High-resolution Bathymetric Surveys Research Articles

Sedimentological, Geochemical, and Environmental Assessment in an Eastern Mediterranean, Stressed Coastal Setting: The Gialova Lagoon, SW Peloponnese, Greece

This study describes the prevalent sedimentological and geochemical patterns and investigates the environmental status of the bottom of Gialova lagoon, a highly vulnerable coastal site of the EU’s Natura 2000 network. For this task, lithological, geochemical, and microfaunal analyses of sediment samples were combined with a high-resolution bathymetric survey. Potential pollution was determined using geochemical-based (EF, I-geo, and PLI) and faunal (Foram-AMBI) indices. We find that sedimentation is mainly controlled by the bottom morphology, hydrodynamic variations, and biogenic productivity of the lagoon. The application of the multivariate factor analysis technique revealed four dominant factors explaining the geochemical processes occurring in the lagoon. The first factor, namely “terrigenous aluminosilicates associated with Corg vs. autochthonous biogenic carbonates”, discriminates the deposition of detrital sediments, related to the high adsorption of heavy metals—versus bioclastic sediments. The “sulfides” factor represents an anoxic phase of the lagoon floor, whereas the “Mn-oxyhydroxides” factor indicates increased manganese content with several compounded trace elements. The “phosphate” factor reveals multiple sources of phosphorus in the lagoon. The lagoon bottom shows negligible to minor contamination in heavy metals, except Mo and Pb, which induce moderate pollution levels. The maximum contamination and environmental stress concern two small-sized, shallow basins within the lagoon.

Výsledky druhej fázy geologického prieskumu hlbokomorských polymetalických konkrécií v prieskumnom území Spoločnej organizácie Interoceanmetal

The exploration rights of the Interoceanmetal Joint Organization for exploration of polymetallic nodules (PMN) are granted from 29 March 2001 to an area located within the Clarion-Clipperton Zone (CCZ) in the eastern central Pacifi c Ocean. Exploration area covers 75,000 km2 and consists of two sectors (B1 and B2). The B2 sector comprises four exploration blocks (H11, H22, H33 and H44). The most prospective area, selected for detailed research, is marked as H22_NE exploitable block and delineated within the H22 exploration block. The article presents results of geological survey, based mostly on the data collected during the second phase of exploration in the licence area (2016–2021, extension of the contract). Results are based on IOM’s expeditions and relevant analytical work. During the IOM-2018 expedition high resolution bathymetric survey of H11, H22, H33 and H44 exploration blocks was carried out. The IOM-2019 expedition provided a new set of the data obtained using the distance methods (side-scan sonar, profi ler) and contact methods (box-corer and gravity corer) in H22_NE exploitable block, H33 exploration block and preliminary delineated Preservation Reference Zone. The study was focused on analytical work based on sediment and nodule analyses of samples in H22 exploration block and H22_NE exploitation block. New estimation of mineral resources in B2 sector was caried out using the geostatistical method of ordinary block kriging with Yamamoto correction. The polymetallic nodule resources have been classifi ed within the Inferred, Indicated and Measured Resources categories of the CRIRSCO classification system.

Discovery of active off-axis hydrothermal vents at 9° 54′N East Pacific Rise

Comprehensive knowledge of the distribution of active hydrothermal vent fields along midocean ridges is essential to understanding global chemical and heat fluxes and endemic faunal distributions. However, current knowledge is biased by a historical preference for on-axis surveys. A scarcity of high-resolution bathymetric surveys in off-axis regions limits vent identification, which implies that the number of vents may be underestimated. Here, we present the discovery of an active, high-temperature, off-axis hydrothermal field on a fast-spreading ridge. The vent field is located 750 m east of the East Pacific Rise axis and ∼7 km north of on-axis vents at 9° 50'N, which are situated in a 50- to 100-m-wide trough. This site is currently the largest vent field known on the East Pacific Rise between 9 and 10° N. Its proximity to a normal fault suggests that hydrothermal fluid pathways are tectonically controlled. Geochemical evidence reveals deep fluid circulation to depths only 160 m above the axial magma lens. Relative to on-axis vents at 9° 50'N, these off-axis fluids attain higher temperatures and pressures. This tectonically controlled vent field may therefore exhibit greater stability in fluid composition, in contrast to more dynamic, dike-controlled, on-axis vents. The location of this site indicates that high-temperature convective circulation cells extend to greater distances off axis than previously realized. Thorough high-resolution mapping is necessary to understand the distribution, frequency, and physical controls on active off-axis vent fields so that their contribution to global heat and chemical fluxes and role in metacommunity dynamics can be determined.

