Bottom Stress Research Articles

Long-term evolution in the location, propagation, and magnitude of the tidal shear front off the Yellow River Mouth

Characteristics and formation of the tidal shear front off the Yellow River Mouth had been previously studied based on observations and models. However, its long-term variability and thus related mechanisms have not been investigated. In this study, a three-dimensional hydrodynamic model was used to examine the long-term evolution on the location, propagation, and magnitude (length, duration, and shear strength) of the tidal shear front from 1976 to 1996. Over the 20 years, a peninsula of about 27km long and 7km wide (above the high water line) was formed off the new river mouth, while the coastline retreated by 5–10km, and the water depths increased by about 2m in the north (the abandoned river mouth). Results also show that for the period of each flood and ebb tidal cycle, the tidal shear front originated in the north of the delta, then propagated southeastwards and finally disappeared at the south of the river mouth. During the two decades, the front had an obvious shoreward movement, and its duration within a tidal cycle increased slightly. Furthermore, the length of the front decreased, while the shear strength of the front increased. The propagation of the front was determined by flood and ebb process in the delta, where both of them were from north to south due to tidal phase distribution. Long-term characteristic variations of the front were mainly caused by morphology change, which had a significant influence on the tidal current gradient and the location of maximum bottom stress gradient area. The morphology change also had a slight impact on the tidal current phase in the front area, which may have caused the slight duration variation of the front.

Numerical simulation of suspended sediment concentration by 3D coupled wave-current model in the Oujiang River Estuary, China

A 3D sediment transport model based on the modified environmental fluid dynamics code (EFDC) and the nearshore waves simulation model (SWAN) is developed to study the change of suspended sediment concentration and bottom shear stress under the actions of pure current and wave-current. After being validated by the field measured data, the proposed sediment transport model is applied in the Oujiang River Estuary, China. The results show that the ratios of both bottom shear stress and suspended sediment concentration of pure current to those of wave-current show a gradually increase from shallow nearshore water to deep open sea. The results also show that the proportion of wave contributions on bottom shear stress and sediment concentration are above 60%, approximately 20–30% and less than 10% for the water depth of less than 5m, 5–10m and more than 20m, respectively. For the waters among islands, the proportion of wave contribution to bottom shear stress and sediment concentration is reduced to 10–20% for −5m water depth and this is more obvious for the waves of large amplitude. The bottom stress and suspended sediment concentration between islands are mainly controlled by tidal current, and the effect of wave is not significant.

Tidal-fluvial interaction in the Guadalquivir River Estuary: Spatial and frequency-dependent response of currents and water levels

This paper presents a study on the tidal-fluvial interaction in the highly regulated Guadalquivir River Estuary (SW Spain), which is occasionally subjected to high discharge episodes that affect navigational conditions and increase flood risks. The study specifically focuses on the processes and controlling mechanisms of the nonstationary response of water levels and currents to high discharges. Measurements show a 60 day postdischarge amplification of tidal current and elevation amplitudes and a clockwise rotation of the tidal ellipse in the upper layers. A decrease of amplitudes and an anticlockwise rotation predominate near the bed. Such episodes significantly increase the tidal wave celerity, and especially at high and low water. These features are due to the suspended sediment stratification triggered by the discharge event. The increase in stratification restricts frictional influence to bottom layers, partially decoupling the overlying flow from the bottom. A nonstationary harmonic decomposition method, intended for identifying which nonlinear terms in the governing hydrodynamic equations control overtide and compound tide generation, shows that quadratic bottom stress contributes the most during high discharge periods. The consequence in the subtidal balance is that, during peak discharge and in the upper stretches, friction is largely balanced by the water level gradient, although the density gradient term becomes comparable to the friction term soon after peak discharge. Advection is also important to the force balance in the lower estuary. For both parts, to correctly explain subtidal dynamics, it is necessary to account for the time variability of the friction coefficient due to flow-sediment feedback.

