Bedform Migration Research Articles

Shaler: in situ analysis of a fluvial sedimentary deposit on Mars

AbstractThis paper characterizes the detailed sedimentology of a fluvial sandbody on Mars for the first time and interprets its depositional processes and palaeoenvironmental setting. Despite numerous orbital observations of fluvial landforms on the surface of Mars, ground‐based characterization of the sedimentology of such fluvial deposits has not previously been possible. Results from the NASA Mars Science Laboratory Curiosity rover provide an opportunity to reconstruct at fine scale the sedimentary architecture and palaeomorphology of a fluvial environment on Mars. This work describes the grain size, texture and sedimentary facies of the Shaler outcrop, reconstructs the bedding architecture, and analyses cross‐stratification to determine palaeocurrents. On the basis of bedset geometry and inclination, grain‐size distribution and bedform migration direction, this study concludes that the Shaler outcrop probably records the accretion of a fluvial barform. The majority of the outcrop consists of large‐scale trough cross‐bedding of coarse sand and granules. Palaeocurrent analyses and bedform reconstruction indicate that the beds were deposited by bedforms that migrated towards the north‐east, across the surface of a bar that migrated south‐east. Stacked cosets of dune cross‐bedding suggest aggradation of multiple bedforms, which provides evidence for short periods of sustained flow during Shaler deposition. However, local evidence for aeolian reworking and the presence of potential desiccation cracks within the outcrop suggest that fluvial deposition may have been intermittent. The uppermost strata at Shaler are distinct in terms of texture and chemistry and are inferred to record deposition from a different sediment dispersal system with a contrasting provenance. The outcrop as a whole is a testament to the availability of liquid water on the surface of Mars in its early history.

Prediction of Flow Regimes and Bedform Migration

Prediction of Flow Regimes and Bedform Migration

Regulation of benthic oxygen fluxes in permeable sediments of the coastal ocean

AbstractLarge areas of the oceanic shelf are composed of sandy sediments through which reactive solutes are transported via porewater advection fueling active microbial communities. The advective oxygen transport in permeable sands of the North Sea was investigated under in situ conditions using a new benthic observatory to assess the dynamic interaction of hydrodynamics, sediment morphodynamics, and oxygen penetration depth. During 16 deployments, concurrent measurement of current velocity, sediment topography, and porewater oxygen concentration were carried out. In all cases the oxyclines were found at depths of 1–6 cm, correlating with the topography of stationary and migrating bedforms (ripples). Different conditions in terms of bottom water currents and bedform migration led to fluctuating oxygen penetration depths and, hence, highly variable redox conditions in up to 2.5 cm thick layers beneath the surface. Volumetric oxygen consumption rates of surface sediments were measured on board in flow‐through reactors. Bedform migration was found to reduce consumption rates by up to , presumably caused by the washout of organic carbon that is otherwise trapped in the pore space of the sediment. Based on the observations we found oxygen penetration depths to be largely controlled by oxygen consumption rates, grain size, and current velocity. These controlling variables are summarized by an adapted Damköhler number which allows for prediction of oxygen penetretion depths based on a simple scaling law. By integrating the oxygen consumption rates over the oxygen penetration depth, oxygen fluxes of 8–34 mmol m−2 d−1 were estimated.

Numerical and experimental study of flow over stages of an offset merger dune interaction

