SAND TRANSPORT PROCESS MEASUREMENTS UNDER LARGE-SCALE BREAKING WAVES

Bedload and suspended load contributions to breaker bar morphodynamics

Dredged sediment is commonly placed as a submerged nearshore berm to nourish the beach or to dissipate high-energy waves, but the lifespan of such features is not easily predicted by existing methods. This paper presents a simple technique for generating order-of-magnitude estimates of the sediment transport rate of nearshore berms using offshore hindcast wave characteristics transformed to the nearshore. Total longshore transport for the entire nearshore region is calculated using eight published longshore transport equations (e.g., CERC equation and Kamphuis equation), which were evaluated for their relative performance. Because nearshore placements occupy only a portion of the cross-shore profile, the total longshore transport rate is scaled by an empirically-based fraction between 0 and 1, which is determined by the nearshore berm’s position in nondimensional space. The cross-shore transport rate is calculated independently using the near-bed orbital velocity from stream-function wave theory. The longshore and cross-shore transport rates are then superimposed to generate a total transport rate for the nearshore berm’s constructed footprint. The total transport rates were calculated at 11 historical nearshore berms and evaluated based on accuracy, inclusion of relevant coastal processes, and sensitivity to input parameters. The recommended total transport rate technique resulted in an average percent error magnitude of 72% and a maximum percent error magnitude of 167% at the historical placement locations. This technique is recommended for generating rapid, order-of-magnitude estimates of nearshore berm deflation rates for project design, particularly in scenarios when application of a full numerical model is prohibitive.

Tidal asymmetry in suspended sand transport on a macrotidal intermediate beach

This paper presents results obtained during a large-scale wave flume experiment focused at measuring hydrodynamics and sediment transport processes in the wave breaking region. The experiment involved monochromatic plunging breaking waves over a mobile bed barred profile consisting of D50 = 0.24 mm sand. Vertical profiles of velocity, turbulence, sand concentration and sand fluxes were measured at 12 cross-shore locations, covering the shoaling region up to the inner surf zone. Particularly high-resolution profiles were obtained near the bed within the wave bottom boundary layer, using an acoustic sediment concentration and velocity profiler (ACVP). Sheet flow concentration and particle velocities were measured at two locations near the bar crest using two conductivity-based concentration measurement tanks (CCM+). Total transport rates, obtained from the evolving bed profile measurements, were decomposed into suspended and bedload transport contributions across the bar. The present paper presents a summary of the key findings of the experiment, which are used to discuss existing approaches for modeling suspended and bed load transport in the surf zone.

The effect of solids loading ratio, particle size, and gas Reynolds number on the pressure drop and flow characteristics of a dilute gas‐solid suspension in turbulent pipe flow has been studied experimentally in both vertical and horizontal test sections. Glass beads of 10 to 60μ diameter were used at air Reynolds numbers of 10,000 to 25,000 and solids loading ratios of up to 2.5 Drag reduction was observed in the vertical test section for all of the particles studied, with the 30μ particles yielding a maximum drag reduction of about 75% at a loading ratio of 1.5. In the horizontal test section, drag reduction was observed only with the smallest sized particles, indicating a gravity effect. An explanation of these results based on the particles interacting with the turbulent structure of the gas near the wall has been proposed.

Modeling of tidal circulation and sediment transport near tropical estuary, east coast of India

