Backscatter Sensors Research Articles

Sediment Flocs: Settling Velocity, Flocculation Factor, and Optical Backscatter

Improved relationships for the flocculation factor and floc settling velocity are proposed. They differ from the existing formulas by accounting for the effect of floc porosity on the hydrodynamic drag and for the fractal properties of the floc projection on the plane. These relationships predict that the fractal dimension d influences the floc settling velocity Ws only if d⩾2. A flocculation effect on optical backscatter is also considered, and a fractal-based relationship for quantifying this effect is proposed. This relationship serves as a basis for a simple optical backscatter sensor (OBS) based technique for estimating some fractal-related parameters of sediment flocs involved in the proposed relationships for the settling velocity and flocculation factor. The technique is tested using data from three field sites. The potential effect of particle size on backscatter efficiency is highlighted, and an approach for accounting for this effect is suggested.

Estimating suspended solids concentrations from backscatter intensity measured by acoustic Doppler current profiler in San Francisco Bay, California

The estimation of mass concentration of suspended solids is one of the properties needed to understand the characteristics of sediment transport in bays and estuaries. However, useful measurements or estimates of this property are often problematic when employing the usual methods of determination from collected water samples or optical sensors. Analysis of water samples tends to undersample the highly variable character of suspended solids, and optical sensors often become useless from biological fouling in highly productive regions. Acoustic sensors, such as acoustic Doppler current profilers that are now routinely used to measure water velocity, have been shown to hold promise as a means of quantitatively estimating suspended solids from acoustic backscatter intensity, a parameter used in velocity measurement. To further evaluate application of this technique using commercially available instruments, profiles of suspended solids concentrations are estimated from acoustic backscatter intensity recorded by 1200- and 2400-kHz broadband acoustic Doppler current profilers located at two sites in San Francisco Bay, California. ADCP backscatter intensity is calibrated using optical backscatterance data from an instrument located at a depth close to the ADCP transducers. In addition to losses from spherical spreading and water absorption, calculations of acoustic transmission losses account for attenuation from suspended sediment and correction for nonspherical spreading in the near field of the acoustic transducer. Acoustic estimates of suspended solids consisting of cohesive and noncohesive sediments are found to agree within about 8–10% (of the total range of concentration) to those values estimated by a second optical backscatterance sensor located at a depth further from the ADCP transducers. The success of this approach using commercially available Doppler profilers provides promise that this technique might be appropriate and useful under certain conditions in spite of some theoretical limitations of the method.

Temporal variability of hydrodynamics, sediment concentration and sediment settling velocity in a tidal creek

Measurements of flow, turbulence and suspended sediment concentration were carried out in a tidal creek that acts as a conduit for water and sediment to an intertidal salt marsh. A laser diffraction system (LISST), optical backscatter sensors (OBS) and acoustic Doppler velocimeters (ADV) were used to measure suspended sediment concentration and particle settling velocity variability throughout various tidal cycles for spring and neap conditions. Results suggest that ADV is a very useful system capable of measuring both sediment concentration and settling velocities. During neap tides, sediment in the tidal creeks is primarily in the form of flocs with a settling velocity ranging between 0.02 and 0.20mm/s and a mean particle size of 25–75μm. This population of sediment is enriched by both larger flocs and larger individual grain particles, during spring tides. The results indicate that in the tidal creek, erosion can occur only during the ebb stage of the spring tides, while during the flood tides the tidal creeks do not experience any significant erosion. On the contrary, they might act as temporary depositories for the small flocs. Sedimentation on the marsh surface occurs predominantly in the form of flocs, which seems to have the same settling velocities (0.24mm/s) both during neap–spring tides. This indicates that the neap–spring tidal cycle is controlling marsh sedimentation through sediment availability for deposition and inundation time. Spring tides provide more sediment and more time for the flocs to be deposited and thus contribute more to marsh accumulation.

