rapid water exchange between the lagoon and the open ocean at majuro atoll due to wind waves and tide current measurements and conductivity temperature depth surveys of the lagoon and ocean at majuro atoll the republic of the marshall islands were made from january 10 to 24 1997 a vertically integrated tidal current model reproduced qualitatively well tidal ellipses calculated from the observed current measurements by fourier transform a three dimensional robust diagnostic residual current model explained the major features observed in the current measurements averaged over the dominant tidal cycles we used the diagnostic model to examine the effects of wind stress radiation stress density gradients and tidal stress on the exchange of water between the lagoon and the ocean wind effectively mixes the lagoon water in approximately 2 weeks tidal flushing appears to be restricted to a small area near the main channels connecting the lagoon to the ocean cross reef flat currents induced by radiation stress and flowing to the deep channels in the center of the northern boundary form the dominant mechanism for exchange between the lagoon and the open ocean causing water to exchange completely with the ocean in about 15 days computer generated particles tracked through the lagoon showed that radiation stress is also the main forcing mechanism for particle export from the lagoon however the coupling of tidal exchange through the calalin channel and wind stress induced mixing in the lagoon could also provide a significant export mechanism particularly for particles originating uniformly inside the lagoon

wave transformation and wave driven flow across a steep coral reef observations of waves setup and wave driven mean flows were made on a steep coral forereef and its associated lagoonal system on the north shore of moorea french polynesia despite the steep and complex geometry of the forereef and wave amplitudes that are nearly equal to the mean water depth linear wave theory showed very good agreement with data measurements across the reef illustrate the importance of including both wave transport owing tostokes drift as well as the eulerianmean transportwhen computing the fluxes over the reef finally the observed setup closely follows the theoretical relationship derived from classic radiation stress theory although the two parameters that appear in the model one reflecting wave breaking the other the effective depth over the reef crest must be chosen to match theory to data

double averaging concept for rough bed open channel and overland flows theoretical background the goal of this paper is to discuss the spatial averaging concept in environmental hydraulics and develop it further by considering transport equations for fluid momentum passive substances and suspended sediments the averaging theorems the double averaged in time and in space fluid momentum equation and advection diffusion equations for a passive substance and suspended sediments are introduced and their limitations and applications for modeling rough bed flows experimental design and data interpretation are discussed the suggested equations differ from those considered in terrestrial canopy aerodynamics and porous media hydrodynamics by accounting for roughness mobility change in roughness density in space and time and particle settling effects for the case of suspended sediments we show that the form of the double averaged equations may depend on the type of decomposition of flow variables and that this difference may have important implications for modeling we also show that the suggested methodology offers better definitions for hydraulic characteristics variables and parameters such as flow uniformity flow two dimensionality and bed shear stress

numerical modeling of low frequency wave dynamics over a fringing coral reef abstract low frequency infragravity wave dynamics on a fringing coral reef were investigated using the numerical model xbeach roelvink et al 2009 first the skill of the model was evaluated in one and two dimensions based on its predictions of short waves 0 04 0 2 hz infragravity waves 0 004 0 04 hz and water level measurements tidal and wave setup obtained during a 2009 field study at ningaloo reef in western australia the model calibration was sensitive to friction coefficients for short waves and current infragravity bed friction which were assumed independent in this model study although the one dimensional cross shore model captured the gradients in the dominant hydrodynamic processes at the site a high current ig bed friction coefficient was required this resulted in an overestimation and a phase lag between the observed and predicted wave setup signal in the two dimensional model a lower more realistic current infragravity wave friction coefficient was required to achieve optimum performance due to the presence of significant reef and lagoon mean flows in the model which led to reduced setup across the reef the infragravity waves were found to propagate from the surf zone across the reef in a dominantly cross shore direction towards the shore but with substantial frictional damping the infragravity waves were strongly modulated also over the reef by tidal depth variations primarily due to the variability in frictional dissipation rates when the total water depth over the reef varied two mean wave driven circulation cells were observed in the study area with cross shore flow becoming more alongshore dominated before exiting the system via the two channels in the reef the results reveal that short waves dominated bottom stresses on the forereef and near the reef crest however inside the lagoon infragravity waves become increasingly dominant accounting up to 50 of the combined bottom stresses

