RAPID: Collaborative Research: Floodplain Sedimentation from an Extreme Event: The 2011 Lower Mississippi River Flood

University Of Texas At Austin, Austin TX

Investigators

Abstract

This rapid-response research project will examine the impact of extreme events on the geomorphology of large river floodplains by analyzing overbank deposits associated with the 2011 flood in the lowermost reaches of the Mississippi River alluvial valley in Louisiana and Mississippi. The investigators will draw on and seek to contribute to two bodies of knowledge in physical geography, fluvial geomorphology, and sedimentology: (1) the role of extreme events on sedimentation processes and floodplain construction of large lowland, embanked, meandering rivers and (2) the modern geomorphic adjustment of the Lower Mississippi River in response to flood control. They will gather materials and date to examine possible answers to the following research questions: How do sediment thicknesses and textural characteristics of overbank deposits derived from the 2011 flood directly compare with a previous, large-scale flood (1973) in the study area? Do overbank sedimentation thicknesses differ for floodplain transects of various embanked (leveed) widths? Do advective (longitudinal) or diffusive (lateral) sedimentation mechanisms control overbank deposition along embanked floodplains? They will engage in field work, laboratory analyses of overbank sediments, geographic information system-based evaluations of LiDAR-derived floodplain elevations, and quantitative analyses. They will determine overbank sediment thickness at locations identical to a published study documenting the 1973 flood. They will infer overbank flow direction from preserved sedimentary structures, quantify sediment texture, associate sedimentary characteristics with floodplain elevation and distance from the channel, and compare the 2011 data with data from the 1973 flood. Their work will focus on a reach of the Lower Mississippi that is the only location to receive drainage from all tributaries and has the highest discharge within the Mississippi River basin, thereby sampling flood deposits associated with record flood stages from one of the largest recorded discharge events in North America. The field study design will permit a direct examination of the impact of flood-control levees on sedimentation. In contrast to the western side of the river, the eastern floodplain reach is not embanked, which permits unimpeded overbank sediment transport from the river to the valley wall. The study area is upstream from the most important flood-control structure in North America, the Old River Control Structure, whose function is partially dependent on stage-discharge relations that adjust with floodplain elevation. Furthermore, the study reach has been shown to have increasingly high stages for a given flood discharge since the 1980s. While it is rare to have an opportunity to sample overbank deposits from large floods on large rivers, it is unprecedented to have the opportunity to sample overbank deposits from two large floods within the same reach, this project will resample the exact locations from one of the most cited studies in the flood sedimentation literature (Kesel et al, 1974). The results of this project therefore are expected to have broader relevance for environmental scientists, including hydrologic and hydraulic engineers, physical geographers, riparian biologists, geologists, and instream-flow specialists. The project will provide field-, lab-, and computationally based education and training experiences for graduate and undergraduate students, and it will provide valuable information and insights to floodplain managers and policy makers.