Wednesday, March 11, 2020

2019 Sewells Point Update

Sea Level Rise Update 2019 

This is the annual look at seawater levels at the NOAA Sewells Point water level gauge.

Caution -
  • There are many nuances to this data and interpretations of it. 
  • Do not make decisions based on the information here
  • Contact me if you plan to use the information presented here.
Some Key Points
  • The average annual water level for the whole year 2019 was the highest since 1928.
  • Hours of nuisance flooding was 5th highest since 1928.
  • The monthly average water level was highest on record in January, May, August, and October. It was the third-highest in July.
  • By 2050 floods that used to happen every 12 years will happen every year.
Note: all plots are referenced to Mean Higher High Water (MHHW).

Hours per year of nuisance flooding

First, the iconic plot of hours of nuisance flooding per year.



Nuisance flooding is a value determined by the National Weather Service in collaboration with regional emergency managers. It is 0.53 meters (1.7 feet) above Mean Higher High Water: the horizontal black line in the lower panel. 

The upper panel shows:

1. There are occasional years with abnormally high hours of flooding. These are typical during a major hurricane or northeasters with long durations in the area.
2. There is a slow, steady increase from about 2005.
3. Based on this plot we can expect at least 40 to 50 hours of nuisance flooding per year in the coming years.

The lower panel shows 


The lower panel shows the hourly water level since 1927. Since it is hourly you can see the twice-daily range in tides of about 1.5 meters or 3 feet. You can also see the occasional higher values well above 1 meter when hurricanes and northeasters pass our region.

Because of sea-level rise, the water levels are slowly increasing. The solid line is the nuisance flooding value of 0.53 m. You can see how the daily tides are slowly reaching more often into the nuisance flooding value. This is why we have more flooding.

Note - the hours of flooding are calculated assuming that any water level point above 0.53 m represents one hour of flooding. Thus, even if the water level was above 0.53 meters for one minute (unlikely) we count it as one hour. 

Hourly Plot for the whole record for 2019




This is a plot of the water level measured every hour so you see the tidal range of a few feet and the variability month to month. 

You can see that the water level can change over weeks to months.   Changes in the ocean, wind fields and passage of hurricanes and northeasters cause these longer-term changes. The extreme values in September through December were caused by hurricanes and northeasters. 


Average Monthly Water Levels 

The above plot shows the average monthly water level for each month since 1928. The heavy line is the 2019 data. The monthly water levels for 2019 were in the top four for the entire record. 


Ranking of months
This table shows the 10 highest months (highest to 10th highest). 2019 was in the first place 4 times, 3rd place once, 4th 4 times and 6th 2 times. 



January      2019  2016  1998  2005  2010  2017  1996  2011  1993  2013  
February    1998  2005  2010  1999  1983  2019  2016  2013  1993  1973  
March         2010  2018  2013  2019  2014  1983  2011  1987  2016  2012  
April            2010  2018  2013  2019  2014  1983  2011  1987  2016  2012  
May            2019  2017  2005  1998  2011  2016  2008  2014  2006  2012  
June           2018  2009  2012  2019  2014  2011  2016  1997  2017  2005  
July            1975  2018  2019  2017  2015  2012  2009  2005  2011  2016  
August        2019  2017  2014  2011  2010  1995  2013  2015  1998  2018  
September 2018  2017  2016  2019  2015  2009  2003  2014  2011  2006  
October      2019  2015  2016  2013  2004  2009  2005  2018  2012  2002  
November 1928  1931  1936  1933  1934  1941  1929  1930  1943  1940  
December 2018  2009  2012  2014  1996  2019  2015  2010  2011  2016 



Average Monthly Water Level Movie

Now we show an animation of the monthly water level values from 1928 through 2019


Yearly Average

In 2019 the average water level was the highest since records began in 1928.

Ranking of years: 
 2019, 2018  2016 2010 2017 2014 2012 2009 2011 2005


Flooding Return Period

Floods 1.5 feet above MHHW occur every 12 years now on average. In 2050, when water levels are predicted to be higher by 1.5 feet flooding will occur every year. 



Trends



The figure above is directly from the NOAA website. The linear trend is about 4.7 mm per year or 1.54 feet per century. Note that the most careful analysis of these records strongly suggest that the rate is not linear.

The Sea Level Rise Report Cards produced at the Virginia Institute of Marine Science are an excellent source of information on acceleration rates and the causes of sea-level rise.

I urge you to go to that site for more information on projected rise rates.

They calculate the current linear rate at 5.33 mm/year and the acceleration rate as 0.13 mm/year. Note that both numbers are slightly higher than last year. 

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