Increased Bed Roughness Research Articles

Representing the impact of Rhizophora mangroves on flow in a hydrodynamic model (COAWST_rh v1.0): the importance of three-dimensional root system structures

Abstract. Coastal wetland vegetation modulates water flow by exerting drag, which has important implications for sediment transport and geomorphic dynamics. This vegetation effect on flow is commonly represented in hydrodynamic models by approximating the vegetation as an array of vertical cylinders or increased bed roughness. However, this simple approximation may not be valid in the case of Rhizophora mangroves that have complicated three-dimensional root structures. Here, we present a new model to represent the impact of Rhizophora mangroves on flow in hydrodynamic models. The model explicitly accounts for the effects of the three-dimensional root structures on mean flow and turbulence as well as for the effects of two different length scales of vegetation-generated turbulence characterized by stem diameter and root diameter. The model employs an empirical model for the Rhizophora root structures that can be applied using basic vegetation parameters (mean stem diameter and tree density) without rigorous measurements of the root structures. We tested the model against the flows measured by previous studies in a model mangrove forest in the laboratory and an actual mangrove forest in the field, respectively. We show that, compared with the conventional approximation using an array of cylinders or increased bed roughness, the new model significantly improves the predictability of velocity, turbulent kinetic energy, and bed shear stress in Rhizophora mangrove forests. Overall, the presented new model offers a more realistic but feasible framework for simulating flows in Rhizophora mangrove forests with complex root structures using hydrodynamic models.

Influence of Bed Roughness on Flow and Turbulence Structure Around a Partially‐Buried, Isolated Freshwater Mussel

AbstractThe present study uses eddy‐resolving numerical simulations to investigate how bed roughness affects flow and turbulence structure around an isolated, partially‐buried mussel (Unio elongatulus) aligned with the incoming flow. The rough‐bed simulations resolve the flow past the exposed part of a gravel bed, whose surface is obtained from a laboratory experiment that also provides some additional data for validation of the numerical model. Results are also discussed for the limiting case of a horizontal smooth bed. Additionally, the effects of varying the level of burial of the mussel inside the substrate and the discharge through the two mussel siphons are investigated via a set of simulations in which the ratio between the median diameter of the (gravel) particles forming the rough bed, d50, and the height of the exposed part of the mussel, h, varies between 0.10 and 0.22. The increase of the bed roughness is associated with a strong amplification of the turbulence kinetic energy in the near‐wake region. Increasing the bed roughness and/or reducing h intensifies the interactions of the eddies generated by the bed particles with the base and tip vortices induced by the active filtering and by the mussel shell, respectively, which, in turn, induces a more rapid dissipation of these vortices. Increasing the bed roughness also reduces the strength of the main downwelling flow region forming in the wake. The strong downwelling near the symmetry plane is the main reason why the symmetric wake shedding mode dominates in the smooth bed simulations with negligible active filtering. By contrast, the anti‐symmetric wake shedding mode dominates in the simulations conduced with a high value of the bed roughness. The mean streamwise drag force coefficient for the emerged part of the shell and the dilution of the excurrent siphon jet increase with increasing bed roughness.

Particle saltation trajectories in supercritical open channel flows: Roughness effect