The effect of suspended sediment concentration on the settling velocity of cohesive sediment in quiescent water

A laboratory study was carried to qualitatively investigate the effect of suspended sediment concentration C on the settling velocity w s of cohesive sediment in quiescent water. A bay mud sample was mixed with water in a cylindrical container, and three optical back scatterance sensors were then used to measure suspended sediment concentrations of the mud–water mixture at three levels every 15 s for 5 h while sediments were settling in the quiescent water. Based on the measured sediment concentrations, the settling velocities at different concentrations were derived from the depth-integrated mass balance equation. This study has found that the settling velocity w s is independent of C in the free settling regime of C<0.3 g/l, and then increases nonlinearly with C in the enhanced settling regime of 0.3< C<4.3 g/l, and finally decreases sharply with C in the hindered settling regime of C>4.3 g/l. The maximum settling velocity occurs at C≈4.3 g/l and is about nine times faster than the settling velocity in the free settling regime. A single empirical formula is also proposed to calculate the settling velocities at different sediment concentrations.

INSSECT—an instrumented platform for investigating floc properties close to the seabed

A new type of tripod, INSSECT (IN situ Size and SEttling Column Tripod), designed for floc measurements in the shallow parts of the continental shelf, is described. INSSECT orientates itself into the current so that instrumentation on it faces upstream, thereby minimizing flow disturbance. The instrumentation includes a digital floc camera, a video settling column, a unique sediment trap designed for capturing flocs intact, a laser sizer, an optical backscatterance sensor, a compass and tilt meter, and a current meter. It is designed for relatively short deployments of up to 2 weeks and can be recovered, turned around, and redeployed rapidly. It was designed for use in semi‐synoptic surveys of the spatial variation in floc properties such as size and settling velocity and the factors that influence them. The paper describes the various components of the INSSECT and presents selected results from the first deployments, demonstrating its ability to turn in the current and capture single flocs intact. Contrary to recent findings on the influence on settling columns on floc size, intercomparison between the video settling column and the digital floc camera reveals that the sizes are similar.

Tidally-induced clouds of suspended sediment connected to shallow-water coral reefs

The Bay of Banten, West Java, Indonesia represents a shallow coastal embayment harboring coral reefs subject to extremely turbid conditions. The sedimentary regime and flow structure of two contrasting coral reef islands in the embayment were investigated to identify the mechanisms responsible for mean and peak levels of suspended sediment concentration (SSC) around the reefs. Arrays of in situ calibrated optical backscatter (OBS) sensors were deployed across the reef slopes. Far field conditions were simultaneously monitored using an acoustic Doppler current profiler (ADCP) and a conductivity temperature and depth (CTD) probe, equipped with an OBS sensor. In addition, hydrographic surveys were conducted using the ADCP attached to a small fishing boat, navigating transects perpendicular to the reef islands. The direct influence of river plumes was found to be insignificant. Waves act only in the uppermost few meters of the reef slopes, where coral growth is most abundant and little sediment is available for resuspension. Tidal currents are the primary cause of high turbidity, which was inferred from peaks in the diurnal and semidiurnal frequency bands of SSC spectra, and the large depths where the turbidity variation sometimes initiates. The entrained sediment disperses upward across the reef slopes, which is likely due to a secondary current associated with bending of the flow along the curved, hydraulically rough reef slopes, in weakly stratified ambient waters. Changes in sediment availability, variable critical shear stresses for erosion and deposition, and the formation and evanescence of island wakes may explain the high variability of SSC at the reef slopes.

Methods for Reducing Biofouling of Moored Optical Sensors

Biofouling is one of the primary limiting factors in terms of measurement accuracy and deployment longevity for oceanographic studies involving autonomous sampling. Copper can significantly reduce marine fouling for long-term optical sensor deployments in coastal and open-ocean environments. Copper can effectively replace previously used highly toxic chemical antifoulant methods. Copper-based antifouling systems can be employed with three types of optical sensors: 1) open, 2) enclosed or semienclosed, and 3) shuttered. Copper plates on open-faced backscattering sensors can enable deployment periods of longer than 60 days in coastal waters without biofouling. In addition, copper tubing on nine-wavelength absorption-attenuation meters (ac-9s) has extended measurement capabilities from about 10 days to greater than 60 days with no signs of biofouling in coastal waters. Implementation of copper shutters on optical sensors in open-ocean waters off Japan has resulted in extended deployment periods (410 days and possibly longer) for optical measurements whereas previous optical measurements in the open ocean were typically degraded within several weeks to at most a few months due to biofouling.

