Naturally Kiawah Magazine Volume 39 | Page 41

Referencing Sea Level Life on Kiawah Island moves with the tidal cycle as the ocean level moves up and down. But how is that measured? There are five common sea level reference standards called datums: • • MLLW, mean lower low water • • NGVD29 (National Geodetic Vertical Datum of 1929) • • MSL, mean sea level (and GMSL, global mean sea level) • • NAVD88 (North American Vertical Datum of 1988) • • MHHW, mean higher high water • • MLLW is the average height of the lower tide recorded at a given tide station each day during a 19-year recording period. NOAA references tides to that datum for navigational purposes. • • MHHW is the average of the daily higher tide at a given station, again during the same 19-year recording period. It generally defines the landward boundary of a salt marsh, that is, the area that is normally flooded on daily high tides. NOAA uses MHHW to define nuisance flooding. For Charleston, nuisance flooding begins when the tide is 0.38 meters (1.25 feet) above MHHW. MHHW is perhaps the most common-sense reference because exceeding it actually starts bringing sea water onto the land. WINTER/SPRING 2018 • VOLUME 39 • • NGVD29 is the old reference used to measure the elevation of most infrastructure such as roads, buildings, and flood maps. It will continue to be used by surveyors until new flood maps and requirements are adopted. • • NAVD88 is the new standard reference for measuring infrastructure and flood maps today. NOAA uses this standard for scientific purposes, and tide elevations, such as king tides, reported in public media are referenced to the NAVD88 standard. For the Charleston area, if you choose MLLW as the standard, then NGVD29 is 2.16 feet higher, MSL is 2.92 feet higher, NAVD88 is 3.14 feet higher, and MHHW is 5.76 feet higher. Using these values, elevations referenced to any of these datums can easily be converted to any of the others. For 2018, the predicted king tides in the Charleston area are: • January 1-4 • August 9-13 • November 23-26 • May 15-17 • September 6-10 • December 23-24 • June 12-16 • October 6-11 • July 11-15 • November 6-8 39