Ripples in intertidal mud—a conceptual explanation

On protected mudflats and along sheltered tidal channel margins, wave- and current-generated ripples are frequently observed on surficial and subsurface mud beds, although such bedforms are generally not thought to occur in cohesive sediments. In this paper, examples of such ripple marks in the German Wadden Sea (back-barrier tidal flats of Spiekeroog island) and also along the west coast of Korea (Baeksu tidal flats) are documented and analyzed. The mud ripples are 5–8 cm in spacing and 0.3–0.8 cm in height, and are composed of slightly sandy to virtually pure mud (80–98% mud content). For the Spiekeroog study area, a comparison of in situ particle-size measurements of suspended matter and of dispersed mud collected from the ripples shows that the former consists of low-density flocs which are considerably larger than the constituent grains of the latter. To assess local wave effects, near-bed orbital velocities and orbital diameters were calculated on the basis of standard wave theory using estimated wave parameters at the time of the study (June 2004) as well as wave data recorded nearby within the back-barrier tidal basin. The relationships between grain size, morphometric ripple parameters, and the near-bed orbital diameter show the wave-generated mud ripples to be of the orbital post-vortex type. It is demonstrated that only short-period shoaling (intermediate water depth) waves with periods of 1.5–2.5 s and heights of 0.1–0.5 m are able to generate and maintain such ripples. Corresponding near-bed orbital velocities range from 8–32 cm s^–1 and near-bed orbital diameters from 6.25–10 cm. It can be anticipated that increased current shear and turbulence associated with higher and longer waves prevent ripple formation due to the resuspension of settled mud, and the breakdown of suspended flocs and aggregates into smaller particles which then tend to remain in suspension. The most plausible explanation for the formation of the mud ripples is that mud flocs and aggregates deposited from suspension around high-water slack tide under moderate weather conditions initially respond as single (non-cohesive) particles which are hydraulically equivalent to ambient very fine sands. During exposure at low tide, gradual loss of water transforms the rippled mud into increasingly more cohesive mud drapes which are more resistant to erosion. Unless destroyed during high-energy events, the mud ripples may remain intact long enough to become buried and thereby preserved. Indeed, occasional but persistent observations of ripples in sub-Recent to ancient mudrocks document their preservation potential.     

The dimensions of sand ripples in full-scale oscillatory flows

New large-scale experiments have been carried out in two oscillatory flow tunnels to study ripple regime sand suspension and net sand transport processes in full-scale oscillatory flows. The paper focuses on ripple dimensions and the new data are combined with existing data to make a large dataset of ripple heights and lengths for flows with field-scale amplitudes and periods. A feature of the new experiments is a focus on the effect of flow irregularity. The combined dataset is analysed to examine the range of hydraulic conditions under which oscillatory flow ripples occur, to examine the effects of flow irregularity and ripple three-dimensionality on ripple dimensions and to test and improve existing methods for predicting ripple dimensions.The following are the main conclusions. (1) The highest velocities in a flow time-series play an important role in determining the type of bedform occurring in oscillatory flow. Bedform regime is well characterised by mobility number based on maximum velocity in the case of regular flow and based on the mean of the highest one tenth peak velocities in the case of irregular flow. (2) For field-scale flows, sand size is the primary factor determining whether equilibrium ripples will be 2D or 3D. 2D ripples occur when the sand D₅₀ ≥ 0.30 mm and 3D ripples occur when D₅₀ ≤ 0.22 mm (except when the flow orbital diameter is low). (3) Ripple type (2D or 3D) is the same for regular and irregular flows and ripple dimensions produced by equivalent regular and irregular flows follow a similar functional dependence on mobility number, with mobility number based on maximum velocity in the case of regular flow and based on the mean of the highest one tenth velocities in the case of irregular flow. For much of the ripple regime, ripple dimensions have weak dependency on mobility number and ripple dimensions are similar for regular and irregular flows with the same flow orbital amplitude. However, differences in ripples produced by equivalent regular and irregular flows become significant at the high mobility end of the ripple regime. (4) Ripple dimensions predicted using the Wiberg and Harris formulae are in poor agreement with measured ripple dimensions from the large-scale experiments. Predictions based on the Mogridge et al. and the Nielsen formulae show better overall agreement with the data but also show systematic differences in cases of 3D ripples and ripples generated by irregular flows. (5) Based on the combined large-scale data, modifications to the Nielsen ripple dimension equations are proposed for the heights and lengths of 2D ripples. The same equations apply to regular and irregular flows, but with mobility number appropriately defined. 3D ripples are generally smaller than 2D ripples and estimates of 3D ripple height and length may be obtained by applying multipliers of 0.55 and 0.73 respectively to the 2D formulae. The proposed modified Nielsen formulae provide an improved fit to the large-scale data, accounting for flow irregularity and ripple three-dimensionality.     

