Sandbar Position Research Articles

Photogrammetric Investigation of Storm-Induced Erosion Process on Sandy Beach Profile in Medium-Scale Flume

In this study, laboratory experiments were conducted to investigate the influence of changes in storm wave height and water level on beach response in a medium-scale wave flume. A schematic storm was simulated (rising, apex, and waning phases). A non-intrusive photogrammetric method was used to collect high-resolution and synchronous data regarding the free surface water elevation and bed level, from which shoreline location, sandbar position, cross-shore sediment transport rates, and nonlinear wave parameters were derived. The cross-shore sediment transport was in agreement with previous laboratory measurements, including the monotonous exchange from foreshore erosion to shoaling zone accretion in most stages of the storm simulation. The surf zone was the main region supplying sediment for beach morphology modification and sandbar generation. The degree of storm erosion was not completely determined by the largest wave height and water level or the cumulative wave power of the apex phase. The largest gradients of the wave parameter sequence change occurred in the rising phase, and this was the main factor generating efficient beachface erosion. It induced an increase in sandbar size, accompanied by the cross-shore motion of maximum velocity amplitude, more violent disturbances of wave nonlinearity, and increased surf zone erosion, with these factors increasing beach instability and leading to more severe storm erosion. The large wave height and water level resulted in shoreline retreat, with a more significant swash zone erosion under a higher runup. The offshore sediment transport turned toward the onshore direction as the original large sandbar deteriorated under the decreasing wave parameter sequence in the waning phase.

Nearshore sandbar switching episodes and their relationship with coastal erosion at the Curonian Spit, Baltic Sea

The longshore realignment of nearshore sandbars is a morphodynamic phenomenon of multiple sandbar systems that has been known about for several decades. However, it is unknown how switching-related nearshore changes influence the evolution of subaerial beaches. This study aims to define the relationship between sandbar switching episodes and the dynamic state of the beach-foredune system along the Curonian Spit coast (Baltic Sea) using decadal satellite-derived and beach profiling data. To define this connection, sandbar switching locations, sandbar cross-shore positions, shoreline positions, and sand volume changes in the beach-foredune system were assessed on interannual and storm-related time scales. Twenty-seven sandbar switching episodes were observed with an average duration of 14.3 months. Most of the switching episodes occurred at preferred locations, coinciding with breaking points of different shoreline orientations where oblique waves and longshore currents prevailed. Shoreline retreat at an average rate of –14.2 m was observed within most of the sandbar switching zones. During major storm events, the average rate of erosion within the sandbar switching zones was significantly higher than the rate outside them. On an interannual time scale, a moderate average rate of erosion was observed within the sandbar switching zones compared to a small accretion rate outside them. Additional case studies of coastal evolution within the switching zones indicated well-correlated rates of switching-determined outer sandbar positions, shoreline positions, and sand volume on the beach and foredune during the switching episodes. The results of this study could be important for the identification of erosional hot spots and coastal prediction.

Headland bypassing timescales: Processes and driving forces

In this paper, a natural headland bypassing is investigated in terms of its short (months to years) and long-term (years to decades) variability and its relationship with wave conditions, climate drivers and anthropogenic interventions. The research is focused on Fingal Head (New South Wales, Australia) where nine detailed topo-bathymetric surveys were undertaken between June 2018 and January 2020. To extend the analysis in time, over 30 years of satellite and aerial images were used to describe the headland bypassing variability based on the shoreline and sandbar position changes. Shoreline and sandbar positions presented moderate to strong correlation between updrift and immediate downdrift of the headland highlighting the influence of the bypassing process for the longshore transport on the study area. Results indicate that the headland bypassing around Fingal Head is governed by two distinct processes and their dominance is controlled by waves and sediment availability. The sandbar-driven bypassing scenario requires a storm wave event to trigger the sandbar system and over seven months to complete the full bypassing cycle. A quicker bypassing cycle (i.e. few months) happens when sediment leaks around the headland following a persistent low energy wave condition that largely accretes the updrift upper beach. Headland bypassing cycles occur in multiple timescales, including seasonal variability of the wave climate to interannual and decadal cycles of shoreline progradation and retreat. Shifts in the large-scale climate drivers such as El Niño-Southern Oscillation, Pacific Decadal Oscillation and Interdecadal Pacific Oscillation were observed to influence on changes to the low frequency of variability of the headland bypassing in the study area by affecting the predominant wave direction and updrift beach sand availability. The understanding of this intermittent nature of the headland bypassing process and particularly considering its periodicity and the related driving forces is crucial to predict future coastal hazards and develop management strategies.

