Bar Migration Research Articles

Nearshore sandbar rotation at single‐barred embayed beaches

AbstractThe location of a shore‐parallel nearshore sandbar derived from 7 years of video imagery data at the single‐barred embayed Tairua Beach (NZ) is investigated to assess the contribution of barline rotation to the overall morphodynamics of sandbars in embayed environments and to characterize the process of rotation in relation to external conditions. Rotation induces cross‐shore barline variations at the embayment extremities on the order of magnitude of those induced by alongshore uniform cross‐shore migration of the bar. Two semiempirical models have been developed to relate the barline cross‐shore migration and rotation to external wave forcing conditions. The rotation model is directly derived from the cross‐shore migration model. Therefore, its formulation advocates for a primary role of cross‐shore processes in the rotation of sandbars at embayed beaches. The orientation evolves toward an equilibrium angle directly related to the alongshore wave energy gradient due to two different mechanisms. Either the bar extremities migrate in opposite directions with no overall cross‐shore bar migration (pivotal rotation) or the rotation relates to an overall migration of the barline which is not uniform along the beach (migration‐driven rotation). Migration and rotation characteristic response times are similar, ranging from 10 to 30 days for mild and energetic wave conditions and above 200 days during very calm conditions or when the bar is located far offshore.

Investigating coastal geomorphological response to the passage of Hurricane Dean 2007 in the Southern Caribbean: Cocos Bay, Trinidad

<span style="font-size: 10.5pt; font-family: 'Times New Roman','serif'; mso-bidi-font-size: 12.0pt; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA;" lang="EN-US">Hurricanes form part of the annual weather phenomenon for many Caribbean territories during the months of June to November. Trinidad and Tobago lies south of the main hurricane belt and direct hits from these events are seldom, although, their associated effects from swell waves and spiral bands are experienced. This paper investigates the passage of Hurricane Dean on 17<sup>th </sup>August 2007, and its associated effects on the beach profiles and sediment of Cocos Bay. While this hurricane did not make landfall, the effects on the coastal system are due to the storm generated swells which travelled outwards from the storm centre toward the east coast of Trinidad. Data was collected from 2005 to 2007 and included littoral processes, beach profiles and sediment data using standard geomorphological techniques. The beach's response to the event varied spatially along the bay. While beach erosion is typical at other Caribbean beaches during the passage of these extreme weather systems, at Cocos Bay, some areas accreted while others eroded. Results indicate that the beach's response to this high energy event is dependent on the state of the beach prior to the event. Morphological change seem to be fuelled by bar formation and migration which is enhanced by the passage of a high energy event. A proper understanding of the effects of high energy events on low-latitude tropical beach systems exposed to the Atlantic is invaluable towards informing proper management in a region forecasted to have increased and enhanced Tropical Cyclonic activity due to global warming.</span>

Network response to disturbances in large sand-bed braided rivers

Abstract. The reach-scale effects of human-induced disturbances on the channel network in large braided rivers are a challenge to understand and to predict. In this study, we simulated different types of disturbances in a large braided river to get insight into the propagation of disturbances through a braided channel network. The results showed that the disturbances initiate an instability that propagates in the downstream direction by means of alteration of water and sediment division at bifurcations. These adjustments of the bifurcations change the migration and shape of bars, with a feedback to the upstream bifurcation and alteration of the approaching flow to the downstream bifurcation. This way, the morphological effect of a disturbance amplifies in the downstream direction. Thus, the interplay of bifurcation instability and asymmetrical reshaping of bars was found to be essential for propagation of the effects of a disturbance. The study also demonstrated that the large-scale bar statistics are hardly affected.

