Morphodynamic response of nearshore bars to a shoreface nourishment

The autonomous nearshore bar behaviour along the barrier island of Terschelling, The Netherlands, is characterised by the presence of net seaward cyclic migrating sand bars generated near the shoreline. In 1993, a perturbation of the cyclic bar system was introduced by the implementation of a 2 Mm³ shoreface nourishment supplied to the nearshore bar zone, filling up the trough between the middle and outer bar. The morphodynamic response of the nearshore bars to the nourishment perturbation is investigated using a bathymetric data set with an alongshore extent of 12 km and sampled for 10 years. Bar behaviour is quantified in terms of bar crest position in relation to morphometric parameters such as bar depth, height, width and volume. Along with a pronounced development of a three-dimensional bar system unseen in the autonomous behaviour, the nearshore bars exhibited a 6–7 year arrest in their migrational behaviour during which bar morphology remained stable at immediate pre-nourishment morphometric values. At the subsequent onset of bar movement, bars resumed their migration at a rate predicted by autonomous behaviour in parallel development with morphometric parameters along their predicted trends. It is shown that the observed onshore transport of nourished sediment in the 6–7 year arrest results from a gradual deepening of troughs. Cross-shore sediment transport modelling is used to assess the effect of the nourishment on yearly averaged onshore (short-wave nonlinearity) and offshore (undertow) sediment transport rates. The gradual reappearance of the pre-nourishment bar-trough morphology is shown to engender a normalisation in the cross-shore distribution of sediment transport rates to pre-nourishment rates.     

Linear evolution of a shoreface nourishment

The morphological evolution of a shoreface nourishment is investigated by interpreting the nourishment as a linear perturbation of the natural system. The nourishment is projected onto the subset of linear eigenmodes with negative growth rates of the morphodynamical system. The evolution of these linear modes then determines the temporal behaviour of the shoreface nourishment. The method is presented, and results are shown for shoreface nourishments of different length scales on a straight coast and subject to normal incidence. Shoreface nourishments are represented by their expansions according to the projection method on a 1:50 plane beach profile. All nourishments are shown primarily to be diffusive features, with long scale nourishments diffusing more slowly than shorter length scale nourishments. Long scale nourishments also exhibit a shoreward movement during their decay. This all indicates that long length scale nourishments may be more beneficial in coastal engineering projects. This study is a first step towards nonlinear projection to study shoreface nourishment behaviour.     

Experimental simulation of shoreface nourishments under storm events: A morphological,hydrodynamic, and sediment grain size analysis

This study focuses on barred beach shoreface nourishments physically simulated in a wave flume. The attack of a schematic storm on three different nourishments is analysed. The apex and waning storm phases lead respectively to offshore and onshore sediment transports. Nourishments in the trough and on the outer bar feed the bar and increase wave dissipation offshore. The bar acts as a wave filter and reduces shore erosion (lee effect). In contrast, nourishment on the beach face leads mostly to shore feeding and reconstruction (feeder effect). With successive nourishments, the beach face clearly becomes steeper and onshore sediment transport is reduced during moderate wave climates. The surface grain size analysis reveals marked variations. Coarser sediments are sorted on the bar and the upper beach face. These locations correspond to large wave dissipation zones during the storm apex.     

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.     

Chronic offshore loss of nourishment on Nice beach,French Riviera: A case of over-nourishment of a steep beach?

