Natural floodplains provide flood risk reduction benefits by slowing runoff and storing floodwater. They also provide other benefits of considerable economic, social, and environmental value that are often overlooked when local land-use decisions are made.
Floodplains frequently contain wetlands and other important ecological areas which directly affect the quality of the local environment. Some of the benefits of floodplains to a functioning natural system include:
- Fish and wildlife habitat protection
- Natural flood and erosion control
- Surface water quality maintenance
- Groundwater recharge
- Biological productivity
- Higher quality recreational opportunities (fishing, bird watching, boating, etc.)
See the Green Guide, published by the Association of State Floodplain Managers, for more information on the benefits of natural systems and the history of floodplain development.
Natural Floodplains and Flood Loss Reduction
Floodplains provide numerous flood loss reduction benefits as a result of their unique natural functions. Rivers and streams shape floodplain topography and influence riparian habitats and riverine ecosystems. Likewise, the physical characteristics of the floodplain shape water flows and can provide flood loss reduction benefits to include the following:
Excess water storage: Except in narrow, steep valleys and areas of coastal bluffs, floodplains allow floodwaters to spread out and temporarily store excess water. This reduces flood peaks and velocities and the potential for erosion. One acre of floodplain flooded 1 foot deep holds approximately 330,000 gallons of water. Flood storage is particularly important in urban areas where even small floods, for example from a 5- or 10-year storm, can cause severe damage.
Flow rate and erosion reduction: In their natural vegetated state, floodplains slow the rate at which the incoming overland flow reaches the main water body in the area. Vegetation also reduces shoreline erosion. In coastal areas, floodplain features such as beaches, sand bars, dunes, and wetlands act as natural barriers to dissipate waves and protect back-lying areas from flooding and erosion.
Slowing runoff: A natural floodplain has surface conditions favoring local ponding and flood detention, plus subsurface conditions favoring infiltration and storage. Slowing runoff across the floodplain allows additional time for the runoff to infiltrate and recharge available groundwater aquifers when there is unused storage capacity. The slowing of runoff provides the additional benefit of natural purification of water as local runoff or overbank floodwater infiltrates and percolates through the floodplain alluvium (flat land area adjacent to a stream).
Flow regulation during non-flood periods: During non-flood periods, groundwater discharge acts to naturally regulate the flow in a river or the level of a lake or pond. In other words, during periods of abundant water, the water can enter the groundwater system whenever there is available capacity rather than contribute to seasonal flood peaks. During low flow periods, the water flows from the higher groundwater system into lower surface waters, so that the frequency and duration of extremely low flows are reduced.
Conserving Wildlife While Reducing Flood Risk
The nation’s coastal and riverine floodplains and surrounding land areas support large and diverse populations of plants and animals by providing habitat and critical sources of energy and nutrients for these organisms. Many species spend their entire lives in the habitats found in and adjacent to the floodplain. The wide variety of plants and animals supported directly or indirectly by floodplains constitutes an extremely valuable, renewable resource important for our economic welfare, aesthetic enjoyment, and physical well-being.
Many communities across the country are recognizing the connection between conserving wildlife and reducing flood risk to their inhabitants, and are engaging in activities that both protect important habitat and help minimize community flood loss. The success stories below provide examples of such community efforts:
- Palm Beach County, Florida developed its Environmentally Sensitive Lands Acquisition Program in the 1980s, which set criteria for ranking and classifying lands with significant native vegetation, including upland and wetland ecosystems, for acquisition and protection as natural areas. Through the program, thousands of acres have been acquired and maintained which protect and preserve hundreds of plant and animal species in addition to providing stormwater runoff control. In addition, the county has preserved natural floodplains through deed restrictions – largely on abandoned farmland – which reduces flood losses by maintaining flood storage and permeable areas as well as protects native species populations.
- The coastal city of Myrtle Beach, South Carolina has reduced its vulnerability to flooding from tropical storms and hurricanes by implementing a model beachfront management plan developed by the state of South Carolina. As a result of this plan, the city codes changed to require a 40-year setback line for private properties to help ensure the shoreline remains preserved mostly as open space. This effort has protected the city’s beaches and other sensitive ecological areas such as dunes, as well as prevented property and structural losses due to coastal storm impacts.
- Regulatory changes in Pierce County, Washington related to development limitations and low-density zoning were implemented for the purpose of protecting life and property from flooding, but have also resulted in a number of environmental benefits, such as allowing the flow regimes of several streams to return to a more natural state. This has enhanced in-stream habitats for fish and macroinvertebrates have diversified, leading to increased biodiversity overall.
*Received CRS credit for these examples. See CRS webpage.
For more case studies on communities that have used ecological approaches to flood risk reduction benefitting both people and nature, please read the Natural Defenses in Action report. For example, Cape May County, New Jersey, restored dune and wetland ecosystems in the footprint of the former resort town of South Cape May, creating water storage and management capacity while also providing habitat for migratory birds.