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 fundamental research important to petroleum interests because river channel deposits can be particularly important 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 how to better predict fine-grained suspended sediment and coarser-grained bedload sediment erosion, transport and deposition rates and patterns. 

Our experimental plan has three parts: studying the (1) hydraulics, (2) suspended sediment transport, and (3) bedload transport of flash flood bores in comparison to steady flow conditions.  To date,  I and several graduate and undergraduate students have made progress on primarily the first two topics.  Most experiments have been conducted in a large and unique outdoor flume at The University of Texas at Austin, which I modified by adding a computer controlled lift gate at the upstream end to allow the instantaneous release of a controlled volume of water. 

Several University of Texas graduate students have benefitted from this grant. Mr. Peter Polito was initially conducting this research, and gained practical and scientific experience planning, designing and instrumenting the flume for these experiments.  Unfortunately, Mr. Polito then decided to withdraw his PhD program and from UT Austin due to personal reasons.  Fortunately, Ms. Kealie Goodwin subsequently started as new a PhD student, advised by me and working on this project (to date she has completed one year of graduate school).  To study flash flood flow hydraulics, Ms. Goodwin has measured instantaneous water velocities using several acoustic doppler velocimeters (ADVs) during the experimental flash floods.  Velocity measurements have proven challenging in our fast, rapidly changing, and aerated (bubbly) flood bores, and Ms. Goodwin continues to perfect our experimental configuration and communicate extensively with ADV technical support to minimize noise and optimize instrument settings.  By measuring how flow velocity changes very close to the bed, we calculate local near-bed shear stresses during the flash floods.  Preliminary measurements and calculations indicate that shear stresses are much higher in the flood bores and then decrease as the flow progresses and becomes more steady, as we hypothesized.  From the velocity data we also calculate turbulent kinetic energy (TKE), and preliminary results similarly indicate that TKE is highest at the flood bore and then rapidly decreases.  Shear stresses and TKE are key parameters because previous work in steady uniform flow indicates that both are important for sediment transport. 

To further understand shear stress distributions in flood bores, Ms. Goodwin and I conducted laboratory experiments described in the proposal at the University of Natural Resources and Applied Life Sciences (BOKU), Vienna, Austria, in collaboration with Dr. Roland Kaitna (Vice-Head of the Institute of Mountain Risk Engineering), in August 2012.  This work was partially supported by this grant and partially by a competitive student research grant Ms. Goodwin received from UT.  These experiments focused on independent measurements of basal shear stresses in and following flood bores, using a unique rotating drum flume in which bed shear stresses can be directly measured using a shear force plate.  In this flume, we were able to compare direct shear stress measurements to those calculated from flow velocity profiles, to evaluate how well this method for calculating shear stresses works in aerated and rapidly changing flood bores.  Our preliminary analysis of the shear stress plate data suggests that basal shear stresses spike at the bore and then decrease quickly, consistent with the velocity-based calculations. Ms. Goodwin has benefitted from this international collaboration in a variety of ways, including working in a different lab with a different experimental design and instrumentation, which is valuable for planning future experiments on this and related topics. 

Masters student Alexander Aronovitz also conducted a different set of experiments related to this project, on sediment size sorting and armoring (coarsening of the surface grain sizes, which tends to stabilize river bed surfaces) under steady uniform flow.  These experiments will be used as a comparison later in the project when we conduct flash flood experiments using coarse sediment.

Finally, Travis Waller, an undergraduate student at North Carolina Agricultural and Technical State University, conducted research for this project through an NSF-supported Research Experiences For Undergraduates (REU) program in which I participate at UT.  Mr. Waller successfully completed this 10-week research program in summer 2012.  Travis's independent project was to calibrate and evaluate whether an optical backscatter sensor (OBS), typically used for measuring water turbidity (cloudiness) caused by very fine sediment, could be used in our flume to quantify suspended sediment concentrations when the sediment in question was much coarser (sand).  We concluded the device can be successfully used in our experiments, but that accurate calibrations at the very high suspended sediment concentrations in our flood bores are difficult to measure.  Travis's work was overseen not only by me but also by Ms. Goodwin, giving her the opportunity to advise undergraduate research.