Sandbar Migration Research Articles

Models and scales for cross‐shore sandbar migration

We investigate the long‐term (months to years) predictability of cross‐shore sandbar migration with two models that operate on different abstraction levels: (1) a coupled, cross‐shore waves‐currents‐bathymetric evolution model and (2) two data‐driven neural network models, based on simplified cross‐shore profile representations and daily averaged wave properties. For model calibration, training, and validation, we use a high‐resolution 15 yearlong profile data set collected at the Hasaki Oceanographic Research Station in Japan. Sandbar behavior at this field site is characterized by cycles of net‐offshore migration with a duration of 1–4 years. We find that all models can produce several general features of sandbar behavior at the studied field site, such as rapid offshore migration, slower onshore return, and net‐offshore migration. However, it is difficult to quantitatively predict the offshore‐directed trends in sandbar location over time scales of months to years. While simple linear models outperform more detailed nonlinear models, for all models it is difficult to predict long‐term sandbar behavior, because of error accumulation in the model's processes over time. Representing processes on a more abstract level (scale aggregation) alleviates error accumulation but does not completely overcome this problem.

On cross‐shore migration and equilibrium states of nearshore sandbars

The location of submerged sandbars in the nearshore zone changes over time in response to variability in wave conditions. Here, cross‐shore sandbar migration is studied with an empirical model consisting of a differential equation relating cross‐shore sandbar migration to wave forcing. Model parameters are fitted to data sets containing multiple years of daily‐observed positions of five sandbars at three field sites. From the fitted model parameters we determine the cross‐shore location of zero‐migration (equilibrium location), the stability of this equilibrium, and the rate (response time) at which a sandbar migrates toward or away from the equilibrium location. We find that for breaking waves a sandbar moves toward a stable, wave‐height‐dependent equilibrium location. For non‐ to slightly‐breaking waves, however, a sandbar moves away from the equilibrium location, implying that the beach profile evolves toward a state without submerged sandbars. The response times can differ up to a factor three for different sandbars at a particular field site, and a factor ten between sandbars at different sites. For breaking waves, response times are found to decrease with increasing wave height from months to days, while for non‐ to slightly‐breaking waves response times exceed several months. In general, response times remain larger than the timescale of variability in wave climate (several days), implying that sandbars spend most of their lifetime out of equilibrium with the wave forcing. The model is able to hindcast the relevant features of cross‐shore sandbar migration on timescales of days to several years, and outperforms a baseline prediction of no change for all sandbars. When the equilibrium location is reached by both the hindcasted and actual sandbar, the hindcast error depends on the accuracy of the predicted equilibrium location only, and no longer on the history of accumulated errors. As a result, sandbars that occasionally reach their equilibrium location can accurately be predicted from the wave forcings over their entire lifespan. However, it is difficult to predict the exact rate of the yearly to interannual trends that are observed at two field sites, because the sandbars at these sites never reach the offshore‐located equilibrium states associated with high waves.

Seasonal-scale nearshore morphological evolution: Field observations and numerical modeling

A coupled waves–currents-bathymetric evolution model (DELFT-3D) is compared with field measurements to test hypotheses regarding the processes responsible for alongshore varying nearshore morphological changes at seasonal time scales. A 2001 field experiment, along the beaches adjacent to Grays Harbor, Washington, USA, captured the transition between the high-energy erosive conditions of winter and the low-energy beach-building conditions typical of summer. The experiment documented shoreline progradation on the order of 10–20 m and on average approximately 70 m of onshore sandbar migration during a four-month period. Significant alongshore variability was observed in the morphological response of the sandbar over a 4 km reach of coast with sandbar movement ranging from 20 m of offshore migration to over 175 m of onshore bar migration, the largest seasonal-scale onshore migration event observed in a natural setting. Both observations and model results suggest that, in the case investigated here, alongshore variations in initial bathymetry are primarily responsible for the observed alongshore variable morphological changes. Alongshore varying incident hydrodynamic forcing, occasionally significant in this region due to a tidal inlet and associated ebb-tidal delta, was relatively minor during the study period and appears to play an insignificant role in the observed alongshore variability in sandbar behavior at kilometer-scale. The role of fully three-dimensional cell circulation patterns in explaining the observed morphological variability also appears to be minor, at least in the case investigated here.

