The Town of Mount Pleasant is considering a ban on new slab-built single-family homes in flood zones. Also known as “slab-on-grade” or “fill-and-build” construction, the method involves placing homes directly on a concrete slab foundation, which can make those buildings vulnerable to flooding.
The practice can also create a domino effect that impacts adjacent homeowners. When trying to ensure new homes reach a certain elevation above sea level, developers often will raise a plot by importing dirt. That practice, multiplied across an entire development, can dramatically shift the hydrology of an area and worsen flooding. Developers often clear-cut trees and other plant life that help capture and control flood water — further complicating an already problematic situation.
Charleston City Council approved an essentially identical ban in April. It was a years-in-the-making policy that involved input from local environmental organizations and developers, according to the Southern Environmental Law Center, one of the ban’s key supporters.
Mount Pleasant’s ban would only bar new slab-built homes in the 100-year floodplain. Those are areas that have a 1 percent probability of flooding in any given year.
The proposed ban would go into effect July 1, six months after Charleston’s takes effect. Katherine Gerling, Mount Pleasant’s floodplain manager, said the proposed timing of the ban was intentional.
“This effective date was chosen to kind of see how the city of Charleston is going to manage their ordinance,” Gerling said at a Dec. 13 meeting of Mount Pleasant’s planning commission, where the proposal was under consideration.
Planning Commission member Adam Ferrara expressed concerns that the ban could make it more difficult to build affordable housing in Mount Pleasant, which has seen rising rents and home costs as a result of a decadeslong population boom. Mount Pleasant’s population has roughly tripled since 1990.
“Just bear in mind, that does kind of go against the narrative of trying to build workforce housing that is single-family,” Ferrara said at the meeting. “That does create a cost burden to builders and to homeowners. I’m not saying that’s right or wrong, I’m just saying that is a result of what we’re doing.”
Despite those concerns, the commission passed the measure unanimously. The ban still needs approval from Mount Pleasant Town Council.
Also at the Dec. 13 meeting, planning commission members voted unanimously in support of a measure extending restrictions on new residential construction in Mount Pleasant. Town leaders enacted that measure in 2019 to curb traffic and strain on local resources in the growing suburbs.
“In response to people that said, ‘We need to have all this development because it’s the only way we’re going to keep real estate reasonable,’ — they are wrong,” commission member Kathy Smith said. “That argument only works when the supply and demand curves are in a state of equilibrium. As long as we are net positive in demand in Mount Pleasant, no matter what we do, prices will go up. You can build until your brains blow out, and the prices will go up.”
The proposed ban would extend the restrictions, which would only permit up to 600 new residences annually, until January 2029. The restrictions also still need approval from the full town council.
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Charleston is going to look a lot different in 2050. A booming population and rising sea levels will reshape the city. One new resource could offer a way for local leaders to glimpse the future.
FUTure Urban-Regional Environment Simulation version three, or FUTURES 3.0, allows researchers to play out hypothetical changes in urban growth and migration in response to rising sea levels. First outlined this month in the peer-reviewed journal Scientific Reports, the method uses “spatially interactive” modeling (that is, what happens in one area impacts another) to determine how the city and its surrounding communities could change in coming decades due to flooding and sea-level rise.
In Charleston, the project’s researchers found a sample case with numerous potential variables.
Sea levels in the region are expected to rise 14-18 inches over the next 30 years, federal climate researchers predict. That increased water level will make the region even more prone to flooding.
“The Charleston Metro Area is also growing three times faster than the U.S. average, putting pressure on the city’s infrastructure and capacity to effectively manage coastal flooding,” the study’s authors note.
The city is weighing numerous projects to combat the inundation, from a proposed $1.3 billion sea wall which would encircle the peninsula to a forthcoming tidal and inland study that will examine flooding-related problems in West Ashley and other parts of the city.
Georgina Sanchez, one of the study’s authors, said she hopes the research will provide a helpful road map to city leaders for navigating Charleston’s flooded future.
“The idea is not to say, ‘Here’s a perfect scenario,’ but, rather, providing a tool for exploration (so) we can understand the long-term consequences of our current development choices,” said Sanchez, a research scholar at North Carolina State University’s Center for Geospatial Analytics.
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So what does Charleston’s future look like according to the FUTURES 3.0 forecast?
“A lot of our projections show areas that will be newly developed and subsequently retreat,” Sanchez said. “Why not protect those areas in the first place? It’s a bit less controversial if it’s an area that has not yet been built up.”
That’s a question city leaders are struggling with. Charleston is in the process of updating its zoning codes to help prevent new developments in wetlands and flood-prone areas. But their hands are largely tied when it comes to years-old development “entitlements” — agreements the city signed with developers outlining, among other things, how many homes can be built on a piece of property. If the city backs out of those agreements, it could trigger a lawsuit, even if the developer hasn’t officially broken ground.
On the other end of the spectrum, researchers also model what a climate change-instigated retreat from Charleston could look like.
Most people (about 76 percent) would stay in the Charleston-Dorchester-Berkeley county region, according to the analysis. They’d just move further inland to escape rising seas, although some would relocate as far as the West Coast. Sanchez said her team plans to analyze “climate-aware” migration patterns more in-depth in the future. She said that data could provide an important planning metric for communities further inland.
“Are these receiving communities prepared to welcome potentially thousands of climate migrants?” she asked.
Sanchez said one of the next steps for the project is to make the open-source framework more accessible to other communities; either through advanced 3D modeling or, in the long run, potentially a 2D computer-based platform.