulation, design, and con-

struction of future projects.

While there were a number of site specific factors that led to the poor performance of some of the projects,

there were some common factors that were identified. These included:

Impacts from debris during storm events

Use of undersized and/or not properly graded stone

Use of improper slopes

Impacts from ice

In addition, although it was not observed directly at any of the sites, the issue of increased erosion on the

leeside of structures from when floodwaters receded was identified by the Sustainable Shorelines Forensic

Analysis Technical Advisory Panel as potentially significant during Sandy and a cause of concern for future

designs. As with the factors related to poor performance, there were a mixture of site specific and general

factors that were associated with strong project performance. Some of the general factors included:

Regular inspections and maintenance

The willingness and ability to use an adaptive management approach to correct identified defi-

ciencies

Proper slopes and stone sizing

Maturity of vegetation

These general performance factors are discussed in more detail below, followed by a set of recommenda-

tion based on observations made during the forensic analysis.

MATURITY OF VEGETATION

The maturity of the vegetation at several of the sites was determined to be a critical factor in the survival

of plantings and the overall resilience of the shorelines. The clearest evidence is provided by the Esopus

Meadows site which experienced heavy damage and erosion during a spring storm in April 2007. The

storm occurred less than one year after the projects completion and before the vegetation had time to ma-

ture. Much of the smaller vegetation was uprooted, while some of the larger vegetation was rearranged as

it slid down slope during the storm. The decision was made to work with nature, and the project was

repaired by reinforcing the storm modified slope with a biodegradable mat, secured with new plantings.

The modified project was thriving when Irene, Lee, and Sandy occurred and experienced only minimal

damage during the storms. The maturity of the vegetation (combined with the modified slope) is believed

to be responsible for the resilience of the site. Hindcasts were not performed for the April 2007 storm;

however, in all likelihood, the conditions during the storm did not exceed those experienced during Irene,

Lee, or Sandy. It should be noted that the site also withstood the March 2010 Noreaster which was a storm

similar to the April 2007 storm.

Mature vegetation also likely helped to stabilize the slopes at Habirshaw Park and Coxsackie Boat Launch

during Post-Tropical Storm Sandy; however, both sites were submerged at the storms peak, reducing the

amount of time the vegetation was exposed to the most damaging conditions. Conversely, the immature

vegetation at Oak Point and Hunts Point provided little resistance to the erosional forces experienced at

those sites.

SLOPE COMPATABILITY

Oversteepened slopes were determined to be one of

the primary factors responsible for the amount of

damage that occured to the Oak Point project during

Sandy. At Oak Point the steep slopes were dictated

to some extent by regulatory requirements. NY State

Department of Environmental Conservation

regulations prohibit filling below the mean high

water line unless a reasonable and neccesary

criteria is met. Flood insurance requirements on the

other hand dictate that the site elevation exceed 13 ft

NAVD 88 (elevation of the neighboring coastal A

zones). Traditional wetland slopes on the order of 1

(Vertical) on 10 (Horizontal) dictate a horzontal

distance of at least 130 feet to achieve the required

Figure 2 - Esopus Meadows site after 2007 storm (Scenic Hudson).

Figure 3 - Esopus Meadows site after Sandy, photo taken on November 3, 2012 (Creative Habitat Corp.).

Figure 4 - Oversteep slope at Oak Point.

elevation change, however some parts of the Oak Point site have less than 30 horizontal feet. As a result

some sections of the Oak Point site were constructed with slopes as steep as 1 on 2. Such steep slopes make

the site extremely vulnerable to erosion not only from waves and storms, but also from runoff (which has

also been identified as a problem). During Sandy, the steep slopes were impacted by a combination of

large waves and extreme amounts of heavy debris that combined to scour away soil and vegetation. While

the upland area was largely protected due to its elevation, the wetlands restoration project at the site was

nearly completely destroyed.

