How high is that seawall? How high is that seawall? Understanding Elevation and Tides in Charleston City of Charleston | IT/GIS Division You may recognize this walkway in downtown Charleston. 'The Battery', as it is known, is a set of man-made seawalls. From the beginning of Charleston's history, these seawalls were built to protect the City's military assets from washing away in the daily tides and annual storms. Go to the Charleston County Public Library Time Machine Story *Lower Battery Seawall breached by Hurricane Irma in 2017 Photo by Jared Bramblett While the need for cannons is now a relic of our past, the Battery has been preserved and fortified over time. It continues to be a part of our iconic present both as a place for some of our most stunning views of the harbor, and as a physical barrier against high tides and storm surge. Sometimes, during extreme events, the Battery seawall isn't enough to hold back the ocean on the other side. What is the highest tide the High Battery seawall was built to protect us from? 12.14' Wait, is that a typo? 12.14 feet?! Does the seawall even go that high? That would be like the height of 2 refrigerators! You're right, the High Battery seawall is nowhere near 12 feet high, even from the harbor/water perspective (seen here). Of course, the question 'how high is that seawall?' implies we know its height. Instead, the key to understand how the seawall stands up to the impact of tides lies in knowing the seawall's top elevation. Let's have a closer look... A surveyor would report that the seawall's top elevation stands at approx. 9 feet above a baseline. What is the baseline? Do surveyors measure from the ground up? The seawall's elevation is based on a specific reference system called NAVD88. This is the baseline used to measure elevations on land. At this location, the NAVD88 baseline appears to match the seafloor. This is not always the case. The baseline does not always follow the physical ground. NAVD88 stands for the North American Vertical Datum of 1988. This standard sets land elevation at 0' based on the mean sea level at a tide station at Father Point/Rimouski, Quebec, Canada; this is the baseline reference for other land elevation measurements. Learn More about Datums So if the seawall's top elevation is 9’ high, how could it stop a 12’ tide? To answer that, we need to cover another important detail: Land is measured one way, using the baseline set by the NAVD88 system. Tides are measured using a different baseline. Why do we measure tides differently? Water’s always moving. It flows in and out of the harbor; it varies a lot between low & high tides. We have multiple baselines to measure from. How do you set a baseline for a moving target like water? Water levels are averaged over a period of time. NOAA measures tides over 19 year periods to get 3 important tide averages (also known as means). Mean Higher High Water (MHHW): the higher of each day’s two high tides averaged over time. Mean Lower Low Water (MLLW): the lower of each day’s two low tides averaged over time. Mean Sea Level (MSL): the final average between MHHW and MLLW. Tide ranges and Mean Sea Levels vary locally, based on the location and shape of coastlines. Our MSL in Charleston is 2.64" (.22') lower than the 0' NAVD88 baseline. Learn more about our Charleston Gauge Which of the averages do we use as the baseline for tides? In Charleston, we use MLLW as the baseline for our local tides. Why do we use MLLW and not MSL? MLLW is the baseline used on most NOAA nautical charts that show water depth. If we used MSL, half the tides would be below MSL and negative numbers. And how does MLLW compare to the 0' NAVD88 baseline? As you can see, a tide close to 0' MLLW is significantly lower than the baseline for land elevation (0' NAVD88). The exact difference (at our Charleston tide gauge) is 3.14'. Are we able to answer how the seawall's 9' elevation can protect us from a 12' tide then? To be able to compare land elevations and tides, we need to do simple math. To convert from any NAVD88 land elevation to a MLLW tide, remember: land starts higher (+) than water, so ADD the difference (3.14'). 9' NAVD88 (the seawall's elevation) +3.14' (the difference between the NAVD88 and MLLW baselines) = 12.14' MLLW This is how a 9’ seawall is meant to protect this area from a 12’ tide. So, if there was a barrier measuring 12’ NAVD88 elevation, it could protect against a 15.14’ tide? 12' NAVD88 + 3.14' = 15.14' MLLW That's it! Of course a barrier lower in elevation is more limited in its protection. This is why the City is raising the Lower Battery Seawall. *Lower Battery breached by Irma in 2017 By the way, we can also apply this math in the other direction: If we see a tide forecast and want to know what land elevation may be at risk of flooding, we'll need to SUBTRACT 3.14' from the tide measurement. Are we sure about these elevations? Great question! While we've rounded our seawall's top elevation to simplify the math, rest assured that its exact elevation is known. Elevations across the US are measured and managed by the National Geodetic Survey. National Geodetic Survey Map NGS Marker Location at the Battery Measured locations, like that on the High Battery seawall, can be identified by these official markers. The official elevation at this marker is 8.55 feet (NAVD88); last verified in 11/07/2019. Marker Data Sheet Why is this important for us to know? Our tides are in constant flux and are influenced by many factors: the moon phase that determines the pull of gravity on our oceans, wind speed and direction, water temperature and currents, and the location and shape of our coastline. Tides influence a lot of what happens along our coast. The timing and extent of tides can impact our safety during recreation activities (fishing, kayaking) as well as our planning and response to high tide events (like King Tides) and storms. NOAA has been collecting data at the entrance to our Charleston harbor for over a century. Long-term studies of our station data show that the average heights of our tides and extreme tide events are increasing. Now, more than ever, it's important for all of us to understand the meaning of our local water levels. Tide Trends Dashboard Sea Level Rise & Elevation Map Ready to keep an eye on our daily tides? Follow TIDEeye, to track real-time tides, weather, and road conditions here in Charleston. TIDEeye TIDEeye is a web & mobile-friendly tool for keeping an eye on our latest local weather and tides, as well as their impact on our traffic and road closures. https://gis.charleston-sc.gov Developed by R. Hauck & E. Paz (City of Charleston GIS Division) with contributions from D. Marcy (NOAA) and M. Fountain (City of Charleston Stormwater).