High Current Velocities Research Articles

Design and tests of a marine current turbine in low flow velocity

Marine current energy is recognized a promising energy source due to its inherent stability and high predictability. In the past decade, several milestones have been achieved in marine current turbine (MCT) research such as MeyGen and O2. However, compared with high-velocity currents, low velocity currents are more commonly distributed worldwide, which presents new challenges. The demand for low starting velocity and high efficiency necessitates some novel structure in the design of blade, power transmission, etc. To address these issues, taking a 50W MCT as research object, this paper firstly presents the MCT design of a new fan-shaped rotor configuration, a compact non-contact magnetic coupling, a low inertial power train and hollow-cup DC generator for easier starting. Furthermore, to realize the maximum power point tracking (MPPT) and sustainable power supply, the electrical system and its control strategy were designed. The hydrodynamic performance of the fan-shaped rotor was simulated and analyzed using CFD, and a simulation model of the electrical and control systems was constructed and tested to verify the performance. Finally, the tank and sea tests were respectively conducted to validate the performance of proposed MCT. The results indicate that the proposed MCT is capable of starting at velocities around 0.2 m/s and achieving a power coefficient (Cp) exceeding 0.4, making it well-suited for providing a continuous and stable power supply in areas with low flow velocities.

Design and control of a parallel-axis twin-rotor counter-rotating marine current turbine for the shallow sea conditions

The ocean contains a large amount of marine current energy which is considered to be one of the most promising renewable energy sources for commercialization. In contrast to the high-velocity currents in straits close to the mainland or islands, the velocities of the currents in most of the sea are low. The exploitation of low-velocity currents is of great significance to the regions with such energy sources. Although the technology for harvesting marine current energy is developing rapidly, the exploitation of low-velocity current energy is often neglected due to its low power density. When the existing marine current turbine (MCT) technology developed for high-velocity currents is used for low-velocity currents, it will result in large rotor diameters and low performance of starting current velocities, which is subject to the water depth and the levelized cost of energy. To solve those problems, a direct-drive parallel-axis twin-rotor marine current turbine (TRMCT) was developed in this research, with a focus on the design of the transmission and generator of the MCT, as well as the improvement of the control strategy. Firstly, the structure of the TRMCT and its characteristics were given. Secondly, the improved optimal torque control strategies for the TRMCT were designed. Finally, to verify the feasibility of the TRMCT and its control strategy, the mathematical model was built and simulated, and the starting performance and operating performance were analyzed. The results show that the proposed TRMCT can start to work at about 0.2 m/s and the conversion efficiency of the whole system can reach about 25%. Compared to the scheme using two identical single-rotor MCTs, the TRMCT's output power can increase by up to 26%. Besides, the torque imbalance caused by the difference in the current velocities on the two rotors can be well coped with.

Exploration of underlap induced high-k spacer with gate stack on strain channel cylindrical nanowire FET for enriched performance

This research explores a comprehensive examination of gate underlap incorporated strained channel Cylindrical Gate All Around Nanowire FET having enriched performances above the requirement of the 2 nm technology node of IRDS 2025. The device installs a combination of strain engineering based quantum well barrier system in the channel region with high-k spacers sandwiching the device underlaps and stack high-k gate-oxide. The underlaps are prone to parasitic resistance and various short channel effects (SCEs) hence, are sandwiched by HfO2 based high-k. This SCE degradations and a strong electric field in the drain-channel region is rendered controlling the leakages. The strain based Nanosystem engineering is incorporated with Type-II heterostructure band alignment inducing quantum well barrier mechanism in the ultra-thin cylindrical channel region creating an electrostatic charge centroid leading to energy band bending and splitting among the two-fold and four-fold valleys of the strained Silicon layer. This provides stupendous electron mobility instigating high current density and electron velocity in the channel. Thereby, the device is susceptible to on-current enhancement via ballistic transport of carriers and carrier confinement via succumbing of quantum charge carriers. The device transconductance, Ion, Ioff, Ion/Ioff ratio are measured and the output performance (ID-VDS) characteristics is determined providing emphatic enrichments in contrast to the existing gate all-around FETs as well as the 2 nm technology node data of IRDS 2025. Hence, the strained channel Nanowire FET device developed here is presented here as the device of the future for various digital applications, RF applications and faster switching speed.

