Resuspension phenomena of benthic sediments: The role of cohesion and biological adhesion

RIVER RESEARCH AND APPLICATIONS

River. Res. Applic. 26: 404–413 (2010)

Published online 29 July 2009 in Wiley InterScience

RESUSPENSION PHENOMENA OF BENTHIC SEDIMENTS: THE ROLE OFCOHESION AND BIOLOGICAL ADHESION

MAURIZIO RIGHETTI* and CORRADO LUCARELLI

Department of Civil and Environmental Engineering, University of Trento, Italy

(www.interscience.wiley.com) DOI: 10.1002/rra.1296

ABSTRACT

The incipient motion conditions of benthic sediments are analysed and an original parametrization for the critical shear stress isproposed. Starting from momentum balance considerations on a single sediment floc, the Shields dimensionless mobilityparameter in incipient motion conditions is expressed also as a function of both cohesive and adhesive forces that concur tostabilize the superficial layer of the benthic sediment. The entrainment model is validated on the basis of laboratory experimentscarried out on natural benthic sediments sampled from three alpine Italian lakes, having different trophic conditions and differentcompositions of the bed. The experimental results show that the cohesive effects decrease as the sediment water contentincreases. Moreover the role of bioadhesion is experimentally evaluated: the critical shear stresses of ‘living’ sediments arehigher than that measured on similar but ‘dead’, poisoned sediments. This stabilizing effect shows a variation in time on aseasonal time scale, accordingly to the literature indications where biological adhesion is related to phytoplankton and bacteriaactivity, the life and growth of which obviously depends on seasonal variations. Copyright # 2009 John Wiley & Sons, Ltd.

key words: adhesion; bioadhesion; cohesion; incipient motion; lacustrine benthic sediments

Received 8 May 2009; Accepted 19 June 2009

INTRODUCTION

The problem of incipient motion of cohesive sediments is important not only from the hydrodynamic point of view,

but also for the impact that movement of benthic substrate—often pollutant rich and oxygen hungry—could have

on the water pollution and eutrophication in rivers, estuaries and shallow lakes. Indeed it has been observed that

nutrient release and oxygen consumption in the water column strongly increase after the benthic sediments are

resuspended (Spagnoli and Bergamini, 1997).

Moreover the resuspension rate of cohesive sediments is often formulated as a function of the critical shear stress

itself (Parchure and Mehta, 1985) and this enforces the need for a clear operative definition of the sediment

threshold conditions.

As highlighted also by Black et al. (2002), the main difficulty in characterizing and parameterizing the sediment

incipient motion conditions stems from the fact that stability of natural, cohesive-adhesive sediment is driven not

only by hydrodynamic and electrochemical forces but also by biological forces. Therefore the uncertainties present

in the definition of empirical threshold curves for non-biologically affected sediments (see e.g. Paphitis, 2001)

remarkably increase when fine grained natural sediments are considered. In fact, such sediment is usually

characterized by a high organic matter content and its structure is organized as a flocculated biogenic matrix, with a

hierarchy of flocs clustered each other by biological bonds of different nature and intensity (see e.g. Sobeck and

Higgins, 2002; Black, 1997).

In this situation, the assessment of the relative importance between physical and biological forces acting on the

sediments is intimately related to the detection of what is at incipient motion conditions: particle or flocs? and at

which characteristic size and density? Among the many procedure proposed, Righetti and Lucarelli (2007)

provided a rational criterion for the definition of the threshold conditions coupled with the evaluation of the

characteristic size of the detached flocs. Righetti and Lucarelli (2007) incipient motion model can be classified as a

*Correspondence to: Maurizio Righetti, Department of Civil and Environmental Engineering, University of Trento, Italy.E-mail: [email protected]

Copyright # 2009 John Wiley & Sons, Ltd.