hydrodynamics of spur and groove formations on a coral reef 1 spur and groove sag formations are found on the fore reefs of many coral reefs worldwide although these formations are primarily present in wave dominated environments their effect on wave driven hydrodynamics is not well understood a two dimensional depth averaged phase resolving nonlinear boussinesq model funwavec was used to model hydrodynamics on a simplified sag system the modeling results show that the sag formations together with shoaling waves induce a nearshore lagrangian circulation pattern of counter rotating circulation cells the mechanism driving the modeled flow is an alongshore imbalance between the pressure gradient pg and nonlinear wave nlw terms in the momentum balance variations in model parameters suggest the strongest factors affecting circulation include spur normal waves increased wave height weak alongshore currents increased spur height and decreased bottom drag the modeled circulation is consistent with a simple scaling analysis based on the dynamical balance of nlw pg and bottom stress terms model results indicate that the sag formations efficiently drive circulation cells when the alongshore sag wavelength allows for the effects of diffraction to create alongshore differences in wave height without changing the mean wave angle

a framework for understanding drag parameterizations for coral reefs 1 in a hydrodynamic sense a coral reef is a complex array of obstacles that exerts a net drag force on water moving over the reef this drag is typically parameterized in ocean circulation models using drag coefficients cd or roughness length scales z0 however published cd for coral reefs span two orders of magnitude posing a challenge to predictive modeling here we examine the reasons for the large range in reported cd and assess the limitations of using cd and z0 to parameterize drag on reefs using a formal framework based on the 3 d spatially averaged momentum equations we show that cd and z0 are functions of canopy geometry and velocity profile shape using an idealized two layer model we illustrate that cd can vary by more than an order of magnitude for the same geometry and flow depending on the reference velocity selected and that differences in definition account for much of the range in reported cd values roughness length scales z0 are typically used in 3 d circulation models to adjust cd for reference height but this relies on spatially averaged near bottom velocity profiles being logarithmic measurements from a shallow backreef indicate that z0 determined from fits to point measurements of velocity profiles can be very different from z0 required to parameterize spatially averaged drag more sophisticated parameterizations for drag and shear stresses are required to simulate 3 d velocity fields over shallow reefs in the meantime we urge caution when using published cd and z0 values for coral reefs

ocean tides in the weddell sea new observations on the filchner ronne and larsen c ice shelves and model validation 1 ocean tides under the large weddell sea ice shelves are among the least well observed on earth here we present new spatially extensive observations of the vertical tidal motion of the filchner ronne and larsen c ice shelves using global positioning system gps data spanning a few weeks to years we pay particular attention to the major tidal constituents m2 s2 o1 k1 as well as important grace aliasing periods k2 and s1 we compare the estimated constituents with recent global and regional tide models and find that no single model is the most accurate across all constituents or ice shelves the root sum square errors are 7 8 cm cats2008a and tpxo7 2 and 11 12 cm got4 7 and fes2004 with the energetic m2 rmse 4 8 cm and s2 4 5 cm generally dominating these statistics the fes2004 k1 is particularly inaccurate near the larsen c ice shelf with errors approaching 20 cm meaning that grace release 4 estimates of mass change in the northern antarctic peninsula will be biased we find tidal energy at 3 4 5 6 and weakly at 7 cycles per day at all of our sites the largest amplitudes within these bands are at m4 mo3 and sp3 and approach 30 mm although significant spatial variations exist we show that they generally do not appear to originate in areas of reduced water column in ice shelf grounding zones comparing model estimates with our m4 ms4 and mn4 values shows that models do not accurately represent these terms

coral reef drag coefficients surface gravity wave enhancement abstracta primary challenge in modeling flow over shallow coral reefs is accurately characterizing the bottom drag previous studies over continental shelves and sandy beaches suggest surface gravi

the dynamics of infragravity wave transformation over a fringing reef a 3 week field study was conducted to investigate the dynamics of low frequency infragravity wave motions over a fringing reef at ningaloo reef western australia short period wave motions 0 04 0 2 hz were observed to dissipate on the reef crest beyond which infragravity wave motions 0 004 0 04 hz gradually dominated toward the lagoon however both the short waves and the infragravity waves were relatively small both 0 3 m on the reef flat owing to the shallow water depth 2 m the results revealed that the surf zone generation of free infragravity wave motions on the steep 1 20 fore reef slope was dominated by breakpoint forcing as opposed to shoaling bound waves which was also supported by detailed numerical simulations of the generation process this is consistent with theory suggesting the efficiency of the breakpoint forcing mechanism should be high in this steep slope regime shoreward propagating infragravity waves traveled across the reef but were damped by bottom friction dissipation however this was at a rate much smaller than experienced by the residual short waves with these rates of frictional dissipation also strongly dependent on the water depth over the reef the infragravity wave heights increased at higher water levels and hence were strongly modulated by the tide due to the strong dissipation of infragravity waves over this wide and shallow reef that is hydraulically rough any seaward propagating infragravity waves that reflected at the shoreline were small leading to the dominance of progressive shoreward propagating infragravity wave motions throughout the reef and lagoon

