Nature-Based Solutions 101

Interest is growing in the development and implementation of nature-based solutions to help communities adapt to a changing climate. Nature-based solutions are practices that weave natural features or processes into the built environment to protect places from floods and other environmental harms. Alternative terminology includes “natural infrastructure,” “green infrastructure,” and “natural and nature-based features,” depending on the specific environmental challenges engineers and policy makers seek to address. The US Army Corps of Engineers uses the term “engineering with nature.” What these approaches have in common is that they strive to work with, not against, the forces of nature.

This explainer provides an overview of the role played by different nature-based solutions in the context of flood mitigation (nature-based solutions are often viewed through a broader lens). Nature-based solutions that address coastal flooding include strategies focused on dunes, salt marshes, and wetlands; in riverine areas, nature-based solutions include forested riparian buffers and natural floodplains; and for urban stormwater management, green roofs, rain gardens, and other approaches to store and retain stormwater runoff comprise the nature-based solutions toolkit.

Nature-based solutions are often evaluated in comparison to traditional “gray” (or hard) infrastructure such as dams, levees, pipes, and tunnels. The use of nature-based solutions can present multiple advantages over gray options. Most importantly, natural approaches often provide a range of benefits, or ecosystem services, beyond flood mitigation. These benefits include water quality improvements, carbon sequestration, temperature regulation, recreational amenities, and protection of wildlife habitat.

Coastal communities are threatened by tidal flooding and flooding from storm surge, both of which are exacerbated by climate change-induced sea level rise. Most coastal nature-based solutions combat encroaching floodwaters by blocking waves, slowing the flow of water over the landscape, absorbing water in plants and soil, and capturing sediment that reduces erosion and builds up defenses over time. Table 1 describes various types of coastal nature-based solutions.

Marshes, which are wetlands that are seasonally or permanently saturated with water, provide important protection from coastal storms. A recent study that analyzed 88 tropical storms and hurricanes that hit the United States between 1996 and 2016 found that counties with more wetlands experienced significantly less property damage and estimated that one square kilometer of wetlands provides, on average, about $36 million in storm protection services over a 30-year period. Mangroves, though limited to tropical and subtropical climates, are similarly beneficial. One study estimated flood protection benefits at $65 billion per year globally.

Coastal erosion, the process where storms and sea level rise gradually wear down rocks, soils, and sand along the coastline, has been estimated to cause over $500 million in property damage per year in the United States. Many nature-based solutions help combat erosion. Wetlands, for example, accumulate sediments, and oyster reefs act as natural breakwaters, both of which help to alleviate erosion problems. One study in a Gulf of Mexico location with high erosion rates found that oyster reefs can reduce wave energies by 76–99 percent and wave heights by 51–90 percent, thereby weakening the forces that lead to erosion.

Living shorelines use combinations of vegetation (including marsh grasses), rocks, and other materials in place of gray structures such as bulkheads (vertical walls constructed from concrete, wood, or steel) to stabilize lands against erosion and reduce flooding. Studies have shown that, in many settings, living shorelines can be more cost-effective than hard infrastructure. In a study of 22 project sites along the US mid-Atlantic and North Atlantic coasts, the average cost per acre protected of a living shoreline project was $84,000 lower than the average cost of a stone revetment (a sloped wall that is a common type of gray infrastructure along shorelines).  An analysis of a project site in the Gulf of Mexico found similar results when comparing a living shoreline to a bulkhead, with most of the difference attributable to avoided bulkhead maintenance and repair costs, which can be substantial.

Nature-based solutions in coastal settings provide myriad ecosystem services beyond flood protection. Mangrove forests have been found to store more carbon per square mile of land area than other ecosystems (such as tropical forests or salt marshes). Most of the storage is in their complex root systems, which also provide critical nursery grounds for several fish species. Coastal wetlands likewise provide critical habitat. For example, many species of birds feed, nest, and raise their young in wetlands, and it has been estimated that more than one-third of threatened and endangered species in the United States live only in wetlands. Wetlands also act as natural water purifiers by trapping and filtering sediment and absorbing pollutants, acting as “nature’s kidneys.”

Rivers support a range of diverse ecosystems and human needs. However, rivers can be destructive when they overflow their banks and spill out onto surrounding areas. Historical solutions to riverine flooding have included dams, which store excess water in reservoirs during periods of heavy rain, and levees, which channelize rivers and shield farms and populated areas from floodwaters. Levees on many large rivers in the United States, such as the Mississippi, have also been built to help make those rivers suitable for navigation.

