Veden virtauksesta aiheutuneiden merenpohjan muutosten kuvailu sedimentin ominaisuuksien ja pohjaeläinyhteisöjen avulla : tapaustutkimus Joskärin hylyllä

Veden virtauksen kohdatessa merenpohjassa olevan objektin, virtauksen dynamiikka muuttuu. Veden liike kiihtyy, minkä seurauksena merenpohjassa olevan objektin ympärille muodsostuu pyörteitä. Virtausten intensiteetti vaihtelee etäisyyden muuttuessa suhteessa pohjassa olevaan objektiin. Vaihteleva virtauksen intensiteetti aiheuttaa sedimentin eroosiota, mikä havaitaan veden virtauksesta aiheutuneina merenpohjan topologisina muutoksina (engl. scour). Virtausten muokkaamaan merenpohjaan keskittyvät tutkimukset kuvailevat virtausten aikaansaamaa vaikutusaluetta usein ainoastaan kaikuluotaimilla. Veden virtauksen aiheuttamien mutosten vaikutus pohjaeläinyhteisöihin ja pienimittakaavaisiin sedimentin ominaisuuksiin ei kuitenkaan tallennu kaikuluotaimien tuottamiin kuviin merenpohjan geomorfologiasta. Tässä maisterintutkielmassa Joskärin historiallisen puurunkoisen purjealuksen hylyn ympärillä olevia virtausten aiheuttamia pohjan muutoksia kuvattiin ensin viistokaikuluotaamalla ja määrittämällä merenpohjan syvyys sukeltamalla. Tämän jälkeen sedimenttinäytteitä kerättiin alueelta, jonka oletettiin olevan suurimman virtausten luoman vaikutuksen alaisuudessa. Näytteitä otettiin kolmen eri etäisyyden päästä hylystä, kahden identtisen hylystä poispäin vedetyn linjan varrelta. Näytteistä analysoitiin orgaanisen aineen pitoisuus, sedimentin raekoko, ja pohjaeläinyhteisöjen lajikoostumus. Pohjaeläinyhteisöistä määritettiin lisäksi yksilömäärät, yksilöpituudet, diversiteetti-indeksi, ja funktionaaliset ryhmät. Kaikki näytteet kerättiin putkinoutimella sukeltajan toimesta. Tässä maisterintutkielmassa hyödynnettyjen menetelmien välityksellä käsitys veden virtausten vaikutuksesta merenpohjan ominaisuuksiin laajeni kaikuluotaimen tuottamasta geomorfologisesta tiedosta kokonaisvaltaisemmalle tasolle, jossa otetaan pohjanmuotojen lisäksi huomioon sedimentin biologiset, geologiset ja kemialliset ominaisuudet. Tämän maisterintutkielman tulokset näyttävät sedimentin ominaisuuksien vaihtelevan Joskärin hylyn ympäristössä suhteellisen pienellä mittakaavalla. Sedimentin orgaanisen aineen pitoisuus oli tehokkain veden virrtausten ja pohjalla olevan objektin aiheuttamien muutosten osoittava ominaisuus Joskärin hylyllä. Sen pitoisuus ilmensi johdonmukaisesti vähenevän kehityssuunnan molemmilla näytteenottolinjoilla etäisyyden hylkyyn kasvaessa. Sedimentin raekoko muuttui hienommaksi kauempana hylystä, mutta sedimenttinäytteiden visuaalisen tarkastelun perusteella sedimentissä olevan kuolleen simpukankuoriaineksen määrä voisi mahdollisesti toimia raekokoa parempana virtausten ja pohjalla olevan objektin aiheuttaman pohjanmuutoksen ilmentäjänä. Pohjaeläinyhteisöjen ominaisuuksien vaihtelu etäisyyden funktiona ei ole toimiva virtausten aiheuttamien muutosten ilmentäjä Joskärin hylyllä, sillä johdonmukaisia vuorovaikutussuhteita pohjaeläinyhteisöjen ominaisuuksien ja hylyn etäisyyden välillä ei paikannettu. Koska tutkimussuunnitelmaan ei sisällytetty kontrollinäytteitä, Joskärin hylkyä ympäröivän vedenvirtauksen vaikutusalueen ominaisuuksia ei voitu verrata tutkimusalueen ulkopuolisen pohjan ominaisuuksiin. Jatkotutkimuksilla voitaisiin selvittää mahdolliset eroavaisuudet virtausten ja pohjalla olevan objektin vaikutusalueen sedimenttien sekä vaikutusalueen ulkopuolella olevien sedimenttien välillä. Tämän tutkielman hypoteesina pohjaeläinyhteisöjen sekä sedimentin ominaisuuksien oletettiin vaihtelevan etäisyyden funktiona Joskärin