Underwater Slope Research Articles

Effect of underwater relief on the dynamics of the autumnal thermal bar

Through numerical modelling techniques, data were obtained on how underwater slopes influence the dynamics of a thermal bar and the related circulations during the water body’s autumn cooling. Different geometries of the bottom topography were analyzed, aligning with configurations such as «gentle slope», «stepped profile», «trench», and «steep slope». It was determined that in the autumn, the flow structure, the formation timeframes, and the thermal bar propagation speed are significantly influenced by the morphological features of the lake bottom.

Study on the influencing factors and evolution of loess bank collapse with physical modelling

BackgroundReservoir bank collapse in loess areas may lead to the siltation of reservoir and bank retreat. Therefore, the study of reservoir bank collapse has practical significance. Almost of the bank collapse width prediction method were based on the classical graphical method which do not consider the process of bank collapse. But practice shows that this method can overestimate the width of bank collapse. Meanwhile, there are few studies specifically focused on the collapse of loess bank slopes.MethodsTo improve the prediction method of loss bank collapse width, the influence of water depth, dry density and bank slope angle on loess bank collapse was studied by physical modelling. The bank collapse width and the morphology of the bank slope were recorded during the experiment.ResultsThe bank collapse width increases with the increase of water depth, increases with the increase of slope angle, and decreases with the increase of dry density. The modeling process shows that the loess bank collapse occurs firstly underwater, the erosion niche will be formed underwater, and then the above water slope is damaged. This process is repeated until the underwater accumulation slope reaches the stable state, and then bank collapse stops. After the bank collapse, the above water slope is polyline, while the underwater slope is curved. When the slope angle is less than 27°, the bank collapse will not occur, and when the slope angle is between 27° and 40°, the bank collapse type is abrasion type. When the slope angle is greater than 40°, the bank collapse type is dumping type or shear type. Based on this, the improved balanced alluvial accumulation approach was proposed, which considers the mechanical equilibrium of above water bank slope and the morphology of underwater slope. The new method can reflect the stage characteristics of loess bank collapse, which is more reasonable than the empirical graphical method.ConclusionsThe experimental results indicate that when predicting the width of loess bank collapse, it is necessary to combine the bank collapse width and process of bank collapse. The relevant conclusions have a certain role in exploring the mechanism of loess bank collapse and bank collapse prediction methods.

Guiana dolphins use mangrove margins as a natural barrier to chase fish prey

AbstractThe behavioral plasticity among and within cetacean species can be driven by their prey sources, local adaptations to environmental features, and/or interactions with human activities. One of the tactics displayed by cetaceans is the barrier feeding, in which individuals or groups herd fish schools against natural and non‐natural barriers to restrict movements of their prey. Coastal odontocetes, for example, are known for using shorelines and underwater slopes to trap their prey. Here, we documented two distinct populations of Guiana dolphin using the mangrove margins as a natural barrier to herd and capture fish. The observations took place in the Cananéia estuary, São Paulo State, southeastern Brazil, and in the Paranaguá estuary, Paraná State, southern Brazil. A total of 63 dolphins over eight encounters were observed performing six different events using mangroves as a natural barrier, summing both study areas. These findings not only expand our understanding of the behavior plasticity of the species but also call attention to the importance of the mangrove ecosystem directly to Guiana dolphins and indirectly to the components of their food web.

