No description available.

Several studies have previously demonstrated that sediment transported as bed load under a standing wave will be moved from between node and antinode towards the node where it accumulates. The end result is the generation of areas of scour in the bed between node and antinode and areas of sediment accretion around the node. However, these studies have failed to provide an adequate description of the mechanisms which led to this so-called "N-type" response. Consequently it has been the purpose of this study to examine the phenomenon of N-type beach response in some detail. As a first step an experimental programme was conducted in a random wave flume using a model beach and vertical, impermeable wall N-type conditions were produced, observations are made on the mechanisms of sediment transport and flow-field measurements are also reported. The experimental results demonstrate that N-type beach response is the direct result of an asymmetry in the main flow-field caused by the superposition of incident and reflected non-linear waves. A second experimental programme examines the N-type equilibrium profile shape using measurements of profiles generated under a range of wave conditions. An equation defined for a given profile amplitude and profile limits is found to predict the underlying characteristic N-type profile shape very well. Finally, a model of N-type beach response is developed. This model follows a relatively simple "grab-and-dump" concept with coefficients based on the understanding gained of the N-type response processes. It is shown that N-type profiles can be predicted reasonably well for the range of wave conditions used in the experiments here. Outside of this range predictions are less good, suggesting a re-examination of the model coefficients is required.

Particle shape is a parameter which has been largely neglected in the study of sediment transport by wind. Many methods of measuring shape have been published. Those which characterize shape in pebble and sand sized sediments are reviewed here. In order to test the influence of shape on aspects of particle movement and on sediment transport rate, two very differently shaped populations were used, (a) a reasonably equant quartz dune sand and (b) a platy dune sand composed largely of shell fragments. Recommendations are made for reasonably fast and accurate methods of shape measurement, including Sneed and Folk's Maximum Projection Sphericity and Winklemolen's Rollability. The possible effects of shape on saltation were examined in terms of grain trajectories and the grain's interaction with the bed at collision using video and high speed photography. The latter enabled individual particles (from a coarse, medium or fine size fraction) to be followed as they impacted the bed, through to ricochet and the possible ejection of previously stationary bed grains. Experiments were conducted over horizontal and sloping beds, the latter representing different parts of the stoss face of a ripple. It was found that shape, in terms of the sphericity of the particles has a marked effect on collision. The near spherical quartz sand causes more dislodgements and more ejecta per collision than the much less spherical platy shell sand. The quartz sand is also more likely to approach and ricochet from the bed at higher angles to the horizontal than the shelly sand, and to rebound more vigorously. Thus, the bed activity generated by collision increases with an increase in particle sphericity. However, high speed photography of grain dislodgement by wind action alone indicates that, as sphericity decreases, there is a greater probability that a grain will be entrained aerodynamically. Shape also influences trajectories. Video films show that grain paths become longer and flatter as sphericity decreases. These observations indicate that the transport rate for grains with a low sphericity will be greater than those with a high sphericity, both in terms of aerodynamic entrainment and of longer trajectories. However, once collision becomes important in the dislodgement of surface grains, the greater bed activity seen with more spherical particles means that their transport rate will increase. These findings are supported by the observations of Williams (1964) and of Willetts, Rice and Swaine (1982), that sediment transport is promoted at low windspeeds by less spherical grains, while the opposite occurs at high windspeeds.

Master of Arts / Geography / Charles W. Martin / Tuttle Creek Dam was built in 1962 after near-record flooding in the Kansas River watershed. It has been in operation for over 50 years. In that time, nearly half of its storage capacity has been filled by sediment, reducing its ability to serve its intended purpose under current operations. The Corps of Engineers authorized a study to examine the sediment in Tuttle Creek Lake and determine management strategies to extend its lifespan. This report examines the erodibility of the sediment as a function of depth and distance to dam. Eight sediment cores were tested and analyzed for two erodibility parameters, critical shear stress and erodibility coefficient. After directly comparing these parameters it was determined that Tuttle Creek Lake sediment ranges from erodible to very erodible regardless of depth or location. Analyzing for locational and depth patterns in erodibility coefficients indicated that both influence the erodibility of sediment, with depth being the stronger factor. These results indicate that locations further upstream from the dam and greater depths are the least erodible. Sedimentation patterns were mapped onto a three dimensional model in ArcMap. It is hoped that this research will assist Corps of Engineers leadership in deciding which management practices to pursue for Tuttle Creek Lake.

