A question we some times receive is “My home’s first floor is supposed to be at 10 feet so when we get a seven foot flood tide will the water be three feet below my floor level?”  The simple answer is No,  but to understand why we must first look at the different ways that land and tidal water is measured.    Because land is static, that is never changes,  there is a datum or reference point that is called the North America Vertical Datum 1988 or NAVD88.   This datum is where “zero” is using NAVD88 reference point.   As tidal water height changes depending on the tide the datum or reference point used is the Mean Lower Low Water or MLLW datum.   This is the point where “zero” in the vertical measurement using the MLLW reference point.   MLLW water is the average of the lower low water heights of each tidal day over a specific 19 year period (currently 1983 to 2001).   The distance the tide rises from low tide to high tide is different at different Latitudes.  for example in Florida the tide will only rise about a foot  from low to high whereas in Maine the tide will rise over 14 feet every low to high tides.   This means that the MLLW is different at every location.   That is why we have different tide tables for each different location.  As not every location has a tide predictions you have to use the closest for your location, so for North Wildwood’s back bay tide we use the West Wildwood tide predictions.   To see tide predictions for different locations (click here) .   Now that we know that land and tidal water are measured using different reference points we need to figure out what the relationship  between MLLW and NAVD88  for our specific area.   To determine the relationship we to look for a National Geodetic Survey or NGS benchmark.   There are many of these located around the area.  One is located on the entrance way to City Hall.   Most show only vertical elevation of the marker that is put there by the NGS.   Only a few have the relationships to tidal water.   The closest one with tidal water is Benchmark Station #8535835.   Looking at the information (see below) we learn that for our area zero feet NVGD88 is equal to 2.75 feet MLLW. So to convert a MLLW height in our area to a NAVD88 height just subtract 2.75 feet or conversely to change a NAVD88 height to MLLW height you add 2.75 feet. Each town will have a different
relationship and number to add/subtract from either MLLW or NAVD88, for example in Ocean City you use 2.06 feet and in Stone Harbor you use 2.66 feet.  Now to answer the original question   “My home’s first floor is supposed to be at 10 feet so when we get a seven foot flood tide will the water be three feet below my floor level?”   As buildings are on land NAVD88  reference point was used to obtain your homes first floor height of 10 feet which after you do the math is equivalent to 12.75 feet above MLLW.      Working from the flood water height of 7.0 feet using the MLLW reference point you find that the water height is equivalent to 4.25 feet NAVD88 or 5.75 feet below your 10 foot floor height.   Even though the water will not enter your house you still have to be concerned with your yard, garage, sheds, etc.  As landscaping, curb heights, driveways can change your yards elevation it is important that you make note of where the water comes up when it is at a specific height.   As wind driven water and at times of heavy rain with flooding can raise the height of water so  it is important to add this information to your notes.   NWOEM posts the tide heights after each flood on the OEM Facebook page.   You can also get the tide heights from the weather station and tide gauge using the link on the left of this page or clicking (HERE)  Remember that the National Weather Service uses the MLLW datum when predicting heights in their Coastal Flood Warnings and Watches.  Also as it is important to boaters to know how much water is below them tide charts are in MLLW datum.   Surveyors use the NAVD88 for land elevations.   A handout with the same information in this post is available on the left hand side of this page at the link called “Vertical Elevations Explained” it also contains a reference chart comparing both datum.    Coming up next year is NAVD22 which will be the first datum using measurement taken by satellite.  Below is a screenshot of the benchmark#8535835