Thursday, February 9, 2017

Sea Level Rise for Hampton Roads 2016

Sea Water Levels at Sewells Point, VA

With the end of the year, it is a good time to recap the data we have on flooding events and sea level rise for the Hampton Roads region. 

Hourly Data at Sewells Point

Average Annual Sea Level at Sewells Point

Because this is hourly data it is highly variable. Here is the annual average of the hourly data from Sewells Pt. 

There is considerable interannual variability caused by El Nino like processes in the global ocean that affect our local sea level.

10 year moving average at Sewells Point

We can take a 10 year moving average of the annual average to smooth out the interannual variations to get a better view of the long term trend.

The long-term average rate of increase is about 0.3 meters per 70 years (approximately) or 0.4 meters per century or about 1.5 feet per century. Note this is the running 10-year average so the first 10 years are truncated (the 1938 value is the average of the past 10 years). 

Hours per year of 'Nuisance Flooding'

The top panel shows the hours per year that water levels at Sewells Point were above the NWS defined nuisance flood level of 0.53 m above MHHW.

The lower panel shows the hourly Sewells Point water level. The solid horizontal line shows 0.53 m - the nuisance flood level. Sea level is rising causing there to be more nuisance flooding hours.

Return Period of Flooding Events

Blue line - Now - A one-meter flooding even occurs about every 3-4 years. A 1.4 meter flooding event occurs about every 33 years now.

Red line - 2050 with 0.5 m (1.5 feet) sea level rise - If sea level rises 0.5 meters as expected by 2050 a one-meter flooding event will be every year or more often and a 1.4 meter flood every 2 to 3 years.

The way to interpret this that flooding events that occurred once during a Hampton Roads resident's lifetime will occur several times during their children's.

Rate of Sea Level Rise

NOAA sea level trend

Quoting the NOAA site  (Link to NOAA Site)
The mean sea level trend is 4.59 millimeters/year with a 95% confidence
interval of +/- 0.23 mm/yr based on monthly mean sea level data from
1927 to 2015 which is equivalent to a change of 1.51 feet in 100 years."
More plots of interannual and seasonal trends are at the NOAA website.

The Past Year - 2016

Here are the Sewells Pt plots for the past year

I've labeled the major storm events. Note I'd like to know what the early May event was called.

2016 compared to other years

Each line is the average monthly temperature for a year. The heavy line is the monthly average data from 2016. Month months in 2016 had near record high average levels at Sewells Point.

You will notice that the highest water levels during any year are often in September. This is because the ocean waters are warmest then and they have expanded - making sea level higher. This is a seasonal effect.

Florida Current Transport

Over the past few years it has become evident that sea level in Hampton Roads is to some extent related to the strength of the Gulf Stream or the Florida Current. (They are the same. It is called Florida Current off Florida). You will note how it varies.

The reduced transport in early October coincided with Hurricane Mathew when is slowed over the Florida Current and apparently the southward winds slowed the Current. More on this from a paper Tal Ezer submitted. 

How this measurement is made can be found at Link to NOAA Florida Transport