Large‐eddy simulation of wave‐breaking induced turbulent coherent structures and suspended sediment transport on a barred beach

AbstractTo understand the interaction between wave‐breaking induced turbulent coherent structures and suspended sediment transport, we report a Large‐Eddy Simulation (LES) study of wave‐breaking processes over a near‐prototype scale barred beach. The numerical model is implemented using the open‐source CFD toolbox, OpenFOAM®, in which the incompressible three‐dimensional filtered Navier‐Stokes equations for the water and air phases are solved with a finite volume scheme. A volume of fluid (VOF) method is used to capture the evolution of the water‐air interface. The numerical model is validated with measured free surface elevation, turbulence‐averaged flow velocity, turbulent intensity, and for the first time, the intermittency of breaking wave turbulence. Simulation results confirm that as the obliquely descending eddies (ODEs) approach the bottom, significant bottom shear stress is generated. Remarkably, the collapse of ODEs onto the bed can also cause drastic spatial and temporal changes of dynamic pressure on the bottom. By allowing sediment to be suspended from the bar crest, intermittently high sediment suspension events and their correlation with high turbulence and/or high bottom shear stress events are investigated. The simulated intermittency of sediment suspension is similar to previous field and large wave flume observations. Coherent suspension events account for only 10% of the record but account for about 50% of the sediment load. Model results suggest that about 60%∼70% of coherent bottom stress events are associated with surface‐generated turbulence. Nearly all the coherent sand suspension events are associated with coherent turbulence events due to wave‐breaking turbulence approaching the bed.

Lattice model for pressure problems in two-dimensional granular columns

In order to make it easier to investigate some problems such as the mechanism of Janssen effect and the stress distribution in granular medium, we simplify a granular column into a lattice system, in which a lattice point represents a small lump of granular medium and only neighbor interactions are considered. To study the disordered granular columns, a force propagation lattice model determined by the absorption coefficient p and the lateral transfer coefficient q is proposed, and this model is analyzed from the theoretical view. Firstly, the equation of force propagation in the matrix form is given, and this equation is determined by a tridiagonal matrix A(p,q) that is called transfer coefficient matrix. Based on the force transfer equation, the bottom force distribution varying with the top force distribution and the layer of lattice system is deduced, and its analytical solution refers to the similarity diagonalization of matrix A(p, q). Then, a method based on the second order difference equations is proposed to obtain the eigenvalues and eigenvectors of the transfer coefficient matrix. The eigenvalues and eigenvectors of A(p, q) can be rigorously deduced for a typical case, and with these results the pressure distribution relationship between the top and the bottom of the container is given. Based on these theoretical expressions, the relationship between the effective mass and the total mass of granular medium is deduced, and it means that the force propagation model and the Janssen model can lead to similar results. Moreover, the bottom stress distribution is calculated without the top load. Calculations show that the stress distribution reaches a maximum at the center bottom and drops down to either side. Finally, numerical calculations are performed to investigate the effects of parameters p and q on the relation between bottom pressure and packing height. Numerical results show that the saturated value of pressure decreases while parameter p or q increases.

A coupled wave-hydrodynamic model of an atoll with high friction: Mechanisms for flow, connectivity, and ecological implications

We present a hydrodynamic analysis of an atoll system from modeling simulations using a coupled wave and three-dimensional hydrodynamic model (COAWST) applied to Palmyra Atoll in the Central Pacific. This is the first time the vortex force formalism has been applied in a highly frictional reef environment. The model results agree well with field observations considering the model complexity in terms of bathymetry, bottom roughness, and forcing (waves, wind, metrological, tides, regional boundary conditions), and open boundary conditions. At the atoll scale, strong regional flows create flow separation and a well-defined wake, similar to 2D flow past a cylinder. Circulation within the atoll is typically forced by waves and tides, with strong waves from the north driving flow from north to south across the atoll, and from east to west through the lagoon system. Bottom stress is significant for depths less than about 60m, and in addition to the model bathymetry, is important for correct representation of flow in the model. Connectivity within the atoll system shows that the general trends follow the mean flow paths. However, some connectivity exists between all regions of the atoll system due to nonlinear processes such as eddies and tidal phasing. Moderate wave stress, short travel time (days since entering the reef system), and low temperature appear to be the most ideal conditions for high coral cover at this site.