Shallow‐water sand bars on the Ruamahanga River delta Lake Wairarapa

In the shallow waters of Lake Wairarapa, a multiple series of sublimnic sand bars has developed around the outer edge of the Ruamahanga River delta. Shore‐normal movements in bar positions occur in response to changing lake level and wave conditions. Unlike many bar systems, most of these movements are lakeward of the breaker zone. Sediments on the bar crests are well‐sorted fine sands; the troughs are a mixture of fine sand and mud settling out of the turbid lake waters during calm conditions.     

Sand waves and other bedforms in lower Cook Inlet,Alaska

Abstract

Lower Cook Inlet in Alaska has high‐ tidal currents that average 3–4 knots and normally reach a peak of 6–8 knots. The bottom has an average depth of about 60–70 m in the central part of the inlet that deepens toward the south. Several types of bedforms, such as sand waves, dunes, ripples, sand ribbons, and lag deposits form a microtopography on the otherwise smooth seafloor. Each bedform type covers a small field, normally a few hundred to a few thousand meters wide, and usually several kilometers long parallel to the tidal flow. High‐resolution seismic systems, side‐scan sonar and bottom television were used to study these bedforms. Large sand waves with wavelengths over 300 m and wave heights up to 10 m were observed. Fields of ebb‐oriented or flood‐oriented asymmetric bedforms commonly grade into more symmetric shapes. Several orders of smaller sand waves and dunes cover the flanks of the very large bedforms. The crest directions of both size groups are normally parallel, but deviations of up to 90° have been observed; local deviations may occur where smaller forms approach the crests of the larger sand waves. Bottom television observations demonstrated active bedload transport in a northerly direction on crests and midflanks of southward asymmetric large sand waves, but not in their troughs. Movement of bedload occurs in the form of small ripples. Although the asymmetry of the large bedforms suggests that migration has taken place in the ebb or flood directions, the very low surface angles (2.5°‐8°) of these bedforms do not indicate regular movements. The large bedforms are probably relict features, or they migrate only under severe conditions, whereas active sand transport by ripples and smaller sand waves and dunes moves bedload back and forth with the tides. An understanding of such movements is essential for determining design criteria for offshore installations and in benthic‐faunal studies.     

Sand ripples generated by regular oscillatory flow

The prediction of ripple geometry is a necessary precursor to the prediction of sand transport under waves for ripple regime conditions. The paper begins with a comparison of four existing methods for predicting the geometry of sand ripples generated by oscillatory flow. The comparison points to substantial differences between ripple dimensions predicted by the methods, especially for field-scale conditions. Ripple geometry experiments carried out in a large oscillatory flow tunnel are then described. The experiments involved a range of sand sizes and sinusoidal and asymmetric flows with periods and velocities typical of field conditions. Comparison of measured and predicted ripple geometries leads to the recommendation that the method of Mogridge, Davies and Willis be used to predict ripple geometry for field-scale oscillatory flows. The Nielsen method yields good predictions of ripple length, but the rapid fall-off in ripple steepness predicted by the Nielsen method at high mobility number is not supported by the measurements. The lengths and heights of symmetric ripples produced by sinusoidal flows are found to be similar to the lengths and heights of asymmetric ripples produced by “equivalent” asymmetric flows. Three-dimensional ripples occur with fine sand in long-period flows typical of field conditions. The dimensions of these ripples cannot be predicted using methods developed for two-dimensional ripples. Previously suggested criteria for predicting the occurrence of three-dimensional ripples fail when tested against a wide range of flow and sand conditions. The occurrence of three-dimensional ripples and the effects of ripple and flow history on ripple geometry require further research.     

