ExEcutivE Summary - Feasibility of Harbor-wide Barrier Systems

EXECUTIVE SUMMARY

Feasibility of

Harbor-wide Barrier

Systems

Preliminary Analysis

for Boston Harbor

Sustainable

Solutions Lab

PRINCIPLE INVESTIGATOR

Paul Kirshen

Professor, School for the Environment;

Academic Director, Sustainable Solutions Lab

PROJECT TEAM

Arcadis

Kelli urson, Brett McMann, Carly Foster,

Heather Sprague and Hugh Roberts

UMass Boston School for the Environment

Mark Borrelli, Jarrett Byrnes, Robert Chen,

Lucy Lockwood, Chris Watson

UMass Boston Urban Harbors Institute

Kimberly Starbuck, Jack Wiggin, Allison Novelly,

Kristin Uiterwyk

Woods Hole Group

Kirk Bosma, Eric Holmes, Zach Stromer,

Joe Famely, Alex Shaw, Brittany Honagle

Woods Hole Oceanographic Institute

Di Jin

PROJECT MANAGER

Rebecca Herst

Director, Sustainable Solutions Lab

PROJECT SUPPORT

Emily Moothart

Climate Resilience Research Assistant,

Sustainable Solutions Lab

Courtney Humphries

PhD Student, IGERT Coasts and Communities Fellow

Robert L. Turner

Senior Fellow, McCormack Graduate School

STEERING COMMITTEE

Boston Harbor Now

Boston Green Ribbon Commission

Boston Planning and Development Agency

City of Boston, Environment Department

Massachusetts Area Planning Council

Massachusetts Department of Environmental Protection

Massachusetts Environmental Policy Act Oce

Massachusetts Executive Oce of Energy and

Environmental Aairs

Massachusetts Oce of Coastal Zone Management

MassBays National Estuary Program

Massport

North Cambridge Consulting

National Parks Service

New England Aquarium

United States Army Corps of Engineers

REVIEWERS

Boston Green Ribbon Commission – Bud Ris

Boston Harbor Now – Jill Valdes Horwood

City of Boston, Environment Department – Mia Manseld, Carl Spector

GZA GeoEnvironmental, Inc. – Chad Cox, Stephen Lecco,

Daniel Stapleton, Bin Wang

Massachusetts Oce of Coastal Zone Management

MassBays National Estuary Program – Carole McCauley

MassPort – Michael Meyran

New England Aquarium – John Mandelman

Stevens Institute of Technology – Philip Orton

Stony Brook University School of Marine and Atmospheric Sciences –

Malcolm Bowman

Tetratech – Bob Daylor, Jason Hellendrung, Mark Williams

UMass Boston – Ellen Douglas

INTERNATIONAL FEEDBACK

Deltares – Martijn de Jong

Acknowledgements

ank you to everyone who was interviewed or who provided

feedback to this project. We are particularly indebted to the

leadership and eorts of Bud Ris.

is report is sponsored by the Boston Green Ribbon Commission

with generous support from the Barr Foundation. It was conducted

by the Sustainable Solutions Lab at the University of Massachusetts

Boston. It is not an ocial document of the City of Boston or the

Climate Ready Boston initiative.

ese are preliminary recommendations from the authors

of this report and do not represent the views of particular City

of Boston agencies, the sponsors of the report or the University

of Massachusetts, Boston.

To read the full report please visit www.umb.edu/ssl/activities.

MAY 2018

Executive Summary: Feasibility of Harbor-wide Barrier Systems

Sustainable Solutions Lab

e Sustainable Solutions Lab (SSL) is an

interdisciplinary partnership among four

schools within UMass Boston: e College

of Liberal Arts, College of Management,

McCormack Graduate School of Policy and

Global Studies, and School for the Environ-

ment. SSL’s mission is to work as an engine

of research and action to ensure that all

residents of Greater Boston, and cities

across the world, are prepared equitably

for the impacts of climate change.

UMass Boston

e University of Massachusetts Boston is

a public research university with a dynamic

culture of teaching and learning, and a

special commitment to urban and global

engagement. Our vibrant, multicultural

educational environment encourages our

broadly diverse campus community to

thrive and succeed. Our distinguished

scholarship, dedicated teaching, and engaged

public service are mutually reinforcing,

creating new knowledge while serving the

public good of our city, our commonwealth,

our nation and our world.

Preliminary Analysis for Boston Harbor

Feasibility of Harbor-wide Barrier Systems

Executive

Summary

Shore-based climate adaptation

solutions have signicant

advantages over harbor-wide

strategies for Boston.