DISTRIBUTION OF SEDIMENTS AT THE MAIN MOUTHS OF THE KILIA DENUBE DELTA AND THEIR SEDIMENTATION VALUE

Purpose: In the article the basic purpose is determination and analysis by distribution natural conformities of deltaic sediments after its movement across every arm, under impact of sea waves separation and influence of the coastal hydrogenetic factors. The noted peculiarities have important practical significance. Additionally, they explain the marine margin dynamics and sediment forming of deltaic types. Data & Methods: This article is based on natural datum of sedimentary material sampling around of marine margin of the dynamical Danube delta, NW Coast of the Black Sea. The sampling was made in region from head of Zhebriyan bay in north side to Musura bay in south side during 2010–2020 period in different environment and synoptical conditions. Laboratory elaborations were made with representative instruments usage and mathematical methods of treatment. Samples of suspended and coarse drifts were elaborated and analyzed within conditions of different sides of marine margin of the Delta. Results: The general definitions of protruding deltas state that the delta cone of protrusion is composed of alluvial sediments. But in the studied delta, a significant place is occupied by coastal-marine facies of wave action. In addition, the newest river sedimentary material changes during the passage through deltaic watercourses (especially long ones). Therefore, on the delta coast within the marine wave field, sediments with a slightly changed composition appear. Therefore, several facies environments can be defined in the Danube delta: a) typically riverine; b) alluvial-delta; c) coastal sea with separated sediments; d) aeolian center of influence of the wind factor; e) typical marine on the outer strip of the sea. We recommend that all the mentioned features be taken into account during the analysis of the Danube’s flow sediments to the bottom of the Black Sea and the determination of sedimentation rates. The main paper outlines provisions of the lithodynamic model of delta-type sediments’ formation in the conditions of a generous influx of river sediments, a steep underwater slope, a non-tidal sea and strong wave action. Fractions of 0.1–0.01 mm dominate along the coastal edge, with integral Md = 0.048 mm, and the concentration of fractions > 1.0 mm is sharply reduced. The content of CaСО3 is lower in comparison with sediments in Zhebriyansk Bay in the area of final dispersion of a strong alongshore flow of sand sediments. Within the studied deltaic natural system, the surrounding physical and geographical conditions actively influence the composition of delta sediments, create a sedimentary diversity of the facies network and tracts. The different composition of sediments leads to different density of the sediment layer, with different amounts of water content, mostly from 1% to 5%, different influence of river and sea factors. Accordingly, specific facies cells of the botanical and zoological substrate are formed in the areas of dominant influence of these factors. In the new parts of the delta, some cells settle first, others later, and the third – last in the life cycle. Therefore, in different areas, we meet the settlement of the primary territories of the delta by different plants and animals that are located in different parts of the warm period of the year.

Shaking table test on underwater slope failure induced by liquefaction

Sand liquefaction is a process in which the excess pore water pressure of saturated sand soils increases and the effective stress of saturated sand decreases under the action of vibration, resulting in the transition of sand soils from the solid state to the liquid state. In this paper, an underwater sand slope model containing the upper sand slope and the bottom non-liquefied clay layer was designed. The whole process of large deformation of flow liquefaction from the solid state to the liquid state was reproduced by the shaking table test and recorded by the high-definition particle image velocimetry (PIV) equipment. Four main influencing factors: the acceleration amplitude of the shaking table, the frequency of the shaking table, the relative density of the sand slope, and the slope ratio of height and width of the sand slope, were considered. During the test, the dynamic response characteristics of acceleration and excess pore water pressure (EPWP) within the underwater sand slope model were monitored and analyzed in the whole deformation process to reveal the mechanism of the sand liquefaction process and the law of development and provide data support for subsequent research.

Internal Waves in the Region of the Akselsundet Strait of Western Spitsbergen Island

Moored measurements of temperature, salinity, and currents at three moorings that operated for a year in the area of strong tidal currents in the area of the Akselsundet Strait at the mouth of the Van Mayen fiord on Western Svalbard Island are analyzed. Tidal currents flowing around an underwater sill in the strait generate intense tidal internal waves. Forced internal waves exist above the underwater slope. Vertical displacements of water particles reach 20 m. Waves rapidly decay as they propagate away from the strait and slope. They are no longer recorded as far as 12 km from the strait. Tidal currents in the strait reach a speed of 3 m/s and form an intense jet when flowing out of the strait. A strong jet of currents during spring tides presses the instruments on moorings to the bottom.

