Suspended Sand Concentration Research Articles

Dynamics of large and small scale bedforms on a macrotidal shoreface under shoaling and breaking waves

Bedform dimensions, bed position changes, near-bed velocities and suspended sand concentrations are analyzed from measurements at a single location on a macro tidal beach in the south west of England. The study was conducted in 0.5-2.25 m water depth under both swell and wind-generated storm waves with both weak and strong currents present. Bed positions and suspended sand concentrations were measured using the 3 transducers of a multi-frequency acoustic backscatter sensor with 5 mm vertical resolution. Two distinct bed types, based on wavelength (λ), were observed, each with two or more subtypes possible: (I) small-scale bedforms ( λ < 20 cm) which include two-dimensional pre-vortex (1), vortex (2), post-vortex (3) and three-dimensional vortex (4) forms; (II) large-scale bedforms ( λ > 20 cm) which include two-dimensional (5) forms and three-dimensional vortex (6) forms. Small-scale forms were dominant under non-breaking conditions while the large-scale forms occurred under both non-breaking and breaking waves; the large-scale forms dominate under breaking conditions. Both types, but particularly low steepness forms, were highly mobile with maximum horizontal migration rates of 5 cm min −1. Large (upto 15 cm) and rapid (upto 3.0 cm min −1) changes in vertical bed elevation were also observed in association with the development and migration of large-scale forms. Large-scale bedforms were also highly variable spatially, often being interspersed with smaller scale forms under decaying flow regimes and with areas of flat bed under increasing regimes. Observations suggest these forms are present even under high energy surf zone conditions (wave Shields ⩾ 1). Models for predicting ripple dimensions did not perform well in this environment. This lack of agreement reflects the complex hydrodynamic regimes associated with random (grouped) shoaling and breaking waves together with the presence of currents, often at large angles to the waves, as well as the rapid rates of change in the wave forcing associated with tidal cycle oscillations in this macrotidal environment. Suspended sediment concentrations and transport rates are particularly sensitive to the bedforms present and also to bed position changes associated with ripple migration. Estimates of transport rates are subject to potentially large errors (upto 30%) without compensation for bed elevation changes relative to sensor position.

Determining suspended sand size and concentration from multifrequency acoustic backscatter

An inversion algorithm for extracting suspended sand size and concentration from simultaneous backscattered acoustic pressure amplitude at three operating frequencies is presented. The algorithm is based on the differences in signal amplitude between different frequency pairs, and is tested using laboratory measurements of multifrequency backscatter from a turbulent sediment-carrying jet. Concentration and size profiles inverted from field and laboratory data are compared with results from a previously developed algorithm based on signal ratios. The difference inversion scheme is less sensitive to errors arising from low signal levels, allowing the size/concentration measurement range to be extended to regions of lower concentration. The concentrations from the field data agree well with independent optically determined estimates. The results demonstrate sensitivity to the backscatter form factor.

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.

Suspension of sand due to wave groups

Field observations of the suspension of sand near the seabed under shoaled waves in the nearshore region are presented and discussed with regard to the influence of wave groups. Sand is found to be suspended at both the incident wave frequency and at lower frequencies corresponding to wave groups. These limited observations indicate the likelihood that wave groups result in higher concentrations of suspended sand than randomly distributed waves of similar heights.

Acoustic measurements of suspended sand on the shoreface and the control of concentration by bed roughness

Acoustic backscatter measurements of the concentrations of sand in suspension on the shoreface, seawards of the breaking zone, during a mild storm event show that sand concentrations increase initially but then rapidly decrease as the wave energy increases: it is suggested that the bed roughness is a major control on suspended sand concentration and that the decrease in concentration is due to decreasing ripple steepness after the break-off bed shear stress is exceeded. No direct measurements of bedforms were available and the combined wave-current interaction model of Grant and Madsen (J. Geophys. Res., Vols. 84 (1979) and 87 (1982)) with extensions to movable bed roughness (ripple dimensions from Carstens et al's. 1969 laboratory data (U.S. Army Corps Eng. CERC Tech. Mem. 28) and to sediment-induced self-stratification (Glenn and Grant, J. Geophys. Res., Vol. 92, 1983) was used to predict the current and suspended sand concentration profile. Matching the measured and modeled concentrations at 2 cm above the bed, the values of the resuspension coefficient γ 0 were found to decrease as excess skin friction increased, in a manner similar to that suggested by Drake and Cacchione (Cont. Shelf Res., Vol. 9, 1989) but were an order of magnitude larger. Using the much smaller ripple dimensions of Nielsen (J. Geophys. Res., Vol. 86, 1981) changed the model results very little. Significant differences were found between the time-averaged suspended sand profiles and those predicted by the model. Sand transport fluxes have also been computed using the instantaneous products of measured suspended concentrations and currents (the current being the sum of steady and wave-induced components). The wave-induced fluxes show considerable variability from run to run but the general pattern is of a transport profile having a shoreward maximum at 5–10 cm above the bed and offshore transport in the few centimetres closest to the bed: little net transport occurred above 15 cm. These fluxes show a weak dependence on the wave energy, becoming more shoreware and the height of the maximum shoreward transport decreasing as the wave energy increased, again consistent with the influence of the ripple steepness.

