Long-term morphological change in estuaries, of the order of 100 years, has developed into an area of significant research interest as a result of increased regulation and management of estuarine environments. The long-term behaviour of estuary morphology results from the net effects of perturbations induced by tidal, seasonal and episodic events, averaged over a longer period. Theoretically a dynamic equilibrium may exist between deposition and erosion when considered over a time period that is sufficiently long to encompass the cyclic variability that exists within an estuarine system. However the assemblage of physical processes required for a stable state to exist, and the causes of deviation from a stable state, are not well understood. The interaction of physical processes of tidal and wave action, and the influence of sea level rise and anthropogenic activity, with estuarine ecology and geology are largely responsible for the evolving state of an estuary. Although the physical processes of tidal movement and wave action are well known and documented, the interaction of these processes with factors controlling estuarine evolution over long time periods is less well understood. This thesis evaluates approaches to analysing historical data and applying computational methods to examine the interaction between factors forcing long-term estuary morphology. Historical data is of considerable value to analysis of long-term morphological change in estuaries, and forms a pre-requisite for developing understanding of the nature and causes of the long-term evolution of estuary morphology. However few data sets exist which cover a period of sufficient duration with sufficient detail to identify the processes forcing morphological change, so recourse to computational methods is required for the purpose of developing understanding of estuary behaviour. Several techniques are employed, including analysis of bathymetric data, calculation of analytical parameters and computational hydrodynamic simulations, to develop a case study of processes causing morphological change in the Mersey estuary over the last century. A major requirement for the approach adopted in this thesis is the identification and reduction of uncertainty. Areas of uncertainty are identified, and the results arising from various computational techniques employing different assumptions are examined within a framework enabling evaluation of the uncertainty arising from analysis and assumptions upon which it is reliant. Volumetric analysis demonstrates that morphological change is dominated by a trend of significant accretion between 1906-1977, with tidal volume reducing by approximately 10% (70Mm3). Previous research has identified the construction of training walls, between 1906-36 to stabilise the position of the low water channel in Liverpool Bay outside the estuary, as a probable cause of perturbation. Changes to tidal flow and related sediment transport patterns outside the estuary resulting from training wall construction are examined with regard to the stability of the estuary system. The results from computational hydrodynamic models representing the years 1906, 1936 and 1977 quantifying potential changes in sediment transport pathways from outside the estuary indicate a significant increase in potential sediment supply to the mouth of the estuary during the period of peak accretion. However, these changes cannot be solely attributed to construction of the training walls, but result from the combined effect of training wall construction and dredging activity in the sea approach channels. Furthermore, it is not simply changes in tidal flow characteristics that cause sedimentation but also the existence of salinity induced gravitational circulation within the estuary and the wider Liverpool Bay system that acts as an important mechanism for importing sediment into the estuary. Evidence for evolution towards a stable estuary state is provided by derivation of a sediment budget demonstrating a negligible net flux of sediment into the estuary between 1977-1997. The establishment of a steady state is attributed to a reduction in the calculated transport of sediment, from west to east, across Liverpool Bay reducing the supply of sediment to the estuary mouth.

Approved for public release; distribution is unlimited / Observations of the temporal evolution of ripples are analyzed in terms of geometry, migration, crest orientation, and their predicted geometry by models using wave orbital velocities. Two weeks of bedform data were obtained in the surf zone during the RIPEX/SBE in April, 2001. Bed sediment consists of medium- to coarse-grained sand (D50=0.43mm). Models capture temporal trends in ripple geometry, but regression analyses show that they do not handle the range in forcing characteristics and geometries in the surf zone well. Transport models of bedload and total load formulated under uni-directional flows qualitatively capture the temporal evolution of observed transport by ripples, suggesting that under low to moderate forcing, bed load and suspended load occur mostly within the bed-following bottom boundary layer, andare measurable by ripple migration alone. Models predict large transport rates when flat beds were observed, so that at higher forcing ripples cannot be used to measure total sediment transport. Using a two-dimensional probability density function (PDF) of vector displacement peaks, a new ripple analysis model is proposed, incorporating a hierarchy of forcing complexity that includes such physical processes as directional spreading, axis rotation, orbital asymmetry, superimposed currents and infragravity wave velocities. The two-dimensional PDF's are compared with concurrent three-dimensional bed maps and are found to assist in describing ripple sizes, types, orientations, and migration velocities. / Lieutenant, United States Navy