Features of the formation of the coasts of Chukotka and the Koryak highlands

Long-term expeditionary studies of the Geography Faculty of Lomonosov Moscow State University of the Arctic coast of Russia (1969 – 2015) have provided extensive material on hydrology, channel morphodynamics and geomorphology of the estuaries of large rivers from the Northern Dvina to the Kolyma. The joint work of the Moscow State University and the M. K. Ammosov North-Eastern Federal University made it possible to signifiantly expand the geography of research to the coasts of the Chukchi and Bering Seas. Based on the analysis of geological and geomorphological data and an array of published and archival materials, it was possible to establish patterns and specifi features of coastal and estuarine processes in the fial phase of stabilization of the post-glacial ocean transgression. Over the past 7-5 thousand years, floding of low-lying hilly coastal plains, river valleys and fjords has occurred. The active impact of the wave activity of the sea led to the erosion of the upper part of the underwater slope, the restructuring of its profie and the retreat of accumulative forms towards the land and the creation of modern outlines of the coastline. The sections of the coast extended into the sea, composed by a thickness of loose quaternary sediments, were partially eroded by waves, and sediments were involved in longitudinal and transverse movements, forming braids and embankments, which led to a general alignment of the coastline. Currently, in conditions of rising sea levels, the general trend towards the restructuring of the underwater slope and the coastal zone persists and can be extremely dangerous for coasts composed of strongly icy sandy-loamy alluvial-lacustrine deposits. A further increase in global temperature will signifiantly soften the climate of the Arctic coasts, cause the degradation of permafrost, thawing of underground ice and accelerate the process of retreat of coastal abrasion ledges, the sea edge of river deltas. Taking into account the regional geological and geomorphological features of the coasts of the Chukchi and Bering Seas, it should be expected that a possible rise in sea level will lead to thawing and degradation of permafrost rocks and vein ice.

SURFACE MORPHOLOGY AND DYNAMICS COASTLINE OF THE DZHARYLHACH SPIT DISTAL END

SURFACE MORPHOLOGY AND DYNAMICS COASTLINE OF THE DZHARYLHACH SPIT DISTAL END

ფოთის პორტის ახალი მოლოს გავლენა სანაპირო ზოლის ლითოდინამიკურ პროცესებზე

In the city of Poti and its maritime region, for several decades there have been of environmental and geomorphological problems, which have been caused by anthropogenic interventions in this region. The impact of the breakwater of the new port of Poti on lithodynamic processes in the coastal zone is discussed in the paper. The sediment mass constancy equation is used in numerical experiments to determine the velocities of overbank and bank-driven sediment transport. To solve this equation, we use the finite element method and Crank–Nicolson schemes. According to calculations, 1,500,000 m3 of sediment migrates along the Nabada delta's banks, 850,000 m3 south of the delta to the harbor, and 65,000 m3 north of the delta to Khobiskal. The remaining 2,500,000 m3 is moved along the shore in a westerly direction before being dumped on the sea's underwater slope. After the new breakwater is built, all fine sediment movement in that direction will be stopped, and solid sediment will start to settle to the north of the new breakwater. Eventually, this will cause the southern branch of the Nabada Canal to become blocked. The total flow of water from the Nabada canal will be transferred to the northern branch, and the formation of a new delta will begin there.

Multivariate analysis and modeling of shoreline changes using geospatial data

Understanding of shoreline dynamics is essential to coastal management and the development of tropical archipelagic countries. Therefore, this study aims to analyze the factors affecting shoreline changes and their future predictions in Bengkayang Regency, West Kalimantan. Research data used includes historical maps, remote sensing imagery, total suspended solids, underwater slope and depth, distance from the estuaries, wind speed, and distance from the coastal constructions. The multiple linear regression models and digital shoreline analysis system were applied. Findings showed accretion occurred in 1981, 1991, and 2021 while abrasion was detected in 2001 and 2011. The shorelines gained 37.05 meters between 1945 and 2021 under intensive accretion in segments 2–4 while abrasion recorded in segments 6–8. Furthermore, the shorelines are projected to be abraded between 2021 and 2041. These findings are recommended to be considered in the development of coastal areas that are adaptive to climate change, sea-level rise, and land subsidence.

