Salt water infiltration in two artificial sea inlets in the Belgian dune area

In the dune area of the Westhoek Nature Reserve, situated in the western Belgian coastal plain, two artificial tidal inlets were made aiming to enhance biodiversity. The infiltration of salt water in these tidal inlets was carefully monitored because a fresh water lens is present in the phreatic dune aquifer. This forms an important source of fresh water which is for instance exploited by a water company. The infiltration was monitored over a period of two years by means of electromagnetic borehole measurements (EM39) and by measurements of fresh water heads and temperature using a large number of observation wells. EM39 observations point to aquifer heterogeneity as a determining factor in the movement of the salt infiltration water. It is shown that part of the infiltration water moves further in the dunes instead of towards the sea. On the long term run, possibility exists that salt water enters the extraction’s capture zone. This issue needs further monitoring and study. Fresh water head and temperature data illustrate that the main period of infiltration is confined to spring tide when large amounts of salt water enter the tidal inlets.     

Tides and mixing in the northwestern East China Sea Part I: Rotating and reversing tidal flows

Bottom-mounted ADV and ADCP instruments in combination with CTD profiling measurements taken along the Chinese coast of the East China Sea were used to study the vertical structure of temperature, salinity, and velocity in reversing tidal currents on a shallow inner shelf and in rotating tidal flows over a deeper sloping bottom of the outer shelf. These two regimes of barotropic tide affect small-scale dynamics in the lower part of the water column differently. The reversing flow was superimposed by seiches of ∼2.3 h period generated in semienclosed Jiaozhou Bay located nearby. As the tidal vector rotates over the sloping bottom, the height of the near-bottom logarithmic layer is subjected to tidal-induced variations. A maximum of horizontal velocity U_max appears at the upper boundary of the log layer during the first half of the current vector rotation from the minor to the major axis of tidal ellipse. In rotating tidal flow, vertical shear generated at the seafloor, propagated slowly to the water interior up to the height of U_max, with a phase speed of ∼5 m/h. The time-shifted shear inside the water column, relative to the shear at the bottom, was associated with periodically changing increases and decreases of the tidal velocity above the log layer toward the sea surface. In reversing flows, the shear generated near the bottom and the shear at the upper levels were almost in phase.     

Long-term evolution of sand waves in the Marsdiep inlet. II: Relation to hydrodynamics

A discussion is presented about the mechanisms that govern the spatial and seasonal variability in sand-wave height and migration speed in the 4 km wide Marsdiep tidal inlet, the Netherlands. Since 1998, current velocities and water depths have been recorded with an ADCP that is mounted under the ferry ‘Schulpengat’. In this paper, the current measurements were used to explain the sand-wave observations presented in Buijsman and Ridderinkhof [this issue. Long-term evolution of sand waves in the Marsdiep inlet. I: high-resolution observations. Continental Shelf Research, doi:10.1016/j.csr.2007.10.011]. Across nearly the entire inlet, the sand waves migrate in the flood direction. In the flood-dominated southern part of the inlet, the ‘measured’ (i.e. based on sand-wave shape and migration speed) and predicted bedload transport agree in direction, magnitude, and trends, whereas in the ebb-dominated northern part the predicted bedload and suspended load transport is opposite to the sand-wave migration. In the southern part, 55% of the bedload transport is due to tidal asymmetries and 45% due to residual currents. In addition to the well-known tidal asymmetries, asymmetries that arise from the interaction of _M2${\mathrm{M}}_2$ and its overtides with _S2${\mathrm{S}}_2$ and its compound tides are also important. It is hypothesised that in the northern part of the inlet the advection of suspended sand and lag effects govern the sand-wave migration. The relative importance of suspended load transport also explains why the sand waves have smaller lee-slope angles, are smaller, more rounded, and more three-dimensional in the northern half of the inlet. The sand waves in this part of the inlet feature the largest seasonal variability in height and migration speed. This seasonal variability may be attributed to the tides or a seasonal fluctuation in fall velocity. In both cases sediment transport is enhanced in winter, increasing sand-wave migration and decreasing sand-wave height. The influence of storms and estuarine circulation on the sand-wave variability is negligible.     

