Mud Concentration Research Articles

Exposure of cod Gadus morhua to resuspended sediment: an experimental study of the impact of bottom trawling

Bottom trawling activities resuspend large amounts of sediment and may be the main agent of sediment transportation in some areas. During periods of intensive fishing by large trawler fleets the high-turbidity environmental conditions they create may become semi-permanent. Little is known about the potential effects of this resuspended sediment on the survival and physiology of marine fish. We carried out a worst-case-scenario simulation whereby groups of cod Gadus morhua were exposed to the highest reported mud concentration behind trawls (550 mg l -1 ) for periods of 24 h, 5 d and 10 d. No mortality was observed in any of the experimental groups. Histological analy- ses of gill lamellae of fish exposed to mud suspension for 24 h showed acute pathological changes in the form of multifocal degenerative lesions. After 5 d exposure, adaptive changes to cope with the turbid environment were observed. These comprised hypertrophy and hyperplasia of the gill epithe- lium and became more marked and widespread after 10 d exposure. The gill lesions observed were similar to those reported in other species exposed to a turbid environment and were in those cases reversible. Analyses of blood plasma cortisol levels showed higher values for fish in the 5 d exposure group than for fish in the 1 d exposure group. No other between-group differences were significant. The within-group variation was high in all groups but especially in the 10 d exposure group.

Processing of Phosphate Mine Tailings by Coagulation Flocculation to Reduce Marine Pollution in Togo: Laboratory Tests

About 2.5 million t of sedimentary phosphorite mine tailings, highly enriched with trace metals such as Cd, Cr, Cu, Ni, and Zn, are dumped annually in the coastal waters of Togo without any pre-treatment, causing serious pollution problems in the region. We conducted laboratory jar tests of a coagulation-flocculation procedure with coagulants RM45U and AN945MPM to clarify the sludge. The efficiency of the method depends particularly on two factors: the amount of coagulant and the solid concentration of the sludge to be treated. Thus, with a mud concentration of 47.7 g/L, the average optimal amount of the two coagulants was 25 mg/L. With both coagulants, water turbidity passed from 60 x 10 3 NTU to approximately 3 NTU after clarification with the optimal amount of the two coagulants. RM45U reduced concentrations of Pb by 40%, Zn by 98.8%, Fe by 80.6%, and Cd by 32.8%. AN945MPM reduced Pb by 20%, Zn by 98.5%, Fe by 48%, and Cd by 32.8%.

Chromate removal from water using red mud and crossflow microfiltration

Red mud was used as an adsorbent for removal of chromate ions from water using crossflow microfiltration. The chromate rejection is a strong function of feed dispersion pH, chromate and red mud concentration ratio, electrolyte concentration, membrane pore size as well as the concentration of co-ions such a sulfate ions used to adjust the dispersion pH. Under certain conditions, especially when pH = 5.2, the chromate rejection reached a maximum of 100% rejection. It was observed that the efficiency of the chromate removal increased with increasing red mud/chromate concentration ratio; however, the steady-state permeate flux decreased with an increasing red mud/chromate ratio. In the presence of chromate (at the same red mud concentration) and electrolyte (at the same chromate and red mud concentrations), permeate flux increased, although the chromate rejections decreased. Furthermore, it was observed that the rejection of chromate decreased with increasing membrane pore size. On the contrary, permeate flux increased with increasing membrane pore size. The method was found to be effective in removing chromate from water.

Suspended sediment concentration-turbidity relationships for Ngarradj – a seasonal stream in the wet-dry tropics

In order to assess the impact of mining-related erosion on stream suspended sediment loads within a catchment, it is critical that suspended sediment movement is accurately measured. The technique of using turbidimeters to monitor stream suspended sediment concentration was assessed using data collected at three gauging stations within a small catchment in the Kakadu region. Turbidity data were strongly correlated with mud concentration data collected at all three sites. Futhermore, analysis of periods of high sediment movement associated with runoff events showed that the use of turbidimeters is considered essential for estimating individual event loads within the region.