Observations and 3D hydrodynamics-based modeling of decadal-scale shoreline change along the Outer Banks, North Carolina

Long-term decadal-scale shoreline change is an important parameter for quantifying the stability of coastal systems. The decadal-scale coastal change is controlled by processes that occur on short time scales (such as storms) and long-term processes (such as prevailing waves). The ability to predict decadal-scale shoreline change is not well established and the fundamental physical processes controlling this change are not well understood. Here we investigate the processes that create large-scale long-term shoreline change along the Outer Banks of North Carolina, an uninterrupted 60km stretch of coastline, using both observations and a numerical modeling approach. Shoreline positions for a 24-yr period were derived from aerial photographs of the Outer Banks. Analysis of the shoreline position data showed that, although variable, the shoreline eroded an average of 1.5m/yr throughout this period. The modeling approach uses a three-dimensional hydrodynamics-based numerical model coupled to a spectral wave model and simulates the full 24-yr time period on a spatial grid running on a short (second scale) time-step to compute the sediment transport patterns. The observations and the model results show similar magnitudes (O(105m3/yr)) and patterns of alongshore sediment fluxes. Both the observed and the modeled alongshore sediment transport rates have more rapid changes at the north of our section due to continuously curving coastline, and possible effects of alongshore variations in shelf bathymetry. The southern section with a relatively uniform orientation, on the other hand, has less rapid transport rate changes. Alongshore gradients of the modeled sediment fluxes are translated into shoreline change rates that have agreement in some locations but vary in others. Differences between observations and model results are potentially influenced by geologic framework processes not included in the model. Both the observations and the model results show higher rates of erosion (∼−1m/yr) averaged over the northern half of the section as compared to the southern half where the observed and modeled averaged net shoreline changes are smaller (<0.1m/yr). The model indicates accretion in some shallow embayments, whereas observations indicate erosion in these locations. Further analysis identifies that the magnitude of net alongshore sediment transport is strongly dominated by events associated with high wave energy. However, both big- and small- wave events cause shoreline change of the same order of magnitude because it is the gradients in transport, not the magnitude, that are controlling shoreline change. Results also indicate that alongshore momentum is not a simple balance between wave breaking and bottom stress, but also includes processes of horizontal vortex force, horizontal advection and pressure gradient that contribute to long-term alongshore sediment transport. As a comparison to a more simple approach, an empirical formulation for alongshore sediment transport is used. The empirical estimates capture the effect of the breaking term in the hydrodynamics-based model, however, other processes that are accounted for in the hydrodynamics-based model improve the agreement with the observed alongshore sediment transport.

Pebble and cobble transport on a steep, mega-tidal, mixed sand and gravel beach

Natural tracer pebbles and cobbles were deployed on a steep (slope: 0.1) mixed sand and gravel mega-tidal beach located at the head of the Bay of Fundy. Two experiments were carried out to investigate the response of natural pebble- and cobble-sized material, both to energetic wave conditions during individual storm events, and to tidal-only forcing conditions between storms. Net displacement of different tracer sizes was the focus of the first experiment, while the effect of different shapes was tested in the second. The pebbles and cobbles were painted in fluorescent colors, labeled, and their positions measured at low tide over periods of 10–12days. The tracers were emplaced at mid-tide level on the beach face at low water, and later recovered and re-deployed on successive low tides. Net travel distance and direction were measured upon recovery. Hydrodynamic forcing conditions and local morphology at mid-tide level on the beach face were monitored continuously during the experiments. In the first experiment, recovery rates were 100% during calm and moderate wave conditions (Hs≤ 0.5m), but decreased significantly in more energetic wave forcing (Hs up to 1m). Recovery rates were lower for smaller tracers. During a three day wind-wave event, net displacements of up to 48m were measured. Comparable net displacements were observed for all tracers. Maximum displacements were 39m onshore, 42m offshore, and 46m alongshore. In calm conditions, maximum displacements were less than 1m, indicating that the bottom stresses due to tidal currents alone are too low to cause significant tracer displacement on time scales of weeks, despite the high tidal range. In the second experiment, recovery rates were lower, due to more energetic wave forcing: significant wave heights reached Hs >0.5m on 80% of the particle tracking days and Hs >1m on 30% of the days. Flat pebbles and cobbles reached the longest transport distances, with a maximum net displacement of 98m. The flat tracers were observed to be flipped by the shore break and to “ride” the swash bore, and were in this manner transported long distances, shoreward up to 66m. In both experiments, the directions of alongshore displacements were consistent with deviations of the angle of wave incidence from the shorenormal. Tracer burial and re-exposure was observed – via rotary imaging sonar – with the evolution and migration of m-wavelength ripples. The study allowed a direct correlation of pebble and cobble tracer displacements to hydrodynamic forcing during individual storm events, with consideration of tracer particle size, and shape.

