Andrew J. Manning

Observation of the size, settling velocity and effective density of flocs, and their fractal dimensions

Abstract In situ instruments, particularly the instrument INSSEV (in situ settling velocity) have given new information on the sizes, settling velocities and effective densities of individual flocs within the spectrum of distribution. The low-density macroflocs (diameter >∼150 μm) contain a mixture of organic and inorganic constituents that become separated when the flocs are disrupted to form microflocs. Representation of the floc characteristics in terms of fractals reveals a range of fractal dimensions representing the distributions varying between 1 and 3, instead of the ideal value of 2. Measurements in estuarine turbidity maxima and on intertidal mudflats show that the fractal dimension is less than 2 in situations where turbulent shearing causes disruption of the flocs. At the same time increasing suspended sediment concentration tends to increase the fractal dimension. Measurements of size using an in situ Malvern sizer show that the floc size distribution is also affected by both turbulent energy dissipation and by concentration. Complementary laboratory studies suggest that, at a constant concentration, flocculation is enhanced by low shear, but that disruption occurs at higher shear. These experiments confirm the relationship between fractal dimension, shear stress and concentration.

A laboratory examination of floc characteristics with regard to turbulent shearing

Abstract Turbulent shear generated within the water column is recognised as having a controlling influence over both the flocculation of fine grained cohesive sediments within estuarine waters, and their respective aggregate break-up. This study examines the inter-relationships between floc characteristics over increasing turbidity (80–200 mg l −1 ) and turbulent shear (0–0.6 N m −2 ) environments, by the use of a laboratory flume within which a suspension can be sheared at a controlled rate, and with an unintrusive macro-lens miniature video camera mounted in a viewing port on the flume channel wall. The camera enables the direct simultaneous measurement of both floc size and settling velocity, from which accurate estimates of floc effective density and porosity can be made. Measurements were made 120 s after the induced turbulence has ceased. The instrument has an upper viewing turbidity limit of 210 mg l −1 , and a lower resolution of 20 μm. The sediment was collected from the inter-tidal mudflats at Weir Quay on the Tamar Estuary in Devon, Southwest England. The results indicated that increasing turbidity at low shear levels encouraged floc growth, but the effect of the increasing turbulent shear (0.35 N m −2 ) together with increasing concentration in suspension causes disruption rather than enhancing the flocculation process. At shears up to 0.35 N m −2 , the largest size and settling velocity flocs were produced at high concentrations, whereas above 0.35 N m −2 disruption caused smaller flocs at higher concentrations. The use of algorithms which were based either on a single floc characteristic (i.e., size or settling velocity) or a mean fractal dimension, were seen not to accurately approximate the experimental data. A multiple regression analysis of the experimental data produced the following formula, based on mean values of the 20 largest flocs sampled under each of the imposed environmental conditions (referred to as max20size mean values): settling velocity=0.301−0.00337 rms of the gradient in turbulent velocity fluctuations−0.000606 SPM+0.00335 floc size, which proved to be the most accurate representation with an R 2 value of 0.95. A similar formula was determined for the average value of the four fastest settling flocs within each sample-group (max4W S ). This highlights the importance of modelling algorithms that are developed from data that take into account effective density variations (i.e., simultaneous size and settling velocity measurement).

A review of sediment dynamics in the Severn Estuary: influence of flocculation.

This paper provides a review and critique of the distributions and characteristics of non-cohesive and cohesive sediments within the Severn Estuary, with particular reference to floc properties. The estuary is hyper-tidal and, consequently, highly turbid along most of its length and it generally has two turbidity maxima. In the upper reaches of the estuary, suspended particulate matter (SPM) concentrations can be in excess of 10 g l(-1) for river flows up to 50 m(3)s(-1), rising to over 50 g l(-1) during periods of lower river flow. The lower estuary turbidity maximum originates in the vicinity of Bridgwater Bay where SPM concentrations may vary between 0.1-200 g l(-1). The formation of fluid mud is coupled to the spring-neap cycle and strong vertical gradients in SPM concentrations produce turbulence damping and drag reduction effects, and hence impair the ability of the flow to transport sediments. Flocculation is an important mechanism for controlling the behaviour of fine sediments and mean settling velocities of flocs vary between 0.8-6 mm s(-1). A secondary consequence of flocculation is the formation of mud:sand mixtures in turbid suspensions. Improved understanding of the significance of flocculation processes is crucial as they may exert an influence on the mechanism by which adsorbed contaminants are transported in the system.

