Backscatter Sensors Research Articles

Tidal asymmetry in suspended sand transport on a macrotidal intermediate beach

Time-series of nearbed horizontal flow velocities and suspended sediment concentrations obtained from a colocated electromagnetic current meter (EMCM) and optical backscatter sensor (OBS), respectively, are used to examine the relative importance of steady and fluctuating components to the total sediment transport over a full tidal cycle on a macrotidal, intermediate beach (Spurn Head, UK). Fluctuating sediment fluxes are decomposed into gravity and infragravity contributions using co-spectral techniques. The relative importance of the oscillatory (gravity and infragravity) and steady (mean) transport components to the total sediment transport is analysed throughout the tidal cycle. A continuum of 34 discrete suspended sediment-cross-shore velocity co-spectra are computed over a full tidal cycle for the OBS and EMCM measurements 0.10 m above the bed. These net transport spectra vary greatly both with cross-shore location and tidal state. In particular, a marked asymmetry in transport processes is evident between the flood and ebb tides, with high levels of sediment resuspension and transport occurring on the ebbing tide approximately two hours after high water (just seaward of the breakpoint). At this time the dominant transport was directed offshore (co-spectral peak, 0.04 kg/m 2/s) at incident wave frequency. Typical patterns are observed in transport spectra outside the surf zone and within the inner surf zone. Outside the narrow surf zone cross-shore transport spectra show weak offshore transport (co-spectral peak = 0.002 kg/m 2/s) associated with bound long waves and stronger onshore transport (co-spectral peak = 0.006 kg/m 2/s) at incident wave frequencies. Conversely, co-spectra computed within the inner surf zone show the offshore sediment fluxes (spectral peak = 0.010 kg/m 2/s) at infragravity frequencies to be greater in magnitude than the corresponding onshore transport (co-spectral peak = 0.008 kg/m 2/s) occurring at incident wave frequencies.

The measurement of constituent concentrations in nonhomogeneous sediment suspensions using optical backscatter sensors

A method is proposed for estimating the concentrations of the constituents of nonhomogeneous sediment suspensions using optical backscatter sensors. The method is predicated on the assumption that constituent interaction (e.g. grain shielding and multiple scattering) does not significantly affect sensor response. This fundamentally linear approach was verified by laboratory tests of a backscatter sensor in a range of silt/sand mixtures. Implementation of the method will require further development of the optical backscatter sensor or possibly the pairing of an acoustic backscatter sensor with an optical sensor. The method has the potential for resolving small-scale spatial and temporal changes in the suspended-sediment population, and therefore could be employed in studies of winnowing and bed armouring, for example.

Measurements of high concentration suspended sediments using the optical backscatterance sensor

An optical backscatterance sensor (OBS™) was used to measure extremely high concentration suspended sediments on the Amazon continental shelf as part of the AmasSeds project (A Multidisciplinary Amazon Shelf Sediment Study). Using a pumping system for obtaining water samples to calibrate the sensor in situ, the OBS output was observed to increase approximately linearly for concentrations < 10 g/l, but decrease approximately exponentially for concentrations > 36 g/l. The OBS response was nearly continuous from 0 to 320 g/l making it possible to obtain continuous suspended sediment concentration profiles through fluid muds.

Vertical profiles of suspended sand concentration and size from multifrequency acoustic backscatter

Vertical profiles of suspended sand concentration and size are obtained from multifrequency acoustic profiling data collected in 1989 during a nearshore experiment at Stanhope Lane Beach, Prince Edward Island. The data were acquired with acoustic sounders operating at 1, 2.25, and 5 MHz. Independent estimates of concentration were made using optical backscatter sensors (OBSs). The algorithm for inversion of the three‐frequency backscatter data to particle size and concentration, based on the ratios of the different signals, was tested in laboratory experiments with an unconfined high Reynolds number suspended sediment jet. Results from the Stanhope Beach experiment are presented for three different surface wave energy regimes, with significant wave orbital velocities ranging from 0.3 to 0.8 m/s and peak wave periods of 4–6 s. The acoustic estimates of mean concentration are shown to be within 10% on average of those determined with the OBS nearest the bottom at 5‐ to 10‐cm height, over time scales ranging from 6.5 min to 4–6 s (one wave period). The acoustic estimates of suspended sediment size near the bottom are within 5–20% of the bottom sediment mean size. The statistical variability of the size estimates is high, with standard deviations in the estimates ranging between 30 and 50% of the mean. The time‐averaged concentration profiles exhibit an exponential decrease with height above an O(10)‐cm‐thick near‐bottom region of nonexpo‐nential decrease. In contrast, the time‐averaged mean size profiles decrease approximately linearly with height, and rather slowly, about 25% in 0.5 m.