Preliminary Analysis for Boston Harbor

The aim of this study is to provide the City

of Boston with a preliminary assessment of

the feasibilities and potential benets, costs,

and environmental impacts of three harbor-

wide barrier congurations.

s sea levels rise and climate change

poses a growing threat, Boston and

neighboring cities and towns along

Boston Harbor and the Massachusetts

coastline need to prepare. In 2016,

the City of Boston began organizing a citywide

response to climate change called Climate Ready

Boston. This project included detailed climate

change projections, a vulnerability assessment,

and proposals for adapting to climate change and

increasing the resilience of the city to sea level

rise, heat stress, and increased precipitation.

One of the recommendations from this project

was to launch a feasibility study for a harbor-wide

ood protection system (Strategy 5.4). This study

on barriers, sponsored by the Green Ribbon

Commission in support of its partnership with

the City of Boston on Climate Ready Boston,

responds to that recommendation. It was funded

by the Barr Foundation.

The aim of this study is to provide the City

of Boston with a preliminary assessment of the

feasibilities and potential benets, costs, and

environmental impacts of three harbor-wide barrier

congurations. The analysis was conducted by a

multidisciplinary team of environmental scientists,

engineers, economists, planners, and lawyers,

drawing upon a wide range of data about engi-

neered ood protection systems, climate change,

coastal ecosystems, and economic impacts of

ooding. We focused on barrier designs and

congurations that would offer protection from

coastal ooding while minimizing interference with

Boston’s main shipping channels and the gains

that have been made in water quality over the

last several decades. We also examined poten-

tial conicts with various harbor uses, and con-

ducted a preliminary comparison with shore-based

adaptation solutions (which include district-level

ood barriers as well as other structural and non-

structural actions), such as those already being

investigated by the City of Boston along the inner

harbor waterfront in East Boston, Charlestown,

and South Boston. The detailed technical report

contains more analysis of the issues summarized

below. Because many of the results of a section

depend upon results from preceding sections,

it is recommended that the sections be read

sequentially.

This analysis yielded these key ndings:

• Thetwomostreasonableoptionsforabarrier

system are an Outer Harbor Barrier (OHB) from

Winthrop to Hull and an Inner Harbor Barrier

(IHB) between Logan Airport and the Seaport

area of South Boston. Each would be a gated

barrier system that would only be closed during

ood conditions caused by storm surge exceed-

ing shoreline levels of ood protection.

• Eitherbarriersystemwiththegatesopenwould

not attenuate the tidal range in the harbor thus

not decreasing tidal ooding (“nuisance ood-

ing”) and also not causing major environmental

impacts compared to the expected changes

due to climate change and sea level rise.

• Intheearlyyearsofoperation,thefrequency

of closure of a barrier would be no more than a

few times per year. Because of rising sea levels,

and assuming the system was designed to be

closed each time the water level is above the

level of protection provided by shore-based

measures, after 50–60 years the frequency of

closure would likely increase so much that the

barrier could no longer function as designed.

• Neitherbarriersystemappearstobecost-

effective. Depending upon assumptions made

on levels of shore line protection and discount

rates and assuming shore-based adaptation is

effective against storm surges, the benet:cost

ratios range from 0.05 to 1.69 with most being

well less than 1.0. This is very unfavorable com-

pared to benet:cost ratios of recently designed

shore-based systems in Boston of 3.22 to 5.3.

• Theanticipatedincreasedwatervelocityin

the barrier openings could cause navigational

and safety issues for both recreational and

commercial vessels near the barrier openings.

The Outer Harbor Barrier could also impact

the abundance, distribution, and behavior of

sh populations, which would in turn impact

both commercial and recreational sheries.

• Thepercentageofsociallyvulnerablepeople

who would remain vulnerable to ooding in the

case of either an IHB or an OHB being built

is the same as that of the total population

in all of Boston. That is, socially vulnerable

populations would not have disproportionate

ooding after an IHB or an OHB was built.

Photo opposite: © iStockphoto/Ken Weidemann

Feasibility of Harbor-wide Barrier Systems

• Shore-basedsystems,includingarangeof

measures from zoning to various kinds of green

and gray protective systems deployed along

the waterfront of the inner harbor, offer many

advantages over harbor-wide barrier systems.

These include cost-effectiveness, community

co-benets, adaptability to changing conditions

over time, and protection against tidal ooding

as well as surge ooding. If over time the per-

formance and implementation of shore-based

systems lag, then decisions about barriers

must be re-revaluated.

international community is able to curb global

emissions, compared to 2013 the Boston area

could experience 6 inches to 1.2 feet of relative

SLR by 2050, and 1.8 to 7.3 feet by 2100.

