Nature-Based Solutions for Coastal Highway Resilience: An Implementation Guide

FHWA-HEP-19-042

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This project was carried out in support of 23 U.S.C. §503(b)(3)(B)(viii), which directs the Department of Transportation "to carry out research and development activities ... to study the vulnerabilities of the transportation system to ... extreme events and methods to reduce those vulnerabilities."

Technical Report Documentation Page
1. Report No. FHWA-HEP-19-042		2. Government Accession No.		3. Recipient's Catalog No.
4. Title and Subtitle Nature-Based Solutions for Coastal Highway Resilience: An Implementation Guide				5. Report Date August 2019
7. Author(s) Bret Webb (SCE), Brenda Dix (ICF), Scott Douglass (SCE), Susan Asam (ICF), Charlotte Cherry (ICF), Beau Buhring (SCE)				6. Performing Organization Code
				8. Performing Organization Report No.
9. Performing Organization Name(s) and Address(es) ICF 1725 Eye Street, NW, Suite 1000 Washington, DC 20006 South Coast Engineers (SCE) P.O. Box 72 Fairhope, AL 36533				10. Work Unit No. (TRAIS)
				11. Contract or Grant No. DTFH6117F00098
12. Sponsoring Agency Name(s) and Address(es) Tina Hodges, Rebecca Lupes, Eric Brown, Rob Hyman, Rob Kafalenos, Heather Holsinger Federal Highway Administration 1200 New Jersey Avenue, SE Washington, DC 20590				13. Type of Report and Period Covered
				14. Sponsoring Agency Code
15. Supplementary Notes This report and related materials are available online on FHWA's resilience website.
16. Abstract This Implementation Guide is designed to help transportation practitioners understand how and where nature-based and hybrid solutions can be used to improve the resilience of coastal roads and bridges. Upfront, it summarizes the potential flood-reduction benefits and co-benefits of these strategies. From there, the guide follows the steps in the project delivery process, providing guidance on how to consider nature-based solutions in the planning process, how to conduct a site assessment to determine whether nature-based solutions are appropriate, key engineering and ecological design considerations, permitting approaches, construction considerations, and monitoring and maintenance strategies. The guide also includes appendices with site characterization tools, decision support for selecting nature-based solutions, suggested performance metrics, and links to additional tools and resources.
17. Key Words Nature-based solutions, natural solutions, green infrastructure, living shorelines, sea level rise, storm surge, wave, erosion, highway, resilience, vulnerability, beach, marsh, breakwater, revetment, oyster reef, ecology, coastal processes, coastal restoration, mangrove, maritime forest, reef, dune, engineering				18. Distribution Statement
19. Security Classif. (of this report) Unclassified	18. Security Classif. (of this page) Unclassified		20. No. of Pages 229		22. Price

LIST OF SYMBOLS
LIST OF ACRONYMS
EXECUTIVE SUMMARY
1. INTRODUCTION
TECHNCIAL FACTSHEETS
2. RISK-REDUCTION PERFORMANCE, ECOLOGICAL BENEFITS, AND MONETARY COSTS
3. PLANNING AND FUNDING
4. SITE ASSESSMENT
5. ENGINEERING AND ECOLOGICAL DESIGN CONSIDERATIONS
6. PERMITTING FOR NATURE-BASED SOLUTIONS
7. CONSTRUCTION
8. MONITORING, MAINTENANCE, AND ADAPTIVE MANAGEMENT
9. CONCLUSIONS
10. APPENDICES
11. GLOSSARY
12. ACKNOWLEDGMENTS
13. REFERENCES

