Charleston area lost more than 10,000 acres of tree cover since 1992, making floods worse

Joshua Robinson recently spoke with Tony Bartelme, senior projects reporter for the Post & Courier, about the importance of trees to the hydrology of the Lowcountry landscape. The resulting article, part of the Rising Waters special reporting series, discusses the flooding of Crosstowne Church and RDE’s work on behalf of the church.

Paul Rienzo is pastor of Crosstowne Church. Bostonian by birth, he moved to Charleston 37 years ago. His church sits in a dip off Bees Ferry Road, not far from Church Creek. Though it's in a low spot, he thought it was safe from flooding. “It didn’t even flood in Hugo,” he said.

But in 2015, the remnants of Hurricane Joaquin brought a lumbering storm, one that dumped 2 feet of rain on Charleston and Mount Pleasant. Off Bees Ferry Road, floodwaters poured into the church. Rienzo grabbed his kayak and paddled inside to save the sound equipment. At the time, then-Gov. Nikki Haley called it “a thousand-year storm.” But floods came again in 2016, 2017 and 2018.

Rienzo wondered why it was happening so often. Was it just chance? “Something had changed,” Rienzo said. The church hired their own hydrologists and engineers, who made a startling discovery.

Joshua Robinson was lead investigator. He runs Robinson Design Engineers in Charleston and Asheville and teaches at the College of Charleston. Their first priority was to get a better handle on the area’s hydrology — how Bear Swamp collected and released water. And they found clues in a nearby forest.

The U.S. Forest Service had done extensive research in the Francis Marion National Forest, the bulwark of green north of Mount Pleasant. Robinson had assumed that when rain fell in the forest, stormwater flowed out via its many creeks and rivers. But Forest Service scientists found something else was going on.

In this flat landscape, about 70 percent went back up into the skies, mostly through the forest's trees. “It seems so obvious," Robinson said in retrospect. "You look at one fully grown tree and how much water it needs.”

In a low-lying area, forests and wetlands had evolved into highly efficient pumps. Pines and oaks and cypresses pumped water from the ground through their roots, which allowed even more water to enter the soil and drain faster. Then the forests released the water through their leaves and needles in a process called evapotranspiration. With their many needles, pine trees were especially efficient water collectors.

Robinson was amazed: “Forests return 7 out of every 10 inches of rain to the atmosphere.”

But as Robinson and his colleagues studied changes in Bear Swamp, they saw how developers and governments had built ditches to guide stormwater toward the Ashley River, and ultimately, the Atlantic. Water flowed faster through the basin. These growing pulses of water overwhelmed ditches and Church Creek. At the same time, some of the ditching allowed tidal water from the Ashley River to flow inland.

“This landscape and its soils formed over eons, and then in a small timescale, a matter of decades, it was turned into rooftops, pavement, more ditches," Robinson said. Area governments had allowed this development "without an awareness — or the tools to understand the power of these actions.”

For Robinson, it was a eureka moment: The Lowcountry's forests and wetlands were amazing pumps; if you got rid of them, stormwater flowed faster through the area. Flooding got worse, requiring more ditches and pipes, a cycle of failure.

He finished the study thinking that the forests and wetlands in Church Creek Basin and elsewhere "should be preserved at all costs.”

Cover photo by Post & Courier. Read the full article here.