Erosion in multiphase flow with entrained solid particles is a complex phenomenon due to existence of different flow patterns. Erosion experiments were conducted on an elbow specimen in a one-inch multiphase flow loop with gas, liquid and sand. Two different elbow specimens were used in the experiments. One placed downstream of a horizontal test section and one downstream of a vertical test section. Erosion tests were conducted at different gas and liquid velocities that showed a difference in erosion rates between the horizontal and vertical specimens. In order to better understand erosion results, the distribution of solid particles within the horizontal and vertical pipes just upstream of the elbows was measured across the pipe with a pitot probe. The results indicate that the distribution of sand in the pipe cross-section plays an important role in the erosion process due to the differences in velocities of the sand particles moving in the gas core and in the liquid film of the annular flow. It is observed that sand distribution in the horizontal test section is different than the sand distribution in the vertical test section for the same gas and liquid velocities. Therefore, the distribution of sand particles affects erosion test results. Based on these observations, a new mechanistic model has been developed to predict erosion in annular flow considering the effects of sand particle distribution and particle velocities in the annular film and gas core region. The experimental erosion results compare well with the model predictions.

Two-phase flow rate measurement is extremely important in many industrial sectors (nuclear, chemical, oil and gas, etc.). Nevertheless, the instrumentation required to perform these measurements is still very limited despite the huge research efforts during the last years. In the frame of the development of instrumentation for the two-phase flow rate measurement, the main aim of this study is the characterization of a spool piece consisting of a Venturi Flow Meter, and two Electric Capacitance Probes mounted in a horizontal test section, where an air-water two phase flow occurs. The probes response is analyzed versus superficial velocities of liquid and gas, flow patterns and void fraction. The test section consists of a nominal ID 80 mm horizontal transparent pipe equipped with a Venturi Flow Meter, a new capacitance probe with concave electrodes and a SIET designed electrical capacitance probe with ten long linear electrodes. In this experimental campaign, the air flow rate is in the range 0.048-0.119 kg/s (superficial velocity from 8 to 19 m/s) and the range of water flow rate is 0.0040-0.0535 kg/s (superficial velocity from 0.0006 m/s to 0.0103 m/s). These ranges of superficial velocities correspond to wavy stratified and annular-dispersed flow patterns along the test section.

The structure of wind-sand flow under different total sand transport rates was measured with field vertical anemometer and sand trap on the crest of typical coastal transverse ridge in Changli Gold Coast of Hebei Province, which is one of the most typical coastal aeolian distribution regions in China and famous for the tall and typical coastal transverse ridges. The measurement results show that, on the conditions of approximate wind velocities and same surface materials and environments, some changes happen to the structure of wind-sand flow with the increase of total sand transport rate on the crest of coastal transverse ridge. First, the sand transport rates of layers at different heights in the wind-sand flow increase, with the maximum increase at the height layer of 4–8cm. Second, the ratios of sand transport rates of layers at different heights to total sand transport rate decrease at the low height layer (0–4cm), but increase at the high height layer (4–60cm). Third, the distribution of the sand transport rate in the wind-sand flow can be expressed by an exponential function at the height layer of 0–40cm, but it changes from power function model to exponential function model in the whole height layer (0–60cm) and changes into polynomial function model at the height layer of 40–60cm with the increase of total sand transport rate. Those changes have a close relationship with the limit of sand grain size of wind flow transporting and composition of sand grain size in the wind-sand flow.

Results are presented of laboratory experiments undertaken to study the dynamics of wave propagation and transformation within the surf zone. The study involved measuring the external flow characteristics of regular plunging waves propagating along a 20 m long flume fitted with a 1:20 plane slope. To achieve this, monochromatic waves of frequency 0.4 Hz and a deep water wave height of 12 cm were generated by a servo-controlled piston-type wave maker. A set of calibrated parallel-wire capacitive wave gauges were employed to measure statistics of the free surface elevation along the slope in order to get an insight into the wave breaking behavior. To characterize the wave field, free surface elevation measurements were made in the vicinity of the break point. The measured time series data were analyzed at each flume position to obtain statistics of the mean water level, wave height, and wave velocity along the flume. Results show that as the wave propagates from deep water towards shallow water, there is an increase in the wave height, reaching a maximum height of about 21.5 cm at the break point, and then decreases sharply thereafter. Wave phase velocity calculations at different flume positions were made from the measured time series. Cross correlation techniques were used to determine the phase difference between the reference wave near the generator and the wave at various points along the flume. The local wave velocity was obtained by taking the phase difference between two points spaced 0.2 m apart. A comparison was made between the measured wave phase velocity, its linear shallow water ((gh)) approximation and the roller model concept wave velocity (1.3(gh)), at various points along the flume. The measured wave velocity c was found to lie in the range (gh)<c<1.3(gh) for most of the positions except near the break point. After the break point, the measured wave velocity is up to 38% higher than the theoretical value predicted using the roller model concept. Also noted is the variability of the phase speed in the breaking region. The present experiments of quantifying the mean macroscopic properties of breaking waves are a necessary prerequisite for more detailed experiments involving internal fluid velocity measurements that will follow.