Sticky stuff: Redefining bedform prediction in modern and ancient environments

The dimensions and dynamics of subaqueous bedforms are well known for cohesionless sediments. However, the effect of physical cohesion imparted by cohesive clay within mixed sand-mud substrates has not been examined, despite its recognized influence on sediment stability. Here we present a series of controlled laboratory experiments to establish the influence of substrate clay content on subaqueous bedform dynamics within mixtures of sand and clay exposed to unidirectional flow. The results show that bedform dimensions and steepness decrease linearly with clay content, and comparison with existing predictors of bedform dimensions, established within cohesionless sediments, reveals significant over-prediction of bedform size for all but the lowermost clay contents examined. The profound effect substrate clay content has on bedform dimensions has a number of important implications for interpretation in a range of modern and ancient environments, including reduced roughness and bedform heights in estuarine systems and the often cited lack of large dune cross-sets in turbidites. The results therefore offer a step change in our understanding of bedform formation and dynamics in these, and many other, sedimentary environments.

A Comparison Of Floc Properties Observed During Neap and Spring Tidal Conditions

It is recognised that in order to properly understand how suspended particulate matter behaves during different tidal conditions within an estuary, high quality in-situ data is of a prime requirement. This paper initially presents floc data sets collected in the upper reaches of the Tamar estuary in south-western England. All floc samples were obtained using the in-situ sampling device INSSEV. The floc data was supplemented by simultaneous time series of near-bed profiles (using the high frequency POST system) of: turbulent shear stress (TSS), suspended particulate matter (SPM) and current velocity. To enable a comparison of typical spring and neap tidal conditions, respective data sets were collected (on a sub-tidal duration) on 24th June 1998 and 5th August 1998. The spring tides experienced nearly twice the annual mean river flow (~ 40 m3s−1), and salinity did not exceed 0.5 at anytime during sampling. The afternoon flood saw surface currents approaching 1.1 ms−1, and a maximum TSS of 0.7 Nm−2 (at 25 cm). Throughout this period a concentrated benthic suspension layer developed, which displayed a peak particle concentration of 6 gl−1 (50 cm above the bed) and a lutocline ~ 40-60 cm above the bed. For the 5th August the annual mean river flow allowed the near-bed salinity at Station A to reach 8 during the afternoon flood. Surface currents did not exceed 0.55 ms−1 and the SPM remained under 190 mgl−1, with the exception of the turbidity maximum (TM) formation at sampling Station A 1.5 hours into the flood, where the near-bed SPM rose to 1.15 gl−1. The maximum flood TSS 25 cm above the bed was 0.74 Nm−2 and occurred just prior to the TM formation. An abundance of fast settling macroflocs (> 160 microns) from spring tides, accounted for a time series average of 89% of the mass settling flux (MSF). Whereas during neap tides, the macroflocs contributed 16% less to the MSF rate. This was partly due to a time series averaged macrofloc settling velocity of 4.6 mms−1 from the spring tidal data; 2.8 mms−1 higher than for neap tide conditions. During the TM passage at spring tides, macroflocs reached 1.5 mm in diameter; these flocs had settling velocities of up to 16.6 mms−1, but effective densities were less than 50 kgm−3, which means they would be prone to break-up when settling to a region of high shear. At the opposite end of the scale, low SPM and quiescent conditions severely restricted floc production. A multiple parametric analysis identified both the TSS and SPM concentration as significant controllers of the settling velocity of the macroflocs, and these parameters must be included within any quantitative empirical algorithms.

