Bar Migration Research Articles

Quantification of bedform dynamics and bedload sediment flux in sandy braided rivers from airborne and satellite imagery

AbstractImages from specially‐commissioned aeroplane sorties (manned aerial vehicle, MAV), repeat unmanned aerial vehicle (UAV) surveys, and Planet CubeSat satellites are used to quantify dune and bar dynamics in the sandy braided South Saskatchewan River, Canada. Structure‐from‐Motion (SfM) techniques and application of a depth‐brightness model are used to produce a series of Digital Surface Models (DSMs) at low and near‐bankfull flows. A number of technical and image processing challenges are described that arise from the application of SfM in dry and submerged environments. A model for best practice is presented and analysis suggests a depth‐brightness model approach can represent the different scales of bedforms present in sandy braided rivers with low‐turbidity and shallow (< 2 m deep) water.The aerial imagery is used to quantify the spatial distribution of unit bar and dune migration rate in an 18 km reach and three ~1 km long reaches respectively. Dune and unit bar migration rates are highly variable in response to local variations in planform morphology. Sediment transport rates for dunes and unit bars, obtained by integrating migration rates (from UAV) with the volume of sediment moved (from DSMs using MAV imagery) show near‐equivalence in sediment flux. Hence, reach‐based sediment transport rate estimates can be derived from unit bar data alone. Moreover, it is shown that reasonable estimates of sediment transport rate can be made using just unit bar migration rates as measured from 2D imagery, including from satellite images, so long as informed assumptions are made regarding average bar shape and height. With recent availability of frequent, repeat satellite imagery, and the ease of undertaking repeat MAV and UAV surveys, for the first time, it may be possible to provide global estimates of bedload sediment flux for large or inaccessible low‐turbidity rivers that currently have sparse information on bedload sediment transport rates. © 2018 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.

MEDIUM-TERM VARIATION OF BAR MOVEMENT AND ITS LINKAGE TO WAVE CLIMATE

Longshore bars are a common site in sandy beaches which is influential on currents, morphology variation and the marine eco system. Longshore bar reacts to the variation of environmental factors and wave properties like wave height, water depth, wave period, wave skewness and it tends to move seaward or shoreward while changing its amplitude. According to Lippmann and Holman, (1990) short term bar migration has been triggered when the wave height or the wave height to water depth ratio are large. Elgar et al. (2001) has shown that short term seaward bar migration was caused by velocity skewness, undertow velocity and acceleration skewness. Influence of the wave breaking on the seaward bar migration has been identified as minimum according to Sallenger and Howd (1989).

Are Bedload Transport Pulses in Gravel Bed Rivers Created by Bar Migration or Sediment Waves?

AbstractBedload transport exhibits considerable spatial and temporal variability, as reflected by large fluctuations of transport rates. Among the various mechanisms proposed for explaining this variability, bedform migration is often cited as the main cause. We took a closer look at this issue by running long‐duration experiments in a gravel bed flume using constant water discharge and sediment feed rates. We monitored bed evolution and measured bedload transport rates at the flume outlet using high‐resolution techniques. The bed was initially flat, but within a few hours bedforms consisting of alternate bars and pools had developed. The bars exhibited a stick‐slip motion: they were stable for long periods but moved episodically (every 10 hr on average). Their downstream migration produced 36% of the sediment volume transported, mostly in the form of intense pulses. Much of the transport was caused by the displacement of sediment waves from pool to pool.

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.