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LIST OF FIGURES

Figure 1-1. Nature-based solutions consist of varying degrees of natural, nature-based, and structural elements depending on the setting, exposure to wave action, and resilience needs (adapted with modifications from SAGE 2017).
Figure 2-1. A constructed, vegetated dune protects this low-elevation barrier island roadway during storm events (photo courtesy of Bret Webb).
Figure 2-2. Thin layer placement of dredged material (photo courtesy of USACE 2019b).
Figure 2-3. Flooding of Great Bay Boulevard during high tide (FHWA 2018c).
Figure 3-1. Map of locations that have implemented Eco-Logical (FHWA 2019c).
Figure 3-2. FHWA Eco-Logical framework (FHWA 2019c).
Figure 3-3. A screenshot from the Maryland Watershed Resources Registry shows how this and similar REFs can be used to identify candidate areas for nature-based solutions. The GIS layers indicate that the segment of MD-261/Bay Avenue circled in red is in (1) the 100-year floodplain, (2) an area of "high marsh protection potential," and (3) a "targeted ecological area." Source: Maryland Watershed Resources Registry.
Figure 3-4. The transportation planning process consists of nine major steps that repeat as a (frequently non-linear) cycle. Stakeholder engagement occurs at every stage of the transportation planning process.
Figure 3-5. Participants at Alabama Peer Exchange (photo courtesy of FHWA).
Figure 4-1. Photos showing site characteristics during (a) low tide and (b) high tide along a coastal road on Long Island, NY with arrows indicating the top of marsh grass for visual comparison (photo courtesy of Bret Webb).
Figure 4-2. Definition sketch for tidal datums and the nearshore profile (NRC 2007).
Figure 4-3. Definition sketch of a wave (FHWA 2008).
Figure 4-4. Profile schematic for selected terrestrial parameters (Miller et al. 2015).
Figure 4-5. Photograph of a storm wrack line in a tidal marsh (photo courtesy of Bret Webb).
Figure 4-6. U.S. Route 1B causeway in New Castle, NH (photo courtesy of New Hampshire DOT).
Figure 4-7. Laguna Shores Road under typical (non-storm) conditions. Note the irregular edge of pavement and proximity to open water (photo courtesy of Corpus Christi MPO).
Figure 4-8. Resilience strategies for coastal highways: (a) complete strategy space, (b) nature-based strategies for flood resilience, (c) nature-based strategies for wave mitigation, and (d) nature-based strategies for erosion mitigation and shoreline stabilization.
Figure 4-9. Example of a fish pond from He'eia State Park in Kaneohe Bay, Oahu, HI (photo courtesy of Bret Webb).
Figure 5-1. Suggested expertise for the implementation of nature-based solutions.
Figure 5-2. Stone sill and constructed marsh at Morris Landing, NC (photo courtesy of Bret Webb).
Figure 5-3. Vertical timber sill protecting a marsh (NCDEQ 2019).
Figure 5-4. Typical vertical timber sill design in shallow water (Bryars 2016).
Figure 5-5. Offshore segmented breakwaters and beach nourishment protecting Louisiana Highway 82 in Holly Beach (source: FHWA 2019a).
Figure 5-6. Headland breakwater design parameters (Hardaway and Byrne 1999; Hardaway et al. 2017).
Figure 5-7. (A) Equilibrium pocket beach shoreline positions as a function of wave conditions; (B) Corresponding breakwater cross-sections for type A, type B, type C, and type BC structures shown in (A). These unique cross-sections demonstrate how site-specific conditions, such as the annual and storm wave conditions, lead to changes in project design (Hardaway et al. 1993; NRC 2007).
Figure 5-8. General diagrams of salt marshes on (A) Protected low energy shoreline, and (B) Open moderate energy shoreline of the Gulf Coast (Stout 1984).
Figure 5-9. Wave tolerance of Spartina alterniflora as a function of significant wave height and frequency of occurrence (based on Roland and Douglass 2005).
Figure 5-10. Contractors placing geotextile fabric for the foundation of an offshore rock breakwater (photo courtesy of Sam St. John).
Figure 5-11. Artificial sand dune constructed seaward of North Carolina Highway 12 to protect the roadway from flooding and damage (FHWA 2008).
Figure 5-12. Active beach nourishment in North Carolina showing the dredge pipeline discharging sand and water directly onto the shore. The dredge ship is visible in the background (photo courtesy of Tina Hodges).
Figure 5-13. (A) Full ADCIRC+SWAN unstructured mesh with local refinement in Mississippi and Louisiana, (B) a detailed view of elevation contours near the study site at Henderson Point, and (C) the location of the nested XBeach model grid relative to the larger mesh (FHWA 2018a).
Figure 5-14. Conceptual reduction in the event return period because of a nature-based solution.
Figure 5-15. Yorktown, VA, location overview (NOAA Nautical Chart 12241 inset, depths in feet).
Figure 5-16. Recent history of shoreline positions at Yorktown, VA (photo courtesy of Google Earth).
Figure 5-17. Conceptual cross-section of the Yorktown Pocket Beach (Hardaway and Byrne 1999).
Figure 5-18. Mobile Bay, AL, location overview (NOAA Nautical Chart 11376 inset, depths in feet).
Figure 5-19. Conceptual cross-section diagram of a constructed marsh and breakwater system in Mobile Bay, AL.
Figure 5-20. Conceptual planform diagram of a constructed marsh and breakwater system for Mobile Bay, AL.
(Credit: FHWA)
Figure 5-21. Great Egg Harbor, NJ, location overview (NOAA Nautical Chart 12316 inset, depths in feet).
Figure 5-22. Cross-section design details for marsh shoreline stabilization in Great Egg Harbor Bay, NJ (Traylor 2017; NJDOT 2009). Details 1 and 3 were used in high wave energy locations.
Figure 5-23: Cross-section design details for marsh shoreline stabilization in Great Egg Harbor Bay, NJ (Traylor 2017; NJDOT 2009). Detail 2 was used along shorelines with the lowest wave energy exposure. The project engineers specifically designed Detail 4 to address increased exposure to boat wake energy as a result of navigation channel realignment.
Figure 5-24. Oregon coast location overview (NOAA Nautical Chart 18561 inset, depths in fathoms).
Figure 5-25. Cross-section diagram of the nature-based protection at Beverly Beach, OR (ODOT 2017).
Figure 5-26. Cross-section diagram of the nature-based protection at Lost Creek, OR (ODOT 2017).
Figure 5-27. Cross-section diagram of the nature-based protection at Ona Beach, OR (ODOT 2017).
Figure 5-28. Sample cross-section of buried revetment and dune restoration (Boudreau et al. 2018).
Figure 5-29. Sample cross-section of an emergent wetland shelf with stone toe protection and sill (diagram and photos courtesy of Brian Majka and Scott Dierks).
Figure 6-1. Summary of Federal policies relevant to the permitting of nature-based solutions.
Figure 6-2. Summary of key points for USACE nationwide general, regional/programmatic general, and individual permits.
Figure 6-3. Typical permitting flowchart for nature-based solutions.
Figure 7-1. Living shoreline construction notification signage for the public (photo courtesy Eli Chen, Delaware Public Media).
Figure 7-2. Typical construction sequence for a simple nature-based solution (adapted from GDNR 2013).
Figure 7-3. General sequence for building a nature-based solution (adapted from Duhring 2016).
Figure 7-4. Example of informational signage installed at a project site (photo courtesy of Dianne Rosensweig, ESA).
Figure 8-1. Hierarchy of metrics in the monitoring framework (adapted from Yepsen et al. 2016).
Figure 8-2. Example of monitoring metrics for a living shoreline used to improve water quality.
Figure . Questions that the monitoring plan should answer (adapted from Yepsen et al. 2016).
Figure 8-4. Mean percentage of cover (columns) and standard error (error bars) of restored and natural Spartina alterniflora at three different sites (Currin et al. 2018).
Figure 8-5. Relative sea level projections for three scenarios from 2015 to 2100 (FHWA 2016b).
Figure 8-6. Conceptual cross-section diagram of a constructed intertidal marsh with segmented toe protection boulders (FHWA 2016b).
Figure 8-7. Conceptual planform diagram of a constructed marsh with segmented toe protection for shore road (FHWA 2016b).
Figure 10-1. Flood hazard decision tree for roads, causeways, and bridge approaches.
Figure 10-2. Erosion and shoreline retreat decision tree for roadways and causeways.
Figure 10-3. Detailed erosion and shoreline retreat decision tree for roads and causeways.
Figure 10-4. Wave hazard decision tree for roads, causeways, and bridges.