RESUSPENSION PHENOMENA OF BENTHIC SEDIMENTS 405

single-particle, point-based threshold criterion, because it analyses the incipient motion conditions of a small size

cohesive–adhesive floc (characteristic size of millimetres) subject to a flow at uniform conditions. It is usually

assumed that such kind of models can be applied also to real, non-uniform flows, provided that the local shear stress

is known. Nevertheless, it has to be pointed out that the application to natural conditions of such kind of models can

be subjected to certain limitations because the spatial heterogeneities of the flow field may become significant, due

to its intrinsic non-uniformity. This aspect has been highlighted in the theoretical framework proposed by Coleman

and Nikora (2008), which is based on appropriate volume averaged momentum balance considerations at a wider

range of scales.

The threshold criterion proposed by Righetti and Lucarelli (2007) is not particularly innovative, and alternative

criteria are proposed by different authors, based on instance on the extrapolation of the shear stress value for which

the erosion rate tends to zero values (see e.g. Houwing, 1999 or Burt et al., 1997 for an exhaustive review).

Nevertheless Righetti and Lucarelli (2007) suggest that the dimensional analysis is a necessary and helpful tool for

a proper distinction between the physico-chemical stabilizing forces and the biological stabilizing ones acting on a

floc. The authors proposed a generalization of the Shields curve to cohesive-adhesive sediments, together with a

parameterization of adhesive biological forces.

In the present work the application of the approach introduced in Righetti and Lucarelli (2007) is proposed and

applied to resuspension tests performed on fresh living sediments and on poisoned sediments. For the former the

resuspension tests were performed in within 3 h after core sampling and with minimum disturbances of sediment

cores. For the latter the resuspension tests were performed after the injection of a copper sulphate solution for at

least 24 h in the interstitial water of the sediment surficial layer. The aim of the poisoning procedure is to kill the

microphytobenthos in the sediment surface layer and thus to annihilate the binding interparticles effects due to

biological activity. The sediment were sampled in different lakes at different trophic levels.

The aim was to try and depict the dependence of biological adhesion on trophic state of the water body and its

seasonal variation.

The lacustrine environment has been chosen because in such water bodies one can better appreciate the

contribution of biological adhesion on flocs stability: the mechanical stress of the flowing water is less than in

rivers, thus allowing the benthic substrate to more thoroughly develop its biological binding.

THEORETICAL CONSIDERATIONS

The incipient motion of a floc belonging to a cohesive-adhesive sediment matrix and exposed to an unidirectional

uniform flow is analysed. The moment equilibrium about the contact point between adjacent particles at

equilibrium is considered (see Figure 1). The gravitational force FG, drag and lift forces induced by the flow, FD and

FL, cohesive and adhesive forces between contiguous flocs FC, FA are taken into account in the moment balance

equation (see e.g. Chepil, 1959; Righetti and Lucarelli, 2007). Dimensional analysis considerations (Yalin, 1972;

Figure 1. Schematic plot of floc incipient motion conditions and forces acting on it

Copyright # 2009 John Wiley & Sons, Ltd. River. Res. Applic. 26: 404–413 (2010)

DOI: 10.1002/rra

406 M. RIGHETTI AND C. LUCARELLI

Righetti and Lucarelli, 2007) allow one to get the Shields parameter for cohesive and adhesive flocs:

#C � t0C

ðrB � rÞgD ¼ #C0 D�ð Þ þ #C0 D�ð Þa3

C

ðrB � rÞgDþ #C0 D�ð Þa3

A

ðrB � rÞgD (1)

where t0C is the critical shear stress (Nm�2), D� is the floc dimensionless characteristic diameter, defined as

D� ¼ D gD�n2

� �1=3

, where D ¼ rB � rð Þ=r is the relative density of the floc; a3 is a floc shape parameter, equal to

p/6 for spherical flocs, #C is the Shields parameter, #C0 is the ‘traditional’ expression of the shields parameter,

referring to non-adhesive non-cohesive sediments, for which Brownlie (1981) gave the following expression:

#C0ðD�Þ ¼ 0:22D��0:9 þ 0:06 exp �17:77D��0:09� �

: (2)

The coefficients appearing in Equation (1), C [N m�2] and A [N m�2], are parameters which refer to the

contribution of cohesive and adhesive forces, FC and FA respectively, on sediment stability (Righetti and Lucarelli,

2007).

