-RHEOLOGICAL PROPERTIES OF SEDIMENT SUSPENSIONS FROM

ECKERNFÖRDE AND KIELER FÖRDE BAYS, WESTERN BALTIC SEA

Richard W. FAAS1 and Stanislas I. WARTEL2

ABSTRACTLocal areas of fine-grained organic-rich sediments in Eckernf6rde and Kieier Förde Bays may

experience disturbances which cause fluidization of the substrate and create a dense suspension (fluidmud) which exists temporarily as a component of the benthic boundary layer before becomingincorporated into the permanent bottom. Laboratory studies indicate this material behavesrheologically as a non-Newtonian substance, and both shear thinning (pseudoplastic) and shearthickening (dilatant) flow behavior can occur (often within the same sample) under low tointermediate shear stresses (2 -40 Pa) and shear rates (0.46 - 122.49 S-I).

Detailed granulometric analyses (1/4 phi intervals) ofthe fraction <63 Ilm show difTerencesin thesilt/day ratio (day <2 Ilm) between the two environments. Little change in the silt/day ratio is seenin the Kieier Förde sediments (from 0.74 to 0.95); however, at EckerntOrde,the ratio changed from0.73 to 2.19. Fine silt partides are lacking or were removed from the 4 to 16 Ilm fraction of theEckernf6rde but not from the Kieier Förde sediments. Both shear thickening and shear thinning flowwas observed in the EckerntOrdesediments. Shear thickening flow behavior was not observed in theKieier Förde sediments.

Samples of organic-rich (10 to 20%) interface sediments from both areas were analyzedrheologically prior to, and after removal of organic matter by H202 treatrnent. Reduction in 'apparent'viscosity occurred through the entire range of shear rates and stresses, shear thickening behavior wasreduced or became nonexistent, and yield stress decreased significantly compared to the naturalsamples. The difTerencesin yield stress and flow behavior of dense suspensions result primarily fromdifTerences in grain size distributions but the role of organic matter on those properties is verysignificant and adds to the efTectsofthe grain size distribution of the sediment.

Key Words: Rheology, Shear strength, Yield stress, Sediment suspensions, Organic matter

1 INTRODUCTIONFluid mud is defined as a dense suspension of fine-grained sediment particles averaging in density

between 1.03 to 1.30 glem3 and between 10 to 480 gil in concentrations by weight (Ingliss and Allen,1957). It overlies a substrate of density > 1.30glem3 and is an integral part of the benthie boundary layer(MeCave, 1984). Fluid muds may oeeur naturally in shallow water environments, e.g., estuaries (Wartel,1977; Faas, 1981; Kirby and Parker, 1977; Van Rijn, 2005) and muddy continental shelves (WelIs andColeman, 1981; Kineke el al., 1996). They mayalso be generated by aetivities that disturb the bottomsueh as fishing and dredging (Gordon, 1974; Parker and Kirby, 1977), bioturbation (Harrison and Wass,1965; Rhodes, 1970), tectonic action (Hein, 1985), and unusual meteorologieal events. Fluid muds carrytoxie chemicals and are eonsidered to be a eoneentrated souree of pollutants (Niehols el al., 1981;Thornton el al., 1995). Reeognition of a navigable bottom is diffieult in fluid mud areas (Parker andKirby, 1982) and maintenanee of navigable dredged ehannels is eomplieated by their presenee (May,1973; Maseh and Espey, 1967).

The potential for the occurrenee of fluid mud exists in the gassy sediments in the fjords of the westernBaltie Sea, northern Germany (Fig. 1). Eekernförde Bay, loeated between 54°28' and 54°32'N latitude,

I Research Scientist, Departrnent of Marine Science, University of Southern Mississippi, Stennis Space Center, MS39529, E-mail: richard.faas@usm.edu

2 Research Scientist, Royal Belgian Institute of Natural Sciences, Vautierstraat 29, B-1040 Brussels Belgium. E-mail: stanislas.wartel@naturalsciences.be

Note: The original manuscript ofthis paper was received in Sept. 2004. The revised version was received in Aug.2005. Discussion open until March 2007.

- 24- International Journal of Sediment Research, Vol. 21, No. 1,2006, pp. 24-41

u ___....

and 9°50' and 10°08' E longitude), is an elongated fjord-type embayment which extends eastward fromthe town ofEckernfórde, Germany for 20 km where it widens out into Kiel Bay in the western Baltic Sea.Eckernfórde Bay is generally flat-floored with depths to 30 m. The floor is bisected by an elongateelevation (Mittelgrund) which rises to within 8 m of the surface. Recent and Holocene sediments arefound overlying Pleistocene glacial deposits to variabIe thickness. This location was chosen by U.S. andGerman scientists to be the site for the Coastal Benthic Boundary Layer Special Research Project(CBBL/SRP) "to study the relationships among the environmental processes responsible for methanebubble formation, and the resultant sediment structure, properties and behavior" (Richardson and Davis,1998).

10'110'

Fig. 1 Generalized location map of Eckernfórde, KieIer Förde, and Kiel Bay

Kiel Harbor, located between 54°20' and 54°25' N latitude, and 10°6' and 10°12' E longitude in thesouthern half of Kieier Förde (depths to 14 m) is a highly urbanized industrial harbor serving themetropolitan area between Kiel and Friedrichsort and includes the entrance to the Kiel Canal which formsa major transportation route connecting the North and Baltic Seas. The geological history andsedimentary distribution in Kieier Förde has recently been discussed in Schwarzer and Themann (2003).Sampling for this work focused on the southern part ofthe harbor, specifically to observe the character of

International Journal of Sediment Research, Vol. 21, No. 1,2006, pp. 24-41 - 25-

the sediments near the mouth of the Kiel Canal. Fluid mud is known to occur near the North Sea entrance

to the canal at Brunsbüttel (Milligan, 1995), and it was thought that fluid mud also may occur at theBaltic entrance.