О золотоносности мелководной зоны шельфа Юго-Западной Камчатки

The materials, obtained from previous (1972-2019) studies of over 300 km of the coastal land strip in Southwestern Kamchatka and the adjoining underwater slope of the Sea of Okhotsk (in the interfluve of the Bolshaya and the Kikhchik rivers) were analyzed. Gold-rich zones (> 60 mg/m3) in the form of subparallel lenses, oriented along the sea shoreline, were identified between 6.3-30 m isobaths in the shallow marine shelf zone. The gold content of the submarine coastal slope correlated directly with the sediments of the upper coastal zone. Higher gold concentrations (up to hundreds of mg/m3) were detected in Holocene marine sediments of the shallow marine shelf in sampling intervals 0 to 2.0 m. They are mostly confined either to the upper, steepest part of the submarine slope or to the first or second terraced level. These are the areas of the most active impact, near the shorelines (modern and submerged), within which glacial and water-glacial formations are often exposed by erosion. Most of the gold concentrate halos related to ancient shorelines, the western boundary of which lies within 25-30 m isobaths, and to paleovalleys, identified by seismoacoustic profiling. In paleovalleys, flooded by the sea and buried under marine sediments, alluvial placers similar to Tikhangou in Primorsky Krai may be revealed. The most promising within the investigated territory was the area of underwater coastal slope within the Severnaya Mitoga - Utka - Khomutina interfluve, between 5.3-30 m isobaths, where concentrate halos with considerable gold concentrations from 60 to 500-1186 mg/m3 had been revealed.

Combined Radar Monitoring of Long Surface Wave Packets in the Pacific Ocean

In order to continue and more detailed study of the manifestations of seismic activity on the World Ocean surface, in this work we verified the complex technique of airborne radar monitoring and research of long surface wave packets in the seismically active region in the Northwest Pacific Ocean near the Kuril-Kamchatka Trench.
 When verifying the technique proposed in the previous work, we used data from two series of radar surveys of the sea surface within the study area in the 3-cm range of radio wave lengths. The first series of radar surveys had included tacks of flights along the north–south direction, to which, for comparison, the results of the second series in the west–east direction were added. These radar images, presented in the work, detect manifestations of two surface wave packets propagating from the same area, in the same direction, with an interval of 16 hours.
 For a comprehensive study of surface wave packets based on a set of radar images of two series and to establish the nature of their origin, a combined spatial and spectral analysis of the nonlinear form of the wave packet components was performed. As a result, the spatial scale of surface wave packets (5–10 km), the lengths of the wave components of the packets (1–5 km) and the speed of packets movement (6.1 m/s) were determined. The analysis involved the parameters of the ocean-atmosphere near-surface layer, provided by operational in situ measurements, which were obtained by the research vessel "Akademik Petrov". This included the direction and speed of the near-surface wind, the state of wind waves and swell, the speed of the surface current, etc. In addition, data on bathymetry along the path of the surface wave packet and seismic activity in the area were used.
 Finally, it was assumed that the observed packets of surface waves are Korteweg – de Vries solitons, which arise as a result of collapses on the steep underwater slopes of the Kuril-Kamchatka Trench due to a seismic shock and the aftershock that followed it. The developed airborne radar technique can also be used in satellite monitoring of the surface of the World Ocean in systems for warning about the approach of potentially dangerous long waves to the coast.

Determining the change in the bathymetry of Saldanha Bay due to the harbour construction in the seventies

Bathymetry is the science to study and measure the depths of the ocean floor. The differences in water depth, underwater slope and ocean floor structure were investigated using a geographic information system (GIS). This article investigates changes to the hydrodynamic sedimentation processes in Saldanha Bay as a result of the harbour constructions that took place in the early 1970s. The construction of the harbour included the massive dredging operations and resultant relocation of 30 million m3 of soil. Bathymetric data from Saldanha Bay before (1957) and after (1977) the construction of the harbour in the early seventies were analysed and compared in ArcGIS. It was found that the overall depth of the inner part of Saldanha Bay increased with about 1.4 m and that the bottom and slopes were smoothed. The physical structure that was constructed for the harbour had a serious impact on the hydrodynamic sedimentation processes. It was found that these interventions altered the depth profile of the Bay and the article finally draws conclusions on coastal and beach formation, based on the change in the hydrodynamic sedimentation processes of Saldanha Bay.