Changes in the future intensity and frequency

of extratropical storms (nor’easters) are uncer-

tain; there is more certainty, however, that the

intensity of tropical storms (hurricanes) may

increase. Even if the region does not see an

increase in storm intensity, the storms that do

occur will cause more ooding when combined

with sea level rise. The biggest unknown in these

projections—the reason why the ranges are so

broad later on in the 21st century—is the amount

of greenhouse gas reduction that will be achieved.

If the global community is able to dramatically

decrease emissions of the greenhouse gases

that cause climate change, the amount of SLR

that Boston will experience can be constrained

to the lower end of the future projections, thereby

decreasing the number of adaptation measures

that will be necessary over time.

Given the preliminary nature of this analysis,

only one scenario of sea level rise and associated

ooding was analyzed compared to 2013. This

is approximately 1 foot of relative SLR by 2030,

3 feet by 2070, and 5 feet by 2100. This is

approximately equivalent to the IPCC RCP4.5

sea level rise scenario, a moderate scenario.

Figure ES.1 shows the present extent of coastal

ooding in Boston Harbor. Figure ES.2 (p. 8)

shows the estimated extent of coastal ooding

with 5 feet of SLR.

Possible Barrier Congurations

This analysis assumes that the goal is not only

to provide ood protection from storm surge to

Boston and neighboring cities and towns along

Boston Harbor, but also to maintain present and

future commercial shipping and other navigation,

and to preserve as much as possible the present

ecological services of Boston Harbor in light of

climate change. Commercial and recreational

navigation is critical to Boston’s historical iden-

tity as a maritime city and to its current economy.

Likewise, hard-won environmental improvements

in Boston Harbor over the past few decades have

provided great benets to the city and its natural

resources. It is worth noting that the project

team considered evaluating in detail a Metro Dike

Barrier which would be an arc in deep water from

Swampscott to Cohasset (see Figure ES.3. p. 9).

This system would have locks that would create

a major impediment to trafc in and out of the

If the global community is able to dramatically

decrease emissions of the greenhouse gases

that cause climate change, the amount of SLR

that Boston will experience can be constrained

to the lower end of the future projections,

thereby decreasing the number of adaptation

measures that will be necessary over time.

KEY RECOMMENDATION

While this study is not comprehensive, and there

are many ways that further research could rene

and extend its ndings, those ndings were clear

enough to justify making recommendations for

next steps. The authors recommend that the City

continue to focus its climate resilience strategy

for the next several decades on the shore-based

multi-layered approach described in Climate Ready

Boston. Shore-based solutions would provide ood

management more quickly at a lower cost, offer

several key advantages over a harbor-wide barrier,

and provide more exibility in adapting and respond-

ing to changing conditions, technological innovations,

and new information about global sea level rise.

These shore-based solutions would be needed

in any case over the next few decades to manage

coastal ooding during the design and construc-

tion period of a harbor-wide barrier if a decision

was made to build one in the future.

Climate Context

The climate projection consensus for Boston

developed by the Boston Research Advisory Group

in 2016 as part of Climate Ready Boston looked

at extreme heat and cold, sea level rise (SLR),

extreme precipitation, drought, and coastal storms

for the region. Depending on how effectively the

Preliminary Analysis for Boston Harbor

FIGURE ES.1

Boston Harbor—Probabilities of Flooding with O Feet SLR

BH-FRM Flood

Probabilities 0 ft SLR

10% Flood Extent

2% Flood Extent

1% Flood Extent

0.1% Flood Extent

Legend

Project Bounds

Study Area Municipalities

Amelia Earhart Dam

Charles River Dam

Neponset River Dam

Sources: MassGIS, UMass Boston, Woods Hole Group, Esri

2.5 1.25 0 2.5 Miles

Feasibility of Harbor-wide Barrier Systems

FIGURE ES.2

Boston Harbor—Probabilities of Flooding with 5 Feet SLR

Source: MassGIS, UMass Boston, Woods Hole Group, Esri

2.5 1.25 0 2.5 Miles

Legend

Project Bounds

Study Area Municipalities

Amelia Earhart Dam

Charles River Dam

Neponset River Dam

BH–FRM Flood

Probabilities 5 ft SLR

10% Flood Extent

2% Flood Extent

1% Flood Extent

0.1% Flood Extent

Preliminary Analysis for Boston Harbor

harbor and hamper water exchange and, as

a result, did not meet the criteria of minimally

impacting shipping and navigation and ecological

services. In addition, this system would be very

expensive ($35–$85 billion) and difcult to

construct. Therefore, this conguration was

dismissed early in the project.

