island wakes in deep and shallow water observations of island wakes from the continental shelf have been used by various authors to suggest that the reynolds number is not an appropriate parameter for wake behaviour in shallow water it is shown here that wake behaviour both in deep and shallow water is well described by the reynolds number if it is derived correctly by taking into account frictional effects in horizontal as well as vertical boundary layers

island wakes and headland eddies a comparison between remotely sensed data and laboratory experiments island wakes and headland eddies in coastal waters are identified from spaceborne and airborne remotely sensed imagery in the visible wavelengths the imagery were obtained for the bristol and english channels united kingdom which are characterized by high tidal currents and levels of turbidity suspended matter in the surface waters is used as a passive tracer for the flow features scaling parameters e g the reynolds ekman and rossby numbers obtained from the depth averaged equation of motion are presented based upon information on oceanographic conditions at the times of the overpasses the parameters are compared then with data obtained from laboratory model investigations presented by other experimenters in the published literature the remotely sensed data demonstrate that in coastal waters with nonuniform obstacles and bathymetry for a given ekman number the flow regimes occur at a lower rossby number than might be expected from laboratory experiments in the absence of a method for the accurate determination of the horizontal eddy viscosity hence the reynolds number it is concluded that an island wake parameter wolanski et al 1984a should be adopted to describe such features in coastal waters

island wakes in deep water abstract density stratification and planetary rotation distinguish three dimensional island wakes significantly from a classical fluid dynamical flow around an obstacle a numerical model is used to study the formation and evolution of flow around an idealized island in deep water i e with vertical island sides and surface intensified stratification and upstream flow focusing on wake instability coherent vortex formation and mesoscale and submesoscale eddy activity in a baseline experiment with strong vorticity generation at the island three types of instability are evident centrifugal barotropic and baroclinic sensitivities are shown to three nondimensional parameters the reynolds number re rossby number ro and burger number bu the dependence on re is similar to the classical wake in its transition to turbulence but in contrast the island wake contains coherent eddies no matter how large the re value when re is large enough the shear layer at the island is so narrow that the ver

zoogeomorphology in the anthropocene abstract the anthropocene embodies the concept of human impacts on the natural environment but disagreements exist as to when to identify its inception starting date in this paper i illustrate that regardless of the proposed starting date of the anthropocene important zoogeomorphic impacts were initiated at each of these proposed starting dates humans have profoundly altered geomorphic pathways through extinctions and the near extirpation of native populations of animal species that strongly influenced hydrology and removal of surface sediment and through the introduction of populations of animals that bring to bear a suite of different geomorphic effects on environmental systems domestication of animals brought its own suite of zoogeomorphic implications introductions of exotic species and the spread of feral species often led to dramatic new geomorphic landscapes because of the absence of natural controls on population expansion in the mountains of the western usa and elsewhere the geomorphic actions of animals are being impacted by human induced climate change climate change in some cases affects the spatial pattern and range of species whereas in other cases it may lead to the extirpation of species with zoogeomorphic impacts

runoff and sediment yield from topsoils with different porosity as affected by rock fragment cover and position abstract interrill runoff and sediment yield from a topsoil having essentially textural pores and from another with structural pores were measured in the laboratory under simulated rain to investigate the effects of cover and positions of rock fragments the results reveal that the relation between the cover of rock fragments and interrill runoff or sediment yield can be negative as well as positive porosity in the soil near rock fragments determines both the type of relation and the degree to which the positions of rock fragments affect this relation rock fragments increase runoff and sediment yield when well embedded in a surface seal i e a top layer with essentially textural pore spaces a negative relation occurs either where rock fragments are partly embedded in a top layer with structural porosity or where the rock fragments rest on the surface of a soil having either textural or structural pore spaces the time at which macropores at the soil surface close is crucial for then the effect of the cover of rock fragments on the generation of runoff switches from reduction to an increase since cover can have an ambivalent effect on the hydrological and erosion response descriptions of topsoils containing rock fragments should include the type of soil porosity as well as the positions of rock fragments