Sea ripple formation: the heterogeneous sediment case

Ripple formation beneath sea waves is analyzed both by experimental and analytical means when the bottom is made up of a mixture of sands. An oscillatory flow is obtained in a closed duct by the oscillations of two rigidly connected pistons located at the ends of the duct. The amplitude and period of the oscillations can be continuously varied. A fixed tray, located at the bottom of the duct and filled with different types of sediments, allows ripple formation to be observed. The presence of graded sediments is found to have a stabilizing effect and causes longer ripples to appear. Moreover a selective sediment transport is observed and quantified which tends to pile up the coarse grains at ripple crests leaving the fine ones in the troughs. As in the companion paper, the theory is based on a linear stability analysis of a flat sandy bottom subject to an oscillatory flow. Because of the presence of a mixture, a modified version of Exner equation is used and an “hiding” factor should be inserted in the sediment transport rate formula. The flow regime in the bottom boundary layer is assumed to be turbulent. The conditions for ripple appearance are determined along with their wavelengths as they form. Good agreement is found between experimental data and theoretical findings.     

In situ measurements of near-surface porosity in shallow-water marine sands

An in situ resistivity profiler was developed to measure with minimal disruption, the near-surface porosity of shallow-water marine sands. Results from a siliciclastic site off NW Florida and two Bahamian carbonate sites (an ooid shoal and coral reef sand flat) suggest the following general features. First, there is a 5- to 15-mm thick zone of elevated porosity adjacent to the sediment-water interface. Porosity in this layer was from 0.05 to 0.25 (decimal porosity) greater than the subjacent values, and would be difficult to resolve using traditional measurement techniques. Second, average porosity at >10-mm depth was 0.38 /spl plusmn/ 0.01 at the siliciclastic site, 0.39 /spl plusmn/ 0.01 at the ooid shoal site, and 0.49 /spl plusmn/ 0.02 at the coral reef sand flat site; consistent with literature values. Third, individual profiles exhibited 0.05-0.15 fluctuations about the mean, with vertical length scales of 5-15 mm. These fluctuations may be the result of grain packing heterogeneities caused by hydrodynamic sorting during deposition and subsequent physical and biological mixing or could be artifacts caused by disruption of the grain framework. Fourth, ripple troughs at the siliciclastic sand site had a significantly higher near-surface porosity compared to ripple crests, due most likely to the presence of detrital material in the troughs.     

Estimation of the wave-related ripple characteristics and induced bed shear stress

A large data set on ripples was collected and examined. A set of new formulas for the prediction of the ripple characteristics is proposed with an emphasis on the disappearance of the ripples. The ripple wavelength was observed to be proportional to the bottom wave excursion but also to be a function of the grain-related Shields parameter and wave period parameter introduced by Mogridge et al. (1994). The ripple steepness was found to be nearly constant for orbital ripples, and with a sharp decrease for suborbital ripples. Two empirical functions are added including the effects of the critical Shields parameters (inception of transport and inception of sheet flow), i.e. giving the boundaries for the ripple existence's domain. The proposed formulas yield better prediction capabilities compared to the previously published formulas, especially when ripples are washed out. The effect of the ripple characteristics on the roughness height and the calculation of the bed shear stress is also discussed. It appeared that the bed shear stress calculation is more sensitive to the empirical coefficient a_r introduced in the estimation of the ripple-induced roughness height or to the limits of existence of the ripples than the ripple characteristics themselves.     

Tidal-cycle changes in oscillation ripples on the inner part of an estuarine sand flat

Oscillation ripples form on subaqueous sand beds when wave-generated, near-bottom water motions are strong enough to move sand grains. The threshold of grain motion is the lower bound of the regime of oscillation ripples and the onset of sheet flow is the upper bound. Based on the relation between ripple spacing and orbital diameter, three types of symmetrical ripples occur within the ripple regime. In the lower part of the ripple regime (orbital ripples), spacing is proportional to orbital diameter; in the upper part (anorbital ripples) spacing is independent of orbital diameter. Between these regions occurs a transitional region (suborbital ripples).