Dynamics of a reflective beach with a low tide terrace

Many beaches are characterised by having steep upper slopes, flat low-tide terraces, and mixed or bimodal sediment distributions. This study compares the surf zone hydrodynamic and sediment transport processes of these two dynamically different regimes on the beach at Teignmouth (UK) as part of the COAST3D project. In-situ measurements were made using pressure transducers, electromagnetic current meters and optical backscatter sensors simultaneously on the steep part of the beach (slope 1/10, D 50 1.1 mm) and on the terrace (slope 1/100, D 50 0.25 mm) during a range of incident wave conditions (H b 0.25–0.75 m). The steep upper beach was characterised by: plunging breakers, high wave height/water depth ( γ) values, large gravity band cross-shore velocity variance, a well-developed subharmonic, strong undertow, high suspended sediment concentrations, and sediment transport dominated by a strong mean offshore component. The shallow shelving terrace was characterised by spilling breakers, low γ values, weak undertow, gravity velocity variance decreasing shoreward, infragravity cross-shore velocity variance increasing shoreward, and stronger longshore currents than undertow. Mean suspended sediment concentrations on the terrace were an order of magnitude less than on the steep beach. The oscillatory cross-shore components of transport were onshore in the incident wave band but increasingly dominated by the infragravity band in progressively shallower water. Longshore sediment transport rates on the terrace were large compared to cross-shore transport rates. The measurements suggest that the dynamics of the terrace/steep beach exhibit characteristics of separate dissipative/reflective sites. However, interaction between the different regions does occur. In particular, the low tide terrace acts to reduce the wave height that is capable of reaching the steep beach, thereby protecting it to some degree from erosion.

The influence of bore turbulence on sediment transport in the swash and inner surf zones

The role of bore turbulence for sediment transport in the swash and inner surf zones is investigated using field measurements of (a) cross-shore velocity using an electromagnetic current meter at z=3 cm, (b) turbulent kinetic energy ( TKE) and cross-shore velocity using a 3-dimensional acoustic Döppler velocimeter at z=7.5 cm, (c) water depth using a pressure transducer at z=0, and (d) suspended sediment concentration ( SSC) using seven miniature optical backscatter sensors at z=1–32 cm, on a steep beach in high energy conditions ( H b ≈1.6 m). High SSC and high TKE values were associated with the passing of the bore front while the near-bed flow was undergoing rapid onshore acceleration. Peak TKE values tended to occur whilst the near-bed velocity was still offshore. The SSC peaked only slightly later than the TKE and therefore no evidence exists to support advection of pre-suspended sediment. Correlation tests are performed to assess the predictive capability of (a) a Bagnold-type model based on shear stress, (b) a similar model based on TKE, and (c) a model combining (a) and (b). When the data from the whole time series are used, the combined model improves r 2 values over the shear stress model by up to 9%. If only data within bore events are included, then the improvement increases to over 55%, suggesting a significant effect of TKE on sediment transport across the bore.

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.

