Bedform Changes Research Articles

Experimental and Numerical Comparison of Flow and scour Patterns around a Single and Triple Bridge Piers Located at a sharp 180 degrees Bend

Placement of bridge piers along rivers creates a complex 3D flow pattern which disrupts normal river flow and the resulted turbulence erodes alluvial sediments around the pier. In this research, SSIIM model has been used to simulate flow and scour patterns when no pier, one pier, and triple piers are placed at a 180 degrees bend and the results are compared with those of experiments. The piers are vertical. The simulated channel was 1 m wide with a U-turn having the relative curvature radius of 2 carrying a volumetric flow of 70 l/s the flow depth at the beginning of the bend is 18 cm. Results showed that SSIIM is well capable of simulating bed form changes and flow patterns such that at the bend with triple piers error in maximum scour and sedimentation was only 4%, in maximum transverse velocity 12%, in maximum longitudinal velocity 13%, and in maximum vertical velocity 19%. In general, SSIIM model satisfactorily simulates the location and value of local scour arising from single and series piers in numerical simulation of the flow and scour. In flow pattern simulation, the errors and differences are greater under moving bed conditions than a rigid bed.

Predicting the occurrence of sandy seabed macroforms in the areas dominated by tidal processes

The main aim of the article is the review study of semidiurnal tides impact into the seabed surface sedimentary structures. Presented results of numerical modeling created on the basis of previous studies is focused on changes in the marine bedforms caused by strong bottom currents and their correlation with previous research in Europe – the southern part of the North Sea and in the USA at the Long Island coast. Particular attention was given to the velocity and frequency of rhythmic tidal events. Described hydrodynamic models include also the values of depth, sediment grain size as well as stresses between marine water and the sandy bottom. Results confirmed the effectiveness of used method in the study of changes in the bedforms height and length and the need to consider other factors to obtain more accurate results. Important from the point of view of the research of seabed macroforms and processes causing their formation is also the fact that intense transport of sandy material may condition human activity, primarily the interference in surface sedimentary structures shape, e.g. during foundation of hydrotechnical constructions, cables and pipelines, offshore wind farms or construction of port infrastructure. For these reasons, there is necessary to investigate what factors affect the areas of modern shelves and determine the sediment transport.

Observations and analysis of giant sand wave fields on the Taiwan Banks, northern South China Sea

Understanding sand wave dynamics is of importance in marine science and also has relevance in engineering. Giant sand wave fields (wave heights up to 10 m) are not common and thus have been relatively rarely studied. This paper presents the first study on bedform changes related to giant sand wave geomorphology on the Taiwan Banks. Using data from three repeated multi-beam bathymetric surveys (2011, 2012 and 2013) and a spatial cross-correlation method, different sand transport patterns are obtained. Classical crest-perpendicular migration is observed in the small sand waves with migration rates of 1–5 m/a, and migration is parallel with the current direction. However, the giant sand waves are immobile over the observed periods, and along-crest transport sand transport has been observed that is perpendicular to the predominant current direction. Statistical results support these findings and thus illustrate a type of sediment transport pattern that differs from other sand wave fields around the world. It is concluded that the giant sand waves act as a bathymetric obstacle, thus changing the direction of bottom currents flowing over them, resulting in a change in sediment transport pattern on the Taiwan Banks.

The effects of flood history on sediment transport in gravel-bed rivers

The transport of coarse sediment during floods often exhibits hysteresis patterns from changes in flow fields, channel geometry, bedforms, or sediment supply conditions. Flume experiments that simulate hydrographs tend to confirm that hysteresis is a consequence of the progressive organization of surface sediments in terms of grain protrusion, imbrication, orientation, and roughness. Hysteretic patterns are also highly dependent on the kind of sediment supply conditions, and the type of simulated hydrograph. A factor that has not been investigated extensively is the effect of the timing and sequencing of floods on bedload transport. Depending on its magnitude and duration, each flood leaves the channel bed in a different condition, which influences the bedload transport of the next event, representing the river bed's memory of past floods, which can determine future responses to natural disturbances. In this study, I investigated the effects of different sequences of events, i.e., the flood history, on sediment transport through a series of flume experiments that simulated three types of stepped and symmetrical hydrographs (ranging from short-duration/high-magnitude to long-duration/low-magnitude events) under sediment recirculation conditions. Hydrographs were simulated as a sequence of the same event, and with events in different sequences, in order to explore the effects of different antecedent conditions on sediment transport. The results show that a previous event decreases the rates of sediment transported by a certain hydrograph by around 40% if a high-magnitude event precedes another one and around 70% if a low-magnitude event precedes another event of similar magnitude. A low-magnitude event does not affect the rate of sediment transported by a subsequent high-magnitude flood, but a high-magnitude event reduces the sediment transported by a subsequent long-duration/low-magnitude event.

