Today’s High Tide is a Flood!!

So when I came to work today, a lot of my work colleagues asked me why the roads on the way to our office were flooded and why the water level in the Cooper River was in the back yard of the Coastal Services Center.  It wasn’t raining, and we weren’t having a major storm.  So what was the deal?

Simple answer is, today’s high tide is a flood!!

We often say the opposite expression when trying to communicate that everyday tidal flooding will happen more often and last longer as sea levels continue to rise - “today’s flood is tomorrow’s high tide.”  In fact, we have worked with the City of Charleston on communicating this concept, when dealing with tidal flooding in low lying areas downtown.  We also try to communicate this concept on our flood frequency tab in our Sea Level Rise and Coastal Flooding Impacts Viewer.

So back to why is it flooding the CSC back parking lot today?  The more complicated answer is that there are several contributing factors.

  1. This week happens to be a Perigean Spring Tide.  This means not only is it a spring tide (exceptionally high and low tides that occur at the time of the new moon or the full moon when the sun, moon, and Earth are approximately aligned), but also the moon is closer to the Earth than it is most other times of the year (Lunar Perigee). By the way, a new moon high tide is actually higher than a full moon, because the moon and sun are on the same side of the earth, causing an extra gravitational force on the ocean. The predicted tide today is 7.14 feet above Mean Lower Low Water (the chart datum for NOAA Nautical Charts). 
  2. When we woke up today and went outside, it was cold in Charleston. We had a cold front come through on Tuesday, and the wind started blowing from the Northeast. The observations from the Charleston Airport showed winds out of the NE at 15mph with gusts to 21mph. Northeast winds push ocean water into Charleston Harbor and add water level to the existing tide. This effect is called surge. Though not associated with a storm, it is still a wind driven rise in water level. This wind caused water levels to be ~1 foot above normal
  3. Throughout the year, a slow seasonal change in sea level can also occur in response to the normal seasonal changes of regional atmospheric pressure, wind patterns, coastal currents, river flows, and heating and cooling of the ocean. Within the U.S., the seasonal change of sea level typically varies by 0.3 to 1 feet. In Charleston, sea levels are on average 0.16 feet higher than normal in November.
  4. Sea level has gradually raised the equivalent of 1.03 feet in the last 100 years in Charleston.  The numbers of high tides that have reached a flood-potential level (defined by the National Weather Service as 7.0 feet Mean Lower Low Water) have increased more than five times over the past 50 years.

Adding all these factors up resulted in one of the highest tides of the year today (close to 8 feet above Mean Lower Low Water) and since much of the SC Lowcountry infrastructure was built just above the mean of the highest tides (not the highest), it is not surprising that many areas flooded. These photos, taken out our back door by my colleague, Mark Finkbeiner, illustrate that when all these factors come together (some episodic and some chronic) today’s high tide is a flood.

References: 

Incorporating Sea Level Change Scenarios At The Local Level

Technical Considerations for Use of Geospatial Data in Sea Level Change Mapping and Assessment

Sea Level Rise and Coastal Flooding Impacts Viewer

Sea Level Trends

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