Heavy rains cause flooding around Crabtree Valley Mall in April
The threat of flooding as devastating as Hurricane Harvey’s impact on Texas is what keeps stormwater management experts awake at night.
Professionals like Ben Brown, floodplain administrator for the city of Raleigh, watch the storm damage unfold to see if there are lessons to be learned from a regulatory or floodplain management perspective.
“It could be a once-in-a-lifetime event, what’s happening in Houston right now,” Brown said. “Fifty inches of rain in 24 hours or so – that’s very scary.”
Were that amount of rain to hit the Triangle, it would be destructive but likely not as bad as in Houston, where the topography is much flatter than in Raleigh, Brown said.
Most of Raleigh’s trouble spots are along Crabtree Creek and Walnut Creek, not downtown. Still, Raleigh requires buildings be 2 feet above the predicted 100-year storm level as precaution.
While restrictions on land use only go so far in preventing disasters, public officials have a good idea where to expect problems, thanks to mapping projects in cooperation with the Federal Emergency Management Agency.
North Carolina embarked on updating flood hazard zones for every county in the state in 2000, which helps determine insurance rates and public safety. Raleigh, for instance, has 23 square miles of regulated floodplain subject to problems from severe storms.
There was extensive flooding in Raleigh in April, when 6 to 9 inches of rain fell throughout the Triangle.
Planning for the future
Coincidentally, as Hurricane Harvey descended on Texas, design experts were convening in Princeville for a five-day workshop focused on how to rebuild that community, which was extensively flooded last fall by the effects of Hurricane Matthew.
It was the second time in 20 years that the Edgecombe County community had been nearly devastated by floods. State officials determined this time to avoid rebuilding in the flood plain.
So the state has purchased a 52-acre tract of elevated land that planners will focus on to develop three options for how to rebuild. Land use planners, engineers, architects and landscape architects have been meeting with local, state and federal officials and residents.
A spokeswoman for the state Division of Emergency Management said Monday that while the product of the workshop will be for Princeville, it could likely produce lessons for how to rebuild in other flood-prone areas around the state.
Local governments and developers have spent decades – and countless dollars – trying to control flooding during the heavy rains that periodically inundate the Triangle.
Cities and towns have created development-free zones and buffers along creeks and streams and limit the amount of water that can run off new subdivisions, office parks and shopping centers. In some cases, they even demolish or relocate structures along waterways to reduce the amount of rooftops, parking lots and other impervious surfaces in a bid to allow more rainwater to soak into the ground.
But at some point, it’s not enough. When the storm in April dropped as much as 9 inches of rain on the Triangle, brown water covered roads and inundated homes and businesses along Crabtree Creek, Walnut Creek and the Neuse River.
“There’s no stormwater control measure that will eliminate flooding with eight inches of rain,” said Bill Hunt, a professor and extension specialist in N.C. State University’s Department of Biological and Agricultural Engineering. “You can’t beat Mother Nature.”
Many of these trouble spots were developed in the 1970s or earlier when local governments did little to control building in the flood plains of creeks and streams. These areas are the first to flood because stormwater that normally would seep into the ground there and upstream has nowhere to go.
“That is because we have cleared out the natural sponge that was the landscape called trees,” Hunt said. “When you replace that with rooftops and parking lots and roads, there’s nowhere for the water to go. You might get a 1/10th of an inch to puddle and that’s it. The water has to go somewhere, and that somewhere is potentially down the drain to a creek.”
And creeks can handle only so much.
As more land in the Triangle is developed, impervious surfaces increase the rate and volume of runoff into waterways unless stormwater controls are put in place. Cities and towns have responded in recent decades with rules that aim to keep more rainwater in place.
Cary adopted new ordinances in the late 1990s and early 2000s that require buffers between streams and new development and don’t allow new structures to be built in the flood plain. The town also requires any new development to have stormwater controls that limit the runoff to pre-development levels and treat it on site.
Developers often can decide how to comply with stormwater control requirements and most choose wet retention ponds, which collect water during storms and slowly release it to limit the flow into creeks and streams during and after a storm. These ponds at shopping centers and subdivisions are smaller versions of big stormwater ponds such as Shelley Lake, Lake Lynn and Lake Crabtree, which help reduce flooding in Crabtree Creek.
Raleigh also limits new developments in flood plains. But it is still allowed, with a flood permit, if no more than 50 percent of the outer area of the flood plain is developed and new structures are flood-proofed, according to the city.
For areas in the Triangle where residents still see flooding, local governments have programs to help, including using federal funding to purchase flood-prone homes and convert the property to public open space. Through FEMA’s Hazard Mitigation Grant Program, Raleigh purchased and demolished two inns on Capital Boulevard in recent years to restore the land to green space. The inns were in the flood plain of House Creek.
Cities and towns also have the ability to replace and repair undersized, damaged or obstructed stormwater pipes that may be overwhelmed by heavy rain.
“We are trying to, over time, to reduce the exposure we have to flooding, and that often gets done one home at a time, one citizen at a time, as we make improvements,” Brown said.
A success story
While wet retention ponds are often the most common choice by developers in the Triangle for stormwater management, constructed wetlands and bioretention cells, or rain gardens, are also common. Wetlands and rain gardens not only hold stormwater, they also filter pollutants from it.
Life Time Fitness Health Club, off Falls of Neuse Road in North Raleigh, uses all three – pond, wetland and bioretention cell – to collect and filter all the rainwater that comes down on its parking lot and rooftop.
Rain that falls in the parking lot runs toward the back of the site, passing through the bioretention cells – which look like decorative medians – where plants and the soil absorb some of the water, along with pollutants such as nitrogen. The water then flows toward the wetlands, which filter more pollutants, and into the pond, where it sits until it is ready to be slowly pumped back into the city’s stormwater system.
The city required this more complex stormwater system because of the property’s proximity to the watershed of Falls Lake, Wake County’s largest source of drinking water.
The system is much more expensive than the more common retention ponds found at most developments. While those ponds can handle two, three or four inches of rain, eight inches is another story, said Hunt of N.C. State.
“People just have to understand that despite engineering’s best efforts, it’s not economically feasible to eliminate flooding,” he said. “It’s impossible to eliminate flooding. The best we can do is limit flooding.”