Characteristics of Systematic Migrations of Multiple Sandbars and Related Cross-Shore Sediment Transport at Chirihama and Adjacent Coasts, Japan

Yuhi, M. and Umeda, S., 2018. Characteristics of systematic migrations of multiple sandbars and related cross-shore sediment transport at Chirihama and adjacent coasts, Japan. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 231–235. Coconut Creek (Florida), ISSN 0749-0208.Characteristics of systematic migration of multiple bar system on the Chirihama and adjacent coasts, Japan, have been investigated based on a set of annual field surveys collected over 11 years. The morphological variations were first separated into time-averaged profiles and deviation components. An EOF analysis was conducted on the deviation components in order to extract the principal modes of periodic bar migration. The migration cycle consisting of generation, migration and degeneration were well reproduced by the combination of first and second modes. The cross-shore sediment transport and sediment volume variation related to bar migration were then examined. An integration of the equation for sediment volume conservation revealed that the principal modes of cross-shore sediment transport was characterized by the spatial integration of spatial eigen-functions and the temporal derivative of the temporal eigen-functions. Corresponding to the cyclic behaviors of sandbar system, the direction of sediment transport changed alternatively between seaward and landward directions. On short-term, cyclic exchange of sediment occurred between the bar migration zone and foreshore, but on long-term the principal modes of bar migration made little contribution to overall sediment volume variation.

Bar Migrations on a Macrotidal Ebb Delta over a Period of Six Years Using Lidar Survey

Pellerin Le Bas, X. and Levoy, F., 2018. Bar migrations on a macrotidal ebb delta over a period of six years using LiDAR survey. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 146–150. Coconut Creek (Florida), ISSN 0749-0208.The migration rates of two swash bars on an ebb delta are studied in a macrotidal environment. A dataset of 12 LiDAR surveys, from 2011 to 2017 is used to compute the movement of the swash bar crests using Digital Shoreline Analysis System (DSAS) version 4.3. Hydrodynamic data are acquired near the ebb-tidal delta during the same period. The relationship between bar migration rate and wave energy flux is analyzed. The correlation is high (r2 = 0.71), but a specific migration rate of the two bars weaken this result. Furthermore, the two swash bars have two different behaviors under the same hydrodynamic conditions. Other analyses are conducted concerning the relationships between the migration of the bar and its volume or the water depth. The correlations are too weak to explain the different behaviors of the bars. Another parameter is highlighted, the number of swash bars in the immediate vicinity of the two bars.

Breaker bar morphodynamics under erosive and accretive wave conditions in large-scale experiments

Differences between breaker bar evolution under high energy (erosive) and subsequent mild energy (accretive) wave conditions are studied based on three data sets from large-scale morphodynamic experiments. Under erosive wave conditions, a clear linear relationship between the height and the cross-shore location of the breaker bar is observed. The similar cross-shore shape of the sediment transport magnitudes underlines the similarity of the bar evolution under erosive wave conditions. Under the subsequent accretive wave conditions, two major evolution patterns are observed during onshore migration (decaying or non-decaying breaker bar). In the presented data, the feedback between the wave conditions and the beach morphology determines if a bar decays during onshore migration. This feedback involves that the wave breaking location depends on the morphology and on the wave conditions resulting in differences in sediment transport patterns between the two types of onshore bar migration. A comparison against numerous other experimental beach profile data sets strongly supports the linear relationship between bar height and bar location under erosive wave conditions. For accretive wave conditions, the number of comparable data sets is very limited. The comparison underlines that breaker bar evolution under accretive wave conditions cannot be condensed to a single pattern as under erosive wave conditions.

Automated extraction of meandering river morphodynamics from multitemporal remotely sensed data

We introduce PyRIS, an automated, process-based software for extracting extensive meandering and anabranching river morphodynamics from multitemporal satellite imagery, including a unique ability to quantify river bars dynamics. PyRIS provides three main computations: (i) detection of planform centerline including complex river patterns, (ii) computation of migration vectors between subsequent centerlines, and (iii) analysis of sediment bars dynamics. PyRIS was validated against several test cases in the Amazon River basin, specifically i) main channel extraction from the anabranching Amazon river, ii) migration analysis following a large cutoff on the Ucayali River and iii) detection of sediment bar migration on the Xingu River. Tests prove the capability of PyRIS to detect the main channel in anabranching structures and chute cutoffs. PyRIS can extract extensive morphodynamic information with unprecedented automation levels and reasonable computational effort (5 h for 28 Landsat images of a 240 km reach of the Xingu River on a 3.20 GHz Intel).

