Let’s Talk About Flooding on Seabrook Island – Part 2

This is the second of a three-part series, “Let’s Talk About Flooding on Seabrook Island”.

What kind of flooding do we get?

There are several types of flooding that can occur on Seabrook Island – – normal rainfall flooding, extreme precipitation, nuisance tidal flooding, and storm surge flooding.

Normal rainfall flooding occurs when the ground is already saturated, allowing no place for the water to run off. The result is standing water in low areas that will drain over time.

Extreme precipitation flooding occurs when the amount of rainfall exceeds the design capacity of our stormwater infrastructure. In general, Seabrook Island stormwater infrastructure is designed to accommodate a 10-year storm event. Charleston County defines a 10-year storm event as 6.5 inches of rain or more in a 24-hour period. It is called a 10-year storm event because it is statistically likely to occur once every 10 years based on historical rain data. Rain data for Charleston County is collected at the Charleston airport. The amount of rain that falls during a storm event on Seabrook can vary depending on where you are on the island.

While extreme precipitation is very unpredictable, Seabrook Island is likely to experience a ‘10 + inches in 31 days’ event almost once a year and a ‘5+ inches in a single day event’ about every 5 years or so. Over the last four years, Seabrook Island has experienced four ‘10+ inches in 31 days’ events, which is about normal.  However, with the help of hurricanes Joaquin, Matthew and Irma, we have experienced three ‘5+ inches in a day’ events in relatively quick succession. For those of us who are relatively new to Seabrook, this sequence of rainfall events may have come as something of a surprise.

Nuisance tidal flooding is some combination of high tide, atmospheric pressure and wind, i.e., high tide of .5’ up to 1’ above the mean higher high water (MHHW) mark; low atmospheric pressure which raises sea level, and sustained SE onshore winds (35 mph can raise sea level by as much as 20 inches or 1.85’).Storm surge flooding is caused by astronomic high tide plus storm surge. Storm surge is the rising of the sea level as a result of atmospheric pressure changes and wind associated with a storm. Storm tide flooding of 1-3’ above the mean higher high water (MHHW) mark causes minor flooding; 3-5’ above MHHW causes moderate flooding; and greater than 5’ causes major flooding. For example, during Hurricane Irma we experienced moderate flooding, which was 3.7’ above MHHW.

Astronomic tides are predicted tidal levels that result from the gravitational pull of the earth, sun, and moon. The mean higher high water mark is 5.76’ for Charleston Harbor, while the astronomic high tide for Charleston Harbor is approximately 7.5’.  

-Submitted by the SIPOA General Operating and Maintenance Committee

 

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