Discover CharlestonFlooding in Downtown Charleston SC: The Past and The Future

In downtown Charleston we live on a peninsula surrounded on three sides by water – two rivers and the ocean to be exact. Much of our city is built on reclaimed land, a lot of which was formally marsh.  Only a fool would believe we have no risk of flooding but just what is that risk and what can we do about it?  Well, we have been busy creating a map that shows you exactly where the vulnerabilities are and exactly what the effects may be. Not only does this map display in great detail the geographically designated areas of Charleston SC that were previously marsh, but it offers an interactive way to reflect how the downtown peninsula might look in years ahead due to projected sea level rises. Interested? Read on.

First of all let’s have a quick look at the causes of flooding of downtown streets on the peninsula:

Rainy Weather – in the summer months especially, we experience intense rainfall as a result of thunderstorms. In October of 2015 we experienced a 1 in 1000 year event and it rained in near Biblical proportions. Over a four day period over 16 inches of rain fell … with the town of Mount Pleasant receiving a record amount of 26.88 inches. The streets flooded as all that water needed time to run off.  It was bad but passable for most vehicles, especially SUVs. This particular rain event was coupled with a high tide which only compounded the issue as it was harder for the rain water to escape as the tides pushed in.

Tide – more specifically, the sea level rise in tide. This is the more insidious event and in the past century the “mean ” high water mark has been rising, something like 10 inches over the last 100 years or so. Anywhere that was once marsh or creek is going to be vulnerable. Hanover, Fishburne, Water, and Market streets to name a few are some examples of downtown streets that are regularly affected and will be doubly so.  The tide does not just pour over a sea wall, it comes up from below as brackish water through old drains or permeable soils. In the 1840’s the City offered a medal to anyone who could solve the downtown flooding problem. Rising tides will remain a serious issue and one we will have to cope with as the climate warms. I read an article recently in the New York Times quoting “dry weather events, ” this means flooding due to tide rather than weather, and for Charleston that happens about 40 times per year. Lockwood Boulevard as it turns into Broad St would be a good example. Try when possible not to drive through streets flooded by high tide, that’s seawater and not great for your car!

Storm – by this I mean “storm surge ” or seawater driven on to the shore by extreme weather conditions. This is your worst scenario but mercifully also very rare. In Hurricane Hugo Charleston was spared unlike McClellanville that experienced a huge rise in sea level inundating the coastal areas. This type of storm surge needs the proverbial “perfect storm ” of events to line up in a particular way that will trigger a catastrophic storm surge. One could worry more about crossing the road with a coffee in hand and being hit by a distracted driver.

So where is the good news?  The Charleston City Council has been spending heavily on flood protection in the form of a huge pumping system with 12 foot tunnels driven 100 feet or more underground.  This has been under construction for a number of years now and the interconnections are being established.  It already moves massive amounts of water and assists in the process of that water finding its way to the ocean.  Drive east on the Crosstown (Septima Clark Parkway) to Bogart St and you will see an example of it being installed.  So there is some good news here and encouraging that the city is simply not sitting around waiting for the inevitable to happen.

Now on to our earlier promise.  Have a look at this link here.  The Charleston Map is a GIS map of the peninsula with a number of interactive layers.  We have added a huge amount of data within this map so spend a while looking around and engaging with the different layers to understand how it works. Find the area you are interested in, then go to “Interactive GIS Maps ” and add the layer “Effect of Sea Level Rise. ”  Next look top right and use the slider to raise the “mean high water ” mark.  Not much happens as you apply the first 12 inches or so (be glad of that!) and then you will notice the blue area expand indicating flood hazards. This map is designed to be easy to use and can be expanded to a very high resolution.  

Note: For fun, move to the “Historic Maps ” layer and add the layer reflecting the map of 1680.  The areas that are susceptible to flooding are exactly those that used to be marsh or creek. That detail is not lost on the observant.  Learn more about The Charleston Map.