To hear or not to hear: selective tidal stream transport can interfere with the detectability of migrating silver eels in a Tidal River

Acoustic telemetry provides valuable insights into behavioural patterns of aquatic animals such as downstream migrating European eels (Anguilla anguilla), so called silver eels. The behaviour of silver eels during the migration is known to be influenced by environmental factors, yet so is the performance of acoustic telemetry networks. This study quantifies the impact of these environmental factors on both, migration behaviour and receiver performance to determine possible limiting conditions for detecting tagged eels in tidal areas. A dominance analysis of the selected models describing migration speed, activity and receiver performance was conducted following 234 silver eels that were tagged with acoustic transmitters and observed by a receiver network in the Ems River during two subsequent migration seasons. The results suggest a passive locomotion of silver eels during their downstream migration by taking advantage of selective tidal stream transport (STST). It is further shown that water temperature, salinity, turbidity, precipitation, and especially current velocity were major parameters influencing migration activity and speed. At the same time, analyses of the detection probability of tagged eels under varying environmental conditions indicated a decreased receiver performance during increased current velocities, meaning that high migration activity and -speed coincides with reduced detection probability. Consequently, there is a risk that particularly during phases of increased activity, migration activity may be underestimated due to reduced acoustic telemetry performance. To avoid bias in telemetry studies, it is, therefore, crucial to conduct range tests and adjust the receiver placement in areas and conditions of high current velocities.

Vertical sediment distribution mechanism in tidal flats -A case study in Zhoushan Archipelago

Tidal flats connect terrestrial and oceanic environments, and play an important role in the evolution of coastal ecosystems and geomorphology. Taking tidal flat on Zhairuoshan Island (ZRS) as an example, we studied the dynamic characteristics of suspended sediment concentration (SSC), sediment resuspension, and transportation in tidal flats. The field data of the waves, currents, and sediments were obtained from the tidal flat on the east of ZRS Island. The results show that current-induced bed shear stress is dominant. Under the combined action of waves, tides, and currents, SSC has two main periods (12.2 h, 6.2 h, 60−80%) and three minor periods (23.5 h, 8.2 h, and 5.1 h, 20−40%). Advection transport is dominant, at approximately 2–3 times the sediment resuspension, and the net sediment flux is onshore. Turbulent intermittent events play an important role in sediment resuspension, and their probability is affected by velocity. The vertical instantaneous transport of sediment depends on wave height. The vertical profile of SSC shows that, at high current velocity, the sedimentation formula that considers flocculation better fits the vertical profile of suspended sediment using the Rouse formula. The settling velocity of a single particle fits better at low current velocities, indicating that the flocculation of sediment is weak at low velocities. This study improves our understanding on near-bottom sediment characteristics and controlling mechanism in tidal flat in muddy coasts.

Improving cathode cleaning and current efficiency by regulating loose scale deposition in scale inhibitor-containing water

For electrochemical water softening in practical water containing scale inhibitor, unsatisfactory cathodic cleaning and low hardness removal effect greatly limit its wide large-scale application. An effective improvement is not only dependent on the performance of external mechanical cleaning, but also closely related to the scale structure itself. In this work, the deposition of loose scale layer in the presence of scale inhibitor and its promotion on current efficiency and cathode cleaning efficiency were investigated from interfacial electrochemical perspective. Results reveal that such scale formation is attributed to the relatively high enrichment degree of scaling ions at cathode-water interface. At a high current density or low flow velocity, a relatively high interfacial pH is obtained and the thickness of this alkaline interface is larger than that at lower current density or higher flow velocity. This leads to the increase of CO32− generated by the reaction of HCO3− with more OH−. Besides, due to the weak chelation ability of the scale inhibitor at the alkaline interface, more Ca2+ are released and interact with cathodically formed CO32–. Corresponding scaling driving force is higher than that at lower current density or higher flow velocity. This leads to the deposition of loose scale layer with less stable morphology and a higher specific surface area. This as-deposited scale layer not only acting as seeds accelerates the crystallization and enhances the current efficiency, but also acting as an fragile pre-deposition layer facilitates the cleaning of cathode and produces the highest cleaning efficiency over 95%. This is beneficial for the stable operation of hardness removal device.

