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United States Department of Transportation United States Department of Transportation

Public Roads - Autumn 2021

Date:
Autumn 2021
Issue No:
Vol. 85 No. 3
Publication Number:
FHWA-HRT-22-001
Table of Contents

Nature-Based Solutions for Coastal Highway Resilience

by Katherine Buckingham and Jessica Torossian

As the climate continues to change, extreme weather events—such as damaging coastal storms, drought, and extreme heat and precipitation—are expected to increase in frequency. According to the U.S. Global Change Research Program’s Fourth National Climate Assessment report, in the United States, there are more than 60,000 miles (97,000 kilometers) of roads and bridges in coastal floodplains and more than $1 trillion in coastal property that may be at risk from these effects of climate change.

"Aerial view of a road running through a marsh. Image Source: © Wangkun JIA / 123RF.com."
Nature-based solutions can offer coastal roads protection from the effects of rising sea levels and extreme weather events. 

Developing a network of resilient coastal roads is a priority for the Federal Highway Administration in order to ensure the safe passage of the American public on coastal roads and to maximize coastal investments. While several nature-based solutions have the ability to innately adapt to rising sea levels, traditional strategies to protect coastal infrastructure that rely on hardening shorelines and other “gray” solutions may not be adaptable or must be specifically designed to do so, which can be costly. These solutions can also have unintended consequences, such as increased erosion or deposition, along other parts of the coastline.

Nature-based solutions that rely on existing or enhanced landscapes help improve roadway resiliency by reducing impacts to coastal roads from hazards such as rising sea level, storm surge, and “nuisance” flooding (such as high tide or windblown flooding). Often these “green” strategies are both more effective and less costly than traditional engineering or gray solutions on their own.

“More States and municipalities are turning to nature-based solutions to protect coastal highways and improve the environment while providing a natural aesthetic and other benefits to coastal communities,” says Elizabeth Habic, an environmental protection specialist with the FHWA Office of Natural Environment.

Considerations for Deploying Nature-Based Solutions 

The term “nature-based solutions” refers to the use of natural materials and processes to reduce erosion, wave damage, and flood risks to coastal infrastructure. These solutions can be used instead of, or in conjunction with, traditional gray methods of shoreline stabilization and protection techniques. Nature-based solutions include the use of naturally occurring features, as well as nature-based features that are created or enhanced by human design, engineering, and construction.

In many coastal areas, naturally occurring habitats and geographic features can provide protection from the coastal processes and storm events that threaten coastal roads. The main habitats involved in nature-based solutions are tidal salt marshes, mangroves, maritime forests, coral and shellfish reefs, beaches, and dunes. In addition to buffering storm effects, these habitats also provide other community benefits (called ecosystem services) related to recreation, water quality, local fisheries, and climate change mitigation.

State and local transportation agencies can capitalize on these ecosystem services—and even enhance them—by deploying nature-based solutions in strategic ways that can use one particular intervention or layer multiple solutions to be as simple or complex as needed. The type of solution that a transportation or planning agency chooses to deploy will be location specific and highly dependent on local geography and ecology as well as the type, frequency, and severity of coastal impacts to road infrastructure. Additionally, each solution may have unique permitting and right-of-way requirements. Successful design and implementation of nature-based solutions often require input from a cross section of expertise, including transportation professionals, coastal engineers, and environmental scientists.

Salt Marshes 

There are three common solutions that can be deployed in regions where salt marshes are common: marsh vegetation, marsh sill, and marsh breakwater. Solutions that use salt marshes generally provide a medium level of protection against both increased wave energy (because of the structure of marsh vegetation) and erosion (because root systems hold soil in place). They also deliver some protection against coastal flooding and are more natural and aesthetically attractive for the community. Marsh-based solutions offer strong environmental benefits because salt marshes provide a variety of ecosystem services, including improving water quality and providing habitat and nurseries for a variety of species. Salt marshes also play an important role in denitrification and help trap atmospheric carbon.

Despite these benefits, there are several challenges associated with deploying marsh-based solutions. Marsh plants can require maintenance, especially during the early stages of life. Project funding should include costs for the removal of dead plants and debris from the marsh and the planting of new vegetation as needed. While the success of these projects is well documented, service life can be hard to predict because a marsh-based solution can be highly dependent on elevation and marsh slope. Tidal elevation may change as sea levels rise and marsh slope and local water conditions can heavily influence shoreline changes. While salt marshes can naturally adapt to many of these changing conditions, in some locations, additional efforts to mitigate for sea level rise and high levels of runoff can help ensure an effective design.