Minimally Invasive Repairs of Pectus Excavatum: Surgical Outcomes, Quality of Life, and Predictors of Reoperation

We sought to examine our institutional experience (1998 to 2014) with minimally invasive repairs of pectus excavatum (MIRPE). We conducted a retrospective review and a mailed survey (quality of life assessment). Associations with reoperation due to bar migration and recurrence after bar removal were evaluated with logistic regression. Three hundred and thirteen patients (79% male) underwent MIRPE at a mean ± SD age of 15 ± 3 years. Bar migration requiring reoperation occurred in 16 (5%) patients (median 26 days, interquartile range 15 to 70 days from repair). Wire fixation (hazard ratio [HR] = 3.16; p = 0.014) and bar stabilizer (HR = 4.57; p = 0.002) use were associated with increased risk of reoperation, and bilateral pericostal suture fixation (HR = 0.15; p < 0.001) and thoracoscopic assistance (78%, HR = 0.23; p < 0.001) were associated with decreased risks. Reoperations rates varied (6% to 26%) during the first 50 cases of each surgeon (n = 6), falling to ≤2% afterward. Of the 101 (32%) patients who have had their bars removed electively, 10 (10%) have required reoperation for recurrence. Patients with a recurrence after bar removal were younger (14.1 ± 3.9 years vs 18.4 ± 3.7 years; p = 0.007) and had their bars removed earlier (2.4 ± 1.2 years vs 3.8 ± 2.1 years; p = 0.036). Of survey respondents (n = 145 [47%]), most (99%) were either very happy (n = 79) or mostly happy (n = 63) with their outcomes. Although excellent outcomes after MIRPE can be achieved, our results highlight identified strategies that are associated with decreased risk of reoperation (eg, use of bilateral pericostal suture fixation, surgeon experience, and thoracoscopic guidance). Our results also suggest that elective bar removal should be delayed until the patient is at least 18 years old and has had the bar in for at least 4 years.

Targeted antibiotic prophylaxis promotes microbial growth and reduces the incidence of necrotizing enterocolitis (NEC)

RESULTS: During the study period, 311 patients (79% M) underwent repair at a median age of 14 years (IQR 13-16). Median Haller index was 4 (IQR 3.5-4.9). Median duration of follow up was 3.8 years (range 0-16 years), reoperations were necessary in 26 (8%) patients and 23 (7%) patients experienced bar migration (of which 16 [5%] required reoperation, median time from repair of 26 days [IQR range 15-70]). Of the 101 (32%) patients who have had their bars removed electively, 10 (10%) required reoperation for recurrence (mean SD, 29 14 vs 46 25 months for non-recurrence, p1⁄40.036). Method of bar fixation was associated with reoperation: wire fixation (HR1⁄43.16, p1⁄40.014) and bar stabilizer (HR1⁄44.57, p1⁄40.002) use was associated with increased risk, while bilateral pericostal suture fixation with decreased risk (HR1⁄40.15, 95% CI: 0.06-0.40, p 50 cases1⁄4 <2%). Thoracoscopic assistance (78%) was associated with decreased risk of reoperation (HR 0.23, p<0.001). Survival free of reoperation at 1 and 5 years was 95% and 93%, respectively.

Predictors of bar migration and reoperation after minimally invasive pectus excavatum repairs

Predictors of bar migration and reoperation after minimally invasive pectus excavatum repairs

Stratigraphic Expression of Intra-Point-Bar Erosion and Rotation

Abstract Meander belts are characterized by a complex amalgam of point bars and associated depositional elements, such as oxbow-lake fills. Point-bar deposits are composite elements often characterized by complex scroll-bar patterns formed in response to processes of expansion, rotation, and translation of a meander bend. Intra-point-bar erosion is commonly observed in modern river systems, yet its product is rarely described from the ancient record. Late Cretaceous meander-belt deposits, consisting of a transition from a point-bar to counter-point-bar deposit, as well as an associated abandoned channel fill, crop out in south central Alberta, Canada. The strata are characterized by widespread evidence for intra-point bar deposit erosion and punctuated rotation. In the stratigraphic record, intra-point bar deposit erosion and punctuated rotation is expressed by steeply dipping discordant surfaces (up to 20°), which truncate previously deposited lateral-accretion packages. Across these erosion surfaces the direction of bar migration often rotates significantly, up to 50°. Subsequent accretion surfaces onlap intra-point-bar-deposit erosion surfaces. The analysis reveals that point-bar surfaces are formed through a complex interplay of erosional and depositional processes along their length, resulting in stratigraphic surfaces that may be highly composite. This study departs from more simplistic process models of point bars that consider deposits to be the product of continuous lateral accretion, and as such has important implications for paleoenvironmental reconstructions and subsurface reservoir characterization.

