Daniel Island scores another good report card after Irma, but why?
Twice in the last year, Daniel Island has felt the impacts of powerful tropical storm systems. First there was Hurricane Matthew and then, just last week, it was Irma’s turn. Both times, flood waters surged on the island, as well as in surrounding communities of downtown Charleston, North Charleston, West Ashley and Mount Pleasant.
But unlike our neighboring locales across the rivers, the rising waters on Daniel Island seemed less obtrusive overall and dissipated quickly - leaving many island residents to wonder why. Is it just good luck? Not exactly.
The answer, to those in the know, is quite simply a blend of good old fashioned foresight and engineering science. Daniel Island drains well from excess water - and that’s no accident.
Tony Woody, vice president and department director for Thomas & Hutton has been working with the Daniel Island Company (DIC) on its development of the island town since the mid-1990s. A professional engineer, Woody attributes the community’s success in handling storms to several factors.
“Number one, there has been one developer for roughly 3,000 acres, which has been a big help,” said Woody. “And that one developer had a vision of creating something special and not just doing the minimum that was required.”
In the early days of planning for the community, the DIC took steps to go beyond the standard of design, added Woody. At the time, that standard was to make sure infrastructure could handle “a 10 year storm,” meaning if you get 7 inches of rain in 24 hours the streets would be passable. But the DIC decided to ensure the island could handle a 25 year storm.
“That meant that they were putting in storm drain pipes and inlets that were bigger than what was required,” continued Woody. “If you had multiple developers out here competing with each other trying to get the lowest price house, you wouldn’t necessarily have had someone willing to spend extra money on infrastructure that you don’t see.”
The different topographies on the north side of the island and the south side also influenced early drainage decision-making. A key factor for both sections was giving water a place to go, explained Woody.
“The north end of the island has a big freshwater wetlands system that runs down the middle,” he said. “…And so what we did was we channeled a lot of the stormwater into the wetlands, which kept the wetlands active and created wetland vegetation, but also served as a nice conduit.”
Large outfalls on the north and south end of the wetland convey clean stormwater runoff from to the marshes and ultimately Charleston Harbor. On the south side, Smythe Lake serves the majority of the area and has a major outfall near the intersection of Hester Park and Wando View Roads.
“The (Department of Health and Environmental Control) requires that the post-development peak rate of runoff for your 10 year storm not exceed pre-development peak rate of runoff for the same storm event,” added Woody.
“This is accomplished by impounding stormwater runoff in the various ponds, which allow it to be released slower,” he continued. “The time in detention allows sediments and pollutants to settle, thus preventing them from contaminating the adjacent marshes and wetlands…We put emergency outflows all around the perimeter of the south end to allow a greater volume of runoff for storms that are larger than the design storm event, which improves the drainage system without sacrificing water.”
It also comes down to keeping outfall areas clear of debris and excess vegetation, added Bill McKenzie, vice president of development for the Daniel Island Company.
“When I first started, we did have some flooding issues, particularly between Etiwan Park and Codner’s Ferry,” he said. “We had a good outfall, but because it got water, it got growth, trees, and grasses, so we got in the process of maintaining those outfalls. …We keep those up particularly during hurricane season and make sure they are in good condition.”
The intentional interconnectivity between the island’s lakes and ponds is also helpful in keeping things flowing, added McKenzie.
“Most of our lakes are interconnected in some form or fashion,” he said. “Which means that if a log from somebody’s house blocks an inlet, there are other ways things can flow and they kind of balance things out.”
When Tropical Storm Irma hit, there were a couple of issues happening, explained Woody. The planners and engineers created infrastructure for Daniel Island based on what they call the “Spring Tide,” which is approximately 6 feet. Irma reportedly brought a 10.25 foot tide, based on tide chart datum, said Woody.
“So what happened was we had that 6 foot tide…plus there was a 4.25 storm surge on top of that,” he added. “…The combination of it hitting at the peak of a Spring tide is what created the extreme event. What the residents probably saw was the water coming over the outfall structures and lower land areas inundated with the tidewater. So you have that happening - and it’s raining. And the water can’t get out. That created minor short term flooding issues on roads and in yards.”
But why did the water come and go so quickly?
“That’s part of being in a tidal influence system,” added Woody. “When the tide went out - everything was open and the water receded. Everything is graded well internally. The only water that didn’t recede was a localized grading problem and not a large scale flooding problem.”
Other than a bit of tidal overflow, water did not collect much in the island’s roadways during and after the storm. There are good reasons for that as well, added Woody. First, engineers designed the roads and streets to be at a minimum elevation of 7.5 feet (equates to an elevation of 10 feet when using the same datum as the tide chart), which is one foot higher than the standard requirement in the City of Charleston when Daniel Island was initially developed in the early 1990s.
After Hurricane Hugo devastated the Charleston region in 1989, FEMA began revising base flood elevations and changed building requirements for homes in flood zones, said Woody. This benefitted Daniel Island properties since the development began after Hugo and its associated storm surge. As an added protection for water quality the developer also created a 20 foot critical line buffer between marsh and high ground. This serves to treat the land immediately adjacent to the marshes.
All in all, added Woody, the DIC has spent millions of dollars to make the community as safe and storm resilient as it is today - through an intricate piping and drainage system that is mostly behind the scenes. While nothing can protect the island against a major hurricane impact - there are plenty of measures in place to at least minimize risk.
“They deserve credit for that,” he said.
Jane Baker, vice president of community services for the Daniel Island Property Owners Association agrees.
“Having worked for City government for 10 years before coming to Daniel Island in 2012,” she said, “I can state unequivocally that the stormwater planning and engineering that the Daniel Island development team did starting day one of its development is what made the difference in how well we fared over the last three years with the incredible amount of water and/or storm surge from the 1000 year flood, Hurricane Matthew and Irma.”
For McKenzie, a longtime Charleston area resident who remembers the devastation of Hurricane Hugo, the positive feedback he and the DIC have received from Daniel Island residents in the aftermath of the recent storms has been encouraging.
“I don’t want to compare those storms to Hugo,” added McKenzie. “…But to twice come back here to everything still in good shape…It feels good when people compliment something that you kind of take for granted.”