AbstractBed load sediment transport governed by particle saltation in supercritical open channel flows causes bedrock incision in high‐gradient mountain streams and hydro‐abrasion at hydraulic structures. Hence, a better understanding of turbulent flow characteristics and particle dynamics is of prime importance for the prediction of river and landscape evolution, and for a sustainable design of hydraulic structures. To this end, we experimentally investigated single‐particle dynamics in supercritical open channel flows over fixed planar and abraded beds covering smooth, transitionally rough, and hydraulically rough regimes. The experiments were performed at the Laboratory of Hydraulics, Hydrology and Glaciology (VAW) of ETH Zürich in a wide range of hydraulic conditions with various channel width‐to‐depth aspect ratios (b/ho) and Froude numbers (F). We investigated the motions of five particle groups differing in shape, hardness, and diameter. By means of a high‐speed camera, the particle velocities and trajectory parameters—such as hop height and hop length—were determined using particle tracking velocimetry. The results reveal that at least 120 individual particles should be used in the tests to determine sound statistics of particle trajectories. In the present test conditions, the particles are dominantly transported in saltation. The effects of particle properties such as shape and diameter on the particle trajectories are negligible, while the rolling probability of the particles for a given flow condition increases with increasing diameter. Bed roughness was found to be the key parameter having a significant effect on the saltation trajectories and horizontal energy transfer. Hop height, hop length, and particle velocities, as well as horizontal energy transfer, increase with increasing bed roughness. Based on the saltation trajectory data, non‐dimensional equations are developed for each hydraulic regime and used to enhance the saltation–abrasion model of Sklar and Dietrich. The present findings contribute to a better understanding of particle motion characteristics under different hydraulic and roughness conditions, and their effect on the mechanics and prediction of bedrock incision and hydro‐abrasion in three‐dimensional supercritical open channel flows occurring in steeply sloped rivers and at hydraulic structures.

An experimental study on the evolution law of roll wave parameters in overland flow

AbstractRoll waves commonly occur in overland flow and have an important influence on the progress of soil erosion on slopes. This study aimed to explore the evolution and mechanism of roll waves on steep slopes. The potential effects of flow rate, rainfall intensity and bed roughness on the laws controlling roll wave parameters were investigated. The flow rates, rainfall intensities and bed roughness varied from 5 to 30 L/min, 0 to 150 mm/h, and 0.061 to 1.700 mm, respectively. The results indicate that roll waves polymerize significantly along the propagation path, and bed roughness and rainfall affect the generation and evolution of roll waves. The wave velocity, length and height decreased with bed roughness, whereas the wave frequency increased with increasing bed roughness under fixed flow rate and rainfall intensity conditions. Rainfall increased the wave velocity and wavelength and decreased the wave frequency. The wave velocity, height and wavelength tended to increase with an increasing flow rate. Rainfall promoted the generation of roll waves, whereas bed roughness had the opposite effect. The generation of roll waves is closely related to the Froude number (Fr) and flow resistance. In this experiment, the range of the Reynolds number for the roll waves generated in the laminar region was 142–416, and the range of the flow resistance coefficient was 0.64–4.85. The critical value of the Fr for flow instability in the laminar region was approximately 0.57. Exploring the generation and evolution law of roll waves is necessary for understanding the processes and dynamic mechanisms of slope soil erosion.

Roughness Effect on Velocity Distribution in Selected Reach of Shatt al-Arab River

Shatt al-Arab is the only navigational artery in Iraq, extending from the city of Qurna to its mouth in the Arabian Gulf at the city of Al-Fao within the governorate of Basrah for a length of approximately 204 km. Its width ranges from 400 m to 2000 m, and its depth ranges from 8 m to 20 m. The southern part of it, 93 km long from Umm al-Rassas Island to Ras al-Bisha, represents the international border between Iraq and Iran, where the Thalweg line represents the border between the two countries, which is the deepest point in the riverbed (according to the 1975 Algiers Agreement). The western bank (the Iraqi side) within the common border of Shatt al-Arab is subject to continuous erosion, which leads to the shifting of the Thalweg line towards Iraqi territory and thus leads to loss of Iraqi land to Iran. Reducing flow velocity along the Iraqi side can lead to reducing or preventing erosion in the river. Increasing the riverbed roughness will reduce the velocity of flow and then reducing the erosion. This principle was adopted in this study to investigate the effect of increasing roughness in a strip along a reach of the riverbed on the distribution of longitudinal velocity in cross-sections at the rest of the selected reach. A reach of Shatt al-Arab with a length of 2500 m, located 34 km north of Fao City, was selected to represent the study area. This reach was simulated by using numerical modeling CFD solver (fluent) with three different roughnesses for an upstream part of the river bed and the velocities compared with the natural (original) roughness of Shatt al-Arab. The results showed an appreciable effect of the increased bed roughness on the velocity distribution and the maximum velocity location by shifting it to the other side.