FC

ðrB � rÞgD3¼ CD2

ðrB � rÞgD3;

FA

ðrB � rÞgD3¼ AD2

ðrB � rÞgD3(3)

The adhesion effects are considered as the mechanical contribution to floc stability due to the production of

biologically-mediated linkages between contiguous flocs, such as the extracellular polymeric substances secreted

by bacteria and microphytobenthos (Black et al., 2002). Quantitative and analytical analyses show that for the

benthic sediments considered in the present work, the contribute of cohesive forces can be neglected with respect to

the adhesive forces (see Israelachvili, 1997; Righetti and Lucarelli, 2007). Therefore in the following the cohesive

force contribution will be neglected.

The methodology proposed by Righetti and Lucarelli (2007) for adhesion forces evaluation is dimensionally

consistent thus it allows one to completely clean up the adhesion force evaluation from unwanted contributions of

other forces. For this reason the adhesion coefficient A evaluated in this way can be regarded as a particular

indicator of the biological activity at the sediment-water interface.

Moreover we can suppose that A can be split into two contributions:

� t

Co

he binding effect related to living material AB, which could be called biological adhesion, and

� t
he adhesion that still persists once bacteria and microphytobenthos are dead, we can call this contribution as
residual adhesion, AR.

Therefore the adhesion coefficient A reported in Equations (1) and (3) can be expressed as the sum of two

contributions:

A ¼ AB þ AR (4)

Practically the distinction between biological and residual adhesion could be made by comparison between the

tests of incipient motion performed on fresh, living sediment samples and sediment samples poisoned with biocide.

In particular, Equations (1) and (2) allow one to evaluate A from the incipient motion tests. Indicating with A and

Apoisoned the adhesion coefficients estimated from tests on living and poisoned samples respectively, we have:

AR ¼ Apoisoned;AB ¼ A� Apoisoned (5)

MATERIAL AND METHODS

Several series of surface sediments cores were collected from three lakes in northeastern site of Trentino (Italy):

Caldonazzo, Levico and Serraia. The first two lakes were in mesotrophic conditions whereas the later was

eutrophic. The cores have been acquired using a gravity corer from the deep zone of each lake during 2004 and

pyright # 2009 John Wiley & Sons, Ltd. River. Res. Applic. 26: 404–413 (2010)

DOI: 10.1002/rra

Figure 2. Scanning electron micrographs of the surface layer of lacustrine beds: (a) Caldonazzo lake-site A in which the presence of benthicdiatoms frustules is evident; (b) Serraia lake benthic sediments, in which inorganic particle are embedded within strands of EPS


2005. Only in Caldonazzo Lake, the deepest of the three study lakes, we have selected two different sites

(site A depth 12m, site C depth 49m) in order to evaluate possible variations in the characteristics of the benthic

materials at different locations and depths in the lake. Low-magnification microscopy with image analysis was used

to estimate structural properties, mean dimension and size distribution of sediment flocs and particles. Sediment

morphology and elemental composition were detected at the sediment surface of some cores coming from

Caldonazzo Lake-site A and Serraia Lake, by means of a scanning electronic microscope (SED-EDX system)

JEOL JSM 5500. It was noted that the nature of the bentic substrate was substantially different between the two

lakes (see Figure 2): Caldonazzo Lake-site A is dominated by the presence of benthic diatoms frustules and

allumino silicates, while in Serraia Lake the presence of blue-green algae prevailed.

Water content was determined by drying ca. 25 g of wet sediment at temperature of 1058C for 24 h; organic

matter was obtained by the loss-of-ignition (LOI, organic matter OM) of dried sediments at 5508C for 5 h. The

densities of sediment particles were determined in laboratory with the pycnometer method; the bulk densities of the

surface layer of each sediment sample were calculated from analytical formulae (see e.g. Roberts et al., 1998) and

are related to floc density, which compose the superficial flocculated sediment matrix (Peterson, 1999).

