Ebb Tide Research Articles

Identification of Marine Debris and Its Distribution Using Unmanned Aerial Vehicle (UAV) on the Cirebon Coastal Area, Indonesia

Marine debris can be a significant problem when it enters the ocean. One of the areas, which has a marine debris problem is Cirebon Coast. Such a problem occurs due to the high human population and activities in this particular coastal area. An effective cleaning method is required to prevent this problem. However, to determine the cleaning method, comprehensive information about the debris condition is required. Therefore, this study aims to identify the dominant types of marine debris and analyze the effect of tides on the characteristics and distribution of debris on the Cirebon Coast. This study carried out marine debris identification by orthophoto obtained from a DJI Phantom 4 Pro Unmanned Aerial Vehicle (UAV)/drone. The dominant types of marine debris on the Cirebon Coast are plastic and styrofoam. Based on the aerial imagery validation results, plastic and styrofoam larger than Ground Sample Distance (GSD) are easily detected. Visually, debris in Karang Anom more than in Rawa Urip. The change in tidal height can affect debris not visible on the orthophoto at the maximum water level. In addition, the tides can also move marine debris varying from 10 to 50 cm from its previous position. The debris area at Rawa Urip Beach when slack before flood tide (55.53 m2) is larger than the area when slack before ebb tide (52.71 m2). The debris area at Karang Anom Beach at low tide (129.89 m2) is larger than when slack before ebb tide (75.79 m2). This study revelaed that the factors affecting debris area on the Cirebon Coast comprise tidal height, seawater visibility, and the beach structure (slope and coast components).

Effective Flood Control Method in River Downstream Affected by Tidal Effect Using Optimal Operation of Estuary Barrage

This article introduces an effective flood control method in the downstream of a river through the optimal operation of a barrage that is connected with a sea. A new operating model for flood control of an estuary barrage that can comprehensively evaluate the intensive discharge through the floodgates at the estuary barrage during low tide and the flood propagation process in the downstream river system is proposed. The optimization method applied to the operating model of the barrage combines a one-dimensional hydrodynamic model to analyze the flood propagation process in river channels and a search algorithm based on an iterative method to organize the maximum discharge at the estuary barrage during the ebb tide periods. The proposed method was applied to the flood control of the Taedong River downstream in the Democratic People's Republic of Korea, and its effectiveness was shown and evaluated through modeling results. By imagining the downstream boundary of the river before and after the construction of the West Sea Barrage, the inundation simulation at the Taean gauging station in the downstream of the Taedong River for floods of different average recurrence interval (ARI) was achieved by employing the proposed method.

Acoustic Monitoring of Tidal Flow and Salinity in a Tidal Channel

Fluvial Acoustic Tomography (FAT) is a powerful hydroacoustic system used in the investigations of estuarine dynamics. This research was designed to explore the effectiveness of using a novel and promising method of monitoring the continuous salinity intrusion and velocity distribution in a tidal channel using the FAT scheme. Four FAT units were installed near the riverbanks using a new zigzag system in a rectangular tomographic domain of 700 m × 170 m. The water velocities and salinities measured by FAT in this study were comparable to the data captured by traditional methods, including moving-boat Stream–Pro ADCP, CTD, and CT sensors. It was found that the delays in time between the maximum salinity and high water level along the channel ranged from 12 min to 1.5 h, with these time lags increasing seaward, primarily due to changes in freshwater flows upstream. In addition, the longitudinal salinity gradient was found to decrease toward the mouth of the river, with tide-driven mechanisms most likely being dominant in the dispersion process. The estuary is ebb-dominant, with an asymmetry in the ebb-tide and flood-tide velocities and the highest velocities occurring during the ebb tide. Furthermore, the residual current was found to be affected primarily by the freshwater discharge from upstream.