LIST OF TABLES

Table 1-1. Full array of coastal risk-reduction measures
Table 1-2: Federal agency nature-based solutions efforts
Table 2-1. Summary of risk-reduction performance and resilience attributes by strategy (adapted with modifications from USACE 2015)
Table 2-2. Ecosystem services provided by structural features and nature-based solutions (adapted with modifications from NRC 2007)
Table 2-3. Summary of conditions and project costs as a function of wave energy (adapted with modifications from Luscher and Hollingsworth 2007)
Table 2-4. Summary of costs for shoreline hardening, structural features, and nature-based solutions
Table 4-1. Key site characterization parameters for nature-based solutions
Table 4-2. Site characterization evaluation criterion and key parameter values
Table 4-3. Nature-based solutions by parameter threshold categories L, M, and H (use in conjunction with Table 4-2)
Table 4-4. Common coastal highway vulnerabilities
Table 4-5. Representative examples of management strategies by vulnerability and infrastructure type
Table 4-6. Regional categorization of coastal States and territories
Table 6-1. Compensatory mitigation opportunities and challenges
Table 7-1. Example measures for performance-based contracts (adapted from AASHTO 2011)
Table 8-1. Representative sample metrics for different project goals
Table 8-2. Suggested actions for routine and preventative marsh maintenance
Table . Comparison of maintenance cost, maintenance interval, and replacement cost for structural and nature-based features
Table 12-1. Tools and resources for site characterization
Table 12-2. Project-type metrics for a living shoreline
Table 12-3. Project-type metrics for tidal wetlands restoration
Table 12-4. Goal-based metrics for erosion control
Table 12-5. Goal-based metrics for habitat enhancement
Table 12-6. Goal-based metrics for water quality improvement
Table 12-7. Goal-based metrics for hydrological enhancement
Table 12-8. Goal-based metrics for socioeconomic improvements
Table 12-9. Nationwide resources for the planning and design of nature-based solutions
Table 12-10. Regional resources for the planning and design of nature-based solutions
Table 12-11. Coastal Zone Management Program governmental contacts by State

LIST OF SYMBOLS

BI	Initial beach width
Bm	Present beach width
cot (θ)	Side slope of the structure
D	Stone dimensions
Ebe	Background erosion
Et	Erosion over time
Gb	Breakwater gap
H	Design wave height
hb	Water depth
Lb	Length of breakwater crest
Mb	Maximum bay indentations
T	Storm return period
V	Dune volume above the stillwater level
Wb	Width of beach
Xb	Distance offshore

LIST OF ACRONYMS

AASHTO	American Association of State Highway and Transportation Officials
BACI	Before-After-Control-Impact
BLM	Bureau of Land Management
BMP	Best Management Practices
BwN	Building with Nature
CatEx	Categorical Exclusion
CEQ	Council on Environmental Quality
CWA	Clean Water Act
CZMA	Coastal Zone Management Act
CZMP	Coastal Zone Management Plan
DEQ	Department of Environmental Quality
DNREC	Delaware Department of Natural Resources and Environmental Control
DOT	Department of Transportation
EA	Environmental Assessment
EFH	Essential Fish Habitat
EIS	Environmental Impact Statement
EO	Executive Order
ESA	Endangered Species Act
EWN	Engineering With Nature®
FAST Act	Fixing America's Surface Transportation Act
FEMA	Federal Emergency Management Agency
FFRMS	Federal Flood Risk Management Standard
FHWA	Federal Highway Administration
GIS	Geographic Information System
GMSLR	Global Mean Sea Level Rise
ILF	In-Lieu Fee
LiDAR	Light Detection and Ranging
LRSTP	Long-Range Statewide Transportation Plan
MHHW	Mean Higher High Water
MHW	Mean High Water
MLLW	Mean Lower Low Water
MLW	Mean Low Water
MMPA	Marine Mammal Protection Act
MPO	Metropolitan Planning Organization
MSA	Magnuson-Stevens Act
MSE	Mechanically Stabilized Earth
MSL	Mean Sea Level
MTP	Metropolitan Transportation Plan
NACCS	North Atlantic Comprehensive Coastal Study
NBS	Nature-Based Solutions
NEP	National Estuary Program
NEPA	National Environmental Policy Act
NERRS	National Estuarine Research Reserve System
NGO	Nongovernmental Organization
NHPA	National Historic Preservation Act
NMFS	National Marine Fisheries Service
NNBF	Natural and Nature-Based Features
NOAA	National Oceanic and Atmospheric Administration
NPS	National Park Service
ppt	Parts per Thousand
PRM	Permittee-Responsible Mitigation
PSU	Partial Salinity Unit
REF	Regional Ecosystem Framework
RFI	Request for Information
RFP	Request for Proposal
RFQ	Request for Quotation
RFT	Request for Tender
RIBITS	Regulatory In-Lieu Fee and Bank Information Tracking System
ROD	Record of Decision
RSLR	Relative Sea Level Rise
SAGE	Systems Approach to Geomorphic Engineering
SAV	Submerged Aquatic Vegetation
SHPO	State Historic Preservation Office
SMART	Specific, Measurable, Achievable, Relevant, and Time Bound
TMDL	Total Maximum Daily Load
USACE	U.S. Army Corps of Engineers
USCG	U.S. Coast Guard
USDA FS	U.S. Department of Agriculture Forest Service
USDOT	U.S. Department of Transportation
USEPA	U.S. Environmental Protection Agency
USFWS	U.S. Fish and Wildlife Service
USGS	U.S. Geological Survey
UV	Ultraviolet
VIMS	Virginia Institute of Marine Science
WwN	Working with Nature

