C.E. Vincent

Measuring suspended sediment concentrations using acoustic backscatter devices

Abstract In recent years the requirement to develop a technique for measuring suspended sediment concentration profiles, with similar temporal and spatial resolution to that of electromagnetic current meter measurements of the flow, has led to the development of an acoustic approach. Acoustic backscatter devices have been constructed and deployed in marine environments and measurements of backscattered signal profiles obtained. However, the translation from acoustic returns to suspended load has been an empirical procedure resulting in some uncertainty regarding the accuracy and reliability of the conversion process. To address this problem a series of laboratory measurements of acoustic backscattering from marine suspensions were conducted using two acoustic backscatter instruments operating at 3.0 and 5.65 MHz. A simple semi-empirical backscattering theory has been developed to predict the form of the scattered signal and comparisons have been made of the concentrations predicted from the theory against concentration measurements made in laboratory tanks. The results show that the main features of the backscattering signal can be explained and accurate concentration estimates can be made from the acoustic returns. It is concluded that acoustic backscatter techniques are potentially powerful tools for examining, at high resolution, the structure of suspended sediment concentration profiles near the seabed.

The composition of snowfall, snowpack and meltwater in the Scottish Highlands ― evidence for preferential elution

Abstract Acidic snows in a small, remote, high-altitude snowpack in the Cairngorms, Scotland, give rise to meltwaters which are proportionally rich in sulphate and nitrate. As a consequence, the within-pack snows become proportionally rich in chloride, even though depleted in solute. Preferential elution appears to be a major process in the chemical evolution of snowfall and snowpack.

Backscattering from a suspension in the near field of a piston transducer

The problem of acoustic backscattering from suspended particulates in the near field of a piston source is examined. A monostatic configuration is used, with the transducer acting as a transceiver. Predictions for the range dependence of the backscatter signal are calculated, and formulated into a nondimensional general form. To compare predictions with observations, a series of laboratory experiments, using a number of transducers insonifying varying homogeneous suspensions, has been conducted. To first order the outcome shows good agreement between prediction and observation.

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 als. 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 γ0 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.

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

Abstract 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.

RECENT CHANGES IN THE LEVEL OF LAKE NAIVASHA, KENYA, AS AN INDICATOR OF EQUATORIAL WESTERLIES OVER EAST AFRICA

A previously unpublished record of lake levels from Lake Naivasha, Kenya from 1880 to 1976 has been analysed and shows little similarity to the level record from nearby Lake Victoria. Level changes from year to year of the two lakes show no significant correlation (at 5%) and spectral analysis of the two records shows no common significant peaks. Both lakes show significant correlations between their level changes and the strength of the North Atlantic winter circulation, with the correlation coefficients in opposing directions indicating important, but different, large scale climatic links.Lake Naivashas major level increases occur during May and September. Lake Victorias level increases mainly in May with a small December increase. East African rainfall is generally during April and November, corresponding with Lake Victorias changes. Rainfall records from Kenyan highland areas, however, show an August rainfall peak and little rainfall in November. Rainfall amounts from Equator, a highland meteorological station, for July, August and September are highly correlated (at 1% significance level) with the change in Naivashas level during September. Winds at the highland stations are westerly during August while the lower level stations experience the drier S.E. Trades. The level changes of Lake Naivasha indicate changes in the extent of the penetration of moist air from West Africa between the Trade winds and the 200 mb easterly jet.

Vertical and horizontal structure is suspended sand concentrations and wave-induced fluxes over bedforms

Abstract High resolution measurements of suspended sand concentrations have been made using a multi-transducer acoustic back scatter sensor over both steep and flat bedforms under low energy swell conditions with weak currents present on a micro-tidal beach in the southwest of England. Similar measurements were made over steep bedforms in a large-scale laboratory wave basin using a wave record simulated from field data. A detailed interpretation of the suspension process associated with steep vortex type ripples was made possible with knowledge of the position of the acoustic sensors relative to the bedform geometry. Analysis of time-averaged, wave ensemble-averaged suspended sediment concentrations, and velocity-concentration phase angles indicates that the bedforms are an important control on the suspension patterns produced by wave-induced flows. Steep asymmetric ripples under shoaling waves produces greater amounts of suspension than low steepness ripples due to the effects of vortex ejection associated with the steep ripples. Over rippled beds, we find evidence for a significant phase couplined between the resuspended sediment and the bedforms in the near bed region (