NATURAL FEATURES AND HYPOTHESES OF ORIGIN NEASHORE BARS IN THE COASTAL ZONE OF NON-TIDAL SEAS

Problems of the research. The article considers the origin, morphology and dynamics of unique forms of coastal-marine relief in the coastal zone of non-tidal seas, under the influence of the hydrogen factor, an exceptionally strong potential of mechanical energy, sufficiently large reserves of sand deposits (fractions of 0.05–1.00 mm), active influence of wave impulses and wave currents in the zone of energy dissipation, active nanoexchange between the shore and the underwater slope. It is shown that this issue has a long history of scientific research in different countries due to its important practical significance. Long years we researching the alongshore sand drifting between C. Bol. Fontain in North to Jebriyan Bay in South as a experimental natural object of the North-Western part of the Black Sea. Theoretical elaborations and methods. Among the natural features of the named landforms, the researchers distinguish 8 leading ones, which are manifested only in the environment of the coastal zone. Therefore, the purpose of the article is to consider the genetic hypotheses of the formation of underwater sand bars in the coastal zone of the seas over the past 100 years, and on this basis – to outline, evaluate, analyze several hypotheses about the formation of bars, show their positives and disadvantages, name the genetic features for the physical geographical. coastal environments As part of the eight most important and most common features, we have identified the following: distribution of wind wave sizes and wave current velocities; the value of the steepness of the underwater slope covered with sediments; value of sediment sizes and hydraulic value of coastal zone sediments; some processes of wave transformation and dispersion of wave energy; certain individual processes of sediment movement of different hydraulic magnitudes in a cell of a sandy coastal zone; steepness of wind waves during storms in the spray source; active action of sand drifts by movement along the coastline and under the influence of the regime of transverse sediments; the nature of underwater swells is different, because one of them moves from the underwater slope to the shore, another – from the shore towards the sea, and the third is dynamically stable. Main results and conclusions. As can be seen, the main hypotheses and the corresponding features of the formation of underwater swells act on their own, separately from the general process of development of the coastal zone of the seas. Such a principle does not meet the elementary requirements for the definition and evaluation of complex geographic natural systems. This is especially true for the coastal zone of the seas, where all the genetic components of the continental, oceanic, and coastal-marine types interact closely in a complex. They act unanimously. It is from such positions, according to this principle, that underwater sand bars and other coastal-marine landforms should be investigated. In other elements of the exogenous part of the geographic envelope, they are not found anywhere and determine the uniqueness of the coastal zone of the seas. The authors of these variants build their hypotheses on separate parts of natural coastal phenomena. Until now, researchers have avoided complex, systematic approaches to this topic. Therefore, the article provides recommendations for improving the theory of the genesis and development of underwater swells within different sites of alongshore sand drifting.

The Method for Evaluating Cross-Shore Migration of Sand Bar under the Influence of Nonlinear Waves Transformation

Sand bar migration on the gently sloping sandy bottom in the coastal zone as a result of nonlinear wave transformation and corresponding sediment transport is discussed. Wave transformation on the intermediate depth causes periodic exchange of energy in space between the first and the second wave harmonics, accompanied by changes in the wave profile asymmetry. This leads to the occurrence of periodical fluctuations in the wave-induced sediment transport. It is shown that the position of the second nonlinear wave harmonic maximum determines location of the divergence point of sediment transport on the inclined bottom profile, where it changes direction from the onshore to the offshore. Such sediment transport pattern leads to formation of an underwater sand bar. A method is proposed to predict the position of the bar on an underwater slope after a storm based on calculation of the position of the maximum amplitude of the second nonlinear harmonic. The method is validated on the base of field measurements and ERA 5 reanalysis wave data.