Laboratory investigation on the effect of particle size, water flow and bottom surface roughness upon the response of an upward-pointing optical backscatter sensor to sediment accumulation

Upward-pointing optical backscatter sensors (OBS) have been recently introduced to measure sediment accumulation in situ. Here, the effect of two potentially important parameters on an upward-pointing OBS response to sediment accumulation measurements is investigated in a laboratory flume: (a) grain-size distribution and (b) bottom surface roughness in relation to water speed. Independent measurements of sediment accumulation were done with a specially built suction chamber. This chamber collects accumulated particles directly with a resolution of the order of 0.01 mg cm −2 and with an accuracy of 70%. The upward-pointing OBS response to sediment accumulation was found to vary (a) with grain size by less than 30% for a range of 15–1000 μm and (b) with water speed on a flat surface by less than 35% for a range of 0–7 cm s −1. Speed had no effect on the OBS response when a rough bottom surface was tested for a range of 0–3.5 cm s −1. This study contributes to the understanding of upward-pointing OBS to sediment accumulation, the only field technique to date able to provide high-resolution measurements of sediment accumulation, with a high temporal resolution but still over extended periods of time.

The influence of oceanic swell on flows over an estuarine intertidal mudflat in San Francisco Bay

In this study, we examine the role that remotely forced ocean waves play in the hydrodynamics of an intertidal, estuarine mudflat. The observations indicate that long-period (10–20 s) ocean waves are a potentially important source of near-bed energy and shear stress in this environment. Over a two-week period in February 2001, we deployed an autonomous SonTek Hydra system on a mudflat in Central San Francisco Bay, and measured velocity and sediment concentration approximately 10 cm from the bed using an acoustic Doppler velocimeter (ADV) and an optical backscatter sensor (OBS). The experiment continued through wet (high tide) and dry (low tide) periods over an entire spring–neap cycle, and thus included the variation of near-bed velocity over a range of timescales. Results show that during large ebb tides, tidally forced flows dominate the near-bed dynamics during calm conditions. Wind waves dominate whenever the wind direction exposes the mudflat to wind coming off the bay (from the south and southwest), as occurs during winter storms. During periods when tidal forcing is limited and wind waves are small, remotely forced ocean swells become an important energy source. These motions appear in the burst samples at frequencies between 0.1 and 0.04 Hz and their energy correlates well ( ρ>0.8) with ocean swell measured from a buoy offshore of San Francisco. Spectral analysis of data shows that the average energy of ocean waves per tide varied between 2 and 15% of total energy load. Moreover, extreme values in the distribution of ocean waves bring episodic bursts of greater energy onto the estuarine mudflat, which may influence local suspension of sediments.

In situ video and diffraction analysis of marine particles

A design for a new underwater video-system to detect and observe suspended particles is presented. Particles are collected and isolated in a rectangular box where they are highly illuminated by a white light plane. The total field of view is determined. The camera, equipped with a remote controlled zoom, can resolve particles sizes ranging from 25 μm to several millimetres. Real-time image analyses are therefore performed. Particle counts and size spectra are calculated and displayed. Total light intensity scattered by the illuminated particles is closely related to the back-scattering values determined by an optical back-scatter sensor. A particle size analyser using diffraction analysis is associated to this video-system on a custom profiler. Hydrological parameters are measured by a standard CTD probe associated to a chlorophyll sensor. Results are acquired and graphically presented in real time. This custom profiler presents numerous advantages in oceanographic research. Two examples of its use in different coastal areas are presented. In an estuary, temporal evolution of particle characteristics was described in relation to the tide cycle. While the video-system allows direct visualization and characterization of the largest particles, the particle-size analyser performs precise quantification of the finest ones. It was shown that the two methods were in accordance for quantification of large aggregates, which were observed around slack tide when salinity decreased. Video analyses cannot be performed above 25 mg l −1 dry weight equivalent. The system reliability, resolution and limits were also demonstrated during a cruise in the Gulf of Finland. A typical profile is presented here showing different layers, one characterized by the association of heterotrophic flagellates and detritals, and another dominated by zooplankton, the surface layer being characterized by cyanobacterial colonies. Video associated to diffraction analyses allows the study of flocculation processes in estuaries and a detailed description of thin layers.