Monitoring gravel augmentation in a large regulated river and implications for process‐based restoration

AbstractThe artificial gravel augmentation of river channels is increasingly being used to mitigate the adverse effects of river regulation and sediment starvation. A systematic framework for designing and assessing such gravel augmentations is still lacking, notably on large rivers. Monitoring is required to quantify the movement of augmented gravel, measure bedform changes, assess potential habitat enhancement, and reduce the uncertainty in sediment management. Here we present the results of an experiment conducted in the Rhine River (French and German border). In 2010, 23 000 m3 of sediments (approximately the mean annual bedload transport capacity) were supplied in a by‐passed reach downstream of the Kembs dam to test the feasibility of enhancing sediment transport and bedform changes. A 620‐m‐long and 12‐m‐wide gravel deposit was created 8 km downstream from the dam. Monitoring included topo‐bathymetric surveys, radio‐frequency particle tracking using passive integrated transponder (PIT) tags, bed grain size measurement, and airborne imagery. Six surveys performed since 2009 have been described (before and after gravel augmentation, and after Q2 and Q15 floods). The key findings are that (i) the augmented gravel was partially dispersed by the first flood event of December 2010 (Q1); (ii) PIT tags were found up to 3200 m downstream of the gravel augmentation site after four years, but the effects of gravel augmentation could not be clearly distinguished from the effects of floods and internal remobilization on more than 3500 m downstream; (iii) linear and log‐linear relationships linking bedload transport, particle mobility, and grain size were established; and (iv) combined bathymetry and PIT tag surveys were useful for evaluating potential environmental risks and the first morpho‐ecological responses. This confirmed the complementary nature of such techniques in the monitoring of gravel augmentation in large rivers. Copyright © 2017 John Wiley & Sons, Ltd.

Bedform migration on Mars: Current results and future plans

Abstract With the advent of high resolution imaging, bedform motion can now be tracked on the Martian surface. HiRISE data, with a pixel scale as fine as 25 cm, shows displacements of sand patches, dunes, and ripples up to several meters per Earth year, demonstrating that significant landscape modification occurs in the current environment. This seems to consistently occur in the north polar erg, with variable activity at other latitudes. Volumetric dune and ripple changes indicate sand fluxes up to several cubic meters per meter per year, similar to that found in some dune fields on Earth. All “transverse aeolian ridges” are immobile. There is no relationship between bedform activity and coarse-scale global circulation models, indicating that finer scale topography and wind gusts, combined with the predicted low impact threshold on Mars, are the primary drivers. Several techniques have been developed to measure bedform changes and are largely dependent on dataset availability and the type of questions being pursued. Qualitative visual inspection can determine whether or not changes have occurred. Offsets registered to fixed tie points yield approximate migration rates of nearby crests and dune lee fronts. To compute volumetric sand flux requires precise orthorectification and registration using a digital elevation model base. Using this technique combined with sophisticated change detection software has the potential to detect changes as fine as 1/3 of a pixel (∼8 cm) or less.

자갈 및 암반 하상 산지하천의 고리형 수위-유량 관계 분석

It is well-known that loop effect of the stage-discharge relationship is formulated based on many field observations especially for the sand rivers. Theoretical understandings of the loop effect for the sand rivers have been widely provided, based on the facts that it is driven by the flood wave propagation and bed form changes over the given flood period. However, very few theoretical studies or field observations associated with loop-rating curves in the gravel or rock-bed mountain streams have been attempted so far, due particularly to the difficulties in the accurate discharge measurement during the flood in such field conditions. The present paper aims to report a unique loop-rating curve measured at a gravel and rock-bed mountain stream based on the flood discharge observation acquired during the typhoon, Muifa that passed nearby Jeju Island in summer of 2011. As velocity instrumentation, a non-intrusive Surface Velocity Doppler Radar to be suitable for the flood discharge measurement was utilized, and discharges were consecutively measured for every hour. Interestingly, the authors found that the hysteresis of the loop-rating curve was adverse compared to the typical trend of the sand bed streams, which means that the discharge of the rising limb is smaller than the falling limb at the same stage. We carefully speculate that the adverse trend of the loop-rating curve in the gravel bed was caused by the bed resistance change that works differently from the sand bed case.