Understanding meso-scale processes at a mixed-energy tidal inlet: Ameland Inlet, the Netherlands – Implications for coastal maintenance

For successful and sustainable management of barrier islands, a thorough understanding of the ebb-tidal delta dynamics and interactions with the adjacent shorelines are of the utmost importance. Such understanding requires detailed observations and interpretations of the morphodynamics of smaller-scale features such as the individual channels and shoals (referred to as intra-delta dynamics). The intra-delta dynamics of Ameland Inlet (the Netherlands) are studied through analysis of sixteen high-resolution bathymetric surveys, supplemented with an extensive dataset of hydrodynamic observations collected in 2017. The observations are compiled into a synthesis of the morphodynamics of the ebb-tidal delta and its neighboring shorelines, to provide a basis for present day and future coastal management.Our observations show that Ameland Inlet as a whole can be classified as a typical mixed-energy, wave-dominated system. However, the ebb-tidal delta contains distinct areas that are wave or tide dominated, and these areas evolve with the changing morphodynamics of the ebb delta. Between 2005 and 2021, large morphodynamic changes have occurred on the ebb-tidal delta and continuous erosion of the island tips occurred. Limited wave-sheltering by the ebb-tidal delta exposes the shorelines of the adjacent barrier islands to significant wave-driven sand transports and sand losses. Sediment supply from longshore transport and the erosion of the updrift island Terschelling contributed to the formation, growth and migration of a series of ebb-chutes and lobes, which eventually led to complete relocation of the main channel on the ebb-tidal delta. This main channel relocation took 15 years to complete and is an example of the ebb-delta breaching model of sand bypassing. Changes in the sediment bypassing patterns result in a sediment starved western island tip of Ameland, necessitating repeated sand nourishments under the Dutch coastal maintenance policy. Our observations also confirm the role the ebb-tidal delta as a sand reservoir for the downdrift barrier island. The delta sand body is not a reservoir for the back-barrier basin, since the basin is predominantly supplied with sand eroded from the updrift island of Terschelling.As demonstrated in this study, the intra-delta dynamics of an ebb-tidal delta are complex and can change drastically through time. Only through detailed measurements and observations can all the intricate interactions that take place be unravelled.

Rapid seafloor changes associated with the degradation of Arctic submarine permafrost

SignificanceTemperature increases in Arctic regions have focused attention on permafrost degradation on land, whereas little is known about the dynamics of extensive glacial-age permafrost bodies now submerged under the vast Arctic Continental shelves. Repeated high-resolution bathymetric surveys show that extraordinarily rapid morphologic changes are occurring at the edge of the continental slope of the Canadian Beaufort Sea along what was once the seaward limit of relict Pleistocene permafrost. How widespread similar changes are on the Arctic shelves is unknown, as this is one of the first areas in the Arctic subjected to multiple multibeam bathymetric surveys. Rapid morphologic changes associated with active submarine permafrost thawing may be an important process in sculpturing the seafloor in other submarine permafrost settings.

Hadal Biodiversity, Habitats and Potential Chemosynthesis in the Java Trench, Eastern Indian Ocean