Modeling on 137Cs Radioactive Dispersion in Gosong Coast as The Candidate Location for Nuclear Power Plant

The study of radioactive dispersion in the ocean should be conducted to prepare the construction of nuclear power plant (NPP) in Gosong Coast, West Kalimantan. This study estimated the distribution of 137Cs radioactive from various scenarios of radioactive waste dumping if nuclear emergency is occurred during NPP’s operation. These scenarios were distinguished based on their volume discharges of radioactive waste into the ocean, included 10 m3 (Scenario I), 50 m3 (Scenario II), and 100 m3 (Scenario III). Model dispersions were constructed for 15 days by Delft3D-Flow module. The simulation showed that ocean current directions were not significantly different among spring and neap tide, instead the ocean current during the spring period dominantly increased rather than neap period. Ocean currents at Gosong Coast flowed parallel to the shoreline towards Singkawang Coastal Area during ebb tide. Meanwhile, during flood tide, ocean currents at Gosong Coast flowed offshore through Burung Archipelagic. The dispersed model showed the distribution of 137Cs radioactive for 15 days reaching to coastal areas of Burung Archipelagic, Singkawang, and Southern Sambas Coast. Each scenario of the disposal system did not influence the marine pollution of the West Kalimantan Sea.

ADV-Based Investigation on Bed Level Changes Over a Meso-Macro Tidal Beach

Beach intra-tidal bed level changes are of significance to coastal protection amid global climate changes. However, due to the limitation of instruments and the disturbance induced by wave motions superimposed on water levels, it was difficult to detect the high-frequency oscillation of the submerged beach bed level. In this study, an observation, lasting for 12 days and covering the middle tide to the following spring tide, was conducted on a meso-macro tidal beach, Yintan Beach, to simultaneously detect the characteristics and influence mechanism of bed level changes at intra-tidal and tidal cycle scales. The collected data included water depth, suspended sediment concentration (SSC), waves, high-frequency three-dimensional (3-D) velocity, and the distance of the seabed to the acoustic Doppler velocimeter (ADV) probe, which were measured by an optical backscatter sensor, two Tide & Wave Recorder-2050s, and an ADV, respectively. The results showed that the tidal cycle-averaged bed level decreased by 58.8 mm, increased by 12.6 mm, and increased by 28.9 mm during neap, middle, and spring tides in succession, respectively, compared with the preceding tidal regimes. The net erosion mainly resulted from large incident wave heights and the consequent strong offshore-directed sediment transport induced by undertows. Moreover, the variations in the bed level were more prominent during a neap to middle tides than during middle to spring tides, which were jointly caused by the wave-breaking probability regulated by water depth and the relative residence times of shoaling wave, breaker, and surf zones that were determined by relative tidal range. In terms of the intra-tidal bed level, it displayed an intra-tidal tendency of increase during floods and decrease during ebb tides, i.e., the intra-tidal bed level changes were controlled by water depth, which modulated the effects of waves on sediment resuspension and vertical sediment exchange. To be specific, waves and SSC were responsible for the intra-tidal bed level changes under low-energy wave conditions, while mean current and bedform exerted important influences on the variations of the intra-tidal bed level under moderate wave conditions, which broke the foregoing interrelation between bed level, waves, and SSC. This study, therefore, emphasizes the usage of ADV measurement to investigate bed level changes in sandy coasts.

Aeolian sand transport influenced by tide and beachface morphology

Beach aeolian sand transport is one of the most complicated but significant processes in beach-dune interactions. Focusing on tidal and morphological effects, field observations over several tidal cycles were carried out successively on two macrotidal and gently sloped beaches to clarify the characteristics, dominant factors and possible mechanism of aeolian sand transport in the intertidal zone. The results show that there is a clear “time window” for aeolian sand transport in the intertidal zone under the joint effect of tidal action and ridge morphology. During ebb tide, the factors controlling the initialization of intertidal aeolian sand transport shifted sequentially from ridge inundation (controlled by tide) to seepage, capillary and drying processes. During the time window, the fluctuations of dew point appear to be a good indicator of the variations in aeolian transport rates. During flooding, the dominant factor for the cessation of intertidal aeolian sand transport is inundation (controlled by the tidal elevation). Further exploration shows that for the wide and gently sloped beachface, the presence of intertidal ridges may be necessary for the aeolian process to entrain and transport sand from the moist intertidal zone. Finally, a conceptual model for the intertidal aeolian sand process under the joint effect of tide and ridge morphology is proposed.