EXECUTIVE SUMMARY

Overview

Nature-based solutions can serve as a first line of defense and improve the resilience of coastal highways. FHWA developed Nature-Based Solutions for Coastal Highway Resilience: An Implementation Guide to help transportation professionals understand when, where, and which nature-based solutions may work for them. The guide follows the project implementation process from planning all the way through construction and maintenance.

How Can Nature-Based Solutions Help Me?

Nature-based solutions provide risk-reduction benefits to coastal highways by reducing coastal flooding, wave heights, and erosion. These reductions are the result of both physical and biophysical processes, underscoring the importance of the ecological components of a nature-based solution. Vegetative features like tidal marshes, mangroves, and maritime forests can reduce coastal flooding and effectively dissipate wave energy that contributes to wave-induced flooding, erosion, and infrastructure damage. Reefs dissipate wave energy and force waves to break further from shore, leading to reductions in wave runup and overtopping, wave energy, and erosion. Beaches and dunes dissipate wave energy and reduce coastal flooding and are known to effectively minimize damage to the built environment during extreme events.

What Other Benefits do they Offer?

Nature-based solutions offer habitat, water quality, and recreational benefits. The natural aquatic edge or shoreline, provision of substrate, and/or use of structures like marsh sills or breakwaters provide habitat for juvenile finfish and shellfish as well as foraging opportunities for birds and mammals. Water quality improvements result from reductions in available nitrogen, phosphorous, and total suspended solids. The habitat and water quality benefits combine to produce beneficial opportunities for recreational fishing, kayaking, paddle boarding, and bird watching.

How Much do they Cost?

Nature-based solutions are often equal to or less than the initial cost of traditional shoreline armoring, such as rock revetments, and have lower replacement costs following extreme events. However, nature-based solutions may require a level of routine maintenance that traditional shoreline protection does not. Unlike traditional engineering approaches that require replacement or retrofit, nature-based solutions can naturally adapt to sea level rise over time when conditions are suitable. Funding for nature-based solutions is available within the transportation sector, as part of coastal restoration grant efforts, and through hazard mitigation programs.

How to Develop Nature-Based Solutions

Planning

Incorporating nature-based solutions into transportation planning enables systematic consideration across a planning area or state and allows for identification of opportunities at an early stage. Transportation planners can facilitate coordination and collaboration with key stakeholders to mobilize larger projects, which increases the project benefits and can reduce costs. Potential partners include state coastal zone management programs, natural resource agencies, national estuarine research reserves, U.S. Army Corps of Engineers District Offices, and non-profit organizations. Coordination includes early engagement with the public as well as appropriate regulatory professionals. Transportation agencies can leverage Eco-Logical, an ecosystem-based approach to transportation planning developed by FHWA and stakeholders, to pre-identify locations where nature-based solutions may be appropriate given existing natural resources and ecological priorities.

Site Assessment

Selecting an appropriate nature-based solution involves site characterization and resilience characterization. This guide describes the characterization process so the practitioner can determine what type of solution will best fit project needs. Site characterization is the process of developing an understanding of historical and present site conditions. This process allows you to answer the simple question, What does nature support at this location? When thinking about nature-based solutions, it is best to let nature be your guide. The types of site characterization parameters addressed in the guide include:

System Parameters		Hydrodynamic Parameters
1. Shoreline Type 2. Infrastructure 3. Erosion Rate 4. Sea Level Rise 5. Tide Range		1. Wind Waves 2. Boat Wakes 3. Currents 4. Ice 5. Storm Surge
Terrestrial Parameters		Ecological Parameters
1. Upland Slope 2. Shoreline Slope 3. Width 4. Nearshore Slope 5. Water Depth 6. Soil Strength		1. Water Quality 2. Soil Type 3. Sunlight 4. Salinity
Additional Parameters
1. Permits 2. End Effects 3. Constructability	4. Species 5. Debris 6. Monitoring

Resilience characterization involves identifying coastal highway vulnerabilities to determine resilience needs. Resilience needs may include flood reduction, wave attenuation, erosion reduction, shoreline stabilization, etc. Nature-based solutions can play an important role in enhancing resilience to events, including long-term sea level rise, ranging from minor and frequent to extreme and infrequent. The guide provides potential nature-based solutions to address the following vulnerabilities:

Highway Hydraulics	Coastal Roadways
1. Sea Level Rise 2. Increased Flooding from Sea Level Rise 3. Storm Surge 4. Sedimentation	1. Sea Level Rise 2. Increased Flooding from Sea Level Rise 3. Storm Surge 4. Wave Action 5. Erosion 6. Shoreline Retreat
Coastal Bridges	Coastal Tunnels
1. Sea Level Rise 2. Storm Surge 3. Wave Action 4. Erosion	1. Sea Level Rise 2. Storm Surge 3. Wave Action

