Development rules ease Triangle flooding but can’t prevent it

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 this week’s storm dropped 6 to 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.”

Flood control

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.

“We think we’ve made a tremendous amount of progress, especially because we adopted these ordinances before a lot of the development has occurred,” said Steve Brown, Cary’s director of water resources.

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.

All of the rain that fell on Life Time’s parking lot and rooftops during this week’s storm made it into the retention pond.

“We had more rain here in the last week than we did during Hurricane Matthew,” said Charlie Stillwell, a Ph.D. student at NCSU who has monitored this stormwater system for more than a year. “From all of this 25-acre site, we weren’t really contributing to any of the flood problems that were in this area.”

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.

“Normally in the Falls Lake watershed, you are only allowed to develop low-density residential because that will limit the amount of driveways and roof space and roads,” Stillwell said.

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, Hunt said, eight inches is another story.

“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.”

This story was originally published April 28, 2017 at 3:27 PM with the headline "Development rules ease Triangle flooding but can’t prevent it."