Protecting Grey Infrastructure with Green Approaches: Using natural channel design approach solutions to achieve cost effective and long-lasting infrastructure protection
David Perry
Education Track: Stormwater Management
Often municipalities and utilities have linear assets, particularly water and sewer lines, that are adjacent to waterways. They become exposed and at risk of failure due to erosion and bank instability, particularly following significant storm events. Natural channel design solutions offer alternatives to service disruptions and costly asset relocation. When evaluated and compared to relocation of the endangered utility lines, natural channel design solutions are often the most cost effective and least disruptive to service. This innovative approach also provides significant other benefits, including: reduced sediment loading, water quality improvements, ecological functional uplift, public education, improved aesthetics, not to mention a more stable long-term solution than more traditional hard armoring techniques.
The Town of Elkin is presented as one case study. As part of the implementation of an Emergency Raw Water Line Installation project, an existing 24” gravity raw water line, located adjacent to the new line, was found to have been entirely exposed by the horizontal and lateral erosion of Big Elkin Creek. The alignment generally follows Big Elkin Creek along an existing greenway trail. An alternatives analysis considered options to protect or replace the exposed 24” gravity line. It was determined that, in addition to the installation of the emergency water line, the most cost effective way to ensure that the gravity line was not compromised was to halt the ongoing erosion by stabilizing Big Elkin Creek’s banks and re-directing its flows away from the existing line. To accomplish this, a natural channel design approach was applied to the eroded areas. Design components included the strategic placement and sizing of root wads, boulder toes and j-hooks. These structures will protect the existing line by restoring and stabilizing the channel banks over and around it, as well as redirecting flows and associated velocity and sheer stress away from it, particularly during storm events. In addition to the work on Big Elkin Creek’s banks and channel, an highly eroded and downcut unnamed tributary, which confluences with Big Elkin Creek directly over the gravity line, is being realigned and stabilized to prevent further erosion and threats to the gravity line. Design challenges include achieving a FEMA no-rise condition, despite significantly altering the channel cross-section and flow path, as well as strategically placing and sizing the flow diversion structures. This was the least expensive of all the evaluated project alternatives