Laboratory Study on Sand Suspension Due to Breaking Waves

ABSTRACTThis paper describes the mechanism of sand suspension due to wave breaking on the basis of detailed laboratory measurements of suspended sand concentration near the breaking point with a light-absorption-type probe. The distribution and the total amount of suspended sand were evaluated using the data obtained, which were carefully corrected for the contamination due to air bubbles entrained through wave breaking. A predictive formula for the amount of sand suspended by a breaker was proposed for a wide range of conditions.

Field measurements of the suspended sand concentration profiles and fluxes and of the resuspension coefficient γ0 over a rippled bed

Measurements of the acoustic backscatter from suspended sand over a rippled bed and of currents were made near the crest of a ridge‐and‐runnel beach on the macrotidal North Norfolk coast of the United Kingdom. Two 12‐min records were taken close to high water in a depth of 1.7±0.2 m. Visual inspection of the data shows that sand was intermittently suspended to heights of 20–30 cm but that these events were not always correlated with strong wave‐induced currents: the structure of some of the suspension events suggest that they are due to vortex shedding from a rippled bed. The observed time‐averaged concentration profiles were compared with predictions generated using Glenn and Grant's (1987) combined‐flow model. The nondimensionalised concentrations showed good correspondence in the first 8 to 10 cm above the wave boundary layer, supporting the use of a value of 0.74, drawn from atmospheric data, for the constant of proportionality between sediment and momentum diffusivity. Above this height, observed values of the proportionality constant tended to approach 1.0, which may have been due to a decrease in sediment settling velocity with height (which is consistent with finer sand being suspended higher in the flow), or alternatively to nonuniform turbulence in the vicinity of the wave ripples. Absolute concentrations measured 2 cm above the bed were consistent with values of O(10−4) for the empirical constant γ0 in Smith and McLean's (1977) pickup function. Concentrations within the wave boundary layer were observed to decrease much more rapidly than predicted by the model. Sand transport fluxes were computed for both the cross‐shore and alongshore components of the flow using a current profile based on the combined‐flow model of Glenn and Grant (1987). For the two records, vertically integrated fluxes of 1.5‐mg/s/cm width and 0.89‐mg/s/cm width directed towards the shore were calculated for the wave oscillatory component of the transport; the mean flow produced shoreward vertically integrated fluxes of 5.1‐ and 10.5‐mg/s/cm width. The uncertainty associated with these estimates is considerable (±50%). The vertical profile of the total flux (wave oscillatory and mean) for both records showed a strong shoreward transport close to the bed, an offshore transport between 5 and 15 cm above the bed, and a further weak onshore flux above this, demonstrating the necessity for measurements of the whole concentration profile (rather than a few point samples) for the calculation of sand transport over a rippled bed in a wave‐dominated environment.

A laboratory evaluation of optical backscatterance suspended solids sensors exposed to sand-mud mixtures

Optical suspended solids sensors were studied in the laboratory in order to obtain calibration factors and evaluate the general behavior of the instrument when exposed to suspensions of mud and sand. The sensors were found to be capable of measuring the time-averaged concentration of either suspended sand or suspended mud, but the relative sensitivity of the sensor to these two different types of sediment varied by approximately an order of magnitude. In mixtures with a constant or known mud concentration, the instrument response to the mud may be treated as an offset, and gains appropriate to sand may be used to estimate the suspended sand concentrations. Semi-quantitative observations of the effects of flow direction relative to sensor orientation indicate that significant concern is warranted.

Prediction of Sediment Sampling Error at Nozzle

Pump sampling can be an accurate method of measuring suspended sand concentrations. When the flow velocity through the pumping nozzle does not match the local stream velocity, however, an error is introduced. A simple equation is presented that can be used to produce correction factors for this source of error. The equation is successfully compared to data that was not used in the development of the theory. The predictive equation is also adapted to include misalignment of sampling nozzle; the adapted equation is checked against data and used to show that these errors are very small.

