Come Hell and High Water

Superstorm Sandy floods West Street

A new report from the National Oceanic and Atmospheric Administration(NOAA), the federal scientific agency responsible for study of oceans, major waterways, and the atmosphere, predicts that Lower Manhattan will, in the next 12 months, experience between double and triple the number of flooding days that it did in 2000.

The same report warns that ten years from now, the number of flood events will total between five and eight times the benchmark set at the turn of the century, and that by the year 2050, there will be local flooding events as often as every three days.

The report, an annual study compiled by NOAA, is titled, “2018 State of U.S. High Tide Flooding with a 2019 Outlook,” and was released last week. It paints a grim picture of what climate change (especially the component consisting of rising sea level driven by melting glaciers) it likely to do to coastal communities throughout the United States, noting that, “tide gauges… are measuring rapid increases in coastal flood risk along U.S. coastlines due to relative sea level (RSL) rise.”

But particularly dire predictions are reserved for the corner of the country in which New York is located: “The Northeast Atlantic coast is projected to experience the most [high-tide flooding, or HTF] in 2019 with the regional-median expected value of 8 HTF days. Individual locations are projected to experience more (likely range): 12-19 days in Boston, Massachusetts, 8-13 days in New York City region, and 10-15 days in Norfolk, Virginia.”

The Battery Underpass in Sandy’s aftermath

For Lower Manhattan, NOAA’s analysis and predictions are driven by a tide gauge located at the Battery. This gauge registered high-tide flooding — defined as water reaching 21 inches above a benchmark known as “mean higher high water” or the average height of the highest tide recorded at a station each day during a recording period — for a total of five days in 2000. For 2018, that figure was 12 days. Before 2019 is out, NOAA predicts that the new total will be a minimum of eight and maximum of 13 days.

Looking farther into the future, NOAA scientists calculate that by the year 2020, high-tide flooding at the Battery will likely occur at least 20 days per year (or more than once a month) and no more than 40 times each year (or approximately once each ten days).

By 2050, NOAA projects, high-tide flooding at the Battery will take place no less than 50 times per year (about one per week) and up to 150 days per year (meaning approximately three days).

These calculations are especially sobering because they exclude extreme-weather events, such as hurricanes, and focus instead on routine tidal action. “Flooding that decades ago usually happened only during a powerful or localized storm can now happen when a steady breeze or a change in coastal current overlaps with a high tide,” the report warns.