Controls on effective settling velocities of mud flocs in the Changjiang Estuary, China

AbstractTheoretical and experimental studies were undertaken to gain insight into physical parameters controlling the flocculation and settling properties of mud flocs in the Changjiang Estuary, China. The Rouse equation is applied to vertical profiles of suspended sediment concentration to determine the bulk mean settling velocity (ws) of sediment suspended in the Changjiang Estuary. Both in situ point‐sampled and acoustically measured profiles of suspended mud concentrations were fit selectively. The calculated settling velocities ws mainly ranged from 0·4 to 4·1 mm s−1 for the point‐sampled data set, and from 1·0 to 3·0 mm s−1 for the acoustically measured data set. Furthermore, the settling velocities of mud flocs increased with mean concentration (C̄) of mud flocs in suspension and were proportional to increasing bottom shear stress (τb) of tidal flow. The best equation for the field settling velocity of mud flocs in the Changjiang Estuary can be expressed by the power law: ws = mC̄n (m, 1·14–2·37; n, 0·84–1·03). It is suggested that C̄ and τb were the dominant physical parameters controlling the flocculation and ws of mud flocs in suspension. Copyright © 2004 John Wiley & Sons, Ltd.

River flooding, storm resuspension, and event stratigraphy on the northern California shelf: observations compared with simulations

Coast-hugging surface flood plumes occur on the inner shelf of northern California during the winter season, generating dense, near-bottom suspensions which may attain fluid mud concentrations as particles settle. The period of storm-heightened waves may continue into the flood period, leading to gravity-driven seaward displacement of the bottom suspension; or the wave regime may ameliorate, leaving the suspension to consolidate as a short-lived, inner-shelf flood bed. Such beds tend to be resuspended within days or weeks by subsequent storm events that may recreate the original high concentrations. The sediment is thus dispersed seaward by gravity flows, to be deposited as a muddy flood bed on the central shelf. The locus of deposition of these “high-concentration regimes” is a function of the relative intensities of river discharge and storm wave height. Greater discharge piles thicker storm beds nearer shore, while intense wave regimes allow deposition of the fluid mud further seaward. During events with high values of both parameters, large amount of fluid mud may bypass over the shelf edge. In contrast, “low-concentration regimes” occur during storm periods when there has been no recent flood deposition on the inner shelf. The shelf floor is better consolidated than in the previous case, and the resulting suspended sediment concentrations are lower. As a consequence, low-concentration regimes are winnowing and bypassing regimes, and the beds deposited are thinner and sandier. Algorithms describing deposition by high and low-concentration regimes have been embedded in a probabilistic model. A simulation of a 400-year sequence of beds deposited by winter storms and floods suggests that on the Eel shelf, the Holocene transgressive systems tract consists of back-stepping, seaward-fining event beds, whose timelines (bedding planes) dip more gently than do their gradational facies boundaries. At these longer time scales, flood beds dominate over storm beds.

Investigation of the separation of scandium and rare earth elements from red mud by use of reversed-phase HPLC.

A chromatographic method has been developed for separation and determination of scandium (Sc) and rare earth elements (REEs) in samples from a red mud (RM)-utilization process. Reversed-phase high-performance liquid chromatography (RP-HPLC) with post-column derivatization using 4-(2-pyridylazo)-resorcinol (PAR) and UV-visible detection at 520 nm was tested using different gradient elution profiles and pH values to optimize separation and recovery, primarily for Sc but also for yttrium and the individual lanthanides, from iron present in the samples. The separation was performed in less than 20 min by use of a mobile phase gradient. The concentration of alpha-hydroxyisobutyric acid ( alpha-HIBA), as eluent, was altered from 0.06 to 0.4 mol L(-1) (pH 3.7) and 0.01 mol L(-1) sodium salt n-octane sulfonic acid (OS) was used as modifier. Very low detection limits in the nanogram range and a good resolution for Sc and REEs except for Y/Dy were achieved. Before application of the method to the red mud samples and to the corresponding bauxites, Sc and REEs were leached from red mud with 0.6 mol L(-1) HNO(3) and mostly separated, as a group, from the main elements by ion exchange/selective elution (6 mol L(-1) HNO(3)) in accordance with a pilot-plant process developed in this laboratory. After evaporation of the eluent to dryness the extracted elements were re-dissolved in the mobile phase. By use of this chromatographic method Sc, which is the most expensive of the elements investigated and occurs in economically interesting concentrations in red mud, could be separated not only from co-existing Fe but also from Y/Dy, Yb, Er, Ho, Gd, Eu, Sm, Nd, Pr, Ce and La. All the elements investigated were individually recovered. Their recoveries were found to be nearly quantitative.