The Java Trench is the only subduction trench in the Indian Ocean that extends to the hadal zone (> 6,000 m water depth), and except for sevenbenthic trawls acquired around the 1950s, there has been little to no sampling at hadal depths undertaken since. In 2019, we undertook a 5-day expedition comprising a scientific dive using a full ocean depth-rated submersible, the DSV Limiting Factor, seven hadal-lander deployments, and high-resolution bathymetric survey. The submersible performed a video transect from the deepest point of the trench, up a 150 m high near-vertical escarpment located on the forearc, and then across a plateau at a depth of ∼7,050 m to make in situ observations of the habitat heterogeneity and biodiversity inhabiting these hadal depths. We found the Java Trench hadal community to be diverse and represented by 10 phyla, 21 classes, 34 orders and 55 families, with many new records and extensions in either depth or geographic range, including a rare encounter of a hadal ascidian. The submersible transect revealed six habitats spanning the terrain. The deepest trench axis comprised fine-grained sediments dominated by holothurians, whereas evidence of active rock slope failure and associated talus deposits were prevalent in near-vertical and vertical sections of the escarpment. Sediment pockets and sediment pouring down the steep wall in “chutes” were commonly observed. The slope terrain was dominated by two species in the order Actiniaria and an asteroid, as well as 36 instances of orange, yellow, and white bacterial mats, likely exploiting discontinuities in the exposed bedrock, that may indicate a prevalence of chemosynthetic input into this hadal ecosystem. Near the top of the escarpment was an overhang populated by > 100 hexactinellid (glass) sponges. The substrate of the plateau returned to fine-grained sediment, but with a decreased density and diversity of epifauna relative to the trench floor. By providing the first visual insights of the hadal habitats and fauna of the Java Trench, this study highlights how the habitat heterogeneity influences patchy species distributions, and the great benefit of using a hadal-rated exploratory vehicle to comprehensively assess the biodiversity of hadal ecosystems.

A novel approach to mapping ebb-tidal delta morphodynamics and stratigraphy

Ebb tidal deltas (ETDs) are highly dynamic features of sandy coastal systems, and coastal management concerns (e.g., nourishment and navigation) present a pressing need to better describe and quantify their evolution. Here we propose two techniques for leveraging the availability of high-resolution bathymetric surveys to generate new insights into the dynamics and preservation potential of ebb-tidal deltas. The first technique is conformal mapping to polar coordinates, using Ameland ebb-tidal delta in the Netherlands as a case study. Since the delta tends to evolve in a clockwise direction around the inlet, this approach provides an improved quantification and visualization of the morphodynamic behaviour as a timestack. We clearly illustrate the sediment bypassing process and repeated rotational migration of channels and shoals across the inlet from updrift to downdrift coasts. Secondly, we generate a decadal scale (1975–2021) stratigraphic model from the differences between successive bathymetries. This stratigraphy showcases the delta's depositional behaviour through space and time, and provides a modern analogue for prehistoric ebb-tidal deltas. During the surveyed period, inlet fills form the largest and most stable deposits, while the downdrift swash platform is the most stable structure over longer periods. Together, these approaches provide new perspectives on ebb-tidal delta dynamics and preservation potential which are readily applicable to other sites with detailed bathymetric data. These findings are valuable at annual to decadal timescales for coastal management (e.g., for planning sand nourishments) and also at much longer timescales for interpreting stratigraphy in ancient rock records.

First considerations on post processing kinematic GNSS data during a geophysical oceanographic cruise

<p class="Abstract">Differential GNSS positioning on vessels is of considerable interest in various fields of application as navigation aids, precision positioning for geophysical surveys or sampling purposes especially when high resolution bathymetric surveys are conducted. However ship positioning must be considered a kinematic survey with all the associated problems. The possibility of using high-precision differential GNSS receivers in navigation is of increasing interest, also due to the very recent availability of low-cost differential receivers that may soon replace classic navigation ones based on the less accurate point positioning technique. The availability of greater plano-altimetric accuracy, however, requires an increasingly better understanding of planimetric and altimetric reference systems. In particular, the results allow preliminary considerations on the congruence between terrestrial reference systems (which the GNSS survey can easily refer to) and marine reference systems (connected to National Tidegauge Network). In spite of the fluctuations due to the physiological continuous variation of the ship's attitude, GNSS plot faithfully followed the trend of the tidal variations and highlighted the shifts between GNSS plot and the tide gauges due to the different materialization of the relative reference systems.</p><p class="Abstract"><span lang="EN-US"><br /></span></p>

Bathymetric observations of an extreme tidal flow: Approaches to the Gulf of Corryvreckan, western Scotland, UK