Design Considerations

Successful nature-based solutions benefit from a wide variety of technical expertise applied throughout the entire implementation process. This guide contains helpful information regarding engineering and ecological design considerations for common nature-based materials and techniques. The guide refers to established methods and techniques for project design. It also provides important lessons learned regarding structure design, selection of materials, addressing ecological needs, and accommodating coastal processes. The guide illustrates the protective benefits of nature-based solutions through examples, which serve as a framework for performing similar analyses on a case-by-case basis. Six design examples demonstrate the wide variety of nature-based solutions, each created for a site-specific application that addresses the relevant vulnerabilities while accommodating the regional setting.

Photograph of a continuous stone sill structure protecting a constructed tidal marsh along a bay shoreline at Morris Landing, NC. — Photo credit: Bret Webb

Permitting

Nature-based solutions require Federal, State, and perhaps even local/municipal review for compliance and permitting. Nationwide, regional, and individual permits exist at the Federal level, each having their own set of requirements when permitting a nature-based solution. Working in consultation with district mitigation leads, nature-based solutions can be used to meet compensatory mitigation requirements.

Construction

Performance-based contracts allow innovation in construction techniques and can be a good option for including maintenance. For land-based construction, soft soils may require a timber mat or lighter equipment. Water-based construction may need to be timed for high tide in shallow areas. You may need to avoid nesting season for sensitive species or time vegetation planting or oyster placement for seasonal growth windows. "Pardon our Mess" signs during construction and interpretive signs explaining project benefits aid public support.

Monitoring, Maintenance, & Adaptive Management

Monitoring a project's performance provides an opportunity to measure and assess impacts. There are three key monitoring plan components: identify project goals; identify relevant performance metrics; and select appropriate measurement methods. Maintenance requirements vary by project type and setting. Once established, routine maintenance costs for nature-based solutions should be minimal but are unlikely to be zero. Transportation professionals can use the monitoring results as part of an adaptive management strategy to help ensure project success and longevity as site conditions, or transportation needs, evolve.

Selecting Nature-Based Solutions

The decision to use a purely nature-based approach, a fully structural approach, or a hybrid solution to mitigate coastal hazards depends on (1) the type of nature-based solution that your site will support (e.g., Is it a beach or a marsh?), and (2) the infrastructure vulnerabilities or risks you seek to mitigate through project implementation. To help understand the options available for a given site, the following table presents common coastal hazards (column 1), relevant transportation asset types (column 2), and representative examples of possible management strategies (nature-based, structural, and hybrid)

Issue	Application	Natural & Nature-Based	Structural	Hybrid (possible examples)
Nuisance Flooding	Roads Causeways Drainage	Dunes Berm	Elevate Bulkhead Seawall Flood Barrier	Dunes + Bulkhead/Seawall/Barrier Berm + Bulkhead/Seawall/Barrier
Storm Flooding	Roads Causeways Bridges Tunnels Drainage	Beach Dunes Maritime Forest Marsh Mangroves	Elevate Bulkhead Seawall Flood Barrier	Beach + Dunes + Seawall/Barrier Marsh/Mangrove + Seawall/Barrier
Wave Runup & Overtopping	Roads Causeways Tunnels	Dunes Marsh Mangroves Reefs	Elevate Revetment Flood Barrier Breakwaters Sill	Dunes + Seawall/Barrier Marsh/Mangrove + Revetment Marsh/Mangrove + Sill Marsh/Mangrove + Reef/Breakwater
Wave Forces	Bridges Revetments	Marsh Mangroves Reefs	Elevate Breakwaters	Beach + Dunes + Reef/Breakwater Marsh/Mangrove + Reef/Breakwater
Erosion	Roads Causeways Bridges	Dunes Marsh Mangroves Reefs	Revetment Armoring Sill	Dunes + Vegetation Marsh/Mangrove + Sill Marsh/Mangrove + Revetment
Shoreline Retreat	Roads Bridges	Beach Marsh Mangrove Reefs	Bulkhead Seawall Revetment Breakwaters	Beach + Reef/Breakwater Marsh/Mangrove + Reef/Breakwater Beach + Revetment/Wall Marsh/Mangrove + Revetment/Wall

For More Information
Visit Nature Based Resilience for Coastal Highways project website for the following resources:

Implementation Guidebook
White Paper
Peer-Exchange Report
Pilot Reports
Resources from other agencies

1. INTRODUCTION

1.1 Purpose and Scope

This guide describes how transportation professionals can implement nature-based solutions that enhance the resilience of coastal highways under conditions ranging from typical to extreme weather events and sea level rise. Here the term coastal highway describes the roads, bridges, and other infrastructure that make up transportation systems exposed to, or occasionally exposed to, tides, storm surge, waves, and sea level rise. Briefly, the term nature-based solution describes a natural or nature-based (i.e., engineered) approach that reduces coastal hazards and damage as an alternative to, or in combination with, traditional engineered solutions.

The purpose of this guide is to provide information to transportation professionals that will enable them to consider nature-based solutions for protecting coastal highways as part of a broader portfolio, or system, of resilience measures. There are many reasons why transportation agencies should consider implementing nature-based solutions:

When appropriately designed, a nature-based solution enhances resilience to flooding, wave action, and erosion while simultaneously facilitating natural ecosystem function, with benefits such as improved water quality, habitat, and fisheries.
Nature-based solutions are often equal to or less than the initial cost of traditional shoreline armoring.
One significant advantage of nature-based solutions is that some can naturally adapt to sea level rise, whereas traditional structural features require replacement or retrofit to achieve similar goals.
The traveling public generally perceives nature-based solutions as more aesthetically pleasing, and they value the tourism and recreation benefits.

