Community Group Tour Trip Report
On Wednesday, March 14, 2007 a group of Community Advisory Panel members, joined with representatives of the MED Project and Alcoa staff took a day trip to Hanover, N.H. the site of the Cold Regions Research Engineering Laboratory (CRREL), operated by the U.S. Corps of Engineers.
The purpose of the trip was to take a look at the testing of structural ice control options for the Lower Grasse River, including a specially constructed physical model of the proposed ice control structure and a flume test chamber designed to evaluate the ice retention capabilities of the proposed MED project; and to observe how broken sheets of ice could be retained by the ice control structures, while the water moves through, successfully passing into the downstream river.
The visit commenced with a comprehensive presentation on how ice control structures are designed to work, showing a number of working examples in the Eastern seaboard (and elsewhere), using photographs and video sequences to illustrate the talk. It became clear that CRREL has accumulated a significant body of information and experience about how river ice can interact with its surroundings - especially during ice break-up. Much of this information base has been incorporated into the various computer modeling efforts that have been applied to the Lower Grasse River, but based on recommendations from the ice expert group retained by Alcoa it was decided that scale model testing of the concepts was warranted to verify expected performance. Our party had time to discuss the presentation with the experts before moving into one of the laboratories to see the demonstration of the Grasse River physical model.
When we think of laboratories, the mental picture is of benches with Bunsen burners and test tubes. CRREL Laboratories are very different; in fact this particular testing facility looks like a hockey arena. Imagine a hockey arena (without the seats and the ice surface). Here, carpenters had created a plywood form, based on the bathymetric readings of the Lower Grasse River and had reproduced the pertinent part of the river on a scale of 1/30th of the actual river. The model is very recognizable as the river from the Massena Power Canal Dam, downstream to several thousand feet below the Alcoa Bridge, you just have to remember that you are 90 feet tall when you stand on the bank!
The proposed ice control structure consists of a series of posts embedded in the river bottom and protruding above the surface - with a span between the posts just sufficient to allow smaller chunks of ice to pass through, while holding up the larger plates until they have melted to more manageable size. The posts are configured across the river just below where the Alcoa Bridge stands, and where the river widens. The plan is to take advantage of the point where the river widens for the diversion of the flow towards the bank. The posts go straight across the river and then curve into a "j" shape to permit the water to pass through, while the ice gets held up.
Consistent with our idea of the hockey rink, the building is maintained at a low temperature in order to "grow" ice in an enormous tank situated along one side of the building. The ice in the tank represents the ice that comes down from the headwaters of the Grasse River at break-up every spring. Various forms of controls and instrumentation allow the researchers to simulate different thickness of ice, different sizes of ice plates different flow rates of the river, and different surges of ice clusters. Roof mounted cameras with an integrated digital clock allow the whole activity to be revisited many times using video playback. The task of re-setting the model to vary the different possibilities of the spring break-up river flows and ice thicknesses is a painstaking task, and many hours are invested in the lab to "run" various permutations and combinations of the spring thaw in the Lower Grasse River.
Ice Breaking Demonstration
When we arrived at the lab, there was an ice surface on the river model, and a tank full of ice ready to enter into the model. Our view as observers was the equivalent of a helicopter hovering over the Lower Grasse River waiting to see the moment of break-up. Care had been taken to have the ice in the model river achieve the same thickness (in scale) as the ice in the Grasse River at break-up. It is also necessary to replicate the strength of the ice in scale. To achieve this, the scientists mix the freezing water with a quantity of urea to achieve the effect of the scale strength of the ice.
When the ice was let go from the tank, our group spent about two hours watching with fascination as the model received the ice plates from the tank. In some cases, the plates slid on top of the existing ice, forming small jams; in other cases they just broke under their own weight and carried on down stream.
The ice control structure performed well, amassing large quantities of ice plates in stacks, while the excess volume of water flowed out of the side of the "j" into the expanded area where the river widens. For demonstration purposes, the scientists increased the flow rate into the model so that we could see some of the different aspects of behavior of ice as the circumstances changed. Panel members were impressed with the way in which the model performed, and individuals noticed how specific points on the river tend to attract an ice build-up, or remain ice-free.
Also available to observe was a working model of the proposed MED Project, which is under evaluation as a means by which to take advantage of the river flow to capture electrical power, restore upstream water levels to what existed prior to the breach of the weir in Massena, and potentially to operate as an ice control structure in support of the Grasse River remediation project. This was on a different scale of 20:1. The MED Project dam worked effectively to hold back the ice, and experimentation is underway to determine the types and location of gates that would be incorporated in the structure to ensure that it could achieve optimum performance.
For those who made the ten-hour round-trip it was an astonishing first-hand demonstration of the amount of work that is involved in developing and testing the most effective methods of avoiding an ice jam in the river, and the scientific expertise available within the team of ice experts that is working on the project design. The results of the demonstration testing gave the clear impression to the tour group that either the pillars of the ice control structure or the barrier of the MED project could be effective in managing ice flows from the headwaters of the Grasse River in support of preventing future ice jam related sediment scour events in the Lower Grasse River.