Artificial Structures Steer Morphological Development of Salt Marshes: A Model Study

Salt marshes are increasingly recognized as resilient and sustainable supplements to traditional engineering structures for protecting coasts against flooding. Nevertheless, many salt marshes face severe erosion. There is a consensus that providing structures that create sheltered conditions from high energetic conditions can improve the potential for salt marsh growth. However, little proof is provided on the explicit influence of structures to promote salt marsh growth. This paper investigates how artificial structures can be used to steer the morphological development of salt marshes. A morphological model (Delft3D Flexible Mesh) was applied, which enabled the analysis of various artificial structures with realistic representation. A salt marsh in the Wadden Sea which has seen heavy erosion (lateral retreat rate of 0.9 m/year) served as case study. We simulate both daily and storm conditions. Hereby, vegetation is represented by an increased bed roughness. The model is able to simulate the governing processes of salt marsh development. Results show that, without artificial structures, erosion of the salt marsh and tidal flat continues. With structures implemented, results indicate that there is potential for salt marsh growth in the study area. Moreover, traditional structures, which were widely implemented in the past, proved to be most effective to stimulate marsh growth. More broadly, the paper indicates how morphological development of a salt marsh can be steered by various configurations of artificial structures.

CURRENT ACTIVITY IN THE THERMAIKOS GULF CONTINENTAL MARGIN , IN RELAÏJON TO MODERN SEDIMENTATION PROCESSES

The area under investigation represents the NW continental margin of the Aegean Sea i.e. the Thermaikos Gulf and the associated NW Sporades Basin. Surficial seabed sediments are of terrigenous origin whilst in terms of grain size, offshore sediments maybe distinguished into silty sediments that cover the northern and western part of the Gulf; clayey that cover the floor of the Sporades basin and relict sands that cover mainly the central and eastern part of the self. Current meter measurements from different water depths were obtained from 10 stations on the shelf, shelf/break, within canyons and in the Sporades Basin. Surface currents over the period of the investigation are dominated by inertial currents generated by wind events. Inertial period currents also dominate the near-bed currents on the continental shelf and in Sporades Basin. On the canyon slope, however, tidal currents are the dominant ones; these relate to the amplification of the internal tide, within the submarine canyon. Measured near-bed currents are below the threshold for sediment movement in the case of flat seabeds, although they are capable to inhibit deposition of the settling clayey particles. This contributes also to a further offshore dispersal of riverine sediments. Moreover, increased bed roughness due to benthic activity can cause resuspension for lower current speeds.

Bed shear stress, surface shape and velocity field near the tips of dam-breaks, tsunami and wave runup

An analytical model is presented for the 2DV flow-structure, bed shear stress and surface shape of the tips of dam-break waves, tsunami- or wave run-up. This model differs from previous analytical models, which have taken the usual ‘hydraulics approach’, describing bed-parallel velocities only and expressing the bed shear stress, τ in terms of a friction factor from steady, uniform flow and an ad-hoc velocity. The 2DV model presented here gives a simple, rational explanation for the fact that the boundary layer is very thin at the contact point. In turn, this explains the measurements of bed shear stresses, which decay with distance s from the tip, or at a fixed point, with time t since passage of the tip. The manner of τ−decay depends on the growth of the boundary layer thickness δ with distance from the tip. The details of this boundary layer growth depend on challenging turbulence features for unsteady, non-uniform flows over rough and usually mobile beds. For illustrative purposes, details are given for the simple example of δ = (vtt)1/2 = (vts/c)1/2, where vt is the nominal eddy-viscosity and c is the speed of the tip, assumed to progress with constant form. With this variation of δ, the model gives τ ∼ t−1/2 = (s/c)−1/2, which is in good qualitative agreement with measurements. Subsequently, this quasi-steady model gives h ∼ s1/4 for the depth on a horizontal bed, also in good agreement with experiments. The 2DV flow pattern includes surface particles drifting towards the tip and eventually impacting on the bed at the contact point with full forward momentum and large bed-normal velocity. As well as large local bed shear stresses, this tip-flow pattern involves large vertical accelerations and associated large localized pressures, which are likely to be important for the sediment entrainment under the tip. The eddy viscosity is the only tuning parameter in this simple initial model. Based on shear-stress and depth measurements from laboratory settings, with tip propagation speeds of the order 2 m/s, one finds that the required eddy viscosity is in the range 1 × 10−6 - 18 × 10−6 m2/s increasing with increasing bed roughness in the range from smooth beds to beds of 2.85 mm fixed sand-grains.