Bar response to tides under regular waves

The morphological time scale for the response of outer bar crest level to tidal water-level changes is found to be remarkably short, namely around 15min for near-prototype wave heights and depths and regular waves. This corresponds to the bar crest level range being 0.77 times the tidal range with a time-lag of only 15min. Only one near-prototype scale experiment with tide and sufficient time resolution is known to the writers, namely Kraus & Larson (1988) Case 911, which had hmax=4.4m, d50=0.4mm, a semidiurnal tide range of ca 1m and regular waves with H=1.34m, T=7.9s. However, the fast crest-level response is also seen in profile development studies without tide, i.e., the bar crest level generally gets close to its final value in an hour or so while the overall profile adjustment (e.g. seaward bar migration) continues for tens of hours.

Stratal Architecture and Morphodynamics of Downstream-Migrating Fluvial Point Bars (Jurassic Scalby Formation, U.K.)

Abstract Assessments of depositional architecture and paleodrainage in planform are increasingly recognized as critical for the reconstruction of ancient fluvial morphodynamics; however, planview exposures of ancient, undeformed meandering-fluvial deposits are scattered, and exceptional occurrences such as the exhumed meander plain of the Scalby Formation (Yorkshire, UK) deserve particular attention. Here we specifically focus on the planview style and morphodynamics of downstream-migrating point bars, providing parallels with modern meandering rivers sharing scale and processes. The Middle Jurassic Scalby Formation lies in the Cleveland Basin of Yorkshire (UK), and accumulated in incised valleys eroded onto shallow-marine strata. Five well preserved downstream-migrating point bars are exposed along a modern tidal platform. Through 1:2000-scale mapping, point bars are analyzed for sedimentology, stratal architecture, vectors of paleoflow and bar migration, and nature of adjacent and underlying deposits. Results indicate that downstream migration was facilitated by—but not strictly related to—the occurrence of resistant outer banks. Meander-bend migration promoted cannibalization of upstream-bar portions, subordinate preservation of central-bar portions, and widespread development of downstream-bar scrolls. The most reliable indicators of regional transport are upstream-bar paleoflow indicators and the dip direction of downstream-bar inclined beds. The generated sectional architecture overlaps in part with that of other fluvial depositional models, such as those for braid bars, indicating that lack of planform exposures may introduce bias when reconstructing ancient fluvial morphodynamics.

Paleohydrology and 3D Facies Architecture of Ancient Point Bars, Ferron Sandstone, Notom Delta, South-Central Utah, U.S.A

Abstract Most outcrops of fluvial deposits consist of a series of cliff exposures, either natural or manmade (e.g., roadcuts). Predictions, and especially observation of plan-form geometry, such as might be made with numerical experiments or from studies of modern rivers, are challenging to test in most ancient outcrops. This study examines ancient exhumed channel belts from the Cretaceous Notom Delta of the Ferron Sandstone Member in south-central Utah. Extensive plan-view exposures with local vertical cliff exposures allowed documentation of channel plan-form, channel-belt dimension, bar migration patterns (translation versus expansion), and cross-sectional facies architecture. Channel-fill thickness and bedding structure, documented from the cliff exposures, were used in paleohydraulic reconstructions. Approximately 270 paleocurrent directions were integrated with grain-size measurements to reconstruct the 3D facies architecture. Paleocurrent measurements are consistent within specific facies architectural units (such as unit bars) and show systematic variation at the channel-belt scale that can be used to infer channel and bar migration patterns. In this example, the migration pattern of the single-thread channel bend was interpreted to change from expansion to translation with a corresponding bend sinuosity that increased from 1.01 to 1.44. Inclined large-scale foresets are interpreted to be indicative of unit bars. Empirical equations result in estimated average channel depths from 1.7 m to 3.6 m with corresponding widths of 23 m to 89 m respectively. These empirical estimates match the dimensions measured in the field. For example, channel widths of around 50 m were measured from abandoned channel fills. Bar thickness, measured from vertical outcrops, ranges from 5.4 m to 6.3 m, which yields a narrower estimate of channel width ranging from 47 to 59 m. Integration of sediment size, bedforms type, and channel depth were used to estimate the discharge of the river, which is on the order of < 400 m3/s. This suggests that the Ferron deltas were characterized by small, steep-gradient “dirty” rivers, which is consistent with the hyperpycnal nature of linked downstream deltaic systems.