The growing interest in tidal energy has led to a need to better understand the interaction between energetic tidal flows and submarine bathymetry. The Gulf of Corryvreckan (GoC) is a highly energetic tidal channel, ∼3.2 km long and ∼1.1 km wide with a maximum depth of 220 m occurring between the islands of Scarba and Jura, western Scotland, UK. A high-resolution bathymetric survey of the region, groundtruthed by sediment samples, has enabled the bathymetry to be related to Lagrangian flow data from GPS-tracked surface drifter buoys. The maximum recorded drifter velocity was 4.75m/s within the GoC during the ebb (east-flowing) tide. Exiting the GoC into open water to the west on the flood tide, the ‘Great Race’ decelerates and subsequently flows in a curve to the north. East and west of the GoC, sedimentary bedforms lie on the flanks of the main flow paths. The northern flank of the Great Race aligns with a 6 km elongate bedform (‘banner bank’) extending from the GoC. East of the GoC, complex tidal bottom flows are suspected to result in uncommon sediment morphology, including suspected sediment wave interference patterns expressed by the mobile bedforms. The novelty of these findings indicates the importance of further research into sediment pathways in energetic tidal zones. A greater understanding of how sediment transport may be modified during offshore construction is an important consideration for offshore installations and the feasibility of marine renewables.

Assessing Subaquatic Mass Movement Hazards: an Integrated Observational and Hydrodynamic Modelling Approach

High-resolution lake and reservoir bathymetric surveys can pinpoint locations that may experience underwater landslides (subaquatic sedimentary mass movements). These can pose a risk to underwater and shoreline infrastructure. This paper outlines an approach for using spatial variation in sedimentary patterns to identify areas susceptible to subaquatic mass movements in lakes and reservoirs. This study focusses on Lake Biel (Switzerland), which has experienced a protracted history of upstream alteration of river flow. Altered flow patterns increase risk of unstable sedimentary features and subaquatic mass movements. Data from sediment traps and cores, Acoustic Doppler Current Profilers and results from a 3D hydrodynamic model gave a consistent picture of spatial and temporal variation in weather-related sedimentation. Erosion caused by short-term rain events contributes the largest proportion of sediments to the lake. Strong rain events combine with typical wind patterns to drive lake circulation. The net effect results in preferential sedimentation onto a steeply sloping shelf prone to subaquatic slides. The integrated approach outlined here incorporates short- and long-term sediment dynamics to provide a systematic assessment of lake sedimentation and potential mass movement hazards. This research represents a first step in developing a risk-evaluation tool for aquatic hazard evaluation.

Mapping of seabed morphology of the Bagnoli brownfield site, Pozzuoli (Napoli) Bay, Italy

ABSTRACT In this study, we present the results of a very high-resolution bathymetric survey carried out offshore the dismissed industrial site of Bagnoli (Pozzuoli Bay, Gulf of Naples). The bathymetric maps and the thematic representation produced, allowed for detailed characterisation of the seabed lineaments and features. Seafloor bedforms and morphological features can be observed at a very high resolution. Moreover, the potential pattern of sediment transport can be inferred from the seafloor bedforms observed in the central sector of the surveyed area. The new morphobathymetric map of Bagnoli-Coroglio offshore provides a valuable tool for stakeholders towards a comprehensive planning and management addressed to environmental revaluation and rehabilitation of this disused industrial area.

Evidence for late Pleistocene volcanism at Santa Maria Island, Azores?

Santa Maria Island constitutes the oldest volcanic island within the Azores Archipelago, with no onshore record of eruptions younger than ≈2.8 Ma. A recent high-resolution multibeam bathymetric survey, however, revealed the presence of a seemingly young submerged wide volcanic edifice at approximately −70/−80 m, on the northeastern sector of the island shelf. The outer flanks of this volcanic edifice are partially eroded by marine erosion, but its general morphology is largely preserved, attesting to its relative youth. The edifice's aspect ratio and crater size are typical of a tuff ring formed by very violent surtseyan to taalian eruptions (with water/magma interaction ratios close to 1), implying extrusion at sea level or in very shallow waters, conditions that are incompatible with the present-day water depth at which this structure occurs. A detailed geomorphological analysis – coupled with a correlation with a modified reference eustatic curve – allowed the formulation and discussion of a formation model for the tuff ring, which involves extrusion during a period immediately preceding a rapid relative sea-level drop, most likely at ≈43 ka. Extrusion during such a period would have allowed for the subaerial consolidation and palagonitization of the tuff ring, increasing its resistance to erosion, before being finally submerged during the Last Glacial Termination. Submersion during the Last Glacial Termination – a period characterised by extremely fast sea-level rise – also helps to explain why this tuff ring was submerged without being completely razed by marine erosion. Our study offers insights on the formation and preservation of tuff rings in coastal environments, and in relation to sea-level oscillations, suggesting that consolidation plays a crucial role in the process. Crucially, our study suggests that Santa Maria's volcanism might have extended well into the very late Pleistocene, raising important hazard implications. Future work is scheduled to confirm this hypothesis, including sampling of the volcanic products by dredging and/or by remote operated vehicle. Our study also emphasises the importance of available high-resolution bathymetric surveys to the formulation of solid volcanic hazard assessments on volcanic islands.

