Bottom Velocity Research Articles

Effects of hanging ice dams on winter movements and swimming activity of fish

Formation of a hanging dam, a large, thick sub-surface accumulation of frazil ice that blocked river flow, altered physical environmental conditions dramatically and common carp Cyprinus carpio, and brown trout Salmo trutta, evacuated the pool in which the dam developed. A mean of 80·1% of the pool volume was filled with frazil ice. Mean and bottom water velocities in the pool increased from 6 to 27 cm s−1 and from 4 to 21 cm s−1, respectively, when the hanging dam formed, and water depth decreased from a mean of 2·25 to 0·45 m. Activity levels of fish leaving the pool varied. Two carp had higher swimming activities when in the over wintering pool, compared to the period when they were out of it, and a third showed opposite results.

Effects of hanging ice dams on winter movements and swimming activity of fish

Formation of a hanging dam, a large, thick sub‐surface accumulation of frazil ice that blocked river flow, altered physical environmental conditions dramatically and common carp Cyprinus carpio, and brown trout Salmo trutta, evacuated the pool in which the dam developed. A mean of 80·1% of the pool volume was filled with frazil ice. Mean and bottom water velocities in the pool increased from 6 to 27 cm s−1 and from 4 to 21 cm s−1, respectively, when the hanging dam formed, and water depth decreased from a mean of 2·25 to 0·45 m. Activity levels of fish leaving the pool varied. Two carp had higher swimming activities when in the over wintering pool, compared to the period when they were out of it, and a third showed opposite results.

Behavior and distribution of upstream-migrating juvenile Rhinogobius sp. (the orange form)

Upstream-migration of juvenileRhinogobius sp. OR was studied along the Ado River, a tributary of Lake Biwa. Juvenile gobies, mainly 18–21 mm in standard length, migrated from the lake in a belt of about 3 m wide in lower reaches and about 10–20 cm wide in middler reaches. Their upstream-migration was restricted to places shallower than 20 cm with bottom current velocities less than 20 cm/sec. Individual migration speed varied between 4.2–8.9 m/min at places with bottom current velocity of 8.8–20.0 cm/sec, slowing down with increasing current velocity. Most active migration occurred between 14:00 and 16:00, number of upstream-migrating juveniles decreasing with distance from the river mouth. Riffles in the mid-and upper reaches, having few slow current areas, probably present barriers to upstream-migration of the juvenile gobies, although both adults and subadults of the species are distributed in more upper reaches.

Sedimentary record of the 1872 earthquake and “Tsunami” at Owens Lake, southeast California

In 1872, a magnitude 7.5–7.7 earthquake vertically offset the Owens Valley fault by more than a meter. An eyewitness reported a large wave on the surface of Owens Lake, presumably initiated by the earthquake. Physical evidence of this event is found in cores and trenches from Owens Lake, including soft-sediment deformation and fault offsets. A graded pebbly sand truncates these features, possibly over most of the lake floor, reflecting the “tsunami” wave. Confirmation of the timing of the event is provided by abnormally high lead concentrations in the sediment immediately above and below these proposed earthquake deposits derived from lead-smelting plants that operated near the eastern lake margin from 1869–1876. The bottom velocity in the deepest part of the lake needed to transport the coarsest grain sizes in the graded pebbly sand provides an estimate of the minimum initial “tsunami” wave height at 37 cm. This is less than the wave height calculated from long-wave numerical models (about 55 cm) using average fault displacement during the earthquake. Two other graded sand deposits associated with soft-sediment deformation in the Owens Lake record are less than 3000 years old, and are interpreted as evidence of older earthquake and tsunami events. Offsets of the Owens Valley fault elsewhere in the valley indicate that at least two additional large earthquakes occurred during the Holocene, which is consistent with our observations in this lacustrine record.

