Thesis (M.Oc.E.)--Oregon State University, 2008. / Printout. Includes bibliographical references (leaves 51-54). Also available on the World Wide Web.

This study set out to investigate the shear stress behaviour at the bed under combined wave and current action. The intention of the study was to make experimental measurements to determine how wave and current shear stresses combine, so that theoretical models describing the combined flow condition could be proposed. Two types of experiment were conducted, and theoretical models for the combined flow were assessed. One set of experiments attempted to use a shear plate to make direct measurements of the combined flow shear stress, and of the shear stresses for the component waves and steady currents. This approach failed because the large correction terms introduced by the non-uniform wave pressure field could not be accurately estimated. The second set of experiments used a laser doppler anemometer to make detailed velocity profile measurements over flat sediment beds. The onset of sediment motion was used as a criterion to carefully control the experiments. It is assumed that the threshold of sediment motion represents a specific shear stress intensity at the bed for sediments of narrow size ranges. As the shear stresses can be determined from the velocity fields under waves and currents, their additive nature under combined flow conditions could be investigated. For each sediment size range, it is shown that the same maximum velocity very near the bed can be used to specify the threshold of sediment motion condition for all flow types, be they under waves, currents, or combined waves and currents. It is also shown that the near-bed velocity under a laboratory wave can be predicted accurately from second order wave theory and that the velocity under a current can be predicted from combining Manning's relation with the universal log velocity law. It is further shown that the near-bed velocity under a combined wave and current can be described by the vectorial addition of the maximum component wave velocity and the average component current velocity. The shear stress for the onset of motion is calculated for the steady current using Manning's relation, for the wave by combining the oscillatory shear stress formula with Kamphuis's rough turbulent friction factor relation, and for the combined wave and current by the simple vectorial addition of the component shear stresses, and is shown to be comparable with Shields's threshold criterion for nearly all conditions tested. / Applied Science, Faculty of / Civil Engineering, Department of / Graduate

Numerous studies have identified the west coast of Canada as an attractive place for the development of wave energy projects. To evaluate the viability of these projects, an accurate description of the wave resource is crucial. Most of the previous efforts to characterize the wave climate in B.C. at shallower waters, where wave energy converters (WECs) are most likely to be deployed, lack the necessary nearshore spatial resolution, and were driven by overly simplistic wave boundary conditions. In addition, none of the previous studies have included the effect of tidal currents, which have been proven to be significant in wave resource characterizations in other locations. This work increased the fidelity of the wave resource characterization and developed an understanding of the impact of tidal currents on the wave conditions in this region by generating two most accurate, long-term (14 years, 2004 to 2017), high resolution (in space and time) datasets of the wave resource for the west coast of Canada. The two datasets were generated using nearly identical SWAN wave models, which their only difference was that one of them (V5), did not incorporate the effect of currents, while the other (V6) included tidal currents as forcing. Thus, the pure influence of the tidal currents on the wave characteristics was able to be identified when comparing the two wave model results. This study developed simple, robust, and objective metrics to support the calibration process and to evaluate the performance of the models. Utilizing these metrics, the V5 and V6 models presented substantial improvements in reproducing the wave conditions of about 18% and 20%, respectively and in relation to the previous most complete and accurate wave model of the region (V4). Their better performance was largely achieved by a significant increment in their ability to reproduce the significant wave height (H_m0) and energy period (T_e). The inclusion of tidal currents in the wave model increased the accuracy of the wave resource characterization, mainly by improving the model’s ability in simulating T_e by 5.1%. The most sensitive wave parameter to the tidal currents was the peakedness of the wave spectrum (Q_p), which was consistently and significantly reduced by values even larger than 2.5. In some regions, directions characterized by the mean wave direction (D_m) and the directional spreading (D_spr) were also noticeably very sensitive to the currents, which even deflected D_m to its opposite direction and drove changes in D_spr that reached values of up to 40°. However, these significant transformations were less frequent and reduced in magnitude at exposed (to swell-waves) sites, where strong currents have affected waves in a reduced part of their trajectory. Typically, tidal currents had the effect of reducing the wave power density (P), but in a relatively small amount, however, during rare events, tidal currents were able to induce changes in this parameter ranging -140 kW/m to 75 kW/m. At these extreme events, it was observed that the peak of the wave spectra became flatter, with some of its wave height variance redistributed to near increasing and decreasing frequencies and directions, regardless to the magnitude and direction of the local tidal currents. Impacts of the tidal currents on P were largely attributed to the induced changes in H_m0 and T_e. Although D_spr and Q_p were greatly transformed by the action the tidal currents, they account very little in explaining the variations in P. These four wave parameters together, and how they are transformed under the presence of currents, can explain a large part of the changes in P, however, other transformations of the wave spectrum due to the currents, not investigated in this study, must account for a considerable part of the changes in P. / Graduate

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Civil Engineering, 1981. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING. / Bibliography: leaves 91-92. / by François-Marc Turpin. / M.S.