Onshore sandbar migration in the surf zone: New insights into the wave‐induced sediment transport mechanisms

AbstractWe present a novel process‐based morphodynamic model, which includes transport processes due to both velocity and acceleration skewness and a new formulation for intrawave motions, that successfully simulates observations of onshore sandbar migration. Results confirm findings of previous studies, in which each process was considered separately and in which sediment transport was computed from the observed water motion. However, our results indicate that accounting for the joint action of both velocity and acceleration skewnesses causes major improvement of the modeled onshore bar migration and is essential to accurately model the evolution of the entire cross‐shore bottom profile, when compared with observations. We also demonstrate that the morphodynamics in the shoaling zone are dominated by velocity skewness (bed shear stresses), while sediment transport induced by acceleration skewness (pressure gradients) controls the morphodynamics in the inner surf zone.

Modelling of sedimentation processes inside Roseires Reservoir (Sudan)

Abstract. Roseires Reservoir, located on the Blue Nile River in Sudan, is the first trap to the sediments coming from the vast upper river catchment in Ethiopia, which suffers from high erosion and desertification problems. The reservoir has already lost more than one-third of its storage capacity due to sedimentation in the last four decades. Appropriate management of the eroded soils in the upper basin could mitigate this problem. In order to do that, the areas providing the highest sediment volumes to the river have to be identified, since they should have priority with respect to the application of erosion control practices. This requires studying the sedimentation record inside Roseires Reservoir in order to assess when and how much sediment is deposited and to identify its source. This paper deals with the identification of deposition time and soil stratification inside the reservoir, based on historical bathymetric data, numerical modelling and newly acquired soil data. The remoteness of the study area and the extreme climate result in coring campaigns being expensive and difficult. Therefore, these activities need to be optimised and coring locations selected beforehand. This was done by combining bathymetric data and the results of a depth-averaged morphodynamic model recording the vertical stratification in sediment deposits. The model allowed for recognising the areas that are potentially subject to neither net erosion nor bar migration during the lifespan of the reservoir. Verification of these results was carried out by analysing sediment stratification from the data collected during the subsequent field campaign.

