Acoustic Backscatter Sensors Research Articles

Inhibition of sediment erosion and phosphorus release by remediation strategy of contaminated sediment backfilling

The management of sediment-water interfaces, especially bed stability, is essential for controlling accumulated contaminants in the sediment. In this study, the relationship between sediment erosion and phosphorus (P) release under the remediation strategy of contaminated sediment backfilling (CSBT) was explored through a flume experiment, i.e. the dredged sediment was calcined into ceramsite after dewatering and detoxification and then backfilled to the dredged area for sediment capping, thus avoiding the introduction of foreign materials via in-situ remediation and the large-scale land occupation associated with ex-situ remediation. Acoustic Doppler velocimeter (ADV) and optical backscatter sensor (OBS) were used to measure the vertical distributions of flow velocity and sediment concentration in the overlying water, respectively, and diffusive gradients in thin films (DGT) was used to measure the P distribution in the sediment. The results revealed that improving bed stability from CSBT can considerably improve the robustness of sediment-water interface and reduce sediment erosion by more than 70%. The corresponding P release from the contaminated sediment could be inhibited with an inhibition efficiency as high as 80%. CSBT is a potent strategy for managing contaminated sediment. This study provides a theoretical reference for controlling sediment pollution, further supporting river and lake ecological management and environmental restoration.

Acoustic Measurement of Ship Wave–Induced Sediment Resuspension in a Large River

This paper presents a proof of concept that acoustic velocity and backscatter intensity measurements can provide essential information on ship wave–induced sediment transport processes in the littoral zone of a large river. Through a case study in the Hungarian section of Danube River, a combined application of an acoustic Doppler velocimeter (ADV) and a formerly calibrated acoustic backscatter sensor (ABS) is introduced for parallel measurements of 3D flow velocities and suspended sediment concentration (SSC) in a characteristic point of the near-bank zone. SSC times series detected by the ABS supported the calibration of the ADV backscatter, which eventually enabled the analysis of the temporal behavior of flow velocity and SSC conditions altered by ship waves. A suitable method for the separation of primary and secondary waves provided essential information on the turbulence intensities and sediment fluxes, and was found that the low-frequency primary waves are rather responsible for the lateral sediment transport instead of the dynamic secondary waves. The introduced data processing techniques and the main findings of the paper mean an important step toward the better understanding of the impacts of ship waves on the riverbanks, inherently playing a crucial role in the hydro-morphological and habitat conditions of river systems.

The influence of ship wave on turbulent structures and sediment exchange in large shallow Lake Taihu, China

Turbulent burst plays a key role in momentum transfer and sediment movement in large shallow lakes. This study addresses the quantitative influence of ship wave on turbulent structures, momentum and sediment fluxes in Lake Taihu, a large, shallow eutrophic lake that experiences heavy shipping traffic. Acoustic Doppler Velocimeter (ADV) and Optical Backscatter Sensor (OBS) were used to obtain high-frequency flow velocities and suspended sediment concentration (SSC) data before and when a cargo ship passed by. We found that ship waves mainly propagate along the horizontal direction. The near-bed turbulent bursting events with ship disturbance increased by 8.42%. Consequently, the vertical momentum and sediment fluxes were increased by 3.4 times and 8 times, respectively. In terms of bursting processes, ejection and sweep possess much stronger capacity in carrying sediment and are largely controlled by the disturbance in the vertical direction, while inward and outward interactions are mainly affected by horizontal disturbance. These findings represent an important step towards understanding the mechanism and effect of turbulent bursting under ship disturbance, and pave the way to future research in order to manipulate turbulent dynamics.

