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Tidal Effects (tidal + effects)

Distribution by Scientific Domains

Physics and Astronomy	33%

Selected Abstracts

Tidal Effects on Ground Water Discharge Through a Sandy Marine Beach

GROUND WATER, Issue 7 2004
Daniel W. Urish
Tidal fluctuations along the salt water boundary of a sandy beach affect the magnitude, location, timing, and salinity of both subaerial and submarine ground water discharge. Detailed studies of shoreline discharge from an unconfined aquifer at two sites in an embayment on the Cape Cod, Massachusetts, coastline provide insight into the highly dynamic spatial and temporal nature of discharge along sandy beaches affected by the tide. The constantly moving tidal boundary over a sloping beach results in a shoreline-perpendicular discharge zone of 10 to 20 m, with ,35% to 55% of the discharge being submarine discharge. The distribution of fresh ground water through a beach face varies greatly, depending primarily on the tidal cycle and range, the heterogeneous characteristics of the beach sediments, and the beach geometry. The estimated relative volume of discharge varies temporally with tidal fluctuations, with the greatest discharge occurring during early to mid ebbing tide and location of greatest estimated discharge moving seaward during ebbing tide. This is determined using net hydraulic head calculations in monitoring wells set in a shoreline-perpendicular transect in the beach. The salinity of discharge varies temporally from near fresh water values of 1 part per thousand (ppt) to near coastal salt water values of 30 ppt, being saltiest at the start of discharge as the tide ebbs and freshest during a low tide period of ,2 h. Of the discharge volume, ,65% to 85% is estimated to be from salt water that infiltrates during high tide episodes. This study highlights the complexity of the dynamic coastal ground water discharge phenomenon and provides insight into the hydraulic mechanisms involved. While there is a general pattern to sandy beach discharge, comparison of results from beaches studied at Cape Cod indicates that the temporal and spatial details of the discharge is very site-specific. [source]

Short-term dynamics of bacterial communities in a tidally affected coastal ecosystem

FEMS MICROBIOLOGY ECOLOGY, Issue 2 2008
Beate Rink
Abstract Tidal effects on the composition of free-living (FL) and particle-associated (PA) bacterial communities were studied in a tidal flat ecosystem in the southern North Sea. Denaturing gradient gel electrophoresis targeting the 16S rRNA gene and the 16S rRNA of Bacteria, Bacteroidetes, Alphaproteobacteria and the Roseobacter clade was applied. Despite strong tidal variations in the quantity and, depending on the season, also the quality of suspended matter as well as variations in bacterial activity, the bacterial community composition remained rather stable. FISH showed some variations of the community composition, but these were not related to typical tidal situations. Variations were higher during tidal cycles in May and July compared with November. Bacteroidetes, Alpha - and Gammaproteobacteria constituted the majority of the bacterial communities but relative proportions of the different groups varied considerably. On particles, Betaproteobacteria were also detected to substantial proportions. The Roseobacter clade constituted up to 90% of FL but only 30% of PA Alphaproteobacteria. Banding patterns of the Bacteroidetes -specific amplicons, and in particular those targeting the 16S rRNA, revealed tidally induced effects, as several bands appeared or disappeared at distinct events such as slack water or resuspension. Sequencing of prominent bands revealed predominantly phylotypes reported previously from this ecosystem. [source]

Inverse Modeling of Coastal Aquifers Using Tidal Response and Hydraulic Tests

GROUND WATER, Issue 6 2007
Andrés Alcolea
Remediation of contaminated aquifers demands a reliable characterization of hydraulic connectivity patterns. Hydraulic diffusivity is possibly the best indicator of connectivity. It can be derived using the tidal response method (TRM), which is based on fitting observations to a closed-form solution. Unfortunately, the conventional TRM assumes homogeneity. The objective of this study was to overcome this limitation and use tidal response to identify preferential flowpaths. Additionally, the procedure requires joint inversion with hydraulic test data. These provide further information on connectivity and are needed to resolve diffusivity into transmissivity and storage coefficient. Spatial variability is characterized using the regularized pilot points method. Actual application may be complicated by the need to filter tidal effects from the response to pumping and by the need to deal with different types of data, which we have addressed using maximum likelihood methods. Application to a contaminated artificial coastal fill leads to flowpaths that are consistent with the materials used during construction and to solute transport predictions that compare well with observations. We conclude that tidal response can be used to identify connectivity patterns. As such, it should be useful when designing measures to control sea water intrusion. [source]

Evaluation of time-space distributions of submarine ground water discharge

GROUND WATER, Issue 3 2005
Makoto Taniguchi
Submarine ground water discharge (SGD) rates were measured continuously by automated seepage meters to evaluate the process of ground water discharge to the ocean in the coastal zone of Suruga Bay, Japan. The ratio of terrestrial fresh SGD to total SGD was estimated to be at most 9% by continuous measurements of electrical conductivity of SGD. Semidiurnal changes of SGD due to tidal effects and an inverse relation between SGD and barometric pressure were observed. Power spectrum density analyses of SGD, sea level, and ground water level show that SGD near shore correlated to ground water level changes and SGD offshore correlated to sea level changes. SGD rates near the mouth of the Abe River are smaller than those elsewhere, possibly showing the effect of the river on SGD. The ratio of terrestrial ground water discharge to the total discharge to the ocean was estimated to be 14.7% using a water balance method. [source]

Bulges versus discs: the evolution of angular momentum in cosmological simulations of galaxy formation

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008
Jesus Zavala
ABSTRACT We investigate the evolution of angular momentum in simulations of galaxy formation in a cold dark matter universe. We analyse two model galaxies generated in the N -body/hydrodynamic simulations of Okamoto et al. Starting from identical initial conditions, but using different assumptions for the baryonic physics, one of the simulations produced a bulge-dominated galaxy and the other one a disc-dominated galaxy. The main difference is the treatment of star formation and feedback, both of which were designed to be more efficient in the disc-dominated object. We find that the specific angular momentum of the disc-dominated galaxy tracks the evolution of the angular momentum of the dark matter halo very closely: the angular momentum grows as predicted by linear theory until the epoch of maximum expansion and remains constant thereafter. By contrast, the evolution of the angular momentum of the bulge-dominated galaxy resembles that of the central, most bound halo material: it also grows at first according to linear theory, but 90 per cent of it is rapidly lost as pre-galactic fragments, into which gas had cooled efficiently, merge, transferring their orbital angular momentum to the outer halo by tidal effects. The disc-dominated galaxy avoids this fate because the strong feedback reheats the gas, which accumulates in an extended hot reservoir and only begins to cool once the merging activity has subsided. Our analysis lends strong support to the classical theory of disc formation whereby tidally torqued gas is accreted into the centre of the halo conserving its angular momentum. [source]

Tidal dynamics of relativistic flows near black holes

ANNALEN DER PHYSIK, Issue 5 2005
C. Chicone
Abstract We point out novel consequences of general relativity involving tidal dynamics of ultrarelativistic relative motion. Specifically, we use the generalized Jacobi equation and its extension to study the force-free dynamics of relativistic flows near a massive rotating source. We show that along the rotation axis of the gravitational source, relativistic tidal effects strongly decelerate an initially ultrarelativistic flow with respect to the ambient medium, contrary to Newtonian expectations. Moreover, an initially ultrarelativistic flow perpendicular to the axis of rotation is strongly accelerated by the relativistic tidal forces. The astrophysical implications of these results for jets and ultrahigh energy cosmic rays are briefly mentioned. [source]