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.