wave runup over steep rocky cliffs wave runup is known to depend on offshore wave conditions and coastal morphology while most field studies on wave runup have focused on low to mild sloping sandy beaches runup measurements on steep and irregular rocky cliff profiles are still very scarce here we investigate the physical processes controlling wave runup in such environments and the range of applicability of empirical runup formula this study focuses on the steep rocky cliffs 0 1 tani i 0 4 of banneg island a small island located in the molene archipelago brittany france occasionally flooded during extreme water level events a statistical parameter for extreme runup is derived from the measurements of pressure sensors deployed in the intertidal zone deep water wave parameters are used to force a high resolution wave model and nearshore wave parameters and high resolution topographic data are analyzed concurrently with runup time series in order to assess the dependence of the runup on hydrodynamic conditions and morphological parameters the wave runup is shown to be strongly related to the square root of the offshore significant wave height times the offshore wavelength the measurements also reveal the depth dependence of the runup which is mainly attributed to the curvature of the foreshore profile in comparison to empirical relation obtained for a mild sloping beach the present data show a significant reduction in normalized wave runup that is attributed to enhanced bottom friction over the rocky bottom

wave induced setup and wave driven current over quasi 2dh reef lagoon channel systems abstract the primary action of near shore waves at many field reef sites is to produce a wave induced setup and 2dh reef lagoon channel circulation cells instead of using wave basin to study the behaviors of the wave induced setup and wave driven flow in such systems an experimental technique was introduced to achieve the same objective through wave flume tests a pipe with a valve was installed underneath the wave flume to connect the lagoon and the ocean side which makes it possible to simulate a rip channel connecting the lagoon and the offshore side experimental results for two reef systems with a fore reef slope 1v 6h were obtained under a series of monochromatic wave conditions one simulating a closed lagoon and the other simulating an open lagoon compared with the reef system with a closed lagoon connecting the lagoon to the ocean side was found to have the following effects i producing a wave driven current on the reef flat ii reducing the maximum wave setup on the reef flat by 10 40 and iii causing the wave setup to decrease continuously along the reef flat all the way to the lagoon for the reef system with an open lagoon the wave driven current was found to increase with increasing wave height and wave period and there appeared to be a peak value of the current within the tested range of reef flat submergences the experimental results for the two reef systems were compared with a set of existing wave basin results for similar reef systems but with a much steeper fore reef slope 1v 1h the dimensionless parameters used in the literature were found to be insufficient to describe both the wave setup and wave driven current on the reef flat for the reef system with an open lagoon a semi analytical quasi 2dh model which was devised according to the balances of momentum and mass was also included to understand how the reef and channel morphology and the bottom roughness affect the wave setup and wave driven current in a reef lagoon channel system the semi analytical model was validated by our laboratory data as well as field observations reported in the literature

coral reef drag coefficients water depth dependence abstracta major challenge in modeling the circulation over coral reefs is uncertainty in the drag coefficient because existing estimates span two orders of magnitude current and pressure measurements from five coral reefs are used to estimate drag coefficients based on depth average flow assuming a balance between the cross reef pressure gradient and the bottom stress at two sites wind stress is a significant term in the cross reef momentum balance and is included in estimating the drag coefficient for the five coral reef sites and a previous laboratory study estimated drag coefficients increase as the water depth decreases consistent with open channel flow theory for example for a typical coral reef hydrodynamic roughness of 5 cm observational estimates and the theory indicate that the drag coefficient decreases from 0 4 in 20 cm of water to 0 005 in 10 m of water synthesis of results from the new field observations with estimates from previous field and laboratory studies indicate that coral r

coral reefs reduce tsunami impact in model simulations 1 significant buffering of the impact of tsunamis by coral reefs is suggested by limited observations and some anecdotal reports particularly following the 2004 indian ocean tsunami here we simulate tsunami run up on idealized topographies in one and two dimensions using a nonlinear shallow water model and show that a sufficiently wide barrier reef within a meter or two of the surface reduces run up on land on the order of 50 we studied topographies representative of volcanic islands islands with no continental shelf but our conclusions may pertain to other topographies effectiveness depends on the amplitude and wavelength of the incident tsunami as well as the geometry and health of the reef and the offshore distance of the reef reducing the threat to reefs from anthropogenic nutrients sedimentation fishing practices channel building and global warming would help to protect some islands against tsunamis