Atlantic sturgeon presence in a designated marine hydrokinetic test site prior to turbine deployment: A baseline study

In 2009, the Fundy Ocean Research Centre for Energy (FORCE) developed a 1.6 x 1.0 km Crown Lease Area (CLA) to test Marine Hydrokinetic devices (MHKs) in Minas Passage (MP), a strait that connects the Bay of Fundy to Minas Basin (MB), Nova Scotia. Minas Basin is an important summer feeding aggregation site for numerous fishes including Atlantic sturgeon (Acipenser oxyrinchus Mitchill, 1815) stocks from Canada and the US. In this study, acoustic tagging technology was used to describe sturgeon presence in the CLA, as well as an adjacent site proposed for MHKs deployment. During 2018 and 2019, 33.9% (n = 19/56 Atlantic sturgeon tagged) and 29.9% (n = 23/77 Atlantic sturgeon tagged) respectively, of sturgeon acoustically tagged in MB during 2014–2019 were detected in these two FORCE sites. Binomial General Linear Models (GLMs) were fit to determine if environmental parameters influenced the presence, or depth of Atlantic sturgeon in the FORCE sites. The only significant predictor of Atlantic sturgeon depth and presence at the FORCE sites was tide type. Atlantic sturgeon was found to have a higher likelihood of being detected during ebb tide (12%) as opposed to flood tide (3%). Apparent decline in the presence of sturgeon during flood tide was probably due to poor signal reception caused by turbulence from higher current speeds. Tagged sturgeon travelled through the FORCE sites pelagically at similar depths (27.5 ± 14.57 m) to those proposed for seabed mounted MHKs (~30 m), travelling deeper during flood tide than ebb, suggesting they may be at risk of spatial overlap with MHKs. While this study indicated that the duration Atlantic sturgeon utilized the FORCE sites throughout the summer appeared to be low, sturgeon from both endangered and threatened stocks migrated through this region during their yearly entry and exit to MB and were present for approximately 12% of the days assessed in this study.

Utilization of an Unmanned Aerial Vehicle and Echo-Sounder for Shoreline Surveying

Park, J.K. and Jung, K.Y., 2021. Utilization of an Unmanned Aerial Vehicle and echo-sounder for shoreline surveying. In: Lee, J.L.; Suh, K.-S.; Lee, B.; Shin, S., and Lee, J. (eds.), Crisis and Integrated Management for Coastal and Marine Safety. Journal of Coastal Research, Special Issue No. 114, pp. 410–414. Coconut Creek (Florida), ISSN 0749-0208. The shoreline is a baseline that defines the shape of the national territory or defines the borders of the seas between countries, and it is important geospatial information to be managed by the nation for international and legal responses. In addition, importance is increasing day by day as a guideline for various laws that determine the spatial scope of the land, coast, and ocean, and closely related to people's lives. In this study, geospatial information was constructed using UAV (Unmanned Aerial Vehicle) and echo-sounder to investigate shoreline changes. UAV was used to survey the coastal area during ebb tide, and echo-sounder was used to measure areas where UAV is not applicable and to integrate with UAV data. The data surveyed by UAV was processed to create a DSM(Digital Surface Model) and ortho image. And echo-sounder was used for depth measurement during high tide, and was used to construct geospatial information for coastal areas by integrating with UAV data. As a result of the study, geospatial information about the study area could be constructed. The UAV results showed X, Y, and H deviations within 5.5 cm compared to the check point survey results using GNSS and it suggesting the possibility of using the results. The shoreline variation survey should be conducted from the existing “line concept” survey to the “plane concept” survey. In this regard, the result of integrating UAV and echo-sounder will be a way to effectively build plane data through data processing. Shoreline change survey using UAV and echo-sounder can be used to establish and manage standardized shoreline data and establish predictable mid- and long-term national maritime policies through continuous shoreline survey. In addition, it will help effective business execution as data for the execution of major marine policies such as coastal management and marine safety.