Relationship between structural and functional aspects of microbial and macrofaunal communities in different areas of the North Sea

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 282:13-31 (2004) - doi:10.3354/meps282013 Relationship between structural and functional aspects of microbial and macrofaunal communities in different areas of the North Sea Ingrid Kröncke1,*, Thorsten Stoeck1, Gunther Wieking1, Ansa Palojärvi2 1Senckenberg Institute, Department for Marine Research, Südstrand 40, 26382 Wilhelmshaven, Germany 2MTT Agrifood Research Finland, Soils and Environment, 31600 Jokioinen, Finland *Email: ingrid.kroencke@senckenberg.de ABSTRACT: Spatial similarities of structural and functional (trophic) aspects of microbial and macrofaunal community patterns were studied in relation to sediment parameters in the German Bight, the Oyster Ground, the Dogger Bank, eastern North Sea, the Skagerrak, and northern Kattegat in May and September 1999 and 2000. Spatial patterns of microbial and macrofaunal communities were congruent. Differences in spatial distribution of communities were caused by utilisation of the different food sources available in the different areas. In the German Bight, the communities were correlated significantly with chlorophyll a (chl a) concentrations in the sediments, an indicator of fresh nitrogen-rich organic matter. In contrast, communities in the Skagerrak and Kattegat were correlated with total organic carbon (TOC) and mud concentrations, indicating the presence of more refractory organic matter for the microbial and macrofaunal organisms. For the communities in the Oyster Ground, the eastern North Sea and the Dogger Bank, no such significant correlations were found. The community in the Oyster Ground was characterised by the ophiurid Amphiura filiformis foraging on relatively refractory macrofloculate organic matter from the benthic boundary layer. Preferred substrates of the microbial communities in this area were macrofaunal excretion products. The Skagerrak and Kattegat stations were also characterised by the A. filiformis community, indicating lower food supply than in the German Bight. Although microbial community patterns were similar to those in the German Bight, lower chl a concentrations also suggested the presence of relatively refractory organic matter. Reflected by the dominance of sand-licking species, macrofaunal communities at the Dogger Bank and in parts of the eastern North Sea seem to rely on benthic primary production. Here, microbial communities were adapted to hydrodynamically induced stress, sediment disturbance and limited food supply. Our results revealed that in areas with high food supply such as the German Bight, no competition for food between microorganisms and macrofauna occurs. But in areas with less or limited food supply from the water column, macrofauna provides the microorganisms with organic matter or microorganisms utilise macrofaunal excretion products and are physiologically adapted to the exposed environment. KEY WORDS: Microorganisms · Macrofauna · Community structure · Trophic relationships · Food availability · Phytopigments · North Sea Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 282. Online publication date: November 16, 2004 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2004 Inter-Research.

Large-scale trends in some mass physical properties of Danish Wadden Sea sediments, and implications for organism-sediment interactions

Mass physical properties (wet/dry bulk densities, mud/sand contents and concentrations, water content, shear strength) were determined on 132 surficial sediment samples from intertidal flats in Ho Bugt and north of Langli Island between the Hjerting Løb tidal channel and the Skallingen Peninsula, Danish Wadden Sea. Except for shear strength, all parameters were found to be highly intercorrelated (r<0.97), and the 12 equations thus generated can be used conveniently for regional modelling purposes. Investigations which rely on small sample numbers can be optimised by prior pilot studies in which average conditions are defined along environmental gradients and, once suitable in-situ calibrations have been performed, the method can be expanded to other parameters. Water content is a particularly useful predictor because it is a universal master variable by which differences in compaction between different environments can be normalised. Shear strength values show large variations at low mud contents, which decrease sharply towards higher mud contents. Nevertheless, shear strength appears to decrease with increasing mud content, a trend observed in spatial plots of comparative data from the German Wadden Sea. This suggests that the data comprise both spatially coherent and incoherent values. Similarly to the German Wadden Sea, counter-intuitive convex mud concentration curves were recorded along sand-mud gradients, peaking at intermediary mud contents in mixed sediments. Based on a growing body of published literature, interpretations of these novel trends suggest far-reaching implications for organism-sediment interactions.

