Thursday, May 26, 2022

Stormwater runoff and development’s increasing impact on the coastal environments of Topsail and Stump Sound

An oyster lease near Permuda Island in Stump Sound. With few nearby inlets to flush out stormwater runoff, Stump Sound is less resilient to the impacts of rainfall than Topsail Sound to the south, causing an increase in shellfish closures. (Port City Daily photo/Mark Darrough)
An oyster lease near Permuda Island in Stump Sound. With no nearby inlets to flush out stormwater runoff, Stump Sound is less resilient to the impacts of rainfall than Topsail Sound to the south, causing an increase in state shellfish closures. (Port City Daily photo/Mark Darrough)

The first in a two-part series examines the impact stormwater runoff has on the waterways near Hampstead and Holly Ridge as development pushes north from Wilmington.

HAMPSTEAD — Paved surfaces and building roofs continue to cover an increasing amount of Pender County’s coastal land as residential development grows north from Wilmington. 

This means more and more rainwater travels across impervious surfaces, rather than soaking into the soil, as it carries pollutants and sediment into the Intracoastal Waterway and its nearby saltwater creeks — an area historically rich with shellfish. When water quality reaches levels that don’t permit healthy shellfish harvests, the N.C. Division of Marine Fisheries (DMF) issues temporary or permanent shellfish closures.

According to Anne Deaton, a habitat program manager for the DMF in its Wilmington office, the number of closures in the Hampstead and Holly Ridge areas — particularly in Stump Sound, well known for its oyster production — is increasing due to higher levels of bacteria found in the water. 

RELATED: New Hanover County considering addition of stormwater utility for unincorporated areas

Deaton said Hampstead once had an opportunity to manage stormwater properly, guided by the mistakes of areas already developed to the south, but today closures “keep inching down the creeks as [the area] gets more developed.”

“What bothers me is that Hampstead was in a position when it was less developed to learn from the areas nearby that are more developed,” Deaton said. “We’ve seen most of New Hanover County’s tidal creeks that used to be open to shellfish harvests are now closed all the way down to the mouth [of the Cape Fear River]. And that’s due to stormwater runoff.”

Effective stormwater management has been hampered by many developers receiving variances that allow them to use land in ways not otherwise permitted by local zoning requirements, as well as doing the minimum required by the state, according to Deaton.

If they do the minimum that’s required by the law and they’re legal, but we’re seeing that’s causing an impact, then that means more needs to be done in terms of [state] requirements,” Deaton said. 

She is also concerned that as development continues and water quality declines, this will negatively impact important fishery species like speckled trout, flounder, clams, and oysters that are dependent on seagrass. This produces a cyclical effect: the sediment from stormwater runoff cuts down on water clarity, blocking the sunlight seagrass needs to survive, in turn damaging habitats like oysters that filter and clean the water.

As these habitats decline, fish lose coverage and protection from predators while less seagrass also means less oxygen is released into the water, according to Deaton.

An N.C. Division of Marine Fisheries map shows current shellfish closures in coastal areas of New Hanover, Pender, and Onslow counties. Areas marked red are permanent closures, while areas marked yellow are temporary. (Port City Daily photo/Courtesy N.C. Division of Marine Fisheries)
An N.C. Division of Marine Fisheries map shows current shellfish closures in coastal areas of New Hanover, Pender, and Onslow counties. Areas marked red are permanent closures, while areas marked yellow are temporary. (Port City Daily photo/Courtesy N.C. Division of Marine Fisheries)

Closures increasing

Sections of North Carolina coastal waters are divided into shellfish growing areas surveyed by the DMF to monitor the bacteriological quality of the water. One area is labeled B8 Topsail Sound, stretching from the New Hanover-Pender county line to the Surf City Bridge. The next is B9 Stump Sound, an area between the Surf City Bridge and the North Topsail Bridge.

Andy Haines, a biologist with the Department of Environmental Quality in Morehead City, said all of the mainland creeks in both growing areas are “either mostly or entirely permanently off-limits to the harvest of shellfish, indicating, in some cases, that they can exceed the bacteriological standards for safe shellfish harvest even in the absence of major rainfall.” 

But where Topsail Sound can tolerate a greater amount of runoff because it includes large bodies of water surrounded by nearby inlets that flush stormwater into the ocean, Stump Sound has no inlets and is less resilient to the impacts of stormwater runoff. Because of this, thresholds that trigger shellfish closures differ in both areas.

According to data provided by Haines going back to 2010, the number of closures and the duration of those closures in both areas are, in general, rising (he notes that certain years such as 2018 see a spike due to above-average rainfall).

Haines said that because each area has different levels of resiliency to runoff, thresholds for the two areas are different, which is why you see a difference in the number of closures between the areas. In Topsail Sound, 2.5 inches of rain in a 24-hour period brings about a closure, while in Stump Sound, that threshold is 1.5 inches in the same period.

The amount of temporary closures is also much higher in Stump Sound, as seen in a real-time shellfish closure map provided by the DMV.

According to Lauren Kolodij, deputy director of the N.C. Coastal Federation, the conventional stormwater management model of down-spouts channeling water from roofs onto driveways, streets, stormwater systems (like ponds or ditches), and into the waterways is not effective.

“[I]f we want to have a viable coastal economy, we really need to start taking this stuff seriously, and reduce the volume of runoff that’s going into our surface waters,” Kolodij said. “Once those shellfish waters close, one of the next things in line are swimming advisories. People may be able to live without oysters, but when you start having swimming advisories, that’s really bad for business.” 

[Editor’s Note: This is the first of a two-part series examining development’s impact on coastal waterways in the Hampstead area. In the second, Kolodij discusses how the low-impact development model can not only reduce the impact on the environment, but also provide certain economic benefits for developers.]


Mark Darrough can be reached at Mark@Localvoicemedia.com or (970) 413-3815

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