hylystä. Sedimentin ominaisuuksien ilmentämät johdonmukaiset kehityssuunnat suhteessa etäisyyteen hylystä varmistavat osan hypoteesista todeksi, mikä vahvistaa sedimentin ominaisuuksien käyttöarvoa veden virtauksen aiheuttamien muutosten ilmentämisessä Joskärin hylyllä. Osa hypoteesista kuitenkin hylätään, sillä pohjaeläinyhteisöjen ominaisuuksia tutkittaessa johdonmukaisia kehityssuuntia ei löytynyt.As water flow encounters an object on the sea floor, its hydrodynamics change. Accelerated currents and vortices develop around the object with changing intensity as a function of distance from its proximity. This leads to erosion and aggradation of sediment, known as scour. Studies focusing on formation processes of scour often involve locating visible scour sites by sonar scanning the geomorphology of the seafloor. However, the effects of scour on macroinfauna and small-scale sediment characteristics are not visible in sonar images. In this Master’s thesis, scour at a shipwreck of a timber-built historic sailing ship, the Joskär shipwreck, was first identified by scanning the study area with side-scan sonar, and by measuring water depth contours around the shipwreck by scuba diving. Sediment samples were then taken inside the area assumed to be under the most pressure from scour. Samples from three separate distances on two transects drawn outwards from the hull of the shipwreck were collected and analysed for sediment grain size, organic content, and species assemblages of macroinfauna. In addition, macrofauna were analysed for individual lengths, number of individuals, diversity index, and functional groups. All samples were collected with a core tube sampler operated by a scuba diver. The methods used in this Master’s thesis widen the concept of scour past the sole physical processes observable with sonar to a more holistic level that considers the quality of biological, geological, and chemical characteristics of the benthic environment. The results of the present Master’s thesis show that the quality of the sediment near Joskär shipwreck varies within a relatively small scale. Organic content of the sediment was the most potent descriptor of scour at the study site, exhibiting a consistent decreasing trend as distance to the shipwreck increased on both sampled transects. Sediment grain size became finer as distance to the shipwreck increased. However, compared to grain size, based on visual observations of the sediment samples, shell debris content of the sediment could possibly act as a better measure of presence of scour. The variability of characteristics of macroinfaunal communities as a function of distance from Joskär shipwreck was not a viable tool to describe the presence of scour, as no consistent trends of the variables were observed. As no control site was included in the study design, the characteristics of the benthic environment inside the scour around Joskär shipwreck could not be compared to the seafloor unaffected by scour. Further research could reveal possible variation between these distinct habitats, and that way produce valuable indicators of scour. The hypothesis in the present thesis was that macroinfaunal assemblages and sediment characteristics would exhibit variation between the sampling sites as a function of distance from the shipwreck. The observed trends of sediment characteristics validated a part of the hypothesis, showcasing the utility of sediment characteristics in describing scour at Joskär shipwreck. However, a part of the hypothesis was rejected, as no consistent trends of macroinfaunal features were present