Acoustic measurements of suspended sand concentration in the C2S2 experiment at Stanhope Lane, Prince Edward Island

A 3 MHz acoustic concentration meter (ACM) was deployed with electromagnetic flowmeters in the nearshore region in October 1984 at Stanhope Lane, Prince Edward Island, Canada, as a component of the Canadian Coastal Sediment Study. The ACM measured the backscatter of acoustic energy in a vertical beam pattern within approximately 1 m of the seabed. Laboratory calibration experiments were conducted to verify a method for converting acoustic backscatter into suspended sand concentration profiles.The ACM functioned well when there were no air bubbles in the water. Both the suspended sand concentration profiles and the bottom location were measured with an approximately 1 s time resolution and a 1 cm vertical spatial resolution. The suspension of sand is related to the wave and current forcing, and is dominated by events of short duration.

Intermittent sediment suspension and its implications to sand tracer dispersal in wave-dominated environments

Recent observations of time series of suspended sand concentration in wave dominated environments indicate the occurrence of intermittent near-bed suspension. The process of intermittent suspension is a significant mechanism resulting in the transport of sand due to waves and currents. Dyed sand tracer measurements which had previously been interpreted to indicate the dominance of bed-load motion are reinterpreted within the context of intermittent suspension by using an order of magnitude stochastic description of suspension and burial events. This analysis indicates that tracer burial and dispersal patterns are consistent with the process of intermittent suspension.

A 3‐MHz acoustic concentration meter that measures suspended sand

A 3‐MHz acoustic concentration meter (ACM), which measures the intensity of backscattered acoustic energy, has been deployed in the laboratory and in the field in order to estimate the concentration of suspended sand. Techniques for measuring the profile of the concentration of suspended sand close to the seabed with a vertical resolution of about 1 cm are discussed. The scattering characteristics of suspended sand in a laboratory tank were used to provide a method of calibration for the ACM. The dependence of the scattering coefficients upon sediment size was also investigated. The ACM was deployed in the surf zone at Starthope Lane, Prince Edward Island, Canada, in October 1984. In the surf zone, the limitations of such profilometers are (a) air bubbles injected into the water by breaking waves, and (b) the possible variations in sand size with height above the seabed. [Work supported by Naval Facilities Engineering Command.]

SEDIMENT TRANSPORT DUE TO BREAKING WAVES

In the surf zone, the agitation of the bed materials by breaking waves is strong and the suspended sand concentration in the vicinity of the wave plunging point is extremely high. Sand movement in this region was observed and sand concentration was measured in a wave flume. The sand movement in the region was divided into the following two categories: 1) sand suspension due to the large vortex which is created by wave plunging, and 2) sand deposition under turbulent flow. The condition for exciting this suspension process was considered and the result was well explained by the two parameters which are the deep water wave steepness and the bottom slope. Then a numerical model of the sediment suspension process was formulated and the process was well simulated by the model.

LABORATORY STUDY ON SAND TRANSPORT OVER RIPPLES DUE TO ASYMMETRIC OSCILLATORY FLOWS

Mechanism of sand movement due to asymmetric oscillatory flows was investigated through experiments. Measurements of bed forms, suspended sand concentration and net sand transport rate were carried out by using an oscillatory flow tunnel. The process of entrainment and suspension of sand above asymmetric ripples was quantitatively described. The geometry of ripples and the net sand transport rate in regular and irregular flows were expressed in terms of hydraulic parameters characterizing the oscillatory flow. Two-dimensionality of ripples was found to be an important factor in the estimation of the net sand transport rate.

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.

Variation of roughness length of a mobile sand bed in a tidal flow

Velocity profiles, suspended sand concentration, and ripple shapes were measured over the tidal cycle on a mobile bed. Variations in ripple shape or dimensions, and the effects of mobile sediment could not account for the variations in bed roughness length, which increased systematically during the tide from 0.5 to 1.3 cm. The effects of acceleration could be the cause as they would lead to the same pattern of variation, but existing theory does not give the correct magnitude.

Measurements of the turbulence characteristics of sand suspended by a tidal current

Measurements made with a new fast-response suspended sand sensor have for the first time enabled the turbulence characteristics of a natural sand suspension to be studied. Suspended sand concentration measurements, together with fast-response current measurements, were made 18 cm above a sandy bed under a strong tidal current. They showed a highly variable concentration field, dominated by clouds of sand which took 2 to 10 s to sweep past the sensor. The concentration spectrum had peak energy at a wavelength of about 3 m, and exhibited a −5/3 power dependence at high frequencies. Damping of the turbulence intensity of the current was observed when sand was suspended. A spectral approach produced more plausible values for the upward Reynolds flux of sediment than the direct covariance technique. The calculated upward fluxes were appreciably smaller than the settling fluxes around the time of maximum current, contrary to the expectation for an equilibrium concentration profile. The turbulence characteristics of the concentration field displayed marked similarities with standard results from atmospheric temperature measurements, strengthening assumptions commonly made in the prediction of sediment transport rates.

