acase@tulane.edu / Though rivers are inextricably linked in our minds with an intermittently flooded overbank environment, surprisingly little is known about the sedimentary processes that operate there, or how they interact with those of the river. The knowledge gap is acute in deltas, where dense populations often necessitate tightly engineered control over flow patterns, leading to disconnected overbank environments that no longer receive input from the main channel. However, the need to understand sedimentary function in the overbank is also acute in deltas, as rising relative sea levels create an urgent need to manage water and sediment resources. This dissertation is presented as three primary chapters, each of which examines a different aspect of the hydrodynamic and sedimentary connection between a river’s channel and its overbank environment. In Chapter 2, my coauthors and I ask which factors enhance overbank sediment retention, and what retention rates might be considered typical in deltas. We compare the sediments stored in a crevasse splay to those transported by the river and conclude that retention rates approaching 100% might be achievable in settings that are not exposed to coastal processes. Chapter 4 is also concerned with spatial patterns of sedimentation on a delta. In it we use physical experiments to examine the influence that floods play in mobilizing sediments from the channel and storing them in the overbank environment. We find, counterintuitively, that an experiment whose input included floods has a lower proportion of floodplain to channel deposits preserved than an experiment with a constant input. Chapter 3 is focused on water and sediment dynamics in the channel in a region where significant flow is lost to the overbank environment. Here we present measurements from channel networks in the Mississippi River’s Birdsfoot Delta and show that flow loss along the channels is a critical control on channel function that causes channels of disparate sizes to behave similarly. We use our field results to inform a numerical model of channel bed evolution in a region with flow losses, and conclude that the modern flood control system in the Lower Mississippi River may have significantly changed the bed morphology. / 1 / Christopher R. Esposito

No description available.

In this study we assessed growth laws of sub-delta crevasses in the Mississippi River delta plain, experimental laboratory deltas, and compared them to previously studied river dominated large deltas worldwide. Metrics for channel and delta geometry for each system were obtained using a combination of geospatial tools, bathymetric datasets, sediment size, and hydrodynamic observations. Most crevasses and experimental deltas appear to obey delta growth laws suggesting that they exhibit planform metrics similar to larger deltas. However, some channels within each system, exhibit outlier behavior (e.g. asymmetric growth) where channel length is much larger than channel width. Hydrodynamic observations and morphodynamic modeling results, support the role of confinement in governing this response, through direct lateral confinement of the receiving basin width and depth thus guiding channels, and indirect confinement caused by sediment cohesion, whereby natural levees guide the systems asymmetric channel growth.

In this study the patterns of sediment transport and deposition in the channels and receiving basin of a crevasse splay in the modern Mississippi River delta are examined, with emphasis on the development of a distributary mouth bar. Simultaneous hydroacoustic and optical measurements on the mouth bar show that the bar conforms to the progradational stage of an existing conceptual model of mouth bar development. This is confirmed by cores dated using Beryllium-7, which provides a record of the deposition on the bar over a 90-day period. Stratigraphic data from cores obtained on the bar are used to extend the conceptual model to account for variable riverine inputs. A numerical model, developed and validated using field data is capable of representing the fundamental sedimentary processes responsible for mouth bar progradation. These results will be of interest to coastal geologists, engineers and coastal managers alike.