OCCURRENCE, BEHAVIOUR AND PHYSICAL

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1 OCCURRENCE, BEHAVIOUR AND PHYSICAL PROPERTIES OF FLUID MUD Christine Habermann* and Andreas Wurpts** * Federal Institute of Hydrology /Dpt. Groundwater, Geology and River Morphology, Koblenz ** Franzius-Institute for Hydraulic, Waterways and Coastal Engineering /University of Hannover Abstract: Fluid mud occurs at most river mouths where the overall hydrodynamic situation aids the culmination of cohesive fines like usually the turbidity zone does. Fines predominantly settle in calm side areas as well as harbors and over deepened parts of the fairway. The presence of fluid mud can affect navigation by means of reduced maneuverability and increased propulsion forces needed. The presence of fluid mud also implements the need for different methods of navigable bottom detection, since already weak and almost fluent layers show up very clear at acoustic echo soundings. The resulting overestimation of material to be dredged by means of fairway maintenance can unnecessarily increase maintenance costs. Fluid mud consists of mainly mineral fines, supplemented by a smaller fraction of organic matter and microorganisms that form a network. The aforementioned mixture is exposed to transport, erosion and deposition processes on different spatial and temporal scales. Its different cohesive contents lead to complex rheological behavior with shear thinning and time dependency. Mainly caused by biological impacts the long term state of fluid mud in sheltered areas can be characterized by slow consolidation combined with low density and strong mechanical strength at the same time. Knowledge of the time dependent and very variable properties of fluid mud can help to improve the treatment of harbors, rivers, and fairways where fluid mud is present, and therefore help to reduce costs and increases the safety of navigational operations. Numerical modeling as a tool for decision support also still lacks functionality in order to correctly predict flow- and sediment dynamics in estuarine waters. I. INTRODUCTION Fluid Mud is composed of loosely bound flocs of silt, clay and organic material with water in the space between. It marks a certain state between dilute suspensions in the upper part of a water body that show strictly Newtonian behavior and solid consolidated and no longer mobile sediment at the river bed. At its upper lutocline, material can be entrained back to the water column whereas the lower part of the mud layer can start to consolidate due to self-weight, see Figure 1. Flow characteristics in the fluid mud layer can be laminar due to its structure where flocs form a network until this network is broken up by the shearing of the flow. Fluid mud can take very different states since mechanisms of strongly varying spatial and temporal scales affect its actual mode. Those differences clearly show up by comparing fluid mud as found in harbors and calm side areas of rivers and estuaries to that found in the deep tidal channels during slack water times. In short, the situation in the deep channels during slack time can be characterized as a temporal state before or at very early consolidation. What is found then is a high concentrated suspension of cohesive fines in a state of hindered settling. Since slack water is oscillatory changing with intense tidal flow the suspension becomes entrained into the flow field before some further increase of mechanical strength (like e.g. consolidation) would be possible. Nonetheless, the fluid mechanical properties of the suspension are already dominated by a complex rheology consisting of a strongly increased kinematic viscosity with shear thinning and thixotropy due to the high amount of (cohesive) inter particle attraction. If the settling takes place at a position less exposed to external forcing like a harbor basin or a calm side area of the estuary, the aforementioned state can be maintained and a continuous increase of mechanical strength will take place by means of consolidation and biological processes. Biological influences are responsible for major deviation from classical consolidation phenomena and also strongly increase the suspensions non- Newtonian properties. The mechanical properties are therefore affected in several ways: Aerobic metabolism of organic contents leads to storage of methane within the fluid mud layer and reduces density/increases the fluid muds buoyancy. Also, after all available oxygen is consumed the 363

