Decrease In Current Velocity Research Articles

Hydrodynamic Model due to Reclamation in Lamong Bay

PT. Teluk Lamong Terminal is one of the busiest port terminals in Indonesia. Such conditions require the development of the facility area for reclamation. This reclamation can potentially change the seabed’s contours and current patterns in the Lamong bay area of Surabaya. Therefore, the Author will conduct a research study to determine the impact of reclamation on the speed and current pattern of the existing and post-reclamation conditions in Lamong bay, Surabaya. This research conducted numerical modeling with Delft3D software. The simulation results show that the current pattern at the highest tide in the existing and reclamation models tends to have a dominant current direction from east to the northwest in the Madura strait. In contrast, some currents from the Madura strait are deflected into the Lamong bay area. On the other hand, during the lowest low tide conditions, both existing and reclamation conditions, the current pattern has a dominant direction coming from the northwest to the east, and the current in the Lamong bay area tends to exit or towards the Madura strait. There is an increase or decrease in current velocity when reclamation is carried out in each tidal condition. The highest increase in current velocity occurs at high tide with a value of 0.02 m/s, while the highest decrease in current velocity occurs at the high tide of 0.13 m/s. At the lowest ebb conditions, the highest current speed increase is 0.12 m/s, and the highest current velocity decrease is 0.03 m/s.

Modeling of Plastic Debris Particle Trajectory During Pre and Post Reclamation in Jakarta Bay

The primary source of environmental problems in Jakarta Bay’s waters is the change in hydrodynamics caused by reclamation and land-derived waste from the 13 watersheds that discharge into the bay. In June and December 2006 and 2015, hydrodynamic and trajectory simulations were conducted in Jakarta Bay to determine the distribution of plastic debris in pre- and post-reclamation conditions. The hydrodynamic conditions and particle trajectory in Jakarta Bay were described numerically using the Hamburg Shelf Ocean Model (HAMSOM). Secondary tidal data and primary ADCP data are used to validate the simulation results. The distribution of plastic debris released during the simulation depicts a distribution that follows the monsoon pattern, moving west during the east monsoon and moving east in the west monsoon and spreading along the coastline in both conditions. The percentage of plastic debris increased by 21.42% in June and 4.07% in December. The increase in the percentage of plastic debris that remains in the waters of Jakarta Bay is due to a 0.03 - 0.05 m/s decrease in current velocity following the formation of the reclamation island. Due to the decrease in current velocity within the bay, trajectory simulations indicate that plastic debris will take an additional 69.52 – 304.25 hours to exit. The accumulation of plastic debris around the reclamation islands demonstrates that the islands act as waste traps, necessitating proper waste cleanup, particularly in the canal area and around the reclamation island.

Influence of shoreface morphological changes since the 19th century on nearshore hydrodynamics and shoreline evolution in Wissant Bay (northern France)

Located at the southern approaches to the North Sea, the Wissant Bay has had the most rapidly eroding shoreline in mainland France in recent decades. Using historical bathymetries spanning the second half of the 19th century to the present, an analysis of the long-term coastal morphological changes and evolution of a prominent sand bank (Line bank), extending across the bay, was carried out. A period of overall seabed erosion with a width-reduction of the Line bank; a deepening of the channel between the Line bank and the coast; and a lowering of the foreshore have been ongoing since the middle of the 20th century. This phase followed a period of channel infill in the western part of Wissant Bay in the early 20th century. Numerical modelling of wave propagation and tidal circulation was performed using the TELEMAC suite of models to get some insights into the influence of these bathymetry changes on coastal hydrodynamics. Two seabed configurations were identified revealing a contrasting hydrodynamic circulation. An erosional one induces an acceleration of tidal currents and an increase in wave height. Conversely, accumulation leads to an hydrodynamic regime weakening with a decrease in current velocity and to more wave energy dissipation in the nearshore. This study highlights the role of morphological feedbacks between nearshore morphology and coastal hydrodynamics. The identification of these feedbacks mechanisms at a historical time scale is important to assess potential drivers of coastal changes.

Field investigation of siltation at a tidal harbor: North Port of Incheon, Korea

To understand siltation processes within the North Port of Incheon, Korea, this study estimated the sedimentation rate and then determined the dominant mechanisms for the siltation. The sedimentation rate was estimated by comparing bathymetric surveys adjusted with the dredged volume, and verified by the penetration depth of 11 gravity cores to the previous dredging surface. The average sedimentation rate was 64 cm/year but varied from 10 cm/year in the interior to about 100 cm/year at the entrance. To determine the dominant mechanisms for siltation, two ADCPs were deployed at the entrance and the interior end of the harbor over a month to estimate sediment flux. Detailed sediment transport processes were examined by two 13-h CTD profiling surveys near the entrance and interior end of the harbor. High siltation at the entrance is attributable to the influx of high turbidity water, the decrease of current velocity, and the formation of an eddy. Low sedimentation inside the harbor was attained by the slow settling of microflocs of about 20–30 mg/l concentration that are maintained regardless of time and space.

