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<br />~~ <br />1 <br />f <br /> <br />1 <br />t <br />1 <br /> <br /> <br />1 <br /> <br /> <br /> <br />forms on water in which the flow velocity plays no role is said to result from static ice formation. <br />This type of ice is also formed on lakes and ponds during periods of low winds. Generally the <br />surface flow velocity must be approximately 1 foot per second or less for static ice to form. <br />Static ice formation starts in a very thin layer of supercooled water at the water surface, and is <br />probably initiated by the introduction of seed crystals. The ice grows at the ice/water interface as <br />heat is transferred from the ice/water interface through the ice and into the atmosphere. <br />Throughout the study reach of the Green River, the formation of a stable ice cover results <br />from the interaction between the transported ice pieces and the flowing water. In this case the <br />cover is said to form dynamically. Ice covers that form dynamically progress in the upstream <br />direction from an initiation point as ice is transported to the leading edge (upstream edge) of the <br />ice cover by the flow of the river. The actual process that occurs at the leading edge depends on <br />the hydraulic flow conditions and the form of the arriving ice. The processes at the leading edge <br />are described in general below in an order which reflects the relative flow velocity at which they <br />occur, from the lowest flow velocity to the highest. However, it is more common to refer to the <br />non-dimensional flow velocity, or Froude number, defined as <br />V <br />gD ' <br />where V =the flow velocity, g =acceleration of gravity, and D =mean depth. <br />At relatively low flow velocities and high concentrations of surface ice (approximately <br />50% coverage or higher) it is possible for the ice cover to spontaneously arch across the width of <br />the open area of the channel and stop moving, a process known as bridging. It is generally not <br />possible to predict where these bridging locations will be without historical knowledge. Ice <br />control booms and/or hydraulic control structures are often used to assure the initiation of ice <br />cover at a specific location. At relatively low flow velocities, ice floes arriving at the leading <br />edge of the bridging location may also come to a stop adjacent to the leading edge. In such cases, <br />the ice cover will progress upstream by juxtaposition. The maximum flow velocity at which <br />juxtaposition will occur depends on floe geometry and channel depth. At higher flow velocities, <br />the ice floes arriving at the leading edge of ice cover may be forced underneath the existing ice <br />cover or underturn. If the flow velocity is not too high these underturned floes will remain at the <br />leading edge of the ice cover. <br />The strength of an ice cover formed from many separate pieces of ice is directly <br />' proportional to its thickness. If the forces acting on the ice cover exceed the ability of the cover <br />to withstand those forces, the ice cover will sometimes collapse in the longitudinal direction and <br />become thicker, a process known as shoving. When shoving occurs the strength of the ice cover <br />' is increased. An ice cover may repeatedly shove and thicken as it progresses upstream. If the ice <br />cover is treated as a "granular" material, the strength characteristics of the cover can be <br />mathematically estimated and the final thickness of the cover estimated. <br />' At relativel hi h flow velocities, the ice floes arrivin at the leadin ed a of the ice <br />Y g g g g <br />cover may be underturned and transported under the ice cover for considerable distances. At this <br />' S <br />