Oscillation ripples develop on a sandy tidal flat in Willapa Bay, Washington, as a result of waves traversing the area when it is submerged. Because wave energy is usually low within the bay, the ripples are primarily orbital in type. This means that their spacing should respond in a systematic way to changes in wave conditions. During the high-water parts of some tidal cycles, ripples near the beach decrease in spacing during the latter stage of the ebb tide while ripples farther offshore do not change. Observations made over several tidal cycles show that the zone of active ripples shifts on- or offshore in response to different wave conditions.

Detailed bed profiles and current measurements taken during the high-water part of spring tides show the manner in which the oscillation ripples change with changes in orbital diameter. Changes in ripple spacing at the study site could be correlated with changes in orbital diameter in the manner suggested by the criterion for orbital ripples. However, there appeared to be a lag time between a decrease in orbital diameter and the corresponding decrease in ripple spacing. Absence of change during a tidal cycle could be attributed to orbital velocities below the threshold for grain motion that negated the effects of changes in orbital diameter.

Because changes in sand-flat ripples depend both upon changes in orbital diameter and upon the magnitude of the orbital velocity, exposed ripples were not necessarily produced during the preceding high tide. In fact, some ripples may have been just produced, while others, farther offshore, may have been produced an unknown number of tides earlier. Therefore, when interpreting past wave conditions over tidal flats from low-tide ripples, one must remember that wave periods have to be short enough to produce velocities greater than the threshold velocity for the orbital diameters calculated from the observed ripple spacings.     

The wave plus current flow over vortex ripples at an arbitrary angle

This work concerns the wave plus current flow over a sand bed covered by vortex ripples, with the current and the waves coming from different angles. Experiments were performed in a basin, where current and waves were perpendicular, in order to determine the conditions (current strength) leading to a regular ripple pattern formation. Numerical simulations were conducted changing the direction between the waves and the current from 0° to 90° and the ratio between the current strength and the wave orbital velocity from 0.2 to 1.5. Close to the bed, the current aligns parallel to the ripple crests, leading to a veering current profile with the vertical coordinate. The current-related friction coefficient was calculated. It was found that it decreases as the angle approaches 90°, while it increases for decreasing values of the current with a trend that can be described by a power law.     

The influence of troughs and crests of ripple marks on the structure of subtidal benthic assemblages around rocky reefs

This study tested the hypothesis that the small-scale topography caused by ripples in sediment would affect benthic macrofaunal assemblages. Ripple marks were measured and macrofauna and sediments were sampled near to and far from subtidal rocky reefs at two sites. Ripples were significantly wider and taller close to than far from reefs. Sediment grain-size was significantly different between crests and troughs. Microtopography clearly influenced the structure of the benthic macrofaunal assemblages in three of the four locations (and 8 of the 12 sites) examined. Numbers of taxa of the benthic macrofaunal assemblages and almost all individual taxa analysed showed significant greater abundances in troughs than in crests. The results strongly support the model that benthic macrofauna are affected by locally varying hydrodynamic environments produced by ripple-beds. These, in turn, were influenced by their proximity to a reef. Models about passive transport of the macrofauna by water-movement and active movements of the fauna are discussed. Furthermore, it appeared that there was a relationship between spatial variability of the macrofaunal assemblages and size of ripples. It is suggested that microtopography should be considered in experimental designs when patterns of distribution of benthic organisms are being evaluated.     

Storm-Generated Sediment Distribution Along the Northwest Florida Inner Continental Shelf

Hurricane Ivan made landfall along the Alabama– Florida coastline on September 16, 2004 as a category 3 storm. Ivan provided a rare opportunity to quantify surficial sediment changes following a significant storm event. Sidescan sonar imagery was collected immediately offshore Santa Rosa Island, FL, five days before and after Ivan's landfall 100 km west of the study area. Particle-size, multisensor core logger, X-radiography, photography, scanning electron microscopy (SEM) grain shape, direct shear, radiocarbon isotope, and lignin–phenol analyses were performed on grab or vibracore samples collected after the storm. Sonar observations before Ivan's landfall revealed a mostly sand bottom with uniform, small-scale wind-wave ripple morphology, and a distinct area of low backscatter trending NW–SE that was interpreted to be a mud swale. Ivan introduced new material to the relict sediments and resulted in the deposition of fine-grained material across the shelf that settled in the bathymetric lows and formed mud flaser deposits. Hardbottoms were draped by sand in some locations, but exposed in others. Ripple morphology changes occurred along sand ridges. Hurricane Ivan created major sediment distribution changes along the near-shore shelf, yet served to reinforce and to maintain the ridge-and-swale topography of the northeastern Gulf of Mexico near-shore continental shelf.      