2 NATURE OF THE PROBLEM

2.10bservationsDuring the sampling of Eckemfórde Bay in the spring of 1993 divers observed turbid c10udsover the

bottom at several sites, indicating possible resuspension. Some otterboard tracks were observed to becovered with sediment on one dive and completely exposed on the succeeding dive several hours later. Inthe first instance, browsing and remolding of the surface sediment by deposit feeding organisms mayhave created areas of fluidized sediment, confined to the upper few cm of the sediment surface.Populations of burrowing organisms in the muddy reaches of the Eckemfórde are high, ranging from60,000 to 70,500 animals1m2 (0' Andrea el al. 1996) and may exceed 100,000 animalslm2 duringpopulation 'bursts' (Lopez, pers. comm). Fluidization is also associated with areas of high interstitial gasconcentrations, particularly those around the 'pock marks' - broad, circular depressions (craters) whichmay result from expulsion of biogenic and/or thermogenic gases generated within the sediment column,or possibly by artesian freshwater flow from an underlying subglacial coarse sand channel (Khandricheand Wemer, 1995). Concentrations of dense suspensions have been observed in pockmarks on seismicrecords (Lambert, pers. comm.) Fluidization mayalso occur through wave/bottom interaction (Ross andMehta, 1991); and storms (Milkert and Wemer, 1992). Friedrichs and Wright (1995) have demonstratedthat resuspension ofbasinal sediments in Eckemfórde is unlikely to occur under normal wave activity.

2.2 PurposeThe purpose ofthis work is 1) to study the rheological behavior of dense nearbottom suspensions; 2) to

determine the specific parameters which control the forms of rheological behavior observed; and 3) tosuggest how these parameters influence the erosion, suspension, and redeposition of the surficialsediments. The parameters inc1udeboth compositional (e.g., grain size, organic matter) and rheological(e.g., 'apparent' viscosity, yield stress). The flow behavior ofthe Eckemfórde suspensions is compared tothe flow behavior of suspensions of bottom samples from the southem part of Kieier Förde (eastementrance to the Kiel Canal). Since organic matter in varying amounts are known to affect sedimentstrength properties (Keiler, 1982; Bennett el al., 1999) an attempt was made to assess the influence oftheorganic matter on the rheological properties ofthe suspensions.

3 GENERAL SEDIMENT PROPERTIES/CHARACTERISTICS

3.1 Eckernförde SedimentsBox cores and gravity cores were taken for various purposes from water depths greater than 20 m at

various locations within the CBBL test area (Fig. 2). One set of samples (200 Series) was taken in April1993 and a second set (600 Series) was taken in July 1994. Sampling locations are listed in Table I. Thesurficial sediments of Eckemfórde Bay are c1assifiedas silty c1aysand c1ayeysilts (Shepard, 1954), withhigh organic matter (between 10 to 20%, Table 2). The non-disaggregated sediment is composed of largeamounts of fecal pellets which are cylindricalor spindle-shaped, showing a broad mode in length, 0.09-0.21 mm, with a mean of O.18 mm, and a narrower range mode in width, 0.08-0.16 mm, with a mean ofO.IO mmo The fauna is dominated by surface deposit feeders (Alba abra, Polydora dUala) and headdown deposit feeders inc1udingCapitella sp., and tubificid oligochaetes (D'Andrea el al., 1996).

3.2 Kieier Förde SedimentsSurface box core samples taken in 1993 from the RV SAGITTA (Kiel University) were almost entirely

dark gray to black, non-pelletal, fine-grained sandy silty c1ayscontaining no living organisms but a few tomany broken and blackened mussel shells. While appearing anoxic, no obvious methane nor hydrogensulphide gas was emanating from the sediment. Samples were taken from the area surrounding theentrance to the Kiel Canal (Holtenau) and the southem portion ofthe fjord (Fig. 3).

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--

09"50' 10"00' 10°10'

Fig. 2 Location map in Eckemfórde showing the CBBLSRP sampling rectangle

Table 1 SamDI'

International Joumal of Sediment Research, Vol. 21, No. 1,2006, pp. 24-41 - 27-

-.. - .--.. ----------Core No Latitude Longitude Depth, m

Eckernförde - 200 Series BC 225 542943.2 09 59 26.8 24BC 232 54 29 28.3 095854.7 26BC 233 542928.7 09 58 54.5 24BC 237 54 29 36.5 10 00 10.4 24BC 238 542935.7 10 00 10.3 23BC 249 542957.7 10 0156.1 25BC 250 542957.7 10 0156.1 24BC251 542937.5 10 00 03.5 20BC 253 54 29 36.4 095918.0 21BC 257BC 260 542757.7 0952 10.7 20BC 262 5427 59.4 09 52 11.0 20BC 264 542937.18 095919.02 24

Eckernfórde - 600 Series BC 601 5423 43.9 095931.0 25BC611 54 29 24.4 095901.2 25BC 619 5432 13.26 10 03 33.50 27BC 620 5421 13.36 100333.50 27BC 629 543000.95 09 58 33.69 25BC631 54 30 10.60 09 58 31.87 21

KieIer Förde Kiel 3 54 21 49 100931 8Kiel 4 54 22 10 10 09 50 11Kiel 6 5421 15 10 09 38 11.5Kiel 7 54 20 16 101004 13Kiel 8 54 1942 10 09 24 13.5

cu-0'E..!2

i21'

23'

24'

22'

KIBLBRFÖRDB

o 1110O. 2000. ., .