Esopus Meadows, Coxsackie, and Hunts Point are

three sites where the survivial of the vegetation can

be at least partly accredited to mild sloping

shorelines. As discussed above, at Esopus

Meadows a spring Noreaster less than a year after

the projects consruction significantly damaged the

original project. The milder, storm-modified slope

was incorporated into the revised project design

and successfully resisted the three histroic storm

events. The Esopus Meadows project is discussed

further in the Adaptive Management section of this

report. The Hunts Point Landing and Habirshaw

sites are additional examples of locations where

mild slopes may have minimized damage during

the three historic storms. Each site benefits from

being in a location where the horizontal space

constraints faced at Oak Point are less of an issue. As a result, more natural slopes on the order of 1 on 10

were used at each of those sites. During the historic storms, there is a tendency for the mild sloping sites

to be innundated and/or have any debris ride up, rather than gouge into the slope. Although not the focus

of this study, the mild slopes likely play a similar role in limiting the damage due to ice.

DEBRIS IMPACT

During Post-Tropical Storm Sandy, erosion and scour caused by large floating debris was determined to

be the primary cause of damage at several of the sites. This was particularly true for the Oak Point and

Hunts Point sites. At both of these locations, even during non-storm conditions debris accumulation is

common. Particularly in urban areas, debris removal can be a common and costly component of the mainte-

nance associated with shoreline projects (Habirshaw Park is another example). During Sandy, the flooding

and storm damage that occurred throughout the region resulted in an extensive amount of floating debris

entering the water. As a result of the intensity of the storm, a significant amount of this debris consisted of

large, heavy objects that when thrust against an unprotected embankment, were capable of causing severe

damage. At Oak Point, the amount of debris which had to be removed from the site after the storm pro-

vided direct evidence of the nature of the conditions experienced. The coved nature of the shoreline at Oak

Point is particularly conducive to the collection of debris. Once inside the cove, the debris becomes trapped

Figure 5 - Naturally sloping shoreline at Habirshaw Park in June 2004 (Photo by: Sven Hoeger).

and the shoreline is abraded continuously over the duration of the storm. At Hunts Point there was less

direct evidence, but plenty of indirect evidence as to the impact of debris during Sandy. While all of the

structural elements of the Hunts Point project survived Sandy intact, much of the vegetation and many of

the ancillary project elements were damaged. Damage to sturdy ornamental project features such as water

fountains and fences provide evidence as to the power and destructive capabilities of the debris laden flow.

It is believed that the sloping terraced nature of the site, combined with the appropriately sized and placed

structural elements limited erosion and damage to the structural features at the Hunts Point site.

LEESIDE EROSION AND IMPACTS

Although leeside erosion was not observed to be a

factor at any of the sites visited for the forensic anal-

ysis, members of the technical advisory panel iden-

tified it as a primary cause of shoreline structure

damage at other Hudson River locations. Leeside

erosion occurs as water and/or waves overtop a

structure and erode the sediment behind the struc-

ture. In areas where overtopping is expected fre-

quently, splash pads (typically concrete pads de-

signed to prevent erosion behind a structure) are of-

ten included as a part of the design to reduce the po-

tential for leeside scour. In areas where overtopping

is not expected, specifications are typically provided

as to the type and placement of backfill and/or veg-

etation, to stabilize the area behind the structure,

Figure 6 - Debris at Oak Point after Sandy (NY Times).

Figure 7 - Damaged fence at the Hunts Point site af-ter Sandy.

Figure 6 - Receding flood water during Sandy.

even though leeside scour is not a particular concern. For

a properly designed structure, leeside erosion tends to be

fairly intermittent and can typically be addressed during

routine maintenance. During Post-Tropical Storm Sandy

leeside scour depressions created when structures were

overtopped, interacted with debris laden return flow as

the surge subsided, creating a secondary hazard typically

not accounted for in design. This was observed by the

Technical Advisory Panel at several sites within the Hud-

son not included in the Forensic Analysis. The scour de-

pressions created a flow path which allowed fast-mov-

ing, debris-laden water to impact the leeside of struc-

tures, effectively dragging them back into the water.

