Bedform Morphology Research Articles

Morphology of Subaqueous Dunes at the Mouth of the Dammed River São Francisco (Brazil)

Traini, C.; Schrottke, K.; Stattegger, K.; Dominguez, J.M.L.; Kacenelenbogen Guimarães, J.; Vital, H.; D'avila Beserra, D., and Aquino da silva, A.G., 2012. Morphology of subaqueous dunes at the mouth of the dammed river São Francisco (Brazil).Economic development of the São Francisco River basin has increased by building seven river dams since the 1950s. Subsequently, strong coastal erosion reaching maximum rates of 98.6 m/year has been recorded at the river mouth, also leading to the destruction of a coastal village. This work combines discharge data with new sediment and hydro-acoustic records, collected in January 2009, to verify the impact of river damming on processes, controlling coastline evolution. Data analysis is focused on discharge evolution, morphology and sediment composition of subaqueous dunes as well as on the impact by tidal currents. Discharge has been strongly regulated since 1986, when the last river dam was implemented. Again, bedform morphology seems to be not affected neither by reduced river-discharge nor by reduced sediment supply. However, migration rates of these dunes, as calculated, are lower due to decreased discharge. At the same time, tidal impact is strengthened at the lowermost part of the river. From this we assume, that the replenishment of sediment at the coast near the river-mouth might be time-delayed, which could support coastal erosion.Le développement économique du bassin versant du fleuve São Francisco a été favorisé par la construction de sept barrages hydroélectriques sur le cours du fleuve. Par suite, une forte érosion du trait de côte atteignant un taux maximal de 98,6 m par an a été enregistré à l'embouchure du fleuve, conduisant à la destruction d'un village côtier. Ce travail allie des données de débit, de prélèvements sédimentaires et de nouveaux enregistrements hydroacoustiques collectés en janvier 2009 pour vérifier l'impact d'un barrage sur les processus contrôlant l'évolution du trait de côte. L'analyse des données est concentrée sur l'évolution des débits fluviatiles, la morphologie et la granulométrie du sédiment composant les dunes du fond du fleuve ainsi que l'impact des courants de marée. Les débits fluviatiles ont été fortement régulés depuis 1986 alors que la construction du dernier barrage débutait. La morphologie des dunes fluviatiles ne semble pas être affectée par la rétention sédimentaire des barrages. Cependant, les taux de migration calculés de ces dunes sont plus faibles en raison de l'affaiblissement des débits. Dans le même temps, l'impact de la marée s'est renforcé dans la portion la plus distale du delta. Nous présumons ainsi que l'alimentation sédimentaire du littoral à proximité de l'embouchure fluviale est retardée et donc contribue à l'érosion de cette portion du littoral.

Extraction of morphometric bedform characteristics from profiling sonar datasets recorded in shallow coastal waters of China

The morphological characteristics of small-scale bedforms were measured by means of an acoustic profiling sonar on the Dafeng tidal flat, Jiangsu, in 2009, and in the Jiulong Estuary, Xiamen, in 2010, respectively. The “multi-threshold value” method was utilized to reveal the morphological undulations along which bedforms were present. Analyses of the datasets obtained show that: (1) sand ripples can have irregular shapes, and (2) changes in bedform morphology are small within a single tidal cycle but may be significant over several tidal cycles. Fractal and variogram analyses of the seabed roughness revealed the existence of a significant relationship between current speed and the fractal dimension of the seabed roughness. As current speed increases, seabed roughness increases with a trend of smaller-scale bottom structures being replaced by larger-scale structures. Furthermore, the surface of the larger-scale bottom structures can either become smooth due to the absence of smaller-scale features or become rougher due to the presence of superimposed smaller-scale structures.

