From The Blog

KICA Responds to Historic Flooding

Last month Kiawah Island experienced two unprecedented flooding events within two weeks. One was caused by over 18 inches of rainfall in two days (with some help from high tides). The second was caused by so-called “King Tides” which reached over nine feet in elevation. Normal high tides on our island range from five-to-seven feet.

Obviously, both were huge challenges for the island drainage system. There was no early forecast of the historic rainfall and the eventual prediction was 50% lower than what occurred.

Kiawah’s lakes and ponds are connected to outfall structures which, allow surface water to drain through the island into the Kiawah River and Atlantic Ocean for several hours before and after low
tides. With several days’ notice, these connected lakes and pond water levels can be lowered some in order to accommodate excess surface water. Since there was no early forecast until one day before the rain, this was not possible. An engineering consultant, Stantec Company, reported that even with lakes lowered, Kiawah would have had flooding with this record rainfall. Their report states, “Results show there was negligible impact in peak water surface elevations when attempting to pre-drain the ponds. The following three factors contributed to the results: The first is the extreme volume of rainfall in a short period of time. Secondly, the outfalls could only drain during low tides, which were approximately two periods of six hours. During high tide, the rainfall could not discharge into the rivers, and instead must store in the ponds until the high tide recedes. Lastly, the low and high tides were approximately 2.5 feet higher than normal during this event. This further squeezed the window of opportunity for Kiawah to drain during low tide.” In a follow up with Stantec, they state: “KICA had no viable options.

On average, when an inch of water falls, its direct impact on the water surface elevation of a pond is roughly an inch. However, when factoring in runoff from roads, yards and surrounding property, there’s a multiplier effect of two-to-three times the rainfall total that winds up in the drainage system. With that in mind, roughly 36-54 inches on average was added to the drainage system. There was four-to-five feet of water to move off the island. Outfall gates were set to block incoming flow, but tides were too high to allow much time for outflow at low tide. The good news is Kiawah’s drainage system worked and it took just two tide cycles for Kiawah’s system to drain all the water after the rain stopped, and tides were lower.

Going forward, KICA will be looking at ways to improve the situation during these very infrequent events. Kiawah’s drainage system is gravity fed. It takes two tide cycles to lower lake and pond levels six inches. In this case, even at low tide, drain pipes were covered with water. It also takes roughly 20 minutes to manually raise and lower each gate. KICA will look at computerized control of the gates.

There also may be ways to relocate drain lines. Individual street drains eventually flow into larger drain pipes, which then flow to outfall structures. With an unprecedented amount of rainfall, this puts significant pressure on even a 48-inch pipe, as many smaller pipes flow into it. KICA will look at the possibility of adding or redirecting some pipes to a closer outfall.

Also, for years KICA has purchased fuel efficient smaller vehicles for the security force. They do not do well in high water. So as the association replaces existing vehicles, it will look at purchasing some larger trucks, which will enable security to move freely about the island to assist owners in need.

The second flooding event was caused by the historic 9.4- foot tide. It happened at the time of a full moon when the gravity pull is greatest. A strong wind from the northeast also helped drive the tide higher on Kiawah. This means the water surrounding Kiawah is higher than some of Kiawah’s roads. Engineers are studying exactly where the most serious flooding occurred to see if any mitigation actions could be undertaken. These locations include Kiawah Island Parkway between Sora Rail and the Fire Station; the Flyway – Governors Drive intersection by the Vanderhorst gate, and the Marsh Edge Lane area. All-in-all, little can be done when the tides on those infrequent occasions reach over nine feet.

Over the last several years, KICA has placed an added emphasis on making repairs to the island’s drainage system. KICA owns and maintains more than 43 miles of underground drainage, and roughly 25% of it consists of metal pipes installed by the original Kuwaiti developers in the 1970s. Forty years submerged in salt water has taken a toll and several years ago KICA redoubled its efforts to reinvest in this critical infrastructure.

Hopefully, both of these floods were “1,000 year” events, but KICA will continue to study and consult with engineers to see if there are ways to reduce the inconvenience to Kiawah members and guests of not having a means to move on, off, or around the island. This will include intense discussion with government officials about improving off-island roads between Kiawah and the mainland.

Article contributed by Digest Member Volunteer Bill Hindman.