Extended characterization of damage in rubble mound scour protections

The analysis of damage in rubble mound scour protections is crucial for the armour stability assessment. Former methodologies focused on the analysis of the maximum damage number. This work introduces a complementary methodology to determine and characterize damage statistical distribution based on a flexible arrangement of sub-areas. The sub-areas are overlapping and can be varied in size, allowing a refined damage analysis. To capture the variation of damage with changing sub-area layout, the methodology is coupled with a statistical evaluation of damage numbers, which is enabled by the definition of a grid ratio between armour stone size and sub-area size, (Dn50)2/Asub. The methodology is applied to high resolution bathymetric surveys from two stability tests of large-scale rip-rap scour protection around a monopile foundation and combined wave and current loading. Results show that the methodology provides a complementary understanding of damage distribution to the maximum damage acquired from previous methodologies. In addition, it allows a comparison of damage characteristics between tests despite different shapes of the foundation. Research concludes that the size of the sub-areas influences significantly the damage description. Characteristic measures as the maximum damage number and standard deviation become stable at a grid ratio (Dn50)2/Asub equal to 1/4.

Non-hydrostatic numerical modelling of nearshore wave transformation over shore-oblique sandbars

Observations from high resolution bathymetric surveys on the coast of North Carolina indicate areas with shore-parallel depth contours and other areas with shore-oblique sandbars. These shore-oblique bars are large-scale morphological features that extend up to 1000 m from shore, and previous studies have shown that they are spatially correlated with higher shoreline change rates and induce erosional hotspots. To investigate the influence the oblique sandbar morphology has on nearshore wave transformation and energy distribution, a non-hydrostatic wave model is used with the bathymetry corresponding to two areas with shore-parallel depth contours and two areas with shore-oblique sandbars. The model is applied with a horizontal resolution of 5 m and a boundary condition representative of the directional spectrum of a typical southeast swell event. An idealized approach with relatively short alongshore model domains and cyclic lateral boundary conditions is adopted due to limited availability of high resolution bathymetry, however this approach elucidates the influence of shore-oblique sandbars on the nearshore hydrodynamics. The numerical results show that higher alongshore variability in depth due to the shore-oblique bars drive large alongshore changes in cross-shore wave energy flux and cause spatial variability in dissipation in the nearshore region. The simulations indicate that alongshore variations in wave energy caused by the shore oblique bars results in a 50–75% reduction in wave setup and a 25–40% decrease in alongshore velocity averaged across the surf zone for the conditions investigated. The results suggest that wave-driven hydrodynamics can influence nearshore sediment dynamics to create erosional zones compared to areas with shore-parallel bathymetric contours, since the morphology of shore-oblique sandbars are an important control on wave energy that impacts the beach.

