Still Water Surge Risks and Hurricane Risks
FEMA’s proposed flood zones. Basically 99% of structures are in the flood zone on Sullivan’s Island
Yearly Risk of Inundation - Analysis of Extreme Water Levels
OK, this type of analysis is similar to a FEMA flood hazard map (top of page), although FEMA includes wave action; I am not at this point. Anyway, if you open the GEOSCIENCE YEARLY RISK MAP you will see the 2020 yearly inundation risk listed in the legend (as well as the 2040). The big difference between the Geoscience and the FEMA products is that you have a bit more idea of what kind of risk you are looking at. On Sullivan’s Island about 99% of the houses are in the flood zone (FEMA) but that, obviously, does not mean all of the risks are the same even in areas with the same BFE (base flood elevation). If you open the map you can find your risk of inundation (some level of water) and also what I call major flooding (2 ft or more, which can move cars). I have simplified it by grouping the risk into roughly 20% bins. In the full product each location has unique value, and if you are really interested in a more exact number please get in touch and I will provided it to you.
A quick review of how it was done may help provide some context to the values. I use only the 1980 to present data and, in this case, used the maximum yearly value from each year. Because this period is smaller than the entire record (like from 1880 to present) the risk values may be a bit more elevated, but I trust this period’s data and feel it is more appropriate than using all of the record in this case.
I ‘normalized’ the data by taking the natural log (ln) of the values and plotted them verses time (see graph below). I computed the equation between years and water levels (it has been going up) and plugged my present year value (2020) or any future one (2040 for example), within reason, into the equation to get the ‘yearly water level’. I then determined the standard deviation (ln) of the record data to get an indicator of the yearly variability and I then took the inverse of the natural log to get a back to real values (in meters NAVD 88). So at the end I have the theoretical yearly water elevation (50% chance) and the sample standard deviation from which to determine the percentages. From there I use the technique that I have detailed in this section.
Yearly maximum water levels since 1980. Note the outlier resulting from Hurricane Hugo, which would be about a 5% yearly risk at present (2020) based on the data used.
From Hurricanes - Based on SLOSH runs
This data is provided mainly for emergency planning. It is intended to provide some level of ‘on the ground’ parcel-level information if a certain category storm is predicted to impact (come ashore in) the Charleston Area. If your home/business is an area shaded at 4o% or more (open map below) then it is likely that your yard will be inundated and appropriate precautions should be taken. I am only providing data for Category 1 and 2 storms; the entire island is at very high risk in a Category 3, 4, or 5 hurricane. To see the risk of being wet (some inundation) for your location open the map below and choose either the Cat 1 or 2 risk tab in the legend.
You may notice that this is a bit different map than most SLOSH outputs provide because it uses all the values in 50+ runs. Lots of times you will see the term MOM, which stands for ‘maximum of maximums’, in a SLOSH output. I don’t believe in providing the worst case but rather let the viewer choose the risk they are comfortable with. A MOM risk would be less than a 5% chance. If you are interested in raw water depths for the average or some specific level of risk, please get in touch and I can send you those.