A flood occurs when water overflows its typical confines and inundates land that is generally dry. This happens when the volume of water flowing through an area exceeds the capacity of the system to drain it. Floods vary significantly in their onset, ranging from a gradual rise over several days to a destructive wall of water appearing in minutes. The mechanisms causing this inundation stem from prolonged meteorological events, intense localized rainfall, or forces originating in the ocean.
Prolonged Weather Systems and River Overflow
Riverine flooding is often characterized by a slow to medium onset, sometimes taking days for the water level to reach its maximum. This type of inundation frequently follows a period of prolonged rainfall that saturates the ground across an entire watershed. Once the soil is fully saturated, it cannot absorb additional moisture, causing subsequent rainfall to immediately become surface runoff.
This excessive runoff is channeled into tributaries and streams, which eventually feed into the main river system. The cumulative volume of water from a large catchment area can overwhelm the river’s capacity, forcing the flow to overtop its banks and spill onto the adjacent floodplain. This build-up is delayed because it takes time for the water from the entire watershed to drain into the main channel.
Another contributor to regional flooding is the rapid thawing of a large snowpack. When temperatures rise quickly, or rain falls onto existing snow, the massive volume of meltwater is added to the drainage system. This rapid snowmelt dramatically increases the water volume in rivers, especially when the ground beneath the snow is frozen and impervious, preventing absorption.
Sudden and Localized Flood Events
In contrast to regional flooding, sudden and localized events, known as flash floods, are characterized by rapid timing and high danger. A flash flood typically begins within six hours of the causative event. The most common trigger is an intense, localized downpour, such as those associated with severe thunderstorms, where the rainfall rate exceeds the ground’s capacity to absorb it.
These events are particularly dangerous in steep terrain or narrow valleys, where water rapidly funnels into a concentrated area, creating a fast-moving, destructive surge. In urban environments, a unique form of localized flooding occurs due to impervious surfaces like concrete and asphalt. These materials prevent water from soaking into the ground, leading to immediate surface runoff that overwhelms storm drains and sewer systems.
Cities can experience severe flooding when drainage infrastructure is unable to handle the sudden volume of water. Sudden water releases from man-made structures also cause rapid inundation. The catastrophic failure of a dam or a levee can instantly unleash a massive volume of stored water, resulting in a flash flood with extreme flow speed and destructive power downstream.
Ocean-Driven Flooding Mechanisms
Flooding is not always caused by precipitation falling inland; it can also be driven by forces originating in the ocean. The most destructive coastal mechanism is the storm surge, an abnormal rise of water generated by a severe storm, such as a hurricane or cyclone, moving over the sea. This rise is separate from the normal astronomical tide and is caused by two primary factors.
First, the storm’s powerful winds push a dome of ocean water toward the shore. Second, the extremely low atmospheric pressure at the storm’s center draws the sea level upward. When this surge coincides with the normal high tide, it creates a storm tide that can reach heights of 20 feet or more, leading to extensive coastal inundation.
Another ocean-driven event is a tsunami, which is entirely unrelated to weather. A tsunami is a series of extremely long waves caused by the sudden displacement of a massive volume of water, typically from an undersea earthquake, landslide, or volcanic eruption. Unlike wind-driven waves, a tsunami involves the entire water column from the ocean floor to the surface, rushing inland as a rapidly rising flood. Unusually high seasonal tides combined with strong onshore winds can also cause minor coastal flooding, sometimes called tidal flooding, even in the absence of a major storm.