Dune bedforms produced by dilute pyroclastic density currents from the August 2006 eruption of Tungurahua volcano, Ecuador

A series of pyroclastic density currents were generated at Tungurahua volcano (Ecuador) during a period of heightened activity in August 2006. Dense pyroclastic flows were confined to valleys of the drainage network, while dilute pyroclastic density currents overflowed on interfluves where they deposited isolated bodies comprising dune bedforms of cross-stratified ash exposed on the surface. Here, the description, measurement, and classification of more than 300 dune bedforms are presented. Four types of dune bedforms are identified with respect to their shape, internal structure, and geometry (length, width, thickness, stoss and lee face angles, and stoss face length). (1) “Elongate dune bedforms” have smooth shapes and are longer (in the flow direction) than wide or thick. Internal stratification consists of stoss-constructional, thick lensoidal layers of massive and coarse-grained material, alternating with bedsets of fine laminae that deposit continuously on both stoss and lee sides forming aggrading structures with upstream migration of the crests. (2) “Transverse dune bedforms” show linear crests perpendicular to the flow direction, with equivalent lengths and widths. Internally, these bedforms exhibit finely stratified bedsets of aggrading ash laminae with upstream crest migration. Steep truncations of the bedsets are visible on the stoss side only. (3) “Lunate dune bedforms” display a barchanoidal shape and have stratification patterns similar to those of the transverse ones. Finally, (4) “two-dimensional dune bedforms” are much wider than long, exhibit linear crests and are organized into trains. Elongate dune bedforms are found exclusively in proximal deposition zones. Transverse, lunate, and two-dimensional dune bedforms are found in distal ash bodies. The type of dune bedform developed varies spatially within an ash body, transverse dune bedforms occurring primarily at the onset of deposition zones, transitioning to lunate dune bedforms in intermediate zones, and two-dimensional dune bedforms exclusively on the lateral and distal edges of the deposits. The latter are also found where flows moved upslope. Elongate dune bedforms were deposited from flows with both granular-based and tractional flow boundaries that possessed high capacity and competence. They may have formed in a subcritical context by the blocking of material on the stoss side. We do not interpret them as antidune or “chute-and-pool” structures. The dimensions and cross-stratification patterns of transverse dune bedforms are interpreted as resulting from low competence currents with a significant deposition rate, but we rule out their interpretation as “antidunes”. A similar conclusion holds for lunate dune bedforms, whose curved shape results from a sedimentation rate dependent on the thickness of the bedform. Finally, two-dimensional dune bedforms were formed where lateral transport exceeds longitudinal transport; i.e., in areas where currents were able to spread laterally in low velocity zones. We suggest that the aggrading ash bedsets with upstream crest migration were formed under subcritical flow conditions where the tractional bedload transport was less important than the simultaneous fallout from suspension. This produced differential draping with no further reworking. We propose the name “regressive climbing dunes” for structures produced by this process. A rapid decrease in current velocity, possibly triggered by hydraulic jumps affecting the entire parent flows, is inferred to explain their deposition. This process can in principle hold for any kind of particulate density current.Electronic supplementary materialThe online version of this article (doi:10.1007/s00445-013-0762-x) contains supplementary material, which is available to authorized users.

Hydrodynamics and sedimentation induced by large-scale coastal developments in the Keum River Estuary, Korea

The macro-tidal Keum River Estuary located in the eastern Yellow Sea has been suffering siltation and morphological change since 1994. To understand the effects of the large-scale coastal developments on the sedimentation processes in the estuary, hydrodynamic and sedimentary data collected from 1985 to 2002 were analyzed and numerical experiments of hydrodynamics were performed. The sedimentation rate in the estuary increased by a factor of 1.9, from 3.5 × 10 6 to 6.7 × 10 6 m 3 y −1, after the construction of a dam in the upper reaches of the estuary in 1994. Large part of the estuary is veneered by the muddy sediments noticeably, which were rarely found before dam construction. Since then, siltation has concentrated in the upper estuary rather than the lower. The upstream transport and accumulation of fine-grained sediments is due to: (1) the change to flood-dominance in the main channel, i.e. the relative intensification of flood current and the flood-directed residual current; and (2) the decrease in transport capacity in the upper estuary, i.e. the marked decrease in current velocity, which was induced by dam construction. The former has resulted in the ebb-dominance of the Gaeya channel, a distributary in the north of the main channel. The tidal pumping of fine sediments was reinforced not by the freshwater/saltwater interaction but by the residual tidal circulation. The sediment fluxes observed in 2001–2002 demonstrate year-round net inflow both at the entrance of the jetties and at the Gaeya channel, which implies that the sediments delivered by the Keum River are entirely confined to the estuary, incapable of escaping to the sea. The net inward transport of fine sediments may accumulate pollutants adsorbed to or absorbed in the sediment grains in the estuary, thus deteriorating the benthic environment gradually and the water quality eventually.