Dynamic Routing of Flow Resistance and Alluvial Bed-Form Changes from the Lower to the Upper Regime

On the basis of a Strouhal number and the definition of the control factor, m, a new routing to calculate the energy slope in the lower and upper alluvial regimes is proposed. The control factor, m, representing the interactions in alluvial rivers, is reckoned as a bed-form index: while the flow evolves through transition, the control factor, m, decreases from m=2, associated with two-dimensional fully developed dunes, to m=1, associated primarily with in-phase waves. The way to predict the value of the control factor, m, is drawn from a previously published criterion for delineating the upper regime and is calibrated with experimental data. On several data from flumes and rivers, the routing is tested and compared with other methods from the literature. It appears that the new routing is the most robust because it allows researchers to obtain low averages of the discrepancy ratio for a wide range of ratios between the water depth and the median sediment diameter. On a selection of contrasted freshet events, the new routing allows for the capture of the primary dynamic of the flow resistance decrease.

Nonlinear mechanism of bed load transport

From the group movement of the bed load within the bottom layer, details of the nonlinear dynamic characteristics of bed load movement are discussed in this paper. Whether the sediment is initiated into motion corresponds to whether the constant term in the equation is equal to zero. If constant term is zero and no dispersive force is considered, the equation represents the traditional Shields initiation curve, and if constant term is zero without the dispersive force being considered, then a new Shields curve which is much lower than the traditional one is got. The fixed point of the equation corresponds to the equilibrium sediment transport of bed load. In the mutation analysis, we have found that the inflection point is the demarcation point of breaking. In theory, the breaking point corresponds to the dividing boundary line, across which the bed form changes from flat bed to sand ripple or sand dune. Compared with the experimental data of Chatou Hydraulic Lab in France, the conclusions are verified.

Assessment of sedimentation processes determined from side-scan sonar surveys in the Buffalo River, New York, USA

An assessment of sedimentation processes was made using side-scan sonar records collected over a 15-year period in the Buffalo River, New York, USA. Spatial and temporal changes in natural and man-made bedforms documented sediment erosion, slumping, and sediment depositional sites. Bedforms and sediment texture were used to divide the mapped portion of the river (9 km) into four sedimentary environments, with Section 1 at the upper end and Section 4 near the mouth where the river discharges into the east end of Lake Erie. Based on annual changes in side-scan sonar records, depositional rates decrease in the downriver direction. Section 2, located 5 km from the river’s mouth, contains sedimentary furrows. The persistence of furrows in this portion of the river requires both sediment erosion and deposition under a bidirectional flow regime. Results from this study confirmed that side-scan sonar is a valuable tool in understanding sediment dynamics and can be useful in river restoration decision making.

Mesoscale changes in present-day nearshore surface sediments and bedforms off the north coast of Ireland

Abstract The distribution and spatial patterns of surficial nearshore sediments off the north coast of Ireland were mapped in detail using three repeated (over one year total duration) dual-frequency (100–500 kHz) side-scan sonar surveys which were ground-truthed using a bucket-grab sediment sampler. In the study area (40 km 2 , < 30 m water depth), three acoustic facies were identified (interpreted as sand, gravel, and bedrock). Planar sand with small sand waves dominates most of the study area; bedforms developed on sand and gravel substrates are located around the margins of a bedrock-floored bathymetric trench. Unusual sea-going gravel ‘ribbons’, which morphologically resemble rip current chutes, feed into the trench. Temporal changes in substrate and bedform patterns, mapped from the side-scan sonar data, suggest bedform development and facies boundary migration (by > 100 m distance) over the winter season followed by sediment migration back again during the summer. Generally, substrate and bedform mobility is likely to be related to changes in the relative strengths of waves and tides along this exposed, mesotidal coast. More specifically, tidal asymmetry and the formation of a headland leeside eddy can account for scouring of the trench, trench-flank bedform patterns developed in sand, and the formation of the gravel ribbons.