the nonnormality of coastal ocean flows around obstacles and their response to stochastic forcing abstract the effect of stochastic variability upon the wake downstream of a real headland is investigated from observations taken at bass point australia three sources of stochastic forcing are identified and quantified these being variability in the wind stress in the flow incident on the headland and that generated in the flow by complex reef topography at the headland s tip these sources of stochastic forcing are found to induce a significant degree of unsteadiness in a nonlinear numerical model of flow around bass point that in the absence of stochastic forcing simulates steady recirculating flow the observed stochastic variability of the flow incident on the headland is found to be larger than is necessary to support unsteadiness in the simulated straining zone the dynamics of the perturbation growth observed in the model are understood by applying the techniques of generalized stability theory to the associated tangent linear model tlm the eigenmodes of the tlm confirm that the system

tidal computations for morecambe bay summary a numerical finite difference technique for computing tides in areas of shoaling water has been established and used to calculate the m2 tide in morecambe bay an essential feature of the method is the representation of drying banks which appear as the water level falls on the ebb and which are re submerged on the flood as the level rises again since the non linear advective acceleration should be important in shallow water schemes for incorporating these terms are examined and the influence of advection on the tide assessed in a series of tests the open sea tides applied to the outer boundary of the numerical bay model were taken first from existing tidal charts then from calculations with a larger scale encompassing model and finally from observations of current in each case the computed tidal elevations within the bay turned out to be in satisfactory agreement with those obtained from observations however it is significant that in the first case inaccuracy of the input tidal distribution deduced from charts gives rise to erroneous currents near the boundary

recirculation in the lee of complicated headlands a case study of bass point it has been argued in recent papers that the nature of headland and coastal island wakes can be highly dependent on the level of turbulence caused by complicated coastal geometry and bottom topography as well as on the loss of energy and momentum to the seabed due to friction a series of field experiments was conducted around bass point a 4 km long headland near sydney australia using moored current meters and an acoustic doppler current profiler the offshore component of currents measured offshore of the point showed a high degree of spectral energy at about the 7 day cycle with a small amount at the diurnal frequency but the reverse was found in the lee of the headland where most of the recirculation was at the diurnal frequency numerical experiments resulted in similar spectra being obtained from locations behind the headland when the numerical model was forced with a spectrum of frequencies resembling the spectrum of the offshore free stream current which was derived from the field results the size and frequency of eddyshedding in the lee of the headland depended on the degree of variability of the inflowing free stream with a diurnal component in the free stream appearing to cause the oscillations in the wake also of diurnal frequency the numerical experiments also show that the size of the recirculating wake was affected by the complexity of the headland s geometry

response of the benthic nepheloid layer to near inertial internal waves in southern lake michigan 1 time series measurements of water transparency water temperature and current velocity were made at a station located in 58 m of water in southern lake michigan during the summer of 1995 currents generated by near inertial internal waves are correlated with variations in the thickness and in the vertical distribution of suspended sediment in the benthic nepheloid layer although a direct causal link between internal wave action and changes in the nepheloid layer could not be established the data suggest that local resuspension by shoaling internal waves maintains the layer during the stratified period the origin and maintenance of the benthic nepheloid layer is most likely the result of local resuspension due to a combination of internal wave action and longer term processes

nepheloid and suspended particulate matter in south eastern lake michigan a persistent benthic nepheloid layer with high total suspended matter tsm and high total particulate surface area was observed in south eastern lake michigan the layer thickens from a few metres near the shelf slope boundary to greater than 10 m at the base of the slope when compared to the hypolimnion tsm increases by a factor of 2 20 at 1 m above the bottom the greatest increase detected at the slope basin boundary sediment trap profiles within the nepheloid layer show that the particulate flux increases exponentially from about 10 m above the bottom to 1 m above the bottom suggesting that a large fraction of the collected material came from resuspension a nepheloid layer is created during the formation of the thermal bar and maintained during the stratified period apparently through the action of weak but persistent currents this layer is supplemented by lakeward transport of fine particles resuspended near the shelf slope boundary due to impingement of the thermocline on the bottom or during higher energy events

field experiments on the transport of rock fragments by animal trampling on scree slopes qualitative observations in an area with steep scree slopes in turkey suggested that trampling by goats and sheep may contribute significantly to the downslope movement of rock fragments in these environments therefore a series of field experiments was set up to determine the rate of rock fragment transport caused by trampling the experimental results allowed the calculation of a unit displacement distance from each trampling experiment which may then be used to extrapolate the measured transport rates over longer time periods and larger areas although such extrapolations should be interpreted with great caution the results demonstrate that trampling contributes significantly to rock fragment transport in such environments as calculated yearly displacement distances are equal to or higher than those reported in the literature for other geomorphic processes the observed travel distance distributions are not well described by a negative exponential distribution trampling causes a weak downslope sorting the coarsest clasts move farther but has a significant effect on vertical sorting and is one of the factors which can explain the layered structure of the scree slopes in the area