This analysis looked at two main options

similar to those proposed by Climate Ready

Boston (2106) (see Figure ES.3):

1. The Outer Harbor Barrier (OHB), a gated barrier

system that would only be closed during ood

conditions caused by storm surge; the OHB

would cover 3.8 miles from Winthrop to Hull,

with additional 9.3 miles of shore-based pro-

tection in Hull, Winthrop, and Revere to prevent

oods from anking the barrier from the ocean.

2. The Inner Harbor Barrier (IHB), a gated barrier

system that would only be closed during ood

conditions caused by storm surge; the IHB

would be in the passage between Logan Airport

and the Seaport area of South Boston. It would

require approximately 18 miles of shore-based

protection systems to its north and south.

This conguration assumes that the barrier

and shore-based system could be designed

for compatibility with Logan Airport operations.

The largest of the two gates of the OHB consid-

ered for this study would be the largest built thus

far and its in-water span length the longest in the

world. Opening and closing gates of these types

of barriers is a cumbersome process that takes

several hours. The gates of these types of barriers

are designed for a small number of closures over

a year or more, and with SLR would be closed more

frequently. For example, the gates discussed in

this analysis are similar in scale to the Maeslant

Barrier protecting Rotterdam. It was designed for

a closure frequency of approximately once every

10 years. Studies suggest that rising sea levels

could increase its closure frequency to once

every 3.2 years in 2050 and once every 1.1

years in 2100.

Conceptual Designs and Costs

OUTER HARBOR BARRIER

We chose a conguration for the OHB that would

make use of Lovells, Gallops, and Georges Islands

and stretches of shallow water, minimizing materials

needed for construction, and avoiding impacts to

shipping channels. It would have two oating leaf

sector gates; the northern one in the President

Roads navigation channel with an average low

tide depth of 35 feet (soon to be dredged to 45

to 51 feet), and the southern one in the Nantasket

Roads channel with an average low tide depth of

32 feet. Each oating leaf sector gate consists of

two leaves that are closed only during storms. The

total width of the northern barrier would be 1500

feet—making it the largest gate system of this type

yet constructed—and the width of the southern

barrier would be 650 feet. This design is based

on the minimum navigation size according to the

US Army Corps of Engineers. Vertical lift gates

(smaller, non-navigable openings that can be shut

during storms but allow some tidal exchange when

open) would also be built into the barrier to miti-

gate some of the localized negative water quality

impacts. Since securing enough clean and com-

patible sediment to build a natural barrier would

FIGURE ES.3

Barrier Alternatives in Boston Harbor

Legend

Inner Harbor Barrier

Outer Harbor Barrier

Both Inner and Outer

Harbor Barriers

Metro Boston Dike Barrier

0 6,500 13,000 Feet

Sources: Arcadis, Esri World Imagery

Feasibility of Harbor-wide Barrier Systems

be a challenge, the barrier would be constructed

of gray (e.g., concrete and steel) features and

then could be “greened” (covered to form a core

of an island or land mass) over time as additional

funds and sediments were identied. It would

be possible to increase the height of the barrier

if necessary after it was constructed, but not

the height of the sector gates. As a result, this

solution is not fully adaptable to the uncertainties

of sea level rise.

$8.7 billion (2017 dollars), including the many

miles of structures and berms needed to prevent

anking. Approximately 60% of the cost is the

oating sector gate. Annual operation and main-

tenance costs are estimated at approximately

1% of total construction costs. Given the proximity

to Logan Airport, Massport and FAA regulations

governing the air space around the airport must

be considered as well. Preliminary analysis indi-

cates that the height of the barrier is likely less

than the air-space requirements at this location,

but this aspect will require further investigation

if more detailed planning and design for a barrier

are ever pursued for this site.

Hydrodynamic Analysis

We applied the Boston Harbor Flood Risk Model

(BH-FRM), used in both the Boston Central Artery/

Tunnel project and Climate Ready Boston, to deter-

mine hydrodynamic conditions with and without

harbor-wide barriers. Conditions were analyzed

for relative SLR scenarios of 0, 1, 3, and 5 feet

since 2013. The 1 and 3 feet scenarios are ap-

proximately the same as used in the vulnerability

assessment conducted for Climate Ready Boston.