Morphodynamics and cobbles behavior in and near the surf zone

The evolution of an initially flat sandy slope and the dynamics of large objects (cobbles/mines) emplaced on it are studied in a laboratory wave tank under simulated surf conditions. Upon initiation of wave forcing, the initially flat beach undergoes bedform changes before reaching a quasi-steady morphology characterized by a system of sand ripples along the slope and a large bar near the break point. Although the incoming wave characteristics are held fixed, the bottom morphology never reaches a strict steady state, but rather slowly changes due to the migration of ripples and bar transformation. When the wave characteristics are changed, the bedform adjusts to a new quasi-steady state after a suitable adjustment time. Studies conducted by placing model cobbles/mines on the evolving sandy bottom subjected to wave forcing show four distinct scenarios: (i) periodic cobble oscillations with zero mean displacement and small scour around the cobbles, (ii) mean onshore motion of relatively light cobbles, (iii) periodic burial of relatively heavy cobbles when their sizes are comparable to those of sand ripples, and (iv) the burial of relatively large cobbles under the bar, when the bar migrates due to changes of incoming waves. Quantitative data on the characteristics and dynamics of the bedform, including ripple-formation front propagating down the slope, ripple growth and drift, and flow around ripples, are presented. Physical explanations are provided for the observations.

Développement végétal dans le lit endigué de l’Isère en amont de Grenoble : du diagnostic à l’évaluation des pratiques de gestion

In a channelized 50 km reach of the Isère river, studies of changes in bedforms and vegetation were undertaken from a series of vertical aerial photographs between 1948 and 1996 and from field measurements of geomorphological and ecological parameters. Results showed a radical change of channel landscape and geomorphological activity during the last decades. Two different periods have been identified. During the first period (pre-1 970), geomorphological activity of Isère channel was intense, bedforms were continually removed, and pioneer vegetation uprooted during frequent floods. During the second period (post-1970), geomorphological activity slowed down, leading to the stabilisation of bedforms (lateral gravel bars) and development of vegetation. Vegetation rate which was less than 5% in 1970 is presently by 20% and new successional mature stages with hardwood species (Fraxinus excelsior, Robinia pseudacacia, Ulmus minor) have progressively replaced pioneer communities and softwood woodlands. Several causes explain this change. The main cause was the drastic decrease in the gravel bedload supply (from 150,000 m3 before 1950 to 40,000 m3 in 1960 and 5,000 m3 in 2000) which resulted in lateral bars pavement processes and narrowing of the active bedload band. Secondary causes such as gravel extraction (responsible for 1-3 meters deep channel incision) and absence of large floods over the last decades have added their effects, favouring establishment of mature plant communities. The vegetation inside the dykes infers new risks for riverside residents : increase of roughness and elevation of flood water levels by promoting denser growth of woody floodplain species within the dykes, and logjams formation in the downstream reach in Grenoble in case of a very large flood. In an effort to reduce the risk of these vegetated bars, river managers propose two types of maintenance. The first one consists in a simple clear cutting of the vegetation. Fine sediments which recover the gravel bars (the thickness is 1 to 3 meters) are left in place as well as tree stumps and roots. In the second type, clear-cutting is followed by additional disturbances such as burning of branches, scraping of the ground, and scouring of fine sediment deposits. First records of species richness and distribution of community diversity and geomorphological changes permitted comparisons between the both techniques. The first one (clear cutting) encouraged bed-forms stabilisation, rapid and denser coppice regrowth, increase of channel roughness, increase of fine sediment deposits during floods, decrease in plant diversity and invasion by exotic species such as Faloppia japonica. As a consequence, this type of operation needs to be systematically repeated with a 3-year rotation. The second technique (clear cutting and scouring of fine sediment deposits) is particularly effective because it favoured the restoration of geomorphological processes (erosion-deposit). Gravel bars are regularly eroded during frequent floods and migrate downstream. The reactivation of geomorphological processes led to the restoration of favourable conditions for regeneration of vegetation in the active channel. Biodiversity is enriched with presence of pioneer species, typical of the alpine rivers with high geomorphological activity (Typha minima, Calamagrostis epigejos, Salix, Populus, Alnus).

