61st Annual Report on Research 2016

Data collection for this project has concluded, and publishing the results is also nearly concluded. This project proposed to test the hypothesis that longitudinal profiles of delta foresets are a function of the distribution of particle excursion lengths over delta brinks in the absence of other types of sediment transport in a receiving basin. Two goals had to be accomplished in order to test the hypothesis.  First, deltas of arbitrary foreset morphology had to be experimentally constructed as a function of associated sediment transport conditions.  Second, individual particles had to be tracked over delta-scale distances to make detailed observations of their excursion lengths.  The first goal was never accomplished.  The second goal was achieved and used to validate the proposed model for particle excursion lengths as a function of Rouse number and relative roughness.

A series of experiments were conducted wherein 2-D delta profiles were constructed under varying depth and discharge conditions, using two separate sediment grain types – crushed corn cob and plastic.  Our goal was to predict the longitudinal profile of these deltas as a function of the sediment characteristics and transport conditions. However, the result of this effort was that all deltas created were Gilbert deltas, i.e. with angle of repose foresets. Despite the wide range of flow and sediment transport conditions, no deltas were created with low-angle foresets. Rather than continue to pursue this aspect of the project in an exploratory fashion, the second goal (measuring excursion lengths and testing the proposed model) became the focus of the project in its final year.

A post-doc developed a low-cost system that employs an off-the-shelf, consumer-grade video surveillance system for tracking motions of individual particles during flights of suspension.  They stitched 8 video cameras together spatially and temporally to allow for individual grains to be tracked over distances up to 1000’s times greater than the size of the particle itself.  Combining the camera system with publicly available particle-tracking software and with a substantial amount of our own software, it became possible to track hundreds of grains per video in an automated fashion.  The limit on this method is with computing power.  It would take approximately one week to process a single video of one minute of sediment transport.  A PhD student assisted with the implementation of this process by helping to adapt a method for using black light on particles coated with fluorescent paint.  In this manner it was possible for the publicly available software to automatically identify moving particles.

Data were collected from 14 experimental sets with multiple hydraulic and sediment conditions that cover a wide range of sediment transport lengths.  All personnel were involved in analysis of the data.  Partial results focusing on the model predictions have been published as part of the River Flow 2016 conference and in its associated volume of papers.  In addition, abstracts have been presented at the Geological Society of America meeting 2016 and will be presented at the American Geophysical Union meeting 2016.  Finally, a manuscript is in submission currently that demonstrates how the proposed model for particle excursion lengths appropriately predicts the motions of sediment from all 14 experimental sets.

Ultimately, this work was successful in demonstrating the first model that is capable of predicting sediment excursion lengths across transport modes for individual grains in a fluvio-deltaic setting.  This result and model will be able to be used to better understand river systems as well as similar transport systems (e.g. bores and turbidity currents).  However, the project failed to directly accomplish its major goal over an experimental delta because no delta could be made to have a low-angle foreset.  The reason for the failure is not currently clear but poses a challenge and opportunity for further understanding the relationship between delta morphology and sediment transport conditions.

Experimental set-up for tracking suspended particles using fluorescent paint and black light with publicly available software applied to video constructed from stitching 8 video cameras together.

Data on excursion lengths of individual particles plotted on top of proposed model predictions.  The models successfully captures the distribution of particle excursion lengths across a wide range of conditions.