Recent and modern marine erosion on the New Jersey outer shelf

Recent chirp seismic reflection data combined with multibeam bathymetry, backscatter, and analysis of grab samples and short cores provide evidence of significant recent erosion on the outer New Jersey shelf. The timing of erosion is constrained by two factors: (1) truncation at the seafloor of what is interpreted to be the transgressive ravinement surface at the base of the surficial sand sheet, and (2) truncation of apparently moribund sand ridges along erosional swales oriented parallel to the primary direction of modern bottom flow and oblique to the strike of the sand ridges. These observations place the erosion in a marine setting, post-dating the passage of the shoreface ravinement and the evolution of sand ridges that form initially in the near shore environment. Also truncated by marine erosion are shallowly buried, fluvial channel systems, formed during the Last Glacial Maximum and filled during the transgression, and a regional reflector “R” that is > ∼ 40 kyr. Depths of erosion range from a few meters to > 10 m. The seafloor within eroded areas is often marked by “ribbon” morphology, seen primarily in the backscatter data as areas of alternating high and low backscatter elongated in the direction of primary bottom flow. Ribbons are more occasionally observed in the bathymetry; where observed, crests exhibit low backscatter and troughs exhibit high backscatter. Sampling reveals that the high backscatter areas of the ribbons consist of a trimodal admixture of mud, sand and shell hash, with a bimodal distribution of abraded and unabraded sand grains and microfauna. The shell hash is interpreted to be an erosional lag, while the muds and unabraded grains are, in this non-depositional environment, evidence of recent erosion at the seafloor of previously undisturbed strata. The lower-backscatter areas of the ribbon morphology were found to be a well-sorted medium sand unit only a few 10's of cm thick overlying the shelly/muddy/sandy material. Concentrations of well-rounded gravels and cobbles were also found in eroded areas with very high backscatter, and at least one of these appears to be derived from the base of an eroded fluvial channel. Seafloor reworking over the transgressive evolution of the shelf appears to have switched from sand ridge evolution, which is documented to ∼ 40 m water depth, to more strictly erosional modification at greater water depths. We suggest that this change may be related to the reduction with water depth in the effectiveness of sediment resuspension by waves. Resuspension is a critical factor in the grain size sorting during transport by bottom currents over large bedforms like sand ridges. Otherwise, we speculate, displacement of sand by unidirectional currents will erode the seafloor.     

Mine Burial Experiments at the Martha's Vineyard Coastal Observatory

Several experiments to measure postimpact burial of seafloor mines by scour and fill have been conducted near the Woods Hole Oceanographic Institution's Martha's Vineyard Coastal Observatory (MVCO, Edgartown, MA). The sedimentary environment at MVCO consists of a series of rippled scour depressions (RSDs), which are large scale bedforms with alternating areas of coarse and fine sand. This allows simultaneous mine burial experiments in both coarse and fine sand under almost identical hydrodynamic forcing conditions. Two preliminary sets of mine scour burial experiments were conducted during winters 2001-2002 in fine sand and 2002-2003 in coarse sand with a single optically instrumented mine in the field of view of a rotary sidescan sonar. From October 2003 to April of 2004, ten instrumented mines were deployed along with several sonar systems to image mine behavior and to characterize bedform and oceanographic processes. In fine sand, the sonar imagery of the mines revealed that large scour pits form around the mines during energetic wave events. Mines fell into their own scour pits, aligned with the dominant wave crests and became level with the ambient seafloor after several energetic wave events. In quiescent periods, after the energetic wave events, the scour pits episodically infilled with mud. After several scour and infilling events, the scour pits were completely filled and a layer of fine sand covered both the mines and the scour pits, leaving no visible evidence of the mines. In the coarse sand, mines were observed to bury until the exposed height above the ripple crests was approximately the same as the large wave orbital ripple height (wavelengths of 50-125 cm and heights of 10-20 cm). A hypothesis for the physical mechanism responsible for this partial burial in the presence of large bedforms is that the mines bury until they present roughly the same hydrodynamic roughness as the orbital-scale bedforms present in coarse sand.     