sediment transport in the presence of large reef bottom roughness the presence of large bottom roughness such as that formed by benthic organisms on coral reef flats has important implications for the size concentration and transport of suspended sediment in coastal environments a 3 week field study was conducted in approximately 1 5 m water depth on the reef flat at ningaloo reef western australia to quantify the cross reef hydrodynamics and suspended sediment dynamics over the large bottom roughness 20 40 cm at the site a logarithmic mean current profile consistently developed above the height of the roughness however the flow was substantially reduced below the height of the roughness canopy region shear velocities inferred from the logarithmic profile and reynolds stresses measured at the top of the roughness which are traditionally used in predictive sediment transport formulations were similar but much larger than that required to suspend the relatively coarse sediment present at the bed importantly these stresses did not represent the stresses imparted on the sediment measured in suspension and are therefore not relevant to the description of suspended sediment transport in systems with large bottom roughness estimates of the bed shear stresses that accounted for the reduced near bed flow in the presence of large roughness vastly improved the relationship between the predicted and observed grain sizes that were in suspension thus the impact of roughness not only on the overlying flow but also on bed stresses must be accounted for to accurately estimate suspended sediment transport in regions with large bottom roughness a common feature of many shallow coastal ecosystems

boundary layer turbulence and flow structure over a fringing coral reef measurements of velocity and rates of turbulence were made across a fringing coral reef in the gulf of aqaba red sea to determine the effect that the rough topography has on boundary layer mixing and flow dynamics observations were made at two fore reef sites and a nearby sandy slope the friction velocity u and drag coefficient cd were determined directly from turbulent reynolds stresses measured using acoustic doppler velocimeters values of cd for the coral substrates ranged from 0 009 to 0 015 three to five times greater than over the sandy bottom site the turbulence dissipation rate e was determined by fitting spectra of vertical velocity to the theoretical 5 3 law expected for the inertial subrange of turbulence there was a local balance between production and dissipation of turbulent kinetic energy signifying that we could estimate u from either the mean velocity profile turbulence or dissipation rate of turbulent kinetic energy estimates from all three measures agreed well with mean u uo ranging from 0 10 6 0 03 to 0 12 6 0 03 indicating that existing turbulent boundary layer flow theory can be applied to flows over the rough topography of coral reefs the bottom topography by enhancing both reef scale and local drag and mixing levels allows reef biota to more effectively exchange dissolved and particulate matter with oceanic waters

holocene climate conditions in central yakutia eastern siberia inferred from sediment composition and fossil chironomids of lake temje abstract a 380 cm long sediment core from lake temje central yakutia eastern siberia was studied to infer holocene palaeoenvironmental change in the extreme periglacial setting of eastern siberia during the last 10 000 years data on sediment composition were used to characterize changes in the depositional environment during the ontogenetic development of the lake temje the analysis of fossil chironomid remains and statistical treatment of chironomid data by the application of a newly developed regional russian transfer functions provided inferences of mean july air temperatures tjuly and water depths wd reconstructed wds show minor changes throughout the core and range between 80 and 120 cm all the fluctuations in reconstructed water depth lie within the mean error of prediction of the inference model rmsep 0 35 so it is not possible to draw conclusions from the reconstructions a qualitative and quantitative reconstruction of holocene climate in central yakutia recognized three stages of palaeoenvironmental changes the early holocene between 10 and 8 ka bp was characterized by colder than today and moist summer conditions cryotextures in the lake sediments document full freezing of the lake water during the winter time a general warming trend started around 8 0 ka bp in concert with enhanced biological productivity reconstructed mean tjuly were equal or up to 1 5 c higher than today between 6 0 ka and 5 0 ka bp during the entire late holocene after 4 8 ka bp reconstructed mean tjuly remained below modern value limnological conditions did not change significantly the inference of a mid holocene climate optimum supports scenarios of holocene climatic changes in the subpolar part of eastern siberia and indicates climate teleconnections to the north atlantic realm

plume dispersion on a fringing coral reef system a field program was implemented on a fringing reef located on oahu hawaii to investigate the dispersion characteristics of near bed dye plumes tracked using an autonomous underwater vehicle auv with an onboard fluorometer the combination of moored and auv based measurements collected synchronously in this study allowed us not only to observe the dispersion characteristics of individual plumes but importantly to isolate the flow structures directly responsible for dispersion under a variety of forcing conditions the evolution of several plumes was observed each influenced by different wave surface and internal current and stratification conditions the observed dispersion was caused by both turbulent dispersion and plume meandering lateral turbulent dispersion coefficients were roughly an order of magnitude greater than previous observations on sandy beach sites which is likely attributable to the enhanced surface wave induced mixing and the large physical roughness of the reef for a large portion of the 2 week experiment the effects of meandering dominated over turbulent dispersion on average 1 5 times greater the meandering was the result of large scale cross reef current variability moored thermistor chain data on the forereef suggest that the source of this cross reef flow variability were packets of solitary waves that accompanied the propagation of baroclinic bores up the reef slope baroclinic bores dissipated before they reached the inner reef when surface wave conditions were large resulting in the reduction of plume meandering during this period