rocking the landscape chacma baboons papio ursinus as zoogeomorphic agents abstract animals sculpt landforms by physically altering the substrate many digging and burrowing animals are then considered zoogeomorphic agents while the significance of soil disturbing species is well established geomorphic impacts are rarely quantified for rock transporting species the chacma baboon papio ursinus a widespread and abundant primate throughout most of southern africa south of the zambezi may be a zoogeomorphic agent through its role in rock displacement while foraging we quantified baboon rock movement along belt transects placed across a catena in a semi arid karoo environment in south africa along each transect we the counted the number of moved rocks and recorded their dimensions mass and shape and related this to rock i e mass shape and landscape i e hillslope obstacles that hinder movement features to assess the drivers of the extent of rock transport baboon rock movement was found to be extensive ranging from 26 to 54 615 kg 10 967 2 2300 9 kg of rock material displaced per ha and year the distance of rock movement was influenced by rock size large rocks were displaced further and rock shape flat rocks were moved further followed by angular and rounded rocks furthermore slope influenced rock movement with rocks displaced greater distances on steeper slopes baboon rock movement brought about a loss in potential energy and a change in landscape entropy through the net downward movement of rocks we show that baboons play an important geomorphic role and probably serve as a keystone species as they are the only species in this environment to intentionally move rocks in this way because baboons are considered pests and are widely persecuted their role in zoogeomorphic processes are vulnerable to being lost

rapid perturbation growth on spatially and temporally varying oceanic flows determined using an adjoint method application to the gulf stream abstract the stability of the gulf stream flow predicted by a nonlinear quasigeostrophic model is examined by employing an iterative method which uses both the tangent linear equations and the adjoint tangent linear equations of the quasigeostrophic model the basic state flow is the model s representation of the gulf stream as observed during january and february 1988 the growth of perturbation energy is examined as a measure of disturbance growth and linear perturbations are found that are optimal in the sense that they maximize the growth of perturbation energy the structures of optimal perturbations are compared with the structure of the normal modes the optimal perturbations are found to be mere localized and to grow much more rapidly than the normal modes optimal perturbations are of interest because they can be used to plan tight constructive upper bounds on the growth of perturbations to ocean currents such as the gulf stream and they provide valuable information about the predictability of

rock fragments in top soils significance and processes this introductory paper reviews various aspects of rock fragments in top soils such information is required for various reasons and in particular for predicting the impact of climatic or landuse changes on the response of these soils particular attention is paid to the definition and measurement of rock fragment content in top soils the density of soils containing rock fragments the spatial distribution and movement of rock fragments in top soils the effects of rock fragments on some key hydrological processes thermal properties of top soils physical soil degradation soil erosion and soil productivity

transient eddy formation around headlands eddies with length scales of 1 10 km are commonly observed in coastal waters and play an important role in the dispersion of water borne materials the generation and evolution of these eddies by oscillatory tidal flow around coastal headlands is investigated with analytical and numerical models using shallow water depth averaged vorticity dynamics eddies are shown to form when flow separation occurs near the tip of the headland causing intense vorticity generated along the headland to be injected into the interior an analytic boundary layer model demonstrates that flow separation occurs when the pressure gradient along the boundary switches from favoring accelerating to adverse decelerating and its occurrence depends principally on three parameters the aspect ratio b a where b and a are characteristic width and length scales of the headland h cda where h is the water depth cd is the depth averaged drag coefficient and uo a where uo and are the magnitude and frequency of the far field tidal flow simulations with a depth averaged numerical model show a wide range of responses to changes in these parameters including cases where no separation occurs cases where only one eddy exists at a given time and cases where bottom friction is weak enough that eddies produced during successive tidal cycles coexist interacting strongly with each other these simulations also demonstrate that in unsteady flow a strong start up vortex forms after the flow separates leading to a much more intense patch of vorticity and stronger recirculation than found in steady flow