Results of a coordinated research effort on unidirectional turbulent flows over canonical barchan dunes at high Reynolds number are presented. Large-eddy simulations (LES) and experiments are conducted under inertial-dominated flow conditions, thus capturing dynamics essential to fully understanding processes responsible for observed patterns of asymmetric erosion and migration. A series of dune field topographies have been considered wherein a small “impactor” dune is positioned at a series of positions upflow of a large “parent” dune, from a spanwise-offset position; this interaction constitutes a so-called “offset interaction” Kocurek and Ewing [14]. The small dune is geometrically similar, but one-eighth the volume of the large dune. Since migration rate is inversely proportional to volume, the prescribed volumetric ratio ensures that the synthetic topographies replicate instantaneous configurations exhibited during actual dune interactions in a laboratory or natural setting. In this sense, the static dune configurations provide a means to understand flow processes responsible for patterns of erosion and deposition that induce interactions. Experimental measurement and LES are both used to study these configurations, with strong agreement reported between resultant datasets. We report that flow channeling in the interdune space between the upflow and downflow dune induces a mean flow heterogeneity – termed “wake veering” – in which the location of maximum momentum deficit in the dune wake is spanwise displaced. We also report elevated turbulent stresses in the channeling region. These results are used to explain the mechanisms responsible for simultaneous merging and ejection between the small and large dunes. Finally, spatial distributions of surface stress from LES have been used to identify locations of elevated erosion and, therefore, to predict bedform migration patterns. Results show that locations of minimal erosion – whether associated with upflow sheltering or with vanishing spatial gradients of dune height – constitute spatial “junctions” of coalescing, proximal dunes. In the absence of upwind sheltering, or in locations with the least upwind sheltering, turbulent mixing facilitates downward fluxes of high momentum fluid. This augments downwind dune erosion, cumulatively accelerating the migration of downwind dunes.

Subaqueous Barchans and Plane Beds from Deposition of Quartz Silt

AbstractThe suspension flow of quartz silt (geometric mean grain size of 4.15 μm), in unfavorable conditions for deposition, is the motivation for the development of different bed morphologies. Particles deposit over a plane nonerodible surface and develop well-defined barchan ripples, barchanoids, and plane beds in flume experiments. Bedload transport of quartz silt by bedform migration is several orders of magnitude smaller than the suspended transport. The final bed morphology is controlled by the suspended sediment concentration and running time of the experiment. The average dimensions of the bedforms after 1-day experiments are 4.93 cm wide, 10.33 cm long, and 0.45 cm high. Cohesive plane beds appear after 2-day and 3-day experiments with very high sediment concentrations (≥22.5 kg/m3). Viscous effects are deemed relevant for the formation of the beds.

Residual currents and bedform migration in a natural tidal inlet (Knudedyb, Danish Wadden Sea)

The morphology and hydrodynamics of the natural tidal inlet Knudedyb in the Danish Wadden Sea were investigated by the analysis of high resolution bathymetric data and hydrodynamic numerical modeling. In contrast to the expected anticlockwise pattern similar to the other inlets in the Wadden Sea, a clockwise tidal residual current was found, which drives the sediment transport and results from the presence of a confluent meander bend. The channel is draped by bedforms of several hierarchical scales (on average, approximately 155m long and 2.3m high), with average sizes decreasing from south to north and seaward (i.e., westward). Primary bedforms in the area are mostly asymmetric ebb-directed and migrate in the ebb direction in the order of 3myr−1. Bedform sections at the northern channel flank show more symmetrical profiles; crests migrate flood-ward, leading to crestal flexing in the central channel. Lateral recirculation cells develop during ebb tide on both sides of the channel seaward of a bend, before the tidal reversal occurs in the rest of the model domain. As a result of the longer flood phase, a main clockwise residual eddy exists in the middle reach of the channel. Bedform migration patterns and hydrodynamic simulations reveal that the pronounced tidal asymmetry in the channel is enhanced by the effects of the channel morphology on the confined flow (at low tide).

First wide-angle view of channelized turbidity currents links migrating cyclic steps to flow characteristics.

Field observations of turbidity currents remain scarce, and thus there is continued debate about their internal structure and how they modify underlying bedforms. Here, I present the results of a new imaging method that examines multiple surge-like turbidity currents within a delta front channel, as they pass over crescent-shaped bedforms. Seven discrete flows over a 2-h period vary in speed from 0.5 to 3.0 ms−1. Only flows that exhibit a distinct acoustically attenuating layer at the base, appear to cause bedform migration. That layer thickens abruptly downstream of the bottom of the lee slope of the bedform, and the upper surface of the layer fluctuates rapidly at that point. The basal layer is inferred to reflect a strong near-bed gradient in density and the thickening is interpreted as a hydraulic jump. These results represent field-scale flow observations in support of a cyclic step origin of crescent-shaped bedforms.

