Recurring flooding on the Platte River in Nebraska presents a demonstrated risk to several high-voltage transmission line towers owned by Omaha Public Power District (OPPD). Protecting three tower foundations exposed to flood waters near Ashland, Nebraska, required installation of sheet pile-encased structures reinforced with structural steel bracing.
One of the transmission structures found itself stranded the middle of a new river channel after historic flooding in March 2019 eroded roughly 600 feet of riverbank and bottom land where the tower once stood west of the river. High water also affected two other nearby towers, although both remained on dry land.
A temporary trestle provides a crane access to reinforce a transmission tower foundation in the Platte River.
The project came with a 16-week window for completion between the end of peak power demand and the start of the next potential flood season. That meant working through the heart of winter.
Design of the reinforcement structures required accounting for wind, river current, seasonal ice jams, and flooding. A reconnaissance airboat trip was scheduled by Olsson, the engineering firm selected to design the protective measures. The team discovered that the current had scoured several feet of sediment from around the structure’s concrete drilled-shaft foundations.
A hydraulic analysis evaluated flood flow forces, including debris, potential scouring, and stream channel degradation, to understand how the river would affect the structure over time. Regulatory parameters were reviewed to devise a strategy for obtaining environmental permits. In addition, a structural analysis was conducted to determine the significant lateral loads on the towers generated by wind movement of the wires.
The solution called for driving sheet piling 40-feet down into the riverbed and backfilling the space between the pile and tower bases with granular material and grouted riprap. A temporary trestle was constructed for crane access to install the sheet pile with structural steel bracing.
Hawkins Construction executed Olsson’s design and completed the construction project in challenging conditions before flood season arrived again. OPPD’s transmission towers are now in a much stronger position to withstand whatever nature decides to unleash on the Platte River.
John T. Henning, PE, leads the Energy Market at Olsson, serving utility, district energy, and renewable energy clients. John is an accomplished geotechnical engineer with experience in project management, quality assurance, and project delivery in power generation, delivery, and storage; pipeline infrastructure, and energy capture. He earned his Bachelor’s and Master’s degrees in civil engineering from Bucknell University and has served as a member the Energy Nexus Committee for the American Society of Civil Engineers and currently serves on the Energy Committee for the American Council of Engineering Companies.