Spectral estimates of bed shear stress using suspended‐sediment concentrations in a wave‐current boundary layer

High‐resolution time series of suspended‐sediment profiles have been obtained using an acoustic backscatter system at an inner shelf site (North Carolina) where flows are dominated by wind‐driven currents and waves. We analyzed the spatial and temporal structure of near‐bed turbulence in particle‐transporting flows and scalar‐like fluctuations of suspended‐sediment concentrations. An important element of our analysis is a new inertial dissipation method for passive tracers to estimate the shear stress acting on the seabed, using the spectral properties of suspended sediment concentrations observed by acoustic backscatter sensors. In flows that provide adequate separation of the scales of turbulence production and dissipation, a sufficiently thick constant stress wall layer, and significant sediment suspension, frequency (or associated wave number) spectra of near‐bed sediment concentration exhibit a −5/3 slope in the inertial subrange that spans frequencies of order 1 Hz. This observation suggests that the suspended sediment is effectively a passive tracer of turbulent fluid motions. Inversion of the relevant, Kolmogorov scaling equations yields estimates of the shear velocity that agree reasonably well with other, independent and widely used measures. High‐ and low‐frequency limits on application of the inertial dissipation method to sediment concentration are related to the inertial response time of sediment particles and the sediment settling timescale. We propose that, in future applications, the inertial dissipation method for passive tracers can be used to estimate either the shear velocity, effective settling velocity of suspended sediment (or equivalent particle size) or dynamic bed roughness if two of these three quantities are independently known.

Suspended sediment concentrations in the Tamar estuary

Suspended sediment concentrations, obtained from the lower Tamar estuary in South West England using optical back scatter sensors, showed a depth-averaged background concentration of 0.02 kg m −3 throughout most of the spring–neap cycle. On spring tides the depth-averaged concentration increased to 0.25–0.40 kg m −3 either side of low water; however, the concentration maxima did not correspond to the time of maximum tidal flow, which suggests the influence of sediment advection. The observations were simulated using two-dimensional depth-averaged models of tidal currents and suspended sediment concentrations. Harmonic constants generated by the tidal model were used to estimate the advective terms and the bed shear stress in the sediment transport model. The sediment model included three size fractions which represented the low settling velocity wash load (2 μm), the cohesive (25 μm) and the non-cohesive (75 μm) suspended loads. During spring tides the simulated fine bed sediment (25 and 75 μm fractions) was resuspended in the upper model region and advected down estuary on the ebb tide. The sediment transport model reproduced the observed low water concentration increases to within a factor of two to five. The quantity of eroded sediment increased from medium tides to spring tides, as maximum bed shear stress increased, and formed a mobile pool of suspended sediment. After spring tides less sediment was remobilised on successive phases of the tide and accumulation occurred. The finer silty material (25 μm) was deposited in the shallower upper model region whereas the sand sized particles (75 μm) accumulated in the deeper parts of the estuary, which was in general agreement with published bed composition data. At neap tides, in accordance with the observations, the simulations showed no evidence of sediment resuspension.

Neutron back-scattering sensor for the detection of land mines

A hand-held neutron back-scattering sensor (NBS) has been developed at the Laboratori Nazionali di Legnaro in the framework of the EU funded project DIAMINE. A description of the prototype and the first laboratory tests for land mine detection are reported in this paper.

Assessing sediment removal capacity of vegetated and non-vegetated settling ponds in prawn farms

Sediment removal capacity is assessed for a constructed mangrove wetland, and a non-vegetated settling pond that are both used for filtering water in tropical aquaculture. The assessment is performed through sediment budget analysis using data of suspended sediment concentration collected from optical backscatter sensors. The sensors were deployed at the pond's inlet and outlet. These data sets provide a measure of trapping efficiency of each pond with different flow regimes and settling areas. The tides influenced flow in the wetland but none was felt in the settling pond. The average trapping efficiency obtained for the vegetated and the non-vegetated ponds was (40±33) and (70±36)%, respectively. The deposition rate calculated for the vegetated and non-vegetated pond ranges between 13–174 g/m 2 per h (average=63 g/m 2 per h) and 10–19 g/m 2 per h (average=14 g/m 2 per h), respectively. The efficiency of vegetated and non-vegetated ponds is likely to be improved by decreasing the aspect ratio (length/width) from the current value of 6 to 1 and of 5 to 1, respectively.