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).

Medium-term dynamics of a middle Adriatic barred beach

Abstract. In recent years, attention has been paid to beach protection by means of soft and hard defenses. Along the Italian coast of the Adriatic Sea, sandy beaches are the most common landscape feature and around 70 % of the Marche region's coast (central Adriatic) is protected by defense structures. The longest free-from-obstacle nearshore area in the region includes the beach of Senigallia, frequently monitored in the last decades and characterized by a multiple bar system, which represents a natural beach defense. The bathymetries surveyed in 2006, 2010, 2011, 2012 and 2013 show long-term stability, confirmed by a good adaptation of an analyzed stretch of the beach to the Dean-type equilibrium profile, though a strong short- to medium-term variability of the wave climate has been observed during the monitored periods. The medium-term dynamics of the beach, which deal with the evolution of submerged bars and are of the order of years or seasons, have been related to the wave climate collected, during the analyzed temporal windows, by a wave buoy located about 40 km off Senigallia. An overall interpretation of the hydrodynamics, sediment characteristics and seabed morphology suggests that the wave climate is fundamental for the morphodynamic changes of the beach in the medium term. These medium-term time ranges during which waves mainly come from NNE/ESE are characterized by a larger/smaller steepness and by a larger/smaller relative wave height, and seem to induce seaward/shoreward bar migration as well as bar smoothing/steepening. Moving southeastward, the bar dimension increases, while the equilibrium profile shape suggests the adaptation to a decreasing sediment size in the submerged beach. This is probably due to the presence of both the harbor jetty and river mouth north of the investigated area.

Unit-bar migration and bar-trough deposition: impacts on hydraulic conductivity and grain size heterogeneity in a sandy streambed

The hydrologic function of riverbeds is greatly dependent upon the spatiotemporal distribution of hydraulic conductivity and grain size. Vertical hydraulic conductivity (K v) is highly variable in space and time, and controls the rate of stream–aquifer interaction. Links between sedimentary processes, deposits, and K v heterogeneity have not been well established from field studies. Unit bars are building blocks of fluvial deposits and are key to understanding controls on heterogeneity. This study links unit bar migration to K v and grain size variability in a sand-dominated, low-sinuosity stream in Nebraska (USA) during a single 10-day hydrologic event. An incipient bar formed parallel to the thalweg and was highly permeable and homogenous. During high flow, this bar was submerged under 10–20 cm of water and migrated ~ 100 m downstream and toward the channel margin, where it became markedly heterogeneous. Low-K v zones formed in the subsequent heterogeneous bar downstream of the original 15–40-cm-thick bar front and past abandoned bridge pilings. These low-K v zones correspond to a discontinuous 1-cm layer of fine sand and silt deposited in the bar trough. Findings show that K v heterogeneity relates chiefly to the deposition of suspended materials in low-velocity zones downstream of the bar and obstructions, and to their subsequent burial by migration of the bar during high flow. Deposition of the unit bar itself, although it emplaced the vast majority of the sediment volume, was secondary to bar-trough deposition as a control on the overall pattern of heterogeneity.

