Tidal flooding (high-tide flooding) is an increasingly frequent phenomenon affecting low-lying coastal regions. This flooding is distinct because it is not caused by heavy rainfall or the destructive winds of a major storm system. Instead, it is a predictable, recurring event driven by the regular astronomical cycles that govern ocean water levels. Tidal flooding causes cumulative damage and disruption, even on days that are otherwise sunny and calm. Its effects are becoming more pronounced as global sea levels continue to rise, raising the baseline water level upon which the tides operate.
Defining Tidal Flooding
Tidal flooding occurs when the predicted high tide temporarily exceeds a specific elevation threshold, causing water to inundate coastal infrastructure. This threshold is often defined locally by agencies like the National Oceanic and Atmospheric Administration (NOAA) as the level that causes minor public inconvenience, such as standing water on low-lying roads or overwhelmed storm drains. The flood event is a function of the astronomical tide, spilling onto the land until recession.
The fundamental difference lies in the driving force behind the water level rise. Storm surge is a sudden, unpredictable event caused by strong winds pushing water inland during a severe weather event like a hurricane. Storm surge is a wind-driven phenomenon that occurs above the predicted astronomical tide, often resulting in catastrophic damage. Tidal flooding, conversely, is the result of the predictable cycle of the tide rising high enough to exceed the elevation of the built environment.
Riverine flooding is caused by excessive freshwater runoff overwhelming river channels, often due to heavy precipitation. Tidal flooding, however, is purely saltwater intrusion from the ocean and typically affects areas immediately adjacent to the coast or tidal rivers. While rainfall can sometimes exacerbate a tidal flood by preventing storm drains from emptying, the primary cause remains the height of the sea level.
The Astronomical Drivers
Ocean tides are governed by the gravitational pull exerted by the Moon and, to a lesser extent, the Sun. The Moon is the dominant force because its proximity to Earth creates a stronger differential gravitational pull across the planet’s surface. This force causes the oceans to bulge outward on both the side of Earth facing the Moon and the side facing directly away from it, creating two high tides daily.
The alignment of the Earth, Moon, and Sun dictates the magnitude of these tidal bulges. When these three celestial bodies align in a straight line (during the new and full moon phases), their gravitational forces combine to produce the greatest tidal range. These are known as Spring Tides, resulting in exceptionally high high tides and very low low tides. The term “spring” refers to the water “springing” forth or surging, not the season.
Conversely, when the Moon and Sun are positioned at a 90-degree angle relative to the Earth, their gravitational forces partially counteract each other. This configuration (during the Moon’s first and last quarter phases) results in a smaller tidal range known as Neap Tides. Subtle variations in tidal height are caused by the elliptical orbits of the Moon around the Earth and the Earth around the Sun. Tides are slightly enhanced when the Moon is at perigee (closest to Earth) and when the Earth is at perihelion (closest to the Sun), which typically occurs in early January.
Frequency and Terminology
Because this flooding occurs without storms or heavy rain, it has earned the name “sunny day flooding.” It is also referred to as “nuisance flooding” because the initial impacts are disruptive rather than life-threatening or immediately destructive. This terminology highlights the public inconvenience caused by the recurring nature of the events.
The most extreme, predictable high tides are often called “King Tides,” a non-scientific but widely used term for the highest tides of the year. King Tides typically coincide with the alignment of the Sun, Moon, and Earth (Spring Tides) when the Moon is also near its closest point to Earth (perigee). These events are predictable months in advance, allowing coastal managers and residents to prepare.
The frequency of tidal flooding events has been noticeably increasing across U.S. coastlines over recent decades. This increase is primarily attributed to the gradual rise in local sea level. As the average sea level baseline rises, the same astronomical forces that once produced a normal high tide now push the water past the local flood threshold more frequently. For many communities, the number of days experiencing high-tide flooding is now double what it was just 20 years ago.
Coastal Areas Affected
Tidal flooding directly impacts the lowest-lying infrastructure in coastal zones, disrupting daily life and causing cumulative damage. Coastal roads and thoroughfares are often the first to be inundated, leading to temporary closures and significant traffic disruptions. Repeated exposure of pavement and roadbeds to saltwater accelerates their degradation, requiring more frequent and costly maintenance.
Storm drains and stormwater systems are also severely affected, as rising seawater enters the outfalls and backs up through the system. This reversal prevents the drainage of any rainwater, effectively turning a minor rain shower into a local flooding event. In areas utilizing septic systems, saltwater intrusion can interfere with the system’s ability to leach wastewater into the ground, posing public health concerns.
The corrosive nature of salt water poses a long-term threat to structural integrity, accelerating the deterioration of metal components and electrical equipment in urban areas. Homes and businesses in low-lying zones experience repeated property damage, and the constant presence of standing water can lead to mold and mildew issues. These localized impacts demonstrate how a slight, predictable rise in water level can trigger widespread, chronic problems for coastal communities.