The things I sit around thinking about. I just did a blog posting on the cool map that NOAA has to predict storm surge, and particular around Jupiter Florida.
Read the actual storm surge for Hurricane Idalie at Cedar Key.
The measured storm surge for Hurricane Ian.
I wanted to see how the forecast surge aligned with my real world observations of the storm surge water levels at my house in Hurricane Jeanne in 2004. This led me to an interesting question. How can I tell how say a 5′ storm surge relates to the elevation of my house?
First my real world observation. I recently went through a whole bunch of work to file for and obtain a Letter of Map Amendment (LOMA) from FEMA for my house east of US-1 in North Palm Beach. During that process I asked the surveyor to locate and flag the 6′ NAVD elevation contour in front yard as this the BFE or Base Flood Elevation shown on the FEMA FIRM. I remember where the water came up to in 2004 during hurricane Jeanne and that was with a reported 5′ storm surge at the FP&L power plant in Riviera Beach. The actual high water line at my house was indeed just below this 6′ contour and in our area the mean high water line is at +1′ by this NAVD datum. I also know the finished floor elevation of my house is +7.5′ NAVD 88. So does that mean that with say an 8′ storm surge, is my first floor is wet or nearly wet?
Intuition would tell us yes and it turns out intuition is right in this case. Storm surge is the water level ABOVE the ocean water LEVEL ON THAT DATE. Storm surge is caused by the temporary effect of the wind and low pressure of a hurricane. It is added to whatever the tide happens to be when the storm hits. If the storm hits at high tide then it’s X feet above the high tide. But tides fluctuates based upon the time of the year and the cycle of the moon. In south Florida we are all aware of this and what we call the ‘King tides’ which happen to occur in, you guessed it, hurricane season.
First we need to ask what is mean high water? Well, at the Lake Worth Pier the MHW is given as +0.37 ft ABOVE the NAVD88 datum. The TYPICAL high water line in at +0.37 ft using the same feerance line used by surveyors to show the elevations of my land and the first floor of my house. However this Mean High Water (MHW) changes base upon where you are. For the body on water near my house, Lake Worth, the LABINS data give Mean High Water at about +1′ NAVD.
Looking at my flood Elevation Certificate (EC) for my house I note that my FF EL = 7.5′ (NAVD 88). Thus, if the forecast is for a 7′ storm surge and that storm surge is forecast to occur on a date and at a time of an otherwise normal high tide then my house finished first floor should be wet as 7′ ADDED to the +1′ mean high water is more than the +7.5′ NAVD of my house. However, if the forecast is for a lesser storm surge but it’s occurring in a tide that just high because for the cycle of the moon I could have a wet floor OR worse. Hide from wind, RUN from water.
If you have an old survey or elevation certificate for your house which give the elevation of your floor in the 1929 datum then THIS POST will show you how to translate this to the newer 1988 datum.