This gray infrastructure has created a set of problems, however. For one thing, the structures have shrunk (in some cases eliminated) natural floodplains—the relatively flat lands adjacent to a river that become inundated in a flood. These natural floodplains collect floodwaters, protecting people and properties behind them, and are home to nutrient-rich sediments, which in turn provide water quality benefits and support wildlife habitat. Dams also degrade ecosystems and habitats by trapping sediment in the reservoirs behind them rather than passing sediment downstream.

Table 2 lists some of the most prominent types of riverine nature-based solutions, which mainly involve restoring a river’s natural flow through tributaries and floodplains and conserving and restoring adjacent natural lands. The restoration of flow and natural floodplains help provide receptacles for the excess water in heavy rainfall events. The natural lands in riparian buffers and adjacent lands also help absorb and store excess water, protecting properties behind them from flooding.

Floodplain reconnection is one of the major nature-based solutions to restore the health of river systems. This strategy involves removing levees, or setting them back from the river, to allow for a wider natural floodplain where the river waters can go during flood events, and restoring hydrological connectivity, that is, the ability of water to move unimpeded through a watershed. These changes improve the distribution of river sediment and organic matter, which not only helps manage flooding but also improves the river’s water quality. The Netherlands’ “Room for the River” program, which involves 30 projects to lower the level of flood plains, set back levees, create vegetative buffers, and increase the depth of side channels, along four rivers, is based around the concept of floodplain reconnection.

The aging of levees, coupled with increases in weather extremes, has led to repeated levee failures in the United States, which has provided opportunities to experiment with setbacks. After floods caused multiple failures of one levee on the Missouri River, for example, setting it back and creating a larger natural floodplain was found to be much more cost-effective than rebuilding the levee in its current location.

Many rivers in the United States have been straightened out, their natural meanders removed to allow for easier navigation and to create lands for development. Straightening a river’s path leads to significant losses in ecosystem services, namely loss of aquatic species habitat for spawning, feeding, and resting. Restoring natural meanders has been found to not only restore these services but also to reduce downstream flooding. River and stream restoration also includes the planting and installation of other natural materials to stabilize riverbanks and the resizing or burying of culverts (structures that channel water under roads).

Conserving and restoring riparian buffers and natural lands adjacent to streams and rivers reduces flooding in two ways: first, by avoiding damages that would result if those lands were developed, and second, by providing trees and other vegetation that collect rainfall and absorb it in roots and soil, slowing the rise of floodwaters. Studies have shown that conservation of lands alongside rivers is cost-effective and can provide additional ecosystem services beyond flood mitigation. However, targeting the right lands—those with relatively high expected flood damages and low property values—is important to ensure benefits outweigh costs.

Conventional gray stormwater infrastructure, which is comprised of gutters, drains, pipes, tunnels, and retention basins, is designed to redirect water in cities away from developed areas and towards nearby water bodies, such as streams and rivers, to reduce flooding. In a world with climate change, however, gray stormwater infrastructure is increasingly unable to handle the more extreme precipitation events that are occurring. Furthermore, stormwater runoff over pavement and other impermeable surfaces picks up pollutants such as heavy metals, suspended solids, nutrients, salts, and oil. When that runoff is sent through pipes and tunnels to nearby streams and rivers, it increases pollution in those water bodies. Green infrastructure in urban areas can simultaneously reduce flooding and water pollution by storing more water on the landscape for natural absorption and filtration.

Table 3 lists some of the most common urban nature-based solutions strategies deployed in US cities.

Some studies have found green infrastructure approaches to be lower cost than gray alternatives. An EPA study of the use of green roofs, urban forestry, and permeable pavement in Lancaster, Pennsylvania, for example, estimated upfront capital cost savings of $25.5 million compared to gray infrastructure and lower annual operating costs from reduced wastewater pumping and treatment. Similarly, a study in Cincinnati, Ohio, found that rain gardens would cost 42 percent less than a proposed tunnel, while leading to a 62–98 percent reduction in flooding.

Some studies have also found that urban nature-based solutions provide broader economic benefits. A study of the impacts of planting one million trees in Los Angeles estimated between $1.35 and $1.95 billion in economic benefits over 35 years. Only 8 percent of the benefits were from stormwater runoff reduction; the rest came from energy savings, air quality improvements, and improved urban aesthetics.

