Defining King Tides

King tide is a colloquial term, leaving the project organizers to formulate their own definitions (Table 1). Conceptually, a king tide is an exceptionally high and naturally occurring tide that causes nuisance flooding (also known as, “sunny day tidal flooding”) (Figure 1). These events occur up to several times per month. The King Tides Project, however, defines these events according to the alignment of the Sun and Moon and states a “king tide” occurs twice per year. Fortunately, many king tides can be predicted, just as any other tidal event can be. Unfortunately, atmospheric disturbances can unexpectedly raise water to king tide levels. Thus, our conundrum: Is a king tide defined by its cause (lunar and solar constituents, atmospheric disturbances) or its effect (observed water levels and flooding)? Table 1 summarizes the different definitions used by 15 of the most active king tide initiatives. It is plainly evident that there is little consistency.

Table 1.

Location, organizers, definitions, and rate of occurrence for the most active King Tide Initiatives. Source: kingtides.net.

Figure 1.

Examples of the effects of king tides in coastal communities. (A) Flooding in a road in Strawberry, California, on 5 December 2017. (B) Flooded dock in Sausalito, California, on 3 January 2018. (C) Flooding in a Charleston, South Carolina, neighborhood on 24 April 2017. (D) Coastal erosion near the Springmaid Pier, Myrtle Beach, South Carolina, on 28 September 2015. Credit: (A) by Jef Poskanzer and (B) by Robert Sanford, California King Tide Photo Initiative ( https://www.flickr.com/groups/cakingtides/). (C) by Luke Schimmel and (D) by Christopher Stout, South Carolina King Tide Initiative ( https://mycoast.org/sc/king-tides/photos).

According to the King Tides Project, king tides occur twice per year when there is alignment and gravitational pull between the Sun and Moon. The exact alignment of the Sun and Moon is not defined, nor is the position of Earth clearly stated. The spring tides occur during linear alignment of the Sun, Earth, and Moon (perigee). Spring tides occur twice per lunar month (twice per ∼28 days, or ∼25 times per year) and are referred to as king tides by the U.S. National Oceanic and Atmospheric Administration ( https://oceanservice.noaa.gov/facts/springtide.html). The highest predicted tides (i.e. those formed from celestial body alignment, not atmospheric disturbances) are proxigean tides (a common scientific term) or perigean spring tides (according to the National Oceanic and Atmospheric Administration [NOAA]). These tides occur when the new or full Moon cycle temporally coincides with the perigee of the Moon (i.e. when the Moon is closest to the Earth). The perigean spring/proxigean tides occur 3–4 times per year and result in higher high tides compared to “normal” spring tides. Inspection of the definitions in Table 1 reveals that many king tide initiatives do not seem to understand the intricacies of the Earth–Sun–Moon relationships.

However, not all king tide definitions rely solely on tidal constituents. Atmospheric disturbances regularly raise water levels beyond those generated exclusively by tidal forcing. As a result, some places recognize a king tide based solely upon the water level. In South Carolina, a king tide is a spring tide greater than 2 m (6.6 ft) above mean lower low water (MLLW). In 2017, the South Carolina Office of Coastal Resource Management predicted 35 king tide events, but observed 128 (SCDHEC, 2018). This fivefold exceedance demonstrates the power of atmospheric conditions to increase sea levels well beyond the predicted tidal levels. The timing of king tides with atmospheric disturbances also plays a significant role in this underprediction; between November and March the Earth is closer to the Sun, resulting in higher spring high tides and increased potential for king tides (Patel, 2006; Woodworth and Blackman, 2004). Other periodic phenomena, such as the El Niño Southern Oscillation, can produce entire seasons of extreme high tides that cause persistent coastal flooding and erosion. For example, in 2015, California Sea Grant warned its communities that king tides and coastal flooding were expected to be even higher than expected due to El Niño (NOS, 2016).

Toward Solving the Conundrum

The King Tides Project is a forward-thinking public engagement initiative that allows scientists and citizens to visualize the impacts of rising sea levels on their communities. As coastal scientists, we should actively participate in this conversation and contribute to the King Tide Initiative and other similar public outreach efforts. However, for the success of this and other king tide initiatives, a “king tide” must be defined consistently and accurately. The definition should include its cause and potential impacts. The definition should refrain from predicting an absolute number of occurrences but may state a minimum number of occurrences based on known celestial circumstances. We suggest: King tides occur during spring tides and atmospheric disturbances, such as the passing of a low pressure system or during an El Niño event. The impact of a king tide includes road closures, overwhelmed stormwater systems, damage to transportation infrastructure, and coastal erosion.