Dynamic equilibrium of sandbar position and height along a low wave energy micro-tidal coast

Nearshore sandbars play an essential role in dissipating incident wave energy and protecting the beach landward. Thus, understanding the dynamic equilibrium of nearshore bars is valuable to beach management and shore protection. This study examines the sandbar equilibrium in terms of bar height and cross-shore bar location, in order to assess how the dynamic equilibrium is maintained and influenced by storms along a low wave energy micro-tidal coast. The bar height and bar position were extracted from 51 beach profiles surveyed every two months, spaced at 300 m along a 15-km stretch of beach from October 2010 to August 2014. For the studied coast, alongshore variation in equilibrium bar position measured from the shoreline ranges between 40 and 80 m and equilibrium bar height between 0.20 and 0.70 m. Greater equilibrium sandbar height tends to occur around a headland, where waves are higher. Alongshore variations of bar behavior were observed during storms, with both onshore and offshore bar migration observed during one storm. Water depth over the pre-storm sandbar crest is a major factor controlling the storm-induced onshore or offshore bar migration. On average, the depth over the onshore migrating sandbar is found to be 0.20 m deeper than that over the offshore migrating bar during both summer and winter storms. There is no significant correlation between incident wave angle and sandbar height changes, while significant correlation exists between wave angle and sandbar movement under certain wave conditions, with more oblique waves being associated with further offshore movement of the sandbar. Energetic storm conditions tend to make the bar higher than the equilibrium height, while post-storm adjustment would restore the equilibrium height within 4–6 months. Although the exact values may vary at different locations, the concept of dynamic equilibrium of bar height and distance to shoreline could apply at many locations.

Observations and modelling of shoreline and multiple sandbar behaviour on a high-energy meso-tidal beach

This contribution describes 10 years of observed sandbar and shoreline cross-shore position variability at a meso-tidal, high energy, multiple sandbar beach. To examine relationships between the temporal variability in shoreline/sandbar position with offshore wave forcing, a simple equilibrium model is applied to these data. The analysis presented in this paper shows that the equilibrium model is skilled at predicting the alongshore-averaged, time-varying position of the shoreline (R = 0.82) and the outer sandbar position (R = 0.75), suggesting that these end members of the nearshore sediment system are most strongly influenced by offshore wave forcing in a predictable, equilibrium-forced manner. The middle and inner bars are hypothesized to act as sediment transport pathways between the shoreline and the outer bar. Prediction of these more transient features by an equilibrium model was less skilful.Model coefficients reveal that these two end members (outer bar and shoreline) in the sediment system act in opposite directions to changes in the annual offshore wave forcing. During high wave events, sediment is removed from the shoreline and deposited in the nearshore sediment system with simultaneous landward retreat of the shoreline and offshore migration of the outer sandbar. While both end member features have cycles at annual and inter-annual scales, their respective equilibrium response factor differs by almost a factor of 10, with the shoreline responding around an inter-annual mean (ϕ = 1000 days) and the outer bar responding around a seasonal mean (ϕ = 170 days). The model accurately predicts shoreline response to both mild (e.g. 2004/05, 2008/09) and extreme (e.g. 2005/06, 2009/10) winter storms, as well as their summer recovery. The more mobile and dynamic outer sandbar is well-modelled during typical winters. Summer onshore sandbar migration of the outer bar in 2005 and 2006 is under-predicted as the system transitioned between a triple (winter) and double (summer) sandbar system. The changing of the number of bars present in the system is something that this simple model cannot predict. Analysis of the data suggests that this multi-bar system adjusts its cross-shore seasonal movement when there is a significant change in the sediment supply to the system (e.g., nourishment projects, severe storms).