Nature-based solutions have many benefits but relying entirely on a nature-based approach may not work to control flooding in some locations. Moreover, legacy gray infrastructure may already be in place and infeasible to remove. Thus, green infrastructure is often used as a complement to, rather than a substitute for, gray infrastructure.

Examples of such hybrid, or gray-green, solutions include horizontal levees, which are wide, gently sloping, vegetated buffers of land in coastal areas, usually sited on the seaward side of conventional levees; living breakwaters that support the growth of coral reefs or shellfish populations; and sea walls or breakwaters located behind mangrove forests (and thus built to lower heights than they would be without the mangroves). In urban settings, green roofs, bioswales, and other green infrastructure may not replace an existing system of stormwater drains, pipes, and tunnels but be used to lighten the load on that system. A study in China found that a hybrid green-gray approach resulted in superior urban flood reduction compared to gray strategies alone, mainly by preventing drainage pipes from being overfilled. In the United States, an urban watershed simulation study in Illinois found that using green infrastructure with existing gray stormwater management systems could cut by half the increased surface runoff and flooding expected to result from urban growth.

Using natural and nature-based features together with gray infrastructure will often yield the ecosystem service benefits of the nature-based option alone. The habitat benefits of wetlands and oyster reefs, for example, are typically achieved even when those approaches are used together with some coastal gray infrastructure. Nature-based features can even protect hard infrastructure behind them, making those built structures more effective and lengthening their useful lives. Sea walls behind mangroves is one example.

Although nature-based solutions, and hybrid gray-green approaches, are increasingly recognized as effective alternatives to gray infrastructure, there are often challenges to implementation. One challenge is overcoming decades of familiarity with gray infrastructure, which leads to it being the default option in many cases. Engineering contractors are more experienced in the construction of gray infrastructure than in natural and nature-based alternatives. They also often perceive nature-based solutions as more costly than gray alternatives because they fail to recognize and account for the co-benefits of nature-based solutions and/or the costs of gray infrastructure that are avoided with a nature-based approach such as maintenance, repair, and replacement costs associated with the gray option. The modeling of flood reduction benefits from nature-based solutions is often more complex than for hard infrastructure alternatives, again because of decades of practical experience with the latter. This complexity can make the engineering community reluctant to take on evaluation of a multi-faceted and dispersed green infrastructure approach for stormwater management, for example, compared to more traditional drains, pipes, and tunnels. Similarly, the benefits of coastal gray infrastructure such as seawalls are relatively easy to model in comparison with various nature-based solutions and hybrid options. In its assessment of options to protect Miami from sea level rise-induced flooding, for example, the Army Corps of Engineers has opted for gray infrastructure (including sea walls) because of the uncertainty in the nature-based approach. The Corps has suggested small nature-based solutions pilot demonstration projects, however, to inform future choices.

For some nature-based solutions, upfront costs may be a barrier to implementation. Restoring oyster reefs, for example, can cost more than $120,000 per acre, according to a 2018 review of 88 projects (oyster restoration projects average between 5 and 50 acres in size). Dam removals can also be costly, depending on dam size and site characteristics. Costs of levee setback projects are typically in the millions. Many nature-based solutions often include extensive up-front planning, siting, and design costs, sometimes more than would be required with a gray infrastructure option. Further, in comparison to gray infrastructure, nature-based solutions may require site-specific specialized environmental engineering expertise that could be less readily available, which can drive up the labor costs of projects and lengthen completion timelines. In some settings, nature-based solutions are deemed to take up too much space and/or be less effective in the most severe storms, especially in hurricane-prone coastal areas.

For nature-based solutions that involve land conservation, such as protection of wetlands or riparian buffers, purchases of private land or easements can be costly in regions where land values are high. Levee setbacks can also involve easement or land purchases, to allow for the larger floodplain. Any nature-based projects that involve private property owners, for that matter, can face costs associated with negotiation, education, and collaboration. Convincing shoreline property owners, for example, to adopt living shorelines instead of bulkheads can be difficult when they (or the local contractors they use) are more familiar with bulkheads. For oceanfront property owners, it may be necessary to educate them about the importance of restoring and maintaining dunes for storm protection, especially in cases where dunes can block ocean views.

Nature-based solutions are promising climate adaptation strategies that offer flood protection along with a number of important ecosystem cobenefits. The diverse range of nature-based techniques and geographic applications can mitigate flooding in coastal, riverine, and urban settings and help overcome the shortfalls of legacy gray infrastructure. Implementation challenges remain, however, and more analysis of the benefits and costs of NBS in specific settings is needed.