Evolution of coarse gravel bed forms: Field measurements at flood stage

Field measurements to investigate the origin and growth of mesoscale gravel bed forms in deep flows were made in the North Fork Toutle River, Washington. Sonar observations of the gravel streambed at a stationary point were recorded during two storm flows in December 1989 and January 1990 with concurrent bed load sampling and continuous velocity measurements. Mean diameter of bed load was about 3 cm, flow depths were 1.4–2.4 m, and bed shear stresses were 2–5 times the critical stress of mean bed load diameter, as computed from the depth‐slope product. These records document the hydrodynamic conditions under which dunelike coarse gravel bed forms were observed. Coarse gravel dunes (height, 20 cm; length, 6–15 m) evolved more than 24 hours after peak stage, primarily by accretion, as inferred from bed form changes revealed in dual sonar records. Dune heights increased to 40 cm as mean trough elevation rose about 50 cm over several hours. Smaller dunes (wavelength, 1–3 m), transitional from bed load sheets, migrated on the backs of the large dunes. The superposed dunes finally became indistinguishable from the large dunes, which diminished in height by increasing the mean level of troughs. Gravel deposition occurred at the observation point in conjunction with migration of gravel dunes. The direct comparison of known bed form regimes and gravel bar facies provides alternative interpretations of gravelly deposits.

Sediment supply and the development of the coarse surface layer in gravel-bedded rivers

THE bed surface of most gravel rivers is considerably coarser than the sub-surface or the gravel load transported over it, a phenomenon affecting river dynamics, morphology and ecology. The coarse surface layer, often called an armour or pavement, has been attributed to an inherent tendency for small grains to settle between larger ones during active transport of all sizes1,2; and to selective erosion or trapping of finer particles when the coarse grains are relatively immobile3,4. Where bedload supply is cut off below dams, selective erosion causes coarsening5,6. Using a simple quantitative model, we propose that surface coarsening develops in gravel-bedded rivers when local bedload supply from upstream is less than the ability of the flow to transport that load. We present laboratory results which support this hypothesis, and show that supply reduction causes changes in bedforms and progressive confinement of active bedload transport to a narrow band of finer bed surface bordered by a coarse, less active bed. It may therefore be possible to relate the degree of river-bed surface coarsening to sediment supply resulting from land use.

Facies, flow and bedform patterns across a storm-dominated inner continental shelf: Proterozoic Kaimur Formation, Rajasthan, India

A comprehensive sedimentation model for an epeiric sea, useful for further comparison and prediction, is derived crom the Proterozoic Kaimur Formation, Rajasthan, India. Facies analysis of a steadily regressive shale-quartz-arenite sequence documents the patterns of flow, sediment transport and deposition from beach to offshore across a storm-dominated gently sloping (< 0.1°) shelf. An alongshore westward drift dominated the sediment transport on the upper shoreface, but the transport turned gradually offshore in the lower shoreface. This change in flow pattern was accompanied by changes in bedforms: steepsided, close-nested, small and shore-parallel erosional bars on the upper shoreface; and higher, wider and gently sloping depositional bars at high angle to the shore on the upper reaches of the lower shoreface and hummocks farther offshore. Depth-related facies transitions, changes in lithologies, bed thickness and geometry, morphometric parameters and rate of decapping of hummocks mark the minimum depth of penetration of the average storms at 40 m, though the effective storm wave base at about 85 m.

Ocean current generated sedimentary facies in the Osumi Strait, South of Kyushu, Japan

An investigation of the surface sediments and bedforms in the Osumi Strait, located between Kyushu and Tanegashima, south of Kyushu, Japan, was carried out. The distribution of some characteristics of the surface sediments and bedforms is clarified. In the Osumi Strait, the surface sediments tend to become finer in size, better sorted and lower in specific gravity from southwest to northeast. The bedform distribution shows a systematic change in the same direction. This direction is the same as the direction of sediment transportation and of the current flowing through the Strait. It is considered that these changes in bedforms and sediment properties are formed by the decrease in the energy of the current. The current generating bedforms and sediment distribution is the Osumi Branch Current, one of the branches of the Kuroshio. The sediment transportation is active under the present hydraulic conditions. The sedimentary facies developed in the Osumi Strait is controlled by a unidirectional ocean current. The ocean current is one of the important factors for sedimentary processes where strong ocean current prevails along or near the continental shelf such as around the Japanese Islands.