stochastic forcing of the north atlantic wind driven ocean circulation part ii an analysis of the dynamical ocean response using generalized stability theory as discussed in part i of this study the magnitude of the stochastic component of wind stress forcing is comparable to that of the seasonal cycle and thus will likely have a significant influence on the ocean circulation by forcing a quasigeostrophic model of the north atlantic ocean circulation with stochastic wind stress curl data from the ncar ccm3 it was found in part i that much of the stochastically induced variability in the ocean circulation is confined to the western boundary region and some major topographic features even though the stochastic forcing is basinwide this can be attributed to effects of bathymetry and vorticity gradients in the basic state on the system eigenmodes using generalized stability theory gst it was found in part i that transient growth due to the linear interference of nonnormal eigenmodes enhances the stochastically induced variance in the present study the gst analysis of part i is extended and it is found that the patterns of wind stress curl that are most effective for inducing variability in the model have their largest projection on the most nonnormal eigenmodes of the system these eigenmodes are confined primarily to the western boundary region and are composed of long rossby wave packets that are doppler shifted by the gulf stream to have eastward group velocity linear interference of these eigenmodes yields transient growth of stochastically induced perturbations and it is this process that maintains the variance of the stochastically induced circulations analysis of the large scale circulation also reveals that the system possesses a large number of degrees of freedom which has significant implications for ocean prediction sensitivity studies show that the results and conclusions of this study are insensitive and robust to variations in model parameters and model configuration

sediment resuspension and transport in lake michigan during the unstratified period instrumented moorings were deployed during the winter of 1994 95 at three depths 28 58 and 101 m in southern lake michigan storms during the observation period were not unusually severe so the processes observed are typical of those that occur during an average winter time series observations of water temperature beam attenuation coefficient a measure of water transparency and current velocity show that local resuspension of bottom sediment occurred frequently after the breakdown of the thermocline resuspension was most frequent close to the shore but was also observed at the 58 m station local resuspension did not occur at the 101 m station but advection to the site of material resuspended at shallower sites was observed these observations do not support the hypothesis proposed by previous investigators that local resuspension at depths of 100 m or greater occurs during the unstratified period it is more likely that fine grained material resuspended by storm action in intermediate water depths 30 60 m is transported into the deeper parts of the lake by the general lake circulation

spatial distribution of surficial sediments in part of the kingston basin of northeastern lake ontario canada the distribution of surficial sediments in the interisland area of northeastern lake ontario is characterized by a generally homogeneous composition of fine grained muds with some local exceptions at the gaps leading into kingston basin and in parts of north channel this pattern reflects principally differing levels of wave energy from the exposed areas of outer kingston basin and upper and lower gaps to the more protected interisland area additional processes responsible for the distribution are currents at the upper and lower gaps and local slumping the observed deposition of surficial sediments in the northern kingston basin shows generally good agreement with the wave base calculations employing wind speeds of 60 km h 1 the study region consists of three sedimentary environments above the wave base related to winds of less than 60 km h 1 continual erosion and transport of all grain sizes leads to a constant source of sediment to the deeper parts of the kingston basin above the wave base related

stone movement on hillslopes in the mojave desert california a 16 year record previous research has shown that either hydraulic action or creep may be the dominant process transporting coarse debris down hillslopes in the american southwest this study analyses the movement over 16 years of painted stones on two hillslopes in the central mojave desert to ascertain which of these two processes dominate in this region the distance moved m is found to be directly related to length of overland flow x and hillslope gradient s and inversely related to particle size d the fact that m is more highly correlated with x than with s suggests that hydraulic action rather than creep is the dominant process it is concluded that this is probably the case over most of the mojave and sonoran deserts on slopes up to at least 24 and that it is only at higher elevations where winters are more severe that creep may become dominant

numerical study of a coastal current on a steep slope in presence of a cape the case of the promontorio di portofino 1 a process study aimed at investigating the winter circulation in the area of the promontorio of portofino northwestern mediterranean is presented motivated by historical current measurements suggesting the presence of an attached recirculating eddy in the lee of the promontorio cape a sensitivity study is first performed in an idealized setting considering the interaction of a steady incoming current with the cape numerical experiments are performed using both two dimensional 2 d vertically integrated and 3 d versions of the pom model from the 2 d results the main controlling parameter appears to be the equivalent reynolds number ref in agreement with previous historical results in the 3 d case the dependence on the vertical ekman number ekv is investigated at fixed ref and a significant intensification of the attached eddy with respect to the 2 d solutions is obtained the main difference between the 2 d and 3 d dynamics is the presence of a resolved bottom ekman layer in 3 d introducing a vertical shear in the incoming current particularly noticeable in the region of the shelf break this shear is likely to be responsible for a secondary circulation dominated by coriolis effect inducing an inshore surface current and associated upwelling in the lee of the cape and causing the observed eddy intensification experiments with realistic bathymetry are performed and the 3 d results are found to be in good qualitative agreement with the measurements the results are expected to be relevant for biological transport pointing out to a mechanism for eddy intensification connected with surface inshore transport and upwelling