Bed shear stresses and the sedimentation of sandy gravels

It is suggested that variations in bed shear stress between the crests and troughs of gravel waves may be the cause of simultaneous deposition of sand and gravel and, coupled with spatial variations in gravel wave steepness, can produce variations in the proportions of sand and gravel in the seabed sediment.     

The effect of bedforms (crest and trough systems) on sediment erodibility on a back-barrier tidal flat of the East Frisian Wadden Sea, Germany

The erosion potential over bedforms in a tidal flat of the East Frisian Wadden Sea was studied by conducting erosion and physical and biological sediment property measurements on the crests and troughs of bedforms. Five stations along a cross-shore transect of 1.5 km length from immediately below the salt marsh to the mid tide-level of the tidal flat were visited during two field campaigns in June and September 2002. Measurements of sediment erodibility were made on both crests and troughs using an EROMES erosion device and quantified in terms of critical erosion shear stress and erosion rate. Surface sediment scrape samples (upper 1 mm layer) were taken from crests and troughs to determine various physical and biological properties of the sediment. The results show that crests are generally more stable (i.e. higher critical erosion shear stresses and lower erosion rates) than troughs. In general, crests contained more chlorophyll a, colloidal carbohydrate, and EPS (extracellular polymeric substance) than troughs. Median grain-size, water content and wet bulk density of the crests showed no statistically significant difference from those of the troughs with the exception at the most landward station immediately below the salt marsh margin, where crests had significantly lower water content and higher wet bulk density than troughs.Two different processes were identified for the difference in erodibility between crests and troughs: (1) At stations with emersion times less than 6 h, the higher benthic diatom biomass (measured as chlorophyll a concentration) on the crests increases the amount of EPS, which is likely to stabilize the sediment surface of these features; (2) in a saltmarsh transition area (most landward station), physical processes such as surface drying and compaction seem to enhance in a synergistic way the sediment stability on the crests.     

Rapid wave-driven advective pore water exchange in a permeable coastal sediment

In this study we present in-situ measurements of pore water flow velocities in a coastal sandy sediment (permeability=3.65×10⁻¹⁰ m²). The advective pore water flows were driven by the interaction of oscillating boundary flows with sediment wave ripples, (amplitude=7 cm, wavelength=30 to 50 cm). The measurements were carried out in the Mediterranean Sea at 50 to 70 cm water depth during a phase of very low wave energy (max. wave amplitude=10 cm). An optode technique is introduced that permits direct pore water flow measurements using a fluorescent tracer. Near the sediment surface (0.5 cm depth) pore water reached velocities exceeding 40 cm h⁻¹. Thus, advective transport exceeded transport by molecular diffusion by at least 3 orders of magnitude. Based on the pore water velocity measurements and ripple spacing, we calculate that 140 L m⁻² d⁻¹ are filtered through the sediment. Pore water visualisation experiments revealed a flow field with intrusion of water in the ripple troughs and pore water release at the ripple crests. The wave-driven water flow through the sediment, thus, was directly linked to the wave-generated sediment topography, and its spatial dimensions. These results show that surface waves cause water filtration through permeable sediments at water depths smaller than half the wavelength. We conclude that surface gravity waves constitute an important hydromechanical process that may convert large areas of the continental shelves into expansive filter systems. Surface gravity waves thereby could affect suspended particle concentration and cycling of matter in the shelf.     

Note on sediment sorting in a sandy bed under standing water waves

We present new quantitative data on the sorting of sediments on a sandy seabed under standing waves. Starting from a flat bed composed of a homogeneous mixture of a coarse and a fine sand with mean diameters 0.11 and 0.21 mm, we observed simultaneous ripple and sand bar formation and sand sorting on the seabed. Over days of wave action, sand bars formed with crests beneath the surface wave nodes and flat plateaus flanked by mounds beneath the antinodes. Bar crests were composed of sand coarser on average than 0.21 mm, while the flat plateaus were covered by sand finer on average than 0.11 mm. Comparison with two experiments involving sand beds of more homogeneous size distributions shows that the mounds are characteristic of the motion of fine suspensions.     