The 4.5 km-long gravel beach fronting the exclusive resort of the city of Nice, on the French Riviera, in southeastern France, was artificially nourished from 1976 to 2005 to the tune of 558,000 m³, making this long-term operation one of the most significant for gravel beaches in the world. Nourishment has ranged from nil in certain years (1979, 1980, 1983–85, and 2001–2002) to a peak of over 97,000 m³ in 2000. Analyses of 50 transects covering the beach highlight no significant change in net beach width over this 30-year period of massive gravel nourishment. A Principal Components Analysis and a Cluster Analysis used to detect patterns in the 87 beach-width measurement dataset show no clear spatial trends in transect groups that can be interpreted in terms of the morphology of the beach and the steep inner shoreface. Significant wave height off Nice shows no change over the period 1979–2005. Since there is no possibility for alongshore gravel leakage on the strongly embayed Nice beach, the relative stability in beach width clearly implies loss of recharged gravel offshore. Gravel loss following nourishment is favoured by: (1) the steep inner shoreface inherited from the geological context of Nice beach at the flanks of the southern Alps, and (2) the practise of artificial beach widening through flattening, in summer, of a narrow (5–15 m-wide) mobile zone of the profile in order to enhance the ‘carrying’ capacity of this highly touristic beach. Beach widening and flattening following nourishment bring close to the very steep inner shoreface zone several cubic metres of gravel for each metre of beach that may be permanently lost downslope during autumn and winter storms. Recharged gravel is redistributed alongshore and offshore leakage is probably enhanced where small narrow submarine canyon heads impinge on the beach, resulting in a very narrow shoreface. Mean beach width shows an oscillating alongshore pattern that may be due to the influence of these canyons as pathways of gravel loss offshore. However, there is no correlation between mean beach width and distance to the 10-m isobath, used as a surrogate for inner shoreface width. Storms are associated with plunging waves that are particularly effective and concentrated, on this almost tideless shore, over the narrow mobile zone of the beach profile where a series of steep reflective berms are built during storms. The high dynamic pressures associated with this narrow zone of concentrated wave breaking, and energy reflection from the steepened profile, are deemed to contribute to the permanent downslope loss of gravel. This situation of long-term gravel loss is probably accepted by the beach management authority because of the low cost of obtaining nourishment material and the advantages derived from a temporarily wider beach in terms of recreational space.     

Process-based modelling of a shoreface nourishment

A state-of-the-art process-based model is applied to hindcast the morphological development of a shoreface nourishment along the barrier island of Terschelling, The Netherlands. The applied morphodynamic model is Delft3D for fully three-dimensional flow and sediment transport in coastal environments. Owing to a complex geomorphological setting of the study area, the curvi-linear model includes adjacent tidal inlets and covers 40×70 km with an increasing grid size resolution towards the nourishment site in the center of the island. In 1993, a total of 2 Mm³ of sand was supplied to the nearshore bar zone off Terschelling, filling up the trough between the middle and outer bar. By spring 1994, most of the nourished sediment had been redistributed onshore and welded onto the middle bar where it remained in the following years. The morphodynamic model is applied to the prediction of this rapid nearshore profile behaviour. The calibration of the model against an extensive set of full-scale hydrodynamic data at several locations throughout the nearshore bar zone shows a good representation of the measured hydrodynamics. Morphodynamic results show a dependency on spatial scale: on the scale of precise bed level evolution with respect to bar migration and growth, model predictions are poor as the nearshore bars are predicted to flatten out. Resorting to bulk volumes integrated over larger spatial scales, the model clearly has skill in predicting the overall effects of the nourishment. The lack of phase shift between sediment transport and bathymetry is identified as the key controlling factor for the poor sandbar predictions.     

Analysis of submerged bar nourishment strategies in a wave-dominated environment using a 2DV process-based model

The present study aims to analyze the effects of different submerged bars nourishment strategies using a 2DV process-based morphodynamical model. A two-barred beach profile typical of the French Mediterranean micro-tidal storm-dominated coastline is chosen as a reference profile. Two different kinds of modified beach profiles are considered. (i) Only the outer bar is nourished, the inner bar being unchanged (ii) both bars are nourished. Three typical wave forcing regimes are considered. The behavior of the natural profile is first investigated under the 3 wave forcing regimes. Then the behavior of the various nourished profiles is analyzed in terms of wave dynamics and bars behavior. On the basis of the model results, the outer bar only nourishment strategy appears to be preferable.     

Seasonal persistence of a small southern California beach fill

Torrey Pines State Beach, a site with large seasonal fluctuations in sand level, received a small shoreface beach fill (about 160,000 m³) in April 2001. The 600 m-long, flat-topped nourishment pad extended from a highway riprap revetment seaward about 60 m, terminating in a 2 m-tall vertical scarp. A 2.7 km alongshore span, centered on the nourishment region, was monitored prior to the nourishment and biweekly to monthly for the following 2 years. For the first 7 months after the nourishment, through fall 2001, significant wave heights were small, and the elevated beach fill remained in place, with little change near and above Mean Sea Level (MSL). In contrast, the shoreline accreted on nearby control beaches following a seasonal pattern common in southern California, reducing the elevation difference between the nourished and adjacent beaches. During the first winter storm (3 m significant wave height), the shoreline retreated rapidly over the entire 2.7 km survey reach, forming an alongshore-oriented sandbar in 3 to 4 m water depth [Seymour, R.J., Guza, R.T., O'Reilly, W., Elgar, S., 2004. Rapid erosion of a Southern California beach fill. Coastal Engineering 52 (2), 151–158.]. We show that the winter sandbar, most pronounced offshore of the nourishment, moved back onto the beach face during summer 2002 (following the usual seasonal pattern) and formed a wider beach above MSL at the site of the original nourishment than on the control beaches. Thus, the April 2001 shoreline nourishment was detectable until late fall 2002, persisting locally over a full seasonal cycle. In an extended 7-year time series, total sand volumes (summed between the back beach and 8 m water depth, over the entire 2.7 km reach) exhibit multi-year fluctuations of unknown origin that are twice as large as the nourishment volume.     