Structure and dynamics of solitary waves in fluid media

This research deals with the study of nonlinear solitary waves in fluid media. The equations which model surface and internal waves in fluids have been studied and used in this research. The approach to study the structure and dynamics of internal solitary waves in near-critical situations is the traditional theoretical and numerical study of nonlinear wave processes based on the methods of dynamical systems. The synergetic approach has been exploited, which presumes a combination of theoretical and numerical methods. All numerical calculations were performed with the desktop personal computer. Traditional and novel methods of mathematical physics were actively used, including Fourier analysis technique, inverse scattering method, Hirota method, phase-plane analysis, analysis of integral invariants, finite-difference method, Petviashvili and Yang–Lakoba numerical iterative techniques for the numerical solution of Partial Differential Equation. A new model equation, dubbed the Gardner–Kawahara equation, has been suggested to describe wave phenomena in the near-critical situations, when the nonlinear and dispersive coefficients become anomalously small. Such near-critical situations were not studied so far, therefore this study is very topical and innovative. Results obtained will shed a light on the structure of solitary waves in near-critical situation, which can occur in two-layer fluid with strong surface tension between the layers. A family of solitary waves was constructed numerically for the derived Gardner–Kawahara equation; their structure has been investigated analytically and numerically. The problem of modulation stability of quasi-monochromatic wave-trains propagating in a media has also being studied. The Nonlinear Schrödinger equation (NLSE) has been derived from the unidirectional Gardner–Ostrovsky equation and a general Shrira equation which describes both surface and internal long waves in a rotating fluid. It was demonstrated that earlier obtained results (Grimshaw & Helfrich, 2008; 2012; Whitfield & Johnson, 2015a; 2015b) on modulational stability/instability are correct within the limited range of wavenumbers where the Ostrovsky equation is applicable. In the meantime, results obtained in this Thesis and published in the paper (Nikitenkova et al., 2015) are applicable in the wider range of wavenumbers up to k = 0. It was shown that surface and internal oceanic waves are stable with respect to selfmodulation at small wavenumbers when k → 0 in contrast to what was mistakenly obtained in (Shrira, 1981). In Chapter 4 new exact solutions of the Kadomtsev-Petviashvili equation with a positive dispersion are obtained in the form of obliquely propagating skew lumps. Specific features of such lumps were studied in details. In particular, the integral characteristics of single lumps (mass, momentum components and energy) have been calculated and presented in terms of lump velocity. It was shown that exact stationary multi-lump solutions can be constructed for this equation. As the example, the exact bilump solution is presented in the explicit form and illustrated graphically. The relevance of skew lumps to the real physical systems is discussed

Dune dynamics under unsteady flows

River bedforms keep translating and deforming perpetually during their migration. During flow field unsteadiness they also change in size and shape over time and in space. However, our knowledge of how bedforms adapt to changing flows remains inadequately understood. Therefore, how do dunes adapt to different changing flows (floods and tides), and how does the coupled sediment transport affect dune morphology and dynamics is urgent to be investigated.Large-scale flume experiments were conducted to simulate dune dynamics during carefully controlled floods with various hydrographs, while field surveys were undertaken in both the middle reach, close to the backwater zone, and within the estuary of the Changjiang (Yangtze) River, in order to examine the combined effect of the flood and tide on dune evolution.The result indicates that the sediment transport mechanisms dominate how dunes adapt to unsteady flows. The analysis of dune three-dimensionality reflects that the generation of the larger dunes is the main factor controlling sediment transport and thereby the bedform adaptation. Moreover, the processes of bedform adaptation to changing flows will be varying under different sediment transport mechanisms due to diverse sediment redistribution over and between dunes.In the tidally influenced area where riverbed is composed of fine sediment, our result implies that clay content is a first-order control on bedform aspect ratio and the specific sediment composition of the riverbed, in some extent, affects the mechanism of sediment transport related to the exchange between suspended sediment and riverbed.This work extends our knowledge on how dunes generate and develop under variable flows and has delineated how variations in transport stage can be coupled with the variation in the dominant sediment transport mechanisms. Moreover, these developments will provide fundamental knowledge that is of significance for a wide variety of purposes, such as improving morphodynamic modelling over large spatio-temporal scales, environmental and engineering management, and more reliable flood predictions