2 metabolism changes to anaerobic mode which intensifies the production of EPS (exocellular polymeric substances) which span over all interstitial space and strongly affect the mechanical properties as well as the further consolidation of the suspension by 'binding' interstitial water. The mechanical strength of the fluid mud is strongly increased by means of the aforementioned biological mechanism. It is known from newer practice, that the aforementioned state can be persistent for longer periods and can also be maintained by periodically adding oxygen to the fluid mud layer. The mechanical properties in the aforementioned state allow flowing/refluidization by means of exceeding a threshold value (yield point) of shear force above which plastic deformation and flow of the fluid mud takes place. The fluidmechanical behavior of the fluid mud in this state can be characterized by shear thinning viscoplastic behavior. Viscosity is also time dependent by means of thixotropy. II. OCCURRENCE AND TRANSPORT PROCESSES The basic sediment material that forms fluid mud is fine sediment (avg. diameter less than 0.01 mm) of mainly mineral and at lesser extent organic origin. It originates from as well the upstream reach of the mounding river as also the open sea and is 'catched' within the turbidity zone of an estuary. The basic mechanism responsible for the existence of a turbidity zone is usually referred to as vertical gravitational circulation which is the largescale result of local baroclinic damping of turbulent momentum and suspended/dissolved matter exchange, see figure 2. On a local scale the aforementioned damping of turbulence is also responsible for local flow stratification in conjunction with fluid mud layers. Figure 1. Vertical section through stratified fluid mud layer. Two horizons correspond to different echo sounding frequencies (Nasner et al., 2007) Figure 2. Schematic of vertical gravitational circulation The mechanism of density gradient induced damping of turbulent mixing stabilizes the fluid mud layer unless shear induced by flow or external forces overcomes the stabilizing effect. The generation of density gradients is strongly affected by flocculation and aggregation effects of the suspended fines, since which the average settling velocity of the suspended particles varies widely with aggregate size. The probability for the permanent existence of major quantities of slowly consolidating fluid mud layers rises at harbors and calm side areas of the rivers and estuaries, because the mechanical strength of the aggregates depends on turbulence criteria. Also one has to consider the strength of vertical gravitational circulation increased by means of deepening the fairways in rivers and estuaries, since this is a necessary task in order to handle nowadays ship sizes in these waters. The aforementioned increase is one of the major 'restoring forces' which tend to bring the hydro-morphodynamic system back to its original state of dynamic equilibrium. Nonetheless, the high concentrated fluid mud layers which are observed in practice are still capable of flowing by means of the aforementioned rheology (visco-plastic behavior including a yield point, shear thinning and thixotropy). Since the density of fluid mud is significantly increased compared to that of clear water, the flow behavior of fluid mud layers can be compared to other density density currents, e.g. stabilization of 364

3 the 'interface' to the upper free flowing layer and also decoupling to its momentum by suppression of turbulent exchange processes. Fluid mud flow therefore is dominantly gravity driven along bottom and interface gradients. III. EXEMPLARY MEASUREMENTS In the past, measurements of the properties of fluid mud in different environments have been made. Measurements in harbors are exemplary for undisturbed settling and consolidation processes where the different phases from high concentrated suspensions to consolidated stationary mud can be observed. During consolidation, dewatering of the fluid mud layer takes place (water is pressed out of the space between) and the flow characteristics of the mud layer change. At the beginning of the measurement at the end of the ebb current a clear stratification of suspended matter takes place (P1 to P3). The amount of suspended particulate matter starts to increase significantly at a depth of 1.5 meters below water surface. A vertical gradient is present in the fluid mud layer with maximum concentrations up to 75 g/l near the bottom. At slack water (P5, P6), two lutoclines become obvious. A second layer of even higher concentrated suspended material (up to 120 g/l) can be observed at a depth of approximately 6 m below water surface. With increasing flow velocities and increasing turbulence in the overlaying dilute suspension, this mobile, not yet consolidated mud layer is eroded, entrained to the water column and removed by means of convection. The lowest fluid mud layer where flocs could already start to form a network can resist longer to the forces as it can be seen in P7. Figure 3. Vertical Profiles of Suspended Particulate Matter around slack water. Tidal part of river Ems. (Schöl, 2006) In free flowing estuaries, the formation of fluid mud layers occurs only temporarily in certain tidal phases and material is entrained back to the water column as flow velocity increases. Depending on the flow characteristics, bulk erosion and surface erosion can also take place at the interface of the overlaying water body and the fluid mud layer. Analyzing echosounder plots it became obvious that parts of the river bed of the tidal Ems are temporarily covered by very mobile fluid mud layers with a strictly horizontal surface that already starts only several meters below the water surface. This layer can overlay structures on the riverbed like ripples or dunes. In an effort to describe the properties and the vertical structure of fluid mud in a free flowing estuary, measurements in the main channel in the turbidity zone of the tidal Ems were carried out. Figure 3 shows vertical profiles of suspended particulate matter for different phases of the tidal cycle. As the time period for deposition is too short for the material to consolidate, the newly built fluid mud structure can not resist as the flow velocities increase (P9). As it was stated before, the sediment concentration is not a sufficient parameter to describe the characteristics of fluid mud.therefore, additional characteristics of the fluid mud in the turbidity zone of the Ems were collected. The fluid mud samples taken in different depths all show the same grain size distribution with solely silt and clay to a lesser extent. This corresponds very well to the sediment samples taken from the river bed. These samples show a low level of consolidation and contain a high fraction of silt and clay as well. Whereas in the dilute suspension in the upper part of the water body the organic content amounts up to 35%, the percentage of organic material (ignition loss) in the fluid mud is down to 10 to 15 %. Because the biodegradability of this material is lower it can be assumed that the fluid mud has a higher age. In the next chapter, the rheological properties of the samples from the free flowing Ems are analysed. IV. MECHANICAL PROPERTIES Fluid mud in general exhibits non-newtonian, shear thinning flow properties (viscosity decrease with shear). Figure 4 shows a comparison of rheological measurement for clear water and a fluid mud suspension taken from river Ems around slack water. Non-Newtonian behavior of fluid mud can be verified at early stages already: Even fluid mud found in tidal rivers during slack water which is only little more than a suspension of colloidal particles exhibits viscosities of an order of magnitude higer than suspended sand particles of the same average diameter with a total density of equal or even higher concentration. This can be seen from figure 5 which compares yield curves of a fluid mud sample taken from river Ems with several suspensions of different concentrations of silica dust. The average particle sizes of both suspensions are of the same order of magnitude (around 5 µm). 365