Comparison of Acoustic to Optical Backscatter Continuous Measurements of Suspended Sediment Concentrations and Their Characterization in an Agriculturally Impacted River

The increased soil loss in an agricultural watershed raises challengers for river water quality and a reliable automated monitoring for suspended sediment concentrations (SSC) is crucial to evaluate sediment budgets variation in systems. The aims of this study were (1) to test if an acoustic doppler current profiler (ADCP) would give similar results to turbidity probe measurements as a high frequency monitoring tool for suspended sediment; and (2) to analyze the relationship between sediment drivers and SSC in a typical agricultural drainage basin. The acoustic and optical backscatter sensors were used to collect SSC data during the ice-free seasons of four consecutive years in the Dunk River (PEI, Canada). The slopes of the relationships between the two SSC indirect measurements were not significantly different than 1. Correlations between SSC and hydro-meteorological variables showed that the high SSC values were more associated with the streamflow and water velocity than precipitation. This study highlighted the great potential of ADCP for the continuous monitoring of suspended sediment in an agricultural watershed. For summer periods the prevalence of clockwise hysteresis (74.1% of measured rainstorm events with SSC > 25 mg L−1) appeared related to rainstorm behaviors.

Impact of intermittent turbulent bursts on sediment resuspension and internal nutrient release in Lake Taihu, China.

Intermittent turbulent bursts have great impacts on sediment resuspension in coastal regions, tidal estuaries, and lakes. In this study, the role of turbulence structure on sediment resuspension was examined at Meiliang Bay of Lake Taihu, the third largest freshwater lake in China. The instantaneous three-dimensional velocity and suspended sediment concentrations were synchronously recorded by Acoustic Doppler Velocimetry (ADV) and Optical Backscatter Sensor (OBS) placed close to the lakebed. Statistical and quadrant analyses results revealed that the coherent structure contributed significantly to sediment particle entrainment. The intermittent burst events (dominant ejection and sweep) were the main energy source for sediment resuspension processes. 99.2% of turbulent sediment fluxes were triggered by ejection and sweep events, whereas the contributions coming from the outward interactions and inward interactions were relatively small. The large-amplitude burst events in the coherent structure dominated the influence on the sediment diffusion. Additionally, it was found that instantaneous sediment particle entrainment occurred earlier than the mean critical shear stress, which was induced by the stochastic nature of turbulence. The amount of sediment flux considering the turbulence characteristics was one or two larger magnitudes than the flux amount assessed by the time-averaged flow field, which indicated the critical shear stress approach might underestimate the sediment resuspension. Therefore, the influence of turbulence performance on sediment entrainment shall be seriously considered when evaluating sediment flux and internal nutrient loads in Lake Taihu.

Turbulent bursting and sediment resuspension in hyper-eutrophic Lake Taihu, China

Turbulent bursting plays a crucial role in sediment resuspension and transportation in many shallow lakes. This study analyzed the relationships between coherent structures and sediment resuspension in regions with and without intense wind waves in Lake Taihu. Acoustic Doppler Velocimetry (ADV) and Optical Backscatter Sensor (OBS) were used to measure field data sets including flow velocities and intense wind waves respectively. The results showed that the intermittent bursting events contributed significantly to the processes of momentum and sediment exchanges. Turbulent bursting effectively controlled the three patterns of sediment suspension during the 512 s observation period. In the windy region, the bursting motions, i.e. “ejection” and “sweep” events mainly accounted for the Reynolds stress generation and made major contribution to sediment resuspension. While in the current-dominant region, turbulent mixing from inward and outward interactions overwhelmed other processes. The vertical sediment fluxes reduced by an order of magnitude due to the less effect of wind waves and the lack of upwelling currents. It is therefore important to study turbulent coherent structures and sediment resuspension to understand the spatio-temporal and frequency characteristics of bursting events.