Evaluating the PEM passive beach drainage system in a 4-year field experiment at Egmond (The Netherlands)

The PEMs' (Pressure Equalizing Modules) passive vertical drainage system was tested at the coast just south of Egmond (The Netherlands) from November 2006 to December 2010. The present study aims at providing qualitative and quantitative insight into the impact of the PEMs on the nearshore morphological development at Egmond. The evaluation is based on a comparison of the annually observed temporal evolution of the nearshore morphology prior, during and after the PEM installation period. To this end a number of aggregated volumetric profile parameters derived from the surveys were compared for the test area and for an adjacent area which acted as a reference. Besides the PEMs, the study area is also influenced by beach and shoreface nourishments. Therefore, the nourishments carried out just north and south of the Test and Reference areas in 2004 and 2005 prior to the installation of the PEMs were specifically considered in the analysis. From 2008 onward the morphological behavior of the nearshore zone was dominated by the alongshore coherent net offshore migration of the multiple bar system and no obvious feeding of sand from the nourishments toward the Test and Reference areas could be identified. The influence of the nourishments, the PEMs, and the natural variability were difficult to distinguish at the Test and Reference areas. Therefore, the morphological development of the Test area relative to that of the Reference area was adopted as the primary evaluation criteria. Interestingly, the installation period (2007–2010) exhibited statistically significant different CSIs (coastal state indicators) compared with the 4 years prior to installation (2003–2006) for both the Test and the Reference areas. Because both the Test and the Reference areas exhibited very similar behavior prior, during and after the PEM installation period, the analysis was unable to identify the impact of the PEMs. No influence of the PEMs on the local beach and dune morphology could be established. Considering the above and given the fact that the PEM system was also not able to induce a measurable accretion at a similar experiment in Denmark we consider it unlikely any efficacy of the PEM system will be identified in future experiments.     

Evaluation of the physical process controlling beach changes adjacent to nearshore dredge pits

Numerical modeling of a beach nourishment project is conducted to enable a detailed evaluation of the processes associated with the effects of nearshore dredge pits on nourishment evolution and formation of erosion hot spots. A process-based numerical model, Delft3D, is used for this purpose. The analysis is based on the modification of existing bathymetry to simulate “what if” scenarios with/without the bathymetric features of interest. Borrow pits dredged about 30 years ago to provide sand for the nourishment project have a significant influence on project performance and formation of erosional hot spots. It was found that the main processes controlling beach response to these offshore bathymetric features were feedbacks between wave forces (roller force or alongshore component of the radiation stress), pressure gradients due to differentials in wave set-up/set-down and bed shear stress. Modeling results also indicated that backfilling of selected borrow sites showed a net positive effect within the beach fill limits and caused a reduction in the magnitude of hot spot erosion.     

The morphological response of a nearshore double sandbar system to constant wave forcing

Results of 2DH morphodynamic computations are presented to quantify the temporal evolution of the crescentic patterns emerging in a double nearshore bar system in response to constant wave boundary forcing. Sixteen different conditions varying both offshore wave height and angle of wave incidence were applied. The mean length scales of the emerging irregular crescentic patterns are linearly proportional to the local longshore velocity over the inner and outer bars. For similar longshore velocities, the length scales of the outer bar are larger than of the inner bar. This is explained by accounting for the difference in water depth above the bar crest. The variable morphological response times can be explained by including additional bathymetrical parameters. The active volume of the bar, defined by the breaker index, plays an important role in this response time. With larger active volumes the bar responds more rapidly to identical boundary conditions. Also, bars with a smaller total volume respond more quickly. This faster response is due to the steeper active volume of the bars. Different initial perturbations resulted in different locations of the emerging features, showing that their location is sensitive to the initial bathymetry. However, the range in length scales and response times due to the different perturbations was significantly smaller than those obtained for the different hydrodynamic conditions. Based on the present findings we hypothesize that morphological length scales in the field are rarely in equilibrium with the concurrent offshore wave height and angle of incidence owing to the slow response of the sandbars under constant conditions relative to the stochastic nature of natural wave forcing.     

