Flood Risks - Monthly High Water Levels
There are a couple ways to look at this aspect of flooding, which has been called “sunny day flooding”, “nuisance flooding”, or “shallow coastal flooding”. I am not going to use the phrase shallow coastal flooding to describe my analysis because there is a certain water level that is set by the local weather offices that signifies shallow coastal flooding.
If you get a weather statement on your phone (I do) warning of shallow coastal flooding the local weather office has determined that the conditions (tide, wind, waves, rain, etc) are such that there is a likelihood that water levels will pass the threshold. The local threshold for Sullivan’s Island is about 0.6 meters above MHHW (about 1.4 meters NAVD88). This value does not change through time, but rather the number of times that it is surpassed does. So if the road in front of your house is at or below this elevation there is a good chance that the road will be wet more frequently in the future.
My analysis, I will call it Monthly High Water, is more of a time/elevation level not a time/frequency analysis. I use the past data to highlight what level of flooding we can expect on a monthly basis. To put it a different way, on average we can expect flooding to this elevation to occur once a month. This is similar to the MHHW flooding study, however, in this case I am not going to use SLR projections - just historic data with a curve that describes the trend (remember I am not using linear trends).
Monthly average (per year) high water since 1980
The curve is similar to the one for SLR - with slightly different constants. In this case the velocity is 1.5 mm/year and the acceleration is 0.00008 mm/yr^2. These are both less than the measured change in SLR for Sullivan’s Island, which makes sense since there are other factors involved that may not change at the same scale as SLR. In this analysis there is no use of ‘SLR curves’ - the change is reflected in the curve based only on the data.
So, in 2020 (40 years since 1980) the projected average monthly high water is about 1.31; that is not quite at Shallow Coastal Flooding level, but it has a 50% probability of occurring every month at least once (and a 99.8% annually). This value is about exactly the same as NOAA uses (1.35) as the 99% yearly water level. I have used this metric of flooding (monthly risk) for a study in Beaufort to look at public infrastructure risk, which is basically what would affected by this level of inundation; there are not many homes/yards that are inundated every month (or that you would let stay that way).
This is what the model outputs look like for one year (2040 in this case). I have tried to keep it simple in the online map and choose one contour (25% risk contour) for each modeled year.
In the map below (click on it and select the monthly flooding risks) the 25% risk of a monthly inundation are shown. Over a year this equates to a risk of about 97% (almost certain) that the shaded areas will be inundated for each year shown. In this case I have modeled the flooding so that the risk is not higher inland than the risk near the water; i.e., no ‘underground’ connections like sewers are permitted. I think this is a reasonable assumption in most places given the short duration and infrequent nature of the event - but it is a conservative estimate in areas where there are sewers/drains that could flow backwards (which I am sure you have seen). If you are interested in looking more closely you can choose one of the google earth layers below.
Monthly 2020 Monthly 2030 Monthly 2040 Monthly 2050 Monthly 2060
Flood Risks on Sullivan’s Island - MHHW
Flooding at Mean Higher High Water (MHHW) is really more of an ecologic measure. This level of flooding happens, typically, about once a day. So, if you lived in this zone your house would really need to be a boat. More likely it will be someone’s yard that becomes more and more ‘salty’ until it really is part of the marsh system or a road that floods daily (where you would have to put a measure out to let people know how deep it is).
It will help show where the edge of the marsh is in the future. And, if the marsh is constrained what level of marsh loss that would have. At this point I am using a 10% chance that the MHHW will be within the contour boundary (think of it as a shoreline that has a 90% chance that the MHHW will be seaward of it). One thing to remember, the tides are often higher than MHHW (it is an ‘average’) so multiple days of flooding are really not out of the ordinary at the 10% risk level. Again, this is more of an ecologic indicator. I have also left on the ‘non-connected’ areas since at this point I don’t know which drains are outfitted with tide-flaps.
Present MHHW location. Red is pretty much always wet at high tide. Blue is probably dry, and green is a good indicator of the area covered at MHHW. In the present case, there are errors in the topographic surface (mainly from vegetation) that drive this ‘uncertainty’.
This is a good image to contrast with the above one. In 2050the uncertainty is mainly from potential SLR scenarios. But note that the culverts next to the roads are generally wet at high tide.