flow around islands in rupert bay an investigation of the bottom friction effect aerial photographs of transverse shear flows around islands in rupert bay a shallow and turbid estuary in northern quebec were studied for the effect of bottom friction the island wake depends on the stability of the transverse shear layers developed along the two sides of the island two basic patterns of wake flows were observed a vortex street wake was formed when transverse shear was large as in flows around small islands on the other hand for small transverse shear as in flows around large islands the shear layers were stabilized by bottom friction and in that case a clear water wake of low turbulence and low sediment concentration was observed laboratory demonstration of the bottom friction influence on the turbulent wake in shallow water was made using a shallow water table a similarity solution was obtained for the wake flow in the far field region and numerical calculations were made of the recirculating flow in the wake bubble a wake stability parameter was introduced for wake classification and for correlation of field and laboratory data in rupert bay vortex street wakes were observed in nine out of 27 events when the wake stability parameter was less than a critical value this result is consistent with the laboratory and numerical simulations

processes and rates of rock fragment displacement on cliffs and scree slopes in an amba landscape ethiopia abstract distinct rock fragment displacements occur on the ambas or structurally determined stepped mountains of the northern ethiopian highlands this paper describes the rock fragment detachment from cliffs by rockfall quantifies its annual rate and identifies factors controlling rock fragment movement on the scree slopes it further presents a conceptual model explaining rock fragment cover at the soil surface in these landscapes in the may zegzeg catchment dogu a tembien district tigray rockfall from cliffs and rock fragment movement on debris slopes by runoff and livestock trampling were monitored over a 4 year period 1998 2001 rockfall and rock fragment transport mainly induced by livestock trampling appear to be important geomorphic processes along a 1500 m long section of the amba aradam sandstone cliff at least 80 t of rocks are detached yearly and fall over a mean vertical distance of 24 m resulting in a mean annual cliff retreat rate of 0 37 mm y 1 yearly unit rock fragment transport rates on scree slopes ranged between 23 1 and 37 9 kg m 1 y 1 this process is virtually stopped when exclosures are established corresponding mean rock fragment transport coefficients k are 32 69 kg m 1 y 1 on rangeland but only 3 9 kg m 1 y 1 in densely vegetated exclosures a conceptual model indicates that besides rockfall from cliffs and argillipedoturbation all factors and processes of rock fragment redistribution in the study area are of anthropogenic origin

eddy formation in unsteady flows unsteadiness of flow at m2 s2 tidal frequencies is demonstrated to be a dominant factor forming eddies in natural shallow water and continental shelf environments numerical hydrodynamic modelling and field measurements show that during the later stages of the half tidal cycle in a low friction environment currents in the lee of an obstruction accelerate with supporting sea gradients opposite to the adjacent inertial free stream which is still decelerating these opposed currents appear as a phase eddy and ideally can endure for 3 hours in m2 s2 reversing currents case studies are cited and numerical hydrodynamic modeling is applied to examine the formation of these circulation patterns

effects of viscosity and bottom friction on recirculating flows a recent set of coastal experiments pattiaratchi et al 1987 found that the horizontal eddy reynolds number did not adequately describe the wakes found in the lee of coastal islands other flow parameters formulated on bottom friction were found to be much better at describing these wakes in this paper it is suggested that these findings are caused by the extraction of energy from the flow due to bottom friction which is the net result of the vertical viscous transfer of momentum through the water column to the seabed this is in contrast to the horizontal viscous transfer of momentum which results in negligible energy loss to the coast because of the far larger horizontal scales it is hypothesized that this difference results in fundamentally different flow regimes and this hypothesis is investigated both analytically and numerically conditions for the existence of steady zones of barotropic recirculation under the influence of both horizontal viscosity and bottom friction in the wake behind a bluff body are examined analytically it is found that such steady recirculation requires the presence of horizontal viscosity and is hindered by the loss of energy due to bottom friction a numerical model is used to verify and quantify this analytical result for the case of two dimensional flow around a circular cylinder and it is concluded that an appropriate bottom friction number based on the ratio of scales of the nonlinear and bottom friction terms may often govern the nature of the recirculation in the lee of coastal islands instead of the eddy reynolds number based on horizontal eddy viscosity as is sometimes assumed