Generally coastal structures are not designed to resist these types of dynamic forces on the backside of the

structure.

MAINTENANCE AND ADAPTIVE MANAGEMENT

Maintenance and adaptive management were identi-

fied as critical factors contributing to the long-term re-

silience of several of the projects. Typical maintenance

requirements consist of removing debris and inspect-

ing and replanting vegetation. Oftentimes this mainte-

nance can be carried out by volunteer groups during

river clean-ups, service projects, etc. An effective

maintenance plan also includes regular inspections.

Regular inspections can identify deficiencies which can

often be corrected before an entire project is compro-

mised. Two of the projects selected for the forensic

analysis provide examples of effective adaptive man-

agement. The previously mentioned Esopus Meadows

project required adaptive management after a spring

Noreaster significantly damaged the original project.

Rather than restore the project to its original design, an adaptive management approach was pursued in

which the shoreline slope was reduced and secured with an erosion control mat and appropriate vegeta-

tion. The modified project has been successful and the shoreline has remained stable through several large

storm events, and icy winters. At Habirshaw Park, site inspections revealed that the original stone sill was

incapable of protecting the marsh and tide pool features behind it. This was attributed to the size of the

stones being too small and the elevation of the sill being too low. An innovative, low cost solution that

capitalized on the available material and labor was to create a gabion using the original sill stones and a

net to secure them (Figure 10). The modified sill has been successful at protecting the marsh and vegetation,

and has survived Sandy and several icy winters.

Figure 8 - Adaptive management of Habirshaw Park site in 2008 (Photo by: Sven Hoeger).

Figure 7 - Typical leeside erosion.

Several of the projects visited during the spring of 2014

including Habirshaw Park are currently in need of

maintenance. At Habirshaw Park, the net that was used

to encapsulate the sill stones is beginning to show signs

of degradation. (Figure 11). In its current condition, the

sill is still effective; however if the deficiency is not ad-

dressed in the near future, the net may tear completely

and the sill will revert back to its original ineffective

state. At the Coxsackie site some of the stones have be-

come dispersed across the site and invasive vegetation

has encroached on the project. The movement of the

stones is likely related to both natural (ice impact) and

human activity (dumping of snow/ice at the site). In its

current state, the project still appears to be effective at

reducing/eliminating the erosion along the edge of the

parking lot. It is recommended that the site be inspected on a regular basis and if the erosion resumes that

the displaced stones and vegetation be replaced.

ADEQUATE STONE SIZING

Stone sizing plays an important role in the survivability of shoreline projects because it is generally the size

and the weight of the stone (or other armoring element) that provides the resistance to erosion. During the

forensic analysis, inadequate stone sizing was identified as a factor limiting the current and future protec-

tion provided by several of the projects. At Habirshaw Park, stones existing onsite were used to create the

original sill/breakwater structure, but were found to be unstable when placed directly on the shoreline.

Fortunately, the site was monitored and the deficiency was recognized early enough that corrective actions

could be taken. More detail is provided above in the Maintenance and Adaptive Management section, but the

creative solution that was found was to bind the stones together to form a low-tech gabion. Gabions effec-

tively allow many small stones to function like larger stones, and are frequently used when large stones

are not available.

During the site visit to Coxsackie in the spring of 2014, stone sizing was identified as a potential problem.

While the sill/terrace structure at Coxsackie survived Sandy, it appears as though ice and/or human im-

pacts (dumping of ice/snow) during the winter of 2013-2014 have displaced several of the smaller stones.

The original design for Coxsackie called for 2 foot diameter stone; however many of the displaced stones

have a substantially smaller diameter (some as small as 6 inches). Although the design for Coxsackie may

have been sufficient, the stones that were ultimately utilized appear to have been too small.