tidal and subtidal flow patterns on a tropical continental shelf semi insulated by coral reefs 1 the present study sets out to describe the tidal and subtidal water motion at the berau coastal shelf which represents a tropical continental shelf area of variable width hosting a complex of barrier reefs along its oceanic edge moored and shipboard measurements on currents and turbulence were made as part of the multidisciplinary east kalimantan research program these results and collected data on sea levels meteorology and bathymetry were used to setup and calibrate a three dimensional barotropic hydrodynamic model in the ecomsed environment which is derived from the princeton ocean model concurrent profile measurements of flow velocity and reynolds stress with an acoustic doppler current profiler were used to infer values of the drag coefficient on locations on the shelf and within the barrier reef environment the drag coefficients determined from shelf measurements differed less than 20 from optimal model settings diurnal and semidiurnal tides have the same character propagating across the isobaths with amplitudes that increase toward the coast the tidal amplification is captured in a one dimensional model revealing an analogy with the amazon shelf regarding the subtidal depth mean flow tidal eulerian residual currents dominate over monsoon driven currents lagrangian mean flows reveal freshwater pathways from the coast to the coral reef environment at a regional scale the reef complex is established to be semitransparent to tides the barrier reef influences the tidal phases rather than the amplitudes especially in the shelf region that is fringed by the reefs this may explain why tidal phases predicted from hydrodynamic models of tropical continental shelves often feature substantial discrepancies from observations

wave setup over a fringing reef with large bottom roughness abstractthe effect of bottom roughness on setup dynamics was investigated using high resolution observations across a laboratory fringing reef profile with roughness elements scaled to mimic the frictional wave dissipation of a coral reef results with roughness were compared with smooth bottom runs across 16 offshore wave and still water level conditions the time averaged and depth integrated force balance was evaluated from observations collected at 17 locations along the flume and consisted of cross shore pressure and radiation stress gradients whose sum was balanced by quadratic mean bottom stresses the introduction of roughness had two primary effects first for runs with roughness frictional wave dissipation occurred on the reef slope offshore of the breakpoint reducing wave heights prior to wave breaking second offshore directed mean bottom stresses were generated by the interaction of the combined wave current velocity field with the roughness elements these two mechanisms acted counter to

the limited growth of vegetated shear layers 1 in contrast to free shear layers which grow continuously downstream shear layers generated by submerged vegetation grow only to a finite thickness because these shear layers are characterized by coherent vortex structures and rapid vertical mixing their thickness controls exchange between the vegetation and the overlying water experiments conducted in a laboratory flume show that the growth of these obstructed shear layers is arrested once the production of shear layer scale turbulent kinetic energy ske is balanced by dissipation of ske within the canopy this equilibrium condition along with a mixing length closure scheme was used in a one dimensional numerical model to predict the mean velocity profiles of the experimental shear layers the agreement between model and experiment is very good but field application of the model is limited by a lack of description of the drag coefficient in a submerged canopy

stokes drift in coral reefs with depth varying permeability in his famous paper of 1847 stokes gg 1847 on the theory of oscillatory waves trans camb phil soc 8 441455 stokes introduced the drift effect of particles in a fluid that is undergoing wa

the characteristics and dynamics of wave driven flow across a platform coral reef in the red sea current dynamics across a platform reef in the red sea near jeddah saudi arabia are examined using 18 months of current profile pressure surface wave and wind observations the platform reef is 700 m long 200 m across with spatial and temporal variations in water depth over the reef ranging from 0 6 to 1 6 m surface waves breaking at the seaward edge of the reef cause a 2 10 cm setup of sea level that drives cross reef currents of 5 20 cm s 1 bottom stress is a significant component of the wave setup balance in the surf zone over the reef flat where waves are not breaking the cross reef pressure gradient associated with wave setup is balanced by bottom stress the quadratic drag coefficient for the depth average flow decreases with increasing water depth from cda 0 17 in 0 4 m of water to cda 0 03 in 1 2 m of water the observed dependence of the drag coefficient on water depth is consistent with open channel flow theory and a hydrodynamic roughness of zo 0 06 m a simple one dimensional model driven by incident surface waves and wind stress accurately reproduces the observed depth averaged cross reef currents and a portion of the weaker along reef currents over the focus reef and two other red sea platform reefs the model indicates the cross reef current is wave forced and the along reef current is partially wind forced