The Removal of Soluble Ions from Melting Snowpacks

Field and laboratory observations have indicated that the “fractionation” mechanism, whereby the bulk of the solute in seasonal snowcover is removed in the early meltwater fractions, is a differential process. Some soluble ions may be removed at faster rates than others. This process is known as “preferential elution”. Since the process is not yet universally accepted amongst scientists who engage in research into snowmelt chemistry, in this chapter we review the pertinent experimental work. Varying experimental conditions in real snowpacks often militate against reproducible elution sequences, although sodium and chloride are consistently found to be the least mobile ions upon melt. Laboratory experiments have provided more controlled conditions under which to examine the relative removal rates of soluble ions in meltwater. Those ions which are more soluble in ice are seen to be less mobile upon melt, whereas the ions which are eluted more rapidly may be regarded as being more strongly partitioned into the solute-rich surficial liquid, or quasi-liquid, layer. In real melting snowpacks, preferential elution of some ions by the meltwater can be masked by snowpack-scale hydrology.

Variability of suspended sand concentrations, transport and eddy diffusivity under non-breaking waves on the shoreface

Abstract Suspended sand concentration profiles and current speed measurements were made in 1.6 ± 0.4m water depth on the seaward side of a bar beyond the break point at Stanhope Lane beach, Prince Edward Island, Canada, during a 4 day period. Two mild storm events occurred and 7 min-long bursts of 4.4 Hz data were recorded approximately every hour. Concentration profiles, with a resolution of 5 mm and 7.5 mm, were obtained using a 3 MHz acoustic backscatter (ABS) instrument. Some data on bed form dimensions were obtained by divers between storm events. Multiple bed echoes from the ABS also provide further information on the ripple heights but no direct observations were available at times of higher waves. During the 4 day period the local position of the bed decreased by about 8 cm, erosion occurring in two short (⋟6 h) periods, one during each storm event. Mean currents at this location were weak ( The resuspension coefficient γ0, calculated from the concentration at 2 cm above the bed, decreased as the wave height increased (in the break-off range) supporting the observations of Vincent et al. [(1991) Marine Geology, 96, 1–18]. Large inter-burst variability was observed in the concentration profiles and in the eddy diffusivity and suspended transport fluxes computed from these profiles. This variability was due to the short length of record (⋟7min) relative to wave groups and to the location of the ABS relative to bed forms; to obtain consistent concentration and transport fluxes it is necessary to average many bursts over a time scale that is long compared to both groupiness and bed form mobility. Except at the beginning and end of events when wave conditions were changing rapidly, averaging over groups of bursts with similar wave conditions produced eddy diffusivity profiles characterized by a linear ɛs gradient of (20 ±2.5) cm s−1 from the sea bed to 20 ± 5cm, beyond which ɛs slowly decreased, and a suspended sand transport which was all below 20 cm and dominated by a jet-like shoreward flow in the lowest 3–4 cm above the sea bed. Bursts obtained when conditions were in, or close to, the equilibrium range of bed forms showed concentration profiles which were partly exponential (hence ɛs was constant with height) and transport profiles with more structure. Sensitivity tests indicate that concentrations and eddy diffusivities may be underestimated by about 20% by using the modal size for the sand in suspension rather than a distribution of sizes. Towards the end of the second storm a “low suspension event” of ⋟6 h duration occurred and suspended sand concentrations about 5–10 cm decreased by 1–2 orders of magnitude while the near bed concentrations remained approximately constant, suggesting that the bed forms responsible for vortex ejection of sand higher into the water column had been wiped out and the sea bed had become (temporarily) flat. Wave heights did not change significantly but wave period increased from 4.8 to 6.2 s, decreasing the maximum bed shear stress. The low suspension period may have a transient response of the sea bed to changed hydrodynamic forcing prior to the subsequent growth of bed forms with a different wavelength.

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

Abstract 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 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.