A floodplain is a generally flat area of land adjacent to a river or stream channel, extending outward to the base of the surrounding valley walls. This landmass is a natural, dynamic component of a river system, built and shaped over long periods by the water that flows through it. It is characterized by its susceptibility to being covered by water during periods of high discharge.
How Floodplains Are Formed
Floodplains are primarily built through the dual geological processes of erosion and deposition driven by a river’s flow. As a river meanders across a valley, it erodes the outer banks where the water flows faster, while simultaneously depositing sediment on the inner banks where the flow is slower. This process, known as lateral accretion, gradually widens the valley floor over time.
The most significant formation event occurs when a river overflows its banks during a flood. Once the water leaves the confines of the channel and spreads across the broader valley floor, its velocity decreases suddenly. This reduction in speed causes the water to drop its suspended sediment load, known as alluvium, across the floodplain surface.
Repeated flooding events leave behind successive layers of fine silt, sand, and clay, which slowly build up the flat landmass. This accumulated material creates the characteristic fertile soil of a floodplain.
The Natural Functions of a Floodplain
When undeveloped, floodplains provide a range of ecosystem services that benefit both the environment and human communities. One of the primary functions is natural flood storage, where the flat area acts like a temporary basin for excess water. By spreading and holding high volumes of water, floodplains reduce the speed and height of flood peaks, which protects downstream areas from more severe damage.
These areas also play a significant role in improving water quality by acting as natural filters. As floodwaters slowly move across the vegetation and soils, pollutants, excess nutrients, and sediment from surrounding runoff are trapped and filtered out. This process helps to cleanse the water before it returns to the river channel or percolates into the ground.
The regular inundation and rich, deposited sediments support diverse and productive ecosystems. Floodplains provide habitat for numerous species of plants and animals adapted to wet conditions, including wetlands that act as nurseries for fish and breeding grounds for migratory birds. The temporary storage of water also facilitates the recharge of local groundwater aquifers.
Regulatory Zones and Risk Assessment
Regulatory bodies map floodplains to assess and manage the risk to human development. These classifications are based on statistical probability to define areas where flooding is likely to occur, with the most significant boundary being the Special Flood Hazard Area (SFHA). The SFHA is the land area that would be inundated by the “base flood,” commonly referred to as the 100-year flood.
The term “100-year flood” signifies a flood event that has a one percent chance of being equaled or exceeded in any given year. This annual chance is the standard used by agencies, such as the Federal Emergency Management Agency (FEMA), to delineate zones on official flood maps. Properties located within this zone are considered to be at a high risk of flooding.
These regulatory maps dictate development restrictions and building codes to mitigate future damage, such as requiring new structures to be elevated above the predicted flood level. The federal government mandates that owners of properties with federally backed mortgages located within the SFHA must purchase flood insurance. The classifications allow communities to understand their level of risk, which influences land-use planning and emergency preparedness efforts.