Douglas J Jerolmack

Department of Earth and Environmental Science

Douglas J Jerolmack

Research Interests - Thresholds, nonlinear dynamics and prediction in sediment transport

Research in this area focuses on threshold processes in a variety of sediment transport systems, including (from left to right in the figure): piles of rice, river bottoms, river deltas and mountain ranges. The presence of a threshold allows for slow storage and rapid release of sediment, leading to complex dynamics that have important implications for prediction, pattern formation and interpretation of the sedimentary record. This research involves laboratory experiments and mathematical modeling.

Large-scale fluctuations in sediment transporting systems

Time series of transport rate in a diverse range of sedimentary systems exhibit fluctuations, which generally have (1) correlations across a wide range of temporal scales, and (2) a well-defined upper limit that we call the decorrelation time. A pile of rice is a canonical example: grains added slowly to the pile will accumulate until the slope exceeds some critical value, at which point the grains topple and initiate an avalanche. These avalanches are not periodic – rather, they occur intermittently across a wide range of time scales, and range in size from one grain to the size of the container. (If you've never played around with a pile of rice, you really have to try it!). This “avalanching” phenomenon appears to be generic in geophysical systems having many degrees of freedom (e.g., earthquakes, forest fires, landslides), and simplified models have been used to demonstrate that avalanching results from the dominance of a threshold process.

Large-scale variability in sediment transport rate under steady conditions has also been observed in “real world” sediment-transporting systems such as bed forms, braided rivers, mountain catchments and river deltas. The figure shows time series of transport rate under steady conditions for a pile of rice, a mathematical model of a river delta, and bed load transport in a river. Recently, such fluctuations have been attributed to nonlinear threshold dynamics. On the Earth’s surface these complex systems also respond to time-varying environmental forcing that may be natural (e.g., precipitation, sea-level cycles, plate motions) or anthropogenic. We are currently using numerical and physical models to examine the response of sediment transport systems with thresholds to periodic variations in sediment (or water) supply. Results show that sediment transport acts as a nonlinear filter that destroys environmental signals over certain frequency ranges. This nonlinear behavior has important implications for interpreting climate from the stratigraphic record and predicting landscape response to climate change.

Collaborators: Chris Paola (U. Minnesota), Liam Reinhardt (Exeter U., UK), Wonsuck Kim (U. Illinois), Pete Sadler (UC Riverside.

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Penn Sediment Dynamics Laboratory

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Department of Earth and Environmental Science
University of Pennsylvania, 254-b Hayden Hall, 240 South 33rd Street
Philadelphia, PA 19104-6316