Wind relaxation and a coastal buoyant plume north of Pt. Conception, CA: Observations, simulations, and scalings

AbstractIn upwelling regions, wind relaxations lead to poleward propagating warm water plumes that are important to coastal ecosystems. The coastal ocean response to wind relaxation around Pt. Conception, CA is simulated with a Regional Ocean Model (ROMS) forced by realistic surface and lateral boundary conditions including tidal processes. The model reproduces well the statistics of observed subtidal water column temperature and velocity at both outer and inner‐shelf mooring locations throughout the study. A poleward‐propagating plume of Southern California Bight water that increases shelf water temperatures by 5°C is also reproduced. Modeled plume propagation speed, spatial scales, and flow structure are consistent with a theoretical scaling for coastal buoyant plumes with both surface‐trapped and slope‐controlled dynamics. Plume momentum balances are distinct between the offshore (&gt;30 m depth) region where the plume is surface‐trapped, and onshore of the 30 m isobath (within 5 km from shore) where the plume water mass extends to the bottom and is slope controlled. In the onshore region, bottom stress is important in the alongshore momentum equation and generates vertical vorticity that is an order of magnitude larger than the vorticity in the plume core. Numerical experiments without tidal forcing show that modeled surface temperatures are biased 0.5°C high, potentially affecting plume propagation distance and persistence.

Sediment provenance and paleoenvironmental change in the middle Okinawa Trough during the last 18.5 ky: Clay mineral and geochemical evidence

We investigated grain size, clay mineralogy, elemental geochemistry, organic composition, and AMS 14C dating ages in Core KX12-3 from the middle Okinawa Trough in order to better understand the sediment provenances and transport processes and also the forcing mechanisms behind their variations over the last 18.5 ky. The geochemical–mineralogical indices reveal three notable phases of change in the sediment provenance of the core, which are related to the development of the Kuroshio Current coupled with sea-level fluctuations, the position shift of the mouths of large rivers, the magnitude of tidal bottom stress, and the East Asian monsoon intensity. The sea-level lowstand deposits of Interval 1 (18.5–9.3 ka) formed from mixed sediments originating mostly from Chinese rivers (the paleo-Huanghe and the paleo-Changjiang) as well as from materials eroded from seafloor by the strong tidal stress. Thereafter during Interval 2 (9.3–8.7 ka), a significant sea-level rise, the moderate tidal bottom stress, and a gradual landward retreat of river mouths led to obviously decreased sediment supplies from both seafloor erosion and mainland China rivers (especially the paleo-Huanghe) into to the study area. After this transitional period, the dominant sediment provenance during the mid-to-late Holocene (Interval 3) changed to the Changjiang and seafloor erosion, resulting in lower detrital linear sedimentation rate and finer grain size. At the same time, some fine-grained materials from Taiwan may have been transported northward to the study area by the Kuroshio Current, as evidenced by higher values of chlorite/kaolinite ratio and chemical index of illite, and lower TOC/TN values. In addition, a prominent decline in chlorite/kaolinite values that occurred at 5.0–3.5 ka may have been linked to a suppression of the Kuroshio Current, probably related to the Pulleniatina minimum event and/or the late Holocene Neoglacial cold event commonly found in the northwestern Pacific.

