All around the United States, a hidden danger lies in channels and waterways — natural gas pipelines exposed by erosion and channel degradation. All it takes is a single heavy branch carried along by rushing water to cause a rupture, resulting in the potential loss of life and property in populated areas.
With tens of thousands of creek and rivers crossing high pressure lines, this is a relatively common issue. And while utilities conduct regular inspections, the sheer number of lines needing inspection sometimes pushes an organization into reactive, rather than proactive, mode.
Speedy, Significant Erosion Causes an Urgent Issue
When a client contacted Burns & McDonnell regarding an exposed natural gas pipeline crossing a channel near in Arlington, Texas, it was clear that an innovative solution was necessary — and fast.
The channel had eroded quickly and significantly, fully exposing the 20-inch diameter high pressure pipeline in less than a decade. Compounding the urgency was the proximity of the exposed pipeline to an adjacent residential neighborhood.
The client considered drilling a new line in the existing easement, but the lack of space and prohibitive cost meant the best approach would be to relocate the pipeline 10 feet under the previous location. Our job was to design channel protection for the new pipeline to prevent exposure in the long term.
Burns & McDonnell was uniquely positioned to handle the issue with the necessary speed and skill. Our Dallas office shares a strong working relationship with the client and offers the permitting experience that’s vital on infrastructure projects. The team was supplemented by channel specialists, geotechnical engineers and structural engineers from our Kansas City, Omaha and South Dakota offices.
The team first obtained a topographic survey to see how quickly the channel was degrading. Green engineering is often the simplest solution to channel erosion, using a combination of rock, vegetation and native plantings to stabilize the banks. But it wasn’t a good option here — because of the speed of the initial pipeline exposure, the project demanded a more significant and permanent solution.
Another issue increased the complexity — the channel in question is a FEMA regulated stream and has an existing floodway. Simply reconstructing the channel and armoring the pipeline crossing was not an option because it would cause a rise in the floodway elevation.
Protecting the Pipeline for the Long Term
Because the channel was in a residential area, Burns & McDonnell developed a solution to stabilize the channel with minimal impact to the residents’ backyards — gabion walls. A gabion is a rock-filled wire basket, and a gabion wall is a retaining wall made of gabions tied together with wire. These walls allowed for an almost vertical side slope, protecting channel banks with limited visual impact.
Where the gabion wall crossed the existing pipelines, the team designed additional structural support to prevent the wall from exerting a point load on the pipeline. A 12-inch gabion mattress — rock-filled wire over the bottom of the channel — armored the channel invert to prevent future erosion.
Considering how quickly the channel had degraded, the team added a cutoff wall at the project’s downstream end for additional protection. The design called for the excavation of a six-foot trench perpendicular to the channel’s flow, into which gabions were installed and anchored to the gabion wall on each side. After the trench was backfilled, the gabion mattress was installed over the top of the cutoff wall.
The design solution employs a number of fail safes to protect the channel for years to come, even with an increase in volume and speed. It’s already been battle tested; the area has experienced three 100-year flood events since installation. Even with rain at historic levels and 15 feet of water in the channel, the system held.
An Increasingly Frequent Problem
As development continues adjacent to streams, rivers and other channels, erosion that exposes pipeline infrastructure is increasingly a problem. And while some solutions are straightforward, many aren’t, encompassing permitting, channel design, geotechnical design, structural engineering and FEMA experience.
I spoke on this issue at the ASCE Pipelines Conference in Kansas City on July 18. Has your utility seen an increase in pipeline exposure by erosion and channel degradation? How have you addressed it? Share your experience in the comments or reach out to me on LinkedIn.