wave driven circulation of a coastal reef lagoon system abstract the response of the circulation of a coral reef system in kaneohe bay hawaii to incident wave forcing was investigated using field data collected during a 10 month experiment results from the study revealed that wave forcing was the dominant mechanism driving the circulation over much of kaneohe bay as predicted theoretically wave setup generated near the reef crest resulting from wave breaking established a pressure gradient that drove flow over the reef and out of the two reef channels maximum reef setup was found to be roughly proportional to the offshore wave energy flux above a threshold root mean square wave height of 0 7 m at which height setup was negligible on the reef flat the wave driven currents increased approximately linearly with incident wave height however the magnitude of these currents was relatively weak typically 20 cm s 1 because of i the mild fore reef slope of kaneohe bay that reduced setup resulting from a combination of frictional wave damping and its re

wave driven flow over shallow reefs long term 1 month observations of waves and currents over a natural reef are presented which show a strong correlation between offshore rms incident wave height and cross reef currents at subtidal frequencies the energy spectrum of the cross reef currents shows a significant peak at twice the semidiurnal tidal frequency while the spectrum of sea surface elevation over the reef flat shows no corresponding peak furthermore experimental results reported by gourlay 1993 show setup over the reef occurs in the absence of a beach and the cross reef transport decreases with an increase in the sea surface slope across the reef flat due to an increase in setup at the top of the reef face analytic solutions for flow forced by wave breaking over an idealized reef explain the above features of cross reef flows in the absence of a beach through the surf zone on the reef face the cross reef gradient in the radiation stress due to wave breaking is partitioned between balancing an offshore pressure gradient associated with setup over the reef and forcing a mean flow across the reef over the reef flat where the depth is constant there is no forcing due to wave breaking and the flow is driven by a pressure gradient which results from the setup through the surf zone the magnitude of the setup through the surf zone is such that the transport across the reef flat matches the transport through the surf zone which is forced by the gradient in the radiation stress solutions are presented for general reef geometry defined by the reef width and slope of the seaward reef face and incident wave forcing defined by the depth at the breakpoint and the depth of water over the reef as the depth over the reef goes to zero the solutions converge to the plane beach solutions described by longuet higgins and stewart 1964 wave setup is maximized and the cross reef transport is zero in other cases the relative magnitudes of the setup and the cross reef transport depend on the geometry of the reef and the incident wave forcing

wave breaking hydrodynamics within coral reef systems and the effect of changing relative sea level this paper considers the hydrodynamics of wave driven flow across a coral reef and the resultant flushing of its lagoon current depth coefficients are introduced that describe the fractional change of across reef and lagoonal current with change in the relative local sea level i e the depth of water over the reef flat the coefficients are derived from an analytical hydrodynamic model and compared with data from two reefs the first is ningaloo reef in northwest australia which is a typical barrier reef with a narrow coastal lagoon and the second is kaneohe bay in oahu hawaii where the reef is both unusually deep and wide the current depth coefficients are shown to be sensitive to the form of the frictional law on the reef flat and can be deduced from measurements of the astronomical tidal currents in the lagoon both quadratic and linear friction laws are considered for the reef flat and good agreement obtained with a linear law based on the high frequency motion due to surface gravity waves

vertical variations of coral reef drag forces modeling flow in a coral reef requires a closure model that links the local drag force to the local mean velocity however the spatial flow variations make it difficult to predict the distribution of the local drag here we report on vertical profiles of measured drag and velocity in a laboratory reef that was made of 81 pocillopora meandrina colony skeletons densely arranged along a tilted flume two corals were ct scanned sliced horizontally and printed using a 3 d printer drag was measured as a function of height above the bottom by connecting the slices to drag sensors profiles of velocity were measured in between the coral branches and above the reef measured drag of whole colonies shows an excellent agreement with previous field and laboratory studies however these studies never showed how drag varies vertically the vertical distribution of drag is reported as a function of flow rate and water level when the water level is the same as the reef height reynolds stresses are negligible and the drag force per unit fluid mass is nearly constant however when the water depth is larger reynolds stress gradients become significant and drag increases with height an excellent agreement was found between the drag calculated by a momentum budget and the measured drag of the individual printed slices finally we propose a modified formulation of the drag coefficient that includes the normal dispersive stress term and results in reduced variations of the drag coefficient at the cost of introducing an additional coefficient

