Nuisance flooding refers to recurrent, minor flood events in coastal areas, often occurring without a major storm or heavy rain. It is sometimes called “sunny-day flooding” because the inundation is driven primarily by the normal cycle of high tides during fair weather conditions. This phenomenon has gained attention as its frequency has increased dramatically in recent decades, disrupting daily life and commerce in low-lying coastal communities. Unlike catastrophic flooding caused by hurricanes or massive storm surges, nuisance flooding is not life-threatening and typically involves only shallow water levels.
Defining Nuisance Flooding
Nuisance flooding is defined by the National Weather Service (NWS) as reaching or exceeding the threshold for minor coastal flooding impacts in a given location. This threshold is generally low, often involving only a few inches of water over dry land, which is enough to cause public inconvenience but not significant structural damage. For instance, a proposed global definition specifies a water depth between 3 and 10 centimeters (about 1 to 4 inches). This low-impact measurement distinguishes it from moderate or major flood events.
The key characteristic of this flooding is its high frequency and predictability, as it is directly tied to astronomical tides. Nuisance floods occur dozens of times a year in some locations, often during the highest tides of the month or year. The National Oceanic and Atmospheric Administration (NOAA) uses water level gauges to monitor when the water level exceeds the local minor flood threshold, tracking the increasing number of these events over time. This makes the phenomenon a chronic problem rather than an acute disaster, with cumulative effects becoming increasingly costly for affected communities.
The Mechanism of High-Tide Flooding
The occurrence of nuisance flooding results from two primary, interacting factors: the predictable mechanics of astronomical tides and the cumulative effect of rising sea levels. Astronomical tides are the regular, twice-daily rise and fall of sea water caused by the gravitational pull of the moon and the sun.
The highest tides, known as perigean spring tides or “king tides,” happen when the sun, Earth, and moon align, temporarily increasing the high-tide water level. Global sea level rise (SLR) acts as an elevated baseline upon which these normal tidal cycles operate. As oceans warm and expand and glaciers melt, the average sea surface elevation increases.
This elevation increase means that what was once a normal high tide now more frequently crosses the minor flood threshold. Sea level rise has effectively narrowed the “freeboard”—the gap between the average sea level and the elevation at which flooding begins—making it easier for minor tidal fluctuations to cause inundation. In many U.S. coastal areas, this mechanism has led to a dramatic increase in flood frequency, with some regions seeing a 300% to 900% increase since the 1960s.
This rise is compounded in some locations by local land subsidence, or sinking, which further elevates the relative sea level. The flooding is thus caused by the combination of an already elevated sea level meeting a naturally occurring high tide, often resulting in “clear-sky flooding.”
Immediate Impacts on Coastal Infrastructure
The frequent, shallow inundation of nuisance flooding causes immediate, localized consequences that impair the functionality of coastal communities. Transportation networks are among the first areas affected, as low-lying streets and roads become submerged, leading to temporary closures and traffic delays. Water depths of just a few inches can be enough to stop traffic flow, forcing commuters to seek alternate, longer routes and disrupting local commerce.
Public utility systems also suffer strain from the recurrent intrusion of saltwater. Storm drains and drainage systems, which are designed to handle rainwater runoff, can become overwhelmed or completely backed up by the incoming seawater. This can diminish their ability to drain the land, creating standing water that persists long after the tide recedes and increasing the risk of localized flooding from subsequent rainfall.
Saltwater intrusion into these systems also accelerates corrosion in metal components and pipes, leading to premature infrastructure degradation. The shallow flooding creates localized impacts on property and public health systems. In areas with septic systems, the rising water table and inundation can cause septic tank backups and failures, introducing public health risks. The repeated exposure to saltwater can also damage the undercarriage and electrical systems of parked vehicles and cause minor damage to the lower foundations of coastal structures.
The cumulative effect of dozens of such events each year strains municipal budgets and disrupts the rhythm of daily life.