Canal wave oscillation: Finding a solution at one-tenth the cost
Finding a solution to a persistent problem is uniquely satisfying. Finding that solution at 1/10th the cost? Priceless.
In the spring of 2009, crews first noticed a problem in the New York Canal near ITD's Interstate 84/Orchard Road project. As the water in the canal rushed past the bridge piers, it created an alternating vortex behind the piers. The frequency of the alternating thrusts of the water, combined with the natural frequency of the channel, created an “alternating wave oscillation harmonics” in the channel that began to erode the banks south of the canal. If left unchecked, the erosion could have compromised the integrity of the bridge.
“The water could have broken through or overtopped the banks in just a few weeks,” said District 3 EIT Marc Danley. ”The water would more than likely have pooled up in the depression areas of the roadway,” causing a whole host of safety problems.
In July 2009, riprap (large rocks) was placed along the banks to prevent further erosion. Later that summer, the water began eroding the soil under the Wright Street Bridge abutments. Last winter, a change order allowed flowable fill (commonly a blend of cement, fly ash, sand and water, often used for bridge reclamation) to be backfilled behind the abutments to prevent further eroding.
To research the cause of the problem, ITD contacted Utah State University for research.
USU has a nationally recognized Water Research Laboratory and is well known for its expertise in water-flow modeling, from a large dam/spillway in California, to testing city water meters, flow measuring devices, and channel revetments.
In October 2009, the university sent Civil Engineering department head, Dr. William Rahmeyer, and Steve Barfuss, a research professor and head of its laboratory, to Boise for an on-site visit with ITD’s State Hydraulics Engineer, Lotwick Reese, and project designers Horrrocks Engineers.
Over the next few months, USU graduate students built a 100-foot-long 1:9 scale model of the N.Y. Canal structure that replicated water velocities, the spacing and height of the piers… even the size of gravel in the channel.
After a number of trial runs with the model, the team found that by placing an angle iron – like a nose cone – to the front of selected piers, the alternating vortices were stabilized, so that they remain fixed in position, and the wave oscillations in the channel stopped.
The team will look at two or three other devices to attach to the piers for further testing to determine which will be the most cost effective. The final solution likely will be a type of a precast-concrete device attached to some selected pier columns.
In 1997, California had a similar problem, and its solution was to build a solid concrete wall between the piers, at a cost of nearly $600,000. The solution to the New York Canal problem is expected to be a fraction of that cost.
According to Reese, ITD may be the only agency in the country to introduce such preventive measure.
“It is an extremely rare phenomenon,” Reese said. “I’m not aware of any other states doing this, and would be surprised if there were any others. This has the potential to save taxpayers a lot of money,” he added.