Thesis advisor: Gail C. Kineke / To determine the role of cohesive suspended particle characteristics on sediment transport patterns in an energetic estuary floc size, density, and settling velocity were investigated in the Connecticut River estuary over three years spanning varying fluvial discharge regimes. Concurrent measurements of in-situ floc size, flow, bed stress, salinity and suspended-sediment concentration (SSC) were used to identify primary influences on floc size variability. Water discharge ranged from 202 to 910 m³/s between the three sampling campaigns, and the timing of major sediment discharge events preceding measurement periods from 23 to 162 days. Two distinct particle populations were observed under high and low sediment discharge regimes. With abundant fluvial sediment input, flocculation occurred resulting in large, loosely-packed flocs dominating the suspended signal (median sizes of 194-209 µm; median excess densities of 13-17 kg/m³). Following an extended period of low sediment discharge, small, dense aggregates resuspended from the bed were observed throughout the water column (median size of 171 µm and excess density of 60 kg/m³). The timing of and partial decoupling of water and sediment discharge led to inter-annual patterns of cohesive particle characteristics controlled by fresh sediment supply. The large, light flocs with lower settling velocities characteristic of high sediment supply regimes likely bypass the estuary. Smaller compact aggregates dominated the low-sediment discharge regimes. However, the similar disaggregated size distribution of the two regimes suggests the same fine source material is reintroduced to the estuary with the intrusion of the salt wedge, which extends farther up-estuary during low discharge regimes and ultimately supplies the off-channel bays and coves. / Thesis (MS) — Boston College, 2017. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Earth and Environmental Sciences.

Rivers sourced in the Himalayan mountains support more than 10% of the global population, where the majority of these people live downstream of the mountain front on the alluvial Indo-Gangetic Plain. Many of these rivers however, are also the source of devastating floods. The tendency of these rivers to flood is directly related to their large-scale morphology. In general, rivers that drain the east Indo-Gangetic Plain have channels that are perched at a higher elevation relative to their floodplain, leading to more frequent channel avulsion and flooding. In contrast, those further west have channels that are incised into the floodplain and are historically less prone to flooding. Understanding the controls on these contrasting river forms is fundamental to determining the sensitivity of these systems to projected climate change and the growing water resource demands across the Plain. This thesis examines controls on river morphology across the central portion of the Indo-Gangetic Plain drained by the Ganga River (the Ganga Plain). Specifically, the relative roles of basin subsidence, sediment grain size and sediment flux have been explored in the context of large-scale alluvial river morphology over a range of timescales. Furthermore, this thesis has developed and tested techniques that can be utilised to help quantify these variables at catchment-wide scales. This analysis has been achieved through combining new sediment grain size, pebble lithology and cosmogenic radionuclide data with quantitative topographic and sedimentological analysis of the Ganga Plain. In the first part of this thesis, I examine the contrast in channel morphology between the east and west Ganga Plain. Using topographic analysis, basin subsidence rates and sediment grain size data, I propose that higher subsidence rates in the east Ganga Plain are responsible for a deeper basin, with perched low-gradient rivers systems that are relatively insensitive to climatically driven changes in base-level. In contrast, lower basin subsidence rates in the west are associated with a shallower basin with entrenched river systems that are capable of recording climatically induced lowering of river base-level during the Holocene. Through an analysis of fan geometry, sediment grain size and lithology, I then demonstrate that gravel flux from rivers draining the central Himalaya with contributing areas spanning three orders of magnitude is approximately constant. I show that the abrasion of gravel during fluvial transport can explain this observation, where gravel sourced from more than 100 km upstream is converted into sand by the time it reaches the Plain. I attribute the over-representation of quartzitic pebble lithologies in the Plain (relative to the proportion of the upstream catchment area likely to contribute quartzite pebbles) to the selective abrasion of weaker lithologies during transport in the mountainous catchment. This process places an upper limit on the amount of coarse sediment exported into the Indo-Gangetic Plain. Finally, I consider the use of cosmogenic 10Be derived erosion rates as a method to generate sediment flux estimates over timescales of 102-104 years. Cosmogenic radionuclide samples from modern channel and independently dated Holocene terrace and flood deposits in the Ganga River reveal a degree of natural variability in 10Be concentrations close to the mountain front. This is explored using a numerical analysis of processes which are likely to drive variability in catchment-averaged 10Be concentrations. I propose that the observed variability is explained by the nature of stochastic inputs of sediment (e.g. the dominant erosional process, surface production rates, depth of landsliding, degree of mixing), and secondly, by the evacuation timescales of individual sediment deposits which buffers their impact on catchment-averaged concentrations. In landscapes dominated by high topographic relief, spatially variable climate and multiple geomorphic process domains, the use of 10Be concentrations to generate sediment flux estimates may not be truly representative. The analysis presented here suggests that comparable mean catchment-averaged 10Be concentrations can be derived through different erosional processes. For a given 10Be concentration, volumetric sediment flux estimates may therefore differ.