measurements of form and frictional drags over a rough topographic bank pressuredifferencesacrosstopographygenerateaform dragthatopposestheflow in the watercolumn and viscous and pressureforcesacting on roughnesselements of the topographic surface generatea frictionaldrag on the bottom form drag and bottom roughness lengths were estimated over the east flower garden bank efgb in the gulf of mexico by combining an array of bottom pressure measurements and profiles of velocity and turbulent kinetic dissipation rates the efgb is a coral bank about 6km wide and 10km long located at the shelf edge that rises from 100 m water depth to about 18m below the sea surface the average frictional drag coefficient over the entire bank was estimated as 0 006 using roughness lengths that ranged from 0 001cm for relatively smooth portions of the bank to 1 10cm for very rough portions over the corals the measured form drag over the bank showed multiple time scale variability diurnal tides and lowfrequency motions with periods ranging from 4 to 17 days generated form drags of about 2000nm 21 with average drag coefficients ranging between 0 03 and 0 22 which are a factor of 5 35 times larger than the average frictional drag coefficient both linear wave and quadratic drag laws have similarities with the observed form drag the form dragis an important flow retardation mechanismevenin the presence of the large frictional drag associated with coral reefs and requires parameterization

episodic circulation and exchange in a wave driven coral reef and lagoon system we examined the role of wave driven circulation relative to wind and buoyancy forcing in a coral reef lagoon system circulation measurements in paopao bay moorea french polynesia during austral summer show the importance of waves in driving flows over the reef crest through the lagoon and out the reef pass tides were comparatively weak due to proximity to amphidromic points and exhibited an unusual spring neap cycle where the major lunar tide modulated the major solar tide and the overall tidal phase stayed approximately constant wind had only a secondary effect compared to surface waves a simple fluid mass balance indicated rapid flushing of the shallow back reef and export through the reef pass and a reef capture zone width of 2 3 km the reef pass circulation dynamics exhibited two layer baroclinic exchange flow when waves were small which was suppressed during large wave events the unusually weak tidal forcing provided an opportunity to more closely investigate wave driven circulation dynamics as expected theoretically there was a wave driven setup of the free surface across the shallow lagoon which drove a highly frictional flow evident by a large drag coefficient cp 0 1 diverging from extant theory the observed setup varied strongly with significant wave height and period overall the circulation and exchange between this coral reef system and the adjacent open ocean were largely determined by episodic remote forcing events and differed significantly from periodic tidal exchange mechanisms

sinkhole formation above underground limestone quarries a case study in south limburg belgium abstract historical records were used for the compilation of a database of sinkholes resulted from collapses of abandoned shallow underground limestone quarries mines in two villages in belgian south limburg during the last 350 years the formation of such sinkholes caused at least 38 casualties but more often it caused a change in topography and damage to public and private property the objective of this study is to better understand the spatial and temporal patterns of quarry collapse related sinkholes in the study area apart from sinkhole locations and ages the compiled database provides information on the dimensions of the area affected and the damage caused by sinkhole formation as well as the causal factors of sinkholes one hundred seventy three sinkholes have been reported since 1665 but most 80 reported sinkholes postdate 1965 sinkhole dimensions provided information on the type of collapse seven large sinkholes displacing a total sediment volume of 480 000 m3 resulted from large scale roof breakdown after pillar failure the smaller sinkholes were the result of fall of the overburden into galleries after local roof collapse or suffosion of a solution pipe in total these small sinkholes displaced 12 300 m3 of sediment at present almost all large underground quarries in the study area have been affected by sinkholes these features were caused by natural and anthropogenic factors and occur in zones with thin roofs where pillars were affected by pillar robbing or on locations with inappropriate sewerage systems above quarries the formation of sinkholes was often reported in spring during years with high moisture contents in the overburden caused by high groundwater recharge or above average precipitation with increasing time since quarry abandonment quarries become more susceptible to pillar creep and bending of the roof hence if no appropriate mitigation measures are taken to reduce such deterioration processes the number of sinkholes is expected to increase

an alongshore momentum budget over a fringing tropical fore reef author s arzeno isabella b collignon audric merrifield mark giddings sarah n pawlak geno

