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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
21

Fluvial Processes Affecting the Texture of a Gravel Bed with an Emphasis on Salmon Spawning Habitat

Meyers, Matthew Armand 14 September 2017 (has links)
<p> Spawning salmon depend on fluvial processes to maintain the bed texture of gravel bedded rivers as suitable spawning and incubation habitat. The coarse texture of a gravel bed is maintained by flow strengths capable of moving the gravel and removing fine sediments, thereby providing loose substrate that enhances its ability to conduct flow (i.e., its hydraulic conductivity). The flow strength that corresponds to the beginning of movement (i.e., entrainment) of a grain on a gravel bed is variable and, therefore, it needs to be measured to predict flow levels capable of bed texture maintenance. Lower flows can deliver fine sediment, which may accumulate in the gravels overlying salmon nests (i.e., redds) reducing the hydraulic conductivity and impairing salmon embryo development. I examine these processes to explain the variability in (i) the rate of change in the proportion of a gravel size fraction entrained as a function of flow strength and (ii) the rate of decrease in hydraulic conductivity as a function of the cumulative transport of fine sediment that depends on flow level. </p><p> I used tracer gravel and cobble grains and a two dimensional flow model to determine the flow strengths capable of gravel bed entrainment using binary plots of the occurrence or absence of tracer movement to approximate the fraction of the bed sediment entrained as a logistic function of increasing flow strength. The method provides an approximation of the flow strength capable of entraining the least resistant through the most resistant grains, thereby providing a new method to approximate the flow strength that is capable of fully entraining the bed. The results are confirmed by comparing the measurements of two study sites. </p><p> To measure the resistance of individual gravel and cobble grains to downstream movement, I used force gages and a theoretical force balance model that incorporates the frictional resistance of a grain and the lift and drag forces applied by stream flow. I measured the frictional resistance of grains at six sites with different morphologies to evaluate influences on grain resistance. Grain resistance varied depending on the grain size and sorting, amount of infiltrated sand, streamwise position along a gravel bar, and degree of fluctuation in the flow strength. The force balance model predicts that the dimensionless instantaneous flow strength capable of entraining a given proportion of a relative grain size varies along a bar. I tested the force balance model predictions by comparison with the tracer measurements from the tracer study. The differences between the force balance predictions and the tracer entrainment measurements are due to the definition of the flow strength as the time-averaged value from the 2D flow model and the instantaneous value from the force balance model. Adjusting the force balance model predictions by a simple factor that quantifies their difference and that corresponds with an index describing the fluctuations of the instantaneous flow strength about its time-averaged value produces agreement with the tracer measurements. This adjustment to force gage measurements from two sites on opposite limbs of a bar indicates that the time-averaged dimensionless flow strength capable of entraining a given proportion of a relative grain size is constant along a bar.</p><p> I constructed artificial redds at five sites to monitor changes in hydraulic conductivity and sand accumulation with cumulative bed load transport. I applied two backfill treatments to examine the effects of the presence and absence of grains smaller than 6.4 mm. Hydraulic conductivity varies temporally due to sand deposition and spatially due to the initial sand content of the backfill and the local sediment supply rate. The rate of change in hydraulic conductivity as a function of increasing cumulative transport is statistically significant but does not differ between backfill treatments or with location. I predicted the effect of flow on egg survival using an empirical relationship between hydraulic conductivity and egg survival with results that indicate hydraulic conductivity is dependent on sediment transport rate as a function of flow rate.</p><p> This work provides additional understandings of the relationship between flow and bed texture maintenance processes and useful tools for managing gravel bed rivers.</p><p>
22

Embauchage intégral définition et théorie

Bélanger, Jean Robert January 1948 (has links)
Abstract not available.
23

The surficial geology and river terraces of Allumette Island and adjacent parts of Ontario and Quebec

Hanley, Peter T January 1972 (has links)
Abstract not available.
24

Thermokarst and related geomorphic processes, eastern Banks Island, NWT

Egginton, Paul January 1976 (has links)
Abstract not available.
25

Morphological response of arid, carbonate watersheds in Oman to climatic and tectonic forcing

Al Abri, Amani 18 February 2022 (has links)
The land surface can be shaped by different forces including climate and tectonics. Here we study how these two drivers affect the morphology of arid, carbonate watersheds in Oman. The tectonic response of a carbonate terrain in the northeastern Hajar Mountains of Oman was studied by quantifying rates of basin averaged erosion and chemical dissolution over late Pleistocene-Holocene time scales. Results indicate that the uplift rate calculated from tectonic flexural modeling (~80-200 mm/ka) and marine terraces (900-6700 mm/ka) is higher than basin averaged erosion rates of 10-15 mm/ka, and chemical dissolution rates of 0.9-1.3 mm/ka. The values of basin averaged erosion and chemical dissolution measured herein are consistent with documented denudation rates in hyper-arid climates that are limited by precipitation. The climatic forcing effects on watersheds was analyzed by studying the relationships among precipitation, vegetation, and morphological characteristics of wadis draining both sides of the Dhofar Mountains in southeastern Oman. At this location, a strong gradient in water availability is present during the monsoon season. A basins analysis shows that the hypsometric curve and statistical moments capture the morphological difference between wet and dry watersheds. Our results shed light on the geomorphic signature of water availability in a semi-arid, carbonate landscape. Finally, the potential teleconnections between climatic indices and cloud cover in the Dhofar Mountains of Oman were explored using statistical cross-correlations and Artificial Neural Networks (ANN). Cross-Correlation results show that cloud cover has a significant positive relationship (|r| > 0.460, p<0.05) with Pacific Decadal Oscillation (PDO) with a lag of 3 months, and has significant negative relationship (|r| > 0.460, p<0.05) with Indian Ocean Dipole (IOD) with a lag of 5 months. The ANN model shows that the Interdecadal Pacific Oscillation (IPO) index yields the closest prediction to the observed values of cloud cover, with the lowest prediction error. The results of this study raise the possibility of predicting in advance summer cloud cover from Pacific climatic indices.
26