Three‐dimensional modeling of tidal currents and mixing quantities over the Newfoundland Shelf

Major semidiurnal (M2, S2, and N2) and diurnal (K1 and O1) barotropic tides over the Newfoundland Shelf are computed using a three‐dimensional nonlinear primitive equation model, with the vertical eddy viscosity calculated from a level 2.5 turbulence closure scheme. Computed elevation cotidal charts for the five constituents are generally consistent with previous knowledge for this region. Comparisons based on a statistical analysis of the differences between the computed elevations and currents and in situ observations indicate good agreement. While M2 tidal currents (up to 20–30 cm/s) are dominant, there are locally intensified diurnal currents (up to 5–10 cm/s) in some outer shelf locations. The diurnal current intensification is attributed to first‐mode continental shelf waves. An examination is carried out for the vertical structure of the computed M2 current and for the temporal and spatial variability of model turbulent kinetic energy, mixing length scale, vertical eddy viscosity, and bottom friction velocity. The examination indicates that large vertical eddy viscosity magnitudes and bottom friction velocities are associated with strong currents in shallow regions, where strong vertical shears produce large turbulent kinetic energy. Solutions with both specified and evolving vertical stratification indicate that the stratification has a significant influence on the vertical profile of tidal mixing parameters and currents in shallow areas. Tidally induced turbulence is substantially reduced in the bottom boundary layer and completely suppressed above it. Tidal currents decrease in the log layer, increase significantly in the rest of the bottom boundary layer, and decrease in the upper and middle water column.

Sediment Resuspension Near the Keweenaw Peninsula, Lake Superior During the Fall and Winter 1990–1991

During the winter of 1990–1991 time series measurements of current velocity, temperature, and attenuation (a measure of water transparency) were made at a site in 91 m of water near Copper Harbor, MI. The observations show that bottom resuspension occurred several times during the unstratified period. The resuspension is the result of the interaction between high bottom current velocities and surface waves generated by strong winds. Transport during the storms was almost entirely alongshore, although some offshore transport of material occurred. Calculations show that suspended material could have been transported eastward several hundred km during the unstratified period.

Relationship between current measurements and sonographs of subtidal bedforms in the macrotidal Tay Estuary, Scotland

Abstract Near bottom current measurements have been compared with sonographs revealing a variety of subtidal dune geometries at eight stations in the middle reaches of the macrotidal Tay Estuary, Scotland. At six stations, dune asymmetries did not support the tidal dominance expressed in terms of maximum or average current speeds. Determination of bottom tidal current dominance according to the length of time during which a speed of 0.5 ms −1 was exceeded, as a proxy for the inequality of the total ebb and flood tide bedload sediment transports, provides a better, but not perfect, correlation with sonographs. Other controls on dune asymmetry and reversal are height, sediment discharge rate and bottom current velocity fluctuations. The study has shown that bedload transport processes and pathways in this relatively small ( c. 8 km 2 ), dynamic part of the estuary are far more complex than previously inferred and near bed flows do not simply correspond with the surface water circulation pattern. Neither sonographs nor current data alone are adequate to gain an understanding of subtidal bedforms in such an environment.

Near-Bottom Turbulence Measurements in a Partially Mixed Estuary: Turbulent Energy Balance, Velocity Structure, and Along-Channel Momentum Balance*

Abstract A set of moored, bottom-mounted and shipboard measurements, obtained in a straight section of the lower Hudson estuary during late summer and early fall of 1995, determine velocity, density, and along-channel pressure gradient throughout the 15-m water column, as well as providing direct eddy-correlation estimates of Reynolds stress and indirect inertial-range estimates of dissipation within 3 m of the bottom. The analysis focuses on testing 1) a simplified turbulent kinetic energy equation, in which production balances dissipation; 2) the Prandtl–Karman law of the wall, which is a relationship between bottom stress and near-bottom velocity gradient; and 3) a simplified depth-integrated along-channel momentum balance involving local acceleration, pressure gradient, and bottom stress. Estimates of production and dissipation agree well throughout the entire record. The relationship between bottom stress and velocity gradient is consistent with the law of the wall within approximately 1 m of the sea...

