60th Annual Report on Research 2015

Introduction and project goals

The goal of this project is to use physical experiments and numerical modeling to explore feedbacks between the creation and consumption of stratigraphy and the morphodynamics of rivers with gravel/sand beds. Such feedbacks between the bed surface and its stratigraphy have largely been ignored in physical experiments, numerical models, and analytical theory for river morphodynamics, in large part because we lack coupled observations of bed surface dynamics and subsurface evolution in gravel-bedded or gravel/sand-bedded rivers and flumes.

This project seeks to fill this gap through two primary activities. The first involves documenting the coevolution of bed surface morphology and stratigraphy by conducting experiments in a laboratory flume where a channel with bar-pool topography is provided a variable sediment supply, resulting in the creation and consumption of its own laterally- and longitudinally-variable stratigraphic signature. The second activity involves the development of a morphodynamic model that couples the two-dimensional (cross-stream and downstream) bed surface evolution with the three-dimensional (cross-stream, downstream, and vertical) creation and storage of stratigraphy.

During the first year of this project, we have nearly completed the first activity – the flume experiments. During the second year of the project, these experiments will be used to guide the development of the numerical model and to test its ability to make realistic and accurate predictions of morphodynamic evolution and stratigraphy.

Summary of research activity during the reporting period

We have conducted flume experiments in the hydraulics laboratory at Colorado State University's Engineering Research Center (ERC). The experimental channel is 1.2 m (4 ft) wide, 0.76 m (2.5 ft) deep, and 18.3 m (60 ft) long. The overall goals of the experiments were to a) develop alternate bar morphology by supplying the channel with a constant water discharge and supply of a gravel/sand sediment mixture, b) to increase the sediment supply and stimulate the development of stratigraphy, and c) to return to the original sediment supply rate, causing the channel to degrade through its self-formed stratigraphy.

During the experiment, we have been periodically collecting detailed measurements of the bed surface topography using structure-from-motion techniques. We also have been photographing the bed surface and using image processing techniques to characterize the bed surface grain size distribution spatially. At the end of the second, aggradational phase of the experiment, we collected sediment cores that were used to characterize the vertical distribution of sediment sizes, along with local bedload transport rates over a bar sequence using a mini Helley Smith bedload transport sampler.

Figure 1 shows the temporal evolution of sediment feed rates and bedload transport rates out the end of the flume, and Figure 2 shows the evolution of the bed surface topography during the experiment. The flume was fed an equilibrium rate of 100 kg/h for 52 h, at which point stationary alternate bars had formed in the channel. The feed was then increased to about 200 kg/h, and again to 300 kg/h, and conditions were at quasi-equilibrium at the end of each of these higher-supply phases. Immediately after the increase to a supply of 300 kg/h, a sequence of small bars migrated over the flume until a new arrangement of steady bars had developed (Figure 2). The feed was then reduced back to 100 kg/h; the experiments are presently continuing at this feed rate and will be completed when equilibrium conditions are once again achieved.

Figure 1. Sediment feed rates and measured sediment transport rates at the flume outlet.

Figure 2. Detrended bed elevation during the experiments. Experiment run time (in hours) is shown to the left of each map.

The stratigraphy measurements taken at the end of the high-supply phase are shown in Figure 3. The channel centerline does not show much vertical sorting, but locations where bars and pools have formed exhibit some vertical fining (bars) or coarsening (pools).

Figure 3. Measurements of vertical sorting collected along the channel centerline (top) and in bars and pools (bottom) at the end of the aggradational phase of the experiment. Plots show the local median subsurface grain size (D50) vs. depth below the surface. Sampling locations are shown in the map on the right, where flow is from top to bottom.

Planned research activity during the next year

As the flume experiments are nearly completed, the major focus in the second year of this project will be the development of the stratigraphic morphodynamic model. As part of a different project, one of my graduate students has made considerable progress incorporating a framework for storing and tracking riverbed stratigraphy in a one-dimensional morphodynamic model. A new student will build upon this, along with my own work, to develop a stratigraphy submodel into the two-dimensional morphodynamic model FaSTMECH, which is part of the freely-available iRIC interface (www.i-ric.org). The results from the flume experiments will be used to validate the model predictions, and the model will subsequently be used to explore surface-stratigraphy interactions for gravel-bed rivers with migrating alternate bars.

Research impacts

As a tenure-track assistant professor, this project has provided valuable resources allowing me to establish new research in the hydraulics laboratory, work with graduate and undergraduate students, and collect valuable experimental data that advance our fundamental knowledge of earth surface processes and stratigraphic development. The project has provided the primary financial support for a graduate student, who has taken the lead in running the experiments and analyzing the data; this has provided him with valuable technical experience and helped develop his independence and critical thinking skills. Many undergraduate students have assisted with the experiments, and this research experience should serve them well as they consider graduate school or careers in industry.