20'

10"08' 09' 10' 11' 12' 13'E. longitude

14' 15' 16' 10"17'

Fig.3 Sample location map for Kieier Förde sediment samples

4 METHODS

4.1 Interface SamplingSamples of the Eckernfórde surface were taken trom the tops of box cores by carefully lifting off the

upper 2 cm of surficial sediment with a simple stainless steel spatuia. The sample was transferred to aplastic sample bag, labeled, and kept remgerated until analyzed. It was not always possibie to sampleonly the upper 2 cm and occasionally deeper material was incorporated. A smalI, stainless steel box corerwas used to take the Kieier Förde samples and the tops of each sample were extracted, using thetechnique described above. Eight samples were taken in July 1993 but only five contained sufticientmaterial for a complete analysis (Tabie 2).

4.2 Grain Size AnalysisGrain size analyses were perforrned on tresh sediments at the Royal Belgian Institute for Natural

Sciences, Brussels (KBIN). Samples were leached with distilled water, treeze dried, organic matter wasremoved with H2û2, calcium carbonate was removed with HCl, and analysis was perforrned with aSedigraph 5100 coupled to a Mastertech 51 using a 50 weight % solution of glycerine as the suspendingmedium. Samples were analyzed at 1/40 intervals to provide a detailed size distribution. Clay-sizedparticles were deterrnined at 2 fJm (9N), silt-sized particles between 2-62 fJm(8-4N), and sand sizes weredeterrnined to be >62 fJm«4N). Sediment was classified according to Shepard (1954). The remainder ofthe sample was used for rheological analyses.

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*ND = No DataSD = Standard DeviationCoV% = Coefficient ofVariation (Standard Deviation/Mean) x100

4.3 Organic MatterSubsamples were taken trom the bulk sample for organic matter analysis using the loss-on-ignition

method. Approximately lOg of bulk sample was dried and ground in a steel mortar to pass a 0.5 mmsieve (U.S. Standard Sieve #35). Analysis of a small (-2 g) sample was performed by placing thepreviously weighed sample (dried at 105°C and stored in a dessicator) in a muffie oven (24 hours at430°C), reweighing the sample and determining the organic matter by loss in weight. Organic matter(OM) contents were observed to be very high, with nearly all samples recording >I0% (Tabie 2).

4.4 ViscometryFresh (3-day old) samples trom Eckemfórde and Kieier Förde were analyzed in a Brookfield RVT 8-

speed coaxial rotational viscometer equipped with the UL adaptor with a narrow gap (2.47 mm), designedspecifically for low viscosity fluids and capable ofproducing shear rates of trom 0.61 to 122.49 S-I(Faas,1990). Occasionally it became necessary to use a smaller adaptor with a wider gap to analyze the higher

International Journal of Sediment Research, Vol. 21, No. 1,2006, pp. 24-41 - 29-

. _..,.... _ _a......a ur.".... ..,..................aur. "".,..,..........................., ......a......,._ ....................................,......_ _............._............. - ..........__a..._.a.... .a ._...

Organic Sediment

Sample Sand Silt Clay Silt/Clay Median matter Carbonate classification Type# % % % % pm % (Shepard,

% 1954)Type 1

BC631 52 33 15 2.2 24 5 5 Silty sand 1Type 2

Kie13 36.56 30.8 32.61 0.95 5.82 12.36 8.2 Sandy silty 2clay.

Kie14 41.09 25.96 32.95 0.79 7.14 13.89 7.1 Sandy silty 2clay.

Kie17 17.46 35.01 47.53 0.74 4.9 17.02 12.3 Silty clay 2Kiel 8 50.15 23.8 26.05 0.91 11.52 10.16 3.1 Sandy silty clay 2

BC 250 22.43 32.51 45.06 0.72 2.17 13.92 0.3 Sandy silty clay 2BC 262 11.41 48.71 37.88 1.22 2.12 15.01 7.5 Silty clay 2Mean 29.85 32.90 37.01 0.89 5.61 13.73 6.42SD 15.05 8.83 8.15 0.19 3.52 2.33 4.19

CoV% 50.41 26.92 22.02 21.02 62.67 16.97 65.32Type 3

BC 225 4.33 52.21 43.46 1.2 1.2 18.03 0.3 Clayey silt 3BC 232 2.35 51.85 45.80 1.13 1.88 17.92 4.2 Clayey silt 3BC 601 4.49 63.24 32.27 1.96 2.16 16.92 2.9 Clayey silt 3BC 611 2.61 53.67 43.72 1.23 1.2 15.67 1.1 Clayey silt 3BC 619 3.39 61.3 35.31 1.74 1.93 15.52 1.9 Clayey silt 3BC 620 5.16 59.77 35.07 1.7 1.93 15.49 1.1 Clayey silt 3BC 629 1.54 62.54 35.72 1.75 1.71 15.94 1.1 Clayey silt 3Mean 3.41 57.80 38.76 1.53 1.72 16.50 1.1

SD 1.31 5.03 5.37 0.33 0.38 1.12 1.34CoV% 38.49 8.71 13.86 21.76 21.96 6.78 74.19

Type 4Kiel 6 8.60 42.78 48.64 0.88 1.03 19.58 4.2 Silty clay 4

BC 237 3.82 44.37 51.81 0.85 2.24 *ND *ND Silty clay 4BC 238 2.75 41.55 55.7 0.74 0.88 20.03 1.1 Silty clay 4BC 249 3.78 43.95 52.27 0.84 2.25 *ND *ND Silty clay 4BC 251 1.42 45.47 53.11 0.86 0.68 10.72 0.2 Silty clay 4BC 253 3.28 43.92 62.8 0.83 0.74 16.80 2.2 Silty clay 4BC 264 1.03 41.51 57.38 0.73 0.64 18.04 1.2 Silty clay 4

