Scale Model Experiment on Local Scour around Submarine Pipelines under Bidirectional Tidal Currents

In nearshore regions, bidirectional tidal flow is the main hydrodynamic factor, which induces local scour around submarine pipelines. So far, most studies on scour around submarine pipelines only consider the action of unidirectional, steady currents and little attention has been paid to the situation of bidirectional tidal currents. To deeply understand scour characteristics and produce a more accurate prediction method in bidirectional tidal currents for engineering application, a series of laboratory scale experiments were conducted in a bidirectional current flume. The experiments were carried out at a length scale of 1:20 and the tidal currents were scaled with field measurements from Cezhen pipeline in Hangzhou Bay, China. The experimental results showed that under bidirectional tidal currents, the scour depth increased significantly during the first half of the tidal cycle and it only increased slightly when the flow of the tidal velocity was near maximum flood or ebb in the following tidal cycle. Compared with scour under a unidirectional steady current, the scour profile under a bidirectional tidal current was more symmetrical, and the scour depth in a bidirectional tidal current was on average 80% of that under a unidirectional, steady current based on maximum peak velocity. Based on previous research and the present experimental data, a more accurate fitted equation to predict the tidally induced live-bed scour depth around submarine pipelines was proposed and has been verified using field data from the Cezhen pipeline.

Coastal Groundwater Dynamics - Investigation of Using Geoelectrical and Hydrochemical Tools for Saline Intrusion Monitoring

<p>With over 65 % of the global population currently living in areas near a coast, increasing fresh groundwater demands within these areas, shifted precipitation patterns, and rising mean sea levels, increased seawater intrusion into coastal aquifers has become a major issue for groundwater resources in many coastal countries all around the world. Although there are many past studies researching the saline interface in affected aquifers from a modelling, laboratory or field perspective with different hydrological and geophysical approaches, little is known about real field dynamics over various time spans and in different geological settings. This PhD project aims at detecting and characterising seawater intrusion into a shallow coastal Holocene sand and gravel aquifer at New Zealand's west coast with respect to seasonally changing aquifer resistivity and hydrochemical tracers, as well as investigating resistivity and hydraulic property changes within a tidal time frame. Seawater mixing behaviour over different time spans was monitored with electrical resistivity tomography (ERT) over the course of two years, and additional hydrochemical sampling was carried out during the second year of the long-term seawater intrusion monitoring study. During two consecutive years, repeat ERT measurements were able to determine seasonal and shorter-term cycles in seawater mixing behaviour within the shallow coastal aquifer. There are strong indications that increasing urbanisation has a major influence on seasonal seawater intrusion patterns, and the dominant processes at the field locations were identified to be tied to the amount of freshwater available over the course of a year, as well as enhanced evapotranspiration / evaporation during summer. Hydrochemical data backed these observations and were also able to characterise seawater intrusion as a function of depth at the field location. Within the smaller tidal cycle investigations, similarly behaving parts of the aquifer could be identified from resistivity ratios and time series analyses. Varying amplitudes in resistivity changes led to the conclusion that saline mixing within a diurnal tidal cycle is strongly dependent on the recharge regime of the aquifer and decreases significantly for times during the year when recharge is continuous as opposed to times with little general recharge but intense, isolated rainfall events. In addition, tidal time series were used to qualitatively infer hydraulic properties of the aquifer and ultimately delineate preferential flow paths for seawater intrusion at the field site. The results of this project support findings of previous modelling, laboratory and short-term field studies, and put the concepts into a much broader time frame. For the first time the dynamics of seawater mixing in a shallow coastal sand aquifer were conclusively studied at the same location and within different time frames, and are thus of great value for sustainable groundwater management in the area of investigation and similar coastal environments.</p>

Coastal Groundwater Dynamics - Investigation of Using Geoelectrical and Hydrochemical Tools for Saline Intrusion Monitoring