식생생장 영향을 고려한 하도변화에 대한 수치모의

본 연구에서는 2차원 흐름/유사이동 모형에 식생생장모형을 추가하여 하도의 식생 활착 및 성장에 의한 지형변화 과정과 특성을 수치모의 하였다. 교호사주가 발달하는 조건에서 식생 이입 및 활착은 사주의 이동을 감소시켰다. 식생면적 및 하폭의 변화는 저유량 지속시간보다 상류유량 변화에 더 크게 영향을 받았다. 상류유량이 감소하면 식생면적은 증가하고 하도폭은 감소하였다. 망상하도가 발달하는 조건에서 하도내 식생 이입 및 활착은 지형변화 특성에 크게 영향을 미쳤다. 망도하도에서 식생은 망상의 수를 감소시키고 결국 상류유량이 크게 감소하면 하도지형을 망상하도에서 단일수로로 변화시켰다. 식생면적은 상류유량이 증가함에 따라 감소하였다. 하도폭은 식생 도입 후 급격히 줄어들었고 상류유량 감소와 함께 감소하였다. 수치모의를 이용하여 홍수량 감소가 하도 내의 식생 이입 및 활착을 가속시키고 이로 인해 하도변화 특성에 미치는 영향을 정성적으로 확인할 수 있음을 보였다. In this study, the numerical simulation regarding the process and characteristics of topography change due to the vegetation recruitment and growth was carried out by adding the vegetation growth model to two-dimensional flow and sediment transport models. The vegetation introduction and recruitment on the condition for developing an alternate bar reduced the bar migration. The vegetated area and channel width changes were more significantly influenced by changes in upstream discharge rather than the duration of low flow. When the upstream discharge decreased, the vegetation area increased and the channel width decreased. The vegetation introduction and recruitment on the condition for developing a braided channel significantly influenced the characteristics of topography changes. In the braided channel, vegetation reduced the braided index, and when the upstream discharge decreased significantly, the channel topography was changed from the braided channel to the single channel. The vegetation area decreased as the upstream discharge increased. The channel width decreased significantly after the vegetation was introduced and it also decreased as the upstream discharge decreased. It was confirmed through the numerical simulation that a decrease in flood discharge accelerated the vegetation introduction and recruitment in the channel and this allowed to confirm its influence on the characteristics of topography changes qualitatively.

Process-based modeling of cross-shore sandbar behavior

A coupled wave–current–sediment transport beach profile model is used to simulate cross-shore sandbar evolution on the time scale from days to months comprising both rapid offshore and slow onshore migrations. The discrimination of four modes of sediment transport driven by velocity and acceleration skewness, mean currents and slope effects allows addressing the dominant hydrodynamic processes governing cross-shore sandbar behavior. Acceleration–skewness-induced transport systematically results in a slow onshore sandbar migration together with a slow bar growth. Velocity–skewness-induced transport can drive onshore and offshore bar migrations with substantially larger rates. Mean–current-induced sediment transport systematically drives an offshore bar migration with either bar growth or decay. Slope effects essentially act as a damping term. The water level above the sandbar crest mainly influences the sandbar migration direction, while wave obliquity regulates the magnitude of the migration rates and is crucial to accurately simulate offshore sandbar migration during energetic obliquely incident waves. The inclusion of acceleration skewness is a necessary requirement to accurately reproduce the onshore migration of shallow sandbars. Detailed inter-site comparison of best-fit model parameters shows large differences meaning that free parameters attempt to compensate some mispecifications of the physics in the model. Although this also applies to other existing beach profile models, this suggests that this model needs further improvements including, for instance, the contribution of the injection of breaking wave turbulence onto the bed to sand stirring.

Facies analysis of a Pliocene river‐dominated deltaic succession (Siena Basin, Italy): Implications for the formation and infilling of terminal distributary channels

AbstractThe recent recognition and description of terminal distributary channels in ancient delta‐front deposits have led to important progress when it comes to understanding: delta construction processes; the plan‐view shapes of both modern and ancient river‐dominated deltas; and sediment dispersal within deltas. Deltaic deposits fed by terminal distributary channels can be observed in the extensive outcrops (approximately up to 300 m wide and 50 to 70 m thick) in the central sector of the Pliocene Siena Basin in Tuscany, Italy. Stratigraphic constraints indicate that the deposition occurred in relatively shallow marine settings, was not affected by tidal effects and was influenced only locally by wave action. As a consequence, these rock exposures provide an opportunity to investigate the facies assemblage of an ancient delta complex, which was built up almost exclusively as the result of river‐related processes. The studied deltaic succession is composed of vertically stacked parasequence‐like units, which are invariably characterized by the lack of coarse‐grained distributary channel deposits at the top and an extensive network of terminal distributary channels that controlled the deltaic deposition. These deposits were interpreted previously as deep‐water turbidites. This study provides new elements for the correct distinction between hyperpycnal‐dominated delta fronts and basinal turbidite deposits. The terminal distributary channel facies documented in this study are more complex than those investigated in previous research. The channels are often passively filled during abandonment phases, and not just due to the aggradation and lateral/upstream migration of mouth bars, as classically proposed. Moreover, the frequent occurrence of mudstone beds just above the basal channel scours may be indicative of an abandonment phase after the channel incision, and before its infilling, with subsequent implications for the formational and infilling mechanisms of terminal distributary channels. Furthermore, the channel fills are often the result of alternating depositional and erosional phases, leading to a complex stratigraphic arrangement.