Mean 3.66 43.06 54.59 0.81 1.23 17.28 1.78SD 2.30 1.55 4.01 0.06 0.69 3.41 1.53

CoV% 62.68 3.61 7.35 7.33 56.37 19.74 85.79

u.._ ... ---_.-

density (> 1.2 glcm3) samples. In those cases, shear rates ranged from 0.37 to 84.0 S-I. The samples werecompletely remolded (but not disaggregated) with a mechanical stirrer and aHowed to settle in a I Isetding tube to create the sediment suspension which was adjusted by dilution with sea water of the samesalinity as when sampled. Viscometric analyses were performed at three different wet bulk densities,ranging from 1.04 to 1.3 I glcm3. FoHowing analysis, the sample was dried, water content and salinity ofinterstitial water determined, and wet bulk density calculated. Organic matter was determined on asubsample of the analyzed sample as previously described. Seventy eight separate analyses were made,not aH of which measured aH parameters. Data are presented as flow diagrams to demonstrate the form ofbehavior, e.g., pseudoplastic (shear thinning) or dilatant (shear thickening) ofthe sediment, rheograms (toquantitatively assess "apparent" viscosity), and shear stress - shear rate curves (to determine Binghamyield stress values). Fig. 4 demonstrates the various forms of tlow behavior which may be exhibited bythese suspensions.

To

Shear rate (Y)

Fig. 4 Diagram showing various forms of flow behavior

5 RESUL TS OF ANALYSES

5.1 GranulometryGranulometric differences exist between the samples from both environments (Tabie 2) . The analyses

reveal four distinctly different grain size distribution patterns (Fig. 5). Type I, a medium-fine silty sand,shows a high sand content (>50%) and low clay content (15%). Type 2, a sandy silty clay, has an averagesand content of 29.85(+/-15)% and an average clay content of 37 (+/-8)%. Most of the Kiel samplesbelong to this sediment type. AH Eckernförde 600 Series (except BC 63 I) and one Eckernfórde 200Series samples (BC 225) belong to the third sediment type (Type 3), a clayey silt, with a sand contentbelow 5%. The remaining sediments of the 200 Series belong to Type 4 which differs from the othertypes by its high clay content (average of 54.59 (+/- 4%).

A ternary grain-size classification was not used for classification purposes inasmuch as it fails torecognize the rheological differences in cohesion of the grain size classes. The mineral part of the mudfraction shows a progression from non-cohesive coarse silt-sized particles (63 to 16 11m) over fine silt-sized particles (16 to 2 11m) part of which shows cohesive behavior to cohesive very fine clay particlesand coHoids. Differences in cohesive behavior between the mud fractions necessitates a set of parametersindicating the relative importance of these fractions in the mud sediments. The silt to clay ratio (Si/Cl)and the coarse silt to fine silt ratio (CS/FS) express this relative importance and have been used for a longtime (Sindowski, 1958). In this paper the Si/Cl ratio is used as a descriptive parameter providing a betterunderstanding ofthe rheological behavior ofthe sediments.

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Eti> Talti><>:::ti>

!i:I<>

..cTmCIJ

....

250 125 63 32 16 8 4 2 0Grain-size in micron

Fig. 5 Grain size distribution curves, showing Types 1,2,3, and 4

The Eckemfórde and KieIer Förde sediments differ in another important aspect. It is shown (Fig. 6)that with increasing cIay content, the Si/Cl ratio of all Eckemfórde sediments decreases trom 2.0 at lowcIay content to less than 1.0 for cIay contents that exceed 50%. This behavior indicates that the observedchanges in the grain-size spectrum result trom an addition of sand and silt to an originally cIay-richsediment. It is different for the Kiel sediments in which the Si/Cl ratio varies only slightly withincreasing cIay-content, indicating that the change in the grain-size spectrum is explained by the changingsand content alone.

2.5.Silt/Clay vs Caly «2mm)

2 . Eck 200 & 600y = -0403x + 3.01R2= 0.80

.. & Kieier fordey = -0.0042x + 1.01R2= 0.23

0.5

oo 10 20 30 40

%<2um50 60 70

Fig. 6 Silt/Clay ratio versus clay content (%)

International Journalof Sediment Research, Vol. 21, No. I, 2006, pp. 24-41 - 31-

15250 125 63 32 16 8 4 2 250 125 63 32 16 8 4 2 15

= Type I: Fine silty-sand = Type 2: Clayey sand

JIJ 1\(BC631) (Kiel 4)

Clay: 15% A Clay: 34% HO...N

'r;;"""0

'5 5...c..

015

tJc:J Type 3: Clayey silt

11

= Type 4: Silty clay(BC 620) (BC 251)

Clay: 36% Clay: 57% HO...N

.;;"""0

'5 5 /'\ B

c..

0250

- < -..- _u < --- - -<-- - - - - --- -

5.2ViscometryDifferences in rheologieal behavior are seen in samples from the different sites and within samples

from the same site. In general, the Eekemfórde 600 Series behave like viseoelastie material whereasEekemfórde 200 Series and Kieier Förde sediment show more viseoplastic behavior. Significant statieyield stress oecurs at lower densities in the Eekernfórde 600 Series than in the 200 Series whereascomparable yield stress occurs at greater densities in the Kieier Förde samples (Fig. 7). Flow behavioralso shows considerabie variation. In particular, the Eckemfórde 600 Series samples generally showdilatant flow in the shear rate interval between land 5 S.Iwhile both Eckemfórde 200 Series and KieierFörde samples show only pseudoplastic flow. This will be discussed more fully in Section 6.