At the two Irvington sites, Sandy exposed a problem with the gradation of the material used to create the

rubble revetment. The larger armor elements (mostly rock, concrete, and brick) in the revetment were

stable; however some of the smaller underlayer material washed out during the storm. An appropriately

engineered and constructed revetment is designed and built in such a way that the finer material at the

Figure 9 - May 2014 state of Habirshaw Park gabion (Photo by: Omar Lopez May 2014).

core of the structure will not wash out through the spaces between the larger armor stones on the surface.

If the core material is washed out, eventually the revetment will collapse on itself and slump, reducing its

effectiveness in protecting the shoreline.

RECOMMENDATIONS

In light of the conclusions reached during the forensic analyses, the following recommendations are made

for improving the storm resilience of future shoreline stabilization projects.

1. More research needs to be done on the performance of ecologically enhanced stabilization ap-

proaches during heavy ice and debris conditions. Currently only minimal engineering design

guidance exists on the consideration of debris and ice forces on structures. The guidance is even

more limited for non-traditional structures. The importance of debris and ice loads are expected

to be more significant as we move away from traditional hard structures towards ecologically en-

hanced shorelines that tend to be softer. Research focused on replacing several existing rule of

thumb relationships with more physically-based design approaches should be emphasized.

2. More research needs to be done in the area of plant material selection. Research should be con-

ducted on the type and placement of vegetation for optimal stability. This is critically important

for shoreline stabilization approaches that rely on vegetation to provide resistance to erosion. In

particular, information on the growth of root systems and how they influence the stability of the

substrate and structural components of hybrid stabilization approaches is lacking.

3. Proper monitoring and maintenance are important to the long-term performance of all projects;

however, it is critical for ecologically enhanced shoreline projects. Adaptive management was

identified as a critical factor in the performance of several of the projects analyzed. The inclusion

of maintenance plans as a part of the design and funding of ecologically enhanced shoreline pro-

jects is suggested. The existing regulations need to be examined, and if necessary modified to allow

Figure 10 - Some of the larger (foreground) and smaller (background) stones at Coxsackie.

Figure 11 - Fine material washed out of the Irving-ton revetment in March 2014.

for maintenance and if necessary adaptive management of ecologically enhance shoreline stabili-

zation projects.

4. Temporary stabilization measures should be provided to allow vegetation to mature. Evidence

collected during this project is consistent with the previous work that indicates that the maturity

of the vegetated components of shoreline stabilization projects plays an important role in deter-

mining their resistance to scour/erosion. Appropriate measures should be taken to protect newly

planted vegetation until it reaches maturity. The type and durability of any temporary protection

measure should be based on the physical conditions expected at the site and the growth rate of the

vegetation. Coir logs are an example of a temporary protection measure that could be used if the

shoreline slope and wave conditions allow.

5. Terracing or other measures should be used to avoid unnatural slopes. Different types of vege-

tation have different preferred growing conditions, with one of the important factors being slope.

When placed on unnaturally steep slopes, wetland vegetation typically does not grow well and is

extremely vulnerable to scour and erosion. Several sites analyzed for this project effectively uti-

lized terraces as a means to achieve the required vertical elevation difference, while also preserving

natural habitat slopes. Terraces are also effective at reducing the potential damage from debris

and ice. The milder slopes of a terrace encourage debris and ice to ride up the slope rather than

cut into the slope as is common on steep shorelines.

6. Leeside forces should be addressed in design/construction of coastal structures. Although

leeside scour and leeside debris flow impacts were not identified as a dominant cause of erosion

for any of the sites considered in the forensic analysis, they were identified by the Technical Advi-

sory Panel as a major problem elsewhere within the Hudson River. It is recommended that these

potentially important forces receive more attention during the design and construction of future

projects. While more research needs to be done to determine the exact magnitude of some of these

leeside forces, there are existing techniques that can be adapted and utilized in situations where

overtopping and leeside scour are expected.

MAturity of VegetationSlope CompatabilityLeeside Erosion And ImpactsMaintenance and Adaptive ManagementAdequate Stone Sizing

Forensic Analysis: Common Project Performance Factors

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