Storm-induced inner-continental shelf circulation and sediment transport: Long Bay, South Carolina

Long Bay is a sediment-starved, arcuate embayment located along the US East Coast connecting both South and North Carolina. In this region the rates and pathways of sediment transport are important because they determine the availability of sediments for beach nourishment, seafloor habitat, and navigation. The impact of storms on sediment transport magnitude and direction were investigated during the period October 2003–April 2004 using bottom mounted flow meters, acoustic backscatter sensors and rotary sonars deployed at eight sites offshore of Myrtle Beach, SC, to measure currents, water levels, surface waves, salinity, temperature, suspended sediment concentrations, and bedform morphology. Measurements identify that sediment mobility is caused by waves and wind driven currents from three predominant types of storm patterns that pass through this region: (1) cold fronts, (2) warm fronts and (3) low-pressure storms. The passage of a cold front is accompanied by a rapid change in wind direction from primarily northeastward to southwestward. The passage of a warm front is accompanied by an opposite change in wind direction from mainly southwestward to northeastward. Low-pressure systems passing offshore are accompanied by a change in wind direction from southwestward to southeastward as the offshore storm moves from south to north.During the passage of cold fronts more sediment is transported when winds are northeastward and directed onshore than when the winds are directed offshore, creating a net sediment flux to the north–east. Likewise, even though the warm front has an opposite wind pattern, net sediment flux is typically to the north–east due to the larger fetch when the winds are northeastward and directed onshore. During the passage of low-pressure systems strong winds, waves, and currents to the south are sustained creating a net sediment flux southwestward. During the 3-month deployment a total of 8 cold fronts, 10 warm fronts, and 10 low-pressure systems drove a net sediment flux southwestward. Analysis of a 12-year data record from a local buoy shows an average of 41 cold fronts, 32 warm fronts, and 26 low-pressure systems per year. The culmination of these events would yield a cumulative net inner-continental shelf transport to the south–west, a trend that is further verified by sediment textural analysis and bedform morphology on the inner-continental shelf.

Morphodynamic characteristics in a tidal inlet: San Blas, Argentina

A tidal inlet on the Argentinean coast is studied. An integrated approach of combining detailed bathymetry survey, seabed sampling and tidal current measurements was used to characterize the tidal inlet morphodynamics. A Phase Measuring Bathymetric System (PMBS) “GeoSwath Plus” from GeoAcoustics Ltd. with centimetric precision and a mean swath coverage of 160 m, was used to recognize bedform morphologies and to determine bedform dynamics by comparison of successive surveys . Tidal velocities were recorded during a complete tidal cycle using an ADCP mounted on a ship operating at a frequency of 650 kHz. Maximum tidal currents, of the order of 2 m s − 1 , exhibit different patterns across the channel. While maximum ebb currents dominate in the central and southern area of the channel, flood maximum currents dominate on the northern flank. The complex morphology of a dune field in an inlet and the resulting 3-D dune movement mechanism are also presented. In the middle of San Blas channel, a field with symmetrical large dunes (10–100 m in spacing) and very large dunes (> 100 m in spacing) 4.5 to 5 m in height was found. Dune migration rates were measured by comparing two successive morphological maps of the field, reaching a rate up to 21 m year − 1 toward the ocean. In contrast, the smaller dunes present on the northern border of the field migrate in an inward direction at a rate of 15.7 m year − 1 .

Parameterization of bedform morphology and defect density with fingerprint analysis techniques

A novel method for parameterizing the morphology of seafloor ripples with fingerprint analysis numerical techniques is presented. This fully automated analysis tool identifies rippled areas in two-dimensional imagery of the seafloor, and returns ripple orientation and wavelength as well as a new morphological parameter, the spatial density of ripple defects. In contrast to widely used manual and spectral parameterization methods, this new technique yields a unique probability distribution for each derived parameter, which describes its spatial variability across the sampled domain. Here we apply this new analysis technique to synthetic and field collected side-scan sonar seafloor images in order to assess the methods capacity to define bed geometry across a wide range of simulated and observed morphological conditions. The resulting orientation and wavelength values compare favorably with those of the existing manual and spectral parameterization methods, and are superior under environmental conditions characterized by low signal to noise ratios as well as high planform ripple sinuosity. Furthermore, the resulting ripple defect density values demonstrate correlation with ripple orientation, wave direction, and the Shields parameter, which is consistent with recent investigations that have theoretically linked this parameter to hydrodynamic forcing conditions. The presented fingerprint analysis method surpasses the capacity of existing methods for ripple parameterization and promises to yield greater insight into theoretical and applied problems associated with the temporal and spatial variability of ripple morphology across a wide spectrum of marine environments.

