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Seabed Integrity Around Objects in a Complex Marine Environment.

紀錄類型:	書目-電子資源 : Monograph/item
正題名/作者:	Seabed Integrity Around Objects in a Complex Marine Environment./
作者:	Yiannoukos, Irinios.
面頁冊數:	1 online resource (320 pages)
附註:	Source: Dissertations Abstracts International, Volume: 84-06, Section: A.
Contained By:	Dissertations Abstracts International84-06A.
標題:	Deposition. -
電子資源:	http://pqdd.sinica.edu.tw/twdaoapp/servlet/advanced?query=30168565click for full text (PQDT)
ISBN:	9798358404816

Seabed Integrity Around Objects in a Complex Marine Environment.
Yiannoukos, Irinios.

Seabed Integrity Around Objects in a Complex Marine Environment. - 1 online resource (320 pages)

Source: Dissertations Abstracts International, Volume: 84-06, Section: A.

Thesis (Ph.D.)--Bangor University (United Kingdom), 2022.

Includes bibliographical references

The ever-increasing need for seabed infrastructure enabling offshore energy extraction and storage requires accurate prediction of erosion and deposition rates of the surrounding seafloor sediments. Seabed erosion and deposition (scour) can become catastrophic for the infrastructure itself but also for the surrounding habitats. Scour predictors largely assume that the bed is composed of unimodal sediments and objects have simple shapes. In mixed non-cohesive beds, however, complexity in the sediment transport process, including the hiding exposure (HE) effect, will likely impact the seabed erosion and deposition. Better understanding of scour dynamics in more complex settings can improve the ability to predict scour development around differently shaped objects sitting on mixed and coarse beds, which are ubiquitous in palaeo-glaciated environments, for instance. In this work, the scour development around the 135 m long wreck of the ship SS Apapa was investigated through time-lapse analyses of multi-beam echosounder surveys over nine years (co-registered bathymetry and backscatter intensity), hydrodynamic measurements and sediment samples. Due to the interaction between the tidal flow and the wreck, flow velocities increased by about ~2.3 times downstream of the wreck, with the highest amplifications where an undisturbed flow first encounters the wreck. The extent of flow disturbance in the wake of the wreck measured between 0.76 and 2.3 times the length of the wreck, with the highest disturbance at slack tides. Vertical flow disturbance of up to 1.66 times the height of the wreck (17.5 m) was also identified directly over the wreck, with nearly a doubling of flow velocities. Laterally, a flow diversion of 90° was identified directly over the wreck. Two consistent 'zones of bed mobility' exist at either side of the wreck, at distances between 0.27 and 1 times the length of the wreck. The extent of these zones was identified by a drop in the bed mobility accompanied by a reduction of the flow speed between 30% and 35% when compared to the undisturbed background flow. The seafloor at the SS Apapa area was composed of mixed coarse sediment, with coarser material present in the deepest parts of the scour mark and finer material present at the depositional features. MBES datasets (bathymetric and backscatter) showed that the depositional feature remained fine over the years but was the most variable in bathymetry. Large variations in bed composition were also observed at the deepest points of the scour mark, where bathymetry varied less over time. A disintegration and shifting of SS Apapa between March 2018 and June 2019 changed the exposure of the wreck to the flow at the north-east and north-west sides of the wreck. This altered the hydrodynamics of the area and subsequently changed the erosional and depositional trends observed until 2018. To investigate the role of the sediment mixture in scour dynamics in more detail, flume laboratory experiments were conducted with two flow speeds over a 9.4 cm long cylinder on a bed, using six sand and gravel mixtures, pure sand, and pure gravel. One lower flow speed mobilised just the sand fraction, whilst a higher flow speed mobilised both the sand and gravel fractions. The bed was acoustically scanned three times for each run and sediment cores were taken to analyse changes to bed composition laterally and with depth. For the lower flow, mobilising only the sand fraction, the scour mark was 66.4% longer, 12.1% deeper and 4.8% wider in a bed composed of 20% gravel and 80% sand, when compared to the pure sand bed.

Electronic reproduction.
Ann Arbor, Mich. :
ProQuest,
2023

Mode of access: World Wide Web

ISBN: 9798358404816Subjects--Topical Terms:

3560744
Deposition.
Index Terms--Genre/Form:

542853
Electronic books.

Seabed Integrity Around Objects in a Complex Marine Environment.
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500 $a Source: Dissertations Abstracts International, Volume: 84-06, Section: A.
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502 $a Thesis (Ph.D.)--Bangor University (United Kingdom), 2022.
504 $a Includes bibliographical references
520 $a The ever-increasing need for seabed infrastructure enabling offshore energy extraction and storage requires accurate prediction of erosion and deposition rates of the surrounding seafloor sediments. Seabed erosion and deposition (scour) can become catastrophic for the infrastructure itself but also for the surrounding habitats. Scour predictors largely assume that the bed is composed of unimodal sediments and objects have simple shapes. In mixed non-cohesive beds, however, complexity in the sediment transport process, including the hiding exposure (HE) effect, will likely impact the seabed erosion and deposition. Better understanding of scour dynamics in more complex settings can improve the ability to predict scour development around differently shaped objects sitting on mixed and coarse beds, which are ubiquitous in palaeo-glaciated environments, for instance. In this work, the scour development around the 135 m long wreck of the ship SS Apapa was investigated through time-lapse analyses of multi-beam echosounder surveys over nine years (co-registered bathymetry and backscatter intensity), hydrodynamic measurements and sediment samples. Due to the interaction between the tidal flow and the wreck, flow velocities increased by about ~2.3 times downstream of the wreck, with the highest amplifications where an undisturbed flow first encounters the wreck. The extent of flow disturbance in the wake of the wreck measured between 0.76 and 2.3 times the length of the wreck, with the highest disturbance at slack tides. Vertical flow disturbance of up to 1.66 times the height of the wreck (17.5 m) was also identified directly over the wreck, with nearly a doubling of flow velocities. Laterally, a flow diversion of 90° was identified directly over the wreck. Two consistent 'zones of bed mobility' exist at either side of the wreck, at distances between 0.27 and 1 times the length of the wreck. The extent of these zones was identified by a drop in the bed mobility accompanied by a reduction of the flow speed between 30% and 35% when compared to the undisturbed background flow. The seafloor at the SS Apapa area was composed of mixed coarse sediment, with coarser material present in the deepest parts of the scour mark and finer material present at the depositional features. MBES datasets (bathymetric and backscatter) showed that the depositional feature remained fine over the years but was the most variable in bathymetry. Large variations in bed composition were also observed at the deepest points of the scour mark, where bathymetry varied less over time. A disintegration and shifting of SS Apapa between March 2018 and June 2019 changed the exposure of the wreck to the flow at the north-east and north-west sides of the wreck. This altered the hydrodynamics of the area and subsequently changed the erosional and depositional trends observed until 2018. To investigate the role of the sediment mixture in scour dynamics in more detail, flume laboratory experiments were conducted with two flow speeds over a 9.4 cm long cylinder on a bed, using six sand and gravel mixtures, pure sand, and pure gravel. One lower flow speed mobilised just the sand fraction, whilst a higher flow speed mobilised both the sand and gravel fractions. The bed was acoustically scanned three times for each run and sediment cores were taken to analyse changes to bed composition laterally and with depth. For the lower flow, mobilising only the sand fraction, the scour mark was 66.4% longer, 12.1% deeper and 4.8% wider in a bed composed of 20% gravel and 80% sand, when compared to the pure sand bed.
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