Qualitative and quantitative acoustic mapping of bedform dynamics

The qualitative and quantitative assessment of bedform dynamics has been a continuous challenge for conventional river monitoring methods due to the vexing problems associated with the limited capabilities to observe the bedforms in-situ and the intrusive nature of the instruments used for quantification of their movement. The advent of the new generation of acoustic- and image-based instruments has brought considerable promise for the experimental investigations in this area by allowing observations of the bedforms from close range along with possibilities of non-intrusive quantification of their movement with unprecedented spatial and temporal resolutions.This paper reports proof-of-concept tests for quantifying the dynamics of the bedform migration using a combination of concepts from two representatives of the new nonintrusive instruments: acoustic survey and processing techniques associated with particle image velocimetry. Given the combination of mapping and velocimetry operating principles, we call the technique acoustic mapping velocimetry (AMV). This hybrid technique has potential to track the planar migration of the bedform and quantify the velocity of the moving bedform fronts. Described herein is a laboratory experiment illustrating the implementation of the AMV concept for quantifying bedform migration.

Acoustic mapping velocimetry

AbstractKnowledge of sediment dynamics in rivers is of great importance for various practical purposes. Despite its high relevance in riverine environment processes, the monitoring of sediment rates remains a major and challenging task for both suspended and bed load estimation. While the measurement of suspended load is currently an active area of testing with nonintrusive technologies (optical and acoustic), bed load measurement does not mark a similar progress. This paper describes an innovative combination of measurement techniques and analysis protocols that establishes the proof‐of‐concept for a promising technique, labeled herein Acoustic Mapping Velocimetry (AMV). The technique estimates bed load rates in rivers developing bed forms using a nonintrusive measurements approach. The raw information for AMV is collected with acoustic multibeam technology that in turn provides maps of the bathymetry over longitudinal swaths. As long as the acoustic maps can be acquired relatively quickly and the repetition rate for the mapping is commensurate with the movement of the bed forms, successive acoustic maps capture the progression of the bed form movement. Two‐dimensional velocity maps associated with the bed form migration are obtained by implementing algorithms typically used in particle image velocimetry to acoustic maps converted in gray‐level images. Furthermore, use of the obtained acoustic and velocity maps in conjunction with analytical formulations (e.g., Exner equation) enables estimation of multidirectional bed load rates over the whole imaged area. This paper presents a validation study of the AMV technique using a set of laboratory experiments.

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.

The role of biophysical cohesion on subaqueous bed form size.

Biologically active, fine‐grained sediment forms abundant sedimentary deposits on Earth's surface, and mixed mud‐sand dominates many coasts, deltas, and estuaries. Our predictions of sediment transport and bed roughness in these environments presently rely on empirically based bed form predictors that are based exclusively on biologically inactive cohesionless silt, sand, and gravel. This approach underpins many paleoenvironmental reconstructions of sedimentary successions, which rely on analysis of cross‐stratification and bounding surfaces produced by migrating bed forms. Here we present controlled laboratory experiments that identify and quantify the influence of physical and biological cohesion on equilibrium bed form morphology. The results show the profound influence of biological cohesion on bed form size and identify how cohesive bonding mechanisms in different sediment mixtures govern the relationships. The findings highlight that existing bed form predictors require reformulation for combined biophysical cohesive effects in order to improve morphodynamic model predictions and to enhance the interpretations of these environments in the geological record.