4 Nonetheless the mechanical strength of the even higher concentrated silica dust suspension is orders of magnitude below the flocculating suspension. The fluid mud sample from the Ems already shows a clear yield point. Shear thinning behavior can also be seen by means of viscosities decreasing with increased shear. Time dependency of the fluid mud's mechanical strength can be shown by repeating the same measurement twice. Figure 6 shows the result two of consecutive shear measurements of the same sample. The decrease in shear strength and the resulting decrease in kinematic viscosity during the second run can be clearly seen. This decrease in viscosity is probably caused by the breaking of the network structure that existed within the mud suspension before the sample was sheared. This network could not be restored before the second run started. Fluid mud which was given enough time to form a network of aggregates and rise its bio-chemical activity differs from that found in rivers by means of increased mechanical strength. The abovementioned basic behavior non the less remains as can be seen from Figure 7 which compares two fluid muds, one sample taken from a tidal river around slack water, see figure 3, and one weak sample taken from a tidal harbor's mouth, where the situation allows the permanent existence of a fluid mud layer of several meters thickness. Both muds are fluid suspensions of comparable density though the more dense matter taken from the harbor mouth is an order of magnitude stronger regarding yield point and absolute value of viscosity around the corresponding yield point. Both muds are capabable of flowing. Figure 4. Shear curves (right axis) and viscosities (left axis) for fluid mud from river Ems and clear water Figure 5. Comparison of fluid mud and several suspensions of silica dust. 366

5 Figure 6. Time-dependent thixotropic behavior of fluid mud Figure 7. Rheometrical comparison of fluid mud from tidal river and harbor entrance V. CONCLUSION Fluid mud can be characterized as a suspension with complex flow behavior which may significantly differ from that of clear water. Due to its dominant transport and deposition modes it can be found in harbors and calm side areas of tidal rivers, mostly at or close to the rivers turbidity zone, where huge amounts of cohesive fines can be expected. The presence of fluid mud has implications on the maintenance of these waters because echosoundings can probably detect fluid mud layers and therefore have to be interpreted carefully in order to avoid misinterpretations of bottom topography. Dredging of fluid mud layers can be very ineffective since fluid mud due to its capability of flowing is much more mobile than e.g. sandy deposits. As a consequence, dredging a local scour into a fluid mud layer immediately induces an interfacial gradient which drives a fluid mud flow to fill the scour. Also most fluid mud layers are liquid enough or at least can be conditioned to be navigable. Knowledge of the occurrence, transport processes and properties of fluid mud can help to optimize the maintenance efforts for fairways and harbors by means of reduced costs, increased safety and even reduced interferences to the natural system. This is also necessary in order to improve morphodynamic 367

6 numerical models for those waters as predictive tools. REFERENCES [1] Nasner, H., Pieper, R., Torn, P. Und Kuhlenkampf, H.: Properties of Fluid Mud and Prevention of Sedimentation, Proceedings of WODCON XVIII, Orlando, 2007 [2] Schöl,A et al. Zusammenhänge zwischen Sauerstoffhaushalt und Schwebstoffverteilung in der Unterems-Naturmessungen und Laboruntersuchungen, Bundesanstalt für Gewässerkunde, Koblenz, Vortrag im Rahmen des BAW/BfG Kolloquiums in Hamburg, November [3] Winterwerp, J.C.: On the flocculation and settling velocity of estuarine mud, Continental Shelf Research, Vol. 22, Pergamon,2001 [4] Bruens,A. Entraining mud suspensions, Communication on Hydraulic and Geotechnical engineering, Faculty of Civil Engineering, Delft University, Report No. 03-1; ISSN , 2003 [5] Toorman, E.: Modeling of fluid mud flow and consolidation. Dissertation, Departement buurgerlijke bouwkunde, Katholieke Universiteit Leuven,