A better understanding of the physics of fine sediment suspension, transport and settling is key to advancing the fields of benthic ecology, biogeochemistry and geology. Controls on seabed erodibility and suspended particle properties are two of the largest unknowns limiting accurate prediction of fine sediment movement. Recent findings suggest that within turbid, biologically active coastal environments, the interactions of flow with the two dominant types of muddy particle packaging are notably distinct. Although they are formed from nearly identical mud grains and are often simultaneously present, loosely bound flocs and biologically compacted pellets erode, settle and respond to circulation very differently. At present, the distinct physical responses to flow of these two common types of particle packages are not adequately documented by observations, have not been sufficiently analyzed, and are not satisfactorily represented within state- of-the art models, especially for cases when they occur together.
Key questions to be addressed in this study include: (i) How do turbulence and particle packaging affect the nature of sediment settling? (ii) How do bed stress and particle packaging affect bed erodibility? (iii) How do feedbacks between hydrodynamics and particle packaging control the locally dominant particle distribution? The associated hypotheses include: (i) Stronger turbulence suspends larger pellets but rips apart flocs, so turbulence increases settling velocity (ws) for pellets but decreases ws for flocs. (ii) With increased stress, the eroded mass of pellets depends on their individual size and density, whereas the eroded mass of flocs depends more on the time that has passed since deposition. (iii) Convergent near-bed flow and/or reduced turbulence more efficiently traps slowly settling flocs relative to heavier pellets, while divergent near-bed flow and/or increased turbulence disperses flocs, leaving a lag deposit of the remaining pellets. The above hypotheses will be tested using a three-pronged approach of observations, analysis, and modeling. Observations of turbulent velocity and sediment concentration time-series, in concert with erosion microcosms and a particle imaging camera, will enable continual estimation of both ws and bed erodibility in a spatially and temporally varying turbid environment. Methods for analysis of fine sediment dynamics will incorporate formal data assimilation techniques, allowing continual objective inference of such key variables as the erosion rate parameter and the size distribution of suspended particles. While accounting for the presence of both pellets and flocs, three-dimensional modeling will be used to extend localized observations and analysis over an entire hydrodynamic system, evaluate the roles of convergent and divergent circulation, and perform more systematic sensitivity analysis.
Broader Impacts: Increased understanding of fine grained sediment transport and deposition will help improve management of human impacts on the coastal zone with applications including contaminant transport (contaminants often are bound to and travel with fine sediment), ecology (muds provide crucial habitat, and host crucial biogeochemical cycles), and engineering of estuaries, harbors, and dredge spoils. Data assimilation has not been widely applied to sediment transport; for this reason, the proposed research has the potential for transformative work. The PIs are active in several major community modeling efforts, providing ready venues for knowledge transfer of research results and products. The project will support two graduate students who will be trained in observations, data analysis and assimilation, and numerical modeling. The PIs will likely mentor undergraduate research pertaining to this project through VIMS? REU and/or William & Mary?s Senior Thesis, as they have done in the past. Research will be incorporated into graduate classes taught by the PIs including numerical methods, sediment transport, and estuarine dynamics. Results of the study will be disseminated to the general public through events at VIMS including a series of public lectures, as well as an annual Marine Science Day that attracts thousands of visitors per year.
