Reports: DNI850789-DNI8: Evaluating Why Flash Floods are Different: An Experimental Investigation of Sediment Transport and Sorting by Rapidly Changing Hydrographs

Joel P. L. Johnson, PhD, University of Texas (Austin)

            The overall goal of this project is to understand how and why sediment sorting and the evolution of river bed grain sizes in channels dominated by flash floods are different from more commonly studied channels with slowly varying flood hydrographs.  These topics are of fundamental importance to petroleum interests because river channel deposits can be permeable reservoir rocks.  Sediment sorting strongly influences deposit permeability through grain size distributions and the development of stratigraphy by patterns of local scour and deposition, which are in turn influenced by flood hydraulics.  In particular, we are exploring controls on sorting in fine-grained suspended sediment and coarser-grained bedload sediment. 

            The key focus of flume experiments conducted in this year was on bedload transport, and in particular what factors influence surface coarsening in gravel channels, also known as armoring.  Flash flood-dominated river channels in arid environments often completely lack surface armoring, yet it is unclear whether increased sediment supply or mobility of all grain sizes prevents armor development. In order to examine armor development in an end-member case of  rapidly changing flow--a flash flood bore--we conducted a series of laboratory experiments. The flume is a concrete channel 32.5 m long, 0.5 m wide, 0.8 m tall, and capable of creating reproducible flash flood bores with a high-speed computerized lift gate (Figure 1a). For each experiment, the gate was lowered as soon as the flood bore traveled the length of the flume, “decapitating” the bore, and to a large extent isolating the effects of the bore from subsequent flow (Figure 1b).  We quantified bedload transport rates and patterns by measuring the transported sediment mass and grain size distribution at the flume outlet, and surface topography using a laser scanner (ground-based LiDAR).  In addition, we used tracer particles embedded with RFID tags, and some with accelerometers ("smartrocks"), to measure the statistics of particle motion in response to flood bores. 

            We ran a series of isolated flood bores, and compared transport rates and surface grain sizes to experiments done with similar initial conditions but steady uniform flow.  Analysis of this extensive data set is not yet complete.  However, our preliminary observations and analyses show that the flash floods actually caused rapid surface coarsening (Figure 1c).  This result was surprising and opposite of what we expected, but also understandable:  in these experiments, the bores disrupted the bed surface and entrained grains of every size class (~2 to 40 mm).  After the initial surface disruption by the bore, high transport rates were not sustained, and so smaller grains were able to fall in between the larger grains, coarsening the surface and preventing the smallest grains from being transported.  This is an example of kinetic sieving.  The bore-disrupted beds were coarser, but also less stable, than beds developed under steady uniform flow.  Future experiments will focus on the effect of the upstream supply of sand; we hypothesize that an abundance of sand will reduce the kinetic sieving effect while increasing gravel transport rates (the latter effect is known from experiments with steady uniform flow).  Implementing the experiments in this way allows us to isolate the variables of surface sorting and sediment supply. 

            One PhD student and two undergraduates worked on this project during this reporting period.  Second year PhD student Kealie Goodwin was fully supported by this grant.  This experimental work is the main focus of her PhD research.  Kealie passed her qualifying exam in April 2013.  In addition to taking the lead on designing and running the experiments described above, during this year of support she also continued to do further analysis on flow hydraulics, turbulent kinetic energy, and suspended sediment transport, using experimental data collected primarily during the first year of the project. 

            Undergraduate Rebecca Rhodes, now a senior at SUNY Brockport, conducted mentored but independent research during Summer 2013, also running the experiments described above and collecting and analyzing data.  Her participation was supported by NSF through an REU grant to the Environmental Science Institute at The University of Texas (Prof. Johnson is one of many co-PIs on this NSF REU grant).  Rebecca is planning to turn this work into a senior thesis.  Kealie and Rebecca will both be giving research presentations on this work at the Fall 2013 AGU meeting.  Rebecca's work was overseen not only by me but also by Kealie, giving her the opportunity to advise undergraduate research.  Finally, UT undergraduate Austin Morrell was supported by this grant for part of Summer 2013, as a Research Assistant.  Austin helped to design, and  manufactured, the "smartrock" housings that hold accelerometers.

            This grant has been important to me (Prof. Johnson), as it allowed me to accept a graduate student (Kealie) and have this work as the focus of a PhD project. Although it unfortunately was not funded, in November 2012, I submitted a collaborative proposal with Dr. Jonathan Laronne (Ben Gurion University of the Negev, Israel) to support continued work on this topic.  I will continue to pursue future funding opportunities for this work.