|Canal water flow averages 20 cubic feet per second (cfs), peaking at more than 30 cfs during times of high use.|
When the Greenfield Irrigation District, headquartered in Fairfield, Mont., decided to rehabilitate a 5-1/2-mile sub-lateral canal in its elaborate network of canals in west-central Montana, they decided to try something new—at least new to them: a geomembrane lining system. Established in 1929 by the U.S. Bureau of Reclamation, the Greenfield Irrigation District provides much needed water to area farmers whose crops would otherwise suffer because of the lack of rain common to the region. Serving more than 500 farmers, Greenfield is one of the largest irrigation districts in Montana with 562 miles of laterals and 131 miles of canals covering 83,000 acres.
"The crops grown in this area are predominantly barley and alfalfa. Alfalfa, in particular, requires deep soil percolation during certain times of the year. If there is no rainfall during those times, the crops rely strictly on irrigation water," said Bob Hardin, manager of the Greenfield Irrigation District. "Our main goal is to deliver the amount of water needed at the time it is required for our farmers to produce a healthy, bountiful crop."
This rehabilitated stretch of canal, referred to as GM47-11, serves five farmers, each having several hundred acres of land, and also supplies water to Freezeout Lake, which is owned by the Montana Fish, Wildlife, and Parks Department. The water source for the sub-lateral canal is Gibson Reservoir, Greenfield’s main storage facility located 60 miles away. The reservoir supplies water to the canal at an average of 20 cubic feet per second (cfs), peaking at more than 30 cfs during times of high use.
|On each bank of the canal, the edges of the geomembrane liner were placed into a keyway, or anchor trench, and covered with dirt to secure it into place.|
The GM47-11 project was funded in part by a grant from the Natural Resources Conservation Service (NRCS). To abide by NRCS rules and regulations, affected landowners hired a technical service provider (TSP) to serve as a private engineer. The TSP then created drawings, designed the canal profile, and specified Firestone EPDM Geomembrane for the lining material. Firestone’s lining products have been used in critical containment applications such as this for more than two decades.
"We were very pleased with the engineer’s decision to use a geomembrane lining system, and this is the first flexible membrane liner we’ve ever used," Hardin said. "We definitely needed a new option based on what is happening with the older sections of the canal."
Of the hundreds of miles of canals and laterals in the Greenfield Irrigation District, some existing sections are lined with concrete. Most, however, are strictly earthen canals. But Greenfield’s earthen canals lose water due to seepage, and some of the concrete-lined canals have cracked because of many years of thermal expansion and frost heaving.
While cost, durability, life expectancy, warranty, expansion and contraction characteristics, and puncture resistance all were considerations in the liner selection, cost was the overriding factor. "We are a non-profit organization and this is a tax-based project," Hardin commented. "The cost of concrete lining has gotten very high compared to what it was in the 1980s when we lined many of our canals as part of an extensive rehabilitation and betterment program. Now however, there is not a more cost-effective method for lining canals than with a product like EPDM Geomembrane, especially given the life expectancy of the material."
|This rehabilitated stretch of canal, referred to as GM47-11, serves five farmers, each having several hundred acres of land.|
Greenfield Irrigation District personnel served as installers on the canal project. The first step was excavating to the engineer’s specifications, which required a canal that was 3-1/2 to 4 feet deep and 30 feet wide, narrowing to 25 feet wide as it approaches Freezeout Lake. Excavation began in September 2007.
Once excavated and cleared of any debris, a geosynthetic fabric underlayment was laid into the canal bed. "The primary reason we used the underlayment material was to protect the geomembrane liner from livestock and animal damage," Hardin stated. "There have been times when cattle drinking out of the canal have fallen in. When they try to climb up the steep sides, their hooves could potentially damage the liner. While this geomembrane liner has high tear resistance, the underlayment will provide additional protection against damage."
Once the geosynthetic underlayment was in place, 30-foot-wide by 200-foot-long geomembrane panels were unrolled down the center of the canal and unfolded up the sides. The panels were then seamed together using Firestone’s QuickSeam Tape System. Winds posed a challenge during the seaming process because seams must be clean and dust free for the adhesive to seal the panels together effectively. "The winds certainly slowed us down a bit, but overall the seaming process was pretty simple," Hardin recalled.
Because of the nature of the project and Firestone’s experience in this arena, the Firestone Specialty Products representative recommended an additional step in the lining process—installing cross trenches and cleanouts at regular intervals down the length of the canal. "Firestone suggested we do this because of the sheer length of the channel and the force at which water travels through it," Hardin stated. "The cross trenches serve the purpose of more firmly anchoring the membrane as well as a means for maintaining a consistent flow during heavy rain or thawing events.
The cross trenches were prepared by digging trenches across the canal at 500-foot intervals. Each trench is 2 feet deep and about 4 feet wide. The liner was then placed across the trenches, and each trench was filled with large rocks to anchor the liner into place. On each bank of the canal, the edges of the liner were placed into a keyway, or anchor trench, and covered with dirt to secure it into place.
Although winter weather settled in during December and put the project on hold until spring, the liner installation was completed and the canal was filled with water in mid-May 2008. A total of 800,000 square feet of geomembrane liner was installed, and a 20-year warranty was issued following inspection.
"There is no doubt in my mind that this is the easiest and most cost-effective way to line a canal and conserve water," Hardin said. "We will definitely be lining additional sections of the canal network with Firestone EPDM Geomembrane."
Jeff PanKonie, a senior sales engineer for Firestone Specialty Products—a division of Bridgestone/Firestone North America, LLC., has more than 25 years experience in flexible membranes and has written numerous technical papers and manuals for Firestone and the lining community. Currently, PanKonie is the presiding secretary of ASTM D35 and has been honored with having three new geosynthetic standards approved within the last three years. He works closely with and advises a wide range of geotechnical professionals including design engineers, regulators, academia, and contractors; and has been a guest instructor at Iowa State University. PanKonie holds a degree in civil engineering from Purdue University. He can be contacted at 800-428-4442.