Ryan Cassotto

Dynamic jamming of iceberg-choked fjords

We investigate the dynamics of ice melange by analyzing rapid motion recorded by a time-lapse camera and terrestrial radar during several calving events that occurred at Jakobshavn Isbrae, Greenland. During calving events (1) the kinetic energy of the ice melange is 2 orders of magnitude smaller than the total energy released during the events, (2) a jamming front propagates through the ice melange at a rate that is an order of magnitude faster than the motion of individual icebergs, (3) the ice melange undergoes initial compaction followed by slow relaxation and extension, and (4) motion of the ice melange gradually decays before coming to an abrupt halt. These observations indicate that the ice melange experiences widespread jamming during calving events and is always close to being in a jammed state during periods of terminus quiescence. We therefore suspect that local jamming influences longer timescale ice melange dynamics and stress transmission.

Quantifying flow and stress in ice mélange, the world’s largest granular material

Significance Ice mélange, a granular collection of broken icebergs ranging from tens of meters to hundreds of meters in size, sits in front of many of the Earth’s most active tidewater glaciers. In addition to influencing heat and mass transport in the ocean, the jam-packed mélange provides a geophysical living laboratory to test principles developed for small-scale granular materials such as sand. By characterizing both flow and mechanical stress using field measurements, laboratory experiments, and numerical modeling, we show that ice mélange is a quasi-2D, creeping granular fluid which constantly jams and unjams as it advances through the fjord. Most importantly, our results show how ice mélange can act as a “granular ice shelf” which buttresses even the largest icebergs that calve into the ocean. Tidewater glacier fjords are often filled with a collection of calved icebergs, brash ice, and sea ice. For glaciers with high calving rates, this “mélange” of ice can be jam-packed, so that the flow of ice fragments is mostly determined by granular interactions. In the jammed state, ice mélange has been hypothesized to influence iceberg calving and capsize, dispersion and attenuation of ocean waves, injection of freshwater into fjords, and fjord circulation. However, detailed measurements of ice mélange are lacking due to difficulties in instrumenting remote, ice-choked fjords. Here we characterize the flow and associated stress in ice mélange, using a combination of terrestrial radar data, laboratory experiments, and numerical simulations. We find that, during periods of terminus quiescence, ice mélange experiences laminar flow over timescales of hours to days. The uniform flow fields are bounded by shear margins along fjord walls where force chains between granular icebergs terminate. In addition, the average force per unit width that is transmitted to the glacier terminus, which can exceed 107 N/m, increases exponentially with the mélange length-to-width ratio. These “buttressing” forces are sufficiently high to inhibit the initiation of large-scale calving events, supporting the notion that ice mélange can be viewed as a weak granular ice shelf that transmits stresses from fjord walls back to glacier termini.

Measurement of fault creep using multi-aspect terrestrial radar interferometry at Coyote Dam

The Calaveras fault passes directly through Coyote Dam located near Gilroy, California. The earthen structure of the dam was constructed to withstand the expected deformation due to fault creep at a rate of 10 to 15 mm/year. As part of a possible dam retrofit, the Santa Clara Valley Water District initiated a series of measurements using a Ku-Band terrestrial interferometer to accurately localize the fault trace through the dam. Measurements over the period 12-May 2016 to 18-Nov-2016 were acquired from 4 different positions situated around the down-stream face. Time series of measurements from each position were obtained after performing corrections for variable tropospheric phase delay. These measurements were combined using least-squares estimation to generate three-dimensional maps delineating both stable and rapidly deforming regions.