Daily to interannual cross-shore sandbar migration: Observations from a multiple sandbar system

The most conspicuous characteristic of the cross-shore behaviour of nearshore multiple sandbar systems in sea-dominated settings is interannual net offshore migration (NOM), while that of similar systems in swell-dominated environments appears to be the on/offshore migration in response to individual wave events. Here we analyse an approximately 7.5-year data set (July 1999–February 2007) of daily 10-min time-exposure video images of the swell-dominated double-barred northern Gold Coast, Australia and show that events with wave heights exceeding three to four times the annual average can trigger NOM behaviour similar to that observed elsewhere. During four such events, we observed how the outer sandbar migrated some 75–100 m further offshore than usual and then decayed during slow onshore migration in the subsequent low-energy weeks. The next, moderately high wave event caused the ( ≈ 20 – 30 m / day ) offshore migration of the inner bar to become the new outer bar and the onshore birth of a new bar. Between the large wave events both the outer and the inner bars responded primarily to the seasonal signal in wave height, whereby both bars were located further offshore during the higher (lower) energy winter (summer) months. We suspect that bar size (height and width) may control whether a multiple sandbar system exhibits episodic NOM (as observed here) or interannual NOM. In contrast to interannual NOM, episodic NOM is steered by the order of storm events.

Onshore sandbar migration at Tairua Beach (New Zealand): Numerical simulations and field measurements

We observed the onshore migration (3.5 m/day) of a nearshore sandbar at Tairua Beach, New Zealand during 4 days of low-energy wave conditions. The morphological observations, together with concurrent measurements of waves and suspended sediment concentrations, were used to test a coupled, wave-averaged, cross-shore model. Because of the coarse bed material and the relatively low-energy conditions, the contribution of the suspended transport to the total transport was predicted and observed to be negligible. The model predicted the bar to move onshore because of the feedback between near-bed wave skewness, bedload, and the sandbar under weakly to non-breaking conditions at high tide. The predicted bathymetric evolution contrasts, however, with the observations that the bar migrated onshore predominantly at low tide. Also, the model flattened the bar, while in the observations the sandbar retained its steep landward-facing flank. A comparison between available observations and numerical simulations suggests that onshore propagating surf zone bores in very shallow water (< 0.25 m) may have been responsible for most of the observed bar behaviour. These processes are missing from the applied model and, given that the observed conditions can be considered typical of very shallow sandbars, highlight a priority for further field study and model development. The possibility that the excess water transported by the bores across the bar was channelled alongshore to near-by rip-channels further implies that traditional cross-shore measures to judge the applicability of a cross-shore morphodynamic model may be misleading.

Multivariate analysis of nonlinearity in sandbar behavior

Abstract. Alongshore sandbars are often present in the nearshore zones of storm-dominated micro- to mesotidal coasts. Sandbar migration is the result of a large number of small-scale physical processes that are generated by the incoming waves and the interaction between the wave-generated processes and the morphology. The presence of nonlinearity in a sandbar system is an important factor determining its predictability. However, not all nonlinearities in the underlying system are equally expressed in the time-series of sandbar observations. Detecting the presence of nonlinearity in sandbar data is complicated by the dependence of sandbar migration on the external wave forcings. Here, a method for detecting nonlinearity in multivariate time-series data is introduced that can reveal the nonlinear nature of the dependencies between system state and forcing variables. First, this method is applied to four synthetic datasets to demonstrate its ability to qualify nonlinearity for all possible combinations of linear and nonlinear relations between two variables. Next, the method is applied to three sandbar datasets consisting of daily-observed cross-shore sandbar positions and hydrodynamic forcings, spanning between 5 and 9 years. Our analysis reveals the presence of nonlinearity in the time-series of sandbar and wave data, and the relative importance of nonlinearity for each variable. The relation between the results of each sandbar case and patterns in bar behavior are discussed, together with the effects of noise. The small effect of nonlinearity implies that long-term prediction of sandbar positions based on wave forcings might not require sophisticated nonlinear models.

Numerical predictability experiments of cross‐shore sandbar migration

Surf zone sandbars, common features along the world's sandy coastlines, continuously change their position in response to time‐variable offshore wave conditions. Process‐based predictions of cross‐shore sandbar migration, relevant to the understanding of autonomous and artificially altered evolution of beaches, are intrinsically imprecise because of uncertainty in the model equations and, potentially, the sensitive dependence on the initial bathymetry. However, the magnitude of the resulting predictability limit and its dominant source are unknown. Here we show that cross‐shore sandbar migration on the time scale of years is deterministically forced rather than deterministically chaotic, and that the unpredictability of sandbar migration results primarily from model inadequacy during major wave events. Because the unpredictability of sandbar migration is related to the stochastic nature of the forcing, the predictability limit is not a fixed value but depends on the timing of the wave event. We anticipate that detailed experiments to understand nearshore evolution from the underlying first principles will eventually pay off by extending the range of plausible simulated behavior and, consequently, will increase our ability to understand and predict how coasts may respond to changing climate.