On the dynamics of the Zapiola Anticyclone

Recent observations obtained in the South Atlantic suggest the existence of a strong anticyclonic flow positioned over a major bottom topographic feature known as the Zapiola Drift. Here a numerical simulation of the South Atlantic is described in which this anticyclone is reproduced, and the model is used to diagnose the dynamics maintaining this flow. With a mean barotropic transport of 140 Sv and bottom velocities of 10 cm s−1, the simulated Zapiola Anticyclone compares well with in situ observations. Furthermore, the model surface eddy kinetic energy shows a local minimum over the drift, in agreement with observations from TOPEX/POSEIDON. Numerical experiments show that the circulation feature is sensitive to the intensity of the eddy field and to the particular value of the bottom friction. Both of these tendencies are in agreement with a theoretical explanation of the Zapiola Anticyclone that has recently appeared elsewhere. Thus we argue that the anticyclone is maintained by eddy‐driven potential vorticity fluxes accelerating flow within topographically closed, ambient potential vorticity contours. As far as we know, this is the first South Atlantic simulation to reproduce the Zapiola Anticyclone in a realistic fashion. The quantitative success of this experiment is attributed to the use of a topography following (sigma) coordinate in a spatially well resolved model.

Magnetic characterization of Holocene sedimentation in the South Atlantic

Surface sediment samples representative for the tropical and subtropical South Atlantic (15°N to 40°S) were investigated by isothermal magnetic methods to delineate magnetic mineral distribution patterns and to identify their predominant Holocene climatic and oceanographic controls. Individual parameters reveal distinct, yet frequently overlapping, regional sedimentation characteristics. A probabilistic (“fuzzy c‐means”) cluster analysis was applied to five concentration independent magnetic properties assessing magnetite to hematite ratios and diagnostic of bulk and fine‐particle magnetite grain size and coercivity spectra. The resultant 10 cluster structures establish an oceanwide magnetic sediment classification scheme tracing the major terrigenous eolian and fluvial fluxes, authigenic biogenic magnetite accumulation in high‐productivity areas, transport by ocean current systems, and effects of bottom water velocity on depositional regimes. Distinct dissimilarities in magnetic mineral inventories between the eastern and western basins of the South Atlantic reflect prominent contrasts of both oceanic and continental influences.

The estimation of shear stresses from near-bed turbulent velocities for combined wave–current flows

High frequency near-bottom velocities measured by a combined pressure gauge and flow-meter are analysed with a view to understanding and modelling waves and sediment transport in shallow waters. Data from five stations in different water depth, wave and current conditions are compared. Wave, tide/surge and turbulence components have been separated from the bottom velocity data and are used to derive a friction velocity and examine the bottom stress parameterisation. The horizontal component of turbulent velocity fluctuations near the bed is found to be well-correlated with the bed shear stress from a combined wave and current model. The model of Christoffersen and Jonsson [Christoffersen, J.B., Jonsson, I.G., 1985. Bed friction and dissipation in a combined current and wave motion. Ocean Engineering, 12, 5, 387–423] is in reasonable agreement with the observed data. The observed turbulence can shed light on the wave bed shear stress as well as the shear stress due to the current. A fitting procedure gives a derived Nikuradse length scale for each station which is similar to that obtained independently from bed grain size estimates.

Frontal structure and Antarctic Bottom Water flow through the Princess Elizabeth Trough, Antarctica

