Tom O'Donoghue

Detailed measurements of velocities and suspended sand concentrations over full-scale ripples in regular oscillatory flow

The knowledge and modeling of wave-induced sand transport over rippled beds still has significant shortcomings, which is partly related to a lack of measurements of the detailed processes from controlled laboratory experiments. We have carried out new measurements of the detailed time-dependent velocity and suspended sand concentration field around vortex ripples for regular oscillatory flow conditions. The fact that the ripples were mobile and the flow conditions were full-scale makes these measurements unique. We made velocity measurements for 14 different flows and concentration measurements for three of these flows. The velocity and concentration field above ripples are dominated by the generation and ejection of vortices on the ripple flanks around the time of flow reversal. Vortex formation results in near-ripple flow reversals ahead of free-stream reversals and velocity maxima near the ripple crest that are much higher than the free-stream maxima. Asymmetry in the free stream produces steady circulation cells with dominant offshore mean flow up the ripple lee slope, balanced by weaker onshore streaming up the ripple stoss slope as well as higher up in the flow. The time- and bed-averaged horizontal velocity profile comprises an offshore streaming near the bed and an onshore drift higher up in the flow. The vortices are responsible for three main concentration peaks: one just after on-offshore flow reversal associated with the passage of a sand-laden vortex followed by two smaller peaks due to advected suspension clouds generated by vortex action at the neighboring onshore ripples. The sand flux field measured for one typical asymmetric flow condition is dominated by an offshore flux associated with the suspended sand cloud generated by vortex shedding from the ripples lee slope around the time of on-offshore flow reversal. The net (time-averaged) current-related and wave-related horizontal sand fluxes are generally offshore directed and mostly contained within 1.5 ripple heights above the ripple crest. The wave-related suspended transport component is larger, but the contribution of the current-related suspended sand transport cannot be neglected. In addition to the measured offshore net transport of suspended sand, there is an onshore-directed transport very close to the ripple surface. The total net transport is in the offshore direction for this specific asymmetric flow condition.

Sand ripples generated by regular oscillatory flow

Abstract 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.

Sand motion induced by oscillatory flows: sheet flow and vortex ripples

A large series of field-scale experiments on turbulent sand-laden flows, conducted in preceding years in the LOWT and AOFT large oscillating water tunnels are reviewed and reanalysed. Using the combined experimental data sets, new insights are obtained on the detailed sand transport processes occurring in sheet-flow and ripple regime conditions. For sheet flow (i) new equations are presented relating maximum erosion depth and sheet-flow layer thickness to the maximum Shields parameter; (ii) detailed analysis of sediment flux data shows the dominance of the current-related flux in the sheet-flow layer and the different characters of the current-related flux for fine and medium sands; (iii) a RANS-diffusion type model is shown to reproduce important trends in net transport rate related to grain size, velocity and wave period and to predict the magnitude of net transport rate to within a factor 2 of measured values. For the ripple regime it is shown that (i) asymmetric waves generate negative (‘offshore’) streaming and the current-related suspended sediment flux associated with this streaming appears to be of the same order of magnitude as the wave-related suspended sediment flux; (ii) time-averaged near-bed transport and time-averaged suspended transport appear to be of about equal magnitude but of opposite sign, and are concentrated on the ‘onshore’ flank of the ripple for asymmetric wave conditions; (iii) near-bed transport along the onshore flank is generated by sand transported over the ripple crest during the ‘onshore’ half-cycle. Net sand transport under asymmetric waves can be ‘onshore’ directed or ‘offshore’ directed, depending on the degree of unsteadiness in the sand flux behaviour during the wave cycle. Dimensionless phase-lag parameters are presented, for sheet flow and ripples, which can discriminate between predominantly quasi-steady behaviour (resulting in ‘onshore’ transport) and predominantly unsteady behaviour (resulting in ‘offshore transport’).

Subsurface processes generated by bore‐driven swash on coarse‐grained beaches

[1] Large‐scale laboratory experiments presented in this paper involved bore‐driven swash on permeable immobile coarse‐grained beaches. Two different sediments were used (d50 = 1.5 and 8.5 mm) resulting in different beach permeability and surface roughness. The experiments yielded detailed measurements of swash depth and velocities, wetting front, pressure, and groundwater levels across the swash zone. This paper is focused on the processes occurring within the beach. The measurements provide the shape of the wetting front and the groundwater table and reveal the behavior of air in the unsaturated region of the beach. Air is initially at atmospheric pressure, but the pressure builds up when air becomes encapsulated between the saturated region formed below the swash and the groundwater table. For the 1.5 mm beach, entrapped air significantly affected the water exchange between the swash and the subsurface. The considerable buildup of interstitial air pressure reduced vertical hydraulic gradients and thus infiltration rates. At the lower end of the beach the hydraulic gradients even became negative, indicating flow reversal and exfiltration. In contrast, for the 8.5 mm beach the rate of infiltration was only slightly affected by the buildup of pore‐air pressure. The vertical hydraulic gradients were more than twice the magnitude of those within the 1.5 mm beach. The results presented in the paper clarify the mechanisms that drive and impede the water exchange between the surface and subsurface flow. In particular, infiltration into the initially unsaturated part of the beach and the resulting air entrapment play a significant role in swash and similar flows.

