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

这个快速反应研究项目将通过分析2011年路易斯安那州和密西西比州密西西比河冲积河谷最下游洪水相关的岸上沉积物，来研究极端事件对大型河流洪泛区地貌的影响。研究人员将利用并寻求对自然地理学、河流地貌学和沉积学两大知识体系做出贡献：(1)极端事件在沉积过程和大型低地、堤岸、蜿蜒河流的洪泛区建设中的作用；(2)现代密西西比河下游的地貌调整以应对洪水控制。他们将收集资料和日期，以检验以下研究问题的可能答案：2011年洪水产生的岸上沉积物的厚度和质地特征与研究地区之前的大规模洪水（1973年）直接比较如何？不同堤岸宽度的河漫滩断面的岸上沉积厚度是否不同？顺流（纵向）或扩散（横向）沉积机制控制沿河岸洪泛平原的岸上沉积吗？他们将从事野外工作、河岸沉积物的实验室分析、基于地理信息系统的激光雷达衍生泛滥平原高程评估和定量分析。他们将在与记录1973年洪水的已发表研究报告相同的地点确定河岸沉积物厚度。他们将从保存下来的沉积结构中推断出河岸上的水流方向，量化沉积物的质地，将沉积特征与泛滥平原的海拔高度和与河道的距离联系起来，并将2011年的数据与1973年的洪水数据进行比较。他们的工作将集中在密西西比河下游的一段河段，这是唯一一个接收所有支流排水的地方，也是密西西比河流域内流量最高的地方，从而从北美最大的有记录的流量事件之一中取样与洪水阶段相关的洪水沉积物。实地研究设计将允许直接检查防洪堤防对沉积的影响。与河流的西侧相比，东部洪泛区没有堤岸，这使得河岸上的沉积物从河流到山谷壁的运输畅通无阻。研究区位于北美最重要的防洪构筑物——老河流控制构筑物的上游，其功能部分依赖于随漫滩高程调整的级流量关系。此外，自20世纪80年代以来，研究河段的洪水流量越来越高。虽然很少有机会从大型河流的大洪水中采样河岸上的沉积物，但有机会从同一河段的两次大洪水中采样河岸上的沉积物是前所未有的，本项目将从洪水沉积文献中引用最多的研究之一中重新采样确切位置（Kesel et al, 1974）。因此，这个项目的结果预计将对环境科学家有更广泛的相关性，包括水文和水力工程师、自然地理学家、河岸生物学家、地质学家和溪流专家。该项目将为研究生和本科生提供实地、实验室和基于计算的教育和培训经验，并将为洪泛区管理者和政策制定者提供有价值的信息和见解。

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.