SUNNY DAY FLOODING: IF YOU’RE NOT FREAKED OUT, YOU’RE NOT PAYING ATTENTION
Today’s Flood Will Become Tomorrow’s High Tide
High tide flooding, also called sunny day, blue sky, or nuisance flooding, occurs when an exceptionally high tide overwhelms infrastructure systems and causes flooding with no associated rain event. Though local definitions can vary, NOAA recently outlined a nationally consistent definition of high tide flooding as when the median water level reaches or exceeds 0.54 m above the long-term average daily highest tide. High-tide flooding events can cause damage to storm and wastewater systems, and they can flood roads and surface infrastructure.
The 2017 NOAA State of the Climate Report indicates that sunny day flooding events are increasing in frequency, depth, and extent. Every coastal region of the country is currently experiencing these increasing tides; however, the effects are more immediate along the eastern and gulf coasts. Cities in these regions are dangerously close to a tipping point, where “nuisance” flooding becomes destructive in nature. Another report recently published by NOAA estimates that under an intermediate/low risk model scenario these regions will see high-tide flooding every other day or more often. The intermediate scenario predicts flooding 365 days per year. The figure below shows the predicted increase in flooding (left) and the percent predicted to be driven by tides alone (right) for these two model scenarios [Image Source] .
The ENSO (El Nino/ Southern Oscillation) cycle has also been linked to high tide flooding. During El Nino, sea level is higher along much of the west coast and northern east coast of the United States, which alone can cause an increased risk of flooding. New data have indicated that, due to a series of ocean-atmosphere teleconnections, we tend to see a corresponding shift favoring onshore winds in regions that experience higher sea levels due to El Nino, thus amplifying the flood risk even further.
Forecasters are looking into the idea of using this information to create an annual flooding outlook, similar to the hurricane outlook issued at the beginning of each hurricane season. Regional planners would be able to use this to inform their budgets and priorities in attempt to be prepared for years that are likely to be worse than others in terms of flooding. In any case, we need to begin preparing for a new reality. The late NOAA Scientist Margarete Davidson coined the phrase “today’s flooding will become tomorrow’s high tide,” and it seems as though we are headed straight into that reality.