Temporal and spatial variations of the Huanghe River mouth bars

1 Introduction T aking the average riverbed elevation ofthe riverm outh from its upstream side to its dow nstream sidew ithin a m outh area as a base line, there is usually asand sw ell standing higher than the base line. If thesw ellis form ed in the distributary channel,itis consid-ered as “a longitudinalbar”;ifitis offornearthe riverm outh,itis called “m outh bar”(Jiand H uang,1995a),such as the cases in the C hangjiang R iver andQ iantang R iverm outh areas(see Figs 1 and 2). Fig. …     

Sediment transport and morphology at the surf zone of Presque Isle, Lake Erie, Pennsylvania

A four-year investigation of surf zone sedimentation at Presque Isle, Pennsylvania, was undertaken in preparation for the design of a segmented breakwater system. Sediment transport calculations were based on hind-cast annual wave power statistics and “calibrated” by known accretion rates at the downdrift spit terminus. 30,000 m³ of sediment reaches the peninsula annually from updrift beaches. The transport volume increases downdrift due to shoreface erosion and retreat of the peninsular neck. At the most exposed point on Presque Isle (the lighthouse) the annual transport is 209,000 m³. East of the lighthouse is a zone of net shoreface accretion as the longshore transport rate progressively decreases.

The downdrift variation in sediment supply, combined with increasing refraction and attenuation of the dominant westerly storm waves produce a systematic change in prevailing surf zone morphology. Storms produce a major longshore bar and trough along the exposed peninsular neck. The wave energy during non-storm periods is too low to significantly alter the bar which consequently becomes a permanent feature. The broad shoreface and reduced wave energy level east of the lighthouse produce a morphology characterized by large crescentic outer bars, transverse bars, and megacusps along the beach. At the sheltered and rapidly prograding eastern spit terminus the prevalent beach morphology is that of a ridge and runnel system in front of a megacuspate shore.

The morphodynamic surf zone model developed for oceanic beaches in Australia is used as a basis for interpretation of shoreface morphologic variability at Presque Isle. In spite of interference by major shoreline stabilization structures, and differences between oceanic and lake wave spectra, the nearshore bar field at Presque Isle does closely correspond to the Australian model.     

The influence of tides,wind and waves on the redistribution of nourished sediment at Terschelling,The Netherlands

A calibrated morphodynamic model of the barrier island of Terschelling, The Netherlands [Grunnet, N.M., Walstra, D.J.R., Ruessink, B.G., 2004. Process-based modelling of a shoreface nourishment. Coastal Eng. 51/7, 581–607], comprising the island and its two adjacent tidal inlet systems, is applied to identify the relative contribution of tides, wind and waves to the cross-shore and alongshore redistribution of a 2 Mm³ nourishment supplied to the nearshore zone along the island. Several model simulations with varying combinations of horizontal and vertical tide, wind and wave forcing were designed to investigate the effect of each individual forcing on a large spatio-temporal scale (order of kilometres and months, respectively). As expected, stirring and transport by waves and wave-induced currents are predicted to be by far the dominant contributor to the net sediment transport along the coast of Terschelling. Because of the strong obliquity of the winds and the relatively small tidal currents in front of the island (≈ 0.5 m/s), alongshore wind-driven currents increase sediment transport rates and horizontal tides virtually have no net transport capacity. This motivated a local model of the study area along the closed coast of Terschelling, not including tidal inlets and further simplifying tidal boundary definitions by omitting the horizontal tides: morphodynamic simulations of the local model show virtually identical results as the larger model predictions. The reduction in complexity in setting up a local model instead of a regional model coupled with the corresponding significant reduction in computational time points to an increasing applicability of complex process-based models.     

Storm-built sand ridges on the Maryland inner shelf: a preliminary report

Several aspects of the Maryland ridge field are pertinent to the problem of ridge genesis in response to Holocene sea-level rise. There is a systematic morphologic change fromshoreface ridges throughnearshore ridges tooffshore ridges, which reflects the changing hydraulic regime. Grain size is 90° out of phase with topography, so that the coarsest sand lies between the axis of each trough and the adjacent seaward ridge crest, while the finest sand lies between each ridge crest and the axis of the adjacent seaward trough. Finally, analysis over a 43-year period on an outer ridge reveals a systematic pattern of landward flank erosion, seaward flank deposition, and seaward crest migration. These relationships support a model which explains the ridges as consequences of the up-current shift of maximum bottom shear stress with respect to the crests of initial bottom irregularities. The oblique orientation of the ridges with respect to the beach may be at least partly due to the more rapid migration rate of the ridges’ inshore ends.     

