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What is a Coastal Flood?

A coastal flood is the inundation of typically dry, low-lying land areas adjacent to coastlines and estuaries by seawater, exceeding established flood thresholds. This phenomenon, driven by a confluence of natural and human-influenced factors, poses significant threats to coastal communities, infrastructure, and ecosystems.

Understanding Coastal Flooding

Coastal floods are far more complex than simply rising tides. They represent a dynamic interplay of factors, and understanding each component is crucial for effective risk management and mitigation.

Factors Contributing to Coastal Flooding

Several factors contribute to the occurrence and severity of coastal floods. These include:

Tides: Astronomical tides, predictable fluctuations in sea level caused by the gravitational pull of the Moon and Sun, are the baseline upon which other flooding factors build. The height of a tide varies throughout the month due to the lunar cycle, with spring tides (higher than average) and neap tides (lower than average).
Storm Surge: A storm surge is an abnormal rise in sea level during a storm, particularly hurricanes and nor’easters. It’s primarily driven by strong winds pushing water towards the shore, combined with the low atmospheric pressure of the storm. This is often the most destructive element in coastal flooding events.
Sea Level Rise: Global sea level rise, driven by thermal expansion of ocean water and melting of glaciers and ice sheets due to climate change, provides a higher baseline for all coastal flooding events. This means that even minor storms and high tides can now reach areas that were previously unaffected.
Wave Action: Large waves, generated by wind, can overtop coastal defenses and cause erosion, exacerbating flood impacts. Even relatively small waves can contribute to flooding if they occur during high tide or storm surge.
Subsidence: Land subsidence, or the sinking of land, can occur due to natural processes like sediment compaction or human activities such as groundwater extraction. Subsidence effectively lowers the elevation of coastal areas, making them more vulnerable to flooding.
River Discharge: Heavy rainfall upstream can lead to increased river discharge into coastal areas. This can contribute to flooding, particularly in estuaries and deltas where freshwater and saltwater meet.
Atmospheric Pressure: Low atmospheric pressure associated with storms can also contribute to sea level rise, although this effect is typically less significant than wind-driven storm surge.

Types of Coastal Flooding

Coastal floods manifest in various forms, each with its own characteristics and impacts:

Nuisance Flooding (Sunny Day Flooding/High Tide Flooding): Relatively minor and frequent flooding events that occur during high tide, even without a storm. These floods can disrupt daily life by inundating roads and infrastructure. The frequency of nuisance flooding has increased dramatically in recent decades due to sea level rise.
Storm Surge Flooding: As mentioned previously, this is a dramatic and potentially devastating form of flooding caused by storm surges. These floods can reach significant depths and extend far inland, causing widespread damage.
Flash Flooding: Rapid flooding events often triggered by intense rainfall, particularly in areas with poor drainage. While not always directly caused by coastal processes, flash flooding can exacerbate coastal flooding problems by overwhelming drainage systems and contributing to runoff into coastal waters.
Estuarine Flooding: Occurs when freshwater runoff from rivers combines with tidal influences, leading to elevated water levels in estuaries. This can impact water quality and ecosystem health.
Inundation: A general term referring to the submergence of land by water. In a coastal context, inundation can be caused by any of the factors listed above.

Coastal Flood FAQs

Here are some frequently asked questions about coastal floods, providing further insight into this critical issue.