潮汐应力-应变对某些地震序列的调制触发

对1960年以来中国大陆浅源地震序列与潮汐应变的相关分析表明, 某些地区某些地震序列, 包括前震序列、 前震-主震序列、 前震-主震-余震序列和主震-余震序列明显受到潮汐应力-应变的调制触发。 如龙陵地震的前震-主震序列, 邢台地震的前震-主震-余震序列、 唐山地震的主震-余震序列等。 这些地震序列受到潮汐力调制触发的特征明显, 对地震预测有意义, 并作了具体的叙述。 同时对地震序列受调制触发的机理作了初步的探讨, 指出不同类型地震序列的调制触发特征与孕育系统地质构造, 地震震源岩石性质和区域构造应力场方向以及地震破裂机制有关, 对地震机理和预测的研究有一定意义。     

Mercury partition in the interface between a contaminated lagoon and the ocean: The role of particulate load and composition

After having estimated the patterns of flow to the ocean and found some seasonal and tidal differences, mainly with regard to the relative importance of dissolved and particulate fractions, mercury partitioning at the interface between a contaminated lagoon and the Atlantic Ocean was investigated during four tidal cycles in contrasting season and tidal regimes. Mercury was found to be located predominantely in the particulate fraction throughout the year, contributing to its retention within the system. Seasonal conditions, variations in marine and fluvial signals and processes affecting bed sediment resuspension influenced the character and concentration of suspended particulate matter in the water column. Variation in the nature, levels and partitioning of organic carbon in the particulate fraction affected levels of particulate mercury as well as mercury partitioning. These results highlight the dominant role of suspended particulate matter in the distribution of anthropogenic mercury and reinforce the importance of competitive behavior related to organic carbon in mercury scavenging.     

Bioelectricity generation and remediation of sulfide contaminated tidal flat sediment

A dramatic decrease in the catch of shellfish has been observed due to the high amount of Acid Volatile Sulfide(AVS) in the tidal flats in Japan.In the current study,an evaluation of simultaneous bioelectricity generation and remediation of sulfide contaminated tidal flat sediment has been done.The sediment samples collected from Tokyo Bay and Yamaguchi Bay,Japan,have been used in the laboratory test.A 2 L cylindrical shaped Sediment Microbial Fuel Cell(SMFC) has been used to evaluate the remediation of both sediment samples in the laboratory.Three different electrode materials carbon felt,carbon fiber and bamboo charcoal were used in the experiments to compare their efficiency to reduce the AVS from the sediment and generate bioelectricity.It was observed that the AVS reduction was higher at 5 cm depths for the Tokyo Bay sediment(100%) compared to the Yamaguchi Bay sediment(60%).The larger grain size for the Tokyo Bay sediment was the probable reason for this.The maximum voltage was around 100 and80 mV for Tokyo Bay and Yamaguchi Bay,respectively.     

A tracer study in an Alaskan gravel beach and its implications on the persistence of the Exxon Valdez oil

Despite great efforts including bioremediation, the 1989 Exxon Valdez oil spills persist in many gravel beaches in Prince William Sound, Alaska, USA. To explore this mystery, a lithium tracer study was conducted along two transects on one of these beaches. The tracer injections and transports were successfully simulated using the 2-dimensional numerical model MARUN. The tracer stayed much longer in the oil-persisting, right transect (facing landwand) than in the clean, left transect. If the tracer is approximately regarded as oils, oils in the upper layer would have more opportunities to enter the lower layer in the right transect than in the left one. This may qualitatively explain the oil persistence within the right transect. When the tracer is regarded as nutrients, the long stay of nutrients within the right transect implies that the oil persistence along the right transect was not due to the lack of nutrients during the bioremediation.     

