Investigating Physical Processes Associated with Chesapeake Bay and Changjiang Estuar

Coastal and estuaries are landforms that not only have great impacts on large marine ecosystem, but also play a significant role in moderating or aggravating natural hazards and erosion risks that are expected to increase with climate change. This dissertation explores some of the concerns associated with coasts and coastal systems. In the second chapter, a thirty seven year wave hindcast (1979--2015) in Chesapeake Bay using NCEP's Climate Forecast System Reanalysis (CFSR) wind is presented. The long-term significant wave heights are generated by the third-generation nearshore wave model SWAN, which is validated using the wave height measurements at buoy stations inside the bay. Validation results show a good agreement between simulations and measurements. Statistical analyses on the simulated wave heights are carried out. Firstly, an Empirical Orthogonal Function (EOF) analysis is performed to study the temporal and spatial variability of significant wave heights in the bay. Secondly, the long-term changing trends of extreme wave heights are examined using regression analysis and empirical cumulative distribution function approach, which reveal a steady increase of extreme wave heights in most parts of the Chesapeake Bay in the past several decades. Finally, extreme value analyses based on generalized extreme value and generalized Pareto distribution functions are applied to evaluate design wave heights with different return periods. The effects of key parameters including threshold value, time span and data length on the design wave heights are extensively studied. Through the comparisons of different distribution functions evaluated by Bayesian Information Criterion and Akaike Information Criterion, it is found that Gamma distribution function and generalized extreme value analysis provide the best fit for annual and monthly data, while generalized Pareto distribution function gives the best fit when peak-over-threshold analysis is conducted. In the third chapter, sediment deposition in the north passage of the Changjiang Estuary, where the Deep-water Navigation Channel (DNC) is located, has been studied. To understand the suspended sediment dynamics and the effects of sediment-induced stratification on sediment flux in the navigational channel, field data on tidal ow and suspended sediment concentration (SSC) are collected and analyzed in this study. It is shown that net sediment transport is dominated by ebb currents in the study area. The net sediment flux is generally toward the ocean and the maximum value is found to be in the middle reach of the passage. In the lower reach of the passage, the net sediment flux is landward in the lower layer and seaward in the upper layer of the water column due to the two-layer feature of the estuarine circulation. Advective flux plays a significant role in transport of sediment in upper and middle reach of the passage by carrying 70~100% of the suspended sediment. However, this amount is reduced to 30~60% in lower reach of the passage where tidal effects become more important. The suspended sediment induced stratification in the north passage is examined by calculating eddy viscosity. It is found that suspended sediment can reduce eddy viscosity by 10~30%. The highest depth-averaged SSC is located in the middle reach of the north passage, where the averaged SSC is 4~15 times higher than that in the upper reach. In this region, bed shear stress is larger at ebb while SSC is higher at food. It is inferred that suspended sediments in the DNC during food are partially transported from a neighboring shoal, which plays an important role in sediment dynamics in the north passage.