Flooding is the overflow of water onto land that is typically dry. This process is driven by an imbalance where the volume of water entering a system exceeds the system’s capacity to contain or drain it away. The mechanisms that cause this overflow are diverse, ranging from prolonged inland rainfall to rapid coastal sea-level rise.
Fluvial and Pluvial Flooding: Overwhelming Inland Systems
Inland flooding most commonly arises from two distinct hydrological processes: fluvial and pluvial. Fluvial flooding, often called riverine flooding, occurs when the volume of water in a river or stream exceeds the physical capacity of its channel banks. This is typically the result of prolonged, widespread precipitation or rapid snowmelt within the river’s catchment area.
The severity of a fluvial flood depends heavily on the soil’s saturation level before the event begins. When soil is already saturated from previous rainfall, it cannot absorb additional water, leading to a phenomenon known as saturation excess runoff. Any new rain immediately flows overland, dramatically increasing the input volume into the river network. Alternatively, during very intense downpours, the rate of rainfall can exceed the rate at which the soil can infiltrate water, causing infiltration excess runoff and a rapid surge in river flow.
Pluvial flooding, or surface water flooding, is distinct because it occurs independently of an overflowing river or body of water. This type of event happens when high-intensity rainfall overwhelms the local drainage infrastructure or the natural permeability of the ground. In areas with poor drainage, rain falls faster than it can be channeled into storm drains or absorbed, causing water to pond in streets and low-lying areas.
This mechanism is particularly common in urban environments where the sheer intensity of a localized storm is a primary factor. Pluvial flooding can occur rapidly and is often categorized as a flash flood, where water accumulates in minutes rather than hours. The movement of water in these events is dictated by micro-topography, flowing along streets and pavements until it reaches a low point or drainage inlet.
Coastal Flooding Caused by Storms and Tides
Coastal flooding is driven by forces from the sea, primarily related to powerful storms and astronomical cycles. The most frequent cause is storm surge, which is an abnormal rise of water generated by a storm, moving above the predicted astronomical tide level. This rise is created by two main factors: the strong, sustained onshore winds pushing water toward the coast and the low atmospheric pressure within the storm center, which allows the sea surface to temporarily bulge upward.
The total water level, known as the storm tide, is the combination of this storm surge and the normal astronomical tide. Storm surges become particularly destructive when they coincide with a naturally occurring high tide, especially a “king tide,” which is one of the highest tides of the year. This combination can push water levels far beyond the reach of normal waves, leading to extensive inundation of low-lying coastal zones and estuaries.
A separate, more rapid form of coastal flooding is caused by tsunamis, which are not related to weather systems. A tsunami is a series of waves generated by the sudden displacement of a massive volume of ocean water, most commonly from a submarine earthquake or underwater landslide. Unlike wind-driven waves, tsunamis have extremely long wavelengths and often appear initially as a rapidly rising tide rather than a breaking wave, but they carry immense destructive power far inland.
How Landscape and Infrastructure Amplify Flooding Events
The severity of any flooding event is frequently amplified by the characteristics of the local landscape and human-built infrastructure. Urbanization, which converts natural landscapes into areas covered by impervious surfaces, significantly worsens surface runoff. Materials like concrete, asphalt, and rooftops prevent rainwater from infiltrating the ground, drastically increasing the speed and volume of water flowing over the surface.
This rapid, concentrated runoff quickly overwhelms storm sewer systems and natural drainages, amplifying the risk of pluvial flooding. The natural topography of a region, particularly the presence of floodplains, also plays a role. Floodplains are low-lying areas adjacent to a river designed to hold excess water during high flow events. When human development encroaches on these areas, structures are placed directly in the path of the river’s natural overflow.
Infrastructure designed to control water can introduce a risk of catastrophic failure. Structures such as dams, levees, and seawalls are built to contain water flow and protect communities. If water levels exceed the design height, these structures can be overtopped, or structural weaknesses can lead to breaches. A sudden failure of these barriers can release enormous quantities of water rapidly, causing a flash flood with much greater destructive force than a gradual inundation.