Soil erosion and sedimentation are key environmental problems in the Upper Yangtze because of the ongoing Three Gorges Project (TGP), the largest hydro-power project in the world. There is growing concern about the rapid increase of soil erosion over the last few decades and its consequence for potential sedimentation in the reservoir. The study aims to examine controls on the spatial and temporal distributions of sediment transfer within the Upper Yangtze and the hydrological consequences of land use changes, using varied approaches at different catchment scales. First, soil erosion and sedimentation are examined using the radionuclide Cs-137 as a tracer within a small reservoir catchment in the Three Gorges Area. The results indicates that soil erosion on sloping arable land and the rates of reservoir sedimentation have been severe during the past 40 years, mainly due to cultivation on steep slopes. Changes in reservoir sedimentation rates are mainly attributed to land use changes. The suitability of the Cs-137 techniques for investigating soil erosion and sedimentation in intensely cultivated subtropical environments is also considered. The use of the technique for erosion investigation may have limitations due to the abundance of coarse soil textures, uncertainty about fallout deposition rates and the high incidence of human disturbance, but the technique shows promising perspectives for sedimentation investigation since a few dating horizons might be identified. Second, sediment and runoff measurement data for around 30 years from over 250 hydrological stations within the Upper Yangtze have been examined within a GIS framework. The dataset has been integrated with catchment characteristics derived from a variety of environmental datasets and manipulated with Arc/Info GIS. The analysis of the sediment load data has permitted identification of the most important locations of sediment sources, the shifting pattern of source areas in relation to land use change and sub-catchments exhibiting trending sediment yields corrected for hydrological variability. The study demonstrates the importance of scale dependency of sediment yield in both the identification of temporal change and the modelling of relationships between sediment yield and environmental variables, suggesting that the treatment of the scale problem is crucial for temporal-spatial studies of sediment yield.

Thesis (M.Sc.), Faculty of Engineering & the Built Environment, University of the Witwatersrand, 2003

Thesis advisor: Noah P. Snyder / The Souhegan River is a tributary of the Merrimack River that drains a 443 km<super>2</super> watershed in southern New Hampshire. The lowermost barrier on the Souhegan River, the Merrimack Village Dam (MVD), was demolished and removed in August and September 2008. The modern MVD impoundment contained at least 62,000 m<super>3</super> of sediment, mostly sand. Analysis of topographic and historical maps, and photographs suggests that approximately twice the area of what is now the modern impoundment has been affected by over 200 years of damming at the site. I use repeat surveys of cross sections and the river longitudinal profile, and sediment samples, to document the response of the Souhegan River to the MVD removal. A base level drop of 3.9 m caused immediate incision of the sand-sized sediment and channel widening. The impoundment later segmented into a non-alluvial, bedrock and boulder controlled reach; and a quasi-alluvial sand and gravel reach with erosion and deposition modulated by the presence of vegetation on the channel banks. One year after the removal, the Souhegan River has excavated 38,100 m<super>3</super> (65%) of the sediment in the modern impoundment. The response of the Souhegan River was rapid and the channel and floodplain continue to evolve toward a quasi-equilibrium configuration. Continued response will be substantially influenced by the establishment of vegetation within the former impoundment and the magnitude and frequency of high discharge events. / Thesis (MS) — Boston College, 2010. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Geology and Geophysics.

Thesis advisor: Noah P. Snyder / Global studies assert that anthropogenic activity now leads to disproportionately higher rates of landscape change compared with background geomorphic processes. This study explores the relative influence of anthropogenic, glacial, and geologic processes on erosion rates (E) in the northeastern United States (NEUS) by analyzing published erosion and sedimentation data across multiple methods and timescales. I compile erosion rates and sediment yields from records of stream gauging, reservoir sedimentation, lake sedimentation, cosmogenic nuclides in stream sediment, and thermochronology. These data serve as a comparison point for quantified volumes of sediment deposited in valley bottoms as a result of European settlement in the NEUS, where glacial history may influence the availability of erodible sediment and, as a result, the relative magnitude of deposited sediment. I hypothesize that E in the formerly glaciated region will be lower than unglaciated E over last century (stream gauging and reservoir sedimentation) timescales due to the erosive power of continental glaciation and resultant thin upland soils, and that there will be an increase in E evident over the last century as a result of human influence. 499 sites with location data were compiled across the NEUS, converted to erosion rate (mm/yr) and sediment yield (Ys; t km-2 yr-1), and analyzed using statistical z-tests to determine whether the population means are significantly different. Mean E from all record types across both the glaciated and unglaciated NEUS exhibits a range smaller than one order of magnitude (0.012-0.055 mm/yr), much less variable than order-of-magnitude differences reported by other researchers comparing modern and geologic erosion, both regionally and globally. Last century timescales exhibit higher E in the unglaciated region than the glaciated region, but only reservoir sedimentation shows a significant difference in E between regions (0.012 vs. 0.055 mm/yr; glaciated and unglaciated, respectively); stream gauging E did not exhibit a significant regional difference, likely due to the large basin sizes, short measurement timescales, and disproportionate spatial distribution of the measurements. E does not increase from geologic to last century timescales: late Quaternary (lake sedimentation and cosmogenic nuclide) records consistently yield lowest E, with geologic (thermochronology) records showing the highest E in both regions, perhaps indicating the relative importance of E over timescales during which major orogenies were occurring in the NEUS. The similarities in mean E and large range of the distributions of all timescales, however, point to the relative stability of E over time in the NEUS. / Thesis (MS) — Boston College, 2018. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Earth and Environmental Sciences.

Thesis (M.C.E.)--University of Delaware, 2007. / Principal faculty advisor: Nobuhisa Kobayashi, Dept. of Civil & Environmental Engineering. Includes bibliographical references.