Hydrodynamical model during east season at Gosong Coastal West Borneo as candidate location of nuclear power plant in Indonesia

Indonesia planned to build the first nuclear power plant in Gosong Coast, Bengkayang Regency, West Kalimantan. This research examined the hydrodynamical conditions in the ocean of Gosong Coast during the eastern season. This hydrodynamic model can be considered in estimating the distribution of various radionuclide wastes that release to the ocean. It was simulated using the Delft3D flow module application for 15 days which included the neap and spring tide conditions. Base on the result of the hydrodynamical model, Gosong Coast had a mixed semidiurnal type of tide with low amplitude. The wind parameters involved higher impacts to the hydrodynamical conditions. The model result did not find significant differences between neap and spring tide periods. There was a flow collision between 2 opposite water currents which was occurred at Burung Archipelagic during flood tide and at the Coastal area of Singkawang City during ebb tide. Therefore, the ocean currents at Gosong Coast flowed directly offshore through Burung Archipelagic during ebb tide. Meanwhile during flood tide, these ocean currents moved to Singkawang and Sambas Coastal area before they deflected toward offshore.

Surface currents modeling based on tidal cycles and monsoon in tropical estuary, Indonesia

The modeling uses a software-based numerical model DHI MIKE user interface developed by The Danish Hydraulic Institute (DHI) Water and Environment, Denmark. Simulation of current uses a 2-dimensional model (averaging to depth) with a finite element approach. Modeling of currents was conducted to know current dynamics by tidal cycle and seasonal variation (west and east monsoon). The models were validated using correlation coefficient value (r) by comparing direct measurement and model output. The r-value for current velocity during west monsoon was 0.653 and east monsoon was 0.697 and for current direct during west monsoon was 0.887 and the east monsoon was 0.857. While the r-value for the tide was 0.858. All these r values showed a strong correlation and these indicated the models were valid. The result of the simulation of the current models showed that the surface currents were strongly influenced by the tidal cycles. The currents direct flowed to the south at flood tide and to the eastern at ebb tide. The maximum current velocity during the west monsoon was 0.50 m/sec and during the east monsoon was 0.40 m/sec. The averages of currents direct were more dominant eastward of Jeneberang estuary.

High and Variable Drag in a Sinuous Estuary With Intermittent Stratification

AbstractIn field observations from a sinuous estuary, the drag coefficient based on the momentum balance was in the range of , much greater than expected from bottom friction alone. also varied at tidal and seasonal timescales. was greater during flood tides than ebbs, most notably during spring tides. The ebb tide was negatively correlated with river discharge, while the flood tide showed no dependence on discharge. The large values of are explained by form drag from flow separation at sharp channel bends. Greater water depths during flood tides corresponded with increased values of , consistent with the expected depth dependence for flow separation, as flow separation becomes stronger in deeper water. Additionally, the strength of the adverse pressure gradient downstream of the bend apex, which is indicative of flow separation, correlated with during flood tides. While generally increased with water depth, decreased for the highest water levels that corresponded with overbank flow. The decrease in may be due to the inhibition of flow separation with flow over the vegetated marsh. The dependence of during ebbs on discharge corresponds with the inhibition of flow separation by a favoring baroclinic pressure gradient that is locally generated at the bend apex due to curvature‐induced secondary circulation. This effect increases with stratification, which increases with discharge. Additional factors may contribute to the high drag, including secondary circulation, multiple scales of bedforms, and shallow shoals, but the observations suggest that flow separation is the primary source.

Suspended sediment floc size in the Cam - Nam Trieu estuary (Hai Phong, Vietnam), in wet season

Median diameters (D50) of suspended particles were inferred in the wet season in the Cam - Nam Trieu estuary (Hai Phong, Vietnam) based on the particle size distribution measured by LISST-100X on five transects along the river from 23 to 26 September 2015. The results showed that floc diameters varied between 3.6 μm to 146.5 μm and averaged 49.14 μm. At high tide, the average floc size D50 was lowest (42.66 ± 11.55 µm). It reached the highest value in the ebb tide (62.87 ± 23.34 µm) and then decreased to intermediate values in the flood tide (48.75 ± 15.72 µm). The coefficient of variation of the mean floc size D50 was lowest in the high tide (27.05%), highest in ebb tide (37.13%), then intermediate in the flood tide (32.12%).