field observations of wave driven circulation over spur and groove formations on a coral reef spur and groove sag formations are found on the forereefs of many coral reefs worldwide modeling results have shown that sag formations together with shoaling waves induce a nearshore lagrangian circulation pattern of counter rotating circulation cells but these have never been observed in the field we present results from two separate field studies of sag formations on palmyra atoll which show their effect on waves to be small but reveal a persistent order 1 cm s depth averaged lagrangian offshore flow over the spur and onshore flow over the grooves this circulation was stronger for larger directly incident waves and low alongshore flow conditions consistent with predictions from modeling favorable forcing conditions must be maintained on the order of 1 h to accelerate and develop the sag circulation cells the primary cross and alongshore depth averaged momentum balances were between the pressure gradient radiation stress gradient and nonlinear convective terms and the bottom drag was similar to values found on other reefs the vertical structure of these circulation cells was previously unknown and the results show a complex horizontal offshore lagrangian flow over the spurs near the surface driven by alongshore variability in radiation stress gradients vertical flow was downward over the spur and upward over the groove likely driven by alongshore differences in bottom stress and not by vortex forcing

observations of spatial flow patterns at the coral colony scale on a shallow reef flat 1 although small scale spatial flow variability can affect both larger scale circulation patterns and biological processes on coral reefs there are few direct measurements of spatial flow patterns across horizontal scales 1 had similar spatial patterns to wakes while higher frequency variations 0 05 0 1 hz kc 1 had no observable spatial structure on the reef flat both drag and inertial forces exerted by coral colonies could have significant effects on flow but within different frequency ranges drag dominates for low frequency flow variations and inertial forces dominate for higher frequency variations including the wave band our scaling analyses suggest that spatial flow patterns at colony and patch scales could have important implications for both physical and biological processes at larger reef scales through their effects on forces exerted on the flow turbulent mixing and dispersion

momentum balance across a barrier reef this paper reports on a combined experimental and numerical study dedicated to barrier reefs hydrodynamics a network of pressure sensors and velocity profilers has been deployed for more than 2 months over the ouano reef barrier new caledonia the primary aim of the study is to assess the relevance of the classical depth averaged momentum balance in such a complex and poorly documented environment the combined analysis of experimental and numerical measurements reveals a specific hydrodynamic behavior contrasting with sandy beaches and fringing reefs the cross reef current induced by wave breaking over the barrier reef plays an important role in the momentum budget in particular through friction processes the hydrodynamic behavior over the barrier reef is thus characterized by the progressive transition from a nearly classical beach type behavior on the forereef where the gradient of radiation stress is balanced by a barotropic pressure gradient associated to the wave setup to an open channel type regime dominated by frictional head loss the reef top wave setup shows a clear depth dependency mainly attributed to the forereef curvature during extreme wave events the measurements tend to indicate a transition toward a critical hydraulic regime above the reef top the numerical simulations involving a non hydrostatic wave resolving model coupled to a urn x wiley jgrc media jgrc23848 jgrc23848 math 0001 turbulence model highlight the vertical structure of the flow over the reef flat a classical log layer profile is observed in agreement with measurements while above the forereef an anticlockwise circulation develops under the breaking zone

variability of depth limited waves in coral reef surf zones wave breaking and transformation on coral reef flats is an important process protecting tropical coastlines and regulating the energy regimes of coral reefs however the high hydraulic roughness shallow water and steep bathymetries of coral reefs may confound common surf zone assumptions such as a depth limited and saturated surf zone with a constant wave height to water depth ratio here we examine wave transformation across a coral reef flat during three separate swell events on both a time averaged and a wave by wave basis we use the relationship between significant wave height and water depth to examine the change in surf saturation across the reef flat and compare the measured wave height decay to results of modelled wave energy dissipation in the surf zone our results show that was not cross reef constant and varied according to location on the reef flat and local water depth on average was greatest at the outer reef flat near the reef crest and progressively reduced towards the inner reef flat near the reef lagoon this was most pronounced in shallow water with large values 0 85 at the outer reef flat and small values 0 1 at the inner reef flat this indicates that there is an increase in wave energy dissipation in shallow water most likely due to increased breaker and bed frictional dissipation the measured wave energy dissipation across the entire reef flat could on average be modelled accurately however this required location specific calibration of the free parameters the wave friction factor f and and further suggests that there is no value for either parameter that is universally applicable to coral reef flats despite model calibration inaccuracies were still observed primarily at the outer reef flat these inaccuracies reflected the observed cross reef variation of on the reef flat and potentially the limitations of random wave breaker dissipation models in complex surf zones our results have implications for the use of wave energy dissipation models in predicting breaker dissipation and subsequent benthic community change on coral reef flats and suggest that careful consideration of the free parameters in such models such as f and is required

connecting flow over complex terrain to hydrodynamic roughness on a coral reef abstractflow over complex terrain causes stress on the bottom leading to drag turbulence and formation of a boundary layer but despite the importance of the hydrodynamic roughness scale z0 in pr