OBSERVATIONS OF SUSPENDED PARTICULATE MATTER IN TIDAL RIVER USING VESSEL-MOUNTED CURRENT PROFILER

An acoustic Doppler profiler (NDP) mounted on a vessel was used to observe distributions of velocity and suspended particulate matter (SPM) in the Ohta Estuary. Salinity, temperature, turbidity, chlorophyll-a and water samples were also collected. The volume scattering coefficient deduced from NDP data correlated with the turbidity from optical backscattering sensor. The concentration and flux of SPM in the longitudinal section were investigated by the vessel-mounted NDP. During the early stage of flood tide, the highest concentration of SPM were found around 1, 000 m upstream. This high concentration was consequent on sediment resuspension caused by strong flood current. During the flood tide, the SPM concentration in lower layer became the maximum around 1, 000 m upstream and then decreased gradually as proceeding upstream. The downstream flux of SPM in upper layer was larger than that in lower layer during the ebb tide.

A simple method for calibrating optical backscatter sensors in high concentrations of non-cohesive sediments

A new method is introduced for calibrating optical backscatter sensors for suspended quartz sand concentrations of up to 200 kg m −3. Due to the high settling velocity of quartz sand in water, considerable difficulties have arisen in the past to maintain a spatially and temporally homogeneous suspension suitable for calibration. Traditional methods are clumsy and prone to errors. Here, the sediment is calibrated in glycerol, a clear fluid with a higher viscosity than water. The settling velocity is reduced by three orders of magnitude. An empirical relationship is obtained which is used to correct for any optical differences in response of the sensors in the two fluids. Any extra errors introduced by calibrating with a different fluid from that found in the field are outweighed by the simplicity and reliability of this method.

Examination of diffusion versus advection dominated sediment suspension on the inner shelf under storm and swell conditions, Duck, North Carolina

A benthic boundary layer tripod supporting six current meters and three profiling acoustic backscatter sensors (ABS) documented storm and swell conditions during the fall of 1996 at a depth of 13 m on the inner shelf off Duck, North Carolina. Sediment concentration was higher in the wave boundary layer (WBL) during storm conditions but higher ∼40 cm above the bed (cm ab) during swell conditions. To test the applicability of a diffusive balance during storm versus swell, ABS data were used to invert the vertical diffusion equation and solve for eddy diffusivity from 1 to 50 cm ab. During the storm period, diffusivity derived from the ABS up to ∼40 cm ab agreed well with viscosity derived above the WBL from observed current profiles and from the Grant‐Madsen‐Glenn (GMG) model. During the swell period, diffusivity derived from the ABS up to ∼40 cm ab did not agree with observed mean current shear above this level nor with the GMG model. Diffusivity did agree with viscosity derived from shear stress due to waves within the WBL extrapolated to a height greater than the modeled WBL. We speculate that during swell conditions, shedding vortices enhanced mass and momentum exchange, extending the eddy viscosity associated with waves above the predicted WBL; during storm conditions, strong currents prevented vortices from penetrating beyond the predicted WBL. Rouse diffusion models with two‐ and three‐layered eddy diffusivity and combined diffusion‐advection models with one and three‐layer were applied to the observational data set. During the storm the two‐ and three‐layered Rouse models including multiple grain sizes and bed armoring reproduced the observed concentration well. During swell (weak current conditions) all the models considered underpredicted the observed concentration if applied with a standard WBL thickness. To correct this, enhanced vertical exchange was represented by a thickened WBL whenever mean currents were weak relative to the estimated jet velocity associated with wave‐induced vortex shedding. The two‐layer Rouse model then reproduced the concentrations observed during swell remarkably well. This implies that mean sediment suspension dominated by wave‐induced advection may still be approximated by a diffusion‐like process under some circumstances.