Thesis: Sc. D., Massachusetts Institute of Technology, Department of Civil Engineering, 1980 / Bibliography: leaves 173-175. / by David William Ostendorf. / Sc. D. / Sc. D. Massachusetts Institute of Technology, Department of Civil Engineering

Tidal variations of gravity were measured at fourteen sites in southeastern North America for periods of between 40 and 199 days. These measurements were used to obtain tidal gravity anomalies that indicate the geologic effect of the earth on tidal gravity. The tidal gravity anomaly is a vector quantity representing the difference between measured tidal gravity and the theoretical tidal gravity on a spherically symmetrical earth model subject to ocean tidal loading. The real part of the anomaly vectors include 8 values in the range of ±0.5 microgals, 4 values in the range of 0.5 to 1.5 microgals, 1 value of 1.5 to 2.5 microgals, and 1 other value in the range of -0.5 to -1.5 microgals, This grouping is consistent with a worldwide distribution of values from regions where the asthenosphere is at intermediate depth, the stress conditions are not excessive, and geothermal heat flow is approximately 60 mW/m². / Master of Science

Sand dune erosion is highly episodic occurring only when storm waves coincide with high tides generating swash that impacts the toe of the dune. Owing to the episodic nature of sand dune erosion, it is difficult to observe in nature. The removal of a structure and rip-rap sea-wall from the Stilwell Hall site located in southern Monterey Bay provided a unique opportunity to study erosion processes at an accelerated rate. A 1-D wave impact line erosion model (Larson et al., 2004) was tested against data acquired at this site between April, 2004 and April 2005. The model was optimally tuned to the data by a dimensionless coefficient that relates the impact force to the rate of recession. The coefficient values ranged from 0.7-1.3x10-3, for this field data, compared with values of 1.0-2.5x10-3 previously obtained for lab and field data. Migrating rip currents create a system of mega-cusps, which are nominally 10m in width and 200m in alongshore wavelength (Thornton, 2005). The presence of megacusps is hypothesized to accelerate sand dune erosion at their embayments where the beach is steeper and narrowest (Short, 1979;Shih and Komar, 1984;Revell, et al., 2002). It was determined that the highest recession occurred at the location of the rip current/mega-cusp embayment. Changes in the surf climate are of great interest to Naval Special Warfare (NSW) and U.S. Marine Corps (USMC) forces tasked with planning and executing operations in littoral areas. Naval history is replete with operations highlighting the importance of understanding and accurate prediction of nearshore dynamics. Without the ability to predict nearshore morphologic processes, providing such support is impossible.

Tides are the rise and fall of water level caused by gravitational forces exerted by the sun, moon and earth. Understanding sea level variation and its impact currents is very important especially in coastal regions. With knowledge of the tide-generating force and boundary conditions, hydrodynamic models can be used to predict or model tides in coastal regions. However, these models are not sufficiently accurate, and in-situ tide gauge data may be used to improve them in coastal regions. The International Hydrographic Organization (IHO) tidal data bank consists of over 4000 tide gauge stations scattered all around the globe, most of which are in coastal regions. These tide gauge data are very valuable for tidal predictions. One drawback of the IHO data is that a considerable number of stations are located in rivers or near man-made structures or small-scale, complex topographic features. Another drawback is the unknown accuracy of the IHO data. To avoid these drawbacks, quality control has been done in the present study. Each IHO tide gauge station has been categorized according to its proximity to rivers, lagoons, man-made harbors, and other factors that may influence tidal elevation. Quantitative metrics such as water depth, distance to the continental shelf break, and horizontal length scale of station site morphology have been computed. Comparisons among IHO data, the output of O.S.U. Tidal Inversion Software (OTIS), and other data sources, such as Global Sea-Level Observing System (GLOSS) data, have been done to test the quality and accuracy of IHO data. Moreover, the characteristics of stations with large error have also been examined. The good comparison of IHO with duplicate GLOSS stations shows that, as far as can be determined, IHO data are reliable and ought to be used in improving coastal tide models. The non-Gaussian character of the errors suggests that further improvements in tidal modeling will require advances in data assimilation which are robust to non-Gaussian data error.