The scope of this guide is intentionally broad. The document aims to provide transportation professionals with relevant, timely, and science-based information regarding the complete project implementation process for nature-based solutions. Where possible, the guide summarizes existing design guidance with the caveat that the design of nature-based solutions requires a complete understanding of the site-specific issues that often drive the design, types of materials, and construction methods. Transportation professionals will benefit from a more fundamental understanding of programmatic implementation issues related to nature-based solutions, particularly considering the lack of coastal ecological and/or coastal engineering design expertise in the broader transportation community at this time. To that end, this guide recommends that transportation professionals engage the services of local practitioners with the requisite expertise in the design of nature-based solutions (Section 5.1).

1.2 What Are Nature-Based Solutions?

Nature-based solutions use natural materials and processes to reduce erosion, wave damage, and flood risks. Nature-based solutions often serve as alternatives to, or ecological enhancements of, traditional shoreline stabilization and infrastructure protection techniques. In this guide, the term nature-based solution is inclusive of both natural and nature-based features. Natural features are created through the action of physical, geological, biological, and chemical processes over time; nature-based features are created by human design, engineering, and construction to provide risk reduction in coastal areas (WIIN Act 2016, Section 1184).

A wide variety of terminology describes nature-based solutions, some of which are listed in the text box at left. The common thread connecting these approaches is the desire to protect or improve the built environment while maximizing the ecological function of the natural system. Because they address a specific ecological or ecosystem function, nature-based solutions are often site-specific and scenario-specific, and their design requires a cross-section of expertise drawing on the fields of coastal ecology, coastal geology, coastal oceanography, and coastal engineering.

This guide addresses the following examples of nature-based solutions: tidal marshes, mangroves, maritime forests, reefs, beaches, and dunes. In this case, the nature-based solutions considered can mitigate storm surge flooding, wave-related damage, erosion, shoreline retreat, and the potential impacts of sea level rise, which pose threats to coastal infrastructure. A nature-based solution may consist entirely of natural elements (e.g., vegetation, beach, dune) or some combination of natural elements, constructed natural elements, and traditional coastal structures (e.g., sill, breakwater, revetment, seawall). A nature-based solution that combines natural and constructed elements is called a hybrid approach. Table 1-1 includes brief definitions and examples for each of these measures.

Along a continuous spectrum from mild to steep slopes, small to large waves, and sheltered shorelines (e.g., estuaries, bays, sounds) to open coast environments, nature-based solutions increasingly rely on some sort of coastal structure or armoring (Figure 1-1). For example, in that figure, as exposure to large waves or the level of desired risk reduction increases, a more substantial coastal structure and/or armoring are likely. The progression of structural armoring might include a small structure close to the shoreline (e.g., a sill); a more substantial structure located some distance from the shore (e.g., a breakwater); or a continuous rock revetment or seawall in some cases. In some parts of the United States, particularly in the Pacific Northwest, the use of dynamic revetments requires an important caveat. While incorporating many of the engineering features of a traditional revetment, a dynamic revetment mimics the behavior of a cobble beach while offering the protective benefits of a revetment (see the Oregon Department of Transportation (DOT) case study in Section 5.7 for more information).

Table 1-1. Full array of coastal risk-reduction measures
Terminology	Brief Description and Examples
Policy Measures	Policy measures, also called non-structural measures, are instituted at the programmatic level through public policy, management practices, and/or regulatory requirements. Examples: acquisition/buyout, relocation, land use, zoning changes
Natural Features	Natural features are created through physical, biological, geological, and chemical processes, over time, by nature. Examples: marshes, mangroves, maritime forests, reefs, beaches, dunes
Nature-Based Features	Nature-based features are created by human design, engineering, and construction to mimic nature and include natural features. Examples: constructed marshes, mangrove restoration, constructed reefs, nourished beaches, restored/constructed dunes
Structural Features	Structural features are engineered structures implemented for the purpose of coastal risk reduction by decreasing flooding, wave action, and/or erosion. Examples: sills, breakwaters, bulkheads, revetments, seawalls
Hybrid Approaches	Hybrid approaches incorporate a combination of natural or nature-based measures and structural measures. Many nature-based solutions, including living shorelines, use hybrid approaches. Examples: marsh and sill, mangrove and reef breakwater, beach and breakwater

When exposure to large waves or the required level of risk reduction decreases, an appropriate nature-based solution may require little to no structure, instead focusing more on natural features or engineered and constructed nature-based features. The example provided in Figure 1-1 includes, but is not exclusive to, beach nourishment with and without vegetated dunes; edging, which is a form of low shoreline bank stabilization typically used in conjunction with newly placed sediment and planted vegetation; and shoreline and/or upland stands of vegetation.

As wave height, fetch, slope, exposure, and level of desired risk reduction increases, greater levels of protection are needed. Natural elements from least to most protection are: vegetation, edging, nourishment, and nourishment with vegetated dunes. — Figure 1-1. Nature-based solutions consist of varying degrees of natural, nature-based, and structural elements depending on the setting, exposure to wave action, and resilience needs (adapted with modifications from SAGE 2017).

As wave height, fetch, slope, exposure, and level of desired risk reduction increases, greater levels of protection are needed. Structural elements from least to most protection are: sill, breakwater, revetment, and seawall. — Figure 1-1. Nature-based solutions consist of varying degrees of natural, nature-based, and structural elements depending on the setting, exposure to wave action, and resilience needs (adapted with modifications from SAGE 2017).