Shoreline recovery on wave-dominated sandy coastlines: the role of sandbar morphodynamics and nearshore wave parameters

This study quantifies and characterises the temporal variability of shoreline recovery on a high-energy sandy coastline using a 10-year dataset of daily shoreline and sandbar positions from a Coastal Imaging station at Narrabeen-Collaroy Beach, Australia. Following a total of 82 individual storm events, rates of the cross-shore return of the shoreline to its pre-storm position were analysed. Observed rates during shoreline recovery were characterised by an overall mean of ~0.2m/day. Temporal variability in rates was most evident at shorter timescales of 1–2weeks and included rates most frequently between 0 and 0.3m/day, less frequent more rapid rates of up to 2m/day and also minor landward movements. This temporal variability was significantly correlated with nearshore forcing parameters describing the ratio of wave height to wave period, the cross-shore proximity (and attachment) of the sandbar to the shoreline and the rate of cross-shore sandbar migration. These findings are summarised in a new conceptual model that characterises temporal phases and rates of shoreline recovery corresponding to stages of onshore sandbar migration following a storm, from fully detached storm-deposited sandbar morphology through to complete sandbar welding with the shoreline. More gradual shoreline recovery rates are observed with fully detached and semi-attached sandbar conditions. In contrast, more rapid rates of shoreline recovery occur when sandbars are closer and attached to the shoreline, observed to be on average 3–4 times greater than for detached sandbars. In conditions with attached and semi-attached sandbars, shoreline recovery rates are negatively correlated to the forcing of nearshore wave steepness and dimensionless fall velocity, and coupled with concurrent rates of onshore sandbar migration. The findings provide insight into key parameters influencing shoreline recovery following storms.

A parametric model for barred equilibrium beach profiles: Two-dimensional implementation

A method is proposed for estimating approximate 2DH bathymetry including longshore variable shoreline and sand bar systems. The method is based on, but extends, a previous 1DH model of Holman et al. (2014) by assuming that this 1DH equilibrium barred bathymetry can be applied for any offshore location if the corresponding mean shoreline orientation is taken to be the local average over a longshore span that is K times the offshore distance. Thus locations close to the beach are sensitive to shoreline and bar details while more seaward locations are steadily less sensitive.The model was tested against 14 ground truth surveys, collected over two years and under widely ranging environmental condition, spanning a 500 by 1000m region. Models inputs for the shoreline and sand bar positions were extracted from measured bathymetries for these tests (but would be derived from other sources in real applications) while deep-water inputs were found from a single deep-water survey. The model yielded complete 2DH bathymetry maps that were a very good approximation of ground truth. The mean bias and rms error over the full region and data set were 0.27m and 0.49m respectively and proxy bathymetries were visually very similar to ground truth. The largest source of error was occasional cross-shore misplacements of otherwise realistic looking sand bars. Results were only weakly dependent on the value of K when tested over a factor of four and the default value of 1.0 is recommended. Performance statistics using input locations for the shoreline and bar crest that were manually digitized from breaking patterns in rectified optical time exposure images were no worse than bathymetry-based inputs. Hydrodynamic predictions using these bathymetries would be a substantial improvement over those from monotonic or even barred 1DH equilibrium proxy bathymetries.

Storm impact on the seasonal shoreline dynamics of a meso- to macrotidal open sandy beach (Biscarrosse, France)

A three-year dataset (2007–2010) of shoreline and sandbar positions derived from video observations of an open sandy beach (Biscarrosse, France) is analyzed, to explore the impact of storms on the seasonal shoreline dynamics. The results indicate that a clear seasonality is observed in the offshore significant wave height and in the occurrence of ‘winter storm’ events that are defined as periods with significant wave height greater than 4m lasting at least 12h. Data indicate that the range of variation of the inner sandbar positions (120m) is two and a half times larger than the range of variation of the shoreline position. Correlation analyses suggest that the offshore conditions do not conclusively explain the position of the inner bar. Shoreline positions show a clear seasonal pattern, displaying a stable position occurring in February and July, providing evidence of seasonal-recovery although analysis of storm activity indicates that the number and intensity of storms is highly variable over the three years. The Biscarrosse data show that rapid erosion can be observed also under moderate conditions, and that beach recovery after the storm season can be relatively rapid.

