When it rains a lot in the Triangle, stormwater can flood the pipes that carry untreated sewage to treatment plants, causing them to back up and overflow, often into nearby creeks and streams.
When a storm dropped 6 to 9 inches of rain on the region late last month, more than 12 million gallons of untreated sewage mixed with stormwater spilled out of municipal sewer systems in the Triangle, with most of it reaching surface waters such as streams, creeks, lakes and wetlands.
In Raleigh alone, 10.5 million gallons spilled, including more than 6.6 million gallons near West Millbrook Road, just upstream of Shelley Lake.
Sewer systems are designed to carry some additional flow when it rains, but excessive stormwater can flood pipes, causing them to back up and spill through cracks or manhole covers. Larger pipes, particularly along Crabtree Creek, could have prevented some of the overflows last month, said Eileen Navarrete, Raleigh’s construction projects administrator for the public utilities department.
That’s why Raleigh is spending more than $400 million over the next 10 years to replace or upgrade the interceptors, or larger pipes, in its sewage collection system, including those along major waterways such as Crabtree Creek, Walnut Creek and the Neuse River. Larger pipes also will provide the city with more capacity to accommodate a growing population.
“This type of interceptor work is probably about 25 percent of our overall capital improvement program in the next 10 years,” Navarrete said. “It’s a significant percentage of the work we do every day.”
Danny Smith, regional supervisor for water quality operations for the state Division of Water Resources, said sewage spills after a heavy rain aren’t unique to Raleigh but occur nationwide. After last month’s storm, an estimated 1.25 million gallons reached an unnamed tributary to the Neuse River in Selma in Johnston County.
“Every collection system has the infill and infiltration, and you have to constantly work on it,” Smith said.
Raleigh’s primary focus is on projects along Crabtree Creek, where city staff see the most sewer overflows. Two pipes – one 33 inches and the other 36 inches in diameter – already run along the creek, and the city is adding a third, 54-inch pipe that will run about four miles from Glenwood Avenue to Capital Boulevard. Work on the $35 million pipeline is underway and expected to be completed in mid-2019.
But the city also plans to spend $195 million on projects along Walnut Creek and the Neuse River in the next 10 years to try to prevent the kind of overflows that are more common along Crabtree Creek.
Along Walnut Creek, an aging 54-inch pipe will be replaced by a 72-inch pipe between Barwell and Sunnybrook roads. Work on the $25 million project is expected to begin by the end of the year and take two years to complete.
Along the Neuse River, 96-inch and 84-inch pipes are expected to take two years and $94 million to put in place, starting in early 2019. They will stretch from Barwell Road to the city’s sewage treatment plant on Battle Bridge Road.
The city hopes the new, larger pipes will last for 75 to 100 years. Had they been in place now, Navarrete said, there likely would have been fewer spills during last month’s storm.
“I can’t say for sure we wouldn’t have had any because there’s only so much we can do, only so much we can oversize our pipes while trying to be fiscally responsible,” she said. “But these types of projects certainly help minimize environmental impacts from storms like this, and they help keep the wastewater in the pipe.”
Heavy rain is only one of several causes of sewer overflows. More common are cracks in pipes or blockages caused by grease, paper products, roots, rocks and other debris.
Municipalities have different ways of detecting cracks in sewer lines, including using smoke or cameras, said Smith, with the Division of Water Resources. Cities and towns also routinely clean sewer lines to remove debris, trash and grease that might stop up a pipe.
Smith said there are ways residents can help reduce keep sewage pipes clear by not pouring grease down the sink and not flushing rags or debris such as wet wipes that can clog the lines.
Kathryn Trogdon: 919-829-4845: @KTrogdon