Repeated assurances by elected officials that Horry County has the strongest stormwater rules in the state has done little to ease residents’ fears their homes won’t flood next when a hurricane strikes.
A lot of folks are tired of hearing the claim.
How can that even be true, when we’ve seen such severe flooding at least four times in the last five years?
What makes our stormwater requirements so darn great?
That’s the first question we asked Thom Roth, deputy stormwater manager for Horry County, during a lengthy interview Friday.
The answers surprised us.
When a new subdivision is created, it must meet standards that start with a basic premise: capture rain runoff, store in retention ponds, then release into the stormwater system downstream at a controlled rate measuring less than before the first shovel of dirt was turned for construction.
That’s right, less runoff.
The state requires counties and municipal stormwater systems be designed to handle two and ten year rain events to control the downstream runoff to prevent flooding.
Horry County’s stormwater program was created in 2000 and implemented stronger measures to handle 25-year rain events.
That’s when new developments were first required to catch and control runoff just to match previous runoff rates, Roth said.
But those rules were strengthened in 2008, requiring that all runoff be at least 20% less for the 25-year rain event.
Roth provided this example: If the runoff rate of rain was 100 cubic feet per second (CFS) before development, then calculated at 180 CFS after all the homes, sidewalks and roads are built, then the end result must be 80 CFS — that’s 20% less than before that land was developed.
In 2017, Horry County rules changed again to include 100-year rain event regulations, mandating that runoff be contained at the same rate it was prior to construction to manage flooding when those destructive storms hit.
“At a minimum, we make sure a 25-year storm does not put so much water into the pond, that it gets too high and puts water on the roads, that’s the way it has to be designed,” Roth said.
“When people talk about flooding, our priority is to keep it out of homes in a 100-year event,” Roth said. “So for the 100-year event, that water isn’t getting into anyone’s home.”
When retention pond waters rise into yards, that’s actually part of its floodplain. It was designed to rise in this bowl during extreme events, so floodwaters would not reach homes, Roth said.
What if there’s a hurricane?
So why did we see flooding in previous years?
Because we had rain events that measured well above the 100-year event; Hurricane Joaquin in 2015, Hurricane Matthew in 2016, Hurricane Florence in 2018, and Hurricane Dorian last month.
Hurricane Joaquin was actually a 1,000-year event, so was Hurricane Florence in some areas of Horry County, Roth said.
“We hear people say now all the time, they don’t know what’s going to happen when we get a hurricane,” Roth said.
“But now we’ve had the hurricanes. We’ve had tropical storms over a 100-year event. And if you were okay then, you are okay now,” Roth said.
What stormwater systems are not designed for, is to handle fast-moving flash flooding events that can drop just four inches of rain in an hour, quickly covering subdivision, county and state roads then drains within a much shorter period of time.
It’s an inconvenience that happens no matter where one lives, but it’s not flooding homes and destroying property.
It can temporarily overwhelm a system, but it quickly moves on, and four inches of rain doesn’t flood homes.
Horry County stormwater systems are designed to handle the big rain events, and they had to go well above state standards in order to do that, Roth said.
How much rain can we handle?
The state’s Department of Health and Environmental Control (DHEC) uses an average rainfall number for the entire state that counties follow to establish stormwater rules.
However, different parts of our region average different amounts of rain, Roth said.
North Myrtle Beach and Surfside get much bigger rain events than Nichols or Galivants Ferry, so the county plans according to federal rain averages collected over the past half century by the National Oceanic and Atmospheric Administration (NOAA) to address each unique area of Horry County.
A 100-year event might get 10 to 11 inches of rain in a 24-hour period across the state, and a 25-year event would see 7 to 8 inches.
But in North Myrtle Beach, a 25-year event might average more with 9.5 inches, and Conway averages 8.2 inches, so each development’s system is designed for that higher amount, Roth said.
“That makes a big difference, so we’re designing specifically to that area,” Roth said.
What happens if a subdivision was constructed when those rules were created and it still didn’t
meet county standards? Roth says they make the developer reconstruct the system and fix the issues.
Where the water goes, and other problems
This is an intricately designed journey that could fill another story, but we’ll be brief.
In subdivisions and developments, the water is captured at the source, then moved through different drainage systems that could be pipes, ditches, culverts swales, or natural streams to a wetland, river, or Intracoastal Waterway and into the ocean.
When people say they’ve got a river running behind their house after a heavy storm, they’re referring to swales constructed on back lots of subdivisions that are designed to convey the water away from the homes and into the drainage system.
The water can run a few inches, but in a flash flood or major storm, it can certainly seem like a little river for a few days, Roth said.
But the point is, the stormwater system is doing what it’s designed to do after a storm, prevent homes from flooding and move massive amounts of water through the system.
At any point along the way, particularly in a hurricane, trees and other debris can block the drainage path and cause flooding problems upstream.
That happened in Hurricane Florence.
Then there’s the beaver dams.
Keeping ditches clear is a frustrating task when problems occur on private property, because not all owners are willing to sign easements to allow the county to clear it, Roth said.
“‘Not only do I like my ditch full of water, I put a blockage in there so I can have a duck pond’ — we’ve actually had that happen,” Roth said.
The county does clean an average of 100 miles of ditches through excavation, they bush hog
another 70 miles, plus they clean about 36 miles by hand, every year, Roth said.
Who do you call with a problem?
The stormwater department averages about 150 complaints a month that seems to follow a particular pattern.
About 75% of calls from the Carolina Forest, Surfside Beach, Socastee and Conway areas turn out to be HOA issues that are the responsibility of that subdivision, and 25% are outfall issues the county can address.
In the western part of the county in the Longs, Aynor, Loris and Bucksport areas, about 90% of calls are outfall issues, Roth said.
“If you have a flood issue during a normal rain event, then they should contact our department, because we want to know what the problem is,” Roth said.
Whether it’s the responsibility of the HOA, or falls under the county’s jurisdiction, the problems can’t be addressed until the county is aware issues exist.
To report a drainage or water quality problem, residents can call the stormwater department hotline at 843-381-8000.