Finally other cores were used for the evaluation of the critical shear stress in a 6m long recirculating sedflume,

following the procedure explained in detail in Righetti and Lucarelli (2007), to which the reader is referred for

further details.

EXPERIMENTAL RESULTS

In Table I, the mean values of the critical shear stresses for incipient motion toc and the sediment floc diameter for

each site of sampling are presented. The experimental results are also shown in Figure 3 as a function of OM, its

representation in a Shields diagram is reported in Figure 4. As reported in Table I, the samples treated with biocide

have a lower measured toc as compared to the corresponding living, untreated, sediment samples.

In Figure 5 the reduction in the critical shear stresses registered between fresh and poisoned sediment

Dt0C ¼ t0Cð Þfresh� t0Cð Þpoisoned is plotted as a function of OM. The data reported in Table I show that the mean floc

diameters of the poisoned dead benthic sediment tend to be smaller than for the living sediment, the reduction in

critical shear stresses Dt0C being proportional to the reduction in mean sediment floc diameter DD. The slight

disaggregation of the flocs and the reduction in its mechanical properties can be interpreted as a secondary effect of

the lethal action of the biocides on the sediment micro-biota, which tends to annihilate the binding effect of the

living microorganisms on the flocs structure. Figure 6 shows living adhesion A of the fresh sediments as a function

ofOM. There were significant differences between the three lakes, which can only be partially explained merely by


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Table I. Comparison between living and biocide sediments. t0C indicate the critical shear stress[N/m2] for incipient motion,D is

the mean sediment floc diameter [mm]. There is no significant variations of the superficial water content (w) and organic mattercontent (OM) after poisoning.

Lake name Site Depth w OM Living sediments Biocide sediments

[m] [%] [% SV/ST] t0c [N m�2] D [mm] t0c [N m�2] D [mm]

Lake Caldonazzo A 12 80.78 10.65 0.117 707 0.091 576C 49 82.55 9.96 0.105 644 0.069 565

Lake Levico M 22 94.16 14.59 0.061 803 0.052 701Lake Serraia G 10 96.17 23.69 0.040 853 0.038 829

(mean values)

Figure 3. Critical shear stresses measured on fresh and poisoned sediment cores as a function of the measuredOM. Filled symbols refers to freshsediments, open symbols refers to biocided sediments:&,&Caldonazzo;D,~ Levico;^,^ Serraia - - - - ,—exponential interpolation curves,Equation t0C ¼ a expðb�OMÞ, continuous line fresh sediments, a¼ 0.287 b¼�0.076; dashed line poisoned sediments a¼ 0.152 b¼�0.073


differences in the OM content. Indeed the Caldonazzo data showed a significant variation of A at almost constant

values of OM. The same trend can be recognized also in the values of biological adhesion, Figure 7. One possible

explanation relates to the different trophic conditions of the lakes and to the different composition of the benthic

substrate, which marks the mechanical properties of the biological binding substances produced by the biological

compartment. Serraia Lake is classified as eutro-hypertrophic, so it can be assumed that in those sediments the


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Figure 4. Dimensionless experimental data and theoretical incipient motion curves for benthic cohesive sediments * fresh sediment,D poisoned sediment; Theoretical Shields curve Equations:—Brownlie formula

Figure 5. Variation of the critical shear stress Dt0C between fresh and poisoned sediments as a function of the measured OM for the three lakes.* Levico; * Caldonazzo; & Serraia


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Figure 6. Variation of the living adhesion A¼ABþAR in fresh sediment as a function of the measuredOM.* Caldonazzo;* Caldonazzo pointC; & Serraia

Figure 7. Variation of the biological adhesion AB¼A-AR as a function of the measured OM.* Caldonazzo;* Caldonazzo point C;& Serraia


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Figure 8. Variation in time of the biological adhesion AB for Serraia and Caldonazzo sediments. * Caldonazzo; * Caldonazzo point C; &Serraia


major fraction of the sedimentary OM is composed by the dead organic detritus falling from the water column,

therefore the stabilizing contributions of living organics factors in lake Serraia are less relevant. These

considerations suggest that, in order to characterize the mechanical properties of the benthic sediments, it is also

necessary to analyse all the biological factors, which are correlated to the lakes trophic state and to the benthic

bacterial communities.