A laboratory investigation of particle size effects on an optical backscatterance sensor

The effect of particle size on an optical backscatterance sensor's (OBS) response to suspended sediment concentration was investigated in the laboratory. Sieved ground glass was used in place of natural sediments to eliminate mineralogical effects. Highly reliable linear regressions of OBS output versus suspended sediment concentration were obtained for suspensions of constant particle size. Regression analyses were conducted to determine an empirical relationship between OBS gain and particle size; an inverse power law regression best fit the data. A regression based upon Mie's theory also fit within the error bands of the data, suggesting the theory may be applicable to OBS performance. Results indicate that the OBS is most sensitive to particle size changes for particles 100 μm or less.

Sea Carousel—A benthic, annular flume

A benthic annular flume (Sea Carousel) has been developed and tested to measure in situ the erodibility of cohesive sediments. The flume is equipped with three optical backscatter sensors, a lid rotation switch, and an electromagnetic (EM) flow meter capable of detecting azimuthal and vertical components of flow. Data are logged at rates up to 10·66 Hz. Erodibility is inferred from the rate of change in suspended sediment concentration detected in the annulus. The energy-density/wave number spectrum of azimuthal flow showed peaks in the energy spectrum at paddle rotation wave numbers (k) of 14 and 7 m−1 (macroturbulent time scales) but were not significant. Friction velocity (U*), measured (1) at 1 Hz using a flush-mounted hot-film sensor, and (2) derived from measured velocity profiles in the inner part of the logarithmic layer gave comparable results for Ū* < 0·064 m s−1. At higher values of U*, method (2) underpredicted by up to 20%. Method (1) showed radial increases in Ū* in the annulus for Ūy > 0·32 m s−1. Radial velocity gradients were proportional to (Ūy − 0·32 m s−1). Maximum radial differences in U* were 10% for Ūy = 0·5 ms−1. Suspended sediment mass concentration (S) in the annulus resulted in a significant decrease (10·5%) in Ū* derived by method (1) over the range 0<S<208 mg l−1. These decreases were not evident in method (2). Method (1) may, therefore, be subject to changes in stress sensor calibration with changes in S. Subaerial deployments of Sea Carousel caused severe substrate disturbance, water losses, and aeration of the annulus. Submarine deployments produced stable results, though dispersion of turbid flume water took place. Results clearly demonstrated the existence of ‘Type I’ and ‘Type II’ erosion documented from laboratory studies.

Notes on the performance of an optical backscatter sensor for cohesive sediments

A comprehensive test on the performance of an optical backscatter sensor (OBS) for cohesive sediments reveals that the OBS responds quite differently for the three basic cohesive sediments, kaolinite, illite and montmorillonite. Its operational ranges (from 0 to 0.5 g/l for kaolinite, from 0 to 0.8 g/l for illite, and from 0 to 1.2 g/l for montmorillionite) are significantly lower than that for silts or sands (up to 20 g/l). It also responds differently with variation in salinity. The color of water, the ambient light, and the scanning rate do not affect the performance. This study demonstrates that calibration of an OBS should be conducted in situ to obtain accuracy.

Estimation of stress and bed roughness during storms on the Northern California Shelf

Turbulent boundary shear stress depends on a roughness length scale which characterizes momentum transfer to the seabed. The effective roughness length is due to physical roughness geometry such as sediment ripples, grain size and processes affecting momentum transfer, such as bedload transport and wave motions. During storms on the California shelf, wave motions dominate the turbulent boundary layer, although feedback through wave-induced bed forms and sediment transport are important. Several intense storms on the Northern California Shelf were monitored with pressure sensors, acoustic current meters and an optical backscatter sensor within 5 m of the bed at 90 m depth. Wave spectra, velocity profiles, turbulent kinetic energy and suspended sediment concentrations were obtained. At 90 m depth, waves were measured in the band of 0.05-0.08 Hz, with nearbed wave velocities of 5–30 cm s −1. In spite of wave-induced currents of up to three times the mean speed, 30 min average velocities yielded typical logarithmic profiles. Roughness, as indicated by the zero intercept of the logarithmic profiles, z 0, varied by a factor of 25 throughout the storms, with a maximum of 18 cm, when mean currents were above 5 cm s −1 and the wave amplitude was maximal. Such large increases in z 0 are predicted by models of wave-current interaction. Effects of sediment transport and bed forms are not easily extracted from the data during storms. But, the fairly large non-storm values of ∼0.5cm indicate the effect of bed ripples.