Bed load transport in a very steep mountain stream (Riedbach, Switzerland): Measurement and prediction

AbstractCompared to lower‐gradient channels, steep mountain streams typically have rougher beds and shallower flow depths, making macro‐scale flow resistance (due to, e.g., immobile boulders and irregular bedforms) more important as controls on sediment transport. The marked differences in hydraulics, flow resistance, and grain mobility between steep and lower‐gradient streams raise the question of whether the same equations can predict bed load transport rates across wide ranges of channel gradients. We studied a steep, glacier‐fed mountain stream (Riedbach, Ct. Valais, Switzerland) that provides a natural experiment for exploring how stream gradients affect bed load transport rates. The streambed gradient increases over a 1 km stream reach by roughly one order of magnitude (from 3% to 38%), while flow discharge and width remain approximately constant. Sediment transport rates were determined in the 3% reach using Bunte bed load traps and in the 38% reach using the Swiss plate geophone system. Despite a ten‐fold increase in bed gradient, bed load transport rates did not increase substantially. Observed transport rates for these two very different bed gradients could be predicted reasonably well by using a flow resistance partitioning approach to account for increasing bed roughness (D84 changes from 0.17 m to 0.91 m) within a fractional bed load transport equation. This suggests that sediment transport behavior across this large range of steep slopes agrees with patterns established in previous studies for both lower‐gradient and steep reaches, and confirms the applicability of the flow resistance and bed load transport equations at very steep slopes.

Salt Dynamics in Well-Mixed Estuaries: Importance of Advection by Tides

AbstractUnderstanding salt dynamics is important to adequately model salt intrusion, baroclinic forcing, and sediment transport. In this paper, the importance of the residual salt transport due to tidal advection in well-mixed tidal estuaries is studied. The water motion is resolved in a consistent way with a width-averaged analytical model, coupled to an advection–diffusion equation describing the salt dynamics. The residual salt balance obtained from the coupled model shows that the seaward salt transport driven by river discharge is balanced by the landward salt transport due to tidal advection and horizontal diffusion. It is found that the tidal advection behaves as a diffusion process, and this contribution is named tidal advective diffusion. The horizontal diffusion parameterizes processes not explicitly resolved in the model and is called the prescribed diffusion. The tidal advective diffusion results from the correlation between the tidal velocity and salinity and can be explicitly calculated with the dominant semidiurnal water motion. The sensitivity analysis shows that tidal advective diffusivity increases with increasing bed roughness and decreasing vertical eddy viscosity. Furthermore, tidal advective diffusivity reaches its maximum for moderate water depth and moderate convergence length. The relative importance of tidal advective diffusion is investigated using the residual salt balance, with the prescribed diffusion coefficient obtained from the measured salinity field. The tidal advective diffusion dominates the residual salt transport in the Scheldt estuary, and other processes significantly contribute to the residual salt transport in the Delaware estuary and the Columbia estuary.