Bottom substrate attributes relative to bedform morphology of spawning site of Chinese sturgeon Acipenser sinensis below the Gezhouba dam

Characteristics of bottom substrates have caused wide concernas to their importance for sturgeon spawning ground requirements. Substrate requirements for spawning of Chinese sturgeon Acipenser sinensis have not yet been well studied because of the difficulties in investigating on a large and fast flowing river such as the Yangtze River. During 2007 and 2008, the underwater video surveys on the present spawning ground of A. sinensis were successfully conducted. Two kinds of substrates were documented: one stable infrastructure consisting of large boulders and cobbles mixed with pebbles and gravels, the other comprise sediments in transportation, especially coarse and fine sands. The large boulders (size range 20–50 cm), cobbles and pebbles (size range 10–20 cm), pebbles or gravels are estimated to account for 50, 30, 20% of the available substrates of the infrastructure on spawning ground according to our underwater video survey points, respectively. The transporting sands can be observed in most area, while the river sand embedded or covered the large structural compounds just occur in certain area and the substrate where spawning occurs annually and occasionally were similarly ‘clean’ of fine sediment. The eggs (embryos) of Chinese sturgeon have been observed ‘hiding’ in or adhering to the interstitial spaces between the clean large materials e.g. boulders, cobbles and pebbles during the 2007/2008 study period. Therefore the degree of cover of the larger particles by sand (embeddedness), which will directly affect the spawning habitat and subsequently influence survival of the new hatched larvae, were assessed by determining the topographic data in relation to water velocity. The multivariable correlation analysis and binary logistic regression analysis showed that the comparably topographic characteristics which had been confirmed to be suitable at the historic and present spawning sites would eventually contribute to the substrate deposition. The embeddedness of the sediment are obviously lower in shallow water with higher water velocity than in the deeper water with slower water current (P < 0.01) and the sites where the embryo occur can be predicted with the three physical variables: embeddedness of the sediment, water velocity and river bottom elevation. The bedform morphology with an adverse slope (channel slope adverse to the bottom elevation grads) formed by larger blocks of substrate, will improve the water turbulence at high current to carry the suspended or embedded fine sediments away from the spawning site and (in turn) provide enough interstitial spaces for effective egg deposition and incubation. Therefore the substrate requirements for the spawning habitat of Chinese sturgeon should be described and assessed more than just rocky and gravelly substrate, taking the embeddedness of sediment in well consider. The concepts of detailed description of the substrate requirements and their relationship with the overall bedform morphology has been shown to be essential to provide benefit for the rehabilitation of the spawning ground while fostering the conservation of the endangered species.

Small-scale sediment transport patterns and bedform morphodynamics: new insights from high-resolution multibeam bathymetry

New multibeam echosounder and processing technologies yield sub-meter-scale bathymetric resolution, revealing striking details of bedform morphology that are shaped by complex boundary-layer flow dynamics at a range of spatial and temporal scales. An inertially aided post processed kinematic (IAPPK) technique generates a smoothed best estimate trajectory (SBET) solution to tie the vessel motion-related effects of each sounding directly to the ellipsoid, significantly reducing artifacts commonly found in multibeam data, increasing point density, and sharpening seafloor features. The new technique was applied to a large bedform field in 20–30 m water depths in central San Francisco Bay, California (USA), revealing bedforms that suggest boundary-layer flow deflection by the crests where 12-m-wavelength, 0.2-m-amplitude bedforms are superimposed on 60-m-wavelength, 1-m-amplitude bedforms, with crests that often were strongly oblique (approaching 90°) to the larger features on the lee side, and near-parallel on the stoss side. During one survey in April 2008, superimposed bedform crests were continuous between the crests of the larger features, indicating that flow detachment in the lee of the larger bedforms is not always a dominant process. Assessment of bedform crest peakedness, asymmetry, and small-scale bedform evolution between surveys indicates the impact of different flow regimes on the entire bedform field. This paper presents unique fine-scale imagery of compound and superimposed bedforms, which is used to (1) assess the physical forcing and evolution of a bedform field in San Francisco Bay, and (2) in conjunction with numerical modeling, gain a better fundamental understanding of boundary-layer flow dynamics that result in the observed superimposed bedform orientation.