Deriving sediment quality guidelines in the Guadalquivir estuary associated with the Aznalcollar minig spill: A comparison of different approaches

Concentrations of heavy metals (Fe, Zn, Cd, Pb, Cu and Mn) and sediment toxicity tests (mortality of amphipods, Ampelisca brevicornis, of clams, Scrobicularia plana, and of fish, Solea senegalensis) were used to derive sediment quality guidelines (SQGs). The approaches are based on the determination of LC50, on the application of a multivariate analysis (MAA), and on the Threshold Effect Level Quotients (TELQs). All approaches lead to consistent SQGs. The range of concentrations established with MAA results in narrower uncertainty ranges. Sediment toxicity estimated by TELQs is in good agreement with that determined experimentally. In terms of the toxic mud concentration, the maximum and minimum LC50s (for fish EC50s) are 1.07% and 0.44% for amphipods, 5.75% and 1.25% for clams, and 7.24% and 1.97% for fish, based on dry weight. However, heavy metal concentrations or ranges should be used only as a first tier in a "weight-of-evidence" approach to determine the environmental quality in aquatic systems. The use of SQGs for the management of these systems should be taken with care, especially those used for the management of dredging processes.

Pilot-Plant Investigation of the Leaching Process for the Recovery of Scandium from Red Mud

A pilot-plant process has been developed based on an innovative laboratory-scale method for the recovery of scandium that exists in economically interesting concentrations in red mud, the main byproduct of alumina production. This method includes acid leaching of the red mud pulp, ion-exchange separation of scandium and lanthanides from the co-leached main elements such as iron, and subsequent liquid−liquid extraction of the eluate for further scandium purification and enrichment. In this work, experimental and theoretical investigation of the pilot-scale leaching process was performed. The following parameters were tested: mode of agitation, solid-to-liquid ratio, acidity of the leachate, number of stages in the process, and pretreatment of red mud with concentrated acids in order to achieve optimum scandium recovery combined with low iron dissolution. Furthermore, by theoretical interpretation of the experimental data, a predictive correlation for the scandium leaching efficiency was developed.

Phase relationships between fine sediment suspensions and tidal currents in coastal seas

In shelf and coastal waters, point measurements of fine suspended sediment concentrations commonly display phase and frequency differences with respect to local current forcing. Phase shifts are often attributed to tidal advection and slow‐settling velocities of cohesive sediments. The phase relationship between tidal current speeds and near‐bed mud concentrations obtained at a site off the east coast of England is investigated. A simple depth‐averaged advection‐diffusion model is introduced and used to explore the influence of relative changes in settling, advection, and local erosion on the temporal evolution of suspension events. The model includes two source terms: local resuspension in phase with current speed and advection of a horizontal gradient in phase with tidal displacement. Incorporating a settling (or sink) term introduces a negative or positive phase shift in the advection or resuspension term, respectively. The settling term accounts for the vertical structure of suspension and becomes important at slack water when the near‐bed concentration, and thus deposition, rapidly increases. Model results are compared with suspended mud concentration data to help explain the observed phase relationships between concentration and current speed.

On the flocculation and settling velocity of estuarine mud

This paper presents a three-dimensional model in an Eulerian reference frame for the evolution of the settling velocity of cohesive sediments in estuarine and coastal environments. The variation in settling velocity is the result of turbulence-induced aggregation and floc breakup processes. The mud flocs are treated as self-similar fractal entities because of which Stokes’ formula relating settling velocity and floc size has to be modified. A new hindered settling formula is proposed which accounts for the gelling processes typical of cohesive sediment at high concentrations. When the floc concentration approaches unity, a space-filling network develops and the settling velocity becomes zero. This state of the suspension is often referred to as fluid mud. The flocculation model is implemented in a 1 dv point model and applied to simulate the processes in a reach of the turbidity maximum in the Ems estuary, The Netherlands, where longitudinal gradients are small. The numerical results compare favourably with observed values of the vertical suspended sediment concentration, of the floc size and the fluid mud concentration. The model predicts large temporal variations in settling velocity and flocculation time, which explains why it was not possible to simulate the observed vertical profiles of suspended sediment concentration properly with a constant settling velocity.

