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Sediment dynamics driven by waves and currents in shallow-water estuarine environments impacts many physical and biological processes and is important to the estuary-wide sediment budget. However, observational restrictions have limited our ability to understand the physics governing sediment entrainment and mixing in these environments. Nonlinear interactions between waves and currents, and sediment-induced stratification, result in complex near-bed physics that impacts the vertical transport of both mass and momentum throughout the water column. An Eulerian based sediment transport model is developed, along with a method for reducing the spin-up time for turbulent channel flow simulations. With proper initial conditions, the method leads to over a factor of six savings in the computational expensive associated with spin-up relative to traditional methods. Direct numerical simulation is then applied to study wave, current, and sediment interactions in combined wave- and current-driven flows with low Reynolds number (laminar) waves. Simulated conditions are relevant to wind-waves propagating into shallow-water, fine sediment environments. In contrast to the effects of high Reynolds number waves, low Reynolds number waves are found to accelerate currents by reducing vertical turbulent momentum transport. However, the wave velocity is unaffected by the currents, and resembles the Stokes theoretical wave solution for all conditions simulated. We also show that sediment entrainment and near-bed sediment dynamics are controlled by waves, although current-generated turbulence is required for vertical mixing. As is the case for steady currents, the downward settling flux is balanced by the upward turbulent flux throughout most of the water column. Near-bed sediment-induced stratification is also shown to suppress vertical transport of mass and momentum by stabilizing infrequent but intense mixing events. The stabilization reduces both the vertical Reynolds stress and vertical turbulent sediment flux, leading to accelerated currents and reduced suspended sediment concentrations. Near-bed reductions in vertical Reynolds stresses also reduce turbulence production but increase vertical shear. The increased shear acts as a feedback mechanism that eventually outweighs suppression of the Reynolds stress and increases turbulence production higher in the water column. Unlike mean shear, if the vertical turbulent sediment flux is restricted at any point in the water column, the magnitude of the mean suspended sediment concentration gradient decreases at all heights above the restriction. As a result, the effects of sediment-induced stratification on vertical turbulent sediment fluxes are more pronounced than on vertical turbulent momentum fluxes.
This research incorporates streambank erosion and failure processes into a distributed watershed model and evaluates the impacts of climate change on the processes driving streambank sediment mobilization at a watershed scale. Excess sediment and nutrient loading are major water quality concerns for streams and receiving waters. Previous work has established that in addition to surface and road erosion, streambank erosion and failure are primary mechanisms that mobilize sediment and nutrients from the landscape. This mechanism and other hydrological processes driving sediment and nutrient transport are likely to be highly influenced by anticipated changes in climate, particularly extreme precipitation and flow events. This research has two primary goals: to develop a physics-based watershed model with more inclusive representation of sediment by including simulation of streambank erosion and geotechnical failure; and to investigate the impacts of climate change on unstable streams and suspended sediment mobilization by overland erosion, erosion of roads, and the erosion as well as failure of streambanks. This advances mechanistic simulation of suspended sediment mobilization and transport from watersheds, which is particularly valuable for investigating the impacts of climate and land use changes, as well as extreme events. Model development involved coupling two existing physics-based models: the Bank Stability and Toe Erosion Model (BSTEM) and the Distributed Hydrology Soil Vegetation Model (DHSVM). This approach simulates streambank erosion and failure in a spatially explicit environment. The coupled model is applied to the Mad River watershed in central Vermont as a test case. I then use the calibrated Mad River model to predict the response in watershed sediment loading to future climate scenarios that specifically represent local temperature and precipitation trends for the northeastern US, particularly changing trends in the frequency and magnitude of extreme precipitation. Overall the streambank erosion and failure processes are captured in the coupled model approach. Although the presented calibration of the model underestimates suspended sediment concentrations resulting from relatively small storm/flow events, it still improves prediction of cumulative loads and in some cases suspended sediment concentrations during elevated flow events in comparison to model results without including BSTEM. Increases in temperature affect the timing and magnitude of snow melt and spring flows, as well as associated sediment mobilization, in the watershed. Increases in annual precipitation and in extreme precipitation events produce increases in annual as well as peak discharge and sediment loads in the watershed. This research adds to the body of evidence indicating that streambank erosion and failure can be a major source of suspended sediment, and thereby a major source of phosphorus as well. It also shows that local climate trends in the Northeast are likely to result in higher peak discharges and sediment yields from meso-scale, high-gradient watersheds that encompass headwater forested streams and agricultural floodplains. One limitation was that we could not drive the model with meteorological data that represented changes in both temperature and precipitation, highlighting the need for improved climate predictions. This coupled model approach could be parameterized for alternative watersheds and be re-applied to answer various questions related to erosion processes and sediment transport in a watershed. These findings have important implications for resource allocation and targeted watershed management strategies.
The physical effects, such as waves and turbulence, associated with the passage of a boat are more pronounced in a narrow, shallow river channel such as the Illinois Waterway, than in the relatively wider and deeper Mississippi River. The bed material in the Illinois below Hennepin (river mile 207) is composed predominantly of silts and clays, which are more easily resuspended by boat traffic and which also take longer to settle, than the sands which largely comprise the bed of the Mississippi. The Illinois thus seems to be especially vulnerable to the physical effects of increased boat traffic. In addition, the areas that are most productive fish and wildlife in the Illinois Valley, the side channels, backwaters, and bottomland lakes, which flank the main channel, are especially vulnerable to siltation, because the current is reduced in these areas and suspended material tends to settle out. The purpose of this report is to evaluate the impact of increased frequency of wave wash and resuspension of sediments, resulting from increased boat traffic, on the biota in the channel and lateral areas of the Illinois River. (Author).