Toward a universal mass‐momentum transfer relationship for predicting nutrient uptake and metabolite exchange in benthic reef communities

AbstractHere we synthesize data from previous field and laboratory studies describing how rates of nutrient uptake and metabolite exchange (mass transfer) are related to form drag and bottom stresses (momentum transfer). Reanalysis of this data shows that rates of mass transfer are highly correlated (r2 ≥ 0.9) with the root of the bottom stress under both waves and currents and only slightly higher under waves (~10%). The amount of mass transfer that can occur per unit bottom stress (or form drag) is influenced by morphological features ranging anywhere from millimeters to meters in scale; however, surface‐scale roughness (millimeters) appears to have little effect on actual nutrient uptake by living reef communities. Although field measurements of nutrient uptake by natural reef communities agree reasonably well with predictions based on existing mass‐momentum transfer relationships, more work is needed to better constrain these relationships for more rugose and morphologically complex communities.

The key technologies for eutrophication simulation and algal bloom prediction in Lake Taihu, China

An integrated system including eutrophication simulation and algal bloom predication was established and applied in Lake Taihu. The mechanic models, which are based on a three-dimensional numerical model coupling hydrodynamic module, pollutant transport module, suspended sediment resuspension module, wind wave module and algal bloom predication module, can simulate not only the single function such as current pattern, nutrient distribution, sediment concentration and wave height field, but also the interaction among them. Using these models, some complex phenomenon such as endogenous pollution can be simulated. In the study, some key technologies are introduced to make the simulation reasonable and feasible. First, a new method for predicting algal bloom is introduced. Differing from the traditional phytoplankton dynamic model, judging the algal bloom through biomass or chlorophyll concentration, the method quantitatively estimates algal growth state switching among bloom, collapse and steady based on algal stability criterion in water column. Second, the sediment resuspension is estimated using calibrated critical shear stresses derived from remote sensing data and the wave-enhanced bottom stress during the windy days. Third, a user-friendly operation environment based on plug-in development and geographic information system is developed. The tests demonstrated that the system could both simulate the reactive solutes such as BOD and DO and the suspended sediment correctly under interaction of gravity and turbulent diffusion. The simulation results demonstrated that the system could obtain reasonable and reliable simulated results in Lake Taihu.

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 one another. Frictional wave dissipation resulted in radiation stress gradients that were predicted to generate 18% (on average) less setup on the reef flat for rough runs than for smooth runs when neglecting mean bottom stresses. However, mean bottom stresses increased the predicted setup by 16% on average for runs with roughness. As a result, setup on the reef flat was comparable (7% mean difference) between corresponding rough and smooth runs. These findings are used to assess prior results from numerical modeling studies of reefs and also to discuss the broader implications for how large roughness influences setup dynamics in the nearshore zone.

Study on anti-freezing functional design of phase change and temperature control composite bridge decks

The bridge decks during the winter time usually present a poor performance of freeze resistance. Using the phase change energy-storing technology and functionally-graded materials design method, we designed and prepared a composite anti-freezing bridge deck structure. Compared with conventional bridge decks, the anti-freezing bridge deck has a phase change functional layer which consisted of high strength seamless steel pipes paved parallelly at a certain interval, and the liquid phase change materials are filled in the pipes. Through theoretical study and laboratory model tests, it has been proven that the large amount of heat given off by the new bridge deck structure during the liquid-solid phase change process has a good effect on melting ice and snow. On the other hand, in order to research the influence the heat has on the mechanical properties of the bridge deck, a simulation analysis has processed by the finite element software. The results show that: the seamless steel pipe buried in the transformation function layer has an effect on the reinforcement of the deck structure, which also greatly enhances the overall strength and stiffness. The bottom layer tensile stress and vertical displacement of the phase change functional layer are about 74% and 92% respectively of those without one. Meanwhile, the average values of tensile stress and the vertical displacement of the former are less than those of the latter. Finally, an optimal layout scheme was found through the comparative analysis of influence of pipe spacing and depth while the bridge deck was experiencing stress.