Bedform movement in a marine tidal delta: air photo interpretations

AbstractMorphological changes in bedforms composing a tidal delta at the northern entrance to Moreton Bay, Queensland have been studied by examining aerial photographs spanning a 26-year time period. The aerial photographs show the movements of 51 different sand-bank crestlines, and the morphological characteristics of both sand banks and sandwaves. From the orientation of sandwave crests to the sand-bank crestlines, zones of ebb- and flood-dominance in sand-transport direction are distinguished. The migration directions of the sand banks are predicted by considering the cross-sectional asymmetry of the sand banks together with their adjacent ebb/flood zones of net sand transport. The reliability of the predictions is tested by comparisons with sequential air photo data. When applied to 53 bedforms, the predictions achieved a high success rate, with 45 predicted migration directions matching those observed on the sequential aerial photographs. Bedform movement can be predicted, therefore, for any water depths in which submarine bedforms can be clearly seen on aerial photographs (&lt; 10 m).Based upon their mobility, sand banks are classified into three categories: dynamic sand banks, which change quickly (within 2 years) and which have migration rates that are non-uniform along the bank crestline; progressive sand banks, which change slowly (from 2–10 years) and have migration rates that are uniform along their crestlines; and immobile sand banks, which change only over time intervals which exceed 10 years. Changes in sand-bank morphology occur by migration of the crestline together with growth and decay, and are considered to be linked with changes of larger ebb- and flood-dominant zones of net sand transport. The three different sand-bank types are characterized by distinctive heights, crestline lengths and wavelengths. They occur in different locations within Moreton Bay, possibly related to distance from external sand supplies and to relative tidal current and surface wave energy levels.

Secondary hydroeruptions in pyroclastic-flow deposits: Examples from Mount St. Helens

Secondary hydroeruptions occur in pyroclastic-flow deposits when water or ice is trapped beneath hot pyroclastic debris and rapidly heated to steam. These eruptions display various styles of activity including fumarolic degassing, tephra fountaining, and explosive cratering. The deposits, which occupy the layer 3 stratigraphic position on the top of pyroclastic-flow units, can be distinguished from ash-cloud material by lateral thickness variation, clast composition, and other sedimentary features. The ejecta of secondary hydroeruptions comprise a subset of hydrovolcanic pyroclastic deposits. A small secondary hydroeruption observed on the Mount St. Helens pumice plain in 1981 produced tephra that was emplaced ballistically, by deposition from base surges, and by fallout from an eruption column. Stratigraphic descriptions and grain-size analysis of the ejecta from several secondary craters on the pumice plain demonstrate that the bedforms produced by a hydroeruption change with crater diameter. In particular, craters of small diameter are surrounded by interbedded ripple-laminated ash horizons and nonstratified, fines-depleted units; large craters have ejecta ramparts comprised of coarse dunes and antidunes. These bedform changes are related to a progressive increase in eruptive energy, which produces base surges of greater power and eruptive columns of greater height. We suggest that the style of activity displayed during a secondary hydroeruption is controlled by both the total thermal energy of the system and the permeability of the pyroclastic overburden.

Medium‐term effects of two landsliding episodes on channel storage of sediment

AbstractTwo landsliding episodes between late 1973 and early 1975 delivered about 60000 m3 of sediment to six small deeply incised streams draining a 2·7 km2 area. About 4700 m3 of logs in the landslide debris formed major log jams in five streams, which impounded large volumes of landslide‐derived sediment. Five years after the landsliding, 42 per cent (25000 m3) of sediment was still in storage behind 35 log jams ranging from 1·4–8·2 m high. The landsliding episodes have produced multi‐stepped stream profiles, aggradation of channel reaches up to 150 m long to mean depths between 1·2 and 4·1 m, reductions in gradient, fining of bed material size, and related changes in bedforms and channel width:depth ratios that seem likely to persist for at least several decades. Sediment presently stored behind log jams is equivalent to between 50 and 220 years normal supply of sediment from hillslopes to stream channels. Long‐delayed, large magnitude impacts on higher‐order channels may occur if sudden failure of log jams is induced by a large storm at some future date.