Residual circulations and wind conditions affect the transport and distribution of Pb

Hydrodynamic conditions affect the dispersion and transportation of pollutants. In this study, 102 surface water samples were collected and numerical simulations were conducted by coupling SWAT (Soil and Water Assessment Tool) and MIKE 21 models to investigate the effect of hydrodynamic conditions on the transport and distribution of Pb in Jiaozhou Bay, China. The tidal current velocity was lowest at flood tide and highest at ebb tide in summer, mainly due to the strong runoff caused by heavy rainfall in summer. The velocity of the residual current near the bay mouth was high, with an average value of 0.32 m/s. The residual currents had alternating clockwise and counter-clockwise circulations of different diameters in the bay mouth and the northern bay. The circulation diameter in the bay mouth area was large, and the circulation extended from the bay mouth to the Yellow Sea. Due to the small circulation diameters and opposite directions of the adjacent circulations in the northern bay, pollutants were easily trapped there. The dissolved Pb particles released from the northern bay mainly migrated to the shore, while the dissolved Pb particles released from the bay mouth tended to migrate to the Yellow Sea. The dominant wind directions and residual currents were the main driving forces affecting the trajectories of the particle tracking simulations. The dissolved Pb concentration in the water was high near shore and in the southwestern bay and under the combined effects of tidal residual current, wind force, and river and wastewater inputs. The low Pb concentration near the bay mouth was mainly influenced by the high residual current velocity. These results can provide improved insights into the mechanism of Pb transport and distribution in a semi-enclosed bay.

MACROZOOBENTHOS DIVERSITY AS A BIOINDICATOR OF WATER QUALITY AROUND THE CENTER POINT OF INDONESIA (CPI)

Benthos are invertebrate animals that live at the bottom of the waters. One biota that can be used as a biological parameter in determining the quality of a waters is macrozoobenthos. The waters around the Center Point of Indonesia (CPI) are one of the waters that have received a lot of additional organic matter and other pollutant materials through the estuaries of the canals that flow into the waters of Losari Beach. The study was conducted by sampling macrozoobenthos at each station using Ejkman Grab at each station 3 times sampling then sieved using a benthos sieve measuring 1 mm. In addition, measurements of environmental factors were carried out at each station with 3 replications directly in the field and analyzed in the laboratory. The results of the macrozoobenthos sampling study obtained 4 macrozoobenthos classes found at the study site, namely the Gastropod Class (4 species), the Crustacea Class, the Ophiuroidea Class, and the Oligochaeta Class each 1 species. The highest diversity and abundance of macrozoobenthos was found at Station 6 which is suspected because this station has a sandy sediment texture and sediment DOM content and high concentrations of water solubility of oxygen which support the life of makzoobenthos. Macrozoobenthos diversity at the study site (waters around the Center Point of Indonesia) is low with a diversity index (H') value of 0.00 - 0.16. The high abundance of makzoobenthos at Station 5 and Station 6 is characterized by high current velocity, sediment DOM and high oxygen concentration.

Environmental Parameters And Phytoplankton Community Quality: Implications For Food Safety In Coastal Communities

