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Posted on January 29, 2016 Updated on January 29, 2016

Last weekend’s blizzard along the East Coast of the United States caused significant flooding along the coasts of Maryland, Delaware, New Jersey, and New York.  Even though this system was not a tropical cyclone, the mechanics of storm surge flooding are essentially the same whether the cause is a hurricane or extratropical storm.   The blizzard provides us an excellent opportunity to delve into the topic of vertical datums, which we promised to tackle in a previous blog post anyway!

Simply put, a vertical datum is a reference level.  Whenever you talk about water levels related to tides or storm surge, that water level needs to be referenced to some datum to provide essential context.  For example, a water surface 2 feet above the floor means something very different than a water surface 2 feet above the roof.

There are many vertical datums out there.  Some are based on tide levels (tidal datums), while some are based on the general shape of the Earth (geodetic datums).  Savvy and more technical experts generally use geodetic datums such as the North American Vertical Datum of 1988 (NAVD88) because they’re more precise and applicable to a large area, such as an entire continent.  For most of us, however, we see water levels referenced to tidal datums such as Mean Lower Low Water (MLLW) or Mean Higher High Water (MHHW).

Some locations along the coast have two high tides and two low tides per day (e.g., the U.S. East Coast), while some areas only have one high tide and one low tide per day (e.g., the U.S. Gulf Coast).  Mean Lower Low Water (MLLW) is simply the lowest of the two low tides per day (or the one low tide) averaged over a 19-year period.  This 19-year period is called the National Tidal Datum Epoch, which currently runs from 1983 through 2001.  So to calculate MLLW for a particular tide station, the National Ocean Service (NOS) took the levels of all the lowest low tides from 1983 to 2001 and averaged them.  Similarly, NOS calculates Mean Higher High Water (MHHW) by averaging the highest of the two high tides per day (or the one high tide) over the same 19-year period.

Historically, MLLW has been used for navigational purposes in the marine waters of the United States and its territories.  Navigational charts from the NOAA Office of Coast Survey show water depths relative to MLLW, or how far the ocean bottom extends below the MLLW line.  If boaters know the tide forecast relative to MLLW, the depth of the ocean bottom relative to MLLW, and the draft of their boat or ship (the distance between the waterline and the bottom of the hull), then they can deduce if the vessel will hit the sea floor.  Since this is the most common way that tides have been referenced, the National Weather Service (NWS) has generally used MLLW as a reference for its water level forecasts, and most tide gauge data is referenced to MLLW by default.  People who have lived along the same stretch of coastline for many years have become accustomed to knowing what type of coastal flooding will occur when water levels reach specific thresholds above MLLW.

But what about people who don’t know those relationships between MLLW–or any other datum for that matter–and coastal flooding (which change from location to location along the coast, by the way).  For this reason, NHC has moved toward providing tropical cyclone related storm surge forecasts in terms of inundation, or how much water will be on normally dry ground.  You can go here for more information on the Potential Storm Surge Flooding Map, issued by NHC when tropical cyclones are forecast to affect the East or Gulf Coasts of the United States.  For the purposes of using water level observations to get an idea of how much inundation is occurring during a storm, NHC uses MHHW.

To answer this question, it’s probably helpful to look at a cross-section of a typical coastline.  Shown below is such a schematic, which depicts both the Mean Lower Low Water line and the Mean Higher High Water line.  Anything seaward of the MLLW line is typically submerged under water.  The region between the MLLW and MHHW lines is called the intertidal zone, and it is the region that is submerged at high tide and exposed at low tide.  Intertidal zones include rocky shorelines, sandy beaches, or wetlands (marshes, mudflats, swamps, and mangroves).  Because intertidal zones are submerged during a typical high tide, people don’t generally live here.