Response of water column to strong wind forcing,southern Brazilian inner shelf: Implications for sand ridge formation

The result of two sequential oceanographic stations of 36 hours each in the area of sand ridges are presented. One station was located in the trough between two sand ridges and the other was at the crest of a sand ridge. At these stations salinity and temperature of the sea water, currents, winds, waves, and barometric pressure were measured each hour.During the observations, a cold front passed; this generated westerly winds that grew in speed from 24 to 52 km h^?1. The average height of the wind generated waves grew from 1.0 to 1.5 m and their periods increased from 7 to 10 s, and the speed of the northeast directed surface current increased from 40 to 82 cm s^?1. A bottom current (also directed northeast) increased from 26 to 34 cm s^?1.After the cold front had passed, the wind backed to the southeast and decreased in speed from 26 to zero km h^?1. The surface current in a northwest direction decreased from 29 to 8 cm s^?1. A bottom current (also directed northwest) decreased from 22 to 3 cm s^?1. Later, swells from the southeast appeared and their periods increased from 5 to 9 s and their heights grew from 1.0 to 1.5 m. After 3 hours, the speeds of the surface and bottom currents increased from 8 to 72 cm s^?1 and 3 to 62 cm s^?1 respectively.This cold front induced strong winds and storm-wave currents able to erode sediments (assuming a threshold velocity of 20 cm s^?1) and transport them in a north-northeast direction.The origin and the maintenance of these sand ridges is thought to be a function of sediments eroded from troughs and piled up at ridge crests during a storm condition. Some eroded sediments are transported north of Verga lighthouse where they are deposited on a smooth bottom.     

Modelling and measurement of sand transport processes over full-scale ripples in oscillatory flow

A new series of laboratory experiments was performed in the Aberdeen Oscillatory Flow Tunnel (AOFT) and the Large Oscillating Water Tunnel (LOWT) to investigate time-averaged suspended sand concentrations and transport rates over rippled beds in regular and irregular oscillatory flow. The wave-induced oscillatory near-bed flows were simulated at full-scale. Five series of experiments were carried out. During the two AOFT experimental series, ripple dimensions, ripple migration rates and net sand transport rates were measured under regular and irregular asymmetric flow for two different sand types. The three LOWT experimental series focussed on measurements of the ripple dimensions, ripple migration rates, time-averaged suspended sand concentrations and net sand transport rates under regular asymmetric and irregular weakly asymmetric flow for two different sand types. From analysis of new and other full-scale data, it is concluded that the lower part of the time- and bed-averaged concentration profile (up to two times the ripple height above the ripple crest level) has an exponential profile. A new reference concentration formula is proposed based on the formula of Bosman and Steetzel [Bosman, J.J., Steetzel, H.J., 1986. Time- and bed-averaged concentration under waves. Proc. 20th ICCE Taipei, ASCE, pp. 986–1000], which includes the grain-size influence. Furthermore, it is shown that the concentration decay length is strongly related to the ripple height and that the simple formula R_c = 1.27η gives good agreement with the data. A new transport model is proposed for the wave-related net transport over full-scale ripples based on a modified half wave cycle concept of Dibajnia and Watanabe [Dibajnia, M., Watanabe, A., 1992. Sheet flow under nonlinear waves and currents. Proc. 23rd ICCE Venice, ASCE, pp. 2015–2028; Dibajnia, M., Watanabe, A., 1996. A transport rate formula for mixed sands. Proc. 25th ICCE Orlando, ASCE, pp. 3791–3804]. The magnitudes of the half wave cycle transport contributions are related to the grain-related Shields parameter, the degree of wave asymmetry and a newly defined vortex suspension parameter P, which is the ratio between the ripple height and the median grain-size. The new model has been calibrated using transport data from the new regular flow experiments and has subsequently been validated using other data, including measurements from irregular flow experiments. The new model is seen to perform better overall than existing practical models for ripple regime net sand transport.