Variability in bedform morphology and kinematics with transport stage

AbstractBedform geometry is widely recognized to be a function of transport stage. Bedform aspect ratio (height/length) increases with transport stage, reaches a maximum, then decreases as bedforms washout to a plane bed. Bedform migration rates are also linked to bedform geometry, in so far as smaller bedforms in coarser sediment tend to migrate faster than larger bedforms in finer sediment. However, how bedform morphology (height, length and shape) and kinematics (translation and deformation) change with transport stage and suspension have not been examined. A series of experiments is presented where initial flow depth and grain size were held constant and the transport stage was varied to produce bedload dominated, mixed‐load dominated and suspended‐load dominated conditions. The results show that the commonly observed pattern in bedform aspect ratio occurs because bedform height increases then decreases with transport stage, against a continuously increasing bedform length. Bedform size variability increased with transport stage, leading to less uniform bedform fields at higher transport stage. Total translation‐related and deformation‐related sediment fluxes all increased with transport stage. However, the relative contribution to the total flux changed. At the bedload dominated stage, translation‐related and deformation‐related flux contributed equally to the total flux. As the transport stage increased, the fraction of the total load contributed by translation increased and the fraction contributed by deformation declined because the bedforms got bigger and moved faster. At the suspended‐load dominated transport stage, the deformation flux increased and the translation flux decreased as a fraction of the total load, approaching one and zero, respectively, as bedforms washed out to a plane bed.

Large-scale sediment waves and scours on the modern seafloor and their implications for the prevalence of supercritical flows

Large-scale (20m to 7km wavelength) bedforms are common on the seafloor, yet there is a lack of consensus on how they form and thus what to call them. We conducted statistical analysis on a dataset of 82 seafloor bedforms that span a range of water depths and environments. The data form three distinct groups: 1) small-scale (20–300m wavelength) sediment waves with mixed relief made of medium sand to cobble-sized sediment that form in confined settings, which we call small sediment waves; 2) large-scale (300–7000m wavelength) sediment waves with mixed relief made of fine-grained sediment that form in relatively unconfined settings, which we call large sediment waves; and 3) large-scale fully enclosed depressions in the seafloor, which we call scours. There is a statistically significant data gap in the size of bedforms between small sediment waves and large sediment waves that does not appear to be a sampling artefact. This data gap probably results from the environments in which sediment waves form being either confined (e.g. channel or canyon) or unconfined (e.g. open slope). Bedform migration direction is available for 36% of the data and includes small and large-scale sediment waves; of these examples all are shown to migrate up-current. Up-current migration is indicative of supercritical flow; thus this data suggests that supercritical flows operate in a wide range of environments and can generate both small and large sediment waves. Therefore, we suggest that small and large sediment waves form by similar processes despite the gap in bedform wavelength and sediment size. The migration direction for scours remains unknown. Scours may form from similar processes to small and large sediment waves, or alternatively they may be a completely separate bedform type that form when erosive flows exploit pre-existing defects in the seafloor. This novel statistical analysis of a global database shows that up-current migrating bedforms associated with supercritical flow are unusually widespread, and are recognised at two distinct scales.

Preservation of meandering river channels in uniformly aggrading channel belts

AbstractChannel belt deposits from meandering river systems commonly display an internal architecture of stacked depositional features with scoured basal contacts due to channel and bedform migration across a range of scales. Recognition and correct interpretation of these bounding surfaces is essential to reconstruction of palaeochannel dimensions and to flow modelling for hydrocarbon exploration. It is therefore crucial to understand the suite of processes that form and transfer these surfaces into the fluvial sedimentary record. Here, the numerical model ‘NAYS2D’ is used to simulate a highly sinuous meandering river with synthetic stratigraphic architectures that can be compared directly to the sedimentary record. Model results highlight the importance of spatial and temporal variations in channel depth and migration rate to the generation of channel and bar deposits. Addition of net uniform bed aggradation (due to excess sediment input) allows quantification of the preservation of meander morphology for a wide range of depositional conditions. The authors find that the effect of vertical variation in scouring due to channel migration is generally orders of magnitude larger than the effect of bed aggradation, which explains the limited impact bed aggradation has on preservation of meander morphology. Moreover, lateral differences in stratigraphy within the meander belt are much larger than the stratigraphic imprint of bed aggradation. Repeatedly produced alternations of point bar growth followed by cut‐off result in a vertical trend in channel and scour feature stacking. Importantly, this vertical stacking trend differs laterally within the meander belt. In the centre of the meander belt, the high reworking intensity results in many bounding surfaces and disturbed deposits. Closer to the margins, reworking is infrequent and thick deposits with a limited number of bounding surfaces are preserved. These marginal areas therefore have the highest preservation potential for complete channel deposits and are thus best suited for palaeochannel reconstruction.