Near‐bed hydrodynamics and turbulence below a large‐scale plunging breaking wave over a mobile barred bed profile

Detailed measurements are presented of velocities and turbulence under a large-scale regular plunging breaking wave in a wave flume. Measurements were obtained at 12 cross-shore locations around a mobile medium-sand breaker bar. They focused particularly on the dynamics of the wave bottom boundary layer (WBL) and near-bed turbulent kinetic energy (TKE), measured with an Acoustic Concentration and Velocity Profiler (ACVP). The breaking process and outer flow hydrodynamics are in agreement with previous laboratory and field observations of plunging waves, including a strong undertow in the bar trough region. The WBL thickness matches with previous studies at locations offshore from the bar crest, but it increases near the breaking-wave plunge point. This relates possibly to breaking-induced TKE or to the diverging flow at the shoreward slope of the bar. Outer flow TKE is dominated by wave breaking and exhibits strong spatial variation with largest TKE above the breaker bar crest. Below the plunge point, breaking-induced turbulence invades the WBL during both crest and trough half cycle. This results in an increase in the time-averaged TKE in the WBL (with a factor 3) and an increase in peak onshore and offshore near-bed Reynolds stresses (with a factor 2) from shoaling to breaking region. A fraction of locally produced TKE is advected offshore over a distance of a few meters to shoaling locations during the wave trough phase, and travels back onshore during the crest half cycle. The results imply that breaking-induced turbulence, for large-scale conditions, may significantly affect near-bed sediment transport processes.

Large-scale laboratory study of breaking wave hydrodynamics over a fixed bar

A large-scale wave flume experiment has been carried out involving a T = 4 s regular wave with H = 0.85 m wave height plunging over a fixed barred beach profile. Velocity profiles were measured at 12 locations along the breaker bar using LDA and ADV. A strong undertow is generated reaching magnitudes of 0.8 m/s on the shoreward side of the breaker bar. A circulation pattern occurs between the breaking area and the inner surf zone. Time-averaged turbulent kinetic energy (TKE) is largest in the breaking area on the shoreward side of the bar where the plunging jet penetrates the water column. At this location, and on the bar crest, TKE generated at the water surface in the breaking process reaches the bottom boundary layer. In the breaking area, TKE does not reduce to zero within a wave cycle which leads to a high level of “residual” turbulence and therefore lower temporal variation in TKE compared to previous studies of breaking waves on plane beach slopes. It is argued that this residual turbulence results from the breaker bar-trough geometry, which enables larger length scales and time scales of breaking-generated vortices and which enhances turbulence production within the water column compared to plane beaches. Transport of TKE is dominated by the undertow-related flux, whereas the wave-related and turbulent fluxes are approximately an order of magnitude smaller. Turbulence production and dissipation are largest in the breaker zone and of similar magnitude, but in the shoaling zone and inner surf zone production is negligible and dissipation dominates.

BEACH RESPONSE IN FRONT OF WAVE-REFLECTING STRUCTURES

A study of alternatives including a shoreline evolution numerical modelization has been carried out in order to both diagnose the erosion problem at the beaches located between Cambrils Harbour and Pixerota delta (Tarragona, Spain) and select nourishment alternatives.

123. SAND TRANSPORT UNDER FULL-SCALE SURFACE WAVES

Existing models for wave-related (cross-shore) sand transport are primarily based on data from oscillatory flow tunnel experiments. However, theory and former experiments indicate that flow differences between full scale surface waves and oscillatory flow tunnels may have a substantial effect on the net sand transport. In this paper, high resolution measurements of both boundary layer flow characteristics and net sand transport rates under full scale surface waves are presented. These experiments were performed for different wave conditions with medium (D50 = 0.25 mm) and fine (D50 = 0.14 mm) sand. It is shown that, especially under sheet-flow conditions, small wave induced net currents are of large importance for sand transport rates.

Experimental Study of Graded Sediments in Sinusoidal Oscillatory Flow

Measurements of concentrations and suspended sediment sizes above well-sorted and mixed sand beds in sinusoidal flows in a large oscillatory flow tunnel are presented. The results show that sediment grading is fundamentally important to the entrainment and suspension of sand from the bed. The depth of bed mobilised by the flow depends on the proportion of coarse sand in the bed, with a greater proportion of coarse sand resulting in a smaller mobilised depth. The distribution of the mobilised sand above the bed strongly depends on the grading. Sand is suspended to higher elevations when the proportion of fine sand is greater. Conversely, a lower proportion of fine sand leads to lower suspended sand concentrations high above the bed and higher concentrations within the sheet flow layer. These observations are intuitively sensible: the experimental results provide quantitative measures of the observations.

Sand transport process measurements under large-scale braking waves

The effects of wave breaking on sediment transport are studied through a new series of mobile-bed experiments in a large-scale wave flume. During the campaign, one experiment involving detailed sand transport process measurements was repeated at 12 different cross-shore location. This procedure allows studying of the cross-shore variation of sand transport processes along the breaking zone. Starting from an initially 1:10 slope followed by a horizontal test section, a breaker bar developed in the breaking region as a result of onshore transport pre-breaking and offshore transport post-breaking. Near-bed suspended sediment fluxes were directed offshore along the complete test section, suggesting that the onshore transport pre-breaking is mainly attributed to bedload. The offshore suspended flux was the sum of an onshore wave-driven component and an offshore current-driven component. The wave-driven contribution to total suspended transport rates seems significant mainly before the breaking point where they account for ~30% of total suspended transport fluxes.