Capturing the spatiotemporal variability of bedload transport: A time‐lapse imagery technique

AbstractMonitoring sediment transport in morphologically complex and labile channels remains a difficult task, even at the laboratory scale. To address this challenge, a fully automated imagery technique for continuously mapping the spatial and temporal variability of bedload transport is proposed. This method uses differentiated time‐lapse imagery taken from a fixed camera to detect bed variations induced by grain displacement. The technique is not based on tracking the individual particles; rather, it evaluates macroscopic colour changes within a region that contains several grains, which depend on the occurrence and intensity of the bedload transport. Image‐derived data were compared with the sediment flux measured during four flume experiments, and produced good correspondence. The method provides continuous tracking of the location of the transporting channels, and enables estimation of local variations in the magnitude of the bedload flux. Moreover, the spatial extent of the monitoring area offers an unprecedented opportunity to aggregate spatially dense and continuous data at the reach scale, as needed to properly capture the full range of variability of morphologically complex and rapidly evolving gravel‐bed rivers. Despite being limited to laboratory‐scale physical experiments, the method provides useful data to investigate fundamental morphodynamic processes such as bar migration, bank erosion, anabranches opening/closure, and the associate spatial and temporal scales. Further, the data obtained have the potential to enhance numerical model calibration and improve our understanding of the complex dynamics of real‐world settings. Copyright © 2017 John Wiley & Sons, Ltd.

Reservoir characterization and multiscale heterogeneity modeling of inclined heterolithic strata for bitumen-production forecasting, McMurray Formation, Corner, Alberta, Canada

Bitumen reservoirs dominated by inclined heterolithic stratification (IHS) formed in large point bars of the Aptian (Lower Cretaceous) McMurray Formation in the northwestern part of the Corner oil sand lease (Alberta, Canada) were investigated to establish their value. Hybrid production technologies were applied to thin pay, typified by homogeneous reservoir sand units thicker than 5 m at the base overlain by IHS (so-called ‘thin pay’), as well as IHS-dominated reservoirs in which the IHS extends down to the base of the reservoir. High-resolution seismic data and well data (core, dipmeter, HMI) were used to map four facies associations, comprising a total of 16 sedimentary facies, as well as various fluid contacts to assist in reservoir characterization and risk assessment. The conceptual depositional model was based on the analysis of the migration and re-orientation history of the IHS-dominated point bars reflecting lateral accretion, downstream migration, rotation and relocation of the bars. Multiple reactivation events, which control the heterolithic nature and reservoir quality of the deposits, create developable “pools”.Seven electrofacies (with generally increasing mud content) were defined and used as input to construct vertical proportion curves that relate the electrofacies distribution to geomodel statistics in the main reservoir zone. At the electrofacies scale, numerical effective porosity-permeability models were created using micromodeling and minimodeling concepts. The geometrical shape of the electrofacies in the geomodel was investigated using non-stationary Truncated Gaussian (TG) facies simulation to enforce the stacking patterns. Each geomodel area was characterized using one variogram to efficiently compute the horizontal and vertical variogram ranges and average azimuths. Sequential Gaussian simulation (SGS) was used to map the distribution of key petrophysical parameters such as effective porosity, effective water saturation and Vshale. Empirically derived saturation versus elevation profiles for each electrofacies were included in the modeling. The distributions from the micro- and mini-modeling were introduced using probability field (P-field) simulation. To investigate the amount of connected resources (the degree of connectivity of good sand as well as IHS) were extractable flow simulation studies were performed at the pad scale. In preparation for reservoir simulation, connectivity calculations within the local pool geomodel realizations were tailored for the reservoir heterogeneities (i.e., IHS) that are expected to have a major impact on the specific thermal and gravity drainage extraction processes. The geomodel realizations were ranked by expected pseudo-dynamic behaviour with connected exploitable pay as a critical parameter.

Storm impacts and shoreline recovery: Mechanisms and controls in the southern North Sea