"An intertidal marsh with a rock breakwater wall in the foreground. Image Source: 2017 NOAA Fisheries."
This marsh in Mobile Bay, AL, has a segmented breakwater to help further attenuate wave energy beyond the marsh’s natural capacity.  

Types of Nature-Based Solutions 

  • Marsh Vegetation: The construction of a marsh, including fill and plantings but without structural elements, in the intertidal zone of a shoreline.
  • 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.
  • Marsh Sill: The construction of a marsh, including fill and plantings, fronted by toe protection in the intertidal zone of a shoreline.
  • Beach Nourishment: The placement of large quantities of good-quality and suitable sand directly on the beach to restore the beach.
  • Pocket Beach: Beach nourishment coupled with the installation of headland breakwater structures to slow the movement of sand out of the project area.
  • Dune Restoration: The placement of good-quality sand to either rebuild existing dunes or create an artificial dune by building up a mound of sand at the back of the beach.

Beaches and Dunes 

In areas where beaches or dunes are prevalent, agencies can deploy solutions such as pocket beach creation, beach nourishment, and dune restoration. All three solutions are highly effective at protecting upland resources during storms, while pocket beach and beach nourishment solutions are also beneficial for reducing erosion. Beach-based solutions offer additional habitat for seabirds and other beach-going animals like sea turtles, and deliver added community benefits for local property owners, tourists, and recreational users.

"Aerial view of a pocket beach. Image Source: © South Coast Engineers."
An example of a pocket beach in Perdido Bay, AL, which uses beach nourishment coupled with the installation of headland breakwater structures to slow sand movement.  

For all three strategies, sand is sourced from a borrow-area located either offshore or from a coastal dredging project or upland sandpit. Sand should have an appropriate grain size that matches the native beach sand. Sourcing high-quality sand can create a challenge as source sand may impact historic and cultural resources or artifacts that should be avoided. Imported sand should be cleared of nonnative species that could potentially be introduced to a new area.

Dune restoration is often coupled with the planting of native vegetation, which may need to be maintained. Design life can be dependent on local conditions and renourishment may be required to maintain the beach or dune. Pocket beaches face the additional challenge of sizing and siting the headland structures, but the addition of these structures can increase the design life almost indefinitely.

Hybrid Solutions 

In certain areas where wave energy may be particularly challenging to dissipate or local conditions lead to more flooding, it may be best to employ a hybrid approach that combines a nature-based strategy with gray infrastructure solutions to provide more robust protection. Hybrid solutions may also be best for other habitats such as mangroves, maritime forests, and intertidal and subtidal reefs.

Mangroves can act in a similar way to marshes by attenuating wave energy and preventing erosion from complex root structures. Upland forests can also be useful in protecting coastal infrastructure, but they can take quite some time to grow into a protective barrier. For this reason, efforts to conserve maritime forests are often more successful than re-planting or encouraging new growth. Nearshore shellfish reefs and coral reefs can help attenuate wave energy but do not provide much protection against any coastal flooding that is not wave induced.

"Photograph of a coastal beach with vegetation in the foreground and a rock wall that extends into the water. Image Source: © Steve Droter, Chesapeake Bay Program."
This hybrid solution involves a living shoreline paired with a pocket beach at the Virginia Institute of Marine Science’s Carl Hershner Teaching Marsh in Gloucester Point, VA.  

Hybrid solutions can include:

  • Constructed marshes with stone or timber sills.
  • Marsh/mangroves with breakwaters, reefs, or habitat devices.
  • Beach nourishment with breakwaters and/or groins.
  • Constructed dunes with reinforced cores.

Corpus Christi’s Resilience Pilot Program 

Laguna Shores Road is a major thoroughfare in the Flour Bluff neighborhood of Corpus Christi, TX. Currently, several locations along Laguna Shores Road are subject to periodic flooding under spring tide and other typical (nonstorm) conditions. Shoreline erosion has also undermined the roadway in multiple locations. The southern end of the project is particularly vulnerable to extreme weather because there is no habitat buffer between the roadway and the open water of the Laguna Madre. These sites are especially susceptible to the impacts of storm surge and extreme weather events, and this vulnerability will increase in the face of sea level rise.