A parametrical study on secondary flow in sharp open-channel bends: experiments and theoretical modelling

The dependence of curvature-induced secondary flow on the curvature ratio H/R (H is the average flow depth and R is the centreline radius of curvature), the Froude number Fr, and the dimensionless roughness coefficient Cf was systematically investigated in a series of 25 experiments in a laboratory flume. The investigated H/R values were (±0.005) 0.050, 0.073, 0.112 and 0.151, the Froude numbers were (±0.03) 0.10, 0.23, 0.32, 0.40 and 0.50, and roughness heights of the sediment bed were 0.002 m and 0.067 m. With width-to-depth ratios of 4.9 to 17.1, these experiments are more representative of small and medium-size rivers than the largest rivers on earth. Commonly used parameterizations for secondary flow predict the normalized magnitude to grow linearly with H/R, and to decrease with increasing bed roughness. This behaviour was only observed in mildly curved flows. The normalized magnitude was found to grow less than linearly with H/R in moderately curved flows, and to reach a maximum value in strongly curved flow. This confirms observations by Blanckaert (2009), who called this phenomenon the saturation of the secondary flow. He attributed the less than linear growth to nonlinear interactions between the primary and the secondary flows. For high values of H/R, secondary flow was found to be stronger over the very rough gravel bed than over the smoother sand bed. This is explained by the nonlinear interactions between the primary and secondary flows that are less pronounced over the rougher bed. The experiments did not reveal any dependence of the secondary flow on Fr. Predictions of the magnitude of the secondary flow with the nonlinear model of Blanckaert and de Vriend (2003, 2010) agreed very well with the experimental data over the entire range of investigated experimental conditions. Nonlinear effects are always important when the parameter Cf−0.25H/R is larger than an indicative value of 0.1, and negligible when this parameter is smaller than 0.05. These values allow a quantitative and objective distinction between mildly, moderately and sharply curved open-channel flow.

Near-bed and surface flow division patterns in experimental river bifurcations

Understanding channel bifurcation mechanics is of great importance for predicting and managing multichannel river processes and avulsion in distributary river deltas. To date, research on river channel bifurcations has focused on factors determining the stability and evolution of bifurcations. It has recently been shown that, theoretically, the nonlinearity of the relation between sediment transport and flow discharge causes one of the two distributaries of a (slightly) asymmetrical bifurcation to grow and the other to shrink. The positive feedback introduced by this effect results in highly asymmetrical bifurcations. However, there is a lack of detailed insight into flow dynamics within river bifurcations, the consequent effect on bed load flux through bifurcating channels, and thus the impact on bifurcation stability over time. In this paper, three key parameters (discharge ratio, width-to-depth ratio, and bed roughness) were varied in order to examine the secondary flow field and its effect on flow partitioning, particularly near-bed and surface flow, at an experimental bifurcation. Discharge ratio was controlled by varying downstream water levels. Flow fields were quantified using both particle image velocimetry and ultrasonic Doppler velocity profiling. Results show that a bifurcation induces secondary flow cells upstream of the bifurcation. In the case of unequal discharge ratio, a strong increase in the secondary flow near the bed causes a larger volume of near-bed flow to enter the dominant channel compared to surface and depth-averaged flow. However, this effect diminishes with larger width-to-depth ratio and with increased bed roughness. The flow structure and division pattern will likely have a stabilizing effect on river channel bifurcations. The magnitude of this effect in relation to previously identified destabilizing effects is addressed by proposing an adjustment to a widely used empirical bed load nodal-point partition equation. Our finding implies that river bifurcations can be stable under a wider range of conditions than previously thought. Key Points Secondary flow in symmetrical bifurcations causes strong near-bed flow curvature A disproportional amount of near-bed flow enters the dominant downstream channel Flow curvature adds a stabilizing feedback on bifurcation evolution.