Video observation of megacusp evolution along a high-energy engineered sandy beach: Anglet, SW France

Birrien, F., Castelle, B., Dailloux, D., Marieu, V., Rihouey, D. and Price, T.D., 2013. Video observation of megacusp evolution along a high-energy engineered sandy beach: Anglet, SW France.We present an 18-month period of video monitoring of both the nearshore sandbars and the megacusps at Anglet Beach, SW France. The study site covers a 2-km long stretch of beach that is constrained to the North by a groin that extends 400 m seaward. The beach is high-energy intermediate, mostly double-barred, with a steep beach face (~1/10) favouring the formation of cusps at a large range of lengthscales, from beach cusps (O(10 m)) to megacusps (O(100 m)). A megacusp is systematically observed against the groin as a result of the persistent presence of a topographic rip. Further away from the groin, about 4–5 megacusps are typically observed. The shoreline dynamics are strongly controlled by the geometry of the surfzone sandbar. In-phase shoreline-sandbar coupling was commonly observed, that is, megacusps in the alignment of the rip channels and seaward bulges in the shoreline facing the surfzone sandbar horns. Megacusps at Anglet Beach have a typical cross-shore amplitude of O(10 m). During a severe storm with significant wave heights over 7 m, a very erosive megacusp (hotspot) formed in the alignment of the rip channel. Prior to this storm, a seaward bulge in the shoreline was observed in this location. Accordingly, 40 m cross-shore variation change in shoreline position at this location was the signature of megacusp dynamics only. This hotspot was systematically observed at the same location during 7–8 months after the storm event, before the fair weather conditions in summer resulted in an overall accretion of the beach. Megacusps were smoothed out during rapid alongshore migration of the sandbar or for high-energy wave conditions. In addition, the evolution of the alongshore-averaged shoreline position shows the exact opposite trend as that of the alongshore-averaged sandbar position, yet with less pronounced cross-shore variations. Overall, our results highlight once again the strong links between shoreline and surfzone sandbar dynamics.

Short-term sandbar variability based on video imagery: Comparison between Time–Average and Time–Variance techniques

Time–exposure intensity (averaged) images are commonly used to locate the nearshore sandbar position ( x b ), based on the cross-shore locations of maximum pixel intensity ( x i ) of the bright bands in the images. It is not known, however, how the breaking patterns seen in Variance images (i.e. those created through standard deviation of pixel intensity over time) are related to the sandbar locations. We investigated the suitability of both Time–exposure and Variance images for sandbar detection within a multiple bar system on the southern coast of Brazil, and verified the relation between wave breaking patterns, observed as bands of high intensity in these images and cross-shore profiles of modeled wave energy dissipation ( x D ). Not only is Time–exposure maximum pixel intensity location ( x i-Ti ) well related to x b , but also to the maximum pixel intensity location of Variance images ( x i-Va ), although the latter was typically located 15 m offshore of the former. In addition, x i-Va was observed to be better associated with x D even though x i-Ti is commonly assumed as maximum wave energy dissipation. Significant wave height (Hs) and water level (η) were observed to affect the two types of images in a similar way, with an increase in both Hs and η resulting in x i shifting offshore. This η-induced x i variability has an opposite behavior to what is described in the literature, and is likely an indirect effect of higher waves breaking farther offshore during periods of storm surges. Multiple regression models performed on x i , Hs and η allowed the reduction of the residual errors between x b and x i , yielding accurate estimates with most residuals less than 10 m. Additionally, it was found that the sandbar position was best estimated using x i-Ti ( x i-Va ) when x b was located shoreward (seaward) of its mean position, for both the first and the second bar. Although it is unknown whether this is an indirect hydrodynamic effect or is indeed related to the morphology, we found that this behavior can be explored to optimize sandbar estimation using video imagery, even in the absence of hydrodynamic data.