Determination of threshold parameter in quadrant splitting for identifying coherent motions in Lake Taihu, China

Coherent motion plays a significant role in sediment resuspension, which is an important cause of eutrophication in Lake Taihu. The most popular approach in detecting coherent motion in wall turbulence is quadrant splitting. However, an important threshold parameter (H) is set by empiricism during inspection above which burst event occurs. This study is aimed at eliminating the erroneous detection caused by this empirical parameter in identifying coherent motions, giving an objective range of H for Lake Taihu and then illustrating the interactions between coherent motion and sediment resuspension. Based on in situ monitoring with Acoustic Doppler Velocimetry (ADV) and Optical Backscatter Sensor (OBS), high-frequency data of three-dimensional velocities and suspended sediment concentration at the bottom of Lake Taihu were obtained. Autocorrelation function and quadrant splitting were jointly adopted to determine parameter H without any empiricism. The value of parameter H varied from 0.72 to 1.28 in Lake Taihu, and an increase of 0.56 in H value led to decreases of 10.7–24.1% in the stress fraction and 14.7–26.5% in the sediment flux fraction. At the Central Zone site, the dominant motions were ejection and sweep, while inward and outward interactions dominated the exchange processes at the East Bay site. Only 16.8, 14.3, and 17.0% of the time fractions of coherent motions contributed up to 58.3, 74.5, and 55.0% to the sediment flux fractions at different observation sites, and the larger time fractions of burst events contributed less to sediment flux fractions due to the less effect from wind waves and the lack of upwelling currents. Burst events with large amplitude and short duration contributed significantly to the processes of sediment exchanges. Calculation results of momentum and sediment resuspension fluxes are very sensitive to parameter H, taking one fixed empirical value for H is unsuitable for Lake Taihu. These findings will provide a useful reference for identifying coherent motions precisely and understanding the interactions between coherent motion and sediment resuspension in Lake Taihu.

Study on turbulence characteristics and sensitivity of quadrant analysis to threshold level in Lake Taihu

The identification of coherent structures is very important in investigating the sediment transport mechanism and controlling the eutrophication in shallow lakes. This study analyzed the turbulence characteristics and the sensitivity of quadrant analysis to threshold level. Simultaneous in situ measurements of velocities and suspended sediment concentration (SSC) were conducted in Lake Taihu with acoustic Doppler velocimeter (ADV) and optical backscatter sensor (OBS) instruments. The results show that the increase in hole size makes the difference between dominant and non-dominant events more distinct. Wind velocity determines the frequency of occurrence of sweep and ejection events, which provide dominant contributions to the Reynolds stress. The increase of wind velocity enlarges the magnitude of coherent events but has little impact on the events frequency with the same hole size. The events occurring within short periods provide large contributions to the momentum flux. Transportation and diffusion of sediment are in control of the intermittent coherent events to a large extent.

Flow and turbulence structure in a hypertidal estuary with the world's biggest tidal bore

AbstractTurbulent and flow structure associated with breaking tidal bores are deliberately investigated on the basis of field measurements. High‐resolution velocity and hydrographic data are collected in the middle Qiantang Estuary by a vertical array of acoustic Doppler velocimeters and optical backscatter sensors, collaborated with a bottom‐mounted acoustic Doppler current profiler. Besides obvious variations in diurnal and spring‐neap tidal cycles, the estuarine dynamics is featured by extreme asymmetry in flood and ebb tides. The flood tide is abnormally accelerated to generate tidal bores at the first 10 min or more, with breaking or undular configurations at the front. The occurrence of peak flow velocity, turbulent kinetic energy (TKE), and TKE dissipation rate (ε) is definitely associated with breaking bores, with their values several times to 2 orders of magnitude larger than the corresponding secondary peak values during the maximum ebb flows. Flow and turbulence dynamics are significantly affected by the tidal‐bore Froude number. A sandwich ε structure is clear exhibited with the maximum value at the surface, secondary maximum near the bed, and the minimum at the intermediate. Dual TKE sources are indicated by an approximate local balance between shear production and dissipation near the bottom, and a top‐down TKE dissipation using the modified Froude scaling in the vertical water column. The highly elevated dissipation by breaking bores is comparable to that by intense breaking waves in the surf zone, and the former potentially penetrates the entire water column to produce extreme sediment‐resuspension events in combination with intense bottom shear stress.