Computer simulations of self-organized megaripples in the nearshore

[1] Megaripples are bed forms with heights of 20–50 cm and lengths of 1–10 m that are common in the surf zone of natural beaches. They affect sediment transport, flow energy dissipation, and larger-scale hydro- and morphodynamics. They are thought to be dynamically similar to bed forms in deserts, rivers, and deeper marine environments. Here a self-organization model (similar to models for subaerial bed forms) is used to simulate the formation and development of megaripples in the surf zone. Sediment flux is determined from combined wave and current flows using stream power and bed shear stress formulations as well as a third formulation for transport based on simple rules, which represent sheet flow. Random bed irregularities, either imposed or resulting from small variations in transport representing turbulence, are necessary seeds for bed form development. Feedback between the bed and the flow, in the form of a shadow zone downstream of a bed form and increasing flow acceleration with elevation over the crests of bed forms, alter the transport such that organized bed forms emerge. Modeled bed form morphology (including cross-sectional shape and plan view) and dynamics (including growth and migration) are similar to natural megaripples. The model can be used to extend the field observations of Clarke and Werner (2004), which suggest that, if conditions remain the same, megaripples will continue to grow. Contrary to many bed form models, this model supports the idea that bed form spacing grows continually.

Experimental study on transient and steady‐state dynamics of bedforms in supply limited configuration

AbstractThe purpose of the present study is to investigate experimentally the development of bedforms in a configuration where the sediment supply is limited. The experimental setup is a rectangular closed duct combining an innovative system to control the rate of sediment supply Qin , and a digitizing system to measure in real time the 3D bedform topography. We carried out different sets of experiments with two sediment sizes (100 µm and 500 µm) varying both the sediment supply and the water flow rate to obtain a total of 46 different configurations. After a transient phase, steady sub‐centimeter bedforms of various shapes have been observed: barchans dunes, straight transverse dune, linguoid transverse dunes and bedload sheets. Height, spacing, migration speed, and mean bed elevation of the equilibrium bedforms were measured. For a given flow rate, two regimes were identified with fine sediment: (i) a monotonic increasing regime where the equilibrium bedform height and velocity increase with the sediment supply rate Qin and (ii) an invariant regime for which both parameters are almost independent of Qin. For coarse sediment, only the first regime is observed. We interpret the saturation of height and velocity for fine sediment bedforms as the transition from a supply‐limited regime to a transport‐limited regime in which the bedload flux has reached its maximum value under the prevailing flow conditions. We also demonstrate that all experiments can be rescaled if the migration speed and height of the bedforms are, respectively, divided and multiplied by the cube of the shear velocity. This normalization is independent of grain size and of bedform morphology. These experimental results provide a new quantification of the factors controlling equilibrium height and migration speed of bedforms in supply‐limited conditions against which theoretical and numerical models can be tested.

Variations in saturated and unsaturated water movement through an upland floodplain wetland, mid-Wales, UK

Summary Variations in inferred water movement are examined during a two-year study of an upland floodplain wetland at Afon Llwyd, in mid-Wales, UK in 2001 and 2002. Soil–water pressures ( ψ ) were monitored by six tensiometer nests, each comprising two sensors at depths of 30 and 60 cm below the surface. Detailed sedimentary sections were produced and changes in saturated and unsaturated hydraulic conductivity (and their relationship to ψ ) were estimated. Seasonal trends in ψ were identified and vertical hydraulic gradients within different tensiometer nests were derived and interpreted in the context of the local channel and floodplain sedimentology, channel bedform morphology and hydro-climatic conditions. Although there is considerable variation in ψ , the results display characteristic trends at several levels: (1) in relationship to precipitation; (2) reflecting changes in river stage (with downstream stage providing base-level control through a riffle-pool couplet); and (3) marked spatial variations in the direction and rate of water movement. These controls influence water redistribution through the wetland, which reflects the relative position of distinct sedimentary units within the floodplain. The results demonstrate that water movement is episodic and characterised by relatively rapid horizontal water fluxes during and immediately following individual rain events, followed by residual seepage to the river and water movement under a local soil moisture gradient. The implications of these results include elucidating the role of upland floodplains in buffering sub-surface drainage, and attenuating patterns of river flow under base flow conditions. This has the potential to advance the basis for future upland floodplain wetland and channel restoration schemes.