A behavior-oriented dynamic model for sandbar migration and 2DH evolution

[1] A nonlinear model is developed to study the time‐dependent relationship between the alongshore variability of a sandbar, a(t), and alongshore‐averaged sandbar position, xc(t). Sediment transport equations are derived from energetics‐based formulations. A link between this continuous physical representation and a parametric form describing the migration of sandbars of constant shape is established through a simple transformation of variables. The model is driven by offshore wave conditions. The parametric equations are dynamically coupled such that changes in one term (i.e., xc) drive changes in the other (i.e., a(t)). The model is tested on 566 days of data from Palm Beach, New South Wales, Australia. Using weighted nonlinear least squares to estimate best fit model coefficients, the model explained 49% and 41% of the variance in measured xc and a(t), respectively. Comparisons against a 1‐D horizontal (1DH) version of the model showed significant improvements when the 2DH terms were included (1DH and 2DH Brier skill scores were −0.12 and 0.42, respectively). Onshore bar migration was not predicted in the 1DH model, while the 2DH model correctly predicted onshore migration in the presence of 2DH morphology and allowed the bar to remain closer to shore for a given amount of breaking, providing an important hysteresis to the system. The model is consistent with observations that active bar migration occurs under breaking waves with onshore migration occurring at timescales of days to weeks and increasing 2DH morphology, while offshore migration occurs rapidly under high waves and coincides with a reduction in 2DH morphology. Citation: Splinter, K. D., R. A. Holman, and N. G. Plant (2011), Abehavior‐orienteddynamicmodelforsandbarmigrationand2DH evolution, J. Geophys. Res., 116, C01020, doi:10.1029/2010JC006382.

Prediction and assimilation of surf-zone processes using a Bayesian network: Part II: Inverse models

A Bayesian network model has been developed to simulate a relatively simple problem of wave propagation in the surf zone (detailed in Part I). Here, we demonstrate that this Bayesian model can provide both inverse modeling and data-assimilation solutions for predicting offshore wave heights and depth estimates given limited wave-height and depth information from an onshore location. The inverse method is extended to allow data assimilation using observational inputs that are not compatible with deterministic solutions of the problem. These inputs include sand bar positions (instead of bathymetry) and estimates of the intensity of wave breaking (instead of wave-height observations). Our results indicate that wave breaking information is essential to reduce prediction errors. In many practical situations, this information could be provided from a shore-based observer or from remote-sensing systems. We show that various combinations of the assimilated inputs significantly reduce the uncertainty in the estimates of water depths and wave heights in the model domain. Application of the Bayesian network model to new field data demonstrated significant predictive skill (R 2 = 0.7) for the inverse estimate of a month-long time series of offshore wave heights. The Bayesian inverse results include uncertainty estimates that were shown to be most accurate when given uncertainty in the inputs (e.g., depth and tuning parameters). Furthermore, the inverse modeling was extended to directly estimate tuning parameters associated with the underlying wave-process model. The inverse estimates of the model parameters not only showed an offshore wave height dependence consistent with results of previous studies but the uncertainty estimates of the tuning parameters also explain previously reported variations in the model parameters.

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.

Recurrent neural network modeling of nearshore sandbar behavior

The temporal evolution of nearshore sandbars (alongshore ridges of sand fringing coasts in water depths less than 10 m and of paramount importance for coastal safety) is commonly predicted using process-based models. These models are autoregressive and require offshore wave characteristics as input, properties that find their neural network equivalent in the NARX (Nonlinear AutoRegressive model with eXogenous input) architecture. Earlier literature results suggest that the evolution of sandbars depends nonlinearly on the wave forcing and that the sandbar position at a specific moment contains 'memory', that is, time-series of sandbar positions show dependencies spanning several days. Using observations of an outer sandbar collected daily for over seven years at the double-barred Surfers Paradise, Gold Coast, Australia several data-driven models are compared. Nonlinear and linear models as well as recurrent and nonrecurrent parameter estimation methods are applied to investigate the claims about nonlinear and long-term dependencies. We find a small performance increase for long-term predictions (>40 days) with nonlinear models, indicating that nonlinear effects expose themselves for larger prediction horizons, and no significant difference between nonrecurrent and recurrent methods meaning that the effects of dependencies spanning several days are of no importance.