Based on the wave flume experiments, the response of excess pore water pressure was studied under the regular breaking wave action on a 1:30 slope sandy seabed. The topographic change was observed in the wave broken zone and its causes were analyzed. The results show that the variation gradient of excess pore water pressure are larger in the surface layer and the changing amplitude of variation gradient of excess pore water pressure is bigger in the wave broken zone. The water depth is the most influential factor of excess pore water pressure and the wave height and period has little influence on excess pore water pressure in the same wave steepness range in the wave broken zone. The topographic change is greatly in the wave breaking zone. Excess pore water pressures changing amplitude at the top are significantly higher than it at the bottom. When the seabed scours, the amplitude of the excess pore water pressure increases; and when the sand seabed accumulates, the amplitude of the excess pore water pressure decreases.

Coastal dunes offer a wide range of valuable ecosystem services such as protection from erosion, flooding, sea-level rise, and provision of specialised habitat for endangered, endemic, or migratory species. Foredune blowouts and landward migrating parabolic dunes play an important role in many coastal dune settings creating ecological heterogeneity associated with inland sand transport, nutrient supply, and geomorphic disturbance processes. However, as coastal dunes globally are being increasingly stabilised by vegetation and declining in their ecological resilience and functionality, anthropogenic interventions, such as the removal of invasive species and excavation of foredune notches, have emerged to simulate and restore critical aeolian processes required to maintain dune morphodynamics and onshore sediment transport between the beach and inland dunes. This study employed computational fluid dynamics (CFD) modelling to investigate key controls on the wind flow dynamics and sand transport potential within idealised foredune notches of varying widths, slopes, and planform shape (rectangular vs. trapezoidal) for perpendicular and oblique incident wind directions. Compared with empirical findings from similarly engineered notches, our results show that notch width significantly influences shear velocity in the excavated notch ‘slot’, with narrower notches (25 m wide) enhancing wind flow acceleration and inland sediment transport potential. Spatial patterns of shear velocity throughout notches were also sensitive to incident wind direction, with maximum shear velocities, and consequent inland sand transport potential, occurring when winds were parallel to the orientation of the notch. On the lobes of the notches, shear velocity and sand transport potential were greatest during oblique winds. Our results suggest that a relatively narrow notch (e.g. 25 m as opposed to 50 m or 100 m), aligned with the prevailing wind direction, creates the most favourable conditions for transporting sediment from the beach to the dune behind. These findings underscore the importance of notch design in coastal dune restoration, offering critical insights for optimising interventions aimed at sustaining aeolian sediment transport from the beach to the hinterdune.