The purpose of a coastal structure in a hybrid nature-based solution is to protect or enhance the natural features and/or provide additional risk reduction for adjacent infrastructure. Functionally, the coastal structure achieves these outcomes by manipulating the wave conditions (i.e., wave height and direction) at the site, by directly stabilizing sediments, or through a combination of both actions. The combination of natural and structural elements in a hybrid nature-based solution provide benefits that traditional armoring does not achieve on its own. Specifically, the combination of features used in hybrid nature-based solutions are effective at attenuating wave energy and provide superior habitat value and ecosystem services (Gittman et al. 2016b).
The decision to use a purely nature-based approach, a fully structural approach, or a hybrid solution depends on the following:

Resilience needs or desired risk reduction
Ecological and geological setting
Exposure to large waves
Project objectives
Project cost
Desired reliability
Local policy and/or regulations

1.3 Linkages to Other Federal Efforts

This guide was developed to be complementary but not redundant to existing guides and sources of information. The U.S. Department of Transportation (USDOT) provides technical guidance and recommendations to transportation professionals charged with the planning, design, and operation of highways. In addition to this implementation guide, the USDOT Federal Highway Administration (FHWA) offers relevant coastal highway information in their Hydraulic Engineering Circulars HEC-25 2nd ed. (FHWA 2008), HEC-25 Volume 2 (FHWA 2014), and also HEC-25 3rd ed. (FHWA 2019a). Other Federal agencies are also including nature-based solutions as part of their systems approach to addressing flood risks and hazards along the coast (Table 1-2).

Additionally, there are international efforts aimed at the implementation of nature-based solutions. For example, the World Bank (2017) provides an international perspective on the principles of implementing nature-based flood protection.

1.4 Other Related FHWA Efforts

This implementation guide is part of the USDOT FHWA effort to develop information for transportation professionals that illustrates how nature-based solutions can protect coastal highways from flood and flood-related risks while also providing environmental benefits. Prior to developing this guide, FHWA partnered with State departments of transportation (DOTs) and others on five pilot projects to assess the potential for nature-based solutions to improve the resilience of coastal roads and bridges (FHWA 2019b). The findings from these pilot projects are used to illustrate key concepts throughout this guide. FHWA also published a white paper that describes the potential use of nature-based solutions for coastal highway resilience (FHWA 2018e); conducted four regional peer exchanges to solicit input from experts, end users, and key stakeholders on nature-based solutions; and later published a report that synthesizes the major findings and outcomes of those meetings (FHWA 2018b).

1.5 Audience

The audience for this implementation guide includes transportation planners, engineering practitioners, operations and maintenance professionals, environmental permitting staff, partnering agencies, and specialized consultants assisting transportation agencies with their work.

Table 1-2 : Federal agency nature-based solutions efforts
Agency	Nature-Based Solutions Efforts
Federal Emergency Management Agency (FEMA)	FEMA describes the benefits of using bioengineering stabilization methods, such as nature-based solutions, as long-term solutions to reduce risk from natural hazards in FEMA (2019). FEMA also supports flood risk-reduction practices as part of their Hazard Mitigation Assistance grant programs (FEMA 2018).
U.S. Army Corps of Engineers (USACE)	As part of their North Atlantic Comprehensive Coastal Study (USACE 2015). USACE developed the report, Use of Natural and Nature-Based Features (NNBF) for Coastal Resilience (Bridges et al. 2014). This report provides an integrative framework that classifies NNBF approaches, characterizes vulnerability, develops performance metrics, places monitoring and adaptive management within a systems perspective, and addresses important policy issues. Through their Engineering With Nature® initiative and international collaboration, USACE is developing international guidelines for NNBF to support engineering functions that achieve sustainability and resilience within coastal systems (EWN 2019).
National Oceanic and Atmospheric Administration (NOAA)	NOAA serves as the program administrator for the National Coastal Zone Management Program (CZMP). CZMP is a voluntary partnership between the Federal Government and U.S. coastal and Great Lakes States and territories (see Appendix D, Table 10-11, for a list of State CZMP agencies). CZMP aims at protecting, restoring, and responsibly developing our Nation's coastal areas and resources. NOAA encourages the use of nature-based solutions to provide, maintain, or improve habitat or ecosystem function and enhance coastal resilience (NOAA 2015).
USACE and NOAA	USACE and NOAA developed a community of practice, the Systems Approach to Geomorphic Engineering (SAGE) group, to help broaden participation in the implementation of nature-based solutions (SAGE 2017). The SAGE group consists of Federal, State, and local agencies; nongovernmental organizations (NGOs); academic institutions; engineering practitioners; and private companies.

1.6 Organization of this Guide

Following this introduction, we provide factsheets on nature-based solutions that give an overview of techniques; a short case study of where each was used; notes on their benefits, challenges, and costs; and some regional considerations. Section 2, which follows the factsheets, provides readers with a summary of the benefits (flood reduction, ecological benefits, long-term performance, and other co-benefits) and typical costs of nature-based solutions. The remaining sections of the guide are intentionally organized to closely mimic the project delivery process. A brief description of each section follows.

Section 3 describes how transportation professionals might incorporate nature-based solutions into their planning process, key stakeholders to engage, and potential sources of funding for project implementation.
Section 4 focuses on the process for selecting an appropriate nature-based solution that addresses local coastal highway concerns.
Section 5 describes engineering and ecological considerations for the design of nature-based solutions, and presents a few selected case studies from around the U.S.
Section 6 describes common regulatory permitting requirements for nature-based solutions, as well as information about National Environmental Policy Act (NEPA) compliance, and compensatory mitigation opportunities.
Section 7 provides information specific to the construction phases of a nature-based solution.
Section 8 address monitoring, maintenance, and adaptive management for nature-based solutions.