<p>With over 65 % of the global population currently living in areas near a coast, increasing fresh groundwater demands within these areas, shifted precipitation patterns, and rising mean sea levels, increased seawater intrusion into coastal aquifers has become a major issue for groundwater resources in many coastal countries all around the world. Although there are many past studies researching the saline interface in affected aquifers from a modelling, laboratory or field perspective with different hydrological and geophysical approaches, little is known about real field dynamics over various time spans and in different geological settings. This PhD project aims at detecting and characterising seawater intrusion into a shallow coastal Holocene sand and gravel aquifer at New Zealand's west coast with respect to seasonally changing aquifer resistivity and hydrochemical tracers, as well as investigating resistivity and hydraulic property changes within a tidal time frame. Seawater mixing behaviour over different time spans was monitored with electrical resistivity tomography (ERT) over the course of two years, and additional hydrochemical sampling was carried out during the second year of the long-term seawater intrusion monitoring study. During two consecutive years, repeat ERT measurements were able to determine seasonal and shorter-term cycles in seawater mixing behaviour within the shallow coastal aquifer. There are strong indications that increasing urbanisation has a major influence on seasonal seawater intrusion patterns, and the dominant processes at the field locations were identified to be tied to the amount of freshwater available over the course of a year, as well as enhanced evapotranspiration / evaporation during summer. Hydrochemical data backed these observations and were also able to characterise seawater intrusion as a function of depth at the field location. Within the smaller tidal cycle investigations, similarly behaving parts of the aquifer could be identified from resistivity ratios and time series analyses. Varying amplitudes in resistivity changes led to the conclusion that saline mixing within a diurnal tidal cycle is strongly dependent on the recharge regime of the aquifer and decreases significantly for times during the year when recharge is continuous as opposed to times with little general recharge but intense, isolated rainfall events. In addition, tidal time series were used to qualitatively infer hydraulic properties of the aquifer and ultimately delineate preferential flow paths for seawater intrusion at the field site. The results of this project support findings of previous modelling, laboratory and short-term field studies, and put the concepts into a much broader time frame. For the first time the dynamics of seawater mixing in a shallow coastal sand aquifer were conclusively studied at the same location and within different time frames, and are thus of great value for sustainable groundwater management in the area of investigation and similar coastal environments.</p>

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 &amp; 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.

Recruitment patterns of freshwater amphidromous fishes (Pisces: Gobiidae, Eleotridae) to the Cimaja estuary, Palabuhanratu Bay

The sustainability of amphidromous fishes is determined by the success of the larval recruitment process from marine to freshwaters habitats. This study aimed to determine the recruitment pattern of freshwater amphidromous fish to the Cimaja River estuary, Palabuhanratu Bay in terms of season and daily. Amphidromous fish were caught monthly on the 25th of Hijri or waning crescent from December 2020 to August 2021. Sampling in one whole day with observation intervals every 4 hours was carried out in June 2021 to reveal the time of larvae and juveniles of the amphidromous fish recruit to the Cimaja River estuary. A total of 13 species of amphidromous fishes representing five genera and two families was captured. The recruitment of amphidromous fishes into the Cimaja River occurs between the peak of the rainy season to dry season. Generally, amphidromous recruit into the estuary waters of the Cimaja River is at the juvenile stage (11.9-21.8 mm body length). However, the recruitment of some amphidromous fish species appears at the postflexion stage (6.9-11.9 mm BL). Based on diel observations, larvae and juveniles of amphidromous fish are generally preferred to recruit to the Cimaja River estuary in the early morning (03:00 am) and afternoon (03:00 pm). Recruitment of amphidromous fish larvae to the Cimaja River follows a semi-diurnal tidal cycle which is a tidal type in Palabuhanratu Bay. Amphidromous fishes use tidal flux to recruit and migrate upstream of the Cimaja River through estuaries.

Reef manta rays forage on tidally driven, high density zooplankton patches in Hanifaru Bay, Maldives

Manta rays forage for zooplankton in tropical and subtropical marine environments, which are generally nutrient-poor. Feeding often occurs at predictable locations where these large, mobile cartilaginous fishes congregate to exploit ephemeral productivity hotspots. Investigating the zooplankton dynamics that lead to such feeding aggregations remains a key question for understanding their movement ecology. The aim of this study is to investigate the feeding environment at the largest known aggregation for reef manta rays Mobula alfredi in the world. We sampled zooplankton throughout the tidal cycle, and recorded M. alfredi activity and behaviour, alongside environmental variables at Hanifaru Bay, Maldives. We constructed generalised linear models to investigate possible relationships between zooplankton dynamics, environmental parameters, and how they influenced M. alfredi abundance, behaviour, and foraging strategies. Zooplankton biomass changed rapidly throughout the tidal cycle, and M. alfredi feeding events were significantly related to high zooplankton biomass. Mobula alfredi switched from non-feeding to feeding behaviour at a prey density threshold of 53.7 mg dry mass m−3; more than double the calculated density estimates needed to theoretically meet their metabolic requirements. The highest numbers of M. alfredi observed in Hanifaru Bay corresponded to when they were engaged in feeding behaviour. The community composition of zooplankton was different when M. alfredi was feeding (dominated by copepods and crustaceans) compared to when present but not feeding (more gelatinous species present than in feeding samples). The dominant zooplankton species recorded was Undinula vulgaris. This is a large-bodied calanoid copepod species that blooms in oceanic waters, suggesting offshore influences at the site. Here, we have characterised aspects of the feeding environment for M. alfredi in Hanifaru Bay and identified some of the conditions that may result in large aggregations of this threatened planktivore, and this information can help inform management of this economically important marine protected area.