Input reduction for long-term morphodynamic simulations in wave-dominated coastal settings

Input reduction is imperative to long-term (> years) morphodynamic simulations to avoid excessive computation times. Here, we introduce an input-reduction framework for wave-dominated coastal settings. Our framework comprises 4 steps, viz. (1) the selection of the duration of the original (full) time series of wave forcing, (2) the selection of the representative wave conditions, (3) the sequencing of these conditions, and (4) the time span after which the sequence is repeated. In step (2), the chronology of the original series is retained, while that is no longer the case in steps (3) and (4). We apply the framework to two different sites (Noordwijk, Netherlands and Hasaki, Japan) with multiple nearshore sandbars but contrasting long-term offshore-directed behavior: at Noordwijk the offshore migration is gradual and not coupled to individual storms, while at Hasaki the offshore migration is more episodic, and wave chronology appears to control long-term evolution. The performance of the model with reduced wave climates is referenced to a simulation with the actual (full) wave-forcing series. We demonstrate that input reduction can dramatically affect long-term predictions, even to such an extent that the main characteristics of the offshore bar cycle are no longer reproduced. This was particularly the case at Hasaki, where all synthetic series that no longer capture the initial chronology (steps 3 and 4) lead to rather unrealistic long-term simulations. At Noordwijk, synthetic series can result in realistic behavior, provided that the time span after which the sequence is repeated is not too large; the reduction of this time span has the same positive effect on the simulation as increasing the number of selected conditions in step 2. We further demonstrate that, although storms result in the largest morphological change, conditions with low to intermediate wave energy must be retained to obtain realistic long-term sandbar behavior. Our input-reduction framework must be applied in an iterative fashion as to obtain a reduced wave climate that simulates long-term sandbar sufficiently accurately within an acceptable computation time. Given its potentially huge impact on the actual simulation, we believe it is imperative to consider input reduction as an intrinsic part of model set-up, calibration and validation.     

Shoreline accretion and sand transport at groynes inside the Port of Richards Bay

The south-western shoreline along the entrance channel inside the Port of Richards Bay has experienced continued erosion. Four groynes were constructed to stabilise the shoreline. Monitoring of shoreline evolution provided valuable data on the accretion adjacent to two of the groynes and on the sediment transport rates at these groynes. Tides, beach slopes, winds, wave climate, current regime, and sand grain sizes were documented. The one site is “moderately protected” from wave action while the other is “protected” according to the Wiegel [Wiegel, R. L. (1964). Oceanographical engineering. Prentice Hall, Inc., Englewood Cliffs, NJ.] classification. The shoreline accreted progressively at the two groynes at 0.065 m/day and 0.021 m/day respectively. The shorelines accreted right up to the most seaward extremity of the groynes. Equilibrium shorelines were reached within about 3.5 years to 4 years, which compare well with other sites around the world. The mean wave incidence angle is large and was found to be about 22°. The median sand grain sizes were 0.33 mm and 0.37 mm. The groynes acted as total traps, the beach surveys were extended to an adequate depth, and cross-shore sediment transport did not cause appreciable net sand losses into the entrance channel. The net longshore transport rate along the study area, which is north-westbound, is only slightly lower than the gross longshore transport. The actual net longshore transport rates are 18 000 m³/year and 4 600 m³/year respectively at the two groynes. A rocky area limits the availability of sand at one groyne. There is fair agreement between the predicted and measured longshore transport rates at the other groyne.     

A parametric model for barred equilibrium beach profiles

Environment predictions for locations for which bathymetric data is missing, poor or outdated requires the use of some sort of representative bathymetric form, usually one that is concave up but monotonic. We propose and test a parametric form that superimposes realistic sand bars (Ruessink et al., 2003b) on a background profile that mixes a concave up nearshore with a planar far field behavior. Implementation at any new site involves estimation of five parameters, three that can be found from approximate information from climatology or old offshore charts, one that can be estimated by almost any remote sensing modality and one, h_sea that is less well understood but mostly affects deeper bathymetry that has little impact on the resulting surf zone hydrodynamics. Tests against several hundred surveys at three diverse locations show that bathymetry is better estimated by the new barred form than with a previous monotonic profiles in about 80% of cases. The remaining cases are usually associated with the parametric prediction of bars that look realistic but are out of phase. The presence of parametric bars has an even greater impact on predicted hydrodynamics since wave breaking is concentrated at sand bar locations. Modeled cross-shore transects of alongshore current and wave height over the measured survey profile are well represented by modeled transects over the barred parametric form but not for results over a Dean profile. The peak alongshore current strength and location are particularly sensitive to the presence of a sand bar.     