Semianalytical analysis of hyporheic flow induced by alternate bars

We investigate the effects of alternate bar morphology on the hyporheic flow in gravel bed rivers. Our goal is to investigate the relations between residence time distribution of a conservative tracer and the parameters controlling bed form morphology. We assume homogeneous, isotropic or anisotropic hydraulic properties of the streambed sediment and constant flow regime in equilibrium with the bed form, which is considered fixed because its formation timescale is much longer than that of the subsurface flow. Under these assumptions, we solve the in‐stream and hyporheic flow fields analytically in a three‐dimensional domain. We approximate the former with the shallow water equations and model the latter as a Darcian flow. The two systems are linked through the hydraulic head distribution, which is predicted at the streambed by the surface model and applied as a boundary condition to the hyporheic flow model. We solve the solute transport equation in the hyporheic zone for a conservative tracer by means of particle tracking. Our model predicts that the mean value and variance of the hyporheic residence time depend on the alternate bar amplitude at equilibrium. This result is found to be applicable also to discharges that are lower (70% in our simulations) than the formative and submerge the bars entirely. Moreover, our analysis shows that 95% of the hyporheic flow is confined in a near‐bed layer, whose depth is about the width of the channel and shallows from low to steep gradient streams. This causes the hyporheic mean residence time to reach a threshold when the alluvial depth is deeper than the channel width. Our results also show that as the stream slope increases, the streamlines compact near to the streambed, thereby reducing the mean residence time and its variance. Finally, we observe that the hyporheic residence time of pulse injections of passive solutes is lognormally distributed, with the mean value depending in a simple manner on the amplitude of the alternate bars.

Climbing and falling dunes in Valles Marineris, Mars

Multiple occurrences of “wall dunes” are found several kilometers above the Valles Marineris canyon floor. Dune slip face orientation and bed form morphologies indicate transport direction and whether the wall dunes are climbing dunes or falling dunes. On Earth, these types of dunes form in a unidirectional wind regime and are strongly controlled by the local topography. Newly acquired Mars Reconnaissance Orbiter (MRO) images and topography of the walls of Valles Marineris show similar sand dune morphologies, as wind blown sediment has interacted with local and regional topography. Primarily found in Melas and Coprates Chasmata, these climbing and falling dunes are relevant for understanding aeolian sediment flux, sediment sources, and wind directions. Falling dunes show photogeologic and thermophysical evidence of their sand being provided by adjacent outcrops.

Sedimentary anisotropy diverges from flute trends in south-east Finnish Lapland

Subglacial flutes are parallel-to-ice flow lineations indicative of glacial streamlining, yet their depositional/erosional origin or sedimentary anisotropy is not fully understood. The flutes, aligned NW–SE with a flow direction to the SE, are superimposed on westerly oriented drumlins, in Kuusamo, south-eastern Finnish Lapland (65°45′N and 29°40′E) and reflect ice stream flow pattern towards the Younger Dryas End Moraines (YDEMs) in Russian Karelia. We investigated bedform morphology (flutes, drumlins and crescentic troughs), as well as sedimentary anisotropy of flute ridges and troughs using digital elevation models (DEMs), airborne radiometric (AR) data, and measurements of azimuthal electrical conductivity ( σ a). The DEM-AR revealed elongation ratios ( L/ W) from 2:1 to 25:1, yet the highest ratios reached L/ W = 48:1. Flutes in the study area fan towards the east and south-east, whereas the drumlins on which they are superimposed show ice flow from the west (280°). The σ a-anisotropy of the flute ridges and troughs indicates sedimentation from 340° to 350°, diagonal with respect to both drumlins and flutes. We found erosional crescentic troughs with down-flow rims indicative of both phases, but failed to find evidence for subglacial meltwater boulder lags or ice-flow erosional bedrock bump sticky spots. We contend that the studied flutes are erosional and date to the YDEM-phase, yet the origin of the precursory sediments may date to Early Weichselian.

Width variations and mid-channel bar inception in meanders: River Bollin (UK)

The extent and pattern of width variations along a meandering channel and its association with variation in bed topography and development of mid-channel bars have been examined through field survey evidence for a reach of the River Bollin in NW England. The wet width has been quantified along the reach by applying a hydraulic model to surveyed cross sections under a range of discharges between low flow and defined bankfull conditions. This approach allows an objective, modelling-based method to compute channel width. The high spatial resolution of the topographical survey allows capture of significant variations in the cross-sectional morphology at the meander wavelength scale. Results disclose some features of longitudinal and stage-dependent width variability in meanders. Width variation is shown to be highly correlated with curvature: at bankfull conditions width peaks in bend apex sections exceed those at inflection sections and can be up to twice greater. The width-curvature behavior is correlated with the pattern of bed and banks morphology, which is different in bend apices and in meander inflections. The survey shows that the bedform morphology can be characterized by a mid-channel bar pattern that is initiated at the inflection section and that the bedform dynamics can be closely associated with channel width variations.