A more in-depth analysis was performed on Caldonazzo and Serraia lake sediments, for which a more detailed

series of resuspension experiments were conducted from spring to autumn season, following the oxic-anoxic cycle

of the water body due to thermal stratification. It has to be pointed out that during the Summer up to the first week of

October the water column was stratified and the bottom was in an anoxic condition in both the lakes. As far as the

deep sediment of Caldonazzo (point C) are concerned, it has to be borne in mind that in that region the lake is

stratified all along the year and in anoxic conditions.

The values of biological adhesion are reported for these two lakes in chronological order in Figure 8. Looking at

Caldonazzo data a certain cyclicity seems to take place, for which the values of the biological adhesion AB tend to

be reduced during the stratification period. This damping seems to be more perceptible in the littoral sediment. This

circumstance could be explained considering that during the stratification period the littoral benthic environment

tends to go towards anoxic conditions. These conditions can be an adverse environment for life of the microbial

component responsible for biological sediment stability. However the experimental data reported in Figure 8 are

too few to clearly prove this assumption and a more extensive series of measurement are needed in order to clarify

and characterize the seasonal behaviour of sediment biological stability.

A more detailed analysis was performed on Serraia data, the main results reported in Figure 9. It can be seen that

during the period of thermal stratification the biological adhesion values tended to reduce, while became greater

during the destratification process, on the other hand residual adhesion values due to ‘dead’ sedimentary organic

matter were almost constant all along the period. This seasonal behaviour of the bioadhesion agrees with the

observations on the seasonal variation of the mud floc size in the water column made by Van der Lee (2000),

confirming the conviction that the mechanical properties of the lacustrine sediments strongly depend on seasonal

phytoplanktonic and bacterial activities.


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Figure 9. Comparison of the seasonal fluctuations of adhesive and bioadhesive coefficients in sediment superficial layers of lake Serraia.& AB¼A-Apoisoned; & AR¼Apoisoned


CONCLUSION

The formulation proposed by Righetti and Lucarelli (2007) for the characterization of the Shields mobility

parameter for cohesive–adhesive sediments was used for the analysis of a series of laboratory resuspension

experiments on lacustrine sediment samples. The bioadhesive effects due to the living benthic communities were

estimated by comparison of experimental data obtained on living and poisoned samples. The novelty of the method

stems from the fact that, being dimensionally consistent, it allows one to clearly separate the different dynamical

contribution to sediment stability. In particular the forces due to biological action can be evaluated without

ambiguity and therefore the adhesion coefficient A can be regarded as a particular biological indicator that could

contribute to the characterizion of biological activity and the state of the benthic environment.

The critical shear stress for poisoned samples tends to decrease together with the mean floc diameter, presumably

because the superficial stabilizing biofilms tend to disappear. The adhesive binding forces, estimated on the basis of

the proposed modified Shields formulation, depend on the lakes trophic conditions and characteristics of the

benthic substrate, having a maximum for mesotrophic conditions. A seasonal analysis of the incipient motion

conditions shows that the bioadhesion coefficient undergoes a seasonal variation, which presumably depends on the

modifications of the biochemical cycle of the water body and probably on the lakes trophic state. The application

presented in the paper, although far from exhaustive, shows the potential of the method, which could be a useful tool

for the characterization of the physico-chemical features of the benthic habitat. The proposed theoretical approach

allows one to generalize the concept of incipient motion conditions for cohesive sediments with living matter.

Nevertheless attention must be paid when the laboratory results are extended to natural conditions, which are

usually characterized by non-linear spatial and temporal variability of the flow field, not considered in present

experiments.

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Resuspension phenomena of benthic sediments: The role of cohesion and biological adhesion

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