The role of biophysical cohesion on subaqueous bed form size

Abstract Biologically active, fine‐grained sediment forms abundant sedimentary deposits on Earths surface, and mixed mud‐sand dominates many coasts, deltas, and estuaries. Our predictions of sediment transport and bed roughness in these environments presently rely on empirically based bed form predictors that are based exclusively on biologically inactive cohesionless silt, sand, and gravel. This approach underpins many paleoenvironmental reconstructions of sedimentary successions, which rely on analysis of cross‐stratification and bounding surfaces produced by migrating bed forms. Here we present controlled laboratory experiments that identify and quantify the influence of physical and biological cohesion on equilibrium bed form morphology. The results show the profound influence of biological cohesion on bed form size and identify how cohesive bonding mechanisms in different sediment mixtures govern the relationships. The findings highlight that existing bed form predictors require reformulation for combined biophysical cohesive effects in order to improve morphodynamic model predictions and to enhance the interpretations of these environments in the geological record.

Assessment of Flocculation Kinetics of Cohesive Sediments from the Seine and Gironde Estuaries, France, through Laboratory and Field Studies

Cohesive suspended particles in a river aggregate into microflocs with average diameters of 10–20 μm , which on their arrival in the estuary aggregate to macroflocs with average diameters of 100–200 μm . We performed a laboratory study on samples from the Seine estuary. The experiments show a more or less systematic behavior of floc size with drivers. Macrofloc size relates to turbulence, η , suspended particulate matter concentration ( C ), salinity ( S ), time ( t ), and mineral and biological composition of primary particles M . Generally, floc size increases with C , S , t , and η . We performed Eulerian field surveys of floc size and the same drivers on various sites in the Gironde estuary. Our field samples show an ambiguous behavior of floc size against salinity and suspended sediment concentration. Floc size does not seem to simply increase with C , S , t , and η , but flocs grow and decay with time.

Numerical modelling of Mud Transport Processes in the Tamar Estuary

Transport processes of fine-grained sediments in the Tamar Estuary, UK, are studied using a combination of two- and three-dimensional numerical models and a comprehensive observational data set, collected during a COSINUS field campaign in 1999. The three-dimensional model is based on a hydrostatic version of MIKE 3, combining models for flow, stratification, turbulence and mud transport. Using a two-dimensional flow model of the whole estuary to provide boundary information, a high-resolution three-dimensional model is set up for a section of the upper estuary, containing a pronounced turbidity maximum. The model is calibrated using the observations. A sensitivity analysis is carried out, where various formulations of flocculation effects and of buoyancy effects on the turbulence are investigated. The conclusions are that the models can provide a realistic picture of the mud transport processes, but are sensitive to the specific parameterisation of flocculation.

CBS layers in a diffusive turbulence grid oscillation experiment

The generation of a Concentrated Benthic Suspension (CBS) of a fluid mud mixture is investigated in the laboratory in a grid stirred experiment. A sediment concentration in the CBS layer in the range 3 g/l to 200 g/l was considered. The sediment concentration, the turbulence properties, and the settling velocity have been measured. The decay of turbulence with increasing distance from the grid is not found to vary with the concentration inside the CBS layer except in the vicinity of the lutocline. The settling velocity decreases rapidely with increasing sediment concentration and appears to be the most sensitive parameter in the experiment. A clear difference in the value of the flux Richardson number at the lutocline is observed depending on the sediment concentration. Rif is about 0.1 for the highest concentrations and increases above 0.5 for the low concentration cases.

The turbidity maximum in a mesotidal estuary, the Tamar Estuary, UK: I. Dynamics of suspended sediment

A series of detailed simultaneous measurements of current velocity, salinity, suspended sediment concentration and turbulence, at two stations in the upper Tamar Estuary, has revealed that the turbidity maximum is centred about 1km upstream of the salt intrusion, and is created mostly from entrainment from the bed. Suspended sediment concentrations at about 0.5m above the bed reach about 6g1 −1 on spring tides, but only about 0.3g1 −1 on neaps. Based on measurements of turbulent energy, the threshold of sediment erosion is estimated as 0.17-0.2Nm −2 . Drag reduction was experienced when the suspended sediment concentration gradients exceeded 4kgm −4 . Analysis of surface water slopes in conjunction with the currents suggests that there is a minimum in velocity between the salt intrusion and the turbidity maximum which coincides with a reduction in the water level, together forming a ‘bow-wave’ effect, upstream of the salt intrusion. These results form the background for detailed analysis of the distribution of floc properties, particularly settling velocity, within the turbidity maximum (discussed in Part II).