TIDAL ATTENUATION

One of the key questions this research sought

to answer was whether building a barrier in the

harbor would impact the tides, as well as provide

protection from storm surge. Would it be possible

to lower the high tide, and as a result, protect

the waterfront from tidal ooding exacerbated by

SLR and moderate storm surge ooding for the

medium term without even closing the gates?

Because the openings are so large, the modeling

indicated that there would be no tidal attenuation

caused by the gate openings in either the OHB or

the IHB. Thus a barrier would not protect Boston

Harbor from nuisance ooding associated with

sea level rise and normal tidal cycles without

closure of the gates.

Since there is no tidal attenuation, the quan-

tity of water entering and leaving the harbor during

tide conditions would not change signicantly.

The openings through which the water would ow,

however, would be much smaller. As a result, sig-

nicant changes in current velocities in the vicinities

of the OHB gates openings would be expected. At

normal ood tide, the peak velocity through the

northern gate could increase from approximately

2 feet per second to 5 feet per second (1.2 knots

to 3 knots). For the southern gate, the peak velocity

could increase approximately 2 feet per second to

The modeling indicated that there would be no

tidal attenuation caused by the gate openings

in either the OHB or the IHB. Thus a barrier

would not protect Boston Harbor from

nuisance ooding associated with sea level

rise and normal tidal cycles without closure

of the gates.

Total design, engineering, permitting, and

construction costs could range from $8.0–$11.8

billion (2017 dollars) with annual operation and

maintenance costs estimated at approximately

1% of total construction costs. Over 60% of the

costs are for the oating sector gates. Given the

extensive time to design, permit, nance, and con-

struct the project, including the several miles of

structures and berms needed to prevent anking

of the barrier to the north and south, the earliest

it could be functioning would likely be 2050.

INNER HARBOR BARRIER

The main channel at the location of the IHB is

approximately 1,200 feet wide with depths of

approximately 35 to 40 feet. This would be span-

ned by one large oating leaf sector gate and

articial islands to support the leaves when the

gate is open. No vertical lift gates would be needed.

Pumps would be needed at the IHB to adequately

control upstream freshwater levels during times

when the IHB is closed because of a storm surge,

as the closed gate would block the egress of ood

water from the inner harbor. The pumps would

maintain the water elevation inside the barrier

with the goal of allowing the Charles River and

Amelia Earhart dams not to close or pump, or

at least to pump less frequently.

Total design, engineering, permitting, and

construction costs could range from $6.5–

Preliminary Analysis for Boston Harbor

8 feet per second (1.2 knots to 4.8 knots). These

high velocities would make navigation challenging

for certain vessels. Therefore, it is unlikely that

entry and exit into the harbor would be available

throughout the entire tidal cycle, especially for

recreational boats with limited power. At the same

time, some new zones of stagnation in the harbor

would be expected.

In our analysis, we found there were no differ-

ences in circulation dynamics outside of the OHB

when the barrier was open under normal tidal

conditions compared to present circulation. With

the gates closed during storms, however, local

circulation dynamics outside of the barrier would

change. In particular, the ood tidal currents with

the gates closed during storms could be perpen-

dicular to the coast of Hull instead of generally

parallel now—potentially increasing erosion on

the Hull coastline.

The IHB would have minimal impact on the

tides and currents in the harbor since the gate

opening is not much less than the width of

the current channel.

CLOSURE ANALYSIS

As described earlier, barriers of the size discussed

in this project are not designed to open and close

frequently. Any increase in closure frequency leads

to higher risks of mechanical failure, environmen-

tal impacts, and shipping disruption, among other

impacts. This analysis assumes that the maximum

number of times the gates could be closed per

year is fty. This is a very high number (approxi-

mately once per week) compared to how often

comparable systems worldwide are designed to

close. We drew upon the historical record of tides

and storms in Boston with projected sea level rise

to forecast how many years after barrier construc-

tion the annual gate closure would exceed this

number.

As shown in Figure ES.4 (p. 12), the closure

analysis found that with no additional shore-based

protection compared to the present (present pro-

tection is assumed to be 10 feet NAVD88—the

approximate elevation of the present 1% storm), a

barrier system under RCP 4.5 would be functional

to approximately 2100 if it were able to close 50

times per year (the number of closures in earlier

years would be considerably less, no more than a

few times per year, if that). If fewer closures were

permitted, the functional life decreases. At the

end of this period, it would no longer be feasible

to close the barrier gate sufciently often to man-

age all storm surge events greater than 10 feet

NAVD88. Similarly, if shore-based protection was

at 12 feet NAVD88, the functional period would

end in approximately 2110 (note in the subsequent

economic analysis, this time was assumed to be

Feasibility of Harbor-wide Barrier Systems

2100). With 14 feet NAVD88 shore-based protec-

tion, the functional period would end in 2130.

