A flood is an overflow of water that submerges land not typically covered by water. While natural processes cause floods, human activities often increase their intensity, frequency, or geographic extent. Understanding human contributions helps explain the growing problem of flooding worldwide. Human interference contributes to flooding through direct alterations of landscapes and waterways, and broader impacts on global weather patterns.
Altering Natural Landscapes
Human activities that change the natural surface of the land reduce its capacity to absorb water, increasing runoff and flood risk. Deforestation diminishes the land’s ability to manage rainfall. Trees intercept precipitation and their root systems absorb water, holding soil in place. Without trees, rainwater rapidly becomes surface runoff, and soil erosion increases, reducing the ground’s water-holding capacity and potentially raising riverbeds with sediment.
The destruction of wetlands removes a natural buffer against flooding. Wetlands function as natural sponges, absorbing and storing large volumes of rainwater and runoff. Their conversion or drainage for agriculture or development eliminates this flood mitigation, leading to higher peak flooding and altered water flow patterns.
Certain agricultural practices contribute to increased runoff. Intensive farming and tilling compact soil, reducing its permeability and ability to absorb water. This compaction, particularly on sloped land, means more water flows over the surface rather than infiltrating the ground, increasing flood susceptibility.
Modifying Waterways and Coastal Areas
Direct human manipulation of rivers, lakes, and coastlines alters natural water flow and elevates flood risk. Dams and reservoirs, while sometimes built for flood control, can disrupt natural river flow. These structures store vast amounts of water, but mismanaged releases or structural failures can exacerbate downstream flooding. Dams also lead to sediment accumulation and increased erosion downstream, changing the river’s characteristics.
Levees and channelization, which involve building artificial banks or straightening rivers, contain water but can increase its velocity. This prevents rivers from naturally overflowing into floodplains, which traditionally store excess water and slow its flow. Concentrating floodwaters within engineered channels can lead to failures or shift flood risk to other areas.
Development along coastlines, including land reclamation and destruction of natural barriers like mangrove forests and coral reefs, removes storm protection. Mangroves and coral reefs dissipate wave energy and reduce storm surge impact. Their removal leaves coastal areas vulnerable to storms and rising sea levels.
Impacts of Urban Development
The growth of cities and their infrastructure contributes to flooding. Impermeable surfaces, such as concrete, asphalt, and rooftops, prevent rainwater from infiltrating the ground. This leads to rapid surface runoff, as water has nowhere to go but across the paved landscape. In urbanized areas, surface runoff can rise from about 10% in natural conditions to over 55%.
Urban drainage systems are often overwhelmed during heavy rainfall. Many existing storm drains may be inadequately designed, poorly maintained, or lack capacity to handle increased runoff from impervious surfaces. This can result in localized flooding as water accumulates on streets and other low-lying areas.
The accumulation of waste and debris complicates urban drainage. Litter, leaves, and other refuse can clog storm drains and culverts, impeding water flow. When drainage pathways are blocked, water backs up, leading to severe and prolonged inundation in urban environments.
Accelerating Extreme Weather Patterns
Human activities accelerate extreme weather patterns that exacerbate flooding. Burning fossil fuels and industrial processes release large quantities of greenhouse gases, such as carbon dioxide, into the atmosphere. These gases trap heat, leading to an increase in global temperatures.
A warmer atmosphere holds more moisture, which contributes to more intense and frequent heavy rainfall. For instance, the air’s capacity to hold water vapor increases by approximately 7% for every degree Celsius of warming. This increased atmospheric moisture can overwhelm natural drainage systems and land absorption, leading to severe floods.
Global warming drives sea level rise through two mechanisms: thermal expansion of ocean water as it warms and the melting of glaciers and ice sheets. Global mean sea level has risen by about 7.8 inches (20 centimeters) since 1901, with the rate accelerating in recent decades. This rise elevates baseline water levels, increasing coastal flooding and storm surges, even during normal high tides.