Using ADV for suspended sediment concentration and settling velocity measurements in large shallow lakes

Sediment resuspension plays an important role to provide nutrient release for algal growth in large shallow lakes. The settling velocity (w s ) is the key parameter for understanding the suspended sediment transport. In this paper, acoustic Doppler velocimeter (ADV) and optical backscatter sensor (OBS) instruments were used to measure in situ velocities and suspended sediment concentration (SSC) simultaneously without affecting the ambient turbulence in the bottom layer of Lake Taihu. The results showed that (1) ADV echo intensity (EI) could be transferred into SSC successfully by using a simple logarithmic relationship with a strong correlation of 0.87. (2) Three methods from a balance of settling and diffusive flux gradients for calculating settling velocities were applied, including Reynolds concentration flux, estimation of eddy diffusivity using the von-Karman Prandtl equation, and Kwon's power equation. The average settling velocity during the observation period was 0.11mm/s for Lake Taihu. (3) The fitted power function, w s = 1.85 × 10-3 C 1.04, could be used to estimate w s according to SSC. (4) Settling velocity changes with SSC, vertical velocities, and wind speed, which describes suspended sediment instantaneous and continuous motions. This paper provides an effective approach for estimating the settling velocity, provides a reference value of settling velocity for hydrodynamic model, and supports a better understanding of sediment transport process and nutrient release in large shallow lakes.

In situ observations of wave‐supported fluid‐mud generation and deposition on an active continental margin

AbstractWave‐supported fluid muds (WSFM) are a type of gravity flow that can rapidly transport sediment across continental margins. They occur when wave‐induced bed stress maintains suspended‐sediment concentrations (SSC) >10 g L−1, and sediment‐induced stratification near the top of the wave boundary layer limits upward diffusion of sediment. Observations from near‐bed instrumentation are used to evaluate the conditions under which WSFMs form on the continental shelf offshore of the Waipaoa River, NZ. An event in July 2010 featured >130 h of energetic ocean conditions, and water discharge >1900 m3 s−1. A calibrated acoustic backscatter sensor at the midshelf measured near‐bed SSC >50 g L−1, with a strong lutocline occurring >15 cm above the predicted wave‐current boundary layer, resulting in ~5 cm deposition. A velocity anomaly occurred during this time, with offshore‐directed currents faster at 1 m above bed (mab) than at 3.5 mab. Using these observations, we empirically solve a simple buoyancy‐drag force balance to estimate the gravity‐driven velocity of the WSFM, which is always <0.03 m s−1. Extending the force balance across a shelf transect suggests that WSFM‐carried sediment can reach the shelf edge in 50–240 h. Spatial and temporal patterns of deposition predicted by the gradient of modeled sediment flux correlate well with seabed observations on the Waipaoa shelf reported in Walsh et al. (2014). This study highlights the importance of WSFMs for cross‐shelf sediment transport, despite relatively slow gravity‐driven velocities and the infrequency with which they occur.

In situ observations of suspended particulate matter plumes at an offshore wind farm, southern North Sea

Suspended particulate matter (SPM) plumes associated with the monopile foundations of the Belgian offshore wind farm (OWF) Belwind I were acoustically profiled by means of a Doppler current profiler (ADCP). Together with the analysis of a bottom lander dataset of optical and acoustic backscatter sensors (OBSs and ADPs respectively), the spatiotemporal SPM plume dynamics were inferred. The fieldwork comprised (1) near-bed measurements of hydrodynamics and SPM concentrations in the direct vicinity of the wind turbines, by means of a bottom lander over a spring–neap cycle in May 2010; this dataset represents a typically tide-driven situation because there was no significant meteorological forcing during the measurement period; (2) additional vessel-based measurements conducted in May 2013 to capture the SPM plumes inside and outside the OWF over part of a tidal cycle. Both in situ datasets revealed that the SPM plumes were generated at the turbine piles, consistent with aerial and space-borne imagery. The SPM plumes are well aligned with the tidal current direction in the wake of the monopiles, concentrations being estimated to reach up to 5 times that of the background concentration of about 3 mg/l. It is suggested that the epifaunal communities colonizing the monopile surface and the protective rock collar at the base play a key role as source of the suspended matter recorded in the plumes. The organisms filter and trap fine SPM from the water column, resulting in predominant accumulation of SPM, including detritus and (pseudo-) faeces, at the base of the piles. When tidal currents exceed a certain velocity, fine particles in the near-bed fluff layer are re-suspended and transported downstream in the wake of the piles.

