Transcript for Drift Ice as a Geologic Agent, segment 04 of 11


{{{Drifting Ice}}}

During the open water season following river flooding and breakup, ice assumes an active role. These five-minute time lapse films taken from towers and barrier islands show how wind and current moves ice around on the shallow shelf where deeper draft flows impact the bottom and bulldoze sediment. Calculations of sediment transport in the surf zone are made based on wind data and ice charts, showing percentage of ice covered. Because ice charts would depict most of these scenes as open water, the method is misleading. Neither waves nor surf are seen in the coastal accumulation of flows. These flows absorb most of the marine energy and transfer it directly to the sea bed. Can they truly be called protective?

Grounded ice flows become flow obstacles, intensifying currents around ice keels as they grind to a halt. The effects of intensified flow are seen here in irregular sand ripples around the contact point. Sediment resuspension is demonstrated by turbid water plumes which commonly trail these massive flow obstacles. The focus of these scenes is on small ice cakes on the shore face, but the same actions occur out to the horizon where water depths are twenty meters and ice flows large. The resulting transport is generally parallel to shelf depth contours. Notice that summer ice groundings are short-term events. Melting decreases ice volume and ice draft, and as a result, the contact point shifts along the bed, Generally into shallower water. This shifting of bottom-disturbing ice keels suspends sediment along the way, resulting in erosion and transport.

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