At the end of these periods, a barrier could still

be used to lessen the impacts of the increasing

number of storm surges, but not eliminate them

as before.

Environmental Impacts

Environmental impacts of an inner and outer

harbor barrier were considered under present and

future (with 5 feet of SLR) conditions. Because of

the tidal attenuation nding mentioned above, it

was assumed that the presence of either barrier

would not affect the tidal range in the harbor, and

that the barriers would be closed for 46 to 84 hours

during a nor’easter to reduce storm surge—less

during a hurricane. This environmental assess-

ment is based on an assumption of several (3-10)

closures per year for major storms. Under future

scenarios of up to weekly closures for regular

tidal ooding, the environmental impacts are

not discussed in detail in this report.

It should be noted that the environmental

condition of Boston Harbor has undergone great

change in its history with slow degradation before

and rapid improvement after 1990 and the Boston

Harbor Cleanup. Boston Harbor is currently under-

going, and will continue to undergo, great change

with expected sea-level rise and a temperature

increase of about 2.7 to 3.7 C by 2100. The

future impacts of a harbor-wide barrier, then, must

be considered in the context of other ongoing and

anticipated changes in the harbor environment.

It does not appear that the construction of

the OHB or the IHB would cause any irreversible

negative transformations of the entire harbor

FIGURE ES.4

End of Functional Period of a Barrier System with Various Levels

of Shoreline Protection

Sea Level Rise (Feet)

2010 2030 2050 2070 2090 2110 2130 2150

Time (Year)

Water Surface Elevation

(NAVD88) and Level of

Shoreline Protection

Earliest

Construction

14 Feet

12 Feet

10 Feet

8 Feet

RCP 8.5

RCP 4.5

Source: Woods Hole Group

Preliminary Analysis for Boston Harbor

environment in terms of water quality, habitat

quality, or ecosystem services. While there are

some foreseeable impacts, most of these are

modest or limited spatially or temporally. For a

great part of the harbor system, 5 feet of SLR and

expected increases in sea surface temperature

could cause more environmental impact than

the construction of a harbor-wide barrier. In a

separate analysis, these overall ndings were

conrmed. The team analyzed the change in the

economic value of ecological services in Boston

Harbor with and without a barrier assuming

marshes could migrate inland as SLR occurred.

This analysis showed some change in services

due to the barrier, but it was not dramatic.

Economic Analysis

The economic feasibility of a harbor-wide barrier

is based upon its benets and costs. Damages

avoided by the barrier system are the economic

benets. The benet:cost analysis was done for

several levels of shore-based protection imple-

mented in different time periods with low and high

estimates of project costs and discount rates.

The results are approximately the same for the

IHB and the OHB. As in the case of the closure

analysis, the benets of a barrier system depend

upon the elevation of shore-based adaptation.

If the shore-based systems are effective in man-

aging ooding, and a barrier is designed to manage

all the events greater than the elevation of the

shore-based protection, the benet:cost ratios

(BCR) of any barrier system are low—ranging from

0.05 to 0.33 for 7% discount rate and from 0.20

to 1.69 for 3% discount rate. If the shore-based

measures are not effective, and a low discount

rate of 3% is used, then in some cases, particularly

if a barrier is built in 2050, the BCR may be more

favorable (as high as in the range of 3.69–5.42).

Under the higher discount rate of 7% and ineffec-

tive shore-based adaptation, most of the BCRs

are less than 1.0. The results indicate a low cost-

effectiveness of barrier systems if shore-based

systems function as designed.

Since this analysis differs from Climate Ready

Boston (2016) in both the approach and data

used due to the size of the study area and project

constraints, the expected benets for some sub-

areas of Boston in this analysis could be as much

as 50% less than the benet values using the

methodology of CRB (2106). Even if this were the

case in all subareas, if shore-based adaptation

is effective, the BCRs are still less than 1.0 in

most cases.

If the shore-based systems are effective in

managing ooding, and a barrier is designed

to manage all the events greater than the

elevation of the shore-based protection,

the benet:cost ratios (BCR) of any barrier

system are low.

The low BCRs for the barrier congurations

we investigated in this study are likely to make

eligibility for federal funding very challenging,

if not impossible.