Previous work in the Gulf of Lions, France has suggested that significant amounts of sediment may be escaping through the western part of this tectonically passive margin, despite it being far-removed from the primary sediment source (the Rhone river, ~160 km to the NE). It is hypothesized that the westernmost Cap de Creus canyon is intercepting the regional sediment-transport pathway and directing it offshore, allowing significant sediment export through this area. The overall goal of this project is to determine pathways and causes of sediment movement into Cap de Creus canyon to determine its role in off-shelf sediment export within the Gulf of Lions. Box cores were collected within the canyon and on the adjacent shelf on five cruises (2003-2005). Geochronology (210Pb-derived accumulation rates), grain-size distributions, and sedimentary structures (x-radiography) were analyzed to assess sedimentation patterns. Results indicate two mid-depth shelf depocenters (30-90 m water depth) separated by a zone of bypassing created by current acceleration around a headland. Within the canyon, the northern flank and mid-depth thalweg are modern depocenters of fine-grained sediments. The canyon head and southern flank are considered non-depositional for fine grains, although the head may be accumulating sands. Material enters the canyon from the northern rim (via advection of shelf benthic nepheloid layers), the southern rim (via dense-water cascading off the shelf), and through the canyon head (primarily coarse-grains). Budget calculations indicate that 9-23% of the sediment input to the Gulf is sequestered on the shelf and 1-4% is accumulating in upper Cap de Creus canyon. An ephemeral mud layer within the canyon axis indicates rapid deposition and frequent flushing, suggesting that sediment is moving through the upper canyon. This is also supported by high suspended-sediment concentrations associated with off-shelf dense-water flows. This study suggests that Cap de Creus canyon is an important conduit of sediment past the shelf break and the western margin is a primary location of sediment export from the Gulf of Lions.

An examination of suspended sediment concentrations throughout the Ganges-Brahmaputra River System was conducted to assess the spatial variability of river sediment in the world’s largest sediment dispersal system. During the high-discharge monsoon season, suspended sediment concentrations vary widely throughout different geomorphological classes of rivers (main river channels, tributaries, and distributaries). An analysis of the sediment loads in these classes indicates that 7% of the suspended load in the system is diverted from the Ganges and Ganges-Brahmaputra rivers into southern distributaries. Suspended sediment concentrations are also used to calculate annual suspended sediment loads of the main river channels. These calculations show that the Ganges carries 262 million tons/year and the Brahmaputra carries 387 million tons/year. These calculations are lower than published values because of either interannual variability and/or sampling artifacts and assumptions in the homogeneity of flow and sediment concentration. The conjoined Ganges-Brahmaputra River carries 530 million tons annually, or only 80% of the sum of the loads that the Ganges and Brahmaputra rivers carry upstream of the confluence. The remaining 20% of sediment is diverted from the main river by the distributaries and deposited along the main river channel during overbank flooding. Suspended sediment concentration is also examined in the north-south oriented tidal channels on the Bay of Bengal to determine whether sediment is delivered to the channels by one of two pathways: (1) sediment is discharged into the Bay of Bengal by the main river channel, carried west by coastal currents, and advected northward into the channels by tidal currents or (2) diverted from the main river bed through the distributaries, migrating southward into the tidal channels. Suspended sediment concentration and salinity data are inconclusive in determining sediment source. Beryllium-7 radioisotope data indicate that newly transported sediment is present in the tidal channels and offshore despite values in the Ganges and Ganges-Brahmaputra rivers being below detection. Sampling artifacts are likely caused by the below detection readings in the Ganges and Ganges-Brahmaputra rivers. Newly transported sediment is observed in a distributary south of the Ganges River and indicates that sediment is actively being transported to the distributary region.

This study investigated distributions and the concentrations of polycyclic aromatic hydrocarbons (PAHs) in the sediment cores in southern continental shelf of Kao-Ping submarine canyon. The sediment cores collected were analyzed for PAHs to reconstruct historical record of PAH contamination and to find out the possible sources, trends and fluxes. The findings demonstrated the average concentrations of PAHs in the northern continental shelf of Kao-Ping submarine canyon were higher than in the southern continental shelf. The average concentrations of £U51 PAHs and £U16 PAHs ranged from 578 to 697 and from 132 to 173 ng/g , respectively. Because particle sedimentation rates and PAH sources were different along cores, vertical distributions of PAH concentrations among cores were not the same. Identification by diagnostic ratios and result of hierarchical cluster analysis (HCA) suggesting PAHs in the core of northern continental shelf were from pyrogenic and petrogenic sources, while those in the cores of southern continental shelf were from petrogenic source. Compared with sediment quality guidelines (SQGs), most of the sample sediment cores are below the outlined criteria. Only the PAH concentrations of the sediments between 2-16 cm in the core L27 were higher than the Threshold Effect Level (TEL) value. Total PAH concentrations in this study were lower than those from other coastal areas in literature. According to the PAH concentrations along cores in the southern continental shelf, significant increase of total PAH concentrations was evident in the early 1900s. Our results also demonstrate that PAH concentrations and sources in the southern continental shelf area related to the increasing industrial activities in Kao-Ping coastal area.