The impact of bedform migration on benthic oxygen fluxes

AbstractPermeable sediments are found wide spread in river beds and on continental shelves. The transport of these sediments is forced by bottom water currents and leads to the formation of bedforms such as ripples and dunes. The bottom water flow across the bedforms results in pressure gradients that drive pore water flow within the permeable sediment and enhance the supply of reactive substrates for biogeochemical processes. This transport‐reaction system has been extensively studied for the case of stationary bedforms, whereas bedform migration—the most ubiquitous form of sediment transport—has been often ignored. To study the impact of sediment transport on pore water flow, we incorporated an empirical model of bedform migration into a numerical transport‐reaction model for porous media, using oxygen as reactive solute. The modeled oxygen flux changes significantly as soon as the sediment divides into an upper mobile layer (migrating bedform) and a stationary layer underneath. The bedform is increasingly flushed with oxic bottom water, whereas pressure gradients and pore water flow reverse at increasing rate underneath the bedform. This suppresses net pore water displacement and reduces the oxygen penetration depth up to 90%. In effect, the overall oxygen uptake decreases significantly with bedform migration although bottom water velocities increase. This counterintuitive effect is systematically described for a range of different sediment types, current velocities, and respiration rates and should be considered in future studies.

The formation and migration of sand ripples in closed conduits: Experiments with turbulent water flows

The transport of solid particles by a fluid flow is frequently found in nature and industry. Some examples are the transport of sand in rivers and hydrocarbon pipelines. When the shear stresses exerted by a fluid flow on a granular bed remain moderate, some grains are set in motion without fluidizing the bed; the moving grains form a layer, known as bed load, that moves while maintaining contact with the fixed part of the bed. Under bed load conditions, the granular bed may become unstable, generating ripples and dunes. Sand ripples are commonly observed in closed conduits and pipes such as in petroleum pipelines, sewer systems, and dredging lines. Although of importance for many scientific domains and industrial applications, the formation of ripples in closed conduits is not well understood, and the problem is still open. This paper presents an experimental study on the formation and migration of sand ripples under a turbulent closed-conduit flow and bed-load conditions. In our experiments, fully-developed turbulent water flows were imposed over a granular bed of known granulometry in a transparent channel, and bed load took place. For different water flow rates and grain diameters, the growth and migration of bedforms were filmed by a high-definition camera, and a numerical code was developed to determine the wavelength and celerity of the bedforms from the acquired images. The obtained results are compared with published stability analyses.

Free surface flows over partially erodible beds by a continuous finite element method

This paper presents a continuous finite element model for shallow water equations coupled with a bedload sediment conservation law. The model incorporates a sign-preserving conservative correction based on the flux correction transport methodology. Complete algorithm permits simulation of flows with evolutionary boundaries and erodible–non erodible interfaces under severe conditions without spurious mass exchanges and oscillations. Numerical experiments explore the efficacy of the procedure for dam-break problems on channels with dry beds, for partially and totally erodible beds, and for migration of small bedforms. Computation of real river morphodynamics under intense flooding illustrates practical application of the model.