Preliminary Identification to Local Coral Bleaching Event in Manjuto Beach, Pesisir Selatan Regency, West Sumatra: Hydro-Oceanographic Perspectives

Highlight ResarchThe cause of local coral bleaching in Manjuto Beash has been addressed.The influence of ebb-tide cycles on salinity mixing and stratification was analyzed.Spatial analysis Digital Shoreline Analysis System (DSASv5) was conducted to determine the coastline changes in Manjuto Beach.Flow model flexible mesh was simulated to determine the flow pattern within Sungai Pinang Bay.AbstractIn October 2019, the local community reported the occurrence of coral bleaching of a colony of Acropora sp. at Manjuto Beach, Pesisir Selatan Regency experienced bleaching. It was published in several local news, becoming a trending topic among local and central government authorities and coastal communities. There were many inaccuracies about the cause of this phenomenon. This study aimed to identify the causes of local coral bleaching in Manjuto Beach based on oceanographic perspectives. The water quality data collected using TOA DKK water quality checker in the surrounding Manjuto Beach were assessed descriptive-statistically. This study also analyzed the spatial changes of the coastline using DSASv5. A time series of tidal data was also used to analyze the tidal range-induced salinity stratification. A flow model with a flexible mesh was also simulated to determine the water mass movement and longshore current patterns in Manjuto Beach. Dissolved oxygen (DO), temperature, and salinity showed anomalies compared to the water quality standard to support marine life. During both flood and ebb tides, it ranged from 5.8-11.2 mg/L, 28-28.3oC, and 25-28 o/oo, respectively. The other parameters measured (pH, conductivity, turbidity, and density) were suitable for marine biota. The findings show that tidal range has a unique influence on salinity stratification. The intrusion of groundwater supply resulted in lowering of salinity, inducing local coral bleaching in Manjuto Beach. Changes in salinity levels were also triggered by tidal current ranging from 0-0.31 m/s resulting in cumulative salinity shock. Currently, Manjuto Beach is experiencing accretion ranging from 2.36-3.17 m/year, altering the water coverage through the flood-ebb cycles. Those states cause cumulative sun rays’ exposures and salinity shock induced by flood-ebb cycles. That is why local coral bleaching event is undoubtedly avoided.

Horizontal variation of microplastics with tidal fluctuation in the Chao Phraya River Estuary, Thailand

Microplastic (MP) pollution in estuarine environments is poorly characterized globally, although they are extensive buffer regions between terrestrial, freshwater and seawater environments. This research aims to investigate MP pollution levels and variations of MPs abundance with tidal fluctuation. Fourteen samples were collected from the surface water of the Chao Phraya River Estuary, Thailand using the Manta net at flood and ebb tides. The average abundance of microplastics at flood tide was 5.16 × 105 particles/km2 and at ebb tide was 3.11 × 105 particles/km2. The abundance of microplastics in the estuary was directly related to the tidal fluctuation, creating an accumulation of microplastics in the study area. Polypropylene, polyethylene, and polystyrene were the most common polymers. The findings provide important information on the pollution status of microplastics in the Chao Phraya River Estuary and the variation of suspended microplastics with tidal fluctuation should be considered in future estuarine microplastic studies.

Study of a Bi-Vertical Axis Turbines Farm Using the Actuator Cylinder Method

Vertical axis turbines, which extract kinetic energy from currents, can produce electricity independently from a current’s direction. Hence, this type of turbines raises interest for harvesting energy from tidal currents, where flow changes direction during flood and ebb tides, and where currents present large variation of direction during tide. Methods for representing vertical axis turbines in tidal farms should be implemented in order to predict correctly power production with an acceptable time cost. The Actuator Cylinder (AC) is one of them. Numerical results in terms of wakes, with the study of velocity profiles, and efforts are compared to experiences, as well as showed that the method is sufficiently accurate and for a reasonable computing time, which is of prime importance for tidal turbines farms studies. The Actuator Cylinder method is implemented in ANSYS Fluent in a 2D stationary resolution. The method is then applied to a double levels of two counter-rotating rotors marine turbine designed by Hydroquest. Wake and power production of a single turbine and several farm configurations are studied under different current conditions (magnitude and direction).