Anthropogenic impact on the shores and the bottom of the Jebriyan bay in the Northwestern part of the Black Sea


 
 
 
 The Jebriyan Bay is located in the northern part of the Kiliya Danube Delta, at a junction of the delta cone and the indigenous coast. This is a zone of very high anthropogenic impact on the Danube Biosphere Reserve. The two opposite shores of this bay are fundamentally different. Along the northern shore, the Northwest coastal sand sediment flows discharge from the Cape of the Great Fontanne to the Jebriyan Bay. That is why the northern coast of the bay is made up of sandy forms of coastal topography (marine accumulative terrace and spit). The southern coast is deltaic; composed of a mixture of muddy, siltstone and sandy sediments. The area of the bay is limited to isobaths –11 m and is about 80 km2. The bottom of the bay has a gentle relief, made up of smooth outlines, with an average depth of 6.2 m. The shape of the transverse profile of the underwater slope is mostly convex. The natural system of the bay was affected by fishing, recreation, shipping and industrial sand production on coastal accumulative landforms. Coastal fishing uses a system of fixed bottom seines and small motorized floating equipment. Recreational facilities are designed to serve about 350 thousand people during the warm period each year. The impact of shipping was expressed in the construction and operation of the seaport of Ust-Dunaysk, together with suitable canal and the technical canal between the sea and the branch of the delta breakthrough the system of the large Ochakov branch. The ladle port had an area of about 1.5 km2, a maximal depth of 16 m, and an average depth of 13.7 m. The trough was connected to the Ochakov branch of the Danube Delta by a technical canal with a depth of 4 m. Vessels could enter the harbor of Ust-Danube through an access navigation channel with a depth of 11–12 m and a bottom width of 125 m. The port was used for the transshipment of large containers, general forest cargo from ocean vessels (displacement of 60–100 thousand tons) on regular sea lines from the countries of Southeast Asia to the Black Sea, to the Danube and further to the countries of Central Europe and to the ports of the North and the Baltic Seas. But it was unfortunate that the port construction site did not last as expected. Between 1980–2010 the harbor and approach canal of Ust-Dunaysk were filled with Danube river sediments. The example of Jebriyan Bay has shown that when executing any type of sustainable nature management project, it is very important to take into account the natural milieu.
 
 
 

Coastal dynamics of the Gulf of Kalamita (Western Crimea)

The performed research showed that since the first third of the 20th century until today the beach width at various sections of the Gulf of Kalamita has decreased two-to fourfold, and at some of them beaches have totally disappeared. The average coast erosion rate for the last 100 years has been 1.3 m/yr, with the maximum being 7.8 m/yr. These processes are accompanied with depletion of debris on the beach and underwater slope, erosion of bench deposits, and destruction of coastal structures. This is due to deficiency of beach-forming material resulted from intense economic management in the coastal area. Implementation of scientifically and technically sound coastal protection projects is essential to preserve the beaches of the Gulf of Kalamita.

Occurrence of Tankhoy field coals in South Baikal bottom sediments

The purpose of the study is to describe the first finds of coal-bearing clays and coals in the bottom sediments of the southern basin of Lake Baikal and compare them with terrestrial coal-bearing deposits of the Tankhoy field. Comparative analysis of the lithological composition and colour of bottom sediments and terrestrial sections, as well as the concentration of organic carbon and conducted palynological analysis allowed their correlation. At the lake’s depth of 900 m the authors discovered a coal-bearing strata in situ (st 56), which later was stratigraphically correlated with the terrestrial coalbearing part of the Tankhoy suite. The fragments of coal found in bottom sediments basically along the entire Tankhoy field, especially bedrock coals on the underwater slope in South Baikal up to 1300 m deep prove the distribution of the coal-bearing part of the Tankhoy suite in the sublacustrine part of the lake throughout the entire slope (from 5 to 10 km offshore) and confirm the distribution area of the Tankhoy paleolake over a significant area of the contour of modern southern basin of Lake Baikal. The finds of coal-bearing strata on these and other various sub-bottom depths, i.e. under various pressure and temperature conditions, suggest that coals themselves and coal-bearing mudstones may be a generation facility of secondary microbial methane. This should be taken into account when searching for gas hydrocarbon and gas hydrate accumulations as well as assessing methane cycles in Lake Baikal.