Canyons of the Eastern Shore of Southern Baikal: Morphology and Genesis

We present new data on canyons and other morphological features of the underwater slope on the eastern coast of the southern basin of Lake Baikal obtained during high-resolution bathymetric surveys of the bottom using multibeam echo-sounders ELAC SeaBeam 1050 and Kongsberg EM710S. Based on the array of bathymetric data generated, we have constructed a digital elevation model for which we used at once several software products responsible for individual tasks. Analysis of the digital elevation model has indicated that tectonic processes played the leading role in the creation of the main, most noticeable forms of relief on the underwater slope. These processes formed a graben-like depression to the south of the Posol’skaya Bank and tectogenic scarps at the foot of the southern slope of the depression as well as canyon-shaped valleys along the entire coast. We have determined that the unusual direction of the Posol’skii canyon across the coastal slope and the relatively small size of the fan in this canyon are due to the history of the tectonic development of this part of the shoreline: in the past, the canyon valley skirted the continental slope of the Selenga river paleodelta from the southeast, and after the formation of a younger graben-like depression it preserved the old direction of its channel. We have suggested that underwater erosion processes initiated tectonic lowering of the floor of the basin. Ultimately, this resulted in the destruction and erosion of the ancient geomorphological surfaces and formation of the ridge and hollow relief and canyon valleys that are currently at different stages of development. Landslide forms resulting from paleoseismic dislocations play a secondary, yet a significant role in the relief of the slope, which undoubtedly complicated the geomorphological surface. It is assumed that the fans created by the activities of the canyons, the ancient channels of canyons and landslide bodies may be promising for the search for hydrate-bearing structures in this part of the lake.

New Evidence on the Relief of the Southern Underwater Slope in the South Baikal Basin

A high-resolution bathymetric survey of the entire underwater slope in South Baikal was carried out for the first time by means of the multibeam echosounder. New elements were revealed in the underwater relief of the southern slope of the basin from the Kultuchnaya river to the Mysovka river, and a relevant regionalization was carried out into three areas according to the presence of ancient landforms and the intensity of the processes of their reworking, with the boundaries along the delta-front and the river fan of the Utulik and Pereemnaya. We reconstructed the formation process of the modern relief of this area of the underwater slope and determined a dominant role of the underwater-erosion processes in the formation of the modern relief, with the tectonic movements served as the “trigger” for them. The study revealed a direct correlation between the reworking of the ancient geomorphological surfaces and the occurrence of separate large underwater elevations or group ridges (remnants), canyons and linear depressions. It is determined that the pivotal role in the appearance of the modern relief of the underwater slope in the second area of extensive plano-convex (in section) and fan-shaped (in plan) delta-fronts of large rivers and debris fans enveloping the remnants. The existence of a steep underwater slope from the mouth of the Pereemnaya to the mouth of the Bol’shaya Tel’naya and of two large benches, and also the proximity of the maximum depths of the southern basin at the foot of this slope are explained by a more intense step-like lowering of this part of the lake when compared with the western part of the basin. This also accounts for the absence of large remnants in this part of the slope. The area of occurrence of lacustrine-fluvial and lacustrine-delta deposits of the Tankhoi paleobasin within the southern basin of Baikal is reconstructed. Four new hydrate-bearing structures associated with these deposits and morphologically pronounced in the vertex parts of the remnants were discovered. The morphological exploration indicators of the presence of new accumulations of gas hydrates on the underwater slopes of Baikal have been expanded.

Glacial geomorphology of the central Barents Sea: Implications for the dynamic deglaciation of the Barents Sea Ice Sheet

Contemporary climate change has resulted in great uncertainty in how glaciers and ice sheets around the Earth might evolve. It has long been appreciated that the contemporary West Antarctic Ice Sheet (WAIS) shares many similarities with the former Barents Sea Ice Sheet (BSIS). Therefore, an increasing number of studies have sought to investigate the Barents Sea glacial record to better understand marine-based glaciation. This paper reports the analysis of a new high-resolution bathymetric survey covering ~55,000km2 in the central Barents Sea. The relative chronologies of newly-mapped glacial landforms such as mega-scale glacial lineations, grounding-zone wedges, moraines, and crevasse-squeeze ridges are used to reconstruct the ice sheet dynamics in the central Barents Sea. Our results show that the ice sheet responded dynamically with different periods of retreat, advance, and stagnation observed. These new landform records have been integrated with other geomorphological records in order to reconstruct the retreat pattern of the BSIS between 17–14ka, when the Fennoscandian Ice Sheet is thought to have uncoupled from the BSIS. Our data shows that the dynamic drawdown of the ice sheet saddle by ice streams was probably the primary mechanism in separating the two ice sheets. After the ice sheets uncoupled, the BSIS is shown to have retreated episodically with several periods of relative stability. Geomorphological records such as those from the BSIS can be used to constrain ice sheet modelling and will help to develop a clearer understanding of marine-based glaciation and the role of ice streams in driving ice sheet evolution.