Novel Acoustic Methods for Directly Monitoring Seabed Sediment Transport, Geohazards & Scour

In the natural environment, sediment transport processes can pose significant hazards to marine infrastructure, such as offshore wind turbines or seabed cables that carry both power onshore as well as carrying over 99% of global data. These processes are often extremely challenging to measure directly because sensors can be easily damaged by the processes themselves. It would, therefore, be highly advantageous to remotely sense and quantify sediment transport via sensors that are located outside the region of sediment transport. One way to do this is via sensors higher in the water column that detect acoustic signals emitted by sediment transport processes closer to the bed. Previous work such as Wren et al. (2015), Marineau et al. (2016), and Le Guern et al. (2021) have started to develop passive acoustic methods to record signals from sediment transport, using tools such as hydrophones and acoustic Doppler current profilers (ADCPs). Normally, ADCPs actively emit their own acoustic pulses, and their reflections are used to monitor flow velocities and concentrations. However, with modification to extend their listening times, ADCP’s can also be used to passively record acoustic signals emitted by sediment transport processes. Thus far, the potential of these passive acoustic methods have not been fully developed, and the fundamental controls that determine the type of acoustic signals produced are not yet fully understood. This PhD sought to understand what controls the nature (frequencies, strength etc) of these signals and, thus, what they can tell us about sediment transport processes (Thorne, 1985,1986,1990,2014; Rigby et al. 2016). It aims to do this using a combination of laboratory experiments (Chapter 2) and detailed fieldwork (Chapters 3 and 4) using acoustic signals passively emitted by sediment flows. In addition, the thesis includes work testing the use of active acoustic methods to monitor sediment transport processes within the natural environment, specifically seabed sediment flows (called turbidity currents) (Chapter 5). Results from this thesis found a general relationship between the strength of self-generated noise and flow speed in some types of sediment flows (Chapters 2, 3 and 4). However, the strength of this relationship changes depending on the frequency and details of the environment investigated. Field data from the Río Paraná (Chapter 3) suggested no relationship between bedload flux and acoustic signal strength, nor between acoustic signal strength and friction velocity. This is unexpected because previous research by Sime et al. (2007), Hossein and Rennie (2009), Hatcher (2017), Hay et al. (2021) and Le Guern et al. (2021) proposed links between flow speed (and bed shear stress and bedload transport) and passively detected noise strength. Passive acoustic signals generated by turbidity currents were used to monitor these flows in a set of submarine canyons, which were Bute Inlet (Canada), Monterey Canyon (offshore California), and the Congo Canyon (offshore West Africa) (Chapter 4). Noticeable variations in the level of passively detected noise between these three field sites were observed. These variations are thought to be related to the main sediment grain size present within each canyon, with lower noise being detected with an increasing mud content of the seabed. In addition, differences in noise down submarine canyons suggest that flow processes and concentration could be controlling the level of sediment-generated noise, with implications of flow field dynamics. Chapter 5 uses one of the most detailed (near-daily) series of multibeam swath bathymetry surveys yet collected, which come from within Bute Inlet, Canada, in September 2022. This unusual set of field observations is used to understand the relationship between flow evolution and the initiation mechanism of turbidity currents. For example, the Bute Inlet study supports the findings from Hizzett et al. (2018) that there is no link between the initiation mechanism and runout distance of a turbidity current. Further research is needed to improve understanding of the controls on acoustic signals in the natural environment, and to also improve our ability to use acoustic signals to monitor sediment transport in a wider range of environments, such as around offshore wind farms

