Floodplain restoration is the process of repairing a river's floodplain to make it as close as possible to its natural state before levees (dikes) were built and wetlands and marshes were drained.

The goals of restoring floodplains are to decrease the number of floods, create homes for water animals, improve the quality of water, and help groundwater refill more effectively.

Description

Human activities often harm floodplains by blocking the natural connection between rivers and their floodplains. To fix this, many restoration projects focus on removing human-made structures that block these connections. One way to restore floodplains is by moving levees farther from rivers or removing dams, either completely or partially, to allow rivers to spread out naturally. Another method is the "beaded approach," which involves restoring small sections of a floodplain to their natural state. Removing levees or weirs can reconnect river channels to floodplains. Building overflow sills or creating openings in river channels can also help rivers connect better to floodplains. Restoring wetlands that have been drained or damaged can improve floodplain connections.

Restoring floodplains and wetlands is a type of nature-based solution (NbS) that helps reduce flood risks. These methods work by using natural ecosystems to absorb and slow down water. Growing more plant life improves soil’s ability to absorb water, which can lower the speed of floodwater. Plants also hold soil in place, trap dirt, and help build up soil over time, making areas more resistant to flooding and rising sea levels. NbS can be used in different areas, such as coastal, lake, and river floodplains, as well as in peatlands.

Increasing the variety of plants in an area can slow down water waves. In salt marshes, plants can reduce wave energy by about 70%, and in mangrove forests, by about 65%. Planting mangroves is a common NbS because they can lower the height of storm surges by about 40–50 cm for every kilometer of forest.

The success of floodplain NbS depends on the specific location and factors like plant density, water flow, and how much sediment is present. These methods work best for frequent, small to medium floods but are less helpful for extreme floods when used alone.

NbS are important for coastal wetlands because they store large amounts of carbon, known as blue carbon. Mangroves, salt marshes, and seagrasses can reduce carbon emissions by up to 0.3 gigatons of CO₂-equivalent each year, according to the IPCC Sixth Assessment Report (2022). While there is strong confidence that coastal wetlands can store carbon, how much more they can store through restoration is uncertain, and how long they will continue to do so under climate change is unclear.

Rewetting peatlands, which are wet, carbon-rich soils, can reduce soil damage and improve ecosystems. Peatlands have the potential to store up to 0.8 gigatons of CO₂-equivalent each year and could reduce emissions by about 0.4 gigatons annually if managed properly.

Although NbS can help floodplains store carbon, the exact amount they can store varies and is hard to measure. However, their role in reducing flood risks and helping communities adapt to flooding is well supported by scientific studies.

Restoring floodplains can bring back important services that ecosystems provide, such as supporting life, controlling floods, providing resources, and offering cultural value. Restored floodplains can help control floods and reduce flood-related damage. They also create or improve habitats for plants and animals, encouraging the growth of native species. Restoring wetlands in floodplains can improve water quality. Connecting rivers to floodplains helps store carbon in soil and supports natural processes in the soil.

Planning and carrying out floodplain restoration can face challenges. These projects often involve many groups with different goals, and disagreements or poor communication can slow progress. Some people may prioritize short-term flood protection or current land use over long-term environmental benefits, which can delay or stop restoration efforts. It is important to consider the social and economic needs of communities affected by these projects. Restoration efforts must be regularly checked to see if they are working and what benefits they provide.

Examples of existing projects

• Waza-Logone Restoration

• The Tarim River case study in China examines the cultural, economic, and environmental factors of the river basin to plan restoration projects.
• The Mekong Delta in Vietnam focuses on restoring wetlands to help protect coastal areas.
• South Korea’s Four Major Rivers Restoration Project aims to restore the Han, Nakdong, Geum, and Yeongsan rivers.

A key reason for restoring floodplains is the EU Water Framework Directive. Early floodplain restoration efforts began in the mid-1990s in areas like the Rheinvorland-Süd on the Upper Rhine, the Bourret on the Garonne, and the Long Eau project in England. In 2007, ongoing projects included Lenzen on the Elbe River, La Basse on the Seine River, and the Parrett Catchment Project in England. Near Lenzen, 420 hectares of floodplain were restored to prevent future floods similar to the 2002 Elbe River flood. After the 2002 flood, 20 floodplain restoration projects were planned for the Elbe River, but only two were completed by 2009, according to the environmental group de:BUND. Another example is the Upper Danube River restoration project in Germany.

The Lower Danube River has suffered major damage over the last century due to the construction of dikes on about 75% of its natural floodplain. This damage has disrupted the ecosystem, lowered the water table, and increased the risk of severe floods, which could worsen with climate change. Human activities, such as gravel mining, dredging, and building dams, have eroded the riverbed and caused water pollution.

The Lower Danube Green Corridor Agreement is a joint restoration project started in 2000 by Bulgaria, Romania, Ukraine, and Moldova. The goal is to create a green corridor along the entire 1,000+ km stretch of the Lower Danube River. This project aims to reduce flood risks in rural and urban areas by restoring natural floodplains, removing invasive plants, and rebuilding wetlands. It also seeks to support local economies through fishing and tourism. Additionally, the project is expected to help the river and nearby cities better withstand the effects of climate change.

Restoring floodplains provides benefits such as reducing floodwater levels, cleaning water, supporting ecosystems, and offering recreational spaces. These benefits also help manage droughts and prevent economic losses from flood damage.

The Waal River project in Nijmegen is a major part of the Dutch Room for the River program. This program was created after floods in 1993 and 1995 showed that raising dikes was not enough to prevent disasters. Near Nijmegen, the river needed more space to handle large amounts of water. Instead of building more dikes, the project restored floodplain space by moving the Lent dike 350 meters inland and creating a 3-km side channel. This change widened the river in winter, lowered water levels by about 35 cm, and reduced flood risks for Nijmegen and Lent. The project also created a new river island that supports nature and recreation. The Waal River project shows a shift in policy to work with natural river patterns. This approach matches recommendations from the IPCC and EU rules like the Water Framework Directive and Floods Directive, which support restoring floodplains, reducing flood risks, and using land in multiple ways.

• Chubut River Restoration Project

In the United States, floodplain restoration is guided by laws such as the Clean Water Act (1972), the Endangered Species Act (1973), and state-level laws.

• Restoration efforts in the Chesapeake Bay catchment area in Maryland.
• The Emiquon Preserve on the Illinois River.
• The Baraboo River in Wisconsin.
• Upper Sandy Creek, a tributary of the Cape Fear River in North Carolina.
• Projects on the Oldman River and St. Mary River in Alberta to restore natural water flow and support plant growth.