Influence of varying tidal prism on hydrodynamics and sedimentary processes in a hypertidal salt marsh creek

ABSTRACTRecent initiatives directing tidal power development in the Bay of Fundy have raised questions about far‐field environmental impacts related to energy extraction. It is understood that commercial scale tidal power installations in the Minas Passage will result in an overall decrease in tidal amplitude in the Minas Basin. Corresponding changes in sedimentation patterns may or may not be within the natural range of variability, and it is hypothesized that intertidal sedimentation rates will demonstrate a non‐linear response to modification of the tidal energy regime. This research considers acoustic Doppler velocimeter (ADV) and optical backscatter sensor (OBS) data from a sheltered tidal creek in the Minas Basin, for analysis of tidal characteristics in a hypertidal creek environment over spring and neap tidal cycles. Sediment deposition in the creek was also measured. Results show a first‐order control of topography on flow magnitude in the tidal creek, which impacts net sediment deposition through resuspension and removal of newly introduced material. This study demonstrates that tides which peak around the bankfull level show reduced early ebb stage turbulence and flow velocity and encourage an extended depositional period. The dynamics of marshfull tides may be responsible for the maximum sediment deposition in tidal creeks, providing large amounts of material that is eventually distributed to and deposited on marsh surfaces. Copyright © 2012 John Wiley & Sons, Ltd.

USE OF DIGITAL HOLOGRAPHIC CAMERAS TO EXAMINE THE MEASUREMENT AND UNDERSTANDING OF SEDIMENT SUSPENSION IN THE NEARSHORE

We present results from a 3-week field experiment measuring surf-zone flows and sediment transport on a steep energetic beach in the south-west UK, including the first reported deployment of an in-line holographic camera ('holocam') in the surf zone, co-located with various optical backscatter sensors and an acoustic backscatter sensor (ABS). The extensive data set provides perhaps a unique opportunity to examine the performance of optical and acoustic backscatter instruments in a wide range of conditions including bubbly flows, as well as some fundamental aspects of sediment suspension processes such as the near-bed size-distribution of suspended sediment. The holocam, deployed with a 1.3cm cubic sample volume approximately 10cm above the bed, provides in-focus well-resolved images of the instantaneous suspended load, making it possible to determine highly-accurate estimates of the concentrations of mineral sand grains, bubbles and organic particles, and their size distributions. Instantaneous estimates of sediment concentration from the ABS compare poorly with the equivalent measure from the holocam. This could be due to various factors such as spatial decorrelation or acoustic insensitivities at larger grain sizes. However, the ABS does a very good job at estimating burst-averaged suspended sediment concentrations when bubble concentrations are low (less than 1ml/l). The error in ABS concentrations (as compared against holocam) appears to be related to relative bubble concentration. The OBS is even more sensitive to bubbles. Suspended sediment grain size distribution is skewed towards the finer grain sizes but shifts to the larger sizes with increased flow intensity.

