High tide flooding is an increasing concern in coastal regions worldwide. This phenomenon involves recurrent flooding during unusually high tides, often without severe weather events. Its frequency has significantly increased in recent decades, prompting communities to adapt.
Defining High Tide Flooding
High tide flooding is commonly referred to as “nuisance flooding” or “sunny-day flooding” because it can occur on clear, calm days. This coastal inundation happens when water levels reach approximately 0.5 to 0.7 meters (about 1.75 to 2 feet) above the daily average high tide. It differs from storm surge, driven by powerful winds and low atmospheric pressure, or riverine flooding, which results from excessive inland precipitation.
The National Oceanic and Atmospheric Administration (NOAA) monitors and classifies high tide flooding events into minor, moderate, and major categories based on water levels exceeding average high tide. Minor flooding begins when water reaches 0.55 meters (1.8 feet) above average high tide, causing disruptions like stormwater backups and road closures. Moderate flooding occurs at 0.85 meters (2.8 feet), potentially damaging homes and businesses. Major flooding, at 1.20 meters (3.9 feet) above average high tide, can be destructive, sometimes requiring evacuations and infrastructure repairs.
Primary Causes
The primary driver behind the increasing occurrence of high tide flooding is global sea level rise. This rise is mainly attributed to the thermal expansion of ocean water as it warms and the melting of glaciers and ice sheets. Global mean sea level has already risen about 21 to 24 centimeters (eight to nine inches) since 1880, providing a higher baseline for tides to build upon.
Local factors can intensify this problem. Land subsidence, the gradual sinking of land, contributes to relative sea level rise, often from over-extraction of groundwater. Natural changes in ocean currents and tidal cycles can also exacerbate high tide events. A notable influence is the Moon’s 18.6-year lunar nodal cycle, which amplifies or suppresses tidal ranges. When its amplifying phase aligns with rising sea levels, projected for the mid-2030s, coastal cities may experience a surge in flood frequency and severity.
Consequences for Coastal Areas
The recurrent nature of high tide flooding impacts coastal communities and their natural environments. Infrastructure is vulnerable, with roads frequently closing, leading to traffic congestion and disrupted commutes. Stormwater drainage systems become overwhelmed, and septic systems can malfunction, potentially affecting public health.
Economic consequences include property damage and interruptions to local businesses. Studies in cities like Annapolis, Maryland, show minor high tide floods can reduce consumer visits to downtown businesses by 37-38 percent, with moderate floods causing even greater reductions of 63-65 percent. Environmentally, saltwater intrusion can contaminate freshwater sources, impacting drinking water supplies and harming coastal habitats. Standing water left behind by these floods can also create breeding grounds for mosquitoes and bacteria, posing additional public health concerns.
Community Responses
Coastal communities are implementing various strategies to adapt to and mitigate high tide flooding. “Grey” infrastructure solutions involve engineered structures designed to withstand water, such as elevating homes and businesses, constructing seawalls and levees, and improving drainage systems with features like one-way back-flow valves.
Complementing these approaches are “green” infrastructure solutions, which use natural systems to manage water. This includes restoring coastal wetlands, creating living shorelines with natural vegetation, and establishing natural buffers that absorb excess water. Practices like permeable pavements, rain gardens, and bioswales help infiltrate rainwater into the ground, reducing surface runoff. Many communities are exploring hybrid “green-gray” approaches, combining engineered structures with natural elements to achieve more resilient and cost-effective outcomes. Communities also benefit from early warning systems, such as NOAA’s Monthly High Tide Flooding Outlook, which provides daily flood likelihoods, supporting proactive planning and preparedness efforts.