Featured in IQ: 2012 Volume 4 Issue 4
A siphon through Norfork Dam is improving the North Fork and White rivers' trout habitat by sending a constant flow of cooler water downstream.
The 42-inch-diameter pipe moves water from Norfork Lake in Baxter County, Ark., carries it through the dam and down the structure's face, and discharges it into the Norfork tailwater (North Fork River). This cooler flow provides a stable condition for the cold-water fish habitat, which contributes to the area's world-class fishery. The nearly 5-mile-long tailwater to the White River is stocked with brown, rainbow, cutthroat and brook trout that struggle to survive and reproduce when water warms and pools.
Constructing a siphon through the Norfork Dam is part of the White River Minimum Flow Project long pursued by the U.S. Army Corps of Engineers and Arkansas Game and Fish Commission. Norfork Dam provides flood control and produces hydroelectric power. However, because the dam only generates electricity during periods of peak energy use, water doesn't regularly flow through the penstocks and turbines. This ultimately impacts the ability for fish to thrive downstream. Adding a siphon system to the dam ensures that the tailwater is continually fed, even when the generators are unused.
"When there are long periods without electricity generation, the water warms up and we experience trout die off," said Michael Armstrong, Deputy Director of the Arkansas Game and Fish Commission. "The purpose of the siphon is to provide temperature stability and raise the base flow of the North Fork. The steady flow increases the wetted perimeter by about 40 percent—which increases food production and makes for fatter, healthier and happier trout."
Garver designed the intelligent siphon system, which is electronically connected to the dam's hydroelectric power system. When the dam is generating power and moving water downstream, the siphon receives a signal from the powerhouse to shut off. When generation stops, the siphon is automatically turned back on. A programmable logic controller (PLC) automates this process and can open or close the siphon valves as required.
"The siphon draws water through one of three intake valves at varying depths in Norfork Lake," said Garver Project Manager Paul Strickland, PE. "The intake valves are separated by approximately 50 feet and fitted with dissolved oxygen and temperature sensors. This enables the dam operators to monitor and determine which intake valve is providing optimal cold-water conditions. Water is then naturally drawn into the pipe because of differences in water pressure and due to the elevation of the intake valves and discharge valve."
After water follows the pipe through the dam and down a spillway abutment, it is sprayed through a fixed cone valve into a stilling basin at 185 cubic feet per second, which is more than 83,000 gallons per minute.
"The siphon is designed with a valve at the export point to spray water for aeration," Armstrong said. "From late August to early December, the tailwater experiences low oxygen, which can be detrimental to trout, and the aeration afforded by the siphon will help correct that."
During construction, the contractor bored a 45-foot-long hole through the dam to insert the siphon pipe. Due to adjacent structural items, the contractor could only bore a 48-inch-square hole to insert the pipe. After coring six pilot holes, wire ropes were passed through the dam for cutting. The concrete section was then removed by jacking.
Sending cold water through a pipe during summer temperatures can cause differential movement in the pipe. To address thermal expansion and contraction, Garver's design needed to find a way to attach the pipe to the dam face while still allowing it to naturally fluctuate.
"There is one fixed support near the siphon's discharge end, but the rest of the pipe 'floats' along the dam face," said Garver Project Leader John Watkins, PE, SE. "This allows the pipe to expand and contract as the temperature changes so the pipe isn't overstressed."
In October, Garver won the American Society of Civil Engineers/Arkansas Outstanding Engineering Design of the Year Award for the project.
"There have been many challenges with the project, including extreme weather events during construction that ranged from high water to extremely dry conditions," said Project Manager Craig Evans, PE with the U.S. Army Corps of Engineers. "Garver worked hard to provide a design that meets Little Rock District and Arkansas Game and Fish Commission requirements. They were very proactive in resolving the issues that arose."
Providing minimum flows by way of a siphon is helping the trout population and is expected to increase the popularity of the North Fork and White rivers as fishing destinations and bring more revenue to the surrounding areas.
Our previous issue of IQ featured another project at Norfork Dam. Garver worked with Mobley Contractors to install a unique maintenance bulkhead. We've added a video to that online article highlighting the award-winning project.