Storm impacts play a significant role in shoreline dynamics on barrier coastlines. Furthermore, inter-storm recovery is a key parameter determining long-term coastal resilience to climate change, storminess variability and sea level rise. Over the last decade, four extreme storms, with strong energetic waves and high still water levels resulting from high spring tides and large skew surge residuals, have impacted the shoreline of the southern North Sea. The 5th December 2013 storm, with the highest run-up levels recorded in the last 60years, resulted in large sections of the frontline of the North Norfolk coast being translated inland by over 10m. Storms in March and November 2007 also generated barrier scarping and shoreline retreat, although not on the scale of 2013. Between 2008 and 2013, a calm period, recovery dominated barrier position and elevation but was spatially differentiated alongshore. For one study area, Scolt Head Island, no recovery was seen; this section of the coast is being reset episodically landwards during storms. By contrast, the study area at Holkham Bay showed considerable recovery between 2008 and 2013, with barrier sections developing seaward through foredune recovery. The third study area, Brancaster Bay, showed partial recovery in barrier location and elevation. Results suggest that recovery is promoted by high sediment supply and onshore intertidal bar migration, at rates of 40ma−1. These processes bring sand to elevations where substrate drying enables aeolian processes to entrain and transport sand from upper foreshores to foredunes. We identify three potential sediment transport pathways that create a region of positive diffusivity at Holkham Bay. During calm periods, a general westward movement of sediment from the drift divide at Sheringham sources the intertidal bar and foredune development at Holkham Bay. However, during and following storms the drift switches to eastward, not only on the beach itself but also below the –7m isobath. Sediment from the eroding barrier at Brancaster Bay, and especially Scolt Head Island, also sources the sediment sink of Holkham Bay. Knowledge of foredune growth and barrier recovery in natural systems are vital aspects of future coastal management planning with accelerated sea-level rise and storminess variability.

Plan-view Paleochannel Reconstruction of Amalgamated Meander Belts, Cretaceous Ferron Sandstone, Notom Delta, South-central Utah, U.s.a.

Abstract Few studies reconstruct paleohydraulic parameters, such as meander amplitude and sinuosity, of ancient rivers based on plan-view outcrop exposures, but rather rely on interpolation from cliff exposures. In this study, extensive plan-view exposures of amalgamated ancient meander belts in the Cretaceous Ferron Sandstone Member of the Mancos Shale Formation, Utah, USA, allow paleohydraulic reconstruction, evaluation of grain-size variability in meander belts, and their component unit and compound bars by incorporating the variability of paleocurrents, facies architectural-element analysis, and bar migration patterns. Three channel belts were identified, marked by scour surfaces, an increase in grain size, and abrupt changes in paleocurrent orientations. The youngest channel was about 2.1 m deep and 118 m wide, and had a meander wavelength of 1 km and a sinuosity of 1.1–2.9. The middle channel was about 3.4 m deep, with a meander wavelength of 2.6 km, and a sinuosity of 1.2. The oldest channel was insufficiently exposed to document its plan-view style. Grain-size trends showed systematic upwards fining in each channel story. In plan view, coarsening occurs towards the bend apex along the bend axis, and fining occurs downstream within some meander scrolls. Plan-form grain-size trends were not obvious, partially due to grain-size variation within unit bars, perhaps also reflecting the fact that the exposures capture variable vertical position in each channel belt. Paleocurrents showed systematically varying trends within individual channel belts (from NE, to SW, to E), and abrupt changes between belts. Grain size and vertical facies associations vary as a function of the style of bar migration, as well as position within a bar (upstream vs. downstream). The outcrop shows a dominance of a meandering-river style, with significant lateral amalgamation of successive point bars within each belt. Compound braid bars, built by overlapping unit bars, constitute the youngest channel deposits, and are probably associated with channel abandonment. Independent measurements of meander wavelength, based on plan-view exposures, match results estimated from empirical equations. The Ferron rivers are small to medium in scale according to calculated paleohydraulic parameters (Qw = 1.6 ∼ 2.8 × 102 m3/s). Architectural-element analysis shows that the last stage, and the most sinuous channel belt (A1), filled largely with downstream accreting unit and compound braid bars, similar to those seen in high-sinuosity meander loops of the modern Red River at the Texas–Oklahoma border. This suggests that both braid bars and point bars may occur within a single meander channel and that braid bars may evolve into compound point bars as they migrate and attach to a channel margin.