This study aimed to evaluate environmental parameters and phytoplankton quality as well as the implications for the food safety of seafood-consuming coastal communities. The study was conducted in coastal waters of South Sulawesi, Indonesia at four sites (three stations/site): Pangkep (PK), Kuri (KR), Maros (MR), and Tallo (TL) during June, August, and October 2020. Observations formed three spatial clusters: MR1, PK; MR2, MR3, TL; KR (92.5 % similarity), and two temporal clusters: June 2020; August and October 2020 (87.9 % similarity). PCA showed parameters most strongly characterizing TL, KR, and MR (except MR1) were high levels of salinity, nitrate, nitrite, ammonium, silicate, temperature, pH, and abundance of both phytoplankton that can form harmful algal blooms (HABs) and non-HAB forming phytoplankton (non-HABs), with low current velocity. PK was characterized by high current velocity, non-HABs, orthophosphate, ammonium, nitrite, and turbidity with low nitrate and HABs. Main characterizing parameters in the temporal PCA were high current velocity and HABs with low orthophosphate in June 2020; high nitrate, ammonium, orthophosphate, and non-HABs with even lower HABs in August 2020; high turbidity, temperature, and salinity with low pH, nitrite, silicate, and HABs in October 2020. Current velocity (low at TL, KR, MR and high at PK) influenced both non-HABs and HABs. The results indicate that faster currents may have impeded HABs development so that the phytoplankton quality was still good. Therefore, the fish from these waters can be considered safe for human consumption, thereby contributing to maintaining the health of coastal communities.

Assessing the Tidal Influences on the Coastal Erosion-Accretion Processes in the Indian Sundarban Delta using Hydrodynamic Models and Geoinformatics

The Sundarbans is currently a subject of serious concern due to the massive coastline erosion that is responsible for reshaping the geomorphological content of the islands. The coastal islands of the Sundarbans have become more susceptible to sea-level rise, cyclonic storms, and coastal flooding, which further accelerate the process of coastal erosion. Identification of erosion-prone regions is highly essential for integrated coastal zone management. Till date, no scientific study has been done to relate the coastal erosion accretion process with tidal velocity and current direction in the Indian Sundarbans. In this perspective, the present study identifies the erosion-accretion zones in the Indian Sundarbans using Landsat imageries of 2000 and 2020 using geoinformatics. It investigates the impact of tidal velocity and direction on the erosion accretion processes. MIKE 21 two-dimensional hydrodynamic model was simulated to predict tidal velocities and directions. The study finds that the sea-facing islands of the Indian Sundarbans experience the maximum amount of erosion. A significant shrinking of the land area takes place in the Bulchery, Bhangaduiani, and Dalhousie islands. The rates of erosion of these islands are 485.99 m/year, 487 m/year, and 480.65 m/year, respectively. Places with high velocity are found to be erosion-prone, while the rate of erosion significantly varied with the flow directions. Two islands, Bulchery and Dalhousie, on the south-eastern margin of the delta, suffered maximum erosion over the past decades due to the impact of high-velocity currents from the southeast and southwest, respectively. Alternatively, minor accretions were observed along with the sheltered island precincts where velocities were comparatively low. Such studies on the current drive erosion–accretion processes along the delta margin during sea-level rise appear to be of utmost importance for sustainable coastal protection and management.

Zoo- and Ichthyoplankton in the Communities of Aquatic Plants and in the Adjacent Sections of the Littoral Zone of Various Water Bodies in the Lower Reaches of the Desna River

Zoo- and ichthyoplankton in the littoral zone of various water bodies of the lower reaches of the Desna River occurring in the communities of aquatic plants essentially differed from those registered in the adjacent sections free of vegetation. The complicated structure of the environment formed by aquatic plants was responsible for higher taxonomic richness, numbers, and biomass of zooplankton communities compared to the shallow water zone free of vegetation. It should be noted that this phenomenon was observed in early summer as well during the period of ichthyoplankton mass development, which is indicative of the sufficient food base. The highest quantitative indices of zooplankton development were registered in multispecies communities of aquatic plants in floodplain water bodies especially under lentic conditions unlike those occurring in mono-species communities of macrophytes observed in the main riverbed of the Desna River, the development of which was limited by high current velocity. Ichthyoplankton was also registered mainly in macrophyte communities. Its highest species richness, quantitative indices, and the number of size-age groups were observed in the communities of aquatic plants in the semi-flow floodplain water body hydraulically connected to the main riverbed.