Hydrographic, current meter and ADCP data collected during two recent cruises in the South Indian Ocean ( RRS Discovery cruise 200 in February 1993 and RRS Discovery cruise 207 in February 1994) are used to investigate the current structure within the Princess Elizabeth Trough (PET), near the Antarctic continent at 85°E, 63–66°S. This gap in topography between the Kerguelen Plateau and the Antarctic continent, with sill depth 3750 m, provides a route for the exchange of Antarctic Bottom Water between the Australian–Antarctic Basin and the Weddell–Enderby Basin. Shears derived from ADCP and hydrographic data are used to deduce the barotropic component of the velocity field, and thus the volume transports of the water masses. Both the Southern Antarctic Circumpolar Current Front (SACCF) and the Southern Boundary of the Antarctic Circumpolar Current (SB) pass through the northern PET (latitudes 63 to 64.5°S) associated with eastward transports. These are deep-reaching fronts with associated bottom velocities of several cm s -1. Antarctic Bottom water (AABW) from the Weddell–Enderby Basin is transported eastwards in the jets associated with these fronts. The transport of water with potential temperatures less than 0°C is 3 (±1) Sv. The SB is shown to meander in the PET, caused by the cyclonic gyre immediately west of the PET in Prydz Bay. The AABW therefore also meanders before continuing eastwards. In the southern PET (latitudes 64.5 to 66°S) a bottom intensified flow of AABW is observed flowing west. This AABW has most likely formed not far from the PET, along the Antarctic continental shelf and slope to the east. Current meters show that speeds in this flow have an annual scalar mean of 10 cm s -1. The transport of water with potential temperatures less than 0°C is 20 (±3) Sv. The southern PET features westward flow throughout the water column, since the shallower depths are dominated by the flow associated with the Antarctic Slope Front. Including the westward flow of bottom water, the total westward transport of the whole water column in the southern PET is 45 (±6) Sv.

Overwinter Habitat Use of Shovelnose Sturgeon in the Kansas River

Overwinter habitat use of shovelnose sturgeon Scaphirhynchus platorynchus in the Kansas River was determined by radiotelemetry from November 1996 to March 1997 at water temperatures of 2–9°C. Eighty percent of the shovelnose sturgeon locations were in water depths of 1.0–2.0 m, where current velocities were 0.01–1.11 m/s at the surface and 0.02–0.79 m/s at the bottom. Depths and surface current velocities at fish locations were positively related to discharge (r = 0.47, P = 0.0001; r = 0.60, P = 0.0001, respectively); bottom current velocities were not significantly correlated (r = 0.08, P = 0.31) with discharge. Water depths, surface current velocities, and bottom current velocities were not significantly related to water temperature (P > 0.05). Ninety-two percent of the shovelnose sturgeon locations were over sand substrate. Inside-bend macrohabitats were used in proportion to their abundance (P > 0.05), whereas channel crossovers were used in greater proportion than their availability (P ≤ 0.05); outside-bend habitats were avoided (P ≤ 0.05). During high discharge (>150 m3/s), shovelnose sturgeon appeared to move near shore or downstream of instream cover. Most shovelnose sturgeon moved less than 2 km during the study period, but one fish moved more than 8 km. Movement (km/d) and directional movement (i.e., upstream or downstream) were not related to discharge or water temperature (P > 0.05). These data indicate that shovelnose sturgeon use channel-crossover macrohabitats and areas with bottom velocities of 0.02–0.79 m/s, independent of discharge. In addition, it appears that shovelnose sturgeon do not congregate in deep areas at water temperatures less than 9°C.

Colonization of new habitats by benthic foraminifera: a review

Colonization of new habitats, which have been established as a result of a catastrophic disturbance of the environment, is one of the characteristic repetitive events throughout the Phanerozoic. In recent years, much attention has been paid to investigations focusing on biological recovery of benthic habitats severely disturbed by human activity. In order to improve our environmental and stratigraphical interpretations of such events, we need a more thorough understanding of the processes involved in colonization by one of the most abundant and useful fossil groups, the benthic foraminifera. The present review focuses on processes governing benthic foraminiferal dispersion and colonization patterns in modern environments. For benthic foraminifera, the only active dispersal mechanism is through self-locomotion on or within the sediment and this is considered to be efficient over short distances only. Several passive dispersal methods have been suggested but two seem to be of more general importance. These are dispersal through release and transport of embryonic juveniles and passive suspension and transport of various growth stages. Both are probably important for most benthic foraminifera but the former is likely to be the main mechanism for attached, tubular and larger foraminifera, which are not easily entrained at a later life stage. The latter seems to be a more important dispersion mechanism for benthic foraminifera than previously realized. The colonization rate of soft-bottom substrates is closely related to the hydraulic regime in, and the transit time from, the source area inhabited by species capable of colonizing the new habitat (as long as food and other environmental characteristics are not limiting factors). The transit time depends on the speed of the transporting medium and the distance from the source area. There seems to be two end-processes which can operate during the colonization, depending on whether physically induced or biological processes are allowed to dominate. They are characterized by different colonization patterns. In high energy environments (bottom current velocities often >20 cm/s), a short transit time may cause the major components of the nearest ambient seafloor assemblages to colonize the new habitat within days. In this case the colonization is simply through a physical transfer of parts of the source community to the new habitat, allowing no time for pioneer, opportunistic assemblages to develop. In low energy environments (bottom current velocities generally <10 cm/s), the transit time is long for most species. Here, colonization follows the classic metazoan successional pattern with an initial, high abundance pioneer assemblage strongly dominated by small opportunists followed by development of assemblages with increasing numbers of specialized species and recovery can take from one to several years. Initial lack of food (e.g., volcanic ash) or `hostile' substrate properties (e.g., recently reoxygenated or severely contaminated sediments) may delay colonization by months or even years. Small, infaunal species (both calcareous and agglutinated) are among the first and most successful colonizers of soft bottom habitats from shallow waters to the deep sea. Throchospiral agglutinated taxa are among the most abundant colonizers on deep sea hard substrates.