Alongshore momentum transfer to the nearshore zone from energetic ocean waves generated by passing hurricanes

AbstractWave and current measurements from a cross‐shore array of nearshore sensors in Duck, NC, are used to elucidate the balance of alongshore momentum under energetic wave conditions with wide surf zones, generated by passing hurricanes that are close to and far from to the coast. The observations indicate that a distant storm (Hurricane Bill, 2009) with large waves has low variability in directional wave characteristics resulting in alongshore currents that are driven mainly by the changes in wave energy. A storm close to the coast (Hurricane Earl, 2010), with strong local wind stress and combined sea and swell components in wave energy spectra, has high variability in wave direction and wave period that influence wave breaking and nearshore circulation as the storm passes. During both large wave events, the horizontal current shear is strong and radiation stress gradients, bottom stress, wind stress, horizontal mixing, and cross‐shore advection contribute to alongshore momentum at different spatial locations across the nearshore region. Horizontal mixing during Hurricane Earl, estimated from rotational velocities, was particularly strong suggesting that intense eddies were generated by the high horizontal shear from opposing wind‐driven and wave‐driven currents. The results provide insight into the cross‐shore distribution of the alongshore current and the connection between flows inside and outside the surf zone during major storms, indicating that the current shear and mixing at the interface between the surf zone and shallow inner shelf is strongly dependent on the distance from the storm center to the coast.

Hydrodynamic modelling and characterisation of a shallow fluvial lake: a study on the Superior Lake of Mantua

This paper presents a numerical modelling framework developed to simulate circulations and to generally characterise the hydrodynamics of the Superior Lake of Mantua, a shallow fluvial lake in Northern Italy. Such eutrophied basin is characterised by low winds, reduced discharges during the summer and by the presence of large lotus flower (&lt;em&gt;Nelumbo&lt;/em&gt; &lt;em&gt;nucifera&lt;/em&gt;) meadows, all contributing to water stagnation. A hydrodynamic numerical model was built to understand how physical drivers shape basic circulation dynamics, selecting appropriate methodologies for the lake. These include a 3D code to reproduce the interaction between wind and through-flowing current, a fetch-dependent wind stress model, a porous media approach for canopy flow resistance and the consideration of wave-current interaction. The model allowed to estimate the circulation modes and water residence time distributions under identified typical ordinary, storm and drought conditions, the hydrodynamic influence of the newly-opened secondary outlet of the lake, the surface wave parameters, their influence on circulations and the bottom stress they originate, and the adaptation time scales of circulations to storm events. Some probable effects of the obtained hydrodynamic characteristics of the Superior Lake of Mantua on its biochemical processes are also introduced.

Variations of bed elevations due to turbulence around submerged cylinder in sand beds

This paper presents the spatio-temporal variations in bed elevations and the near-bed turbulence statistics over the deformed bed generated around the submerged cylindrical piers embedded vertically on loose sediment bed at a constant flow discharge. Experiments were carried out in a laboratory flume for three blockage ratios in the range of 0.04–0.06 using three different sizes of submerged cylinders individually placed vertically at the centerline of the flume. Clear-water experimental conditions were maintained over the smooth sediment bed surface with a constant flow discharge (\(Q = 0.015\,{\rm m}^3/{\rm sec}\)), thereby giving three different cylinder Reynolds numbers \(Re_{D_c} = \frac{U_mD_c}{\nu }\) (=10200, 12750, 15300) away from the cylinder locations, where \(U_m\) is the maximum mean velocity, \(D_c\) is the cylinder diameter and \(\nu\) is the kinematic viscosity of fluid. Instantaneous sand bed elevations around the cylinders were recorded using a SeaTek 5MHz ultrasonic ranging system of net 24 transducers to estimate bed form migration, and the near-bed velocity data at transducer locations over the stable deformed bed around the pier-like structures were collected using down-looking three-dimensional (3D) Micro-acoustic Doppler velocimeter to estimate the bottom Reynolds shear stresses and the contributions of bursting events to the dominant shear stress component. The flow perturbation generated due to relatively lower flow blockage ratio favored to achieve the stable bed condition more rapidly than the others, and larger upstream scour-depth and deformed areas were noticed for greater flow blockage ratio due to larger cylinder diameter. For larger blockage ratio in the upstream of scour-hole near the bed, occurrences of probabilities of both boundary-ward interactions (Q1 and Q3) were the dominant; whereas in the downstream of the scoured region, occurrences of probabilities of second and third quadrant events (Q2 and Q4) were dominant. On the other hand, for the lower blockage ratio, quadrant (Q2) was dominant over Q4 in the downstream of scour-hole, and in the upstream of scour-hole, quadrant Q4 was the dominant.