Storm-induced inner-continental shelf circulation and sediment transport: Long Bay, South Carolina

Long Bay is a sediment-starved, arcuate embayment located along the US East Coast connecting both South and North Carolina. In this region the rates and pathways of sediment transport are important because they determine the availability of sediments for beach nourishment, seafloor habitat, and navigation. The impact of storms on sediment transport magnitude and direction were investigated during the period October 2003–April 2004 using bottom mounted flow meters, acoustic backscatter sensors and rotary sonars deployed at eight sites offshore of Myrtle Beach, SC, to measure currents, water levels, surface waves, salinity, temperature, suspended sediment concentrations, and bedform morphology. Measurements identify that sediment mobility is caused by waves and wind driven currents from three predominant types of storm patterns that pass through this region: (1) cold fronts, (2) warm fronts and (3) low-pressure storms. The passage of a cold front is accompanied by a rapid change in wind direction from primarily northeastward to southwestward. The passage of a warm front is accompanied by an opposite change in wind direction from mainly southwestward to northeastward. Low-pressure systems passing offshore are accompanied by a change in wind direction from southwestward to southeastward as the offshore storm moves from south to north.During the passage of cold fronts more sediment is transported when winds are northeastward and directed onshore than when the winds are directed offshore, creating a net sediment flux to the north–east. Likewise, even though the warm front has an opposite wind pattern, net sediment flux is typically to the north–east due to the larger fetch when the winds are northeastward and directed onshore. During the passage of low-pressure systems strong winds, waves, and currents to the south are sustained creating a net sediment flux southwestward. During the 3-month deployment a total of 8 cold fronts, 10 warm fronts, and 10 low-pressure systems drove a net sediment flux southwestward. Analysis of a 12-year data record from a local buoy shows an average of 41 cold fronts, 32 warm fronts, and 26 low-pressure systems per year. The culmination of these events would yield a cumulative net inner-continental shelf transport to the south–west, a trend that is further verified by sediment textural analysis and bedform morphology on the inner-continental shelf.

Experimental investigation of the impact of macroalgal mats on flow dynamics and sediment stability in shallow tidal areas

This study aims to quantify the impact of macroalgal mats of Ulva intestinalis on flow dynamics and sediment stability. Such mats are becoming increasingly common in many coastal and estuarine intertidal habitats, thus it is important to determine whether they increase flow resistance, promote bed stability and therefore reduce the risk of erosion leading to tidal flooding or to degradation of coastal lagoons. The study has been carried out through a systematic series of experiments conducted in the large open-channel flume of the Total Environment Simulator (TES) facility, University of Hull, UK. The experimental facility was set up with a bed of fine sand, partially covered by strands of U. intestinalis; living individuals attached to large clasts were collected from Budle Bay, in the Lindisfarne National Nature Reserve, UK, and transplanted to the flume. The TES was equipped with acoustic doppler velocimetry (ADV) and acoustic backscatter (ABS) sensors, which measured current velocity, water level, bed level, and suspended sediment concentration. The experiments consisted of several unidirectional flow runs, firstly with a mobile sediment bed covered with U. intestinalis, then with a bare sediment surface, conducted at three different water depths. Under the investigated experimental range of velocities, typical of tidal environments, the macroalgal filaments were bent parallel to the sediment bed. The resulting velocity profile departed from the classical logarithmic trend, implying an increase of the overall roughness. This result reflects the different vertical Reynolds shear stress profiles and energy spectra features of the turbulent flow with respect to a bare sandy bed configuration. Macroalgae are also found to affect the morphological configuration of bedforms. The overall result is significant bio-stabilization, with increased flow resistance and reduced sediment transport.

Fish activity: a major mechanism for sediment resuspension and organic matter remineralization in coastal marine sediments

We quantify sediment resuspension due to groundfish activity in a partly anoxic basin using acoustic backscatter sensors, transmissometers, and remotely operated cameras on stationary and mobile platforms. Where these fish were present, a distinct benthic nepheloid layer (BNL) devel- oped despite minimal bottom currents ( 100 disturbances m -2 d -1 resulting in complete surface reworking every 2.5 d and a daily resuspension of 1.3 ± 0.7 l bulk sediment m -2 d -1 . Preliminary geochemical measurements suggest substantial impact of fish resuspension activity, the immediate effect being an instantaneous increase in nutrient concentration in the benthic boundary layer and a drop in oxygen concentration. Over longer time scales (hours to days), the freshly exposed organic matter is oxidized and additional nutrients (silica, ammonium, phosphate) are regenerated and released to the water. The increase in benthic oxygen demand suggests that fish activity reduces organic carbon sequestration by 9 ± 5 mmol C m -2 d -1 , equivalent to ~40% of its downward flux in this basin. To date, these processes are missing from geochemical models and require further investigation, particularly considering the depletion of groundfish stocks and the likely effects on global biogeochemical cycles.