Radionuclides in the Loire river estuary (France) : Sources, transport and fate

Anthropogenic radionuclide inputs in the Loire estuary (French Atlantic coast) consist of radioactive releases from 14 nuclear reactors located along the Loire river basin, and of fallout from nuclear weapon tests and from the Tchernobyl accident. To estimate to what extent radionuclides associated with sediment accumulate in the estuary, three complementary approaches were used: field surveys, laboratory experiments and numerical modelling. Sampling of bottom sediments, water and suspended solids was carried out at 8 different dates over a 15 month-period. Analysis covered 14C, 90Sr, 3H, the naturally occurring gamma-emitters (uranium and thorium decay chains, 7Be and 40K), and the artificial gamma emitters (mainly cobalt and cesium isotopes). Processes of radionuclide transport and exchange between dissolved and particulate phases were included in a previously developed estuary specific 2D-hydrodynamic model. Equations of sorption and desorption kinetics were derived from laboratory experiments conducted at different salinities. Simulations carried out for two river discharge conditions (low summer flow, high winter flow) allowed to follow radionuclide desorption in the estuary. For long term simulations, a simplified model was developed. It provided estimates of the amount of radionuclides expelled out of the estuary under dissolved and particulate forms, of the transit time for both forms and of the variations in radionuclides concentrations in the fluid mud. Based on computed results and observations, contributions from different origins (natural, military, industrial) to the inventory of radioactivity in the estuary are presented.

Radionuclides in the Loire river estuary (France): Sources, transport and fate

Anthropogenic radionuclide inputs in the Loire estuary (French Atlantic coast) consist of radioactive releases from 14 nuclear reactors located along the Loire river basin, and of fallout from nuclear weapon tests and from the Tchernobyl accident. To estimate to what extent radionuclides associated with sediment accumulate in the estuary, three complementary approaches were used: field surveys, laboratory experiments and numerical modelling. Sampling of bottom sediments, water and suspended solids was carried out at 8 different dates over a 15 month-period . Analysis covered I4 C, 90 Sr ,3H, the naturally occurring gamma-emitters (uranium and thorium decay chains, 7 Be and 40 K), and the artificial gamma emitters (mainly cobalt and cesium isotopes). To gain information on the contamination history of the estuary, sediment cores were also collected at different locations inside and outside the estuarine zone. Processes of radionuclide transport and exchange between dissolved and particulate phases were included in a previously developed estuary specific 2D-hydrodynamic model. Equations of sorption and desorption kinetics were derived from laboratory experiments conducted at different salinities. Simulations carried out for two river discharge conditions (low summer flow, high winter flow) allowed to follow radionuclide desorption in the estuary. For long term simulations, a simplified model was developed. It provided estimates of the amount of radionuclides expelled out of the estuary under dissolved and particulate forms, of the transit time for both forms and of the variations in radionuclides concentrations in the fluid mud. Based on computed results and observations, contributions from different origins (natural, military, industrial, marine, continental) to the inventory of radioactivity in the estuary are presented.

Sediment accumulation in a shallow-water meadow carpeted by a small erect bryozoan

Abstract A marine meadow covered 80–90% by a turf of the erect bryozoan Cellaria occurs over an area >104 m2 west of Banjole Island (Croatia) in the northeastern Adriatic Sea. Surrounding areas are bare to patchily covered by isolated clumps of bryozoans, sponges, and ascidians attached to scattered shell and rock debris. Silt and clay comprise >90% of the mass of sediment within the meadow, and Cellaria branch fragments are the next-most-abundant component. From 13-cm to at least 25-cm depth in the meadow sediment coralline algal branch fragments are abundant, and silt and clay comprise over 50% of the sediment's mass. In contrast, sediment accumulated in the bare area adjacent to the meadow is predominantly coarse carbonate sand composed of diverse skeletal grains. Mud comprises Concentration of mud in the sediment below the meadow apparently is due primarily to baffling by the erect bryozoans and earlier by the coralline algae, because composition and texture of the muds and fine sand are identical within and outside the meadow. The proportion of bryozoan skeletal fragments to mud in the upper sediment is similar to that of erect delicate bryozoans to sediment in shallow ramp buildups of Late Ordovician age in the southern Appalachians and of fenestrate bryozoans to mud plus cement in bryozoan-rich cores of Carboniferous mounds. The Paleozoic mounds also are surrounded by coarser sediment. By analogy with the Cellaria meadow, trapping and deposition of muds by erect bryozoans can be inferred for many of these Paleozoic mounds, even though bryozoans are a relatively small proportion of the total sediment.