Bedforms and depositional sedimentary structures of a barred nearshore system, eastern Long Island, New York

The depositional sedimentary structures and textures of a single-barred nearshore system on the Atlantic coast of eastern Long Island, New York, were studied along seven shore-normal transects. Data along these transects consisted of textural analysis of 160 sediment samples, temporal bedform observations, and 42 can cores for the analysis of sedimentary structures.

Six sedimentary subenvironments were observed, based on distinct combinations of sediment color and texture, bedforms, physical, and biogenic sedimentary structures, and benthic infaunal communities. The shoreface environment is divided into the upper shoreface, the longshore trough, and the longshore bar. The divisions of the inner shelf environment are the shoreface-inner shelf transition, the offshore, and the coarse-grained deposit. The first five subenvironments are arranged in bands parallel to the shoreline, whereas the coarse-grained deposit occurs in patches across the inner shelf.

The location of fair-weather wave base, coinciding with a reduction in slope (3.0–0.3°) from the shoreface to the inner shelf, is characterized by the cessation of debris surge in the troughs of ripples, the formation of a “rust layer” of microorganisms over the bedform surface, and a sediment color change caused by an increase in organic detritus. The sequence of bedforms and physical sedimentary structures observed in this system fits well with existing wave-generated (oscillatory) flow regime models. These models explain the observed sequences as a response to the degree of asymmetric flow created by shoaling waves. Distribution of biogenic structures and assemblages of infaunal organisms is influenced by the distance landward or seaward of fair-weather wave base.

The overall relationships of this nearshore system can then be summarized as a hypothetical prograding stratigraphic sequence. The entire sequence is underlain by organic-rich, bioturbated, offshore deposits. Overlying the offshore is the planar-laminated sediments of the transition. Grading upward from the transition are the cleaner, planar-laminated, seaward slope deposits of the longshore bar. Above this, is a distinct erosional surface indicating the base of the massive to cross-laminated coarse sediments of the longshore trough. Capping the sequence are the cross- to planar-laminated, clean sands of the upper shoreface and foreshore.     

Sediment flux and equilibrium slopes in a barred nearshore

Estimates of time-integrated values of total (ITVF) and net (INVF) sediment volume flux and the associated changes in bed elevation and local slope were determined for a crescentic outer nearshore bar in Kouchibouguac Bay, New Brunswick, Canada, for eight discrete storm events. A 100 × 150 m grid of depth-of-activity rods spaced at 10 m intervals was used to monitor sediment behaviour on the seaward slope, bar crest and landward slope during the storms, at which time winds, incident waves and near-bed oscillatory currents were measured. Comparisons between storm events and between these events and a longer-term synthetic wave climatology were facilitated using hindcast wave parameters. Strong positive correlations between storm-wave conditions (significant height and total cumulative energy) and total volume flux contrasted strongly with the zero correlation between storm-wave conditions and net volume flux. ITVF values ranged up to 1646 m³ for the experimental grid and were found to have power function relations with significant wave height (exponent 2) and cumulative wave wave energy (exponent 0.4); values of INVF ranged from 0 up to 100 m³ for the same grid indicating a balance of sediment volume in the bar form through time. Sediment reactivation increased linearly with decreasing depth across the seaward slope and bar crest reaching maxima of 20 cm for the two largest storms; bed elevation, and thus slope, changes were restricted to the bar crest and upper landward slope with near zero morphological change on the seaward slope. The latter represents a steady-state equilibrium with null net transport of sediment under shoaling waves. Measurements of the asymmetry of orbital velocities close to the bed show that the energetics approach to predicting beach slope of Inman and Bagnold (1963) is sound. Gradients predicted vary from 0.01 to 0.03 for a range of angles of internal friction appropriate to the local sediment (tan ø = 0.3–0.6). These compare favorably with the measured seaward slope of 0.015 formed under average maximum orbital velocities of 1.12 m s⁻¹ (landward) and 1.09 m s⁻¹ (seaward) recorded during the period of the largest storm waves.