On the relationship between flow and suspended sediment transport over the crest of a sand dune, Río Paraná, Argentina

The links between large-scale turbulence and the suspension of sediment over alluvial bedforms have generated considerable interest in the last few decades, with past studies illustrating the origin of such turbulence and its influence on flow resistance, sediment transport and bedform morphology. In this study of turbulence and sediment suspension over large sand dunes in the Río Paraná, Argentina, time series of three-dimensional velocity, and at-a-point suspended sediment concentration and particle-size, were measured with an acoustic Doppler current profiler and laser in situ scattering transmissometer, respectively. These time series were decomposed using wavelet analysis to investigate the scales of covariation of flow velocity and suspended sediment. The analysis reveals an inverse relationship between streamwise and vertical velocities over the dune crest, where streamwise flow deceleration is linked to the vertical flux of fluid towards the water surface in the form of large turbulent fluid ejections. Regions of high suspended sediment concentration are found to correlate well with such events. The frequencies of these turbulent events have been assessed from wavelet analysis and found to concentrate in two zones that closely match predictions from empirical equations. Such a finding suggests that a combination and interaction of vortex shedding and wake flapping/changing length of the lee-side separation zone are the principal contributors to the turbulent flow field associated with such large alluvial sand dunes. Wavelet analysis provides insight upon the temporal and spatial evolution of these coherent flow structures, including information on the topology of dune-related turbulent flow structures. At the flow stage investigated, the turbulent flow events, and their associated high suspended sediment concentrations, are seen to grow with height above the bed until a threshold height (ca 0·45 flow depth) is reached, above which they begin to decay and dissipate.

Fine‐scale relief related to Late Holocene channel shifting within the floor of the upper Redondo Fan, offshore Southern California

AbstractErosional and depositional bedforms have been imaged at outcrop scale in the upper Redondo Fan, in the San Pedro Basin of offshore Southern California in ≥600 m water depths, using an Autonomous Underwater Vehicle developed by the Monterey Bay Aquarium Research Institute. The Autonomous Underwater Vehicle is equipped with multibeam and chirp sub‐bottom sonars. Sampling and photographic images using the Monterey Bay Aquarium Research Institute Remotely Operated Vehicle Tiburon provide groundtruth for the Autonomous Underwater Vehicle survey. The 0·3 m vertical and 1·5 m lateral bathymetric resolution and 0·1 m sub‐bottom profile resolution provide unprecedented detail of bedform morphology and structure. Multiple channels within the Redondo Fan have been active at different times during the Late Holocene (0 to 3000 yr bp). The currently active channel extending from Redondo Canyon makes an abrupt 90° turn at the canyon mouth before resuming a south‐easterly course along the east side of the Redondo Fan. This channel is floored by sand and characterized by small steps generally &lt;1 m in relief, spaced 10 to 80 m in the down‐channel direction. A broader channel complex lies along the western side of the fan valley that was last active more than 850 years ago. Two distinct trains of large scours, with widths ranging from tens to a few hundred metres and depths of 20 m, occur on the floor of the western channel complex, which has a thin mud drape. If observed in cross‐section only, these large scours would probably be misidentified as the thalweg of an active channel.

Changes in a large regulated tropical river: The Paraná River downstream from the Porto Primavera Dam, Brazil

Dams disturb in the fluvial dynamics by changing the natural cycle of the rivers, interfering with the transport-deposition processes and affecting river ecology. The Brazilian part of the Paraná Basin has the largest hydrometrical power potential of the country with more than 150 hydroelectric power dams that transformed the Paraná River and its tributaries in a succession of lakes. This research aims to analyze the changes in certain fluvial variables in the last natural downstream reach of the Upper Paraná River after closure of the Porto Primavera Dam. Data acquired before and after dam construction showed changes in water discharge, bank erosion, flood pulse, bed load grain size discharge, concentration of suspended load and bedform morphology and size. Those alterations generated changes in river ecology especially on fish reproduction, benthic community, and rotifers variety and density. A review of the obscure regional literature, generally in Portuguese, is also presented.