Study on Hydrodynamic Characteristics and Environmental Response in Shantou Offshore Area

As a coastal trading city in China, Shantou has complex terrain and changeable sea conditions in its coastal waters. In order to better protect the coastal engineering and social property along the coast, based on the numerical simulation method, this paper constructed a detailed hydrodynamic model of the Shantou sea area, and the measured tide elevation and tidal current were used to verify the accuracy of the model. Based on the simulation results, the tide elevation and current in the study area were analyzed, including the flood and ebb tides of astronomical spring tide, the flood and ebb tides of astronomical neap tide, the high tide, and the low tide. In order to find the main tidal constituent types in this sea, the influence of different tidal constituents on tide elevation and tidal current in the study area was analyzed. At the same time, the storm surge model of the study area was constructed, and the flow field under Typhoon “Mangkhut” in the study area was simulated by using the real recorded data. Typhoon wind fields with different recurrence periods and intensities were constructed to simulate the change in the flow field, the sea water level, and the disaster situation along the coast. The results showed that under normal sea conditions, the sea water flows from southwest to northeast at flood tide and the flow direction is opposite at ebb tide. The tidal range is large in the northwest and small in the southeast of the study area. The tides in the study area are mainly controlled by M2, S2, K1, and O1 tidal constituents, but N2, K2, P1, and Q1 tidal constituents have significant effects on the high water level. The water level caused by typhoons increases significantly along the coast of Shantou City. In the west area of the Rong River estuary, a typhoon with a lower central pressure than 910 hPa may induce a water increase of more than 2 m.

Modeling Tidal Characteristics in a Creek-Marsh Drainage System: Implications for Stormwater Management

The traditional goal of stormwater management is to reduce the threat of flooding to life and property, and so most landscapes are engineered to maximize the speed at which the unwanted water leaves the watershed. This has been effective in landscapes with some topographic gradient. This often involves the installation of drainage ditches that disperse runoff from urban areas to receiving water bodies; in coastal areas this means a tidal creek, estuary, bay, sounds, or the coastal ocean. This practice reduces flood hazards in some cases but results in unintended effects on the natural hydrology in the watershed and downstream tidal dynamics. For low-gradient watersheds in humid climates, ditch systems also lower the water table of an area, increasing infiltration to recharge and groundwater discharge to streams (baseflow), and larger volume of freshwater delivered downstream yearround. Ditches also create unintentional avenues for the incoming tide from a tidal creek or tidally-influenced waterway to reach further inland, thus reducing the hydraulic gradient between the inland areas and the receiving water body. The combination of these effects can exacerbate compound flooding events, increasing the flood probability if high tide and storm events coincide. Additionally, coastal communities face the challenge of mitigating more complicated flood hazards while land development increases to meet the needs of a growing population. This study analyzed the tidal influence within an inland drainage ditch in the central coast of South Carolina USA that is representative of thousands of artificially-drained coastal watersheds. The ditch-creek system investigated here is 12 km long in a 753-hectare (1860-acre) watershed of Church Flats Creek, a first-order tidal system. We monitored for 13 months a 0.75-km reach of the lower ditch portion of the system, just above the relatively undisturbed tidal creek and marsh. Prior to ditching in the 1960s this system had a wetland-rich floodplain but is now partially tidal. Field data collected were stream stage (depth), discharge, tidal range, tidal volume, incoming (flood) and outgoing (ebb) tidal durations, and water table hydrograph at a location about 50 m of mid-reach of the ditch. Multiple linear regressions were performed to best predict the flood and ebb tidal durations of the system based on tidal characteristics within the ditch. The mean values were 229 ± 2.5 and 182 ± 2.1 minutes for flood and ebb tide durations, respectively and the models explained 84% (residual standard error (RSE) of 25 minutes) and 80% (RSE of 23 minutes) for the flood and ebb conditions, respectively. The models were simulated for sea levels in 1993 and 2050, and results indicate that the flood tide within the drainage ditch is predicted to increase an average of 66 minutes and the total tidal duration (flood and ebb) an average of 139 minutes by 2050. These results suggest a loss in drainage functionality as sea level rises. Increases in the duration of tidal influence will induce a lower capacity for stormwater volume than the drainage infrastructure was constructed to manage, therefore resulting in an increased frequency of compound flooding events because of the lower storage volume and decreased hydraulic gradient in the system. This study fills a knowledge gap of tidal dynamics within coastal ditch-creek systems and we urge stormwater managers to consider the unintended consequences of using traditional stormwater methods in a region that does not benefit from gravity drainage practices like in other regions.