Role of wave forcing, storms and NAO in outer bar dynamics on a high-energy, macro-tidal beach

Outer sand bar dynamics on a high-energy macro-tidal beach were investigated using long-term (multi-year) field datasets of intertidal morphology and offshore bathymetry. Utilising a 15-year time-series of Argus video images, five distinct outer bar types were identified: Mega Rip, Longshore, Crescentic, Crescentic Attached and Welded. The most common classification was Crescentic Attached, with the outer bar oscillations being out of phase with the inner bar oscillations, as would be expected for the shore-normal wave approach at this site. The outer bar had a typical amplitude of 0.5–1m and a longshore wavelength of 600m. Changes in outer bar morphology were related to measured and modelled nearshore wave data. However, the outer bar morphology changed over a much longer timescale (monthly-to-annual) than the daily-to-weekly variations in wave height and period. An extended duration of energetic wave action was required to bring about an upstate bar transition to the Longshore or Mega Rip state, where the bar then remained arrested for a significant amount of time, requiring several months of low wave conditions to induce a down state transition through Crescentic to Welded. This slow morphological response is explained by extended relaxation times attributed to the large tidal range at the study site where the outer bar morphology is only active for part of the tidal cycle (several hours around low tide). The configuration and position of the outer bar were related: the more upstate (downstate) bar types being associated with a more offshore (onshore) bar position. The detrended outer bar position was significantly related to a forcing term based on wave power and disequilibrium of the dimensionless fall velocity with offshore (onshore) bar migration occurring when wave conditions were more (less) energetic that the antecedent conditions. The upstate end-member (Longshore or Mega Rip) was attained sometime during the winter months for 14 out of the 16years of monitoring; the outer bar remained attached to the low tide shoreline over the winter 2005/2006 and 2010/2011. These two winters, with incomplete upstate cycles, were characterised by the lowest winter wave conditions and negative winter North Atlantic Oscillation (NAO) indices, suggesting that the winter NAO is correlated with both beach state and nearshore bar configuration.

Experience with FiberWire for pectus bar attachment

BackgroundMinimally invasive repair of pectus excavatum has become an established method for repair of pectus excavatum. Bar displacement or rotation remains the most common complication of this repair requiring return to the operating room. MethodsRetrospective review of all patients at a single institution who underwent repair of pectus excavatum using FiberWire for bar stabilization between December 2009 and March 2013 was undertaken. Results93 patients underwent minimally invasive pectus repair using FiberWire during the study period. The patients included 73 males and 20 females, with an average age of 14.6years (range 7–21years). Mean operative time was 102minutes (range 56–198minutes). No patients developed wound complications, two patients developed pain because of bar migration and required return to the OR, and no patients had recurrence of their pectus defect because of bar migration during the study period. Median length of follow-up was 17months (range 3–36months). ConclusionStabilization of pectus bars using circumferential rib fixation with FiberWire at multiple points on both sides of the bar appears to be effective in preventing bar rotation and displacement, and requires minimal change to the operation as it has been previously described. Early experience shows a low rate of complications.