20.00

'"'"~'" 10.00-0

Q).>.o

.~Cl)

y = 0.1693x28.3S. Eck600. Eck200. Kiel

15.00

Eck600

5.00

0.00

1.05 1.1 1.15

Density(g/crn3)

1.2 1.25

Fig. 7 Yield stress versus density for Eckernförde -200 Series,Eckernf6rde - 600 Series, and Kieler Förde sedirnents

5.3Lossof Organic MatterOrganic matter content is quite high and is known to affect the geotechnical properties of marine

sediments, usually causing a change in water-dependent parameters, e.g., strength and index properties(Rashid and Brown, 1975; Booth and Dahl, 1986; Bennett el al., 1999). However, to the authors'knowiedge, no experiments have been performed to examine the before-after effects on physicalproperties following removal of organic matter from previously organic-rich suspensions. Therefore,experiments were done to determine those effects. Subsamples were taken from the original analyzedsample, organic matter was removed by gently boiling the sediment samples in 35% H202 for 24 hoursafter which they were rinsed and resuspended in water of their original salinity to form a densesuspension. The OM-free samples were analyzed in the same fashion as the natural samples. Only theEckemforde 200 Series and the Kieier Forde samples were analyzed and compared. Table 3 shows theentire data set for aH the samples analyzed in this analysis, and Table 4 shows the effects of removal oforganic matter. It shows that the greatest change occurs in the strength properties ofboth sets of samples,with a 79.6% loss of static yield stress and 82.8% loss of Bingham yield stress in the Eckemfórde 200Series and 44.2% loss of static yield stress and 70.0% loss of Bingham yield stress in the Kieier Fördesamples. The change in flow behavior is shown by 50.9% and 54.2% loss in the low shear slope and6.2% and 5.2% in the high shear slope of Eckemfórde 200 Series samples and Kieier Förde samples,respectively. While both sets of analyses indicate the samples to be pseudoplastic, the degree of

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--. - .. - - .-

pseudoplastieity inereases by more than 50% in the 'cleaned' sediment over the natural sediment. Thisindieates that the 'cleaned' samples were likely enhaneed with silt and clay-sized particles (previouslybound up in the feeal pellets, or adhering to larger silt and sand-sized particles) and may have behaveddifferently from the natural samples.

6 DISCUSSION AND INTERPRETATION OF RESUL TSThe data indieate that differenees in rheologieal behavior oeeur in similar stress tields in a variety of

sediments from the different environments. Plots of statie yield stress (ty) versus sediment density (Fig. 7)indieate eonsiderable variability. The data seatter around a trend line whieh rises exponentially as densityinereases to 1.21 glem3.The curve rises more steeply for the Eekemförde sediments than those from theKieier Förde. Eekemförde 200 and 600 Series are moderately weIl eorrelated whereas the Kieier Fördesediments are weIl eorrelated. The ratio between the Bingham yield stress and the statie yield stress forthe Eekemförde 200 Series and Kieier Förde sediments shows a mean of2.98 and 2.62 indieating a nearlyidentieal relation. However, a mean of 3.57 for the Eekemförde 600-Series indieates a different relation(Tabie 3).

6.1 Yield StressVield stress is eonsidered to be "a material property denoting a transition between solidlike and

liquidlike behavior" (Nuygen and Boger, 1983). Three types ofyield stress are detined: (I) the elastie-limit yield stress, Ty,the stress above whieh the material exhibits permanent strain, (2) the statie yieldstress, Tsothe minimum stress neeessary to bring about steady state strain or deformation of the material,and (3) the dynamie yield stress, TB,the value obtained when extrapolating to the zero-shear rate on ashear stress-strain rate curve (also known as Bingham yield stress). Both the statie yield stress (Ts)andthe dynamie yield stress (TB)were used in this work. The statie yield stress was determined by rotatingthe spindie within the cup eontaining the sample at the lowest speed (0.5 rpm) and reeording themaximum torque reaehed as the suspension began to flow. The dynamie yield stress (TB)was determinedgraphieally as shown in Figs. 8a(3), 8b(3), and 8e(3).

6.2 Flow Behavior and 'Apparent' ViscosityAnalysis of the flow behavior of the samples through the density range from 1.08 to 1.31 glem3

indieates that Eekemförde sediments exhibit pseudoplastie flow behavior, often interrupted by an intervalof dilatant flow oeeurring at shear rates between 1.22 S.I to 12.34 S.I after whieh pseudoplastie flowresumes to the end of the eycle at 122.36 S.I.At this point a qualitication must be introduced which willallow slight deviations from the detinitions of flow behavior mentioned above. While recognizing thevalue of 1.0 as the limiting slope separating dilatant (>1.0) flow behavior from pseudoplastic «1.0) flowbehavior, only a few of the samples tested had slopes> 1.0 and they occurred only in the lower shear rateportion of the analysis and aU were from the Eekemförde 600 Series. Figs. 8a(l), b( I), and c(l) showflow diagrams of samples from BC 60I, BC 235, and Kiel 8 and are typical of all Eckemförde and KieierFörde samples. Nearly aU other samples showed pseudoplastie flow behavior throughout «1.0) and itbecame neeessary to differentiate between relative degrees of pseudoplasticity through the use of theterms shear thickening and shear thinning, e.g., sample 'X' showed shear thickening behavior ascompared with sample 'V' whieh showed less 'thickening' and behaved in a shear thinning fashion. Inthis case, shear thiekening occurs when the "apparent" viscosity increases more rapidly than the rate ofshear. Shear thinning occurs when the "apparent" viscosity either decreases, or increases less rapidlythan the rate of shear. The shear rate range was also divided into low (0.61 S.I to 12.25 S-I)and high(12.25 S.Ito 122.5 S.I)segments. Shear thickening flow in the low shear range was characteristic of nearlyall samples, and pronounced shear thinning was observed in the high shear range. This phenomenon wasreported by Faas (1994) and recognized by Silva et al. (1996) in their shear vane measurements ofshearstrength. The change from shear thickening to shear thinning is very pronounced in the Eckemförde 600-Series samples. This is apparent when examining the slopes resulting at low shear rates versus slopesresulting at high shear rates (Fig. 8a(l) and Table 3). The mean low shear rate slope is 0.780, nearly 7xgreater than the mean high shear rate slope of 0.111 (Tabie 3).

