The projected increase in high tide flooding in 2018 may be as much as 60 percent higher across U.S. coastlines as compared to typical flooding about 20 years ago, according to a report released on Wednesday by the National Oceanic and Atmospheric Administration (NOAA).
The U.S. broke records for tidal flooding this past year as storms, combined with rising sea levels, inundated downtown areas of major cities. The top five cities to see the highest number of flood days and break records included Boston; Atlantic City, New Jersey; Sandy Hook, New Jersey; Sabine Pass, Texas; and Galveston, Texas.
These cities saw more than 20 days of high-tide flooding between May 2017 and April 2018, the “meteorological year” covered by the report. NOAA scientists write that not only did these cities face the brunt of powerful nor’easters and hurricanes – but much of it was driven mainly by sea level rise fueled by climate change.
The average number of high tide flooding days was the highest measured at 98 NOAA tide gauges. Over 25 percent of the coastal locations tied or broke their individual records for high tide flood days.
New, national flooding thresholds
NOAA’s assessment of high-tide flooding is based on a new, national flooding threshold released in February. The new threshold allowed NOAA scientists to focus on more impactful, deeper high tide flooding in some locations than in previous reports.
In turn, this caused the number of flood days tallied in the report to be lower in some cities compared to previous years. However, this does not mean there are less flooding days.
The new report uses a higher threshold elevation at some locations such as Annapolis, Maryland, Washington, D.C., Wilmington, North Carolina and San Francisco, meaning water must get higher at the tide gauge before it counts as flooding.
With the oceans rising at about 3.0 millimeters per year, due primarily to melting land ice and warming water, which, by the way, expands – the rate is accelerating, and it has led to a steady increase in U.S. coastal flooding in recent decades. So while cities like San Francisco may have minimal above-ground infrastructure impacts from flooding at the present time, this is not to say things won’t change.
But using the new threshold method has helped in establishing a national coastal flooding vulnerability assessment that is an aid in identifying flooding that would typically impact infrastructure throughout a region, rather than at a single location. This kind of information can help communities in establishing high-tide flooding mitigation measures.
High-tide flooding mitigation efforts
Even though many coastal cities are beginning to address rising sea levels and the resultant flooding, the report highlights the fact that much still needs to be done.
Boston, for example, is into a city-wide initiative to protect itself from the effects of climate change and rising seas. The city has already begun efforts to protect vulnerable areas by building new flood walls and elevating streets. The initiative, called Climate Ready Boston, was started in 2016.
Norfolk, Virginia is another city that has taken on efforts to mitigate the damage from high-tide flooding and sea level rise. With 14 days of flooding recorded over the past year, Norfolk has tried to weave adaptation to rising seas into all of its planning and operations, including rewriting the zoning code to include new buildings are built higher up and are more resilient to flooding.
However, Norfolk’s low elevation and proximity to waterways make it extremely vulnerable to precipitation flooding (rain intensity exceeds storm drain capacity), storm flooding (surge from storms like hurricanes), and tidal flooding (related to elevation and proximity to the coastline).
Then there is the problem with impacts from sea level rise and subsidence, which only magnifies the effects of flooding events. This is why the Coastal Resilience Strategy, one of a series of efforts in Norfolk to revise the city’s flood and coastal zone ordinances, was put into place.