Sand and mud flux estimates using acoustic and optical backscatter sensors: measurements seaward of the Wash, southern North Sea

Abstract Optical and acoustic backscatter sensors, more sensitive to fine and sandy sediment respectively, were used to measure the mud and sand components of a mixed suspension at a site seaward of the Wash embayment, in the southern North Sea. Data were acquired from a free-standing instrument frame during a five-week deployment in 12 m water depth about 6 km offshore. Suspended mud at this site was characterized by tidal advection of fine sediment along the coast resulting in semi-diurnal peaks in concentration near slack water. Suspended sand concentrations correlated well with tidal current speeds indicating local resuspension behaviour. Predicted sand flux direction followed the residual current while mud fluxes at the site were different in direction to both the residual current and sand flux. Residual fluxes may be biased by cumulative errors resulting from instrument calibration and inferred vertical concentration profiles. These factors are assessed in relation to both predicted flux magnitudes and directions.

Devices to measure settling velocities of cohesive sediment aggregates: A review of the in situ technology

Suspended cohesive sediments in coastal water bodies occur mainly as amorphous aggregates of single particles and microflocs. The fragile nature of these aggregates demands that their settling velocities be measured under field conditions to minimise their disruption. This review presents the main devices available to measure settling velocities of cohesive sediments in situ. These include hand-operated settling tubes and automated settling columns equipped with video systems, optical and laser instruments and an underwater balance. Additionally, non-intrusive techniques can be used, such as holographic and acoustic backscatter sensors. Regarding their operational principle, these in situ devices can apply direct or indirect methods for computing the settling velocity distribution in terms of particle size, mass or concentration. These devices are briefly described emphasising some important logistical and scientific issues that arise when these instruments are used in the field, in order to facilitate the selection of an appropriate technique for a particular research application. Due to the complexity involved in the in situ settling velocity measurements, a simultaneous application of different types of instruments seems to be the more appropriate approach to understand the aggregate dynamics.

Examination of reference concentration under waves and currents on the inner shelf

We examine reference concentration using three different data sets of near‐bed suspended sediment concentration observed under combined waves and currents. The data include observations made at 15 and 20 m depth off Dounreay, Scotland, UK, and observations obtained at 13 m depth off Duck, North Carolina, USA. These data accommodate different dynamic conditions (from wave‐dominated conditions at Dounreay to wind‐driven, current‐dominated conditions at Duck) and sediment properties (median size of bed sediment ranging from 120 to 350 μm). Near‐bed concentration profiles to elevations of about 80 cm were obtained using acoustic backscatter sensors with 1 cm resolution. The reference concentrations (Cr) at 1 cm were then evaluated by regressing the observed suspended sediment concentrations against a Rouse‐type model. Bed shear stresses associated with each estimate of Cr were estimated using the wave‐current interaction model of Grant and Madsen. Existing equations for reference concentration based on shear stress alone fail to accommodate all Cr estimates from different environments. We introduce a new empirical relationship between Cr and the product of Shields and inverse Rouse numbers. These dimensionless parameters represent the ratio of bed shear stress and submerged particle weight and the ratio of shear velocity and particle settling velocity, respectively. The new formula adjusts the amount of mobile sediment at the bed (related to the Shields number) to that available for suspension at the reference height (related to the inverse Rouse number). The new formula for reference concentration accommodates observations from different environments, suggesting that it may have wide applicability on sandy inner shelves.