Tidal Floods and Moon Wobbles
August 20, 2021 - Jared Bramblett
As of July 31st of this year, there have been 18 tidal floods (tide levels > 7-ft MLLW) in Charleston Harbor, with one reaching the moderate tidal flooding threshold (>7.5-ft MLLW). Over the same period last year, Charleston Harbor experienced 22 tidal floods and one moderate tidal flood. Analyzing the historic tidal flooding data cataloged in the NWS Coastal Flood Event Database, 2021 is tied with 2014 for the 5th most tidal floods observed from January to July. The table below on the left shows the top 10 years with the most tidal floods over January to July. Historically, September through November are the most active months for tidal floods, with approximately 48% of all tidal floods occuring during the three months. The below table on the right shows the percentage of tidal floods that have occured over each month. It will be interesting to see how the frequency of tidal flooding increases over the coming months.
In early July, a NASA report was released that predicted a dramatic increase in the number of tidal floods as we enter the 2030s. The is predicted because of a naturally occuring wobble in the moon’s orbit that occurs every 18 years paired with predicted sea level rise. I’ll post some additional thoughts on the future implications of this, but another interesting note in the article is that we are currently coming out of a phase of the wobble that increased the tides. Perhaps this helps explain the dramatic increase in tidal floods we’ve seen recently (particularly 2019 & 2020), and maybe we'll see the number of yearly tidal floods decrease over the next five to ten years.