The appendices of this guide provide site characterization tools and resources, decision trees for selecting nature-based solutions, suggested performance metrics, and lists of other tools and resources for nature-based solutions.

TECHNCIAL FACTSHEETS

These technical factsheets for select nature-based solutions can be helpful to practitioners considering implementing one of these strategies. The factsheets cover the following solutions:

Marsh Vegetation
Marsh Breakwater
Marsh Sill
Beach Nourishment
Pocket Beach
Dune Restoration

Marsh (Vegitation)

The construction of a marsh, including fill and plantings but without structural elements, in the intertidal zone of a shoreline.

Design Schematics

Overview of Technique

Materials

Habitat Components

Salt marsh habitat

Durability and Maintenance

Maintenance is most important early in the life of a marsh. Waterfowl can damage young plants, and lost plants must be replaced quickly to ensure the health of the entire marsh. After the early stages, invasive species and runoff issues can affect long-term success. Frequent removal of debris and large items in the wrack line will reduce plant breakage.

Design Life

Because of changes that are difficult to predict, such as sediment supply and ability to keep pace with sea level rise, it is difficult to predict the design life of a marsh. There are many examples of constructed marshes in Maryland and Virginia that are thriving 30+ years after construction. If marsh slope and elevation maintain appropriate values, marshes should be resilient with considerable longevity.

Ecological Services Provided

Marshes provide a number of ecological benefits. They can filter water, including runoff, and they increase the uptake of nutrients, filtration, denitrification, and sediment retention. The root systems of marsh vegetation help stabilize the soil they inhabit, reducing erosion. The marsh platform and plant stems attenuate wave energy, further reducing erosion risk.

Case Study

"The Teaching Marsh at the Virginia Institute of Marine Science (VIMS) in Gloucester Point, VA is a one-acre site restored to marshland for both practical and educational purposes. The marsh is designed and maintained by VIMS wetlands experts to naturally remove contaminants from Coleman Bridge stormwater runoff, improving water quality in the York River. The Teaching Marsh also provides a demonstration area for regulated wetland plant species identified in the Tidal Wetlands Act of 1972." http://ccrm.vims.edu/wetlands/teaching_marsh/

A river running through a grassy field Description automatically generated — Photo courtesy of VIMS

Project Proponent	Virginia Institute of Marine Science (VIMS)
Status	Completed (1999)
Permitting Insights	Establish open and direct communication with local, State & Federal regulatory authorities, rather than assume permit requirements or exemptions will apply to government public education and wetland restoration project.
Construction Notes	Excess fill excavated to the appropriate marsh elevation and used to create berms between salt/fresh marshes and walkways. 12,000 plants of various species.
Maintenance Issues	Some species died out and were replaced with saltmarsh cordgrass.
Final Cost	$25,000
Challenges	Failure of some species may be because of salinity incompatibility.

Photograph of marsh grass abutting a body of water, with trees on the opposite shoreline. — Photo courtesy of Bret Webb

A large body of water Description automatically generated — Photo courtesy of Bret Webb

Implementation Notes

Summary of Benefits

Marshes provide a variety of ecological benefits, generally improving water quality and reducing erosion risk while providing habitat for a variety of species.

Challenges

The flood risk reduction and wave attenuation benefits of marshes depend heavily on the water level. Marshes best attenuate waves when the water surface elevation is below the tops of the plantings.

Initial Cost

Upfront costs include the procurement of sediment and plantings for the marsh area, as well as any preventative measures such as bird exclusionary fencing. Upland stormwater BMPs and pruning of shade trees may also factor into initial cost.

O&M Cost

Maintenance costs will likely include the removal of dead plants and debris from the marsh and the planting of new plants, especially early in the life of the marsh.

Transportation and Other Benefits

Issue	Description
Flooding Reduction	Low, some reductions to flooding caused by reducing wave action
Wave Attenuation	Medium, marsh platform and plant stems break waves
Erosion Reduction	Medium, root systems hold soil in place
Multi-Benefits	High, creates/restores habitat, educational opportunity
Locale	Various, adaptable to site
Elevation + Slope	Low/Medium + Low/Medium, most valuable in the intertidal zone
Width	High, the area of marsh created is an important metric of success

Regional Factors

Issue	Description
Gulf of Mexico	Intertidal marsh slope, elevation, selection of appropriate plant species are keys to success. Some marshes vulnerable to high rates of RSLR.
Southeast	Intertidal marsh slope, elevation, and selection of appropriate plant species are keys to success.
Mid-Atlantic	Intertidal marsh slope, elevation, and selection of appropriate plant species are keys to success. Marshes vulnerable to high rates of RSLR.
Northeast	Large tidal fluctuations may preclude intertidal marshes in some areas. Ice formation may damage plants.
Great Lakes	Large lake-level fluctuations may preclude intertidal marshes. Ice formation may damage plants.
Pacific Northwest	Large tidal fluctuations may preclude intertidal marshes in some areas. Ice formation may damage plants.
Pacific Southwest	Select appropriate species based on salinity and marsh elevation.
Hawaii & Pacific Islands	Mangroves more common than intertidal marshes.

Marsh (Breakwater)

The construction of a marsh, including fill and plantings, in the intertidal zone of a shoreline, including segmented breakwaters to reduce incident wave energy.