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Floods usually occur when water inundates areas that are typically dry land. There are very few regions on the planet where people do not need to be mindful of the potential for floods. In most cases, floods develop gradually over hours or days, allowing residents time to make preparations or evacuate the area. However, in some situations, floods can arise rapidly with minimal warning. After wildfires, floods are the second most pervasive natural disaster that impacts the Earth. Their widespread nature makes them a significant concern across many parts of the world. A flood can develop in many ways; let’s take a look at the most common types of floods.
Flash floods
Flash floods can develop rapidly, within just hours after heavy rainfall. These sudden flood events are extremely hazardous, capable of transforming a gentle stream into a powerful, thundering wall of water that sweeps away everything in its path. Most flooding fatalities are a result of flash floods. Unlike other flood types, there is no classification system for the magnitude of flash floods.
Desert environments are particularly vulnerable to flash flooding. Dry river beds known as wadis and arroyos only flow during heavy rain events. Wadis can be especially dangerous during flash floods because they lack riparian zones to slow the energy of the floodwaters.
Due to their sudden onset and the high velocity of the water, flash floods represent an immense danger. The forceful floodwaters can carry large objects like rocks, trees, and vehicles. It is never advisable to drive through a flash flood, even if it appears shallow, as the sheer speed of the water can easily sweep a car away. The hazardous nature of flash floods arises from their rapid development and the tremendous power of the moving water.
Coastal flooding
Coastal flooding refers to the sudden and rapid inundation of coastal areas caused by a short-term rise in water levels due to storm surges and extreme tides. The magnitude and extent of coastal flooding depend on the topography of the coastline, the intensity of the storm surge, and the broader underwater depths near the coast.
While coastal flooding is a natural process critical to coastal ecosystems like salt marshes and mangrove forests, in areas with human development it poses a major challenge that can lead to property damage and loss of life. Over the past two centuries, an estimated two million people have perished in flood events, primarily in South Asia. With growing coastal populations and sea level rise from climate change, the world faces an increasing potential for devastating coastal flood disasters.
Extreme weather systems and high tides can cause sea levels to rise, sometimes resulting in coastal flooding. Low-lying coastal areas typically have defences like man-made barriers or natural sanguine buffers to protect against the influx of water. However, as global warming continues, coastal flooding is expected to become a recurrent and increasingly severe problem worldwide.
Urban flooding
Urban flooding differs significantly from rural flooding due to the developed catchment areas in cities, which can increase flood peaks by 1.8 to 8 times and flood volumes up to 6 times higher. As a result, flooding occurs rapidly in urban areas, often within minutes due to faster flow times. Urban centres have high population densities and people living in vulnerable areas face severe impacts from flooding, sometimes including loss of life. Beyond the flooding event itself, urban residents also suffer secondary effects like exposure to infectious diseases, loss of livelihood, and in extreme cases, loss of life.
Urban areas are also economic hubs with critical infrastructure that requires 24/7 protection. In most cities, damage to vital infrastructure has ramifications not just locally but can impact the broader state, country, or even globally. Major cities in India have experienced loss of life and property, disruptions to transportation and power systems, and outbreaks of epidemics due to flooding. As such, managing urban flooding must be a top priority.
The increasing trend of urban flooding is a universal phenomenon that poses great challenges for urban planners worldwide. Urban flood problems range from relatively localized incidents to major events that inundate entire cities for hours or days. The impacts can be widespread, including temporary relocation of residents, damage to civic amenities, deterioration of water quality, and increased risk of epidemics. Effective strategies are needed to address this growing issue.
River flooding
River flooding is one of the most common types of inland flooding, occurring when a body of water like a river exceeds its capacity and overflows its banks. This typically happens when there is high rainfall over an extended period, causing the river to essentially “burst its banks.” When a river floods in this manner, the localized overflow can lead to considerable damage to surrounding properties and infrastructure, as well as pose significant safety risks to people in the area. To help prevent river flooding, especially in flat or highly populated regions, robust flood defences and river bank reinforcement are needed.
A river flood arises when excessive water levels cause a river to rise over the top of its banks and spill over. The high water levels can be caused by heavy rainfall from landfalling tropical storms or persistent thunderstorms over the same area for a prolonged duration. River flooding can also result from a combination of rainfall and snowmelt runoff entering the river basin. Additionally, ice jams along a river can restrict water flow and lead to upstream flooding. Implementing adequate flood prevention measures is crucial for mitigating the destructive impacts of river flooding events.
Groundwater flooding
Groundwater flooding occurs when the natural underground drainage systems cannot remove rainfall quickly enough, causing the water table to rise above the surface level of the ground. This type of flooding happens when the level of water held within the soil or rock underground – known as the water table – rises and reaches the ground surface level. Once the water table breaches the surface, water starts seeping out and flooding can occur. This means water may emerge up through floors or underground spaces like basements or cellars. Unexpectedly, the water does not always appear in low-lying valley bottoms, but can also surface on hillsides.
In contrast to river flooding which happens more rapidly, groundwater flooding is a much slower process that unfolds over days, weeks or even months after periods of heavy or prolonged rainfall saturate the underground aquifers. Once groundwater flooding begins, the flooded conditions may persist for weeks or months until the water table subsides. The gradual onset and long duration of groundwater floods distinguish them from other more short-lived flooding events tied directly to excessive surface runoff. Monitoring groundwater levels is important for predicting and preparing for this slower type of widespread inland flooding.
Drain and sewer flooding
Drain and sewer flooding occurs when wastewater and/or surface runoff cannot properly enter or gets backed up within a drainage or sewer system, causing it to overflow and remain above ground or enter buildings. Drains and sewers can also experience surcharge conditions, where the wastewater is held under pressure within the system but does not overflow to the surface and cause flooding.
While heavy rainfall is often a culprit, sewer floods are not always directly caused by weather events. They can also result from blockages or failures within the drainage infrastructure itself. Drain and sewer flooding can impact both the interior of buildings as well as outdoor areas.
Even without water breaching the surface, surcharge conditions within drain and sewer lines, where the pipes are overwhelmed and water backs up under pressure, still constitute a flood event. Though weather is a common trigger, sewer backups and overflows can occur due to non-weather-related obstructions or system failures as well. The escaped water can inundate indoor spaces like basements or overflow outdoors onto streets and grounds. Monitoring drainage capacity and maintaining sewer systems is important for preventing both interior and exterior flood damage.
Floods are a pervasive and significant natural disaster that impacts regions across the globe. They can develop through various mechanisms, including flash floods, coastal flooding, urban flooding, river flooding, groundwater flooding, and drain and sewer flooding. Each type of flood presents unique challenges and risks to communities, infrastructure, and the environment.
This article has been modified using Artificial Intelligence (AI) tools.
