Success in CSRM projects can be fostered by using a framework that integrates NbS into the design of infrastructure planning and engineering design, providing guidance on how NNBF fit into the larger project development process. One such appropriate framework is the International Guidelines on Natural and Nature-Based Features for Flood Risk Management (Bridges et al., 2021a). The Guidelines are based on two complementary frameworks: the USACE NNBF coastal resilience framework (Bridges et al., 2015) and the World Bank (2017) framework, which concentrates on implementing nature-based flood protection measures (see also Sayers et al., 2013). Collectively, these frameworks provide complementary guidance for conducting infrastructure projects that incorporate NNBF and therefore can serve as a road map for incorporating NNBF designs into the City of Boston CSRM study.

The NNBF guidelines framework is composed of five phases: scoping (blue), planning (green), decision-making (yellow), implementation (red), and operations (purple), as represented by the circular arrows in Figure 2 . Within those five phases there are 11 steps that when combined are designed to better enable successful NNBF project outcomes. The framework is designed to be iterative, so when new information becomes available during a later project phase, a previously completed phase can be revisited.

The framework as developed is sufficiently flexible to accommodate USACE studies and projects that manage flood risk. The flood risk management business line of the USACE includes flood risk management in situations where the flooding source is primarily riverine and CSRM where the flooding source is coastal waters (including the Great Lakes). The latter is described in greater detail below as it can be integrated with the NNBF framework. The first two phases (scoping and planning) and their associated steps are presented briefly below as they relate to the status of the City of Boston CSRM study. More details describing the framework can be found in Bridges et al. (2021a, 2022).

Scoping involves performing an initial assessment of project needs and objectives, as well as identifying, organizing, and meaningfully engaging stakeholders and partners to integrate their local economic, social, and environmental knowledge into the project design. The problem is also identified and defined during scoping.

In step 1 (preparation), stakeholders and partners are identified, organized, and assembled to define problems and objectives. In steps 2a (establish analysis funding strategy) and 2b (establish implementation funding strategy), funding strategies are established for analysis and implementation. This includes identifying funding mechanisms and cost estimates for planning and engineering activities, design and implementation plans, construction activities, monitoring and maintenance, and adaptive management activities. While an NNBF funding strategy is initiated in scoping, the funding process continues in decision-making and concludes in implementation. The two intended outcomes of the scoping phase are to engage stakeholders and the local community early and often, and to identify the flood-related challenges and encourage stakeholder commitment to actively support the project.

During the planning process, an improved understanding of the existing system is generated, and NNBF alternatives that meet project goals and objectives within that system are developed. Subsequently, they are advanced from scoping to being investigated, communicated, and rescoped (as needed) to acquire the information and data needed to support decision-making. Planning consists of steps that can achieve greater clarity regarding the selected alternative and what type of NNBFs are most appropriate for the project (i.e., greening the gray, hybrid, prompted recovery, and natural infrastructure; Suedel et al., 2021). As more data become available through activities such as studies, testing, and modeling, the evaluation will transition from more qualitative to more quantitative. Through a systems approach, planning is advanced iteratively so that the most cost-effective and practical NNBF-integrated solution is identified. As related to USACE CSRM studies, in 2023 the USACE released ER 1105-2-103 (USACE, 2023), which included in Chapter 5 policies and guidance specific to CSRM. Under this authority, USACE “may provide federal assistance to manage risk to people, infrastructure and associated resources located along the ocean, estuarine and Great Lake coastlines from erosion, waves and inundation due to coastal storms, along with abnormal coastal and lake flooding when such projects serve the public interest.” (USACE, 2023).

During step 3 (determine risk), the existing system including local and regional conditions is evaluated for hazard vulnerability and flood risk. To do this, coastal and riverine models, such as Advanced Circulation (ADCIRC), Adaptive Hydraulics (AdH), and modeling tools (see examples in Bridges et al., 2018, 2021b) may be needed. The modeling results offer a means of generating data that can support the need for NNBF and highlight the potential environmental, economic, and social benefits of the NNBF alternatives under consideration.

Step 4, Assess NNBF and hybrid options, involves the initial and iterative identification of NNBF options integrated with structural measures that align with project objectives. This step typically engages the multidisciplinary team in data gathering, stakeholder and partner engagement, and workshops. The analysis associated with this step includes determining the extent of NNBF use and evaluating the NNBF or hybrid options. This step of the process may include 1) evaluating the ability of an alternative to reduce natural hazards and vulnerabilities of the systems to flood risk, 2) how NNBF could be integrated as part of multiple lines of defense, 3) how NNBF interact with the geomorphology of the system, and 4) the ability of the system’s physical, biological, and chemical processes to support NNBF in the context of CSRM.

