CAREER: The role of organic particulates in controlling the growth of river deltas: a field, experimental, and numerical modeling study

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A non-technical description of the project, which explains the project's significance and importanceCoastal land loss through the combined effects of sea-level rise and land subsidence is a major concern of the 21st century. The retention of sediment upstream behind dams and confinement of major rivers by levees have resulted in diminished sediment delivery to coastal regions, exacerbating problems of land subsidence. Along the US Gulf Coast this loss of land may be lessened through engineered diversions of the Mississippi River that would deliver sediment to a larger portion of the delta plain. Simulation modeling is an essential tool for planning such diversions and evaluating impacts of different management scenarios on rates of coastal sedimentation and erosion. However, a gap in knowledge of the different processes contributing to coastal sediment budgets could lead to inaccurate model predictions of these rates. One of these unknown processes is the direct capture of particles by the stems and leaves of coastal vegetation, which in deltaic marshes might add substantially to overall sedimentation rates. This research will quantify the importance of vegetation capture for coastal sedimentation budgets, leading to improved prediction of coastal growth or submersion.A technical description of the projectThis project addresses three outstanding topics in documenting the role of fine sediment in coastal land building processes: (i) It develops a quantitative relationship between hydrodynamic properties, biofilm and vegetation properties, and sedimentation through direct capture by vegetation. (ii) It tests the hypothesis that the presence of biofilm within vegetation arrays substantially increases sedimentation rates. (iii) It improves the representation of particle capture by vegetation in models of delta land dynamics and uses the model to evaluate the overall significance of particle capture in large-scale delta evolution. A combination of laboratory and field experimentation and numerical modeling will be used to elucidate these topics. Laboratory experiments in the Biogeomorphology Research and Teaching (BRAT) flume will test emerging theory about the functional relationship between flow conditions, particle characteristics, and vegetation canopy structure. Biofilms cultivated within the flume will enable testing of the effects of different surficial properties on sedimentation. These results will be tested in the field, within in situ flumes constructed around intact vegetation communities in the Wax Lake Delta, an actively growing portion of the Louisiana coast. Functional relationships developed in the field and laboratory flumes will be incorporated into a Delft 3D model of delta sedimentation dynamics. Sensitivity analyses conducted with the model will evaluate the importance of fine sediment-vegetation interactions for large-scale delta growth. Educational components of the project include development of experiential course modules that use the BRAT flume, development of a summer short course on fine sediment-vegetation interactions at the National Center for Earth Dynamics, and development of an exhibit for the San Francisco Exploratorium on the role of coastal marshes in land building processes. Delta modeling efforts will be developed in collaboration with coastal resource managers and disseminated broadly for Louisiana coastal marsh restoration planning.