The geological evolution and sedimentary dynamics of Hout Bay, South Africa

Includes bibliographical references.Hout Bay is situated on the Atlantic seaboard of the Cape Peninsula, in the Western Cape Province of South Africa approximately 17 km southwest of Cape Town. Hout Bay is a southward opening bay that hosts a fishing harbour and coastal residential town. This study was initiated to map the marine geology of Hout Bay and to quantify and explain the sediment dynamics of the area. This is important as Hout Bay has the only substantial accumulation of Quaternary sediments on the Atlantic Seaboard of the Cape Peninsula. The Hout Bay study area was saturated with the latest in cutting-edge geophysical techniques to collect detailed and comprehensive bathymetric, sidescan sonar, magnetic, seismic and beach profiling data. Collectively these data can be used to map offshore geological units as well as infer how Hout Bay has responded to the varying changes in sea-level throughout the Quaternary and allow for the reconstruction of the geological evolution of the Hout Bay seafloor

ACOUSTIC METHODS FOR MAPPING AND CHARACTERIZING SUBMERGED AQUATIC VEGETATION USING A MULTIBEAM ECHOSOUNDER

Submerged aquatic vegetation (SAV) is an important component of many temperate global coastal ecosystems. SAV monitoring programs using optical remote sensing are limited by water clarity and attenuation with depth. Here underwater acoustics is used to analyze the water volume above the bottom to detect, map and characterize SAV. In particular, this dissertation developed and applied new methods for analyzing the full time series of acoustic intensity data (e.g., water column data) collected by a multibeam echosounder. This dissertation is composed of three separate but related studies. In the first study, novel methods for detecting and measuring the canopy height of eelgrass beds are developed and used to map eelgrass in a range of different environments throughout the Great Bay Estuary, New Hampshire, and Cape Cod Bay, Massachusetts. The results of this study validated these methods by showing agreement between boundaries of eelgrass beds in acoustic and aerial datasets more in shallow water than at the deeper edges, where the acoustics were able to detect eelgrass more easily and at lower densities. In the second study, the methods developed for measuring canopy height in the first study are used to delineate between kelp-dominated and non-kelp-dominated habitat at several shallow rocky subtidal sites on the Maine and New Hampshire coast. The kelp detection abilities of these methods are first tested and confirmed at a pilot site with detailed diver quadrat macroalgae data, and then these methods are used to successfully extrapolate kelp- and non-kelp-dominated percent coverages derived from video photomosaic data. The third study examines the variability of the acoustic signature and acoustically-derived canopy height under different tidal currents. Submerged aquatic canopies are known to bend to accommodate the drag they generate in response to hydrodynamic forcing, and, in turn, the canopy height measured by acoustics will not be a perfect representation of canopy height as defined by common seagrass monitoring protocols, which is usually measured as the length of the blade of seagrass. Additionally, the bending of the canopy affects how the blades of seagrass are distributed within the footprint of the sonar, changing the acoustic signature of the seagrass canopy. For this study, a multibeam echosounder, a current profiler and an HD video camera were deployed on a stationary frame in a single eelgrass bed over 2 tidal cycles. Acoustic canopy heights varied by as much as 30 cm over the experiment, and although acoustic canopy height was correlated to current magnitude, the relationship did not follow the predictive flexible vegetation reconfiguration model of Luhar and Nepf (2011). Results indicate that there are significant differences in the shape of the return from a deflected (i.e., bent-over) canopy and an upright canopy, and that these differences in shape have implications for the accuracy of bottom detection using the maximum amplitude of a beam time series. These three studies clearly show the potential for using multibeam water column backscatter data for mapping coastal submerged aquatic vegetation while also testing the natural variability in acoustic canopy height measurements in the field

Holocene sedimentology and stratigraphy of coastal sediments in an arid climate, Al Qahmah, Southern Red Sea, Saudi Arabia