The impact of Hurricane Sandy on the shoreface and inner shelf of Fire Island, New York: Large bedform migration but limited erosion

We investigate the impact of superstorm Sandy on the lower shoreface and inner shelf offshore the barrier island system of Fire Island, NY using before-and-after surveys involving swath bathymetry, backscatter and CHIRP acoustic reflection data. As sea level rises over the long term, the shoreface and inner shelf are eroded as barrier islands migrate landward; large storms like Sandy are thought to be a primary driver of this largely evolutionary process. The “before” data were collected in 2011 by the U.S. Geological Survey as part of a long-term investigation of the Fire Island barrier system. The “after” data were collected in January, 2013, ~two months after the storm. Surprisingly, no widespread erosional event was observed. Rather, the primary impact of Sandy on the shoreface and inner shelf was to force migration of major bedforms (sand ridges and sorted bedforms) 10’s of meters WSW alongshore, decreasing in migration distance with increasing water depth. Although greater in rate, this migratory behavior is no different than observations made over the 15-year span prior to the 2011 survey. Stratigraphic observations of buried, offshore-thinning fluvial channels indicate that long-term erosion of older sediments is focused in water depths ranging from the base of the shoreface (~13–16m) to ~21m on the inner shelf, which is coincident with the range of depth over which sand ridges and sorted bedforms migrated in response to Sandy. We hypothesize that bedform migration regulates erosion over these water depths and controls the formation of a widely observed transgressive ravinement; focusing erosion of older material occurs at the base of the stoss (upcurrent) flank of the bedforms. Secondary storm impacts include the formation of ephemeral hummocky bedforms and the deposition of a mud event layer.

The negligible effect of bed form migration on denitrification in hyporheic zones of permeable sediments

AbstractBed form celerity, the migration rate of ripples along a sediment bed, has previously been shown to have dramatic effects on oxygen distribution and transport within the hyporheic zone of permeable sediments. This has the potential to influence denitrification rates—in particular by increasing the coupling of nitrification and denitrification. To further understand this, we numerically modeled nitrogen cycling under migrating ripples. While the simulated oxygen profiles match with expected behavior, almost no effect on denitrification or coupled nitrification‐denitrification was observed with increasing celerity. Instead, denitrification rates were dominantly controlled by the flow velocity of water overlying the sediment.

Bedform contributions to cross-shore sediment transport on a dissipative beach

Field measurements of hydrodynamics, suspended sediment transport rates and bedform sediment transport rates were made in the intertidal section of a dissipative sandy beach (D50=0.26mm, slope=1/80) at Perranporth (UK). Pressure transducers, acoustic Doppler velocimeters, optical backscatter sensors and an acoustic sand ripple profiler were deployed for 12 tides, measuring in a range of wave heights from 0.5 to 2.2m, water depths from 1 to 6m, and in current strengths up to 0.4m/s. Data were analysed in terms of the distance to shore (x) normalised by the surf zone width (xs), and spanned the region 0.4<x/xs<3. Bedform heights up to 30cm and wavelengths 0.5 to 2.7m were recorded. Maximum wavelengths were observed just shoreward of the breakpoint. Bedforms were classified as sub-orbital, vortex ripples. Bedform migration was mostly onshore directed, and correlated with positive (onshore) wave skewness. Migration rates increased through the shoaling zone to a maximum of 1.5cm/min just shoreward of the breakpoint (x/xs=0.8). The bedform component of sediment transport was generally onshore directed, and maximum just shoreward of the breakpoint (0.021kg/m/s). Point measurements showed that the cross-shore suspended sediment transport 25cm above the bed was dominated by the mean component, with an offshore directed maximum at x/xs=0.5. Contributions to onshore transport were only made by the incident wave (gravity band) component. The total depth integrated suspended sediment transport was offshore directed and maximum in the mid surf zone (−0.16kg/m/s). The depth integrated suspended sediment transport dominated over the bedform sediment transport in the inner to mid surf zone (x/xs<0.5) and in the outer shoaling zone (x/xs>1.5). The fractional contribution of the shoreward directed bedform transport to the total absolute transport was up to 100%, and occurred broadly in the region of the breakpoint (0.5<x/xs<1.5). However, spatial averaging in the cross-shore indicated that a more realistic bedform contribution was up to 15% of the transport, with a maximum at x/xs=0.9. Results from this dissipative beach experiment generally agree with previous findings on intermediate beaches, steep beaches, and offshore sandbars.