Modeling cross‐shore sandbar behavior on the timescale of weeks

We compare predictions of a coupled, wave‐averaged, cross‐shore waves‐currents‐bathymetric evolution model to observations of onshore and offshore nearshore sandbar migration. The observations span a 10‐ and 44‐day period with onshore/offshore bar migration at Duck, North Carolina, and at Hasaki, Kashima Coast, Japan, respectively, a 3.5‐month period of onshore bar migration at Duck, and a 22‐day period of offshore bar migration at Egmond, Netherlands. With best fit parameter values the modeled temporal evolution of the cross‐shore bed profiles agrees well with the observations. Model skill, defined as 1 minus the ratio of prediction to no‐change error variances, ranges from 0.50 at Egmond to 0.88 for the prolonged onshore bar migration at Duck. Localized (in time and space) reductions in model skill coincide with alongshore variations in the observed morphology. Consistent with earlier observations, simulated offshore bar migration takes place during storms when large waves break on the bar and is due to the feedback between waves, undertow, suspended sediment transport, and the sandbar. Simulated onshore bar migration is predicted for energetic, weakly to nonbreaking conditions and is due to the feedback between near‐bed wave skewness, bedload transport, and the sandbar, with negligible to small effects of bound infragravity waves and near‐bed streaming. Under small waves and conditions, when breaking and nonbreaking conditions alternate with the tide, the sandbar is predicted to remain stationary. The intersite differences in the optimum parameter values are, at least partly, induced by insensitivity to parameter variations, parameter interdependence, and errors in the offshore wave forcing.

Cross‐shore sediment transport on natural beaches and its relation to sandbar migration patterns: 2. Application of the field transport parameterization

A cross‐shore sediment transport parameterization (shape function) based on data from intensive field experiments carried out on five European beaches is incorporated into a simple time dependent model to explore its capability for reproducing sandbar generation and evolution in the medium term (O(months)). Model results are compared with bar‐crest migration patterns observed at Duck, North Carolina. The model comprises a simple wave transformation routine that accounts for linear shoaling and assumes a saturation law for wave decay inside the surf zone. Second‐order statistics derived from the cross‐shore distribution of wave heights are used to scale the sediment transport parameterizations which estimate the cross‐shore structure of the third and fourth velocity moments. Cross‐shore gradients in sediment fluxes, calculated with an energetics approach, produce bed elevation changes. The updated morphology affects the wave propagation in the next time step forming a fully coupled model for predicting sandbar evolution. Bar generation close to shore and the details of the sandbar evolution are successfully reproduced (R2 = 0.86) over the 77‐day period of the observations using default values for drag coefficients and efficiency factors. In accordance with the observations, the model shows rapid offshore sandbar migration in response to individual storms, and weak onshore bar movement for lower energy conditions. Owing to its simple nature, the model is not capable of reproducing the whole profile morphology especially in the trough (no trough generation), and it is only valid for replicating bar crest position. The results suggest that a convergence of sediment near the breakpoint (breakpoint hypothesis) combined with morphological feedback, can successfully explain the evolution of bar crest over periods of months from the details of forcing (incident waves and water levels).

A Field Study on the Characteristics of Sand Bar Migration under Eroding Conditions

A Field Study on the Characteristics of Sand Bar Migration under Eroding Conditions

Wave-induced sediment transport and onshore sandbar migration

The 25-m onshore migration of a nearshore sandbar observed over a 5-day period near Duck, NC, is simulated with a simplified, computationally efficient, wave-resolving single-phase model. The modeled sediment transport is assumed to occur close to the seabed and to be in phase with the bottom stress. Neglected intergranular stresses and fluid–granular interactions, likely important in concentrated flow, are compensated for with an elevated (relative to that appropriate for a clear fluid) model roughness height that gives the best fit to the observed bar migration. Model results suggest that when mean-current-induced transport is small, wave-induced transport leads to the observed onshore bar migration. Based on the results from the simplified phase-resolving model, a wave-averaged, energetics-type model (e.g., only moments of the near-bottom velocity field are required) with different friction factors for oscillatory and mean flows is developed that also predicts the observed bar migration. Although the assumptions underlying the models differ, the similarity of model results precludes determination of the dominant mechanisms of sediment transport during onshore bar migration.

Cyclic Sand Bar Migration on a Spit-platform in the Danish Wadden Sea—Spit-platform Morphology Related to Variations in Water Level

Cyclic morphology is related to water levels on a shallow spit-platform in the Danish Wadden Sea. Water levels were used as a dynamic parameter because low water levels, water levels at moderate wind set ups and high water levels occur at different wind situations and they result in different morphology. Low water levels result in a flat sand bar with one deep and narrow ebb channel. Water levels at moderate wind set ups result in several wide and shallow ebb channels. After a storm the spit-platform is flat and low with no dissecting ebb channels. Sand bars on the spit-platform migrate up to 35 m/month. A model proposing the different morphology at the three different water level stages is presented.