Tidal and Physicochemical Effects on Phytoplankton Community Variability at Tagus Estuary (Portugal)

The Tagus Estuary is one of the largest estuaries in Europe and merges large urban and industrial areas. Understanding phytoplankton community variability is key for an appropriate assessment of the estuarine ecological status. The objective of the present study was to assess the importance of the tidal influence over the phytoplankton community and to evaluate its main drivers of variation. Weekly sampling was performed at two stations on the Tagus Estuary with different anthropogenic pressures (Alcântara and Barreiro). The sampling covered periods with different tidal amplitude. Alcântara presented both the lowest and highest concentrations of dissolved inorganic nitrogen (DIN) and orthophosphate concentration (DIP), depending on the tidal height. Such high variability in this sampling station is probably due to its proximity to a sewage treatment station outfall and to the estuary mouth. In the present study, both seasonal and tidal variations influenced the chlorophyll a concentration of which the tidal cycle explained up to 50% of the chlorophyll a variations. Chlorophyll a displayed a seasonal trend with two peaks of phytoplankton biomass between spring and mid-summer. The main drivers of chlorophyll a variation were radiation, water temperature, tidal amplitude, salinity, river discharge, and the inorganic nutrients DIN and DSi. The estuarine phytoplankton community was mainly dominated by Bacillariophyceae, especially at Alcântara. Bacillariophyceae were less important at Barreiro, where communities had a higher representation from other phytoplankton groups, such as Cryptophyceae and Prasinophyceae. The drivers of variability in the community composition were similar to those influencing the total biomass. In conclusion, the spring-neap tidal cycle strongly influenced the phytoplankton community, both in terms of biomass and community composition. Of the several tidal conditions, spring tides were the tidal condition that presented both higher biomass and higher Bacillariophyceae representativity in the community.

Tidal mixing of estuarine and coastal waters in the Western English Channel controls spatial and temporal variability in seawater CO₂

Surface ocean CO2 measurements are used to compute the oceanic air–sea CO2 flux. The CO2 flux component from rivers and estuaries is uncertain. Estuarine and coastal water carbon dioxide (CO2) observations are relatively few compared to observations in the open ocean. The contribution of these regions to the global air–sea CO2 flux remains uncertain due to systematic under-sampling. Existing high-quality CO2 instrumentation predominantly utilise showerhead and percolating style equilibrators optimised for open ocean observations. The intervals between measurements made with such instrumentation make it difficult to resolve the fine-scale spatial variability of surface water CO2 at timescales relevant to the high frequency variability in estuarine and coastal environments. Here we present a novel dataset with unprecedented frequency and spatial resolution transects made at the Western Channel Observatory in the south west of the UK from June to September 2016, using a fast response seawater CO2 system. Novel observations were made along the estuarine–coastal continuum at different stages of the tide and reveal distinct spatial patterns in the surface water CO2 fugacity (fCO2) at different stages of the tidal cycle. Changes in salinity and fCO2 were closely correlated at all stages of the tidal cycle and suggest that the mixing of oceanic and riverine end members determines the variations in fCO2. The observations demonstrate the complex dynamics determining spatial and temporal patterns of salinity and fCO2 in the region. Spatial variations in observed surface salinity were used to validate the output of a regional high resolution hydrodynamic model. The model enables a novel estimate of the air–sea CO2 flux in the estuarine–coastal zone. Air–sea CO2 flux variability in the estuarine–coastal boundary region is dominated by the state of the tide because of strong CO2 outgassing from the river plume. The observations and model output demonstrate that undersampling the complex tidal and mixing processes characteristic of estuarine and coastal environment bias quantification of air-sea CO2 fluxes in coastal waters. The results provide a mechanism to support critical national and regional policy implementation by reducing uncertainty in carbon budgets.

The Impact of Fortnightly Stratification Variability on the Generation of Baroclinic Tides in the Luzon Strait

The impact of fortnightly stratification variability induced by tide–topography interaction on the generation of baroclinic tides in the Luzon Strait is numerically investigated using the MIT general circulation model. The simulation shows that advection of buoyancy by baroclinic flows results in daily oscillations and a fortnightly variability in the stratification at the main generation site of internal tides. As the stratification for the whole Luzon Strait is periodically redistributed by these flows, the energy analysis indicates that the fortnightly stratification variability can significantly affect the energy transfer between barotropic and baroclinic tides. Due to this effect on stratification variability by the baroclinic flows, the phases of baroclinic potential energy variability do not match the phase of barotropic forcing in the fortnight time scale. This phenomenon leads to the fact that the maximum baroclinic tides may not be generated during the maximum barotropic forcing. Therefore, a significant impact of stratification variability on the generation of baroclinic tides is demonstrated by our modeling study, which suggests a lead–lag relation between barotropic tidal forcing and maximum baroclinic response in the Luzon Strait within the fortnightly tidal cycle.