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Each NBS strategy summarized in this roadmap includes information on barriers and potential solutions that are particularly important for that specific strategy. There are also several barriers common across many of the NBS strategies covered in this document, which are reviewed below. Some of these barriers are not unique to NBS projects; however, it is important to acknowledge frequently cited barriers and the possible solutions that can be used to overcome them.
- Cost: NBS projects are perceived to be expensive; however, lifetime costs of implementing NBS strategies may be lower than gray infrastructure alternatives. Careful cost-benefit analysis, cost-effectiveness analysis, or similar approaches should be used for a holistic expense evaluation. NBS projects can be expensive, especially if they involve large-scale construction (e.g., river connectivity restoration, living shorelines) or work by technical experts (e.g., prescribed burns). However, nature-based solutions are frequently less expensive to install and operate than gray infrastructure projects that accomplish the same primary objectives, though the distribution of costs over time may differ (Van Zanten et al. 2023; Vineyard et al. 2015). Comparing the lifetime cost of the selected NBS strategy with alternative approaches can help to put the expense in perspective. Nature-based solutions also tend to create cobenefits, such as recreational opportunities, cleaner air or water, habitat, and carbon sequestration. While these cobenefits cannot always be quantified in monetary terms, it is important to recognize them as part of the cost-benefit balance of the project (Seddon et al. 2020; Van Zanten et al. 2023; Viti et al. 2022).
- Capacity: Lack of staff capacity to implement NBS projects is another frequently cited barrier (e.g., Schultz and Moseley 2021; Beaury et al. 2020), but new government programs are in place to help train a new and expanded workforce that can carry them out. Lack of capacity can be particularly challenging when strategies, such as prescribed burns, require work by specially trained personnel. There is also a shortage of contractors with the required knowledge and skills to carry out some emerging NBS techniques, such as living shorelines or thin-layer placement for marsh restoration.
- Public opinion: Public opinion, either positive or negative, can drastically affect the efficiency and implementation of an NBS project. Negative public opinion of a project can slow down implementation or even stop it altogether, while positive public opinion can be a significant asset. Early, frequent, and transparent communication with the local community helps to avoid misunderstandings and address any concerns during project design and implementation. See the Community Engagement section for more details and best practices.
- Conflict with other land uses: While NBS implementation may conflict with other land uses, communicating the potential benefits and avoided risks provided in the long-term can help express why choosing a nature-based solution over another land use makes sense. Many NBS approaches require significant areas of land, which may displace or prevent other land uses from occurring. For example, there may be existing agricultural land along a river that would need to be restored for floodplain reconnection to occur, and installing living shorelines prevents future development directly along the coastline. However, the areas along these shorelines may be at heightened risk for flooding, erosion, or saltwater intrusion, making them less productive or useful lands. Depending on land ownership, these types of issues may increase project cost, raise local opposition to a project, or make a project infeasible to complete.
- Regulation and permitting: Like conventional projects, many NBS projects— especially those involving coastal habitats, wetlands, or waterways—require permits. The permitting process is frequently time-consuming and expensive, and specific requirements vary from state to state, making it difficult to provide relevant general guidance. However, identifying the permitting agencies and engaging in early and transparent communication can help to plan for the requirements for the specific project and avoid costly surprises later. Designing projects to be eligible for streamlined permitting processes, such as US Army Corps of Engineers (USACE) nationwide general permits, also helps to limit the time and cost required. See the Regulatory Processes section that follows for more details.
- Lack of effectiveness evidence: Some NBS strategies that have been in use for many years have sufficient effectiveness data, while others are less proven. However, there are efforts to gather and synthesize effectiveness data for a wide variety of NBS strategies to make them more accessible and easier to use. Information about a particular project’s likely effectiveness, performance, and reliability is often needed to justify using a nature-based solution, especially in place of a gray infrastructure alternative. While some nature-based solutions are well-established and understood, others are newer or evolving and research on their outcomes is nascent or ongoing. There are many published studies for certain NBS strategies, but others lack strong supporting evidence. For these strategies, it is difficult to extrapolate results from one project or location to another, making it hard to know exactly how projects will perform and what cobenefits will be achieved. Monitoring and evaluation of project performance using a common set of credible metrics is key to filling critical evidence gaps, providing information needed for adaptive management, and ensuring projects achieve satisfactory and measurable results (Conroy et al. 2011). Applying adaptive management designed to test and iterate on design and management of less-established nature-based solutions, can help manage uncertainty and build knowledge on how to best design and build these projects.