Jere A. Mohr and John B. Swenson Geology
Erosion and deposition in rivers are generally thought to be controlled by local conditions within the fluvial system or by upstream and downstream factors, such as sediment/water supply and sea level, respectively. But coastal-plain rivers are strongly coupled to offshore shelf and slope transport systems, creating the sigmoidal sediment prisms (clinoforms) that border most continents. We identify the toe of the steep subaqueous part of the clinoform (the foreset) as a critical point in clinoform dynamics. Increasing sediment transport at the toe “unlocks” sediment flow across the updip transport system, causing sediment bypass or erosion in the fluvial system, reduced shoreline progradation, and transfer of coarse sediment to the deeper offshore. We present results from flume experiments that demonstrate how plunging (hyperpycnal) flows in combination with pre-existing basin topography can prevent sediment deposition at the toe and ultimately lead to offshore transfer of sediment from the fluvial system with no change in sea level.
To test our hypothesis that erosion and deposition in river systems is strongly controlled by offshore processes, we carried out a series of experiments in a glass-walled flume. The flume measured 2 cm wide by 4 m long. We placed a ramp in the flume that had a horizontal portion and a sloping portion. In each experiment, we built a delta by feeding sand and water into the flume while holding the water level (sea level) in the flume constant. Initially, the delta built out across the horizontal platform and deposition of sediment at the foreset toe was not inhibited. Then, after the delta reached the sloping portion of the ramp, we attempted to prevent sediment deposition at the foreset toe. In the first set of experiments, the ramp slope was greater than the angle of repose for the sand (the angle at which the sand begins to avalanche under the influence of gravity). This prevented sediment from being deposited at the foreset toe and acted to “freeze” the delta in place. With the delta frozen, there was no sediment deposited in the fluvial system. A second set of experiments expanded on the first set by adding turbidity currents to the delta system. Turbidity currents are flows of sediment-laden water that are driven by their excess density compared to the surrounding water. In natural delta systems, turbidity currents are commonly initiated at mouths of rivers that have high concentrations of suspended sediment. In our experiments, we created turbidity currents by introducing silt in addition to the sand and water already being fed into the flume. Turbidity currents act to reduce the angle of repose of the delta foreset, meaning that the delta could be frozen with the sloping ramp set at a much lower angle.
Digital imaging played an integral role in our project. During each experiment, we took high-resolution digital photographs of the delta at 15-second intervals. These images allowed us to measure the position of various features, such as the shoreline, as a function of time throughout the experiment. In order to automate this process, we wrote an image-processing program in Microsoft Visual C++ that located the top surface of the delta in each image and generated an Excel spreadsheet with the data. We used Viz Lab machines to batch process many of our images, which greatly reduced computing time. Before processing the images, we used Adobe Photoshop to correct each image by making rotational adjustments and eliminating lens distortion. We also used Adobe Premiere and Adobe AfterEffects at Viz Lab to create time-lapse movies of our experiments to facilitate easier visualization of our results. The computing power available at Viz Lab was very useful because it allowed us to work with numerous very large files simultaneously. The video editing software available allowed us to create effective ways for others to visualize our data.