Modern pollen rain and lake mud–water interface geochemistry along environmental gradients in southern Chile

This paper describes the vegetation–environment relationships deduced from descriptive and numerical analysis of pollen and spore percentages and mineral concentrations in mud–water interface samples from 47 small lakes in both forested and non-forested landscapes of the remote island region of southern Chile. The study shows (1) the spatial variation in pollen and geochemistry data derived from lake mud–water interface sediments in the Chonos Archipelago in southern Chile, and (2) the relative influence of a range of environmental variables (altitude, latitude, vegetation cover, fire, edaphic conditions) on lake sediment geochemistry concentration and pollen and spore percentage data. Multivariate analysis (Principle Components Analysis, Redundancy Analysis and Canonical Correspondence Analysis) of lake geochemistry and pollen and spore data show that there is a significant influence from both altitudinal and latitudinal gradients in the region. This is most likely reflecting plant and sedimentation response to temperature gradients.

RED MUD AND WASTE BASE: RAW MATERIALS FOR COAGULANT PRODUCTION

Proposal for coagulant production using red mud and waste base as row material, by-products of abandoned alumina factory near Obrovac in Croatia, is described. Basic physico-chemical characteristics of red mud and waste base were also given. Elemental concentrations of red mud, waste base and wastewaters before and after treatment were measured using tube excited EDXRF method. Coagulant production consist of partial dissolution of red mud with diluted sulphuric acid (30% wt), separation of liquid from residual red mud by centrifugation or filtration through the filter press, and neutralisation of acid red mud (pH = 0) with the waste base to pH = 8. This process of red mud neutralisation results in heavy metals removal and also neutralisation of waste base to pH = 8, which could be, after treatment, discharged directly into the environment. Resulting red mud, after neutralisation, is in gel-like state and is suitable for heavy metals and turbidity removal from industrial wastewaters. As an example, wastewa...

Rapid Smothering of Coral Reef Organisms by Muddy Marine Snow

Estuarine mud, when resuspended in nutrient-rich near-shore water, aggregates to marine snow, and within minutes to hours can exert detrimental or even lethal effects on small coral reef organisms. In a pilot study, estuarine mud was suspended in near-shore and off-shore waters of the Great Barrier Reef to a final concentration of 170 mg l −1. The short-term responses of a coral ( Acropora sp.) and coral-inhabiting barnacles (subfamily Pyrgomatidae), exposed to either near-shore or off-shore water, were microscopically observed and video recorded. In the off-shore water treatment, flocculation was minor, and aggregate sizes were c. 50 μm. The organisms were able to clean themselves from these small settling aggregates at low siltation (<0·5 mg cm −2), and struggled and produced mucus only at high siltation (4–5 mg cm −2). In contrast, in near-shore, nutrient-enriched waters, the suspended mud aggregated into large sticky flocs of marine snow (200–2000 μm diameter). The organisms responded to a thin coat of deposited flocs with vigorous cleaning by cirri and tentacle beating. After 5 min struggle, the barnacle stopped moving, calanoid copepods were entangled in the aggregates, and thick layers of mucus were exuded by the coral polyps. Both barnacle and copepods died after <1 h exposure; a short time compared with natural occurrences of marine snow deposition on coral reefs. Enhanced nutrient concentrations are known to contribute to enhance biologically mediated flocculation. This pilot study suggests that the concentration of suspended mud, and extent of stickiness and flocculation, can synergistically affect reef benthos organisms after short exposure. The enclosed macro video recordings clearly visualize these effects, and help convey the important implications for managers: that inshore reefs of the Great Barrier Reef cannot be sustainably managed without managing the adjacent land.

Zone optimale du soutirage

The technique of sediments drawing by the sluices of draining can reduce the speed of sedimentation in reservoir of dam and consequently increase the time of life. It must be realised rigorously whereas it is not the case in the majority of the Algerian dams. It has then engendred important problems of management, and sometimes the stopping of the exploitation of the dam. The study of the mud concentration in different dams has permitted to observe a critical value for every type of mud, and to determine something like an optimum of drawing off where the suspension density permits the drawing off.