Numerical Model for Bank Erosion in the Brahmaputra River

A method is proposed to predict bank erosion and sand bar migration in river reaches where suspended sediment transport is dominant. The method focuses on the influence of the lateral bed slope on the erosion and deposition rate of suspended sediment, as well as on the profile of lateral bedload transport, assuming that geometric similarity holds in the bank region. In the proposed model, the erosion and deposition rate can be evaluated using either the bed shear stress at a reference location or the average bed shear stress in the bank region. In order to simulate bank erosion and associated bank shifting with a depth-integrated-base treatment, stretchable grids were added to the conventional coarse grid system near the bank. The proposed method, including the bank erosion model, is applied to the lower reach of the Brahmaputra River, which is ∼90 km long and ∼12.50 km wide. The computed results on bank shifting, sand bar migration, and sediment transport rates are compared with data obtained from field investigations and remote sensing. These results suggest that the proposed method is applicable for predicting sediment issues in river reaches dominated by suspended sediment.

Multidecadal dynamics of alternate bars in the Alpine Rhine River

AbstractWe report on a multidecadal analysis of alternate bar dynamics in a 41.7 km reach of the Alpine Rhine River, which represents an almost unique example of a regulated river with fixed levees, straight reaches, and regular bends in which alternate gravel bars spontaneously formed and migrated for more than a century. The analysis is based on freely available Landsat imagery, which provided an accurate and frequent survey of the dynamics of the alternate bar configuration since 1984. Bars were characterized in terms of wavelength, migration, and height. Longitudinal and temporal patterns are investigated as a function of flood occurrence and magnitude and in relation to the presence of local planform discontinuities (bends and ramps) that may affect their dynamics. Bars in the upper part of the reach are mostly steady and relatively long (about 13 channel widths); bars in the lower part of the reach are migrating and shorter (about 9 channel widths). Bar height is rather uniform along the reach, ranging between 3 and 4 m. The temporally long hydrological data set allowed the investigation of bar migration during flood events, showing that bars migrate faster for intermediate floods. The observed relationship between bar migration and wavelength was consistent with linear theories for free migrating and steady forced bars in straight channels. The comparison of theories with observations highlights the key role of theories to support interpretation of observations, for a better understanding of the morphodynamic processes controlling bar formation and dynamics.

Nuss bar migrations: occurrence and classification.

Pectus excavatum results from dorsal deviation of the sternum causing narrowing of the anterior-posterior diameter of the chest. It can result in significant cosmetic deformities and cardiopulmonary compromise if severe. The Nuss procedure is a minimally invasive technique that involves placing a thin horizontally oriented metal bar below the dorsal sternal apex for correction of the pectus deformity. To identify the frequency and types of Nuss bar migrations, to present a new categorization of bar migrations, and to present examples of true migrations and pseudomigrations. We retrospectively reviewed the electronic medical records and all pertinent radiologic studies of 311 pediatric patients who underwent a Nuss procedure. We evaluated the frequency and type of bar migrations. Bar migration was demonstrated in 23 of 311 patients (7%) and occurred within a mean period of 26days after surgery. Bar migrations were subjectively defined as deviation of the bar from the position demonstrated on the immediate postoperative radiographs and categorized as superior, inferior, rotation, lateral or flipped using a new classification system. Sixteen of the 23 migrations required re-operation. Nuss bar migration can be diagnosed with careful evaluation of serial radiographs. Nuss bar migration has a wide variety of appearances and requires exclusion of pseudomigration resulting from changes in patient positioning between radiologic examinations.