Shipping and Recreational Use Analysis

One of the guiding assumptions of this analysis

was the importance of nding a solution that

would minimize the disruption of the various uses

of Boston Harbor. Many commercial and recre-

ational activities occur within Boston Harbor. This

analysis determined that the proposed inner and

outer barriers could have both positive and nega-

tive impacts on these activities. Generally speaking,

the proposed barriers would provide added pro-

tection to activities occurring within the harbor—

including commercial shipping and shing, and

recreational boating and shing—as they would

protect shoreside infrastructure and vessels

from storm turbulence and ooding.

The openings to the barriers would generally

accommodate federal requirements for navigation

channels, minimizing impacts to commercial ves-

sels entering and exiting Boston Harbor (including

the new post Panamax vessels for which Massport

is enlarging its facilities at Conley Terminal).

Vessels would not be able to enter or exit when

the barriers are closed, and would have to plan

travel in advance of closing.

The anticipated increased water velocity in

the barrier openings could cause navigational and

safety issues for both recreational and commercial

vessels near the barrier openings. Additionally,

there could be greater vessel congestion near

the openings in the OHB, especially the northern

barrier opening as its water velocity is expected

to be more manageable than the southern barrier

opening. The OHB could also impact the abundance,

distribution, and behavior of sh populations,

which would in turn impact both commercial

and recreational sheries.

Feasibility of Harbor-wide Barrier Systems

Social Vulnerability Analysis

The social vulnerability analysis sought to deter-

mine the impact an IHB or an OHB would have

on socially vulnerable populations as compared to

the broader population. In particular, the analysis

sought to understand if a barrier system would

inadvertently disproportionately impact socially

vulnerable populations.

Importantly, the analysis found that there is

not a disproportionate negative impact on vulner-

able populations from either the IHB or OHB. More

specically, the percentage of socially vulnerable

people who would remain vulnerable to ooding

if a barrier were to be constructed is not different

from the percentage of socially vulnerable popu-

lation in Boston as a whole. This analysis did not

look at different factors that would allow socially

vulnerable populations to recover from a storm

or take into account the disparate challenges that

different groups have after an emergency event.

Instead, the focus was on exposure to ooding

caused by storm events.

Comparison to Shore-Based Adaptation

While this study focused primarily on the feasibility

of different harbor-wide barrier systems, a decision

about whether or not to build a barrier should

not be made in isolation but in comparison with

other options. Our analysis identied several

key advantages that shore-based solutions

have over a single harbor-wide barrier.

MULTI-FACETED OPTIONS

Shore-based adaptations can fall under the

general categories of protection, accomodation,

and retreat. Within each of these categories, a

mix of different strategies exists. These include

policy-level actions such as ood insurance,

zoning, or managed retreat from the coast. Shore-

based protection systems can include “green”

and/or “gray” approaches to ood walls, elevation

of land using berms and other features, additions

of transparent ood barriers, and temporary

ood walls that can be deployed in advance of

impending oods. They can be employed at the

regional scale or the individual asset scale, and

if designed correctly, can provide multiple layers

of effectiveness and safety. In addition, they

can provide management of high tide nuisance

ooding, which harbor-wide barriers do not.

Most of the shore-based solutions provide many

co- benets such as recreation, public access,

open-space, and urban heat island cooling.

These co-benets might be particularly important

in communities suffering from environmental

and social injustice.

FLEXIBILITY AND ADAPTABILITY

Another advantage of shore-based solutions

is that they provide a exible, adaptive manage-

ment approach to coastal protection. As a result,

responses can be implemented over time as

SLR and ooding increases, projections improve,

and more is known about future socio-economic

conditions.

RISK MANAGEMENT

The risk of singularly relying on a barrier, even

if technology could be developed to ameliorate

the concerns around closure frequency, is that

if completion is delayed or the barrier is less

effective than designed, then the City and the

region may be left completely exposed, and in

the words of Climate Ready Boston, having

“catastrophic” results.

BETTER BENEFIT-COST RATIOS

The benet-cost ratios at a 7% discount rate

of harbor-wide barriers that effectively manage

ooding above the level of shoreline protection

range from 0.05 to 0.33.

For the same level of protection at the

same discount rate (7%) and a shorter func-

tional lifetime (20 years), Climate Ready Boston

estimates a benet-cost ratio of 3.22 to 5.3 for

a shore-based ood protection system in the

Greenway/Border Street area of East Boston,

and a benet-cost ratio of 4.3 to 7.9 for a shore-

based protection project for Charlestown. There-

fore, shore-based adaptation approaches, at

least for these two districts, appear far more

cost effective.

