The morphodynamic influence of base-level change and waves on lake sediment

Abstract

Lakes continuously accumulate sediment, a process that has been ongoing since their formation. The sediment consists of soil particles and biological remains that runoff from the watershed and into the lake. The sediment may also contain particles from aeolian transported material from the “airshed” and biological and inorganic precipitated minerals. Thus, lake sediment is a continuous environmental archive, containing information about the history of the lake, and its surroundings. Unfortunately, lake sediment is subject to remobilization and mixing due to a variety of sediment transport processes, such as mass wasting. There has been limited research on how base-level change influences mass wasting, in particular, slope failure. Some studies suggest slope failures are correlated with base-level fall and/or base-level rise, while others suggest they don’t correlate at all. To address this controversy, I conducted flume experiments at the Saint Anthony Falls Laboratory (SAFL) to observe slope failure in sediment mixtures containing four different walnut-shell/kaolinite ratios. Three base-level change rates were tested, producing 12 base-level change experiments. Another set of experiments was also designed to investigate what effect, if any, the addition of waves would have on mass flow. Video recordings were taken during each experiment and were analyzed to study the geometry of each profile before and after base-level change had occurred. Results suggest that there is consistent sediment transport when waves are present, aiding in the transportation of walnut-shell downslope. Without waves, transport is limited, and the clinoform is subject to compaction, or slump formation, depending on clay content. This difference depends heavily on a sediment’s composition, where hydraulic conductivity (K) plays an important role in water transmission through the clinoform. My experiments suggest that slope failures in the field would occur in lake sediment with higher clay content. Base-level change itself has no effect on the occurrence of failures, but slope failures should occur during base-level change if clay content is high. With only base-level change, we can expect to see compacted lake sediment with lower clay content (~15-33% clay). Slope failures (slumps) occur in lake sediment with higher clay content (~50%). With the presence of waves, however, wavebase plays an important role in sediment transport. Wave-base greatly influences sediment transport downslope, which is seen in my experimental work. This suggests that waves in the field also transport lake sediment downslope while washing away any finegrained particles (i.e. clay).