International Journal of Sediment Research, Vol. 21, No. 1,2006, pp. 24-41 - 33-

I

Wol'>I

Table 3 Rheolo2ical analvsis of Eckemtbrde and Kieier Förde sed'

5'ft3~ö'::s~'-oc::3~o...,V:J(1)P-3'(1)a

~Cl>(1)~;r<:~N-

Z?"Noo.0\'0'?Nt-

---------

ECKERNFÖRDE200seriesNatura! Cleaned

Static Bingham Static BinghamLabel yield yield Bi/St Densi Lowshear Highshear Mean

yield yield Bi/St Densi Lowshear Highshear Meang/cm slope slope slope g/cm slope slope slopePa Pa Pa Pa

225(1) 0.42 1.75 4.22 1.075 0.398 0.27 0.334 0.12 0.95 6.40 1.084 0.175 0.452 0.313

225(2) 3.65 9.20 2.52 1.280 0.417 0.119 0.303 0.43 0.54 1.25 1.121 0.097 0.301 0.199

232(1) 0.31 1.62 5.18 1.087 0.523 0.244 0.383

232(2) 1.96 7.50 3.83 1.130 0.695 0.111 0.403237(1) 1.41 2.90 2.06 1.087 0.725 0.24 0.483 0.04 0.13 3.23 1.091 0.199 0.476 0.39

237(2) 2.19 7.00 3.19 1.106 0.564 0.22 0.392

249(1) 1.49 3.00 2.02 1.084 0.647 0.235 0.441 0.47 0.64 1.36 1.103 0.052 0.222 0.137

249(2) 9.00 9.90 1.10 1.147 0.236 0.249 0.243 0.63 0.72 1.15 1.110 0.128 0.264 0.196

249(3) 9.51 11.47 1.21 1.196 0.289 0.257 0.273 1.41 2.90 2.05 1.143 0.341 0.144 0.246

251(1) 0.31 1.10 3.51 1.038 0.416 0.284 0.35251(2) 1.96 7.50 3.83 1.051 0.515 0.143 0.329 0.00 0.00 0.00 1.038 0.178 0.097 0.137

251(3) 4.88 13.50 2.77 1.147 0.219 0.382 0.196

252(1) 0.31 1.75 5.59 1.080 0.624 0.243 0.434253(1) 1.33 3.45 2.59 1.108 0.429 0.198 0.313 0.48 0.62 1.35 1.102 0.084 0.247 0.165

253(2) 5.73 14.00 2.44 1.137 0.523 0.182 0.352 1.33 1.65 1.24 1.129 0.113 0.164 0.138

253(3) 0.33 1.07 3.28 1.078 0.394 0.184 0.197 2.12 3.65 1.73 1.149 0.176 0.097 0.137

257(1) 5.21 14.10 2.71 1.132 1.188 0.432 0.31 0.86 0.80 0.93 1.102 0.166 0.214 0.19

257(2) 1.17 2.00 1.70 1.136 0.334 0.16 0.247

260(1) 1.88 4.00 2.13 1.092 0.623 0.115 0.369 0.71 0.60 0.85 1.160 0.288 0.116 0.202

260(2) 8.86 20.50 2.31 1.212 0.222 0.342 0.282 4.38 5.70 1.30 1.275 0.119 0.282 0.201

264(1) 1.56 3.00 1.92 1.096 0.382 0.213 0.298 0.04 0.08 1.92 1.102 0.566 0.477 0.522

264(2) 5.99 15.20 2.54 1.141 0.243 0.673 0.458

264(3) 14.59 20.00 1.37 1.162 0.267 0.301 0.284 0.45 0.78 1.74 1.144 0.398 0.128 0.262

Mean 4.46 7.89 2.83 1.121 0.434 0.256 0.338 0.91 1.36 1.76 1.124 0.213 0.24 0.230SD 3.83 6.18 1.17 0.056 0.168 0.124 0.078 1.09 1.54 1.41 0.051 0.138 0.13 0.104

CoV 85.93 78.31 41.39 4.98 39 51 21 108.1 113.50 71.80 4 38.5 34.2 45.22

--

Table 4 P h foll f .tt,

Eckemförde 200 Series samples present a different rheological profile (Fig. 8b(I». The mean lowshear rate slope is 0.434 which is nearly 1.7x the mean high shear rate slope (0.256). Thus, although the'apparent' viscosity of the suspension through the low shear rate interval increases (thickens) morerapidly than the 'apparent' viscosity through the high shear rate interval, it does so at about one-halftherate of the Eckemförde Series 600 samples.

On the other hand, the KieIer Förde samples show a mean low shear interval slope of 0.483 with amean high shear interval slope of 0.273, more than twice that of the Eckemfórde 600 Series samples butcomparable with the Eckemfórde 200 Series samples (Fig. 8c(l) and Table 3). Both the statie yield stressand Bingham yield stress are significantly lower than the Eckemfórde 200 Series samples by 60% and47% respectively, however, their Bingham/Static yield stress ratios are nearly identical. Figs. 8a(2),8b(2), and 8c(2) are rheograms ('apparent' viscosity versus shear rate) which show the sharp anddramatic changes in 'apparent' viscosity which occurs in the Eckemfórde 600 Series samples as theirflow behavior changes from shear thinning to shear thickening and back again to shear thinning. Thiscontrasts sharply with the flow behavior in BC 253 (Fig. 8b(l» and Kiel 8 (Fig. 8c(l» which is nearlyshear thinning throughout. Figs. 8c(l), 8c(2) and 8c(3) show the method of obtaining the Bingham yieldstress (rB)for each sample analyzed.