Morphological diversity and complex sediment recirculation on the ebb delta of a macrotidal inlet (Normandy, France): A multiple LiDAR dataset approach

The shoreline in the vicinity of inlets can exhibit considerable variability in morphology in both space and time. Most studies on inlets and their adjacent shores have focused on the morphodynamics of sediment by-passing mechanisms generated by longshore transport. For the first time, the morphology, sedimentary features, sediment budgets and patterns of evolution of the shoreline and ebb delta in a macrotidal inlet system have been investigated using seven LiDAR topographic surveys in Normandy, France, over a period of 3.7years from February 2009 to October 2012. The ebb delta shows strong development on the northern flank of the inlet, expressed by a large sand spit and two types of superimposed dynamic sandy features: eight long-crested and highly mobile transverse bars and a large swash bar. Sand transport from N–S on the updrift beach feeds the growth of the distal part of the spit. This sand supply is further augmented by the onshore movement of a large swash bar welding to the upper foreshore. However, the main topographic changes were induced by the northward migration of the transverse bars on the ebb platform. This is driven by strong northward-directed tidal currents parallel to the shore. The bars exhibit a more complex morphology and dynamics along the seaward margin of the ebb delta where their mobility is controlled by wave action. Topographic measurements suggest a clear sand recirculation pattern. In this morphodynamic model, sand coming from the updrift upper beach is transported southward and deposited at the distal end of the spit, where it serves to construct transverse bars close to the tidal inlet. Transverse bar migration ends in the wave-exposed northern margin of the ebb delta, where they are integrated into the shallow dissipative shoreface sand sink. This sink nourishes the southward longshore transport to feed growth of the large swash bar and southward spit elongation. This semi-circular recirculation cell model involves an inversion of sand movement close to the inlet and emphasizes the combined role of tidal currents and waves in the large-scale 3D ebb–delta sediment dynamics in this macrotidal setting, in contrast to the much more commonly reported alongshore sediment by-passing mode of microtidal inlets.

Interactions between flow structure and morphodynamic of bars in a channel expansion/contraction, Loire River, France

AbstractThe study of the relationship between flow structure and morphodynamic of bars in a channel expansion/contraction is essential to better understand the processes that control the evolution of rivers. Thus, multibeam echosoundings and Acoustic Doppler Profiler (ADP) measurements were performed with a high temporal resolution in an expansion/contraction zone of the Loire River (France) occupied by bars. During the monitoring period, the macroforms presented successively an alternate, a lateral and a transverse configuration. Field data were analyzed to study how the primary and secondary velocities, the flow directions, the bed shear stresses, and the bed roughnesses (associated to dunes) evolve as a function of the water discharge and bars configuration. The bars modify the flow structure imposed by the channel width variations. In fact, the bars induce a topographic forcing which enables the separation and reducing of the mixing of two currents formed in the upstream channel expansion. This forcing is enhanced by the turbulence formed by the large dunes superimposed on the bars. Therefore, the bars promote a nonuniform flow in the channel. In turn, in the channel expansion/contraction, the migrating bars' morphodynamics are affected by the downstream channel narrowing which stops their downstream migration and forced the bars in the system. Then the nonuniformity of the flow encourages the lateral migration of the macroforms until they reach a bank and become nonmigrating. Finally, the nonmigrating bars are eroded by the flow deflected during the migration of a new bar in the channel expansion/contraction.

Sediment supply to beaches: Cross‐shore sand transport on the lower shoreface

AbstractMany beaches have been built by an onshore supply of sand from the shoreface, and future long‐term coastal evolution critically depends on cross‐shore sediment exchange between the upper and the lower shorefaces. Even so, cross‐shore sediment supply remains poorly known in quantitative terms and this reduces confidence in predictions of long‐term shoreline change. In this paper, field measurements of suspended sediment load and cross‐shore transport on the lower shoreface are used to derive a model for sediment supply from the lower to the upper shoreface at large spatial and temporal scales. Data collection took place at five different field sites that exhibit a wide range of wave conditions and sediment characteristics. Data analysis shows that both suspended sediment load and cross‐shore sediment transport scale with the grain‐related mobility number which ranged up to ψ ≈ 1000 in the measurements while the effect of orbital velocity skewness is more limited. A 1 year long simulation of sediment transfers between the lower and the upper shorefaces on a natural beach compares well with transport rates estimated from long‐term bar migration patterns and aeolian accretion on the same beach.