Bioengineering and Sustainable Tools for Emergency Streambank Repair at Salem Creek
The Salem Creek Streambank Repair project received an Engineering Excellence Honor Award from ACEC North Carolina in fall 2022.
Salem Creek meanders along expansive greenspace, agriculture and urban lands within Winston-Salem’s city limits. Changes in hydraulic and geomorphic conditions had begun to impact the streambanks at a location south of the city, resulting in streambank erosion and sediment loss. Due to its location alongside a major force main and fiber optic line serving the city, this area of the creek remained under close monitoring.
When the City of Winston-Salem experienced a severe rainfall event in spring 2020, high water flows magnified the erosion issues. In just three or four days, Winston-Salem received five to seven inches of rain, and downstream gauge stations registered flows upward of 35,000 cfs (cubic feet per second) with base flows typically around 3,000 cfs. The damage was substantial: Salem Creek experienced severe erosion and debris jams, 15-foot-high banks collapsed, and the force main and fiber optic line were exposed, leaving both utilities in imminent danger of damage and major disruptions to critical services in the city.
Even in this emergency stream repair situation with a compressed schedule, Freese and Nichols was able to help the City successfully use bioengineering and a variety of other natural and sustainable tools to restore the stream, resolve sediment challenges and reduce future erosion.
Site Assessment Utilizes Drone Technology and Flow Modeling
After receiving a call from the City of Winston-Salem, Freese and Nichols’ first step was an initial assessment that included numerous site visits and the use of drone technology combined with modeling. This approach served as an effective technique to identify site conditions, determine improvement options and select the most effective repair technique to serve the City and provide long-term bank stabilization. The team utilized combined assessment data to design and direct the stream’s alignment for a successful proposed solution – bioengineering streambanks with a hard-armored solution on the toe to stay in place.
Drones allowed for the initial assessment of erosion damage and provided a footprint of sandbars that had developed downstream over time. The team also used the drone for monitoring stream movements throughout the project. Even as design progressed, more sediment continued to gather downstream, further showing the significance of addressing stream flow and the negative impact of sediment deposition (sandbars) and debris blockage within the channel. The Freese and Nichols team then completed a sediment transport analysis to better understand the system and guide their design solution.
As one of FNI’s first stormwater projects to utilize drone imagery in combination with modeling and design techniques, the team reviewed drone photographs to develop the channel alignment, map sediment movement and effectively address underlying issues in the channel causing streambank erosion.
As a result, the design provides a favorable stream restoration solution that addresses the redirection of water flow in order to rebuild the bank and protect crucial Infrastructure while also lessening future sediment accumulation, facilitating better conveyance of water and sediment along Salem Creek and lessening the chances of future erosion at this location.
Natural Fill Solutions
With 10-15-foot-high banks, the project involved a significant amount of fill material for repair and protection of the exposed lines. In developing repair solutions on a compressed timeframe, the team created a matrix of fill material options combined with other repair techniques (stacked wall, bioengineering, etc.). This approach allowed for team collaboration and client input, ultimately leading to a combination of techniques for a long-lasting solution.
Using the river’s natural flow on bedrock, FNI designed a system to work with the natural landscape and guide the river along the best path. Use of drones post-construction confirmed the positive impact of improvements.
Sustainable Design Considerations
In addition to rebuilding the streambank and preserving wastewater and communication service by protecting the exposed force main and fiber optic line, the project design reduced sediment loads in the stream, thereby improving water quality. By decreasing nitrogen and phosphorus contained in streambank sediments, the completed project contributes to the reduction of sediment-bound pollutants.
These benefits combined with the team’s focus on bioengineering provides a more sustainable, long-lasting approach. Use of native trees will mature to further stabilize the banks, promoting ecological enhancement and providing a streambank design that fits within the rural, agricultural setting, while also being aesthetically pleasing.
Future Value to Engineering Profession
FNI’s approach emphasizes that bioengineering approaches can be used in emergency situations, favoring sustainable, environmentally friendly approaches as stormwater solutions versus hard-armored, concrete solutions as a “quick fix.” The completed project is an example of how blending solutions can create a balanced design to serve our natural environment.
The amount and types of material used as fill along the 10-15-foot-high banks shows the ability to provide feasible options outside of soil only. The team’s approach displays a good example of how sustainable materials and methods can achieve same or better results as concrete and riprap.
The design provides sustainable, long-lasting approaches to rebuild the streambank, preserve wastewater and communication service, reduce sediment erosion, and improve water quality.
Complexity
Working on a compressed, emergency timeframe to protect the exposed force main and fiber optic line and restore streambank stability, FNI completed design in a one-month timeframe. This included geomorphic site assessment with identification of sediment deposits and completion of flow modeling followed by immediate design to 100% while incorporating City reviews and comments. The team added efficiency by using a simplified plan set once the site footprint was confirmed.
Additionally, as design progressed, the FNI team worked in collaboration with the contractor, North State Environmental, by meeting onsite to discuss several feasible design approaches, confirm constructability and add efficiency to the construction timeframe.
Early coordination also allowed for approval from regulatory agencies as an emergency restoration scenario to maintain sewer service to the City.
Serving as Trusted Advisor, Team Exceeds Client Needs
The project budget accounted for the efficient use of materials when bringing in higher-than-normal quantities to fill the 10-15-foot streambank.
In this emergency streambank restoration scenario, FNI served as the Trusted Advisor on what is now the City’s second bioengineering solution. In everything from site assessments to 100% design completion, the team exceeded client expectations. And although the project was completed on a compressed schedule, the team didn’t take any shortcuts. By combining the use of drone imagery with modeling of sediment deposits, our team was able to help preserve the natural, agricultural surroundings and build back Salem Creek stronger than it was before.
Stakeholder coordination continued alongside a quick schedule that included numerous onsite meetings with the City and Contractor to discuss restoration solutions in real time and ensure that construction could successfully begin as soon as the design was delivered. Design completion within one month and construction completion in the four months that followed exceeded the client’s needs and allowed for the force main and fiber optic line to continue service without any shutdowns.