Hurricane Sally made landfall near Gulf Shores, Alabama as a Category 2 storm with sustained winds of 105 mph. Many of us warned several days in advance that a slow-moving Category 1 or 2 storm would likely create a flood disaster along the Gulf Coast. Here is the problem. A lot of the messaging for this hurricane (and many others) centers around the category and winds with the storm. It did not matter whether Sally was a category 1 or 2 storm, it was obvious to me and colleagues that slow movement was going to cause a compound flood event. What is that anyhow?
Compound flood events happen when inland flooding associated with rainfall is combined with storm surge flooding and river discharge. According to the U.S. Climate Resilience Toolkit website, “The severity of compound events….has increased in many coastal cities” and will continue to do so as sea level rise amplifies extreme flooding with coastal storms. When a storm like Sally barely moves, it allows for a sustained push of water (storm surge), a deluge of rainfall, and a swelling of the rivers. When you couple these processes with impervious surfaces and stormwater engineering designed for obsolete rainstorms, compound flooding events can cause significant loss of life and economic damage.
Steve Bowen, a meteorologist with Aon, shared the graphic above on the morning September 16th noting rainfall totals from Hurricane Sally in excess of 20 inches. with isolated totals exceeding 30 inches. When you couple these rainfall totals with 3 to 7 feet of storm surge and cresting rivers, it is easy to understand some of the stunning images being reported across western Florida and southern Alabama. For example, the Baldwin County Emergency Management Agency tweeted Wednesday morning, “DANGEROUS FLOODING is predicted for ALL of Baldwin County. Our rivers are quickly rising. The Fish River is predicted to reach major flood stage later today. @BaldwinEMA #SALLY #Staysafe #turnarounddontdrown.” As with many compound flood events, Sally hit the region with a multi-faceted flood threat: Sustained rainfall, a gradual and then episodic storm surge, and the lagging river crests.
Professor Matthew Bilskie knows a thing or two about compound flooding events. He is a professor in the School of Environmental, Civil, Agricultural and Mechanical Engineering at the University of Georgia. Bilskie specializes in real-time storm surge forecasting capabilities. He also co-authored a 2019 review of compound inundation models in low-gradient coastal watersheds. He told me via email, “Hurricane Sally will be the new storm of record for the Mississippi and Alabama coast with its dual punch of coastal surges and intense and prolonged rainfall.” Bilskie said such events can cause the water levels in the coastal rivers to rise from rainfall as well as water to rise in the bays and river mouths from the high winds. He concluded by saying, “This causes a very complex situation in coastal areas, especially regions like Mobile, AL that sit at the confluence of a river system and coastal/back-bay region.”
By the way, NOAA has a really interesting tool called nowCOAST. It is excellent tool for these types of compound flood events.