Step 5, socioeconomic analysis, includes an evaluation of the economic, social, and ecological costs and benefits of the NNBF or hybrid options. Consideration of socioeconomic and ecosystem services co-benefits is crucial for informing the selection of options, whether they are structural, nonstructural, NNBF, hybrid, or integrated. For CSRM studies, planning models are utilized to support the appropriate economic and environmental analyses.

The two primary outcomes from planning are identified sources of funding and agreed-upon benefits of implementing NNBF for the system at risk. One outcome product would be an evaluation table that identifies, weighs, and rates categories that can inform numerical rankings for each alternative (see Bridges et al., 2021a for a specific example). When project decision−making requirements are considered, the results of the alternatives analysis allow the high-priority NNBF alternatives to be identified for further deliberation in step 6 (decision-making).

The following case study presents information developed during the planning and scoping phases of the East Boston CSRM study with particular emphasis on considering multiple lines of defense and how they could be integrated and visualized into a cohesive whole for reducing flood risk in the area.

Boston is a coastal city with substantial built infrastructure and ecological resources that faces challenges because it is vulnerable to sea level change ( Figure 3 ). These threats could disrupt transportation networks, strain utilities, and compromise the reliability of essential services (City of Boston, 2016). Since 1991, Boston has experienced 21 events that triggered federal or state disaster declarations (City of Boston, 2016). By 2050, Boston is faced with up to 0.45 meters of potential sea level rise and the area at risk of a 1% annual exceedance probability could expand to include over 2,000 buildings, representing $20 billion in real estate value (City of Boston, 2016). Until recently, Boston had primarily used its zoning power to allow new development along an already vulnerable coastline, and the newly developed buildings have become some of the most vulnerable (Fitzgerald and Hobbs, 2022). Consequently, extreme weather events (e.g., nor’easters and hurricanes) have the potential to cause significant damage to Boston’s waterfront areas, infrastructure, and communities. For example, Winter Storm Grayson caused historic flooding throughout the city when it hit on January 4, 2018. Floodwaters reached new highs in Boston Harbor, and future storms could produce similar floods (Nordio and Fagherazzi, 2023).

Boston’s coastal ecosystem includes salt marshes and other wetland and wildlife habitats. Sea level change, increased water temperatures, and ocean acidification have the potential to impact this ecosystem’s biodiversity, fisheries, and recreational resources. Addressing these interwoven challenges has required a comprehensive and multidimensional approach (City of Boston, 2017, 2022; Fitzgerald and Hobbs, 2022). Planning for coastal resilience in Boston began a decade ago with several overlapping planning efforts. In 2014, Boston joined the 100 Resilient Cities program and in 2017 the city released its resilience strategy (Woodruff et al., 2021). Boston also prepared its first comprehensive plan in 50 years, releasing Climate Ready Boston in 2016. This comprehensive plan included a citywide climate change vulnerability assessment, updated climate projections, and an outline of strategies to address extreme heat, stormwater flooding, and coastal flooding from sea-level change and storms (City of Boston, 2016).

USACE CSRM studies are initiated through partnerships between the federal government and non-federal entities to manage storm risks and sustain our nation’s coastlines by predicting impacts from future storm systems, sea level change, and shoreline fluctuations to inform mitigation options. These studies develop and evaluate CSRM alternatives that manage risk and offer coastal resiliency for residents, industries, businesses, and infrastructure. CSRM often includes layered solutions executed by non-federal sponsors, federal agencies, local agencies, and/or non-governmental organizations (NGOs). Collaborators may include the National Planning Center of Expertise for CSRM, local USACE Districts, states, cities, and NGOs. As NNBF are intended to interact with the environment and society in different ways as compared to traditional approaches, values of stakeholders in the design, planning, and implementation phase are important to capture specific regional needs (Yamanaka and Nakagawa, 2022). The purpose of this case study is to provide context and an illustrative real-world example of how NNBF can help inform the existing CSRM scope to inform future design and implementation.