The Role Of Suspended Sediment In Assessing Coastal Wetland Vulnerability

Coleman, Daniel J. 01 January 2020 (has links)
Coastal wetlands sequester carbon, attenuate waves and storm surge, filter out nutrients and pollutants, and act as nursery habitat for important fisheries. The value of these ecosystems is underscored by their vulnerability to climate change, especially sea level rise. To persist under the threat of rising sea level, coastal wetlands must build elevation vertically. Delivery of sediment to the marsh during tidal flooding is a key component in the ecogeomorphic feedbacks that lead to elevation gain. Despite the importance of suspended sediment to assessing coastal wetland vulnerability, many questions remain unanswered. This dissertation addresses the impact of suspended sediment concentration on wetland geomorphology from fine-scale processes to global patterns and from thriving systems to those experiencing significant environmental change. In Chapter I, I explore alterations to sediment transport and geomorphology caused by an acute vegetation disturbance in a Georgia saltmarsh. My results showed that the loss of vegetation was reversed the trajectory of the site from a prograding marsh to an eroding marsh. In Chapter II, I investigate how suspended sediment travels across the marsh platform using high frequency, long-term measurements in the Plum Island Estuary, Massachusetts. In contrast to the current paradigm, I found that sediment supply in the marsh interior is largely decoupled from channel sediment supply. Chapter III focuses on the role of sediment transport in mangrove encroachment into salt marshes in Australia. My work suggests that mangroves do not inhibit the ability of salt marsh to accrete vertically and that the removal of mangroves to preserve salt marsh would be ineffective. In Chapter IV, I analyze the relationship between suspended sediment concentration, tidal range, and accretion in salt marshes from around the world. My work emphasizes the importance of mineral accretion and marsh elevation when making predictions about marsh response to sea level rise. These results help bridge the gap between numerical models which predict marshes are capable of surviving high rates of relative sea level rise and field studies which suggest drowning at much lower rates. As a whole, my dissertation demonstrates that physical processes and the ways in which biology mediate these processes are critical to the ability of coastal wetlands to persist. As the rate of sea level rise continues to accelerate, it is increasingly important to understand the controls on vertical elevation growth in coastal wetlands at the scale of several meters to thousands of kilometers and in pristine systems to degraded environments.
27

Quantitative Analysis of Beach Sand Movement, Virginia Beach, Virginia

Boon, John D. 01 January 1968 (has links)
The present study describes a new approach to quantitative sediment tracing with dyed (fluorescent) particles, which can be used to obtain estimates of grain velocity, estimates of mobile layer thickness, and estimates of the rate of longshore sand transport.A simplified stochastic-process model was employed Involving the use of statistical procedures to analyze tracer displacement data on a time-stratified basis. The procedures provide meaningful estimates of the direction and magnitude of the movement of beach sand through set Intervals of time. Results of a single tracer experiment are reported from a beach located near Virginia Beach, Virginia. The tracer particle-displacement data obtained wares 1) subjected to preliminary tests to determine the need to separate the data according to two dynamic zones; 2) treated as normally distributed populations in which mean particle displacements were computed for specific times following tracer release and compared with one another using "t" tests to determine significant mean differences;3) time-stratified where successfully tested so that weighted mean dis­placements representing the time Interval 0 to 50 minutes could be computed; and 4) converted to mean particle velocities by dividing adjusted mean displacements by average travel time. For the morning of September 17, 1966, during moderate wave conditions at high tide, average particle velocity was determined for the swash and surf zone®. In the swash zone, V (X) • 0.44 ft./min. (0.22 cm./sec.) in the longshore direction. In the surf zone, V (Y) * 0.37 ft./min. (0.1# cm./sec.) in the offshore direction. Computed mean grain velocities and certain environmental data gathered prior to the time of the tracer experiment were used to make an estimate of the thickness of the mobile layer (the depth interval in which semi-continuous motion of the grains occurs). The thickness,based on grain velocity in the offshore direction and the measured rate of sand accretion seaward, was about 0.5 in. (1.3 cm.); assuming this to be the thickness of the mobile layer in the swash zone as well,an estimate of the longshore rate of sand transport was then computed using known values of swash zone width and longshore grain velocity.A transport value of 440 yd. 3/day (336 m3 /day)» in a longshore direction was obtained.
28

Geomorphic Processes of a Drowned River Valley, Lower York River Estuary, Virginia

Carron, Michael Joseph 01 January 1976 (has links)
No description available.
29

An Investigation of the Late Quaternary Morphology of Mobjack Bay, VA and Application of a Facies Model

Milligan, Donna Angela 01 January 1994 (has links)
No description available.
30

Geographical geomorphology : historical development, contemporary problems, and future prospects /

Beach, Gary L. January 1981 (has links)
Thesis (M.S.)--Oregon State University, 1982. / Typescript (photocopy). Includes bibliographical references (leaves 96-105). Also available via the World Wide Web.

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