매립으로 인한 광양만의 유동변화 수치실험

광양만역 종합개발로 인한 만 내 유동 변화를 정량 파악하기 위해, 광양만 전역에 대한 계절별 해황조사, 계절별 유동 조사 및 진단 다중 모델을 이용하여 광양만 순환류 구성 요소 에 대한 변화량을 운동에너지와 위치에너지 계산을 통해 규명하였다. 1) 개발이 진행됨에 따라 유역 면적이 줄어들고 유동도 약해진다 조류 에너지는 광양만이 개발되기 전에 비해 종합 개발 10년 후, 약 10% 감소한다. 마찰 손실은 약 50% 감소한다. 2) 광양만 개발로 인한 조석 잔차류는 약 35~40% 감소한다. 3) 조석 잔차류의 약 1/10 정도인 밀도류는 하계에 크고 추계에 작으며, 개발로 인해 약 10% 증가한다. 4) 취송류는 풍속이 작은 하계에 작고 풍속이 강한 동계에 크다. 취송류는 개발로 인해 약5% 감소한다. 5) 조석 잔차류+밀도류는 하계에 크고 추계에 작다. 개발에 따른 변화는 현재~10년 후 약 5~l2% 증가하나, 개발 전 과거~현재는 9~13% 감소했다. 6) 조석 잔차류+취송류는 개발에 따라 약 10% 감소한다. 7) 조석 잔차류+밀도류+취송류의 잔차류는 개발10년 후가 개발 전에 비해 10% 감소한다. 이는 2차원적 조류 에너지 Flux 변화 결과와 일치한다. 계절별로는 하계가 가장 크게 감소하여, 종합 개발 후 성층강화와 잔차유속 감소로 오염이 크게 진행될 가능성이 있다. 8) 광양만내의 순환류 요소 중 계절 변화가 가장 큰 흐름은 잔차류이고 그 다음이 조석 잔차 취송류, 취송류, 조석 잔차 밀도류 그리고 가장 작은 것이 밀도류이다. 잔차류의 계절 변화는 주로 취송류 변화에 의한다. This study presents an investigation of the changes of the currents in Kwangyang Bay due to the construction of harbor, reclamation and coastal developments. Currents were simulated by the numerical experiments with a diagnostic multi-level model and using the seasonal oceanographic data of temperature, salinity and ocean current. The values of kinetic and potential energies for the currents were calculated in cases of three topographical changes; before coastal developments, the existing state and after completion of the development project in Kwangyang Bay. The changes of currents due to the coastal developments are as follow; Kinetic energies of tide induced residual currents and wind driven currents decreased by 35~40 percent and 5 percent respectively, however those of density currents increased by 10 percent since the decrease of the coastal areas. Kinetic energy of residual currents including tide induced residual currents, density currents and wind driven currents reduced by 10 percent compared with before the coastal developments. Decrease of current velocity was greatest in summer. Therefore, in summer it was assumed that the Kwangyang Bay is more easily polluted by stratification and decrease of residual current than before the coastal developments carried out.

Sedimentology of the Bengal shelf, Bangladesh: comparison of late Miocene sediments, Sitakund anticline, with the modern, tidally dominated shelf

