About 30 min after the heaters in the heater box have been turned on and the steel box has been heated up, the air compressor is turned on. The combination of heat from the heater box and circulating water to melt the ice is far more efficient than just the heaters alone. Since the skinplate heaters for one gate use 20 kW, considerable energy savings can be realized by operating the heaters less when used in conjunction with the air bubbler. Details of the lower trunnion arm heater and the support channel heater are shown in Figure 4 . Cartridge heaters are enclosed in watertight cavities by steel plates or angles for mechanical protection.
Insulation is placed so that heat is directed toward the surfaces most affected by ice.
These heaters are also operated before the gates are moved. It is a time-dependent program that includes phase change so that a moving melt boundary can be easily identified. A sketch of the model is given in Figure 5 . Initially, ice is attached to the heater box, and there is a linear temperature gradient through the thickness of the ice. The program is for heat conduction only and does not include convection in the water.