Methodology in Setting-Up a Three-Dimensional Flow Model for the Strait of Malacca, Malaysia

As a rapidly developing and expanding country, Malaysia is expected to see an increase in its electricity consumption in the near future. Although known for its abundance of natural resources, specifically petroleum and natural gas, Malaysia has pledged to reduce its dependency on conventional resources and aims to become a carbon-neutral nation by the year 2050. This can be achieved by unlocking sustainable alternatives from the ocean, such as waves and tidal current energy, which are known to be abundant, continuous, and clean. Although various studies had identified several locations along the Straits of Malacca with potential to be used as deployment sites for tidal stream turbines, most of them were focused only on theoretical resource assessment. Since detailed three-dimensional flow models for the Malacca Strait have yet to be thoroughly developed, examined, and discussed, this study aims to provide a preliminary methodology for setting up a three-dimensional numerical model for this region. The analysis of the study consists of three steps: pre-processing using Blue Kenue; processing with Telemac 3D; and post-processing, which visualizes the simulation outcome. The output from the simulation is validated against published measurement data to ensure the accuracy and robustness of the numerical model. The simulation outcome reveals that the southeast part of the Malacca Strait could be a promising area for deploying tidal stream turbines due to the high tidal current velocity in that area. Additionally, it is also observed that the kinetic energy flux increases towards the southeast part of the strait due to the strait's narrow size in that area. Overall, a detailed procedure for setting up a three-dimensional flow model for the Strait of Malacca is presented, and it is hoped that this work could highlight some of the complexity involved in developing an ocean-scale model for this region.

Identification of potential areas for upwelling based on characteristics of eddies event in the Bone gulf

The Bone Gulf is one of the waters with massive potential for catching pelagic fish. The potential for catching fish is associated with oceanographic events in the waters. This study aims to see the potential for upwelling in the eddy occurrence areas. The spatial analysis method will be used to see the potential for upwelling in the area of the eddy occurrence and will be associated with sea surface temperature parameters and chlorophyll-a as a fertility variable. The results of this study indicate that eddy events occur every month in the Bone Gulf waters. In addition, it can be proven that every eddy occurrence is not a guarantee for the emergence of upwelling areas simultaneously. However, several eddy events with high current velocities were seen to cause a decrease in temperature in the middle of the eddy and increase the chlorophyll-a concentration in that area. In addition, the suitability of this event is also shown from the time of upwelling, which occurs several months before the peak month of fishing in the Bone Gulf. This research is expected to be the basis for the sustainable development of fisheries.

Dark diversity in Amazonian stream fish communities: What factors determine species absence along environmental gradients?

Abstract Species distribution models often fail to predict observed patterns of species diversity, and this is because some species within a regional pool that are tolerant of conditions at a given location may nevertheless be absent from the local community. These missing species have been termed “dark diversity”. In the present study, we investigated which factors explain dark diversity among fish assemblages in Amazonian streams. We sampled 71 streams in areas with different types of land use within two river basins and estimated dark diversity from patterns of species co‐occurrence, using Beals’ index, along environmental gradients. From this procedure, taxa are designated as dark diversity components when they are absent from a given stream, but often co‐occur with the local species at other streams, indicating similar ecological requirements. We used generalised linear models both to determine whether environmental or landscape variables, connectivity, instream environmental heterogeneity or some combination of these factors explained dark diversity of fishes, and to evaluate whether ecomorphology is associated with the extent to which a species contributes to dark diversity and which specific traits contribute the most to explaining variation in dark diversity. Mean local diversity exceeded observed dark diversity. The magnitude of dark diversity was directly associated with the proportion of secondary forest in the immediate catchment and with the index of proximity to anthropogenic impact. Species that have high affinity for environments with higher current velocity, low swimming ability and that capture food mainly on the surface contributed more to dark diversity, which suggests that swimming ability, habitat preference and aspects related to diet are key predictors of the probability that a given species will be present at locations with suitable habitat. Our findings reinforce the idea that dark diversity results from interactions between species traits and environmental factors, including anthropogenic impacts. Understanding the interplay among environmental factors and species traits that contribute to dark diversity provides targets for improved ecosystem restoration and sustainability of native species assemblages.

