USING QUALITY SCIENCE TO ESTABLISH A QUALITY MITIGATION BANK
Significant amounts of sediment deposited by floods hindered the ability for fish and aquatic life to thrive before the stream restoration of Gregorys Creek in South Carolina. Deep in the woods of South Carolina, native vegetation and aquatic wildlife begin to flourish across 530 acres of a forested watershed. Only a few years ago, this ecosystem looked much different. Generations upon generations of agricultural and outdated land use practices took their toll — degrading miles of streams, eroding large amounts of sediment along the banks, and diminishing aquatic habitat, among other impacts.
The client has long recognized its responsibility to keep forests healthy and wanted to restore these ecologically significant freshwater streams and wetlands within the Lower Broad River Watershed by establishing a mitigation bank on the land, as a part of the company’s sustainability ambitions.
Ecologically significant mitigation and high-quality work were top considerations for the client in selecting a partner. The team of ecological restoration engineers now at SWCA collaborated with the client throughout the entire process, including identifying potential mitigation sites, developing concept designs, providing engineering expertise and permitting support, and overseeing construction and monitoring.
“The client’s mitigation efforts across the state of South Carolina have targeted vulnerable watersheds and large parcel areas that allow them, and SWCA as their partner, to protect and restore huge areas of valuable stream and wetland habitat. While the impacts we are offsetting are often isolated, degraded wetlands, or segments of eroded streams along road corridors, the mitigation that we provide results in continuous, high-quality stream and wetland habitat protected in perpetuity,” said Scott Miller, an associate restoration engineer at SWCA. “You can see the positive impact by the extent of enhanced and protected habitat, the uplift to fish passage and aquatic biological function, and the restored diversity throughout the project area. Every project we team up with this client in South Carolina means another protected spot in our state, ensuring incredible benefits to fish, wildlife, and watersheds in the region.”
The absence of a meandering path indicated that Gregorys Creek and its tributaries were unstable before the ecological restoration project.MORE THAN 47,000 FEET OF STREAM RESTORATION AS A NATURAL CLIMATE SOLUTION
The first step of implementing the restoration plan included reconfiguring Gregorys Creek to be a more stable and healthier stream shape. Restoring Gregorys Creek involved grading back the banks and reestablishing a low floodplain. A few notable features marked the mitigation site; first is the sheer size of the area, as 530 acres make it one of the largest mitigation banking sites in the state. It’s also nestled between two parcels of disconnected Sumter National Forest land. But the main features were a 2.3-mile stretch of Gregorys Creek that travels west to east and bisects the site before eventually flowing into Broad River and the 13 tributaries within the site boundaries that flow into it.
Such a unique site required creative solutions backed by sound science. The team immediately set to work mapping and measuring the ecological elements of the site including existing streams and wetlands, vegetative species and communities, soil type, stream water quality, and other geomorphic data. Following the U.S. Army Corps of Engineers’ Guidelines for Preparing a Compensatory Mitigation Plan, the team calculated the number of credits that would be available according to the restoration activities.
While the on-site wetlands qualified for preservation, the streams told a different story. Incised channels with actively eroding vertical banks exceeding 10 feet, lack of appropriate channel dimensions, and the absence of a meandering pathway indicated that the streams were unstable. This instability allowed floods to rush downstream, carrying significant sediment and limiting opportunities for fish and aquatic life to thrive. Based on this in-depth assessment, the team designed and implemented a mitigation plan focused on stream restoration, enhancement, and preservation.
Using a variety of natural channel design techniques — adding meanders, raising the channel beds up to the original floodplain, installing in-stream log structures and woody debris, and re-vegetating the riparian buffer with over 100,000 native trees — our engineers navigated the restoration of more than 47,000 feet of stream.
“We applied Priority One restoration along with a fresh construction and restoration practice we termed ‘augmentation’. With this new approach, we kept the tributary channels in their existing planform while raising the bed profiles using a series of brush riffles, log structures, and toe wood,” said Kyle Halchin, an ecosystems restoration engineer at SWCA. “The augmentation approach proved to be a highly effective way to restore deeply incised first to third order streams with a minimal impact on the surrounding forested buffers. Plus, we sourced all materials used in the construction from the site.”
Restoring the Gregorys Creek Mitigation Site provided a timely, cost-effective, and ecologically significant mitigation solution for other unavoidable impacts to streams and wetlands.
Monitoring the Gregorys Creek Mitigation Site is a critical part of the project lifecycle to understand the stream restoration outcomes. A HEALTHY FOREST, A HEALTHY WATERSHED
SWCA employees monitoring the streams and planted buffers in the Gregorys Creek Mitigation Site.Returning the Gregorys Creek Mitigation Site to a more natural ecosystem creates a ripple effect well beyond the site boundary. With the restoration complete, the site serves as a natural bridge between the two separate Sumter National Forest sections, offering environmental continuity and increased connectivity. Wildlife, including threatened and rare species like the tricolored bat (Perimyotis subflavus), northern long-eared bat (Myotis septentrionalis), and the robust redhorse fish (Moxostoma carinatum), will now be able to travel across a much wider expanse of protected land.
Engineering healthy and stable streams across the Gregorys Creek Mitigation Site has improved water quality, enhanced aquatic habitat, strengthened the overall watershed, and brought wetlands back to previously drained valleys.
“Within the first year after construction, we’re seeing the ground water table return to natural levels across the site, in effect, restoring wetland habitat,” said Halchin. “We’re already seeing a return of native fish, turtles, and other aquatic life.”
SWCA will continue to monitor the site annually for the next several years to ensure that the streams and planted buffers are meeting or are on a trajectory to meet the performance standards and goals.
An image of the ecological restoration from the monitoring phase.