Hydraulic characteristics and habitat suitability in presence of woody debris: A flume experiment

Data from a flume experiment were used to explore the modified hydraulic conditions and habitat suitability in streams where feeding of large woody debris (LWD) is present. Feeding of LWD was simulated by insertion of wood dowels with varying diameter and length. Two processes were mimicked, namely (i) lumped LWD load, and (ii) distributed LWD load. Lumped load may occur for wood coming either from upstream or from a tributary, and entering the stream of interest in one only section. Distributed load occurs for wood entering along the considered stream, in several sections. Distributed wood income resulted in homogeneously increased bed roughness, leading to increased flow depth and decreased velocity, whereas lumped input of wood from upstream resulted in larger local clustering and change of the flow properties, but with less influence on the distributed hydraulic properties. A method is proposed to predict bulk flow properties in presence of LWD. Then, a simple approach is used based upon the concept of wetter usable area WUA to investigate modified habitat conditions for fish species in presence of woody debris. An application to a real world case study from the literature is then shown, where increasing density of wood increases habitat availability for colonization by fish guilds.

Evaluating the controls of shear stress, sediment supply, alluvial cover, and channel morphology on experimental bedrock incision rate

We explored the dependence of experimental bedrock erosion rate on shear stress, bed load sediment flux, alluvial bed cover, and evolving channel morphology. We isolated these variables experimentally by systematically varying gravel sediment flux Qs and water discharge Qw in a laboratory flume, gradually abrading weak concrete “bedrock.” All else held constant, we found that (1) erosion rate was insensitive to flume‐averaged shear stress, (2) erosion rate increased linearly with sediment flux, (3) erosion rate decreased linearly with the extent of alluvial bed cover, and (4) the spatial distribution of bed cover was sensitive to local bed topography, but the extent of cover increased with Qs/Qt (where Qt is flume‐averaged transport capacity) once critical values of bed roughness and sediment flux were exceeded. Starting from a planar geometry, erosion increased bed roughness due to feedbacks between preferential sediment transport through interconnected topographic lows, focused erosion along these zones of preferential bed load transport, and local shear stresses that depended on the evolving bed morphology. Finally, continued growth of bed roughness was inhibited by imposed variability in discharge and sediment flux, due to changes in spatial patterns of alluvial deposition and impact wear. Erosion was preferentially focused at lower bed elevations when the bed was cover‐free, but was focused at higher bed elevations when static alluvial cover filled topographic lows. Natural variations in discharge and sediment flux may thus stabilize and limit the growth of roughness in bedrock channels due to the effects of partial bed cover.

Flow over a fixed rough dune

Using a laser Doppler velocimeter (LDV) system, experiments were carried out to obtain detailed information on the flow over a long train of fixed two-dimensional rough-wall dunes. The experiments provide quantitative data on conventionally averaged velocity and turbulence parameters, as well as insights on the flow structures that play a crucial role in sediment transport. The results indicate that the shape of the dunes has a major influence on the flow features, whereas the effect of near-bed roughness is limited to the wall region extending to a distance of about 80% of the dune height about the crest. However, the length of the separation eddy seems to be influenced by the near-bed roughness. As in flow over a smooth dune, the turbulence data reveal large peaks along a line roughly coinciding with the shear layer emanating from the separation at the crest. However, the location of the peak turbulence is farther extended into the flow away from the bed with increasing bed roughness. Moreover, with increasing roughness the magnitude of the peak is also reduced. A secondary peak in the streamwise turbulence profiles is noticed at some distance from the bed. This feature is not influenced by the near-bed roughness and indicates sustenance of turbulence generated by the previous dune and relates to the convection of the upstream flow history. Based on the results obtained at several stations between two successive dune crests, one can conclude that the quantitative effect of roughness itself is limited to the near-bed region in the measurements at all levels.