Research on an All-Flow Velocity Control Strategy for a 120 kW Variable-Pitch Horizontal Axis Tidal Current Turbine

Horizontal-axis tidal current turbines have considerable potential to harvest renewable energy from ocean tides. The pitch control system is a critical part of variable-pitch tidal turbines. Existing control strategies for tidal turbines mainly rely on flow measurement devices to obtain tidal velocities, which are costly and subject to many limitations in practical applications, making them unsuitable for small off-grid tidal turbines. In this paper, we propose a pitch control strategy for a 120 kW horizontal-axis tidal current turbine based on the output power of the generator. The torque of the turbine was calculated based on the blade element momentum theory, and a dynamic model of the tidal turbine was established. The dynamic characteristics of the turbine and generator were studied under various flow rates and pitch angles. On the basis of the characteristic analysis, the generating efficiency of the unit was improved under a low flow rate, and the output power was limited to a rated value under high-current velocity by regulating the pitch angle. Furthermore, a novel protection and start up strategy is proposed to protect the unit and make full use of the tidal energy when the tidal current velocity exceeds the limit value. We performed simulations, the obtained results of which demonstrate the effectiveness and advantages of the designed control strategies.

Simulating Dynamics and Ecology of the Sea Ice of the White Sea by the Coupled Ice–Ocean Numerical Model

In this paper, a numerical model of the White Sea is presented. The White Sea is a small shallow sea with strong tidal currents and complex ice behavior. The model is the only comprehensive numerical model for the White Sea. It consists of several coupled submodels (for water, ice, pelagic, and sympagic ecology). In this work, the focus is on the dynamics of sea ice and its ecosystem. The model is described and its results are compared to available sea–ice data, mostly satellite data. The spatial resolution of the model is 3 km. High current velocities require the time step of 3 min. The model is shown to reproduce sea–ice concentration well; in particular, timing of the sea ice is perfect. The dynamics of the sea–ice ecosystem also looks reasonable. Chlorophyll-a content agrees well with measurements, and the ratio of algal, bacterial, and faunal biomass is correct. Sympagic biomass is underestimated. Light is limiting at the early stage of sympagic bloom, nutrient limitation is for the second half. We show that sympagic component influences the spring bloom (in terms of timing and height of the peaks) but has little effect on the dynamics during the warm period of the year.

Planktonic scenario of tributaries of the river Ganga in upper stretch

River Nayar, River Havel and River Balganga are small but important tributaries of river Ganga in its upper stretch. Their water quality and planktonic structure influence biodiversity of river Ganga and may play important roles in biodiversity conservation of valuable fauna and flora of these small rivers. Water of all these tributaries revealed high dissolved oxygen (7.3 - 10 mg L-1), very low BOD (0.4 - 2.4 mg L-1), high current velocity (0.6 - 1.7 m s-1). High silica content of the tributaries was reflected in diatom abundance anddiversity. Plankton population was dominated by diatoms only and no zooplanktonwere recorded. Plankton abundance ranged between 80-1250 u L-1 in Balganga with 15 genera, *140-1290 u L-1 in Havel with 14 genera and 1290-8010 u L-1 in Nayar with 28 genera. In river Nayar plankton population contributed to only three algal groups, viz. Bacillariophyceae, Chlorophyceae and Myxophyceae, which ranged from 45.7 - 97.8%, 1.5 - 54.2% and 0.6 - 2.2 %, respectively. River Havel and river Balganga depicted diatoms as planktonic flora. River Nayar exhibited maximum planktonic diversity followed by river Havel and Balganga. Dominant taxa in summer season were Asterionella, Cymbella, Melosira (diatoms) Spirogyra, Ankistrodesmus, Cosmarium (green algae) andPhormidium (blue green). Dominant taxa in monsoon were Microspora, Pinnularia, Tabellaria and Cymbella. Winter taxa were Tryblionella, Navicula, Melosira, Tabellaria, Denticula, Diatoma and Eunotia. River Nayar may be considered as Hotspot for phytoplankton diversity conservation as physical condition of river is suitable for diatom growth and species richness. River Havel is influenced by tourist activity/ anthropogenic effects, as reflected by higher TDS and specific conductivity but Balganga is havingclear water. Periphytic populations in Balganga, Nayar and Havel were maximum in winter followed by summer and monsoon, and dominated by various species of small diatoms like Eunotia, Cymbella, Navicula and Gomphonema.