Characterisation of latest Miocene facies of the Bay of Bengal reveals a range of tidal and fluvially dominated sediments that represent sedimentation on a broad, shallow shelf that was strongly influenced by tidal and seasonal cyclicity. These tidally influenced sediments are excellently exposed within growth anticlines of the westward-migrating Chittagong–Tripura Fold Belt (CTFB), and permit detailed study of the late Miocene Bengal shelf for comparison with the modern subaqueous deltaic sediments. At the outcrop scale, facies associations record a subtidal shelf, migrating intertidal/subtidal bedforms, tidal and incised channels. The subtidal shelf comprises extensive sand sheets interbedded with thick clay horizons, while the migrating intertidal/subtidal bedforms exhibit a decrease in bedform size from large dunes to small ripple-scale cross-stratification that reflect a decrease in current velocity and sand/silt grade sediment supply as the bedforms migrated. Rootlet horizons within these intertidal/subtidal bedforms may represent a lower delta plain environment. The tidal channel facies consists of metre-scale sandstone beds that are rich in locally derived clay clasts, with up to 0.5 m of incision into a rhizolith horizon below. Incised channel facies records a coarse, pebbly sandstone with clasts derived from a reworked clastic source, with metre-scale scour at the base of this facies association into laminated and rhizolith clays. A 534-m one- and two-dimensional section within the Sitakund anticline, north of Chittagong, is characterised by 20–100-m thick, tidally dominated, progradational parasequences. Mudstones with isolated sand lenses and abundant trace fossils of the Cruziana ichnofacies, coarsen upwards to fine–medium-grained sandstones deposited in shallower, high-energy environments. These sands were deposited under both uni-directional and bi-directional currents resulting in dune-scale cross-stratification, and are overlain by 2–5-cm thick clay drapes, considered to be of a thickness greater than that typical of deposition from a single slackwater episode. The main controls on sedimentation are the seasonally influenced river and sediment discharge onto the shelf from the Himalayan source region, combined with the influence of daily, spring–neap tides, channel and bedform migration and minimal wave action over the shallow shelf. The modern Bay of Bengal shelf provides an analogue for interpreting processes recorded within the Miocene succession, allowing construction of a depositional model for the late Miocene shelf of the Brahmaputra/Ganges delta system, with sedimentation dominated by tidal and seasonal cyclicity.

Cross-Frontal Mixing in a Meandering Jet

A Gulf Stream-like meandering current is kinematically modeled using a streamfunction ψ = Uλ(1 − tanh[(y − yc)/λ cos(α)]. Under suitable parameter values a “front” exists along the velocity maximum across which there is no transport, but with mixing occurring on either side. An analogous barrier exists in the Gulf Stream but appears to fade with depth. The conditions for mixing across the model front are studied using the Chirikov overlap criterion, which indicates the front breaks down for meander amplitude above a critical threshold that is inversely dependent on the ratio of meander phase speed and current speed, c/U. It is suggested that the increase in cross-frontal mixing in the deeper levels of the Gulf Stream is the result of current meandering and the decrease of current velocity with depth. The mechanism for this is interaction of different meander modes traveling along the Gulf Stream. These ideas are shown to be consistent with field measurements of tracers in the ocean.

Development time and growth rate of the marine calanoid copepod Temora longicornis as related to food conditions in the oosterschelde estuary (Southern North Sea)

The development time of the larval stages of the marine calanoid copepod Temora longicornis was estimated by applying the multi-cohort model of Bossicart (1980) on observed densities of 3 successive cohorts in the open Oosterschelde estuary (SW Netherlands) during 1983 and 1984. The shortest development time of nauplii + copepodids of 16 to 25 days occurred in spring; the maximum development time of both series of stages was 27 to 43 days and was found in summer. This longer time of development could be ascribed to a smaller food supply during this period. The development time of the larval stages was significantly correlated with smaller phytoplankton cells (spherical equivalent diameter ⩽ 50 μm). Lowest daily exponential k-values (0.04) were calculated for summer periods (17 to 20°C) and highest values (k=0.23) were found during spring (∼10°C). From 1986 onwards, the tidal regime in the estuary has been modified by an artificial storm-surge barrier, implying a decrease in current velocity and leading to shifts in phytoplankton composition and densities in the eastern part. It is hypothesized that normal conditions for growth of nauplii and smaller copepodids of Temora in this section will continue in the future, but that those for growth and development of adults may become less favourable.

Grain-size characteristics of turbidites

Summary Detailed sampling using a very small sample size and grain-size analysis with a Coulter Counter of three fine-grained turbidites enabled a distinction to be made between the well-sorted single-grain Stokes’-deposited and unsorted ‘whole suspension’ floc-deposited grain-size populations. The results indicate that each turbidite is a continuous sequence deposited from the same source suspension. Particles settle to the bottom as flocculated masses. Initially flocs are broken up by near-bottom shear forces and only the coarsest silt and sand remains on the bed. The remaining mud forms a temporary mud suspension near the bottom which reflocculates and intermittently deposits at some critical concentration producing mud interlayers between silt laminae. Decrease in current velocity eventually allows simultaneous deposition of single grains and flocs with the latter becoming progressively more abundant resulting in formation of graded beds. Eventually, all deposition occurs in the form of mud flocs and a massive sediment results. The study essentially confirms the basic mechanism of the fine-grained turbidite deposition model proposed by Stow & Bowen (1978).