Spatial-temporal changes of macrophytobenthos in the coastal zone of the Reserve “Karan’sky” (Sevastopol City, Black Sea)

For the first time, a comparative analysis of the specific diversity, changes in the biomass of macrophytobenthos in the reserve “Karan’sky” coastal zone from 1964 to 2016 was performed on the basis of a landscape approach. A landscape map of the underwater coastal slope has been drawn up, three bottom natural complexes (BNC) have been identified. The BNC of psephite deposits dominated by C. crinita and Tr. barbata (up to 89.7–78.6% of the total biomass of macrophytes) is the main one. This complex is characterized by high floristic diversity and maximum values of the biomass of macrophytobenthos. The BNC of psephite deposits with C. crinita and Tr. barbata to predominate and with pebble-gravel deposits with broken shells alternate, where Ph. crispa is the dominant species, is characterized by high floristic diversity and a decreased proportion of C. crinite and Tr. barbata, the contribution of Ph. crispa to the total biomass of macrophytes is 14.4%. The BNC of psammitic deposits with Ph. crispa to predominate and with separately randomly located blocks, where Nereia filiformis and Zanardinia typus predominate, is characterized by low floristic diversity, the predominance of Phyllophora crispa (74.3% of the total biomass of macrophytes), and the preservation of perennial species. For more than 50 years, there was a reorganization and degradation of the plant component in all BNC (depth 0.5–10 m), which was probably due to the intensification of anthropogenic activity in the coastal zone. Eudesme virescens, Dictyota dichotoma, Feldmania irrgularis, Dasya baillouviana and Rhodochorton purpureum, which live in clean, open areas of the coast, have disappeared from the bottom vegetation, and macrophytes (Cladophora laetevirens, Ectocarpus siliculosus and C. virgatum), typical for eutrophic waters, have appeared in the duodenum. The biomass of epiphytes has increased (from 0.1 to 42.6% of the total biomass of macrophytes), and the role of dominant species has decreased: C. crinita, Tr. barbata (from 99.9 to 78.6%) and Ph. crispa (from 52.9 to 14.4% of the total biomass of macrophytes). The organization of the “Karan’sky” Reserve contributed to the preservation and partial restoration of the unique natural complex of marine flora and bottom vegetation, which is confirmed by the increased floristic diversity of algaeindicators of clean waters and the increased proportion of the Red Data Book species.

Characteristics of Underwater Topography, Geomorphology and Sediment Source in Qinzhou Bay

Human activities for exploitation and utilization of coastal zones have transformed coastline morphology and severely changed regional flow fields, underwater topography, and sediment distribution in the sea. In this study, single-beam bathymetry coupled with sediment sampling and analysis was carried out to ascertain submarine topography, geomorphology and sediment distribution patterns, and explore sediment provenance in Qinzhou Bay, China. The results show the following: (1) the underwater topography in Qinzhou Bay is complex and variable, with water depths in the range of 0–20 m. It can be divided into four underwater topographic zones (the central (outer Qinzhou Bay), eastern (Sanniang Bay), western (east of Fangcheng Port), and southern (outside of the bay) parts); (2) based on geomorphological features, the study area comprises four major submarine geomorphological units (i.e., tide-dominated delta, tidal sand ridge group, tidal scour troughs, and underwater slope) and two intertidal geomorphological units (i.e., tidal flat and abrasion platforms); (3) sandy sediments are widely present in Qinzhou Bay, accounting for 70% of the total sediments. From the mouth of the Maowei Sea to the central and northern part of Qinzhou Bay, the sediments gradually become coarser, shifting from sandy mud to muddy sand, and then to fine sand and medium–coarse sand, especially inside the trench. The detrital minerals contained in the sediments mainly consist of quartz, feldspar, ilmenite, leucosphenite, tourmaline, and detrital minerals, whereas the clay minerals are dominated by kaolinite, followed by illite and smectite. The sediment provenance is mainly terrigenous input from near-source river. With sea reclamation and dam construction, outer Qinzhou Bay has experienced enormous morphological variation of its coastline. Human activities for exploitation and utilization of coastal zones have transformed coastline morphology and severely changed regional flow fields, underwater topography, and sediment distribution in the sea. Together with the channel effect where the velocity of ebb tide is greater than that of flood tide, the underwater topography is characterized by increased scale and height difference of troughs and ridges as well as enhanced offshore deposition.