NHC and NOS consider anything landward of the MHHW line (marked as the supratidal zone in the graphic) as normally dry ground.  Only in the most extreme high tide cycles and during storm surge or tsunami events does that region become submerged under water.  Seawater that rises past the MHHW line is considered inundation, and therefore water level measurements relative to MHHW can be considered as proxies for measurements of inundation.  NOS has deemed MHHW as the best approximation of the threshold at which inundation can begin to occur.  While safe navigation of boats is a downward-looking problem that requires the use of MLLW, coastal flooding is an upward-looking problem that is best communicated using MHHW.

Dr. J. D. Boon, Professor Emeritus of the Virginia Institute of Marine Science, probably puts it best in his book Secrets of the Tide:

We should mention that use of other vertical datums is in no way wrong.  There are some very good uses for datums such as MLLW or NAVD88, but NHC uses MHHW when referencing storm tide observations to put things into a frame of reference that is understood by the majority of people at risk for coastal flooding.  If we see a water level observation of 7 feet above MHHW, there’s a pretty good chance that some location in that area is being or was inundated by as much as 7 feet of water on ground that would normally be dry.  This relationship worked quite well during Hurricane Sandy in 2012.  Peak water levels measured by NOS tide gauges at the Battery in Manhattan and Sandy Hook, New Jersey, were between 8 and 9 feet above MHHW, and high water marks surveyed by the US Geological Survey after the storm indeed supported inundations of 8 to 9 feet above ground level in places like Sandy Hook and Staten Island.

Since we said the recent blizzard provides a great case for us to explain vertical datums, let’s take a look at some of the water level observations during the event and how they compared to documented flooding.

Some of the worst storm surge flooding from the event occurred in extreme southern New Jersey and Delaware.  So let’s look at the area around Cape May, New Jersey.  The NOS tide gauge at Cape May measured a peak water level of about 9 feet above MLLW (8.98 feet to be exact).  But does that mean that residents of Cape May and surrounding communities had as much as 9 feet of water on their streets?  No, it just means that the water surface got about 9 feet higher than the “imaginary” line that marks the average of the lowest of the two low tides per day.

At the Cape May gauge, the difference between MLLW and MHHW is 5.45 feet, which means that the peak water level was only about 3.53 feet above MHHW (8.98 minus 5.45).  Nearby, the peak water level observation from the NOS gauge in Atlantic City, New Jersey, was 3.42 feet above MHHW.  So does that mean that residents of Cape May, Atlantic City, and surrounding communities had as much as 3 to 4 feet of water on their streets?  Actually, yes it does. Pictures obtained via Twitter from West Wildwood, North Wildwood, and Atlantic City appear to support an estimate of 3 to 4 feet of inundation.  See below for the evidence.

Incidentally, if you’re ever watching water level observations during a storm from the NOS Center for Operational Oceanographic Products and Services (CO-OPS) website, you can choose which vertical datum you’d like to use.  The default will come up as MLLW, but you can change it to MHHW (as we do at NHC) or another datum such as NAVD88 or Mean Sea Level.  Alternatively, NOS CO-OPS also provides a real-time “Storm QuickLook” website during coastal flooding events, and the default vertical datum on this page is MHHW.  Below is a comparison of the water level data from Lewes, Delaware, during the blizzard using MLLW (top) and MHHW (bottom) as reference levels. Notice that the curves don’t change, only the reference numbers on the left vertical axis.

And finally, if you’re ever looking at storm surge forecast guidance online, make sure you know which vertical datum you’re looking at!  For example, the NWS’s Extratropical Storm Surge (ETSS) model is available on the Meteorological Development Laboratory website, and although data shown is relative to Mean Sea Level (MSL), the vertical datum can be changed to MHHW or MLLW.

Thanks go out to Cody Fritz, Shannon Hefferan, and Jamie Rhome from the NHC Storm Surge Unit, as well as the folks over at the National Ocean Service, for their assistance in putting together this blog post.

The finale ends on a rather humorous note, with Jae - eon commenting that the two should raise a cat together and Nabi responding that he is

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