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.
Normal 0 false false false EN-US X-NONE X-NONE PROJECT OUTCOMES REPORT FOR THE GENERAL PUBLIC NATIONAL SCIENCE FOUNDATION (NSF) AWARD #1151719 Collaborative Research: Floodplain Sedimentation from an Extreme Event — The 2011 Lower Mississippi River Flood Co-PI: Dr. Paul F. Hudson, Leiden University, The Netherlands This report documents project outcomes to the general public for NSF award #1151719 — Collaborative Research: Floodplain Sedimentation from an Extreme Event — The 2011 Lower Mississippi River Flood. The 2011 flood along the Lower Mississippi River was one of the largest discharge events (66,000 m3/s at Vicksburg, Mississippi) in recorded history in North America. The flood was responsible for new stage records between Vicksburg, Mississippi (17.4 m) and St. Francisville, Louisiana (16.3 m), and overbank conditions lasted for over 45 days from early May to mid-June. Efforts to relieve flooding in Baton Rouge and New Orleans, Louisiana, included opening both the Bonnet Carré and Morganza Spillways, the latter of which had not been used since 1973. RESEARCH IMPORTANCE Research efforts associated with this project focused on how much and what type of sediment was deposited by overbank flood waters in adjacent floodplain settings in the Fort Adams / Lake Mary area of Wilkinson County, Mississippi, and the Cat Island area of West Feliciana Parish, Louisiana. Floodplain sediment deposition is important along the Lower Mississippi River because: · flood stages (levels) of the Mississippi River have progressively increased for a given streamflow (discharge) (m3/s) in the study area and discerning how rapidly nearby floodplain surfaces are building upward helps determine the total available area for flood water occupation. · flood-control engineers and managers require information about the nature and rates of changes to river-controlled landforms adjacent to a couple of the most important flood-control structures in the United States, the Old River Control Structure and the Morganza Spillway. · riparian zones adjacent to the river support a diverse assemblage of bottomland habitats whose ecological health are dependent on timely pulses of nutrient-rich suspended sediments. · land-loss in South Louisiana is offset with sediments derived from the Mississippi River and understanding where and how much sediment is deposited during large floods informs policy directed at restoring coastal environments. RESEARCH FINDINGS The investigators for this project visited 55 sites in the study areas in August – September 2011 to measure the thickness of overbank sediments deposited during the prolific flood and determine the type (i.e., particle size) of sediments deposited. Furthermore, most of the sites were sampled in 1973 by the Co-PI, Dr. Richard Kesel, for identical measures of thickness and type associated with the large flood during that year. Findings include: · considerably less overall sediment deposited during the 2011 flood when compared to the 1973 flood. · sediment thickness and particle size generally decreased with distance away from the main channel. · sediment thicknesses are comparable to those measured in floodplains along the Upper Mississippi River following the great 1993 flood. · a high-water event (February – March 2011) just prior to the large flood exhibited a higher peak transport of suspended sediment, which indicates available sediment was exhausted before the larger flood occurred. · sediment size and thickness data indicate relatively high flow velocities across the floodplain and, in some locations, scour of the floodplain surface. IMPLICATIONS The findings associated with this project are important for a variety of reasons. · Extreme flood events are commonly not responsible for the greatest amounts of floodplain deposition and associated landform modification. Alternatively, these large floods can flow fast enough to erode (strip) soil particles from floodplain surfaces, a phenomenon not usually considered present in large, lowland river systems. · The minimal amount of floodplain deposition corroborates other research efforts that indicate a greatly decreased amount of suspended-sediment transport in the Lower Mississippi River during approximately the last 50 years. · The minimal amount of floodplain deposition corroborates other research efforts that indicate channel-bed aggradation (i.e., a rise in the bottom of the river) is the mechanism responsible for observed increases in flood stages along the Lower Mississippi River in the study area. · Efforts to predict volumes of sediment necessary for coastal restoration efforts in South Louisiana probably should exclude overbank floodplain deposition during large floods upstream of the deltaic plain (i.e., Old River Control Structure).
