K.R. Dyer

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.

inssev: An instrument to measure the size and settling velocity of flocs in situ

Abstract An instrument has been developed to observe the settling of individual flocs in turbid water in order to measure size and settling velocity spectra of estuarine cohesive suspended sediments. inssev —IN Situ SEttling Velocity instrument—is bed mounted and comprises a computer controlled decelerator chamber that collects a sample of water from which some of the suspended matter is allowed to enter the top of a settling column. The settling flocs are viewed using a miniature video system. Subsequent analysis of video tapes can provide direct measurements of size and settling velocity of individual flocs down to 20 μm. From this information floc density can be estimated. The main feature of the instrument is its ability to video flocs in situ, irrespective of the concentration in the estuary, with as little disturbance to their hydrodynamic environment as possible. Preliminary field testing in the Tamar Estuary has given useful results in flow velocities up to 0.4 m s −1 and in concentrations up to 200 mg 1 −1 . The video recordings show large numbers of low density flocs with multiple structures linked by fine organic fibres.

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).

The effect of geomorphological structures on potential biostabilisation by microphytobenthos on intertidal mudflats

The chlorophyll a and colloidal carbohydrate content of sediments were measured at Skeffling mud-hat in the Humber estuary, UK, in July 1997 as part of a fieldwork experiment carried out within the framework of the INTRMUD project. The aim was to analyse the spatial variations of Chi a and colloidal carbohydrate concentrations within the surface 1 cm of sediment (together with physical variables) in the different macroscopic sedimentary structures found at four stations along a cross-shore transect. The underlying assumption was that epipelic microalgae (Chl a) produce extra cellular polymeric substances (EPS), largely comprised carbohydrates, when migrating vertically at the sediment surface. This organic material binds sediment particles and thus contributes to enhance sediment cohesiveness/stability. Therefore, the shape and the strength of the relationship between Chi a and colloidal carbohydrates are fundamental for assessing the role of autotrophic microbial communities in biostabilisation processes. At station A, the highest level of the mudflat, there were no obvious sedimentary features, while a ridge (crest) and runnel (trough) system was present at mid-tidal stations (B and C), At station D, the sediment was sandier; crests and troughs were obvious but did not form a ridge and runnel system as at stations B and C, Taking all data together, a significant positive linear relationship between colloidal carbohydrates and Chi a was found, but analysing data separately by station indicated that there was no relationship between variables at the sandy station (D). At stations B and C, there was a difference in the Chi a-carbohydrate relationship between ridges and runnels: (i) there was no relationship in runnels, i.e. carbohydrates concentration was roughly constant whatever the mud Chi a content, and (ii) there was a positive linear relationship in ridges. This indicates that the increase of epipelic biomass on ridges increases the amount of EPS, which is likely to stabilise the sediment surface of these features. The biomass level in runnels is lower and does not enhance the amount of EPS. Therefore, the activity of epipelic microalgae in runnels does not contribute to sediment stability. This observed difference between ridges and runnels does not mean that epipelic microalgae from these two features necessarily behave in a different way; carbohydrates produced by microalgae in runnels are very likely to be dissolved because of the higher water content. Thus epipelic algae cannot build up a pool of carbohydrates in runnels. As a conclusion, it is clear that geomorphological features of intertidal mudflats influence biological processes in a way which exacerbates the physical processes: (i) ridges are regularly exposed and the sediment surface is stabilised, which apparently favours microphytobenthos growth and carbohydrates production with a further increase in sediment stability (according to our initial assumption); (ii) runnels are drainage structures with a high water content, which prevents microphytobenthos from building up a carbohydrate pool. Therefore, there seems to be a synergistic effect between physical and biological processes on ridges to stabilise the sediment surface. [KEYWORDS: intertidal mud flat; microphytobenthos; epipelic diatoms biostabilisation; Extra cellular Polymeric Substances (EPS); chlorophyll alpha Ems estuary; sediment; diatoms]

The influence of bedforms on flow and sediment transport over intertidal mudflats

Abstract A range of bedforms of different types are found at a range of scales on intertidal mudflats. The different types include (a) channels, creeks and gullies, (b) ridge–runnel systems, (c) ripples and other micro-topography, and (d) cliffs. The main features of these bedform types are illustrated with examples found at a number of NW European mudflat sites studied during the EC INTRMUD project. Some comments are made on their environment of formation and observations of bed sediment properties and flow and suspended sediment processes are presented. Finally, the influence of bedforms on the hydrodynamics, shallow water tidal flow and wave propagation, and sediment processes are discussed.

Size and settling velocity distributions of flocs in the Tamar estuary during a tidal cycle

Data extracted from video recordings of individual estuarine flocs near the estuary bed during the advance and retreat of the salt intrusion show changes in size and settling velocity distributions. The recordings were taken using INSSEV —IN Situ SEttling Velocity instrument. Size coupled with effective density variations due to both changes in floc structure and ambient salinity result in changes in the settling velocity during the tidal cycle. In particular, just after high water slack, the appearance of high settling velocity medium size flocs and individual particles suggest that the lower density flocs have been broken up by the intense vertical shear in the currents caused by the salt wedge intrusion. Current shear is shown to have a significant influence on floc effective density.

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.

Floc population characteristics measured with INSSEV during the Elbe estuary intercalibration experiment

Abstract Individual floc data obtained with the in situ video camera settling velocity instrument (INSSEV) during the Elbe estuary Intercalibration Exercise, June 1993, are presented in a variety of scattergraph formats and processed to determine individual floc dry mass. The dry mass data are displayed in spectral formats with respect to size, and settling velocity. Similar floc population characteristics to those found in UK estuaries are shown; in particular there is a wide range of effective densities in the smaller floc sizes whereas all the largest flocs have a low effective density. The data are specifically assembled for comparison with results from a settling tube, deployed at similar time and depth. The data indicate that the INSSEV system is able to measure a higher proportion of larger flocs than the settling tube, supporting claims that bottom withdrawal settling tube systems are destructive of the larger, lower density flocs.

Deriving fluxes of Suspended particulate Matter in the Humber estuary, UK, using airborne remote sensing

This study exploits the use of airborne remote sensing for the assessment of coastal and estuarine environments, in particular the Humber Estuary. A Compact Airborne Spectrographic Imager was used to acquire a series of flightlines across the mouth of the Humber estuary during the flood on 10 August 1995. The flightlines encompassed the flux curtain, which consists of five moorings set out across the estuary to monitor flux of Suspended Particulate Matter (SPM). The images were radiometrically, atmospherically and geometrically corrected and SPM concentrations in surface waters derived. SPM flux was assessed by incorporating image-derived SPM concentration values with velocity results from a hydrodynamic model. These results were integrated over depth and an estimate of total flux determined for a 2-h section of the flood.

Sediment distribution, circulation and provenance in a macrotidal bay: Garolim Bay, Korea

Abstract A study of the sedimentary regime in Garolim Bay has been carried out in connection with a feasibility study for tidal power development. At the mouth of the bay, sand is shown to be circulating round a central bank between an ebb and a flood channel. In the middle bay, clay and heavy-mineral distributions indicate a zone where finer sediments derived from offshore mix with those originating from the local streams. Quantities of land-derived sediments are presumed to be deposited near the shores during the rainy summer season and these are redistributed during the windy winter season, but the concentrations of suspended sediment are generally low. The influx of sediment appears to largely balance the increase in sea level. Siltation over the general area may not be seriously increased after construction of a barrage, but local siltation and scour near the barrage could be serious problems.