acase@tulane.edu / Though rivers are inextricably linked in our minds with an intermittently flooded overbank environment, surprisingly little is known about the sedimentary processes that operate there, or how they interact with those of the river. The knowledge gap is acute in deltas, where dense populations often necessitate tightly engineered control over flow patterns, leading to disconnected overbank environments that no longer receive input from the main channel. However, the need to understand sedimentary function in the overbank is also acute in deltas, as rising relative sea levels create an urgent need to manage water and sediment resources. This dissertation is presented as three primary chapters, each of which examines a different aspect of the hydrodynamic and sedimentary connection between a river’s channel and its overbank environment. In Chapter 2, my coauthors and I ask which factors enhance overbank sediment retention, and what retention rates might be considered typical in deltas. We compare the sediments stored in a crevasse splay to those transported by the river and conclude that retention rates approaching 100% might be achievable in settings that are not exposed to coastal processes. Chapter 4 is also concerned with spatial patterns of sedimentation on a delta. In it we use physical experiments to examine the influence that floods play in mobilizing sediments from the channel and storing them in the overbank environment. We find, counterintuitively, that an experiment whose input included floods has a lower proportion of floodplain to channel deposits preserved than an experiment with a constant input. Chapter 3 is focused on water and sediment dynamics in the channel in a region where significant flow is lost to the overbank environment. Here we present measurements from channel networks in the Mississippi River’s Birdsfoot Delta and show that flow loss along the channels is a critical control on channel function that causes channels of disparate sizes to behave similarly. We use our field results to inform a numerical model of channel bed evolution in a region with flow losses, and conclude that the modern flood control system in the Lower Mississippi River may have significantly changed the bed morphology. / 1 / Christopher R. Esposito

acase@tulane.edu / Land-surface subsidence can be a major contributor to the relative sea-level rise that is threatening many coastal communities. Loosely constrained subsidence rate estimates across the Mississippi Delta make it difficult to differentiate between subsidence mechanisms and complicate modeling efforts. New data from a nearly 40 m long, 12 cm diameter core taken during the installation of a subsidence monitoring superstation near the Mississippi River, southeast of New Orleans, provides insight into the stratigraphic and geotechnical properties of the Holocene succession at that site. Stratigraphically, the core can be grouped into four units. The top 12 m is dominated by clastic overbank sediment with interspersed organic-rich layers. The middle section, 12-35 m, consists predominately of mud, and the bottom section, 35-38.7 m, is marked by a transition into a Holocene-aged basal peat (~11.3 ka) which overlies densely packed Pleistocene sediment. Radiocarbon and OSL ages are used to calculate vertical displacement and averages subsidence rates as far back as ~3.5 ka, yielding values as high as 8.0 m of vertical displacement (up to 2.34 mm/yr) as obtained from a transition from mouth bar to overbank deposits. We infer that most of this was due to compaction of the thick, underlying mud package. The top ~80 cm of the core is a peat that represents the modern marsh surface and is inducing minimal surface loading. This is consistent with the negligible shallow subsidence rate as seen at a nearby rod-surface elevation table – marker horizon station. Future compaction scenarios for the superstation can be modeled from the stratigraphic and geotechnical properties of the core, including the loading from the planned Mid-Barataria sediment diversion which is expected to dramatically change the coastal landscape in this region. / 1 / Jonathan G Bridgeman