Frazil flocculation and secondary nucleation in a counter-rotating flume

The processes of secondary nucleation and flocculation of frazil ice are complex and as yet relatively poorly understood. In order to better understand the effect that turbulence intensity has on these processes, a series of experiments were undertaken at the Hydraulics Research & Testing Facility in the University of Manitoba using a counter-rotating flume. Five sets of bed plates ranging in roughness from smooth PVC to 20 mm gravel were used to generate the turbulence in the flume. Velocity measurements under ice-free conditions were made using a constant temperature anemometer with a conical hot-film probe. The ability to rotate the flume walls at any given rate enabled the researchers to perform experiments where the average velocity was kept constant, whereas the turbulence intensity increased with increasing bed roughness. Measurements of water temperature, air temperature, and digital images taken during ice formation were analyzed. A multiple linear regression model using turbulence intensity and the minimum water temperature as the two independent variables was found to reasonably model the rate of secondary nucleation, and indicated that higher levels of turbulence and colder temperatures increased the rate of secondary nucleation. Initial analysis of the digital images demonstrates that turbulence intensity also increases the uniformity of the vertical distribution of frazil particles and tends to inhibit frazil flocculation.

Bed roughness effects on boundary‐layer turbulence and consequences for odor‐tracking behavior of blue crabs (Callinectes sapidus)

We employed laser‐induced fluorescence (LIF) measurements of odorant plume structure and behavioral observations to examine how turbulence affects the three‐dimensional structure of odorant plumes and subsequently mediates olfactory search efficiency and success in our model organism, the blue crab (Callinectes sapidus). The turbulent characteristics of two laboratory flumes (one for behavioral quantification, one for signal‐structure quantification) were systematically varied by changing the bed substrate roughness to create smooth, transitional, and fully rough flow conditions. Generally, increasing bed roughness caused greater mixing, decreased the time‐averaged odorant concentration and concentration variance, and increased the plume width and homogeneity. Foraging success and speed of blue crabs attempting to locate the odorant source both declined consistently with increasing bed roughness. The variation in signal structure at the height of the antennules among bed‐roughness treatments explains the observed behavior differences in crab foraging speed. In contrast, steering (path linearity) appeared to be unaffected by bed‐roughness‐induced turbulence. The transverse correlation function for odorant concentration at sensors separated across the width of the plume was examined among bed‐roughness treatments, and, ultimately, the correlation function was found to be related to the spatial position of tracking blue crabs. The spatial arrangement of blue crab chemosensors combined with the three‐dimensional structure of odor plumes account for the differential effects of turbulence on the speed and success of crab‐tracking behavior.

Effect of Cerastoderma edule density on near-bed hydrodynamics and stability of cohesive muddy sediments

Flume experiments were performed to investigate the impact of cockle density ( Cerastoderma edule — 0, 47, 141 and 312 individuals m − 2 ) on near-bed hydrodynamics and sediment erodability. Undisturbed cohesive muddy sediment was collected by coring at an intertidal site in the Tamar estuary (SW England) and placed in the annular flumes. Cockles at the required density were added to the surface sediments, allowed to bury, and were then exposed for 24 h to sinusoidal cycles of currents (3 to 18 cm s − 1 ) which simulated 6-h tidal cycles comparable to the Tamar site. During this period the cockles adopted their usual behaviour of burrowing, suspension feeding and shell valve adductions. After 24 h, current speeds were increased from 5 to 50 cm s − 1 in 11 steps, and current velocities, turbulent kinetic energy (TKE), and suspended sediment concentration (SSC) were measured. Current speeds decreased and turbulence increased within 3 cm of the sediment surface as a function of cockle density. Sediment resuspension increased with increasing cockle density: SSC at 0.5 m s − 1 = 156, 574, 1045 and 2253 mg L − 1 for 0, 47, 141 and 312 animals m − 2 respectively. Enhanced sediment erosion was due to increased bioturbation and bed roughness. Critical erosion velocity decreased from 26 to 8 cm s − 1 with increasing cockle density. Shear stress, measured in terms of TKE, increased at 0.5 and 1 cm above the bed for 141 and 312 animals m − 2 and up to 2 cm above the bed for 312 animals m − 2 , reflecting the increased bed roughness due to bioturbation. However, the critical erosion shear stress was relatively independent of cockle density (0.225 and 0.151 N m − 2 for 0 and 312 animals m − 2 respectively). The valve adduction frequency of the cockles was also measured in response to increasing SSC. The frequency increased from 1.2 to 16.3 adductions h − 1 with increasing SSC from 13 to 308 mg L − 1 . It represents a significant behavioural response, creating positive feedback with increased SSC further enhancing sediment disturbance and resuspension.