Contrôle topographique du courant circumpolaire antarctique dans l'océan Indien sud

The influence of bottom topography on the separation of the Antarctic Circumpolar Current (ACC) in the Crozet Plateau region has been studied using an inertial jet model. A series of simulations with realistic velocity profiles of the current clearly shows that the northward separation of the subantarctic branch of the ACC is controlled by the regional bathymetry, in particular by the South-West Indian Ridge. A bottom velocity of 2 to 4 cm·s −1 has been found to be necessary to reproduce this separation.

The Influence of Sediment Load on Tidal Dynamics, a Case Study: Cádiz Bay

A two-dimensional, non-linear, finite-difference, hydrodynamic model was applied to Cádiz Bay to study the influence of sediment load on tidal dynamics. The sediment load effect is represented parametrically as a dependence of the drag coefficient on the relative settling velocity (the ratio of the settling velocity of suspended particles to the bottom friction velocity) and the relative friction velocity (the ratio of the bottom friction velocity to its critical value at which sediment particles begin to go into suspension). This dependence is derived from a solution of the equations describing the vertical structure of the sediment-stratified bottom logarithmic layer. A comparison of the model predictions with and without allowance for the sediment load effect shows that the latter is responsible for small local changes of the amplitude and phase of tidal elevation and the maximum depth-averaged tidal velocity, the result counting in favour of the conventional approach whereby the influence of sediment load on tidal dynamics is considered to be negligible. However, it is apparent after close inspection of the model predictions that the sediment load effect tends to enhance the time-space variability of the tidal characteristics. In particular, it results in an increase in the maximum depth-averaged velocity and a decrease in the drag coefficient for the periods of flood and ebb currents, thus reducing the shear bottom stress and the tidal energy dissipation by about half.

Opposing gradients of permanent strain in the aseismic zone and elastic strain across the seismogenic zone of the Kodiak shelf and slope, Alaska

An industry‐acquired seismic reflection line from the Aleutian Arc to the Alaska Trench was prestack depth‐migrated and further constrained with wide‐angle ocean bottom hydrophone seismic velocity data and swath mapping. From the seismic reflection image, individual thrust slices, the backstop, and the forearc basin were resolved sufficiently well to balance the section. Tectonic shortening and age estimates provided rates of permanent contraction in five discrete segments over ∼150 km during the past 3 Myr. About 70% of the plate convergence is measured as contraction across the ∼40‐km‐wide Neogene accretionary prism which composes less than 10% of the forearc. Permanent contraction is minimal above the 1964 Alaska earthquake aftershock zone indicating dominently elastic strain. Since elastic strain is poorly recorded in the geological record, it was approximated from the historical record of the 1964 Alaska earthquake. The 1964 coseismic vertical deformation displays an inverse pattern to the permanent strain, consistent with the earthquake cycle concept. Elastic deformation is dominant beneath the shelf and upper slope, rapid permanent deformation is concentrated beneath the lower slope, and a transition between them occurs beneath the middle slope.

