A flood is defined as an overflow of water that submerges land not normally covered by water. This natural event occurs when a river, lake, or ocean exceeds its capacity, or when heavy rainfall saturates the ground faster than it can drain. Understanding the sequential progression of a flood is important for public safety and preparedness. The process involves distinct phases, moving from atmospheric predictions to the physical movement of water.
Flood Watches and Warnings
Before the physical inundation of land begins, meteorological services issue alerts to inform the public about the potential for flooding. These alerts are categorized to communicate the level of threat and the urgency of the required response. The distinction between a “watch” and a “warning” is based on the likelihood and imminence of the event.
A Flood Watch is issued when conditions are favorable, meaning flooding is possible but not yet certain. This alert serves as a notification to “Be Prepared,” giving residents time to monitor weather reports, gather emergency supplies, and make plans to move to higher ground. The watch indicates that the necessary ingredients for flooding, such as heavy rain or rapid snowmelt, are present.
Conversely, a Flood Warning is the highest level of alert, signifying that flooding is either imminent or already occurring. This is a call to “Take Action” immediately, as the hazard is a reality. A warning is issued when water levels reach or exceed a point that poses a threat to life and property.
The Ascending Stage
The ascending stage marks the physical beginning of the flood event, where water levels begin to rise noticeably. This phase starts with the “action stage,” where the water approaches the top of the riverbanks but remains contained. This initial rise may only cause minor inconvenience, such as the flooding of low-lying fields adjacent to the water body.
As the water continues to rise, the event escalates into the “minor flood stage.” This stage is characterized by the inundation of secondary roads and potential isolated property impacts. The river’s discharge has increased significantly, pushing water out of its normal channel and onto the floodplain.
The progression to the “moderate flood stage” occurs when the rising water seriously threatens structures and infrastructure. Roads become impassable, and buildings in low-lying areas start to take on water, often necessitating localized evacuations. The expansion of the floodplain becomes more pronounced, and the speed of the current increases, creating a significant hazard.
Cresting and Peak Flood Duration
The crest is the point where floodwater reaches its maximum height at a specific location before the level begins to recede. This elevation is measured against a fixed reference point, known as a datum, and represents the flood’s peak intensity. The maximum water level is often the most destructive moment, combining depth and velocity to create extreme hydraulic forces.
During the cresting phase, the water is at its highest velocity, capable of transporting large debris, including vehicles and structural components. This period causes the most significant structural damage, as foundations are undermined and buildings are subjected to maximum hydrostatic pressure. Even a few feet of moving water can exert tons of force.
The duration of the peak flood can last from a few hours to several days or weeks. This sustained period of maximum inundation prolongs damage to saturated materials, increasing the cost and complexity of recovery. Forecasting the exact height and timing of the crest is a primary goal for hydrologists and is central to effective emergency management.
The Recession Stage
The recession stage begins immediately after the water crests and levels start to fall, marking the end of the immediate physical threat. In many large river systems, the rate of recession is significantly slower than the rate of ascent. The water flows back into the main channel, slowly uncovering previously submerged land.
Even as the water recedes, a new set of hazards emerges. The receding water leaves behind a thick layer of contaminated sediment and debris, posing a serious public health risk.
The ground and structures remain saturated, leading to instability and the rapid growth of mold and mildew. Damaged electrical systems and gas lines also present hidden dangers long after the visible floodwaters have withdrawn. The recession stage requires continued caution as cleanup and recovery operations begin.