Holocene sedimentary deposits in Al Qahmah coast, southern Red Sea, in Saudi Arabia have been accumulated on an arid siliciclastic coast in a rift tectonic setting. This study provides a detail interpretation and analysis of sedimentary facies, stratigraphy, depositional environment, and major geological processes. Two studies have been undertaken. First, petrological study of composition and texture of modern sediments and facies mapping delineate facies distribution and mechanisms of sediment transport and deposition. A unique suite of complex riverine, wave, tidal, wind, and chemical and biological processes affect the composition, texture, and distribution of the facies. Second, the depositional environments of the Holocene deposits were interpreted on the basis of detailed lithofacies analysis and lithostratigraphic correlation, along with absolute age dating, using subsurface trenching and seismic refraction tomography (SRT) and electrical resistivity tomography (ERT) methods. Two major facies, underlying carbonate and overlying siliciclastic facies, were delineated. The carbonate facies are interpreted as shallow normal marine deposits. The siliciclastic facies are marginal marine and nonmarine coastal deposits. A disconformity and an erosional unconformity are identified, across which significant changes in tectonic and climatic conditions had occurred. This study presents a detailed sedimentologic, stratigraphic, environmental, and paleoclimatic reconstruction in modern and recent arid siliciclastic coast in a rift tectonic setting --Abstract, page iv

Experimental, Numerical and Field Approaches to Scour Research

This book presents fourteen state-of-the-art research papers prepared by research scientists and engineers around the world. They explore the subject of scour related to bridge piers, monopiles, propellers, turbines, weirs, dams, grade-control structures, and pipelines. Their works are based on three different research methodologies, namely experimental, numerical, and field approaches

The taphonomy of historic shipwreck sites

An understanding of the extent to which materials and energy are free to exchange across boundaries at shipwreck sites is fundamental to the archaeological interpretation of these unique resources. The limited previous work on the dynamics of shipwreck sites suggest that they can act as either near-closed systems (e.g. Mary Rose), or open systems at some state of dynamic/quasi- equilibrium with respect to their surroundings’ (e.g. Stirling Castle). Nonetheless, our understanding of the temporal evolution of shipwreck sites and thus, whether they are open or closed systems, is extremely limited. This thesis presents repeat (intra-annual; annual; and decadal) Multibeam Echosounder (MBES) surveys for five shipwreck sites (the largest published collection of shipwreck site MBES time-series to date) from a range of environments: the Richard Montgomery, tidally dominated (weakly asymmetrical); the Scylla, storm dominated; the Burgzand Noord site, tidally dominated (strongly asymmetrical); the Stirling Castle, dominated by large-scale geomorphological processes; and the Algerian, sheltered. By quantifying the temporal variability (through bed-level change plots) and the Metocean, geological and geomorphological conditions of these wreck sites, the impact of the differing marine environments on the wreck site’s taphonomic pathway was constrained. Through the collation of these MBES time-series the importance of being able to account for the uncertainty of the data when comparing two time steps was realised. To this end, a robust methodology for assessing the uncertainty of the MBES data was developed for the use with marine MBES data. The spatial patterns of scouring and deposition were accounted for through the application of the simple principles of scouring around bluff obstacles (cylinders, cuboids and piers etc.). Those sites which experienced a disturbance during the observation period (e.g. a storm event at the Scylla, sandbank migration at the Stirling Castle and the implementation of physical protection at the Burgzand Noord site) underwent a larger range of bed-level change and altered dramatically in their scour/deposition arrangement. Those sites at quasi-equilibrium (SS Richard Montgomery, Algerian and Scylla for the final time-step) underwent no perceivable net bed-level change over the observation period and had stable scour and deposition features. The comprehension of shipwreck site taphonomy gained through this thesis is fundamental to the efficacy of heritage management, allowing protective measures to be site-tailored and fills a large data- and knowledge-gap in the long term (multi-annual) evolution of scour around marine anthropogenic structures.<br/