Observations of Multiple Internal Wave Packets in a Tidal River Plume

AbstractRemotely sensed images document the occurrence of multiple packets of internal solitary waves (ISWs) in the Rhine River plume at the same time. We use a combination of field observations, and non‐hydrostatic and hydrostatic modeling to understand the processes that lead to the generation and retention of multiple ISW packets within the Rhine plume. Previous numerical modeling shows that the tidal plume front is trapped in the mid‐field plume for more than one tidal cycle due to tidal straining and recirculation within the plume, resulting in the presence of multiple fronts in the near‐and mid‐field plume regions. In this work, we show how variations in the strength of these fronts can lead to the release of ISW packets. We conclude that the retention of the fronts in the mid‐field region of the plume and modulation in the strength of the fronts can explain the presence of multiple ISW packets. A frontal Froude number analysis shows that fronts generated during the previous ebb tide can release ISWs in addition to the newly released tidal plume front.

Human Impacts on Estuarine Erosion-Deposition in Southern Central Vietnam: Observation and Hydrodynamic Simulation

This paper aims to identify the causes and sources of erosion and deposition at small estuaries in southern central Vietnam under human intervention. The jetty built at the Tam Quan river mouth (Binh Dinh Province, Vietnam) serves as the base for the study. After its completion at the end of 2009, the hydrodynamic and erosion-deposition processes in the region have been significantly altered. Inside the estuary, the waves are not influenced, but the currents are increased during the ebb tide period and decreased during the flood tide timeframe. During the southwest monsoon, the jetty could cause an increase in the deposition process in both frequency and area, whereas the erosion process tends to narrow the area and increase the frequency on the north coast. In contrast, both deposition and erosion processes are increased on the southern coast. About 5859 m3 of sediments are deposited in the channel gate mainly by local sources. During the northeast monsoon, both deposition and erosion processes are located over a narrow area with frequency increased on the north coast, whereas the deposition process is narrowed with higher frequency on the southern coast. The total amount of sediment deposited at the estuary is 56,446 m3, of which 74.2% is from the onsite erosion material, 15.8% from the river and 10% from the longshore transportation. Generally, due to mainly erosion-deposition processes, sediment volume is accumulated during the northeast monsoon with amount 9.6 times more than that the southwest monsoon. The erosion-deposition processes are contributed to by poor practical management and local human activities inland and in the coastal regions, as well as the natural situation, resulting in serious impacts on society, the economy and the environment. Hence, the governance of the erosion-deposition processes and sediment load in small estuaries appear to contribute to the master plan for the local sustainable development of society and the economy.

Exchange Flows in Tributary Creeks Enhance Dispersion by Tidal Trapping

The North River estuary (Massachusetts, USA) is a tidal marsh creek network where tidal dispersion processes dominate the salt balance. A field study using moorings, shipboard measurements, and drone surveys was conducted to characterize and quantify tidal trapping due to tributary creeks. During flood tide, saltwater propagates up the main channel and gets “trapped” in the creeks. The creeks inherit an axial salinity gradient from the time-varying salinity at their boundary with the main channel, but it is stronger than the salinity gradient of the main channel because of relatively weaker currents. The stronger salinity gradient drives a baroclinic circulation that stratifies the creeks, while the main channel remains well-mixed. Because of the creeks’ shorter geometries, tidal currents in the creeks lead those in the main channel; therefore, the creeks never fill with the saltiest water which passes the main channel junction. This velocity phase difference is enhanced by the exchange flow in the creeks, which fast-tracks the fresher surface layer in the creeks back to the main channel. Through ebb tide, the relatively fresh creek outflows introduce a negative salinity anomaly into the main channel, where it is advected downstream by the tide. Using high-resolution measurements, we empirically determine the salinity anomaly in the main channel resulting from its exchange with the creeks to calculate a dispersion rate due to trapping. Our dispersion rate is larger than theoretical estimates that neglect the exchange flow in the creeks. Trapping contributes more than half the landward salt flux in this region.