Biostratigraphy and depositional environment of algal stromatolites from the Mescal Limestone (Proterozoic) of central Arizona

The Apache Group of central Arizona is subdivided into, from base upward, the Pioneer Formation, the Dripping Spring Quartzite and the Mescal Limestone. Radiometric age determinations by Silver, and Livingston and Damon indicate an age of 1.2–1.4 billion years. Within the Mescal Limestone, algal stromatolites form a conspicuous biostrome, commonly 20–25 m thick. The basal 1–5 m of the biostrome consists of up to three zones of digitate stromatolites, which often form discrete bush-like bioherms. These forms are interpreted as Baicalia baicalica, Parmites sp. and Tungussia sp.; the latter form previously reported by Cloud and Semikhatov (1969). The form Parmites is interpreted as a modification of digitate stromatolites probably by decrease in current velocity within the shallow marine environment, which allowed discrete heads to coalesce. Basal digitate forms are replaced upward in the biostrome by domal and undulatory laminated (stratiform) stromatolites, interpreted to represent gradual upward shoaling, with lower intertidal and subtidal forms (digitate morphology) being replaced by upper intertidal and possibly supratidal forms (stratiform types). The digitate form B. baicalica is suggested by Soviet workers to be indicative of Middle Riphean time (1350-950 m.y.). While many empirical data suggest the possibility of gross subdivision of Late Proterozoic time on the basis of algal stromatolite “zones”, the intercontinental applicability and the ultimate validity of this concept in unresolved.

Simulation Experiments on the Migration of Gammarus Zaddachi and Gammarus Chevreuxi

The migratory activity of Gammarus zaddachi and Gammarus chevreuxi is investigated in a current chamber, in which a tidal cycle can be simulated. In fresh running water the number of animals drifting with the current is of about the same magnitude as the number of animals actively swimming against the current. A diurnal periodicity is recorded in the migratory activity. The migratory activity is influenced by the population density and the food supply. In the current chamber a tidal cycle can be simulated by varying the factors current velocity, current direction, salinity, and temperature. The combination of a decrease in current velocity, followed by a slow current in the opposite direction, an increase in the salinity, and a rise in the temperature of the medium causes a significant increase in the activity of both species investigated. An increase in only one of the environmental factors mentioned above can also produce an increase in the migratory activity, but less pronounced than when all the factors coordinate in simulating a complete tidal cycle. Both G. zaddachi and G. chevreuxi react in a similar way on the simulation of a tidal cycle. In both species no great differences have been found between juveniles and adults with regard to the migratory activity. The results of the simulation experiments are discussed in connection with the migration cycles of G. zaddachi and G. chevreuxi as recorded in the field.

Reversed-Climbing Ripples and Sand-Wave Deposition in Arkansas River: ABSTRACT

This study of modern river deposits substantiates and extends flume studies of regressive and sand waves by Jopling (1961) and Allen (1965). As first proposed by Jopling, regressive are a sequence of climbing oriented in a direction opposite that of the general current flow. At the risk of introducing another new term, they are herein termed climbing ripples rather than regressive ripples, because regression has a specific meaning in geology. Four types of sedimentary features were observed in an exposure of a large (4.5 ft high and 35 ft long) sand wave in a cut bank of the Arkansas River near Tulsa, Oklahoma. In downstream sequence they are: (1) accretionary foresets with tangential bases; (2) avalanche foresets with abrupt basal contacts; (3) reversed climbing ripples; and (4) normal or ripples. The accretionary foresets show a progressive decrease in mean grain size down the foresets, have an angle of slope (28°) less than that of the avalanche foresets (31.5°), and contain no reversed climbing ripples. In contrast to this, the avalanche foresets show a progressive increase in mean grain size down the foresets, a steeper slope, and development of reversed climbing ripples. The change from accretion to avalanche foresets apparently was the result of a decrease in current velocity. As velocities decreased, excessive deposition took place near the head of the foresets resulting in oversteepening and avalanching. At the same time a reverse eddy formed near the toe of the sand wave and produced the reversed climbing ripples. The reversed climbing are finer grained (2.82 vs. 2.43^phgr), poorer sorted, and more strongly fine-skewed than the foreset deposits. In the accretion foreset sands the sorting decreases progressively from top to bottom. The accretionary foreset sands tend to be more positively skewed near the base than they are near the top, whereas the avalanche foreset sands tend to be more positively skewed near the top and more nearly symmetrical near the base. End_of_Article - Last_Page 552------------