Restoration of a boulder reef in temperate waters: Strategy, methodology and lessons learnt

Anthropogenic impacts on marine habitats are a global problem, particularly in coastal areas. While boulder reefs in temperate waters hold high biomass and biodiversity, and may be unable to recover from anthropogenic stressors without restoration efforts, little is known about how to restore and conserve this important marine habitat. Limited knowledge is a serious impediment to projects aimed at restoring boulder reefs that have been degraded or removed by substrate extraction. In 2008, a boulder reef was restored in Kattegat, the transitional waters between the North Sea and the Baltic Sea, using differently sized boulders. The restored reef covered approximately 27,600m2 seafloor and included 100,712 tons of boulders added at depths ranging between 4 and 11m. This paper describes methodology and lessons learned during the restoration project. Before the restoration, geological and geotechnical surveys confirmed that the sea bed could support added boulders, and high resolution bathymetric surveys provided input for the design of the reef, particularly for numerical modelling of the hydrographic and sediment transport conditions. Numerical modelling was used to derive hydrographic design conditions for boulder placements and further, to ensure that the restored reef would not affect the sea bed morphology and hydrographic conditions at a local harbour and at a protected habitat, both situated in the vicinity of the restoration area. Data on the physical structure of the restored boulder reef, collected in 2009, demonstrated that cavernous structures and shallow reef areas were restored. Moreover, data collected in 2012 confirmed the stability of the restored reef. Finally, results highlighted the importance of stakeholder mapping at the outset, appropriate timing of stakeholder involvement and ongoing consideration of stakeholder perceptions. Charting strategy and introducing a checklist for marine restoration projects, this paper outlines important considerations and methodology needed to ensure that restoration of temperate reef structures meet the objectives, without having undesirable effects on existing hydrographic and morphological conditions, including nearby coastal areas and protected marine habitats.

P45 伊豆大島沿岸域の高分解能海底地形調査速報(ポスターセッション)

P45 伊豆大島沿岸域の高分解能海底地形調査速報(ポスターセッション)

The role of mass-transport deposits and turbidites in shaping modern lacustrine deepwater channels

Subaquatic canyons are an important pathway for sediment transport into oceanic and lacustrine basins. Understanding the mechanisms governing their geomorphological evolution is a key to predict the sediment distribution patterns through these sediment conduits as well as to implement geo-hazard assessments. Submerged channels developed in large lacustrine basins offer a small-scale natural laboratory to understand the sedimentological processes operating in submarine channels. For this reason, a multidisciplinary research initiative -including time-lapse, high-resolution bathymetric surveys, innovative coring using submersibles, in situ geotechnical tests, and geophysical and sedimentological analyses-was applied to unravel the factors controlling the geomorphological evolution of the Rhone delta channels in Lake Geneva during the last decades. The morphology of the lacustrine Rhone Delta consists of a freshwater delta system deeply incised by nine canyons (C1C9). Geotechnical measurements in proximal areas and sediment cores retrieved in the distal fans at the end of each canyon revealed complex sediment dynamics. No turbidity current events have occurred in the easternmost canyons (C1C4) during the last decades while the western canyons sediment record (C5C9) indicated repeated flushing events during the 20th century. The main “active” canyon C8 has been dominated by turbidite activity on the canyon floor with frequent overspill events along the levees. A large 6.2 × 106 m3 Mass-Transport Deposit (MTD) that resembles a debrite in its upper section was found in the distal area of the active channel. The MTD was dated at 1998–2000 CE and most likely originated from proximal delta areas affected by frequent slope failures of the steep channel walls. In situ geotechnical tests on the modern proximal channel floor showed an unconsolidated soft top-layer that might have served as a low-friction surface favouring the MTD long run-out distance to the distal part of the channel. The MTD has had a major effect morphological evolution of the distal channel by filling the existing conduit, indirectly promoting the formation of a new channel. The role of MTD emplacement in subaquatic channels has important implications for hydrocarbon exploration as they control channel avulsion processes and the location of sand-prone deposits. This study gives a detailed insight on poorly investigated short-term sedimentological dynamics that affect the long-term evolution of turbidite systems and channel migration processes. This detailed model of a river-dominated deep-lacustrine depositional system can be used as an analog for similar modern and ancient deep-water systems.