As part of an applied research deployment, the Corpus Christi Metropolitan Planning Organization (Corpus Christi MPO) received $110,770 from FHWA to support the design of an innovative, nature-based shoreline protection feature to enhance resilience to extreme weather. The city of Corpus Christi is currently initiating the design phase of a project to rebuild three separate portions of Laguna Shores Road to improve level of service and reduce susceptibility to inundation.

The Corpus Christi MPO conducted an alternatives analysis to assess the living shoreline. A multidisciplinary consultant team gathered data and conducted a metocean analysis. Metocean conditions refer to the combined wind, wave, and climate conditions found at a certain location, and may include measurements and statistics such as air temperature, humidity, wind speeds, water level fluctuations, bathymetry, salinity, and stratification. The team also conducted a site visit and habitat assessment, hydrographic surveying, and preliminary geotechnical testing.

"Photograph of Laguna Shores Road. The road has an irregular edge of pavement and is located in close proximity to open water. Image Source: © Corpus Christi MPO."
A pilot project in Corpus Christi, TX, aims to improve the resilience of Laguna Shores Road, shown here under typical (nonstorm) conditions, to extreme weather events.  

The team developed two breakwater concepts, a riprap breakwater and a reef ball breakwater, to provide wave protection to Laguna Shores Road and to support hard substrate habitat. The crew also developed a marsh fill concept to provide additional habitat and living shoreline benefits. They designed both the breakwater concepts and the marsh fill concepts to be constructed with equipment typical for roadway construction and without additional specialized equipment.

The next step was constructing the pilot shoreline protection project as part of the roadway reconstruction project and monitoring effectiveness in terms of habitat development and shoreline condition. The monitoring will help the agency to evaluate the utility of the pilot techniques to enhance the durability of other segments of Laguna Shores Road and other similarly vulnerable transportation infrastructure.

“This pilot project has been and will continue to be a collaborative effort of all the partners,” says Robert MacDonald, PE, MPA, the transportation planning director for the Corpus Christi MPO. “The MPO managed an interdisciplinary team of specialized experts from the academic, environmental nonprofit, municipal, and private sectors. This effort provides a real-world example of combining the elements of resiliency with a planned roadway reconstruction project.”

A Pocket Beach in Perdido Bay, AL 

In Perdido Bay, AL, a single homeowner created a pocket beach in 2018 between two timber and sheet-pile bulkheads to restore a beach on a stretch of shoreline that is heavily armored by individual bulkheads. The two bulkheads act as headland structures, separating the beach from the adjacent coastline and creating the pocket beach—a type of isolated littoral cell. Neighboring bulkheads were used as the landward anchor points for the headland structures.

At a final cost of $40,000, the project generated multiple benefits. One is that the beach fill provides additional habitat for shorebirds and sea turtles. The structural elements attenuate waves and improve the stability of the beach, which can also decrease waves during storm events for upland resources. As a small, homeowner-level project on private land, the responsibility falls on the homeowner to maintain the beach going forward, and the largest challenge will be ensuring that the beach will be monitored in the future.

Beach Nourishment in Dauphin Island, AL 

In 2016, the town of Dauphin Island, AL, completed the Dauphin Island East End Beach and Barrier Island Restoration Project to prevent erosion and protect upland ecosystems and infrastructure in coastal Alabama. The project involved the placement of 320,000 cubic yards (240,000 cubic meters) of sand on an eroding beach that protects a maritime forest, freshwater lake, and substantial upland infrastructure. The $6.7 million project required an extensive, detailed survey of cultural resources in an offshore borrow area. The project also involved the interaction of State law and Federal policy because of a U.S. Coast Guard facility within the project limits. The beach nourishment included rehabilitating existing, detached rock groins perpendicular to the shore and converting them into breakwater headlands, which mitigates ongoing erosion. The project has successfully endured several tropical storms and hurricanes.

A major challenge was that the initial project grant funding was inadequate to cover all project costs. Fortunately, the town of Dauphin Island secured additional grant funding to cover the shortfall without compromising project design. The benefits of this project include increased property value, recreation, tourism, and habitat. The restored beach provides habitat to a variety of species, such as nesting habitats for shorebirds and sea turtles.

