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Uncharted territory: Why the magnitude of the flooding yet to come is so uncertain

SCDNR Map and Disclaimer

The images I’ve seen from flooded areas over the past few days are heartbreaking. I can’t pretend to know what it’s like to watch everything you’ve worked for your entire life gradually get consumed and ruined by slowly rising water. For some, this is the second time in two years. That's incomprehensible to me. While the waters begin to slowly recede in some areas, it's rising elsewhere and for many along the Waccamaw River and Intracoastal Waterway, the wait for what's to come is agonizing.

And it's not just the wait, it's also the unknown.

We're in uncharted territory. Never before in recorded history have we seen the Waccamaw River and Intracoastal Waterway get to the levels they are now or are forecast. We know the flooding is or will be as bad as what we had with Matthew in 2016, but exactly how much higher is unknown. I know reading that is extremely frustrating, but it's the truth.

Many of you have seen the SCDNR Flood Map. Hopefully, you've read the disclaimer:

"The South Carolina Department of Natural Resources (DNR) developed this mapping tool to roughly approximate potential flooding, and for DNR planning purposes. The mapping tool presents broad scale approximations of potential flooding and is not intended for site-specific or local prediction or forecasting. The mapping tool should not be used in making personal evacuation or property preparation decisions. For evacuation and property preparation decisions, please rely on local authorities. The mapping tool is updated periodically and is not a real-time reflection of flooding. Mapping tool projections are subject to change."

Here's the bottom-line, it's a model. Before it can ever produce a forecast, it has to know what the initial state is of the entire region. At a moment in time, it takes into account many variables including total rainfall, river levels, flooded areas, water movement, the geography of the entire area, etc. Once this data is assimilated into the model, it uses very sophisticated physical and mathematical equations to produce a forecast.

The problem is, you're starting with an incomplete picture. It's impossible to know exactly what these initial conditions are at every location of eastern SC/NC. If the initial picture is incomplete, it's impossible to make a perfect forecast. Even though the forecasts on the map are very specific (mapping down to street level and water to within a foot), precision doesn't always equal accuracy. Thus the reason SCDNR says it's a rough approximation of potential flooding and is not intended for specific forecasts. In other words, the model may be right in some areas, but way overestimating flooding in others.

While its impossible to know exactly how high the water gets where you are, we do have an idea of the timeline. These current stages and forecasts are as of Sunday at 1 p.m.:

  • Waccamaw River at Conway is at 20.1 feet now and will likely crest Tuesday/ Wednesday at 22 feet.
  • Intracoastal Waterway at Socastee is now at 17.5 feet and expected to rise to 19.2 feet (Matthew record) Monday then crest near 23 feet by Wednesday or Thursday.
  • Intracoastal Waterway at Bucksport is now at 22.3 feet and expected to rise to 23.7 feet (Matthew record) Monday then crest near 28 feet by Wednesday.

There are gauges, but no specific forecasts for the Waccamaw River at Pawleys Island or Hagley landing although the waters are rising. According to Georgetown County Emergency Management, flooding to the level of King tides would be noticeable Tuesday, reach moderate levels by Thursday then crest next weekend.

So what do you do? You know I am not an alarmist, but as I said earlier, we are in uncharted territory. Because there is much that is unknown, I think you have to plan for the worst case scenario portrayed by the SCDNR map and be ready to take action. As painstaking as it is, the slow nature of the flooding gives you plenty of time to prepare.

PLEASE heed any warnings from our local emergency managers. Over the years, I have been fortunate to attend conferences with Randy Webster (Horry County EMD) and Sam Hodge (Georgetown County EMD). Throughout the emergency management and the hurricane forecasting communities including the National Hurricane Center, they are highly respected and considered among the best in the entire country. Randy and Sam have said many times over the past few days that they have the very best GIS Professionals and Hydrologists here to interpret and tweaks the maps. As it becomes more clear who may or may not flood, they will let us know. They have their game plan together and are constantly releasing information that will prepare and protect you.

Both offices are updating LIVE on Facebook:
Horry County: https://www.facebook.com/horrycountyemergencymanagementdepartment
Georgetown County: https://www.facebook.com/GCEMD

I know these are incredibly stressful time and for some, the worst is yet to come. Hang in there. We are all in this together and we will get to the other side of this mess together.

Rest assured that ABC-15 and I are working around the clock to bring you the most up to date information possible. Thank you for your trust in us.

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