Model Predictive Control-Based Collision Avoidance for Autonomous Surface Vehicles in Congested Inland Waters

Compared with open waters, congested inland waters have narrow waterways, many river-crossing bridges, a high density of navigation, and high current velocity in some sections. In this study, an improved collision avoidance algorithm based on model predictive control (MPC) is proposed to solve the problem of collision avoidance for autonomous surface vehicles (ASVs) in congested inland waters. First, considering the influence of current, the collision avoidance problem of ASVs is transformed into a nonlinear programming problem, and the kinematics of ASVs and the boundary of the channel are regarded as its inequality constraints. Next, since ASVs cannot perform large-scale collision avoidance in congested inland waters, the strategy of reducing the speed and slightly changing the yaw angle is adopted to realize collision avoidance. Then, an improved dynamic bumper model is used to model the safe zone of ASVs and dynamic obstacles, which improves the efficiency of the algorithm and the safety of ASVs. Finally, the collision avoidance rules and the evaluation function of the collision avoidance maneuver are constructed in the cost function of the algorithm. The simulation experiments in different encounter scenarios show that the proposed algorithm significantly improves the rationality and compliance of ASVs’ autonomous collision avoidance in congested inland waters.

Study on the Methods to Estimate the Bed Shear Stresses in Yangtze Estuary

High spatial and temporal current resolution velocities were measured by use of an acoustic Doppler velocimeter (ADV) in the tidal bottom layer of North Passage Deep water Navigational Channel (DNC) of Yangtze Estuary in China. And the vertical profiles of mean velocity were measured by using a pulse-coherent acoustic Doppler profiler (PC-ADP). The measured data show the vertical profiles of mean velocity are almost logarithmic. For discussing, the turbulent kinetic energy (TKE) method based on the velocity for ADVs-based is chosen and a new logarithmic profile (LP) method ignoring the influence of complex variation of the parameter Z 0 is suggested to estimate the bed shear stresses respectively. The calculated results show that the bed shear stresses estimated by LP method are larger than the ones estimated by TKE method mostly in Yangtze estuary. At the moment of ebb peak, the maximum error can be about twice occurring and the error of ebb tide is almost greater than the one of flood tide. These phenomena are explained in this study by the deduction of formula for steady and unsteady flow. It is concluded that the vertical logarithmic profile of velocities will be influenced by unsteady flow and there is not the same bed shear stress even if there is the same logarithmic velocity profile in vertical. Generally, this has led to a necessary adjustment of the drag coefficient Cd of steady flow in flow model to avoid overestimating the bed stress for unsteady flow in estuary based on the observed data.

Natural and anthropogenic influences on the development of mud depocenters in the southwestern Baltic Sea

The morphological evolution of two mud depocenters in the southwestern Baltic Sea is investigated by comparison of numerical model results to geological and oceanographic data. The pathways of dense currents during episodic dense-water inflows from the North Sea are shown to correspond to current pathways inferred from contouritic depositional geometries in the flow-confining channels within the study area. A favorable comparison of model results to published current speed observations shows that the mesoscale dynamics of individual inflow events are reproduced by the model, indicating that external forcing and basin geometry rather than internal dynamics control the mesoscale dynamics of inflow events. The bottom current directions during inflows show high stability in the flow-confining channels and explain the contouritic depositional geometries. Asymmetric depositional features in the channels are qualitatively reproduced in the model. Bottom currents are less stable in areas without contouritic features, possibly resulting in an overall diffusive effect on sediment distribution in those areas. In a simulation of resuspension by bottom-contacting fishing gear, inter-basin sediment transport is increased by 4–30%, depending on the area, compared to the case of natural hydrodynamic resuspension. The model predicts an increased winnowing of the finest sediment fraction due to bottom trawling, leading to an overall coarsening-to-fining trend in the direction of net sediment transport. The results show that rather than hemi-pelagic background sedimentation, episodic events with high bottom current velocities as well as bottom-trawling induced resuspension are responsible for the present-day and future morphological configuration of the mud depocenters in the southwestern Baltic Sea.