Step 4 offers opportunities to consider ways in which NNBF can be incorporated into the design of the City of Boston CSRM study. Landscape architecture drawings and renderings provide an opportunity to graphically illustrate how NNBF can be integrated into flood risk infrastructure (Holmes et al., 2022; King et al., 2022) ( Figures 5 , 6 ). Considering the risk drivers and space restrictions of the region, green-gray and hybrid designs (see Figures 1 , 4 ) offer the best opportunities to include NNBF into flood risk reduction infrastructure measures into the Boston CSRM study. To illustrate this point, a series of renderings were developed to show how the design of traditional infrastructure can be modified to enhance the benefits of the structures ( Figures 5 , 6 ) as discussed below.

For areas where floodwalls are being considered, low-maintenance native vegetation (Gaskin and Thomas, 2025), including woody vegetation such as shrubs with high drag coefficients, can be used in place of turf grass ( Figures 5A, B ) to soften the view-scape, provide sound-deadening benefits, and attenuate waves (Dalrymple et al., 1984; He et al., 2019);. Considerations for how levee safety buffer zone designs can be modified to enhance the benefits derived from this infrastructure feature is another consideration. Sidewalks made from pervious materials can also be integrated on either side of the flood wall to offer recreational opportunities as well as space for surface water to infiltrate. Project teams can work with local communities for the floodwall to serve as a canvas for locally developed artistic mural designs that reflect the area’s history and community values ( Figure 5B ). In this manner. meaningful engagement of the local community can enable the integration of an aesthetic component to the floodwall, greatly contributing to the visual appeal of the structure.

A bin wall (e.g., designed as a living levee) or other hybrid designs appropriate for space-limited urban areas could also be designed to manage flood risk (USACE, 1989b). Such a structure could be designed wide enough to serve its intended engineering function while creating an elevated walkway for residents. If an adjacent roadway is present, design elements can be included to reduce the spatial extent of impervious surfaces by replacing these with natural infrastructure materials such as native plantings in a roadway median, further increasing the area for groundwater infiltration, and thereby reducing local flooding ( Figure 5C ). The concept of a living levee is well known to the Boston area with several proposed designs including a proposed living levee along Bennington Street (Woods Hole Group, 2023); however, there remain challenges including permitting and space constraints. Permitting living levees (and other NNBF) in the Boston area is anticipated to be complex and time consuming, potentially taking years. Key issues include concerns about habitat conversion, especially when levees are proposed behind existing marshes, and the site’s regulatory designations (e.g. Area of Critical Environmental Concern) which may impose additional constraints. Coordination with multiple agencies and compliance with local, state, and federal regulations can lengthen the process. However, well-justified, ecologically beneficial projects can be permitted with diligent planning and early regulatory engagement.

Other opportunities to incorporate NNBF into flood infrastructure are available along the water’s edge. Again, considering space constraints prevailing in the Boston CSRM study area, a traditional floodwall could still be constructed, but designed to incorporate several NNBF to generate benefits beyond the engineered ( Figure 6 ). In areas that border residential neighborhoods, a bin wall could be designed to serve the primary engineering objective but could be bordered street-side by a levee with a lower departure angle serving as a footprint for native vegetive plantings that require minimal maintenance ( Figure 6 , top panel).

Seaside, riprap armoring could be interspersed with concrete blocks or similar structures whose surfaces and designs have been textured to create habitat niches (Perkol-Finkel and Sella, 2015; Sella et al., 2021; Miller et al., 2022; and projects found in Bridges et al., 2021b; Tritinger et al., 2024). The blocks would offer the desired flood risk reduction benefits while increasing the habitat values associated with that aspect of the structure. The bin wall could be topped with a walkway and guardrails to offer community aspect to the structure and waterfront. In scenarios where roadways or commercial developments border the bin wall, vegetative plantings would serve the added benefits of softening the wall while reducing noise and increasing biodiversity ( Figure 6 ). Here, material dredged from a federal navigation channel could be placed against the bin wall to serve as a foundation for creating marsh habitat while reducing the wave energy impacts on the wall itself.

A similar floodwall walk structure was designed and built in Richmond, Virginia in response to severe flooding that occurred in the 1970’s and 1980’s. The 5 km elevated floodwall system, called the Manchester Floodwall Walk, consists of gravel walking paths on top of the floodwall with safety rails and an overlook, all providing views of the James River, its bank habitats, and the City of Richmond (Manchester Floodwall Walk, 2014). In 2007, Diversity Park was added to the Manchester Floodwall Walk, creating additional recreational opportunities for the local community.