A fiber optic sensor for monitoring suspended sediment

A new instrument that can be used to continuously monitor concentration of suspended particulates in close proximity to the sea bed ( z< 3.5 cm) is described. The instrument is composed of a narrow band source light, a photodiode detector, and glass fibers that transmit and receive light. Individual sensors can be sampled rapidly (8 Hz) and respond linearly to increasing concentrations of sediment over the range 0–100 gl −1. An array of sensors can be deployed from z=0.5 to 3.5 cm above the seabed providing simultaneous time series of suspended sediment concentration at five discrete elevations. Results of a field deployment in the swash zone of a dissipative beach indicate that the performance of the instrument is comparable to that of the standard optical backscatter sensor (OBS). The data show that se the standard optical backscatter sensor (OBS). The data show that sediment resuspension in the swash zone is strongly correlated with low frequency cross-shore flows with peak concentrations exceeding 200 gl −1.

An instrument system for profiling suspended sediment, fluid, and flow conditions in shallow marine environments

A new instrument system (sediment profiler) is described which is designed for making suspended sediment transport observations in shallow marine waters. The instrument system is capable of obtaining simultaneous profiles through the water column of suspended sediment concentration ( Cz), fluid properties (conductivity, temperature, depth) and flow conditions ( Uz, Vz), and of collecting water/suspended sediment samples at selected depths. Additionally it can be used in a calibration mode to intercompare multiple optical backscatter sensors (up to eight sensors). The sediment profiler is a relatively small aluminum tripod (91 cm high) that can be lowered through the water column in strong flows (∼2m s −1), measure suspended sediment concentrations between 5 mg l −1 and∼10g l −1, and can be lowered directly to the seabed (sensor elevations∼20cm). It contains internal battery power and on board digital logging capabilities and has no electrical connection with the vessel during operation. Since July 1989 the sediment profiler has been used extensively on a study of the Amazon shelf transport system (AmasSeds). Numerous profiles have been collected from hydrographic surveys off the northeast coast of Brazil to investigate spatial distributions of suspended sediment and to monitor net particle flux over 25 h anchor stations. The instrument system is rugged and reliable and has improved our ability to document the relationships between fluid properties, flow conditions, and suspended sediment characteristics in a complex shelf environment.

Laboratory apparatus for calibrating optical suspended solids sensors

An apparatus for testing and calibrating miniature to medium-sized optical suspended solids sensors is described. Stable suspensions of sedimentary particles ranging in size from 10 −6 to 4 × 10 −3 m can be produced in water with concentrations exceeding 100 g 1 −1. Flow velocities in the test volume can be varied continuously from 0.1 to 1.5 m s −1 and samples of suspensions can be withdrawn directly with a peristaltic pump for gravimetric and particle size analyses. The apparatus has been used to evaluate the effects of particle size, flow speed and air bubbles on the performance of optical backscatterance sensors and transmissometers.

Suspended sediment transport in the surf zone: Response to cross-shore infragravity motion

A field experiment to investigate the magnitude and temporal and spatial scales of suspended sediment transport in the inner surf zone was conducted at San Marine, Oregon on October 7–13, 1984. Instrumentation consisted of five electromagnetic current meters, three strain gauge-type pressure transducers, and 25 optical backscatter sensors to measure the concentrations of suspended sediment. The instruments were deployed approximately 70 m ( x) seaward of the mean high water line (mean depth h ̄ = 1.1−1.3 m ) in a closely spaced cross-shaped pattern. Data were collected for 68 min periods spanning high tide during the 7 day experiment. During the experiment, waves varied in angle of approach from shore-normal to southwesterly. Offshore significant swell heights were 3–5 m with periods of 12–14 s. Local sea conditions with wind waves of approximately 8 s also occurred. A visual estimation of the width of the surf zone indicated it to be approximately 500 m and the breaker pattern suggested that there were two longshore bars at approximately 200 and 500 m offshore. The data resulting from the experiment encompass a broad range of wave and current conditions which included periods of extreme infragravity motion (30 < T < 300 s) and weak longshore currents, and periods of low infragravity motion and strong longshore currents. Periods of incident wave dominance also occurred. The field study section of this paper qualitatively describes the sediment response to strong (± 240 cm s −1), low-frequency (0.01 Hz), cross-shore fluid motions which dominated the inner surf zone during two of the data runs. The infragravity band accounted for 85% of the cross-shore velocity variance, the peak (0.01 Hz) of which was associated with shore-normal standing waves. Concentration gradients of suspended sediment appear to be associated with diffusive processes and develop and decay on time scales of the infragravity motions rather than those of incident waves. Suspension events during times of infragravity dominance often persisted for periods of 30–45 s and reached peak concentrations of 20–40 g l −1 at elevations in excess of 26 cm. Mean suspended load was 3–4 times larger than that associated with incident wave motions. The modeling section of the paper attempts to quantitatively describe the observed time-dependent, vertical suspended sediment distribution with a one-dimensional, turbulent diffusion model. Formulations of eddy diffusion and erosion rate coefficients associated with peak cross-shore flows are found to agree with or fall within the range of values from previous laboratory and field studies and are used in the model. The model is evaluated by its ability to reproduce the time-dependent vertical distribution of suspended sediment and to estimate the observed mean suspended load, and the correct direction and magnitude of the mean longshore and cross-shore flux (level above the bed z = 31 cm) of suspended sediment.