Creative Dune Solutions for Maximum Protection in Saco, ME 

Relatively high beach and dune erosion—approximately 3 feet (1 meter) per year—prompted the Ferry Beach Park Association in Saco, ME, to undertake a dune restoration project to protect roads and homes from flooding and erosion. Given the high erosion rate, the organization constructed an 800-foot-long (240-meter-long) secondary dune ridge landward of the existing dune crest and 1 foot (0.3 meter) above the effective 100-year base flood elevation determined by the Federal Emergency Management Agency. This approach helps protect upland resources from floods and waves during storms, enables native vegetation to grow, and provides additional dry sand habitat. Volunteers also planted native American beachgrass (Ammophila breviligulata) and installed fencing to help trap sand in the constructed dune.

The agency completed the project by the spring of 2009 at a final cost of $29,000. The benefit of this project is that the added sediment from dune restoration supports the protective capacity of the entire beach system (dune, beach, and nearshore area). Any sand eroded from the dune during a storm supplies a reservoir of sand to the fronting beach and nearshore area. Sand dunes provide a unique wildlife habitat, and dunes also act as a barrier to storm surges and flooding, protecting landward coastal resources and reducing overwash events.

FHWA’s Implementation Guide 

Implementing projects like these examples can help individuals and organizations address and mitigate the effects of climate change. To help, FHWA is producing research and offering technical assistance to enable transportation agencies to use natural and nature-based features to improve the resilience of transportation systems.

In addition to sponsoring pilot projects to assess the potential for nature-based techniques to protect specific locations along coastal roads and bridges, FHWA developed Nature-Based Solutions for Coastal Highway Resilience: An Implementation Guide (FHWA-HEP-19-042) to help transportation professionals understand when, where, and which nature-based solutions may work for them. The guide is available at www.fhwa.dot.gov/environment/sustainability/resilience/ongoing_and_current_research/green_infrastructure/implementation_guide.

"Cover of FHWA’s Nature-Based Solutions for Coastal Highway Resilience Implementation Guide. Image Source: Source: FHWA."
FHWA’s implementation guide provides transportation practitioners with step-by-step recommendations for integrating green infrastructure for coastal resilience. 

The majority of the guide is organized around how nature-based solutions can be developed through the transportation project delivery process. The document provides guidance on how to consider nature-based solutions in the planning process and how to conduct a site assessment to determine whether nature-based solutions are appropriate. It also describes key engineering and ecological design considerations, permitting approaches, construction considerations, and monitoring and maintenance strategies.

FHWA’s Hydraulic Engineering Circular No. 25: Highways in the Coastal Environment, 3rd Edition (FHWA-HIF-19-059) also provides information on the planning and design of nature-based solutions for use on highways and bridges in coastal environments. HEC-25 is available at www.fhwa.dot.gov/engineering/hydraulics/pubs/hif19059.pdf.

"Aerial photograph of East End Beach in Dauphan Island, Alabama, showing freshly placed sand along the extended shoreline. Image Source: © Sam St John, FlyTheCoast.com."
The town of Dauphin Island, AL, undertook a beach nourishment project in 2016 to prevent erosion and protect upland ecosystems and infrastructure in coastal Alabama through the placement of 320,000 cubic yards (240,000 cubic meters) of sand on an eroding beach.  

FHWA’s Elizabeth Habic says, “Working with nature and incorporating nature-based solutions into transportation projects is a proven and effective way to develop resilient infrastructure while simultaneously enhancing the surrounding environment with benefits to local ecosystems, residents, tourism, and recreation.”

Katherine Buckingham is an environmental protection specialist at the Volpe Center National Transportation Systems Center. She holds a master of city planning from Massachusetts Institute of Technology and a B.A. in politics from Oberlin College.

Jessica Torossian is an operations research analyst at the Volpe Center National Transportation Systems Center. She holds a Ph.D. in ecology, evolution, and marine biology from Northeastern University.

For more information, see www.fhwa.dot.gov/environment/sustainability/resilience/ongoing_and_current_research/green_infrastructure/index.cfm or contact Elizabeth Habic, an environmental protection specialist in FHWA’s Office of Natural Environment, at elizabeth.habic@dot.gov.