1Determining the largest wave height H which can occur in water of depth d without breaking is a fundamental reference quantity for the design of coastal structures. The ratio of breaking height (Hb) to depth (Hb/d), is known as the breaker index and has been the subject of much research over the past 150 years. Current design guidelines are based on investigations which, predominantly used monochromatic waves, thereby neglecting group effects. Groupiness or height modulation in wave trains is inherent to the free propagation of waves in deep water. Whilst our understanding of the formation and propagation of wave groups remains limited, significant progress has recently been made in the prediction of the onset and strength of breaking of wave groups in deep water. Recent two-dimensional laboratory studies have also shown that wave group effects have a marked affect on wave shoaling and breaking, influencing both the position and form of the breaking waves. These studies have used idealised wave spectra to generate modulating wave groups which are repeatable on a characteristic wave group time scale. Some scenarios yielded breaker indices up to 35% systematically above current design guideline values. Due to limitations of conventional wave probes when used near and within the breaking zone, a laser induced fluorescence (LIF) technique has recently been developed at the Water Research Laboratory. The technique yields high spatial and temporal resolution measurements of propagating wave forms from deep water, through the breaking region and inner surf zone. This technique has been verified using conventional capacitance wave probes and high resolution digital video images with resolution of better than 0.05mm obtained giving errors less than ±0.1 mm. Extensive investigations using repeatable wave groups and incorporating the LIF technique have been completed for a uniform bed slope of 10:1 in which the location of a shoaling bed was incrementally adjusted along the wave tank. In this manner, the shoaling and breaking properties of individual waves within the group could be carefully examined. It was observed that, as a wave group encounters a sloping bed, the behavior of the shoaling wave group is critically affected by the spatial phasing of the group relative to the bed. A critical depth between L0/5 > d > L0/8, where L0 is the deepwater wavelength, has emerged as the primary factor delineating two types of group shoaling. When a wave crest is coincident with the group energy maximum at this critical location, wave breaking tends to occur earlier, in deeper water and in a more gentle manner. However, where there is coincidence of a wave trough with the group energy maximum at this depth, delayed shoaling is observed. Notably, delayed shoaling stabilises the waves within the group, delaying the initiation of breaking and can yield Hb/d ratios up to 38% greater than present design guidelines. Our results show that present design guidelines based primarily on monochromatic waves are not conservative when wave group effects are considered.

The beach studied in this paper spans a length of 51 km and is one of several long sandy beaches in the southern Bohai Strait. Due to the obstruction of islands in the northeast and the influence of the underwater topography, the wave environment in the offshore area is complex; beach types and sediment transport characteristics vary along different coasts. The coastlines extracted from six aerial photographs in different years were compared to demonstrate the evolving features. Seven typical beach profiles were selected to study the lateral beach variation characteristics. Continuous wind and wave observation data from Beihuangcheng ocean station during 2009 were employed for the hindcast of the local wave environment using a regional spectral wave model. Then the results of the wave hindcast were incorporated into the LITDRIFT model to compute the sediment transport rates and directions along the coasts and analyze the longshore sand movement. The results show that the coastline evolution of sand beaches in the southern Bohai Strait has spatial and temporal variations and the coast can be divided into four typical regions. Region (I), the north coast of Qimudao, is a slightly eroded and dissipative beach with a large sediment transport rate; Region (II), the southwest coast of Gangluan Port, is a slightly deposited and dissipative beach with moderate sediment transport rate; Region (III), in the central area, is a beach that is gradually transformed from a slightly eroded dissipative beach to a moderately or slightly strong eroded bar-trough beach from west to east with a relatively moderate sediment transport rate. Region (IV), on the east coast, is a strongly eroded and reflective beach with a weak sediment transport rate. The wave conditions exhibit an increasing trend from west to east in the offshore area. The distribution of the wave-induced current inside the wave breaking region and the littoral sediment transport in the nearshore region exhibit a gradual weakening tendency from west to east, which is opposite to the trend of the wave conditions outside the breaking region. The presence of submerged shoal (Dengzhou Shoal), deep trough (Dengzhou Channel), islands and irregular topography influnces the wave climate, beach types, wave-induced current features, littoral sediment transport trends and coastline evolution patterns in the southern Bohai Strait. Human activities, such as the sand exploitation of Dengzhou Shoal and other coastal engineering projects, also influence the beach morphology and coastline evolution.

Hydrodynamics and sediment transport under a dam-break-driven swash: An experimental study