bottom stress generation and sediment transport over the shelf and slope off of lake superior s keweenaw peninsula 1 data from near bottom instruments reveal that the mechanisms responsible for generating bottom stresses and resuspending sediment over the shelf and slope off of lake superior s keweenaw peninsula exhibit distinct seasonal variations notably near bottom flows over the slope are persistently weak 10 cm s 1 during summer but frequently attain high speeds in excess of 20 cm s 1 in autumn and winter during the intense storms of autumn and winter the generation of bottom stress is enhanced by the action of near bottom orbital velocities due to surface waves even at 90 m depth orbital velocities can increase bottom stress by a factor of up to 20 during storms where the seasonal thermocline intersects the lake floor bottom stress is also considerably enhanced often by more than a factor of 2 by high frequency motions in the internal wave band over the keweenaw slope sediment resuspension is largely confined to autumn and winter episodes of high bottom stress our analysis indicates that this resuspended material tends to be carried offshore a phenomenon that is partly due to the coincidence of the direction of the buoyancy driven component of the keweenaw current with downwelling favorable alongshore winds as a result of this coincidence currents and bottom stresses tend to be greater during periods of downwelling as opposed to upwelling circulation a potential challenge to modeling storm driven resuspension in the study region is indicated by observations that the minimum stress required for resuspension may vary significantly with time over the autumn and winter index terms 4558 oceanography physical sediment transport 4211 oceanography general benthic boundary layers 4239 oceanography general limnology keywords sediment transport sediment dynamics bottom boundary layer dynamics

island wakes in shallow waters field laboratory and numerical results suggest that the free shear layer behind flow separation points and the internal circulation generated by rotation in the eddy are important processes in island wakes in shallow coastal waters

the dynamics of error growth and predictability in a model of the gulf stream part i singular vector analysis abstract the recently developed ideas of generalized linear stability theory for dynamical systems are applied to time evolving flows of the gulf stream using a quasigeostrophic numerical model the potential for the growth of perturbations arising from errors and uncertainties in forecasts of the gulf stream is investigated by computing the singular vectors or so called optimal perturbations of time evolving flows using a linearized model and its adjoint the authors calculations reveal that the potential for rapid perturbation growth is greatest in regions of developing straining and shearing flows such regions include developing meanders and regions where eddies form and where existing eddies interact with the main gulf stream it is found that the optimal perturbations of the gulf stream share many features in common with those of the jet streams in the atmosphere the ideas and techniques developed in this paper have been applied to the problem of ensemble prediction of the gulf stream in part ii

does local resuspension maintain the benthic nepheloid layer in southeastern lake michigan time series observations of water temperature water transparency and current velocity at four stations in southeastern lake michigan show that the benthic nepheloid layer is probably not maintained by the local resuspension of bottom sediment local resuspension may occasionally occur in the deep parts of the lake but it seems likely that vertical mixing and offshore advection of sediment laden water maintain the benthic nepheloid layer during most of the year although sediment trap data have been interpreted as indicating that local resuspension does occur it is more likely that the increased sediment fluxes observed near the bottom are due to vertical redistribution of material already in suspension

island wakes in shallow coastal waters rattray island northeast australia is 1 5 km long 300 m wide and lies in well mixed water approximately 25 m deep its long axis is inclined at about 60 into the direction of the dominant semidiurnal tidal current the length of the wake in the lee of the island as documented by aerial photographs and satellite imagery appears to equal that of the wake behind a flat plate in a two dimensional flow at a reynolds number of about 10 however current metering drogues measurements and temperature mapping indicate internal wake velocities much greater than would be consistent with such a simple low reynolds number model further estimates of the turbulent eddy coefficient suggest an effective reynolds number more in the vicinity of 103 to reconcile these observational differences and to explain the observed upwelling in the core of the wake an ekman pumping model is proposed it is postulated that the ekman benthic boundary layer driven by rotation in the wake allows the vertical vorticity introduced into the water at the tip of the island at the point of separation to be negated by the vorticity of opposite sign introduced at the bottom further it is shown that a large fraction of the kinetic energy of the upstream flow facing the island is used to drive the wake eddies leading to the conclusions that the trapping of water in the lee of islands greatly increases head losses on continental shelves with numerous islands coral reefs and rock outcrops