Lake Rotorua has become increasingly eutrophic over the past 2 to 3 decades. The sediments of the lake have been shown to exert an important influence on this eutrophication process. Chemistry of the sediments has been studied to determine the nature, composition and distribution of elements, through a 1.5 year coring programme. A geophysical survey together with sub-bottom profiling has provided stratigraphic information related to the bathymetry of the lake. Lake Rotorua has two types of sediments: coarse, dense (density c. 0.5 g/cm3) sediments comprised of clastic erosion products and coarse rhyolitic airfall components covering approximately 60% of the lake area; and fine, low-density (approximatly 0.02 g/cm3) diatomaceous ooze that covers the remaining 40% of the lake, accumulated from deposition of biota, predominantly diatom frustules of Aulacoseira granulata. The sediment contains a record of volcanic eruptions, with the Tarawera Tephra typically found 0.5 m below the sediment water interface and Kaharoa Tephra typically between 2 to 3 m depth, in water depths of 10-15 m. Phosphorus concentration in Lake Rotorua sediments decreases with sediment depth. In the centre of the lake phosphorus concentrations in the top 2 cm can exceed 2500 g/tonne and decline to 800 g/tonne at 20 cm depth. Accumulation rate of phosphorus in the sediment based on the nutrient budget is approximatly 29.6 t/yr. Iron and manganese concentrations in the sediment depend on the availability of the element and the sedimentation rate of diatom frustules, and are controlled by the redox conditions in the sediment. The average concentration of iron and manganese in the sediment is approximately 8000 g/tonne and between 300 and 400 g/tonne, respectively. Iron accumulates in the sediment at a rate of 385 t/yr and manganese at 17.9 t/yr. Maximum concentrations of arsenic in the sediment are 250 g/tonne but are generally between 50-100 g/tonne, depending on the water depth. Lead concentrations are typically below 15 g/tonne. Sediment concentrations of both arsenic and lead are highly correlated with iron and manganese concentrations in the sediment and mimic the respective concentration profiles. Arsenic and lead accumulate in the sediment at a rate of 3.71 and 0.49 t/yr, respectively. All elements show a peak in concentration in the tephra layers. The bathymetry of Lake Rotorua is dominated by a curved depression extending from Sulphur Point and almost reaching the Ohau Channel. This depression is probably a structural feature likely associated with the collapse of the caldera, but could be an ancient drainage channel. A series of conical depressions clustered to the north of Sulphur Point and to the east of Mokoia Island are likely to be hydrothermal explosion creators. In the north in the lake at water depths less than 10 m, a series of near-shore terraces are preserved in the sediment. Sub-bottom echo-sounding shows no return of sonic and seismic signals from most of the lake floor, indicating total absorption by the methane gas-filled sediment. In the shallow lake margin environments, generally less than 10 m water depth, gas is absent and a detailed stratigraphy of multiple reflectors from tephra layers was observed with sub-bottom profiles. The basin sediments of Lake Rotorua are significantly pockmarked, with deep, circular flat-bottomed depressions c. 20-60 m diameter and 0.5-6 m depth. The pockmarks are located on the lake floor in areas where the sediment is saturated with gas.

The incipient motion of sediment particles in gravel-bed rivers is a very important process. It represents the difference between bed stability and bed mobility. A field study was conducted in Oak Creek, Oregon to investigate incipient motion of individual particles in gravel-bed streams. Investigation was also made of the incipient motion of individual gravel particles in the armor layer, using painted gravel placed on the bed of the stream and recovered after successive high flows. The effect of gravel particle shape was examined for a wide range of flow conditions to determine its significance on incipient motion. The result of analysis indicates a wide variation in particle shapes present. Incipient motion and general transport were found to be generally independent of particle shape regardless of particle sizes. A sample of bed material may contain a mixture of shapes such as well-rounded, oval, flat, disc-like, pencil-shaped, angular, and block-like. These are not likely to move in identical manners during transport nor to start motion at the same flow condition. This leads to questions about the role of shape in predicting incipient motion and equal mobility in gravel-bed streams. The study suggests that gravel particles initiate motion in a manner that is independent of particle shape. One explanation may be that for a natural bed surface many particles rest in orientations that give them the best: protection against disturbance, probably a result of their coming to rest gradually during a period of decreasing flows, rather than being randomly dumped. But even when tracer particles were placed randomly in the bed surface there was no evident selectively for initiation of motion on the basis of particle shape. It can be concluded from analysis based on the methods of Parker et al. and Komar that there is room for both equal mobility and flow-competence evaluations. However, the equal mobility concept is best applied for conditions near incipient motion and the flow-competence concept is best applied for larger flows and general bedload transport. Furthermore, with an armored bed, such as that at Oak Creek, there is a tendency for a more-nearly equal mobility (or equivalent) for the normalized transport rates for the various size fractions when incipient motion and moderate bedload transport occurs. / Graduation date: 1995