Hummocky Cross-Stratification and Megaripples: A Geological Double Standard?

ABSTRACT Hummocky cross-stratification in ancient sedimentary deposits is considered to be diagnostic of inner shelf storm deposits. The geometry of the cross-strata sets implies a low-relief oval bedform of a few meters spacing; orientation is poorly defined or random. Hummocky cross-strata sets have been attributed to the action of strong storm-wave surges. Megaripples of 2-5 m spacing are typical responses of the inner Atlantic Shelf of North America to storm flows. Side-scan sonar records have shown that megaripples on the inner shelf are commonly hummocky (oval to circular in plan view) and lack asymmetry. Box cores and vibracores reveal gently inclined to horizontal lamination as the characteristic primary structure. The megaripples am produced by combined-flow storm currents. These are alongcoast geostrophic flows with a mean flow component of 20-60 cm sec-1. A high-frequency wave orbital current of similar or weaker intensity is present, but its role in megaripple formation is subordinate to that of the mean flow. The high-velocity coastal margin of the flow is often jetlike and downwelling in nature, and is characterize by a down-current decrease in sand flux and rapid deposition of sand. The bottom boundary layer of such flows is characterized by dense sediment suspensions (40-60 mgl-1) and is highly turbulent due to the interaction of the mean flow and wave orbital current components (eddy diffusivity up to 200 cm2 sect at 40 cm above the bed). Sand transport in this coastal boundary zone is obliquely offshore, but neither the density of the sediment suspension nor storm-surge ebb appear to be important driving forces. It is difficult to account for either modern hummocky megaripples or ancient hummocky cross-stratification as responses to purely oscillatory flows. Combined-flow currents are the typical response of mostmodern shelves and epicontinental seas to storms and presumably were so in the past. Hummocky cross-stratified beds are generally too thick (20-80 cm) to be accounted for by in-situ resuspension; they are more likely to have been despoited by combined-flow currents in areas where these currents were experiencing a downstream velocity decrease and sediment deposition throughout much of the storm's duration. Evidence for strong scour in swales between hummocks and the smoothly rounded surface of the hummocks suggests a transitionlike bed regime. Measurements on the Atlantic Shelf likewi e suggest a transitionlike, regime, whose boundary layer is characterized by lower velocity than the transitional regime of unidirectional flow in flumes, but more intense turbulence and a higher suspended sand concentration. The lack of normal cross-bedding in bothmodern and ancient hummocky bedforms indicates that these features did not migrate. This characteristic may be a consequence of the complex trajectories followed by near-bottom water particles in response to combined-flow currents.

On the relationship between bedload and suspended sand transport on the inner shelf, Long Island, New York

Data obtained by a near‐bottom Profiling Concentration and Velocity system (PCV), deployed in 10 m water depth at a site 1 km from the Long Island coastline, are used to examine links between bedload and suspended sand concentrations. Calculations of bedload areal concentration C* are based on the empirical results of Vincent et al. (1981) and use the theoretical formulations of Grant and Madsen (1978, 1979) to describe the interaction between wave and current boundary layers. Suspended sand concentrations were obtained directly from an Acoustic Concentration Meter (ACM). Average suspended sand profiles Cz were found to fit closely to a log‐linear profile Cz = C1(1 ‐ A loge z/Z1), where C1 is the sand concentration at a height z1 = 1 cm from the bed and A is empirically determined as 0.22±0.005. A linear correlation is observed between the areal bedload concentration C* (the volume of bedload per unit area of the bed) and the suspended sand concentration 1 cm above the bed C1, with a correlation coefficient of 0.82 (significant at the 1% level), and supports the hypothesis of Einstein (1950) that bedload and suspended load are related through bedload concentration. It is also shown that the sedment threshold criterion of Komar and Miller (1973), when expressed as a ratio (here called the Komar ratio), can be used as a useful predictor for C1 under conditions where the wave orbital currents are much greater than the mean flow. These relationships offer the opportunity for the calculation of both bedload and suspended sand transport rates from measurements of the steady current velocity and wave parameters, combined with information defining the surficial sediments and local bottom topography.