Late Maastrichtian chalk mounds, Stevns Klint, Denmark — Combined physical and biogenic structures

Upper Maastrichtian chalk exposed at the Sigerslev quarry, Stevns Klint, Denmark is characterized by wavy and mound-like bedding geometries outlined by bands of black flint nodules. Four morphological elements are recognized, although bedding geometries are highly variable: southward migrating mounds, eastward migrating mounds, chalk waves and evenly bedded chalk. The mounds are interpreted as having been formed by currents carrying fine-grained suspended sediment which was primarily deposited on the up-current mound flanks. Bryozoans were prolific on the up-current flanks and mound summits, which stabilized the mounds, increased bed roughness and the overall accumulation rate. However, accumulation thicknesses do not correlate consistently with bryozoan density. The bryozoans were therefore important for the formation of the mounds, but the distribution of bryozoans did not solely determine depositional thickness across a mound and thus mound growth pattern. Relatively long wavelength wavy-bedded chalk show gentle convex-up geometries and would probably be described as sediment waves if recognized in seismic sections. The chalk waves were deposited under weaker current velocities than those active during mound formation. The exposed succession is topped by more evenly bedded chalk which was deposited by quiet pelagic fall-out of fine-grained material. The whole succession was deposited on the upper part of the northern flank of a large WNW–ESE trending 3 km wide depositional ridge with an amplitude of 35−40 m formed by contour-parallel WNW-ward flowing bottom currents. The mounds may have been deposited by regional bottom currents, or by spill-over currents from the valley south of the large ridge. The succession was deposited during varying bottom current intensities and the depositional architecture indicates a complex and dynamic environment. The depositional style seems to be controlled by the interplay and relative importance of two end-member processes; those that were purely physical and related to the interaction between available grain sizes and hydrodynamics, and those that were largely biological and associated with benthic growth and sediment trapping of mainly bryozoans. Examples of end-member conditions are known from Haute Normandy, France and the lower Danian of Stevns Klint. The structures described here adds to the considerable complexity of depositional styles in chalk.

Estimation of bed shear stress using the turbulent kinetic energy approach—A comparison of annular flume and field data

Annular flumes are often used to quantify the interactions between hydrodynamics, biological activity and sediment dynamics. Therefore it is essential that experimental laboratory flume systems adequately replicate natural conditions occurring in the field. This paper applies the turbulent kinetic energy (TKE) approach to the determination of bed shear stresses ( τ 0) on natural sediments from several sites in southern England and briefly discusses the advantages of this method compared to alternatives (log-profile) in the context of a combined flume and field study. Sediments studied varied in roughness length from 0.0013–1.18×10 −3 m (drag coefficient 0.0017–0.0204) and exhibited a non-linear increase in bed shear stress with increasing current velocity. The slope of this relationship increased also with increasing bed roughness. Several ‘smooth’ sediments (roughness length <0.0036×10 −3 m, C D <0.0024) showed very similar relationships between bed shear stress and current velocity, enabling estimation of bed shear stress from velocity measurements alone. Results from flume studies showed the suitability of open cell polyurethane foam to act as a surrogate smooth sediment in laboratory investigations. Data presented here for natural sediments shows that comparison of bed shear stress versus current velocity in a laboratory annular flume with data for the same sediments in the field reveal very similar relationships, highlighting the suitability of laboratory annular flume systems as platforms to study critical bed shear stress and critical erosion velocity of natural sediments under near-natural conditions.