CHARACTERISTICS OF VELOCITY VERTICAL DISTRIBUTION AND BOTTOM SHEAR STRESS OF TSUNAMI IN A STRAIT

Tsunami causes erosion and accretion in a harbor or a bay, which may result in the collapse of structures and the damage of harbors. It is important to calculate the accurate sea bottom changes by Tsunami. The strong pressure gradient transforms the velocity vertical distribution in a strait or a bay mouth where the flow is accelerated and decelerated rapidly. It is necessary to study the propriety of the method that the bottom shear stress is estimated by the mean flow formula.Characteristics of the velocity distribution and the bottom friction velocity of tsunami were carefully investigated by experiments in a narrow strait where the Shields number is more than 10. The velocity distribution becomes uniform in the acceleration phase, and the bottom friction velocity becomes large by the strong velocity gradient near the bottom. Based on the results, the method of evaluation of the bottom sheer stress by the log-wake law is proposed.

Taxon-specific tidal resuspension of protists into the subtidal benthic boundary layer of a coastal embayment

Sediment resuspension has widespread effects on microbial processes, primary and secondary production, and nutrient cycles, but its influence on protists other than microalgae is largely unknown. Distributions and abundances of protists in subtidal benthic boundary layers (BBL), in particular, are poorly known. We measured vertical profiles of protists in the BBL and underlying sediment at a subtidal silty site in Buzzards Bay, Massachusetts, USA, to determine cell-resuspension patterns. Tidal flow produced maximal bottom shear velocities of 1.4 to 2.2 cm s -1 . Near-bottom turbidity increased during each slack tide, when the suspended load settled, and it decreased during tidal exchange, presumably after a thin veneer of sediment resuspended from the sediment-water interface (SWI) and mixed into the upper water column. Tidal periodicities in protistan vertical profiles were taxon- and functional-group specific. Heterotrophic nanoflagellates (HNan) and ciliates, including scuticociliates, oligotrichs, and hypotrichs of the genera Euplotes and Urostrongylum, showed periodicities in distribution consistent with cycles of resuspension and deposition. BBL concentrations of HNan and scuticociliates were elevated during tidal exchange by factors of ≤2.1 and 4.6, respectively, within 5 cm of the SWI; oligotrichs were found consistently in the BBL but were in the sediment only during slack tide; Euplotes was present consistently in the sediment but was in the BBL only during tidal exchange. Total resuspended cells in the bottom 1 m were of the order 10 8 to 10 9 HNan m -2 and 10 5 to 10 6 ciliates m -2 , and in some cases the measured cell disappearance from surficial sediment during tidal exchange balanced the increase in the BBL. In contrast, pigmented nanoflagellates, pennate diatoms, and ciliates, including karyorelictids and other hypotrichs, maintained constant profiles throughout tidal cycles. Specificity of results among protistan groups might be due to behavioral adaptations such as depth zonation in the sediment, associations with particles, and vertical migration. We know of no other documentation in the field of cyclical emergence of heterotrophic protists and re-entry into sediment. Our data suggest complex taxon-specific linkages between sedimentary and water-column protistan communities that may be controlled by flow in the BBL, potentially influencing food-web dynamics.

Improved Design of Energy Dissipators for Pipe Outlets

This paper presents an experimental study on thedevelopment of an efficient energy dissipator ascompared to Garde's energy dissipator (Garde et al.1986) for pipe outlets. The recommended designs werelaboratory tested on scale models with pipe outletdiameter of 10 cm and 7.5 cm for Froude number rangingfrom 1.70 to 5.50. The performance of the newdissipator improved tremendously by using new shapes,sizes and locations of the appurtenances for the samelength of stilling basin. This was possible due tobetter spreading of efflux jet by using a propersplitter block. The formation of a strong vortexinfront of the solid impact wall, in place of a grid,and an additional horizontal shear at the bottomproduced more fine grained eddies and turbulence,which finally reduced the energy of the outgoing flow. The low bottom velocities at the end of the basinresulted in reduction of the scour. The performanceof various models has been compared by estimating thescour index based on maximum depth of scour and itslocation at the end of run time.