6.3 Effect of Particle Size and Composition on Flow BehaviorWhat is causing the near total shear thinning in the KieIer Förde and Eckemfórde 200 Series sediments

and the extreme shear thickening interval in the Eckemfórde 600 Series sediment? lnitially, the presenceof fecal pellets in the Eckemfórde 600 Series sediments and their absence from the KieIer Fördesediments and apparent absence from the Eckemfórde 200 Series sediments appeared to point toward ananswer. A bimodal model (Sengun and Probstein, I989a,b) that would incorporate particles of quartz('hard sphere') and fecal pellets ('soft sphere') as determinants ofrheological behavior proved to be bothinconclusive and extraordinarily complicated. Rather, a more likely indicator ofrheological behavior wasthe silt/clay ratio, with clay particles defined as all particles < 2 ~m (> 9 <1»in diameter. The answer liesnot with any obvious bimodality but more likely between the absolute values of silt, clay, and colloidal-sizes and the packing together of those particles of various shapes into two- and three-dimensionalstructures under different shear stresses.

International Journal of Sediment Research, Vol. 21, No. 1,2006, pp. 24-41 - 35-

n________ _________ _____ --- ---- -- -- ------ --- --

Location Sed.type Parameter Before After %Loss

Eck 200 4 Static yield (Pa) 4046 0.91 79.6

4 Bing, yield (Pa) 7.89 1.36 82.8

Kieler Förde 2 Static yield (Pa) 1.72 0.96 44.2

2 Bing. yield (Pa) 4.85 lAl 70.9

Eck 200 4 Low shear slope 00434 0.213 50.9

4 High shear slope 0.256 0.24 6.2

Mean shear slope 0.338 0.23 32

Kieler Förde 2 Low shear slope 00483 0.221 54.2

2 High shear slope 0.273 0.259 5.2

Mean shear slope 0.378 0.24 36.5

-36-

IOO~

I0.1

(l) Flow diagram - BC 60I

12.00

,-., 8.00-Cl)

d~~'e;;otJ

~ 4.00

(2) Rheogram - BC 60 I

0.00-1o

-+- 1.122glcm3

1.114glcm3

1.051glcm3

100

40

,20

,40 60Shear rate (S.I)

80

(3) Shear stress - shear rate curve - BC 601

32 pa

'25Cl)

~ 201~ Ia -,-lU..c

rn

~..-......-----

40

16.2pa

1.051glcm3

-- 1.114glcm3

1.122 glcm3

80 120Shear rate (S.I)

Fig.8a Flow diagrams, rheograms, and shear stress-shear rate plots ofBC 601 samples

International Journal of Sediment Research, Vol. 21, No. 1,2006, pp. 24-41

-- 1.051 glcm3

....... 1.114glcm3- 1.122 glcm3I I

--10 100 1000

Shear rate (5.1)

(l) Flow diagram -BC 253

201 (3) Stress-straincurve- BC253t-----------------------------------------

--. 16r'"t:l.';;;' 12~'"o

~ 8r..cen

18.2Pa

-+- 1.078 glem3

1.108 glem3

1.137 glem3

4.0 Pa

1.8 Pa

IW 14020 40 60 80Shear rate (s-])

100

Fig. Sb Flow diagrams, rheograms, and shear stress-shear rate plots of BC 253 samples

International Joumal of Sediment Research, Vol. 21, No. 1,2006, pp. 24-41 - 37 -

1.078 glem3

1.108glem3

1.137glem3

O'bl.

.1 1 10 100 1000Shear rate (S-I)

8.00-

6.001(2) Rheogram - BC 253

--.J

'"«ie:-.€ 4.00-'"

\

-+- 1.137 glem3

:> 2.001.--- 1.108 glem3

....... 1.078 glem3I

. ; , , ,120 40 60 80 100 120 140

Shear rate (S-l)

-38-

100

0.10.1

10.001I

I

8.00j,-..~ 6.00,e:::-i:>.;;;o~ 4.00:>

2.00,

o 00 L<.. 0

30

25

~20e:::-'"'"g 15'"1;1<1)

~IO

(I) Flow diagram -Kiel 8

-+- 1.073 glem31.139 glem3

-- 1.204glem3

10Shear rate (S.l)

100 1000

(2) Rheogram -Kiel 8

(3) Stress-strain curve - Kiel 8

8.2Pa

-+- 1.073glem3

-+- 1.139 glem3

1.204 glem3

2.9 Pa

20 40 60 80Shear rate (S.l)

100 120 140

Fig. Sc Flow diagrams, rheograms, and shear stress-shear rate plots of Kiel 8 samples

International Journal of Sediment Research, Vol. 21, No. 1,2006, pp.24-41

-+- 1.204 glem3........1.139 glem3....... 1.073 glem3

I- II . 120 40 60 80 100

Shear rate (S.l)

......