WAVE-INDUCED FLOW AND NEARSHORE SUSPENDED SEDIMENT

It has been realized for nearly one hundred years that the transport of sediment is related to the characteristics of a wave, in particular its shape. Cornish (1898) noticed that the shoreward velocity associated with a wave crest was more effective at moving coarse sediment than was the seaward velocity associated with the wave trough. Cornish's observation was consistent with the theory of Stokes (1847), which predicts the onshore velocity associated with the wave crest is stronger and of shorter duration than the offshore velocity associated with the wave trough. This horizontal asymmetry of the cross-shore flow, which is a reflection of the wave shape, is known as velocity skewness. It has been suggested that "the existence of the beach depends on small departures from symmetry in the velocity field balancing the tendency for gravity to move material offshore"(Bowen, 1980). Although the concept of velocity skewness has been incorporated into detailed predictors of sediment transport (Bowen, 1980; Bailard and Inman, 1981) it is only one of many facets that needs to be understood in order to make the accurate prediction of sediment transport realizable. A comprehension of sediment transport is hampered by both an incomplete knowledge of the hydrodynamics and a lack of instrumentation to directly measure instantaneous sediment concentration and the accurate prediction of sediment transport is probably the most enigmatic problem in coastal engineering. Occasionally, suspended sediment concentration has been inferred from in situ pumps and hand-held tubes, but these methods lack the temporal and spatial resolution necessary to elucidate the details of the interaction between the waveinduced flow and the sediment. Recently, a miniature optical backscatter sensor (MOBS), which provides a time series of suspended sediment concentration at a "point", was developed by Downing et al. (1981). During a recent field experiment a vertical array of 5 of these optical backscatter sensors and a colocated flow meter was deployed close to the sea bed. These colocated measurements provide a unique opportunity to investigate the response of near-bed suspended sediment concentration to the wave-induced flow.

Continuous measurements of suspended sand concentration in a wave dominated nearshore environment

A miniature optical backscatter sensor (MOBS) and an electro-magnetic flowmeter were deployed seaward of the surf zone at Pte Sapin, New Brunswick during the first Canadian Coastal Sediment Study (C 2S 2), autumn 1983. The MOBS had sensing elements at five vertical locations above the sea bed, and was sampled at 10 Hz. The suspension of sand is well correlated with the passage of individual waves and also with wave groups, with the influence of wave groups progressively more dominant at higher elevations above the bed. Suspension appears to be stronger during the onshore directed phase of the wave motion than during the offshore motion and there is evidence that initiation of suspension may be determined by fluid acceleration more than velocity. Time lags between suspension events at different heights suggest that vertical gradients in sediment flux are more important than horizontal gradients.

New instrumentation for the investigation of sediment suspension processes in the shallow marine environment

An instrumentation system has been developed that continuously measures: flow velocity, water depth, and suspended sediment concentration profiles in the nearshore zone. The system consists of four components: (1) an array of electronic sensors, including: an electromagnetic current meter, a surface-piercing resistance wave gauge, and a five-element array of optical back-scatterance sensors; (2) a signal processing electronics unit; (3) a guyed mast support structure; and (4) a data acquisition system. The small size of the sensors and their response characteristics make the array suitable for monitoring sediment transport to within a few centimeters of the bed under moderately large breaking waves (∼2 m). Results from an experiment on an exposed ocean beach on the coast of Washington, U.S.A., indicate that sand suspension events of long duration (15–40 sec) relative to the period of incident waves contribute a significant quantity of sand to the total littoral drift. Work is now in progress to modify the system for self-contained operation on the continental shelf. Field investigations with the modified system will focus on the acquisition of a data set to test the validity of the suspended sediment distribution equation.