Movements of Ilyanassa obsoleta (Gastropoda) on an intertidal sandflat

The gastropod Ilyanassa obsoleta (Say) is native to the east coast of North America where it is locally abundant on sandflats, mudflats, and in saltmarsh creeks. The local disturbances created by snails and their movements affect soft-sediment community composition. Movements of individually marked snails were followed on an intertidal sandflat on Cape Henlopen, Delaware, U.S.A. In June 1991, 1,200 snails that had tested as trematode-uninfected were released and over 5 months 554 were sighted 971 times. Mean daily net distance moved was 1.7 m, but snails often moved 10–20 m day−1 and one snail was 180 m distant after 130 days. Net dispersal of the released population was attained in ≈10 days, by which time, a typical distance from release was 15–20 m. Snails were not found crossing sandbars and most moved away from shore into a tidal gully. In June 1993, 500 snails, both uninfected and trematode-infected, were released at the same position and over 6 months, 350 snails were sighted 949 times. Sandbars were again barriers to movement, but their changed positions allowed wider dispersal. Net dispersal was complete in ≈20 days by which time a typical distance from release was 30–40 m. Mean daily net distance moved was 2.2 m, but within 10 days snails had moved 50–100 m. In both years, following initial dispersal, snails (infected or not) took up random directions from move to move. Infected and uninfected snails dispersed equal distances, but had different mean final dispersal directions. Dispersal of I. obsoleta individuals was extensive and affected by shifting sandbar positions and parasitism. Recognizing this will be important in appreciating the ecological dynamics of this gastropod and in determining its effects on soft-bottom communities.

Effect of hydrodynamics and bathymetry on video estimates of nearshore sandbar position

Remote sensing of wave breaker patterns, clearly visible as high‐intensity bands in time exposure video images, has become a powerful tool to obtain large‐scale (kilometers) and long‐term (years) time series of nearshore sandbar position. However, intensity‐based bar crest positions xi differ from directly measured positions xb by a time‐varying distance Δx, which is of O(10 m) and depends on the offshore wave height H0, the water level η0, and the bathymetry itself. The effect of these parameters on Δx was investigated from simultaneous video observations and bathymetric surveys, obtained in the double‐barred system at Egmond aan Zee, Netherlands, and from wave model predictions, assuming that the roller energy represents image intensity. When the wave field over a bar was predicted to be nonsaturated, xi was observed and predicted to move offshore as either η0 decreased or H0 increased. Under saturated conditions, Δx only responded to changes in η0. Additional model investigations showed that an increase in outer bar crest depth, similar to that observed during interannual bar behavior, significantly reduced the Δx variability at the outer bar and increased the Δx variability at the inner bar. Implications of our observational and model findings for studying sandbar position from video imagery are outlined.

A simple model for interannual sandbar behavior

Time and length scales of beach variability have been quantified using 16 years of beach surveys sampled at the Army Corps of Engineers' Field Research Facility, located on the U.S. Atlantic coast. Between 50% and 90% of the bathymetric variability at this site was explained by alongshore‐uniform response over the approximately 1 km alongshore span of the surveys. Although the incident wave height variance was dominated by frequencies at or higher than 1 cycle/yr, more than 80% of the bathymetric variance at all cross‐shore locations was explained by frequencies <1 cycle/yr. Interannual cycles consisting of sandbar formation, migration, and decay contributed to the low‐frequency variability. The observed behavior can be explained by a simple, heuristic model. The model assumes that bars migrate toward a wave height dependent equilibrium position. This position was shown to coincide with the wave “breakpoint.” Additionally, the rate of bar response is taken to be variable and was empirically determined to be proportional to the wave height cubed. The net effect of a variable response rate is to shift the expected long‐term mean sandbar position offshore, toward the equilibrium position associated with the largest waves. The model explained up to 80% of the observed bar position time series variance and up to 70% of the variance of bar crest velocity time series, which were extracted from three different sandbars. Characteristic bar response times (related to the inverse of the response rate) were found to be long relative to the characteristic timescale of the forcing (1 year in our case). As a result, transient response (i.e., bar position far from equilibrium) tended to persist for many cycles of the forcing. Transient bar behavior appears in the observations when bars formed near the shoreline or when outer bars decayed and inner bars faced a changed wave climate. While the present model is able to explain the evolution of these transients, it does not contain a mechanism for their introduction.