Sandbar Migration and Shoreline Change on the Chirihama Coast, Japan

Sandy beaches play a key role in regional tourism. It is important to understand the principal morphological processes behind preserving attractive beaches. In this study, morphological variation on the Chirihama Coast, Japan, an important local tourism resource, was investigated using two sets of field surveys. The objective was to analyze and document the multi-scale behaviors of the beach. First, long-term shoreline changes were examined based on shoreline surveys over the last two decades. Then, the middle-term behavior of multiple bar systems was analyzed based on the cross-shore profile surveys from 1998 to 2010. An empirical orthogonal function (EOF) analysis was conducted to capture the principal modes of the systematic bar migration. The shoreline analysis indicated a long-term eroding trend and showed that the seasonal variation has recently tended to increase. The profile analysis demonstrated that net offshore migrations of bars have been repeated with a return period of approximately four years. This general behavior of the bar system is similar to the net offshore migration phenomena observed at other sites in the world. EOF analysis revealed a relationship between bar configuration and middle-term variations in shoreline location; when a new bar is generated near the shoreline and a triple bar configuration is established, the shoreline tends to temporarily retreat, whereas the shoreline experiences an advance when the outer bar has most evolved.

Composition and Architecture of Braided and Sheetflood-Dominated Ephemeral Fluvial Strata In the Cambrian–Ordovician Potsdam Group: A Case Example of the Morphodynamics of Early Phanerozoic Fluvial Systems and Climate Change

Abstract: Two fluvial facies associations, braided and ephemeral fluvial, are recognized in strata of the Cambrian–Ordovician Potsdam Group in the Ottawa Embayment and Quebec Basin in northeastern North America. Braided fluvial sedimentation was dominated by the migration of coarse-grained sandy dunes and rare unit bars and local gravels that built up low-relief 0.4–5-m-thick downstream-accreting compound bars in very wide and shallow braided channels. Rare meter-scale confluence scours, incision, and filling of decimeter-thick bypass channels, and thin (≤ 7 cm) overbank deposition also occurred. Ephemeral fluvial sedimentation was dominated by the progradation of lobate terminal splays, most of which were deflated and/or reworked into thin (10–95 cm) sheets by eolian processes during the long recurrence intervals between floods. These strata were locally incised by rare, 0.3–1.9-m-thick scour-filling high-energy supercritical bedform strata and decimeter-thick bypass channels. Both braided and ephemeral fluvial deposits consist of meter-scale stacked, high-order stratal units separated by horizontal, laterally extensive surfaces recording the recurring buildup, avulsion, and finally abandonment of channel belts. In braided fluvial strata of the Potsdam the dominance of thin (≤ 2 m) elongated compound bars with low-relief (≤ 5°) bar margins and internal accretion surfaces (≤ 15°) suggest that braided channels and in-channel compound bars expanded laterally with negligible increase in flow depth during periods of increased discharge in the absence of bank-stabilizing vegetation, similar to other examples of the pre-vegetated “sheet-braided” style. However, in areas where bedrock topography inhibited the lateral expansion of channels, thicker compound braid bars (1.2–3.5 m) with higher-relief accretion surfaces (up to 35°), like those observed in post-Silurian and modern systems, preferentially formed in relatively deep channels. Ephemeral fluvial deposits here are similar to ephemeral examples throughout geologic history in that they mostly comprise poorly confined sheet-like terminal splays that accumulated in topographic lows. However, compared to several post-Silurian examples, strata in the Potsdam exhibit considerably more eolian reworking of splays and preservation of supercritical-bedform strata, a manifestation of greater rates of surface-water infiltration due to a lack of sheltering by rooted plants and propensity for unconfined flooding. Finally, in the upper part of the Potsdam two regional ephemeral fluvial units are interstratified with two braided fluvial units, providing evidence for shifts in regional climate. The sharp, areally extensive surfaces that separate these fluvial systems are interpreted to approximate time lines, and therein provide a means for relative and absolute chronostratigraphic correlation. These changes are correlated with documented Late Cambrian to Earliest Ordovician global climate fluctuations, with semiarid conditions and related ephemeral fluvial systems corresponding to global cooling events at ca. 491 and 487 Ma.