Asymmetric fluxes of water and sediments in a mesotidal mudflat channel

The hydrodynamics of a small tributary channel and its adjacent mudflat is studied in Willapa Bay, Washington State, USA. Velocity profiles and water levels are simultaneously measured at different locations in the channel and on the mudflat for two weeks. The above tidal flat and channel hydrodynamics differ remarkably during the tidal cycle. When the water surface level is above the tidal flat elevation, the channel is inactive. At this stage, the above tidal flat flow is predominantly aligned along the Bay axis, oscillating with the tide as a standing wave with peak velocities up to 0.3 m/s. When the mudflat becomes emergent, the flow concentrates in the channel. During this stage, current velocities up to 1 m/s are measured during ebb; and up to 0.6 m/s during flood. Standard equations for open-channel flow are utilized to study the channel hydrodynamics. From the continuity equation, a lateral inflow is predicted during ebb, which likely originates from the drainage of the mudflat through the lateral runnels. Both advective acceleration and lateral discharge terms, estimated directly from the velocity profiles, play a significant role in the momentum equation. The computed drag coefficient for bottom friction is small, due to an absence of vegetation and bottom bedforms in the channel. Sediment fluxes are calculated by combining flow and suspended sediment concentration estimated using the acoustic backscatter signal of the instruments. A net export of the sediment from the channel is found during ebb, which is not balanced by the sediment import during flood. When the mudflat is submerged, ebb-flood asymmetries in suspended sediment concentration are present, leading to a net sediment flux toward the inner part of the Willapa Bay. Finally, a residual flow is detected inside the channel at high slack water, probably associated with the thermohaline circulation.     

Waves and tides responsible for the intermittent closure of the entrance of a small, sheltered tidal wetland at San Francisco, CA

Crissy Field Marsh (CFM; http://www.nps.gov/prsf/planyourvisit/crissy-field-marsh-and-beach.htm) is a small, restored tidal wetland located in the entrance to San Francisco Bay just east of the Golden Gate. The marsh is small but otherwise fairly typical of many such restored wetlands worldwide. The marsh is hydraulically connected to the bay and the adjacent Pacific Ocean by a narrow sandy channel. The channel often migrates and sometimes closes completely, which effectively blocks the tidal connection to the ocean and disrupts the hydraulics and ecology of the marsh. Field measurements of waves and tides have been examined in order to evaluate the conditions responsible for the intermittent closure of the marsh entrance. The most important factor found to bring about the entrance channel closure is the occurrence of large ocean waves. However, there were also a few closure events during times with relatively small offshore waves. Examination of the deep-water directional wave spectra during these times indicates the presence of a small secondary peak corresponding to long period swell from the southern hemisphere, indicating that CFM and San Francisco Bay in general may be more susceptible to long period ocean swell emanating from the south or southwest than the more common ocean waves coming from the northwest. The tidal records during closure events show no strong relationship between closures and tides, other than that closures tend to occur during multi-day periods with successively increasing high tides. It can be inferred from these findings that the most important process to the intermittent closure of the entrance to CFM is littoral sediment transport driven by the influence of ocean swell waves breaking along the CFM shoreline at oblique angles. During periods of large, oblique waves the littoral transport of sand likely overwhelms the scour potential of the tidal flow in the entrance channel.     

强震成组活动与潮汐力调制触发

本文应用构造场体成组震模型的研究结果，探讨了中国大陆Ms≥7．0地震的成组活动的引潮力的调制触发关系。研究表明：我国陆区7级以上强震明显受潮汐力的调制触发，且主要表现为对活跃期内（即成组发生的）7级强震的调制触发。平静期内的7级以上地震基本不受潮汐力触发，而活跃期内82．4％的7级强震发生在受潮汐力触发的月相期，比本底概率高出36．4％。因此，潮汐力的调制触发对于强震活跃期的判断和活跃期内强震的临震（时间）预测有实际意义。文章还分析了潮汐力触发与强震活动的区域动力学环境之间的关系，并用构造场体成组孕震模型讨论了潮汐力对强震活动触发作用的力学机制。