Characterization of bedform morphology generated under combined flows and currents using wavelet analysis

The wavelet transform (WT) has been successfully implemented in many fields such as signal and image processing, communication theory, optics, numerical analysis, and fluid mechanics. However, the application of WT to describe bedform morphology in coastal areas, oceans, and rivers is rare. The present study demonstrates the capability of WT analysis to fully represent the space–frequency characteristics of signals describing bed topography generated in marine and river environments. In this study WT is used to examine the morphological characteristics of bedforms generated in two separate laboratory facilities: a wave tank and a meandering channel. In the wave tank a set of ripples superimposed upon large wave ripples were generated; while in the meandering channel, 2D and 3D migrating ripples and dunes were observed. The WT proved to be a useful tool in detecting the complex variability of the generated bedform structures. The size distribution of the bottom features such as ripples, large wave ripples and sandbars were first examined along a 2D bed profile. Later analysis studied the variability of features in the transverse direction by using the power Hovmöller. Experiments in the wave tank were conducted for a mobility number of ψ=(10, 28), and a Reynolds wave number of R ew =(17,500, 83,500) which correspond to waves alone (WA) and to combined flow (CF) scenarios, respectively. Experiments in the meandering channel were conducted under a morphological regime that produced mainly migrating sandbars.

A bedform morphology hypothesis for spawning areas of Chinese sturgeon

Results from previous research suggested that geomorphic characteristics may be important controlling factors among other microhabitat variables for spawning fish. We investigated the bedform morphology of seven spawning areas (five historic and two present spawning areas) of Chinese sturgeon (Acipenser sinensis) by analysis of relief maps of the riverbed and by field surveys. We conclude that the topographic characteristics of the spawning areas include rocky or gravelly substrate river bend, large variations in water depth and river width and a slope with moderate length and slope. The turning structure and the adverse slope may be the most important topographic characteristics for the spawning areas. Combined with the reproductive occurrence of the fish in these spawning areas, we tentatively propose a bedform morphology hypothesis for the spawning areas. We consider that a spawning area of A. sinensis should consist of three functional areas: Mating area (M), Dispersal area (D) and Incubation area (I). M, D and I are distributed along the water current and their bedform characteristics are clearly different. M is the place for mature fish to mate and spawn. It is usually below a riffle with rolling terrain. D is the place for fertilizing and dispersing fertilized eggs. It is usually in a river bend with sharp variations in terrain, such as deep pools or steep slopes. I is the place for dispersal of fertilized eggs. It is usually in long point bars or broad shallow areas with rocky substrate. This hypothesis could serve as a guideline for improving the present spawning areas or constructing new spawning areas, so as to rehabilitate the wild stock of the sturgeon.

Streamlined bedrock terrain and fast ice flow, Jakobshavns Isbrae, West Greenland: implications for ice stream and ice sheet dynamics

This study investigates the marginal subglacial bedrock bedforms of Jakobshavns Isbrae, West Greenland, in order to examine the processes governing bedform evolution in ice stream and ice sheet areas, and to reconstruct the interplay between ice stream and ice sheet dynamics. Differences in bedform morphology (roche moutonnee or whaleback) are used to explore contrasts in basal conditions between fast and slow ice flow. Bedform density is higher in ice stream areas and whalebacks are common. We interpret that this is related to higher ice velocities and thicker ice which suppress bed separation. However, modification of whalebacks by plucking occurs during deglaciation due to ice thinning, flow deceleration, crevassing and fluctuations in basal water pressure. The bedform evidence points to widespread basal sliding during past advances of Jakobshavns Isbrae. This was encouraged by increased basal temperatures and melting at depth, as well as the steep marginal gradients of Jakobshavns Isfjord which allowed rapid downslope evacuation of meltwater leading to strong ice/bedrock coupling and scouring. In contrast to soft-bedded ice stream bedforms, the occurrence of fixed basal perturbations and higher bed roughness in rigid bed settings prevents the basal ice subsole from maintaining a stable form which, coupled with secondary plucking, counteracts the development of bedforms with high elongation ratios. Cross-cutting striae and double-plucked, rectilinear bedforms suggest that Jakobshavns Isbrae became partially unconfined during growth phases, causing localised diffluent flow and changes in ice sheet dynamics around Disko Bugt. It is likely that Disko Bugt harboured a convergent ice flow system during repeated glacial cycles, resulting in the formation of a large coalesced ice stream which reached the continental shelf edge.