On Flooding in CharlestonNovember 3, 2021
The National Weather Service (NWS) Advanced Hydrologic Prediction Services is predicting significant tidal flooding in Charleston over the next few days. At the time of writing, peak tides of 8.1’ at 8AM Thursday, 8.4’ at 9AM Friday, and 8.8’ at 10AM on Saturday (all reference mean lower low water – MLLW datum) are predicted. These will likely change as the weather forecast changes, but if the 8.8’ tide occurs, it will be the 6th highest on record. This will be equivalent to the tidal flooding we saw around Thanksgiving of 2018.
I received several question on an Instragram story I shared yesterday about the tide predictions asking how significant the flooding would be and if it would be as bad as what we saw earlier in June 2021. I think a quick summary of the various flood risks we face is the area is needed, and that we need to do a better job communicating the risk associated with the type of flood events.
1. Tidal Flooding – Flooding associated with tides that exceed the flood thresholds set by the NWS, which is 7’ for minor tidal flooding, 7.5’ for moderate tidal flooding, and 8’ for major tidal flooding (all reference the MLLW datum) in Charleston Harbor. These floods have been occurring at an increasing frequency, with a record of 89 occurring in 2019. Most tidal floods can be predicted in advance, and with many of them, particularly the minor ones, you may not notice the flooding unless you are in lower areas adjacent to the tidal creeks and marshes.
2. Storm Surge – Flooding associated with tropical storms and hurricanes where the storms push water into the harbor, rivers, and creeks and flood streets, roads, and building. Storm surge can be classified as tidal flooding, but due to the potential significant impacts and relative infrequent probability of occurrence, I’ve separated it into its own category. Surge flooding also has the potential for devastating waves over land, which can be extremely damaging and potentially fatal. Hurricane Hugo’s storm surge in Charleston Harbor reached 12.52’ MLLW, which is the flood of record for Charleston. These floods are predicted with hurricane forecasts.
3. Stormwater Flooding – Flooding that occurs when it rains with greater intensities and/or volumes than our drainage systems (inlets, pipes, pumps, etc.) can handle, resulting in flooding of streets, roads, and sometimes buildings. While rainfall can be predicted, it can be difficult to predict when and where stormwater flooding will occur. This type of flooding is not limited to areas along the shoreline and can happen throughout the City.
4. Combination Flooding – Flooding that occurs when rainfall occurs at or around high tide or during surge events. When tide levels in the rivers and harbor are at or near high tide, they can further limit the ability of the gravity drainage systems to remove rainwater from the roadways. Heavy rains on top of high tides can result in significant flooding.
The flooding that occurred on the evening of June 12 was primarily a stormwater flooding event. Looking at rain gauges on the Weather Underground network, over three (3) inches of rain fell over a few hours, with peak rainfall intensities greater the four (4) inches per hour. The tide in Charleston harbor was approaching low tide, so the tides did not factor into the severity of the flooding. In contrast, the flooding predicted for this upcoming weekend is associated with a low-pressure system that is passing off the coast. There is not significant rainfall predicted in the Charleston area, so the flooding that may occur will be mainly tidal flooding. Hopefully the rainfall predictions do not increase.
As for what to expect for an 8.8’ MLLW tidal flood, the exhibit below presents an analysis of Charleston County LiDAR elevation data. Everything within the blue area is at an elevation of 8.8’ or less. Without any rainfall, these areas should be the maximum flood extents associated with an 8.8’ MLLW tide.