“What’s the Frequency, Kenneth?” (With Coastal Flooding That Is)


shallow coastal flooding charleston south carolinaLiving in a VERY coastal area has many benefits, one of which is shallow coastal flooding that occurs in low-lying coastal areas during extreme high tides, often referred to as “nuisance flooding.” Maybe it’s because I work, live, and play in the coastal hazards realm that I have noticed more and more the amount of times that these types of “nuisance” events are happening. Did a little research and, for us Charlestonians, that would be 27 times for the 2012 calendar year with a total duration of 46 hours, or almost two full days at flood stage levels. How do I know this? Read on to learn how to become a flood geek like myself.

But Wait…Isn’t Sea Level Rising Too?

Ignorance can be bliss in many facets of coastal hazards. Oftentimes I seem like a crazed meteorologist, warning family and friends of flooding events with nary a rain cloud in sight. Wouldn’t it seem that nuisance coastal flooding events will continue to increase in frequency, duration, and severity with rising sea levels???

albert einstein pointing to a graph of mean sea level trend for charleston south carolina

That little voice inside my head is correct for once. Add a half-meter (1.6 ft) of sea level rise to current levels and the “nuisance” flooding would have happened 469 times last year with flood thresholds breached a total combined duration of 56 days. Want more doom and gloom? Take it up a notch to a one-meter rise (3.3 ft) in current sea level for Charleston and you have 697 events lasting a combined duration of 156 days. Oh my. At what point does the flooding stop becoming a “nuisance” and become the new normal?

Help Is Here….Well Sort Of.

Earlier in the post I told you I’d tell you how to calculate this magical information. If you made it this far you’re in luck, as there are a few new tools by NOAA that make it possible and quite easy. One way is to check out the Sea Level Rise and Coastal Flooding Impacts Viewer’s Flood Frequency section where we have done most of the work for you (in available areas). Or calculate your own scenarios using the new Inundation Analysis Tool from NOAA’s Center for Operational Oceanographic Products and Services (CO-OPS).

The latter option requires a little research and hands-on work for your part. First you need to contact your local NWS Weather Forecast Office (WFO) to obtain the flood threshold guidelines used in your study area for NOAA monitored tide gages. Many WFO’s often use non-NOAA tide gages that may not be able to be used for calculating frequency with this method. The warning coordination meteorologist or staff hydrologist should have the tide gage and threshold information available for you. In some cases the flood thresholds may be online at each WFO’s Advanced Hydrologic Prediction Service (AHPS) section. That’s a mouthful of geeky science wordage…now let’s get started.

I’ll use the previous Charleston, SC analysis as an example. I know that there is only one tide gage to use located in the Charleston Harbor vicinity (#8665530) and its flood threshold is reached at 7 feet MLLW (2.134 meters) as defined by the CHS WFO. Open the Inundation Analysis Tool, navigate to the Charleston, SC tide gage, and click on the link in the pop-up to access the tool for this location. There you’ll see a lot of datum-based water level information. Locate, but don’t select the value for the MLLW datum (0.843m).

datum-based water level information from NOAA's Inundation Analysis Tool

Next, to get the total water level, we add our NWS provided flood threshold of 2.134 meters (7 ft) with the station MLLW datum of 0.843 meters for a total of 2.977 m. The 2.977 m MLLW can be thought of as the total water level used for flood advisories for this location. Hence, when this elevation is reached, or predicted to be reached, the NWS will issue a Coastal Flood Advisory Statement. Type 2.977 into the User Specified Elevation number box. Select the begin date as Jan 1, 2012 and end date as Jan 1, 2013. Click submit and voila!

Inundation analysis for Charleston South Carolina from NOAA's Inundation Analysis Tool showing current flood

Inundation analysis for Charleston South Carolina from NOAA's Inundation Analysis Tool showing current flood

Notice it lists each tide that exceeds the threshold along with tide elevation, type, and individual duration. Remember, this is verified historical tide data so not much debate about what the data is saying. This is hard science people!

What about the increased sea level rise scenarios you say? The secret ingredient is simply a little math. Subtract 0.5 m (0.5 meter sea level rise) from the 2.977 meters to get 2.477 m. But “Why subtract?” you say. Try to think of it this way…in the datum conversion world, going up (from MLLW to MHHW) is negative and going down (MHHW to MLLW) is positive, which is why we subtract the SLR value increment. With SLR, the whole base is shifting up all while the flooding threshold stays the same, which is why the threshold becomes a smaller number. Not the easiest subject to comprehend, but trust me on this one. For clarity on this, I encourage you to contact the folks at CO-OPs if you tackle this yourself. So re-running the Inundation Analysis Tool with 2.477 m (half-meter SLR scenario) as the user-specified input with the same time period dates, you should get the values as shown below. Cover your eyes now!

Inundation analysis for Charleston South Carolina from NOAA's Inundation Analysis Tool with half meter sea level rise

Ouch, huh? 469 high tides at flood stage and 56 total days of flooding? That outta get your attention. Repeat the process again by subtracting another 0.5 meters for the one-meter sea level rise scenario. The user input should now be 1.977 m and the dates the same. The results should be like the image below that you should NOT look at.

Inundation analysis for Charleston South Carolina from NOAA's Inundation Analysis Tool with one meter sea level rise

Head For the Hills!

697 tides at flood stage and 156 total days of flooding. At this point our charming southern city slowly becomes Venice, Italy. By the way…did I mention onshore flow, low pressure, or precipitation compounding the effects? Now that this analyst has dropped some science on you, I’m passing the buck to those who can do something about it. Have you hugged your coastal adaptation strategists, planners, resource managers, politicians, or climate progressive community today?

P.S. Bonus points if you are old enough to know what the title of this blog is in reference to.

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