Trends Spring 2016 - Page 9

Willows were grown in the Colorado State University to use as part of the research project. “Some resource agencies such as state departments of natural resources tend to discourage, and in some cases actually prohibit, the use of rock for streambank protection; they push for using some kind of vegetation,” Lagasse said. “But we feel that using vegetation and hard material together is best.” David Reynaud, senior program officer with the NCHRP, agreed that more and more entities are pushing for environmentally sensitive techniques. “There’s a movement in the environmental area to use natural materials rather than stone or other man-made materials,” Reynaud said. Using vegetation provides environmental and aesthetic benefits, improves conditions for fisheries and wildlife, and helps improve water quality. However, vegetation also has its limitations. Plants can fail to grow or can die in drought conditions, they can get eaten by wildlife or livestock, and they may require significant maintenance, to name a few potential problems. Therefore, the study recommends that “vegetation alone should not be seriously considered as a countermeasure against severe bank erosion where a highway facility is at risk” but should be used in combination with other “hard” engineering approaches. TIME FOR TESTING To conduct their study, the NCHRP team devised a test in cooperation with the hydraulics laboratory at Colorado State University (CSU) in Fort Collins, Colorado. First, the team selected two bank protection treatments from the original 2005 NCHRP report to test with the CSU flume, which is a full-size model of an open channel that simulates real river flow conditions. Using a flume to test bank protection treatments with a live vegetative component had never been done before, Lagasse said. What made the test even more revolutionary was that the team grew willows to maturity at real-scale in a climate-controlled greenhouse at CSU and then transferred them to the flume. “We were able to take the biotechnical treatments to the big outdoor flume and subject them to water discharges to simulate various levels of river flow, and then see how the vegetation reacted to the flow,” Lagasse explained. The team conducted two tests. The first test, a biotechnical A full-size flume at CSU’s campus allowed the Ayres team to test streambank protection measures in real-world conditions. approach, used a rock toe (base of the bank) with locally harvested willows. The second treatment tested a combination of vegetation with a natural fabric material configured in soil lifts with willows inserted between the lifts, a technique commonly called vegetated mechanically stabilized earth (VMSE). To transport the willows from the greenhouse, the willow trays were put on rollers and pulled out with tractors. A crane put the trays into place in the flume. For each test, water ran for four hours through the flume, and velocity was measured at various points. Following each test, changes or damage to the treatment were examined and documented for analysis. “This type of test had not been done before. No one had this idea or this type of facility; we were truly using real-world conditions,” Lagasse said. “This was a major quantum leap forward.” RESULTS REVEALED In the end, the tests showed that of the two treatments tested in the laboratory, one (live siltation and live staking with a stone toe) met or exceeded all performance expectations. The second treatment