Fixing Knoxville's Grand Canyon

The City of Knoxville, in partnership with Summit Medical Group and S&ME engineering consultants, undertook the city’s largest erosion-control effort to stabilize a severely degraded stream reach known locally as the “Grand Canyon of Knoxville.” This site was identified as the No. 1 pollutant source in Knoxville, due to excessive sediment discharges impacting downstream infrastructure and water quality. Through a comprehensive restoration strategy that combined traditional stabilization methods with innovative technologies—including Flexamat—the project successfully reduced sediment export, improved water quality, and restored ecological and hydrologic function.

Project Background
• Location: Tributary 2 to Fourth Creek, near Kingston Pike and Summit Medical Group property, Knoxville, TN.
• Problem: Over decades, stormwater runoff from urban development caused severe channel erosion. The stream expanded to nearly 130 ft wide and 30 ft deep, losing over 8,000 cubic yards of soil and dropping more than 7 ft in elevation between 2001 and 2014. The site was covered in invasive kudzu, a non-native plant that spreads aggressively, outcompeting native species by blocking sunlight and choking out native vegetation.
• Impact: Sediment discharge was identified as the primary pollutant source in Knoxville, clogging culverts, worsening flooding, and degrading water quality in Fort Loudoun Reservoir.
• Regulatory Context: Under the Clean Water Act, sediment is treated as a pollutant. The scale of this erosion created the city’s single most urgent water-quality challenge.

Project Partners
• Partners: City of Knoxville, Summit Medical Group (landowner), S&ME (engineering and contracting). Two regulatory agencies were involved with this jurisdictional water, the Tennessee Department of Environment and Conservation (TDEC) and the US Army Corps of Engineers (USACE). There was also the requirement of the National Flood Insurance Program “no-rise” requirement.

Scope of Work:
• Re-deposited approximately 7,000 cubic yards of on-site soil as foundational fill
• Placed 2,300 tons of fill to raise and stabilize the channel bed
• Incorporated native vegetation to restore riparian stability and habitat
• Cost: Approximately $820,000, with the City contributing $200,000
• Schedule: Construction began July 2019 and was targeted for completion by fall 2020

Significant Engineering Challenges:
• Significant storm flow in an otherwise almost dry channel
• No-Rise NFIP requirement
• Geotechnical considerations
• Regulatory opposition to riprap
• Mitigation cost
• Site covered in invasive kudzu
• Continued erosion of the bank

Design Approach & Restoration Elements
The project utilized a hybrid restoration strategy:

Traditional Elements:
• Cross-vanes and riffles to restore stream morphology
• Riparian vegetation plantings for bank stabilization

Innovative Technologies:
• Two-dimensional hydraulic modeling to optimize design.
• Flexamat tied concrete block mats for flexible hard-armor protection for the steep stream section
• Bionic soil media and high-performance growth substrates to accelerate vegetation establishment
• Bio-retention features to manage runoff and improve water quality while receiving a mitigation credit.

Why Flexamat Was Chosen
• Durability with Flexibility: Provides long-term erosion protection while adapting to irregular channel contours without cracking.
• Vegetation Integration: Block spacing enables grass and native vegetation growth, balancing hard engineering with ecological recovery.
• Regulatory opposition to rock rip rap
• Efficient Installation: Roll-based delivery allows rapid deployment in constrained construction zones.
• Low-Maintenance Longevity: Engineered to withstand high flows, reducing long-term maintenance costs.

Measured Pollutant Reduction Improvements
• Total Suspended Solids (TSS): Reduced to non-detectable levels downstream at Walden Drive (July 2020).
• Sediment Export: Thousands of cubic yards of soil loss arrested, eliminating Knoxville’s single largest sediment pollution source.
• Water Quality: Fourth Creek showed measurable improvements in clarity and sediment load, reducing pollution to Fort Loudoun Reservoir.
• Flood Mitigation: Improved conveyance and reduced clogging of culverts, protecting infrastructure and reducing localized flooding.

Integrated Project Impact
• Stabilized channel bed and banks
• Substantially reduced sediment pollution
• Improved downstream water quality and aquatic habitat
• Enhanced riparian vegetation and ecological resilience
• Long-term infrastructure protection with reduced maintenance burden

Conclusion
The “Grand Canyon” restoration project demonstrates how combining Flexamat with rock structures, bioengineering, and innovative modeling can transform Knoxville’s most degraded stream reach into a stable, resilient system. By addressing the city’s top pollutant source, this project not only protects infrastructure but also improves regional water quality and ecological function—serving as a model for large-scale urban erosion control and stream restoration.