Nearshore sandbar migration predicted by an eddy‐diffusive boundary layer model

We simulated the erosion and accretion of a natural beach using a wave‐resolving eddy‐diffusive model of water and suspended sediment motion in the bottom boundary layer. Nonlinear advection was included in this one‐dimensional (vertical profile) model by assuming that waves propagated almost without change of form. Flows were forced by fluctuating pressure gradients chosen to reproduce the velocity time series measured during the Duck94 field experiment. The cross‐shore flux of suspended sediment beneath each field‐deployed current meter was estimated, and beach erosion (accretion) was calculated from the divergence (convergence) of this flux. Horizontal pressure forces on sediment particles were neglected. The model successfully predicted two bar migration events (one shoreward bar migration and one seaward) but failed to predict a third (seaward migration) event. Simulated seaward sediment transport was due to seaward mean currents. Simulated shoreward sediment transport was due to covariance between wave‐frequency fluctuations in velocity and sediment concentration and was mostly confined to the wave boundary layer. Predicted seaward (shoreward) bar migration was driven by a maximum in the current‐generated (wave‐generated) flux over the sandbar. A wave‐generated downward flux of shoreward momentum into the wave boundary layer contributed to shoreward sediment transport and often had a local maximum over the bar crest. Second‐order nonlinear advection of sediment, mostly representing shoreward advection by the Stokes drift, also often had a local maximum over the bar crest. Together, wave‐generated momentum fluxes and the Stokes drift substantially increased shoreward transport and were essential to predictions of shoreward bar migration.

Wave-induced sediment transport and sandbar migration.

Onshore sediment transport and sandbar migration are important to the morphological evolution of beaches but are not well understood. Here, a model that accounts for fluid accelerations in waves predicts the onshore sandbar migration observed on an ocean beach. In both the observations and the model, the location of the maximum acceleration-induced transport moves shoreward with the sandbar, resulting in feedback between waves and morphology that drives the bar shoreward until conditions change. A model that combines the effects of transport by waves and mean currents simulated both onshore and offshore bar migration observed over a 45-day period.

The Cambrian-Ordovician siliciclastic platform of the Balcarce Formation (Tandilia System, Argentina): Facies, trace fossils, palaeoenvironments and sequence stratigraphy

The Lower Palaeozoic sedimentary cover of the Tandilia (Balcarce Formation) is made up of thick quartz arenite beds together with kaolinitic claystones and thin fine-grained conglomerates. The Balcarce Formation was formed in the nearshore and inner shelf environments of a tide-dominated and storm influenced open platform. It shows many features suggesting tidal sedimentation. Coarse-grained facies were formed by sand bar migration and accretion. Heterolithic packages are interpreted as interbar (swale) deposits. Subordinated HCS sandstones indicate storm events. The recognition of thick progradational clinoforms allows to confirm that the Balcarce sea was open to the south, as suggested years ago through palaeocurrent interpretation. The great abundance and variety of trace fossils is among the most outstanding characteristics of this unit. The ichnotaxa that have been recognised so far are: Ancorichnus ancorichnus, Arthrophycus alleghaniensis, Arthrophycus isp., Bergaueria isp., Cochlichnus isp., Conostichus isp., Cruziana furcifera, Cruziana isp., Daedalus labeckei, Didymaulichnus lyelli, Didymaulichnus isp., Diplichnites isp., Diplocraterion isp., Herradurichnus scagliai, ?Monocraterion isp., Monomorphichnus isp., Palaeophycus alternatus, Palaeophycus tubularis, Palaeophycus isp., Phycodes aff. pedum, Phycodes isp., Plagiogmus isp., Planolites isp., Rusophycus isp., Scolicia isp. and Teichichnus isp. Trace fossils have traditionally been used to assign the Balcarce Formation to the Lower Ordovician, due to the presence of Cruziana furcifera. However, Plagiogmus is typical of Cambrian successions world-wide.

Nearshore sandbar migration

Field observations suggest that onshore sandbar migration, observed when breaking‐wave‐driven mean flows are weak, may be related to the skewed fluid accelerations associated with the orbital velocities of nonlinear surface waves. Large accelerations (both increases and decreases in velocity magnitudes), previously suggested to increase sediment suspension, occur under the steep wave faces that immediately precede the maximum onshore‐directed orbital velocities. Weaker accelerations occur under the gently sloping rear wave faces that precede the maximum offshore‐directed velocities. The timing of strong accelerations relative to onshore flow is hypothesized to produce net onshore sediment transport. The observed acceleration skewness, a measure of the difference in the magnitudes of accelerations under the front and rear wave faces, is maximum near the sandbar crest. The corresponding cross‐shore gradients of an acceleration‐related onshore sediment transport would cause erosion offshore and accretion onshore of the bar crest, consistent with the observed onshore migration of the bar crest. Furthermore, the observations and numerical simulations of nonlinear shallow water waves show that the region of strongly skewed accelerations moves shoreward with the bar, suggesting that feedback between waves and evolving morphology can result in continuing onshore bar migration.