Table 2 indicates that the Si/Cl mean values for Eckemfórde 200 Series and Kieier Förde samples are0.90 and 0.85, respectively, and 1.68 for Eckernfórde 600 Series sediments. The average silt content forthe Eckernfórde 200 Series samples is 44.26% and 31.87% for the Kieier Förde sediments. The siltcontent for the Eckernfórde 600 Series sediments is 60.12%, almost twice that of the Kieier Förde andalmost one and a quarter times that of the Eckernfórde 600 Series sediments. Both Eckernfórde 200Series and Kieier Förde samples are classified as Type 2 or Type 4, with one exception (BC 225). TheKieier Förde sediments are bimodal, flocculated, type 2 clayey sand, silty clays and sandy muds with theprimary mode consisting ofvery fine quartz particles at 109 J1m, a secondary fine sand mode at 156 J1m,and the remainder «62 J1m) composed of nearly equal quantities of fine silt, clay, and colloids (Fig. 5).Nearly all the samples that exhibited only shear thinning flow had clay contents greater than their siltcontent and were classified as either Type 2 or 4 (this includes only the Eckernfórde 200 Series andKieier Förde samples). At rest, these finer particles form open aggregates composed of links and chainsof particles held together by electrostatic forces which tend to form aggregate structures with yield stress,while Brownian forces ofnearly the same magnitude tend to break them apart (Bergström, 1994). Shearthinning occurs when, under increasing shear rate, the flocculated aggregates are reduced to smaller flocsand individual particles. As aggregates are broken apart, their contained water is added to the solventwater and the relative balance between solid volume and liquid volume changes toward the fluid,effectively diluting the mass being sheared. As shear rates increase, the dispersed clay particles tend torealign themselves parallel to the direction of flow. This face-to-face orientation increases the double-layer repulsion effects and the 'apparent' viscosity decreases, leading to shear thinning flow. In the KieierFörde suspensions, behavior of the primary and secondary mode quartz sand particles is also shearthinning (Tadros, 1990)with the cumulative effect being that of a totally shear thinning suspension.

On the other hand, Eckernfórde 600 Series sediment suspensions are dominated by silt-sized particleswith a secondary mode of coarse silt and fine sand sized particles, probably composed of fecal pellets andmore likely to be influenced by hydrostatic rather than electrostatic forces. A generally accepted model(not without controversy) contends that dilatancy arises in changing from an ordeted arrangement ofparticles in normal flow to a disordered arrangement in dilatant flow (Frith and Lipps, 1995). At somecritical shear rate dependent upon volume fraction and size distribution, the particles would tend to form2- and 3-dimensional framework structures through direct physical interactions, increasing the mass to besheared. Such microstructures have been observed in dense suspensions of uniform particles (Chow andZukoski, 1995). This will cause an increase in 'apparent' viscosity resulting in an interval of shearthickening flow behavior. The fine clay and colloidal particles fill the interstices and do not contribute tothe net viscosity until higher shear rates disrupt the framework of silt-sized particles. The fine clay andcolloid particles become dispersed among the larger silt particles and serve as a lubricant between them.The entire mass then behaves as a shear thinning suspension at the higher shear rates as previouslydescribed. Mewis (1990) indicates that small colloidal particles cause all varieties of non-Newtonianeffects and Probstein el al. (1994) indicate when colloidal particles are involved, there will be a nonlinearrelation between shear stress and shear rate that will result in non-Newtonian behavior.

7 CONCLUSIONS(a) Rheological behavior of dense fine-sediment suspensions from Eckernfórde and Kieier Förde

behave as non-Newtonian fluids. The properties (flow behavior, yield stress) are similar betweenEckernfórde 200 Series samples and Kieier Förde samples. Eckernfórde 600 Series samples show quitedifferent rheological behavior.

(b) Although granulometric analyses of the samples classifies them into four (4) different types, onlytwo different siltJclay ratios are observed. Ratios > 1.00 are found primarily in the Eckernfórde 600Series sediments, whereas ratios < 1.00 are found in both the Eckernfórde 200 Series and Kieier Fördesamples.

(c) Shear thickening (dilatant) flow behavior and high Bingham yield stress observed in theEckernfórde 600 Series appears to result from an abundance of silt-sized particles (SIC> 1.00). Shearthinning flow behavior, reduced static yield and Bingham yield stress in the Eckernfórde 200 Series andKieier Förde samples is controlled by the abundance of clay- and colloidal-sized particles (SIC < 1.00).

(d) Removal of organic matter content from organic-rich suspensions of the Eckernfórde 200 Series

International Journal of Sediment Research, Vol. 21, No. 1,2006, pp. 24-41 - 39-

and Kieier Förde sediments results in lowered static yield stress and extreme shear thinning(pseudoplastic) flow behavior. The Eckemförde 600 Series sediments were not analyzed in this fashion.

(e) Yield stress measurements of sediment suspensions measure only the cohesion of the individualparticles due to electrostatic forces which increase with time as more particles approach each other(Allam and Sridharan, 1984). Most ofthe measurements are made in suspensions ranging between 1.05and 1.20 g/cm3. They correspond roughly to measurements of remolded sediment strength made with avane shear apparatus in that the effects of time on strength development is minimal.

(f) Flow behavior will control the timing and amount of resuspension. Kieier Förde sediments willresuspend easily under low shear stresses due to shear thinning flow behavior and low static yield stresses,while Eckemförde sediments will tend to remain in place until shear stresses exceed their static yieldstress. Yield stress will control the depth of erosion in the near bottom boundary layer. Inhyperconcentrated suspensions, the yield stress will effect the stability of slopes incised in them (e.g., thesides of otterboard marks or dredged navigation channels).

(g) The analysis of the strength behavior of natural suspensions must be clarified with more dataconceming the role(s) played by the type and amount of organic matter and clay mineral mixes, e.g.,kaolinite/smectite, illite/kaolinite, etc., of the suspension.

ACKNOWLEDGEMENTSThanks are extended to Chuck Nittrouer, Glenn Lopez, and Jobst Hülsemann for providing many ofthe

samples for this work; to Tom Orsi for help in sampling the box core tops; to Roland Paepe and IFAQ(Free University of Brussels, VUB) for providing technical assistance; and to the Royal Belgian Instituteof Natural Science for laboratory space and sediment analyses. Mike Lambert (Naval ResearchLaboratory - Stennis Space Center) performed the counting and statistical analysis of the fecal pellets.Richard Bennett and Mike Richardson provided informal reviews of the paper. Funding was provided bythe Office of Naval Research (ONR) through the CBBLSRP, directed by Mike Richardson through theNaval Research Laboratory, Stennis Space Center, Mississippi.

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