More evidence of the cost effectiveness of

shore-based adaptation compared to harbor-wide

barriers can be shown by examining a potential

choice point the city of Boston could face in the

future. If the city is protected to 14 feet NAVD

by only shore-based protection, at a certain

point that amount of protection will no longer

be sufcient. Leaders will need to decide on

additional protections.

Assuming shoreline protections can be built

up, this would cost an estimated $508 million

(2017 dollars based on $4,500 per linear foot

for additional walls and $2,250 to expand exist-

ing walls) to provide protection equivalent to the

Outer Barrier for Boston. Even assuming that the

City of Boston would not pay for the entire cost

Preliminary Analysis for Boston Harbor

of building a barrier, the cost of shore-based

protections is dwarfed by the potential cost of

a barrier which could be $8–$12 billion.

Findings and Recommendations

Based upon the analyses conducted for this

report, it is clear that shore-based adaptation

strategies, if effective, have signicant advan-

tages over harbor-wide strategies for Boston,

at least for the next few decades when a decision

on a harbor-wide barrier could be re-examined

if shore-based systems are not effective. The

same nding likely applies to other municipal-

ities in Boston Harbor.

KEY FINDINGS

The analysis has shown that while a harbor-wide

barrier system could manage some coastal ood-

ing with perhaps minimal environmental impacts

and moderate impacts on harbor users, its cost-

effectiveness is low and its operational life would

be limited. With limited potential to adapt or

adjust the barrier once it is in place, it could be

challenging to respond to the uncertainties of

climate change over time. The alternative of

a wide spectrum of shore-based, district-level

solutions located around the inner harbor water-

front, however, has the potential for high cost-

effectiveness, and has several key advantages.

With proper planning and design, these solutions

have the potential to incorporate multiple levels

of protection, manage coastal ooding, provide

exibility and adaptability, offer co-benets that

address social justice, endure for long opera-

tional lifetimes, and carry minimal impacts

to the environment and harbor users.

KEY RECOMMENDATION

The authors recommend that the City continue

to focus its climate adaptation strategy for the

next several decades on the multi-layered, shore-

based approach described in Climate Ready

Boston (2016). Within a few decades, more will

be known about the rate of sea level rise, the

effectiveness of shore-based solutions, and tech-

nological advances that could improve the feasi-

bility and cost of harbor-wide barrier systems. In

the meantime, focusing on shore-based solutions

will provide ood protection more quickly at less

cost. These shore-based solutions would be

needed in any case over the next few decades

to manage coastal ooding during the design

and construction period of any harbor-wide barrier

if it is decided to build one in the future. Shore-

based solutions are also more adaptive and can

provide substantial co-benets, while protecting

the harbor’s surrounding communities from sea

level rise and storm surge. Any future barrier

would probably best be used to complement

shore-based systems by managing very large

oods with the shore-based systems managing

smaller events and helping to manage the very

large events. This would limit the annual number

of closures of a future barrier system. The deci-

sion regarding a barrier is very much dependent

upon the future risk tolerance of the city and

the performance of shore-based systems.

It will be especially important to monitor the

actual and projected pace of sealevel rise in

Boston Harbor over the next several decades

to determine whether shore-based solutions

being implemented in Boston and adjacent

cities will be adequate.

ADDITIONAL RECOMMENDATIONS

While moving forward with a harbor-wide barrier is

not prudent, we recommend that the City continue

to monitor climate, environmental, economic, and

social changes, the risk tolerance of the city, the

continuing evolution of the technology of harbor-wide

barriers, and the global experience with existing

storm surge barrier systems, to determine if

the feasibility of a harbor-wide barrier should be

re-examined at some point in the future. It will

be especially important to monitor the actual and

projected pace of sealevel rise in Boston Harbor

over the next several decades to determine whether

shore-based solutions being implemented in Boston

and adjacent cities will be adequatefor the

remainder of the century and beyond.

If the feasibility of a harbor-wide barrier is

reexamined at some point, there are several

engineering, hydrodynamic, environmental, climate,

economic, and planning analsyes that would war-

rant more detailed examination than was conducted

for this study. Regardless, the City should under-

take strong greenhouse gas mitigation actions in

concert with cities and nations globally to lessen

the rate of climate change. Strong mitigation

starting now could limit SLR by 2100 to 2 or 3 feet

or less. This would greatly reduce the need for

future consideration of harbor-wide barrier systems

in this century and early next century.

Sustainable Solutions Lab

University of Massachusetts, Boston

100 Morrissey Blvd, Boston, MA 02125