Mullet Key is a right angle barrier island located at the mouth of Tampa Bay, west-central Florida. Based on historical shoreline data from 1873, the Gulf (west)-facing section of the beach has been dynamic illustrating large beach advances and retreats of up to 500 m on a decadal scale, while the south (channel)-facing section of the beach has shown to maintain a stable shoreline. This study focuses on the morphodynamics of the Gulf-facing beach. Since the 1920s, most of the Gulf-facing beach has been accreting except at the southern end near the Tampa Bay main channel. However, over the past 17 years, severe beach erosion has occurred along the northern portion of the island while accretion occurred along the middle portion. The southern end of the island has been maintained through artificial beach nourishments. Analysis of 27 aerial images from 1942 to 2014 revealed that the above large shoreline variations can be explained by the initiation, emergence, landward migrating, shoreline attachment, and post-attachment beach adjustment of the swash-bar complex on the Bunces Pass ebb delta. Two cycles of the swash-bar complex attachments with a period of approximately 30 years were identified from the aerial photos spanning 72 years. Twenty-eight beach-profiles spanning the 4 km Mullet Key Gulf-facing beach were surveyed 7 times on a bi-monthly basis from March 2014 to February 2015 to quantify the recent rapid changes, and to assess a yearly rate of shoreline change. Beach-profile analyses showed that the 120 m beach at the north-most tip in the immediate vicinity of Bunces Pass has lost a small amount of sediment. The 360 m beach to the south has gained some sediment. The 670 m stretch of beach further south has had significant shoreline retreat at a rate of 10-15 m/year. The 2,400 m section southward has experienced some gain of sediment, while the 370 m nourished beach at the southernmost tip has had slight retreat. This beach change pattern illustrates a diverging longshore sediment transport. Nearshore wave and current conditions were measured during a cold front passage in December 2014 to quantify the hydrodynamic processes that induced the diverging longshore transport. Three wave and current gauges were deployed along the eroding and accreting sections. The hydrodynamic data reveal that the longshore transport divergence is caused by diverging flood tidal flow into Bunces Pass to the north and Tampa Bay channel to the south. Furthermore, the waves in front the eroding beach were higher than the adjacent accreting beach.

Tidal currents and rivers are promising sources of renewable energy given that suitable turbines for kinetic energy conversion are developed. To be economically and technically feasible, a velocity distribution that can give a high degree of utilization (or capacity factor), while the ratio of maximum to rated velocity is low would be preferable. The rated velocity is defined as the velocity at which rated power is achieved. Despite many attempts to estimate the resource, however, reports on the possible degree of utilisation from tidal currents and rivers are scarce. In this paper the velocity distribution from a number of regulated rivers, unregulated rivers and tidal currents have been analysed regarding the degree of utilisation, the fraction of converted energy and the ratio of maximum to rated velocity. Two methods have been used for choosing the rated velocity; one aiming at a high fraction of converted energy and one aiming at a high degree of utilisation. Using the first method, with a rated velocity close to the maximum velocity, it is unlikely that the turbine will reach the cut-out velocity. This results in, on average, a degree of utilisation of 23% for regulated rivers, 19% for unregulated rivers and 17% for tidal currents while converting roughly 30-40% of the kinetic energy. Choosing a rated velocity closer to the mean velocity resulted in, on average, a degree of utilisation of 57% for regulated rivers, 52% for unregulated rivers and 45% for tidal currents. The ratio of maximum to rated velocity would still be no higher than 2.0 for regulated rivers, 1.2 for unregulated rivers and 1.6 for tidal currents. This implies that the velocity distribution of both rivers and tidal currents is promising for kinetic energy conversion. These results, however, do not include weather related effects or extreme velocities such as the 50-year velocity. A velocity factor is introduced to describe what degree of utilisation can be expected at a site. The velocity factor is defined as the ratio U-max/U-rate at the desired degree of utilisation, and serves as an early indicator of the suitability of a site.