Turbulent production in an internal wave bottom boundary layer maintained by a vertically propagating seiche

AbstractInternal seiches, which supply the energy responsible for mixing many lakes, are often modeled as vertically standing waves. However, recent observations of vertical seiche propagation in a small lake are inconsistent with the standard, vertically standing model. To examine the processes responsible for such propagation, drag and turbulent production in the bottom boundary layer of a small lake are related to the energy supplied by a propagating seiche (period 10–24 h). Despite complex and fluctuating stratification, which often inhibited mixing within 0.4 m of the bed, bottom stress was well represented by a simple drag coefficient model (drag coefficient 1.5 × 10−3). The net supply of seiche energy to the boundary layer was estimated by fitting a model for internal wave vertical propagation to velocity profiles measured above the boundary layer (1–4.5 m above lakebed). Fitted reflection coefficients ranged from 0.3 at 1 cycle/d frequency to 0.7 at 2.4 cycles/d (cf. near‐unity coefficients of classical seiche theories). The net supply of seiche energy approximately balanced boundary layer turbulent production. Three of four peaks in production and energy flux occurred 0.8–2.2 days after strong oscillating winds, a delay comparable to the time required for seiche energy to propagate to the lakebed. A model based on the estimated drag coefficient predicted the observed frequency dependence of the seiche reflection coefficient. For flat‐bed regions in narrow lakes, the model predicts that reflection is controlled by the ratio of water velocity to vertical wave propagation speed, with sufficiently large ratios leading to weak reflection, and clear vertical seiche propagation.

Effect of river discharge and geometry on tides and net water transport in an estuarine network, an idealized model applied to the Yangtze Estuary

Tidal propagation in, and division of net water transport over different channels in an estuarine network are analyzed using a newly developed idealized model. The water motion in this model is governed by the cross-sectionally averaged shallow water equations and is forced by tides at the seaward boundaries and by river discharge. Approximate analytical solutions are constructed by means of a harmonic truncation and a perturbation expansion in a small parameter, being the ratio of tidal amplitude and depth. The net water transport results from an imposed river discharge and from residual water transport generated by nonlinear tidal rectification. Two new drivers are identified that contribute to the net water transport in tidal estuarine networks, viz. the generation of residual water transport due to gradients in dynamic pressure and due to a coupling between the tidally averaged and quarter diurnal currents through the quadratic bottom stress. The model is applied in a case study on the Yangtze Estuary, to investigate tides and division of net water transport over its multiple channels during the wet and dry season, as well as before and after the construction of the Deepwater Navigation Channel. Model results agree fairly well with observations. Process analysis reveals that the decrease in tides from dry to wet season is due to enhanced bottom stress generated by river-tide interactions. Also, the seasonal variations in net water transport are explained. It is furthermore shown and explained that due to the Deepwater Navigation Channel tidal currents have increased and net water transport has decreased in the North Passage. These changes have profound implications for net sediment transport and salinity intrusion.

Cruise observation and numerical modeling of turbulent mixing in the Pearl River estuary in summer

The turbulent mixing in the Pearl River estuary and plume area is analyzed by using cruise data and simulation results of the Regional Ocean Model System (ROMS). The cruise observations reveal that strong mixing appeared in the bottom layer on larger ebb in the estuary. Modeling simulations are consistent with the observation results, and suggest that inside the estuary and in the near-shore water, the mixing is stronger on ebb than on flood. The mixing generation mechanism analysis based on modeling data reveals that bottom stress is responsible for the generation of turbulence in the estuary, for the re-circulating plume area, internal shear instability plays an important role in the mixing, and wind may induce the surface mixing in the plume far-field. The estuary mixing is controlled by the tidal strength, and in the re-circulating plume bulge, the wind stirring may reinforce the internal shear instability mixing.