The city of Boston CSRM study offers opportunities to consider how the beneficial use of dredged materials can be used to enhance the value of flood infrastructure measures. Beneficial use opportunities could include contributing to hybrid designs where sediment can be placed in front of hardened infrastructure features such as floodwalls and seawalls to dampen wave energy impacts, reduce flood risk, and to also create marsh habitat features that enhance ecological value associated with the structure (Narayan et al., 2017; Baptist et al., 2019). Serving as a natural resource (Ousley et al., 2024), sediment could be sourced from a local federal navigation channel dredged during routine maintenance. However, implementation of beneficial use is not without its challenges to implement in practice. Barriers to overcome include technical (site access, timing, sediment compatibility), economic (cost of material testing, planning, scoping), and institutional (permitting, Federal standards) issues (Searcy et al., 2021; Ousley et al., 2024). In recent years, much progress has been made to overcome many of these common beneficial use barriers through strategic partnerships, field demonstration of practices, long-term performance data, and quantification of economic, social, and environmental benefits (Searcy Bell et al., 2021; Ousley et al., 2024).

A key opportunity that the CSRM study provides is the potential to leave space for natural infrastructure and associated habitats. Such ecological habitat would not only contribute to managing flood risk but also enhance the economic, social, and environmental benefits associated with the structures ( Figures 5 and 6 ; Bridges et al., 2021a). The presence of Belle Isle Marsh within the study area also presents an opportunity to utilize this natural infrastructure to enhance resilience. The marsh is currently being impacted by sea level change and storms but represents an opportunity to restore this valuable local resource. While the marsh itself may serve a limited capacity as a feature in the Boston CSRM study, sediment TLP is a management action that if implemented can serve to enhance the resilience of the marsh through habitat uplift well into the future.

Space limitations to implementing NbS in coastal marine environments is a challenge that should be overcome. Using a global modeling approach Schuerch et al. (2018) determined that the main driver of global wetland resilience is the availability of space to accommodate sediment accretion and subsequent vertical and lateral growth of wetlands. Yet, built infrastructure in the coastal zone is expected to increase over the remainder of this century, thus serving as a key factor contributing to space limitations. The authors suggested that large-scale loss of coastal wetlands might be avoidable if sufficient space is made available for NbS to be used to manage coastal environments. Therefore, hybrid NbS could be a contributor to limiting and perhaps reversing coastal wetland loss.

The performance and intended function of NbS in urban and other developed coastal areas remain key areas of study, particularly in the context of global coastal risks affecting countries in transition. In this study, there are numerous sources of uncertainty that may limit the future implementation NbS in the constraints of highly urban regions, including lack of physical or ecological performance data, lack of economic data, and uncertainties associated with long-term (life cycle) benefits. The uncertainties identified in this study are not unique, but a common barrier to implementation experienced globally. For instance, Paxton et al. (2024) reviewed 252 studies on NbS interventions for coastal protection to better understand what key data gaps need to be prioritized in future efforts. Results from the review indicated that long-term performance of NbS was rarely evaluated, with most studies occurring over short timeframes (less than one to five years). Additionally, most studies reviewed rarely incorporated social or economic metrics (Paxton et al., 2024). Similarly, Ruangpan et al. (2020) analyzed 146 studies on NbS efficacy for flood risk reduction and found that key data gaps included long-term effectiveness, performance when combined with gray infrastructure. Notably, 82% of these studies reviewed in Ruangpan et al. (2020) focused on urban flood risk reduction, indicating the global opportunities and interest in populated urban regions.

A recurring theme in the literature, as noted by Ruangpan et al. (2020), is the need for standardized frameworks to select, evaluate, and design hybrid NbS approaches. Yet, the benefits provided by “prescriptive” standardized approaches need to be balanced with methods that are appropriately flexible and iterative to be applicable to a wide range of practitioners, geographic regions, funding, and project scales. Recent advancements, such as the development of NNBF guidelines (Bridges et al., 2021a, 2022) have made substantial progress in addressing these aspects of an adaptable, transparent, and repeatable framework for NNBF implementation. In addition, this case study highlighted that many Nbs lack the at scale applications and the effectiveness of NNBF for mitigating flood risks from inundation (e.g., flood risks from tidal surge and storm surge; see Table 1 ). However, there remain many questions of the cumulative benefits that can be achieved by multiple NNBF and NBS at a “system” or regional scale. In a recent example of a large-scale coastal barrier island restoration, Tritinger et al. (2025) demonstrated that through sustainable engineering principles and integration of multiple natural and engineered ecosystem components at a regional scale, enhanced social, economic, and environmental outcomes were achieved. While there remain many uncertainties, information from real-world case studies - like presented here - are essential for demonstrating practical applications and facilitating NNBF implementation in practice.