Working with Natural Cohesive Sediments

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K. S. BlackSediment Ecology Research Group, St. Andrews Univ., Fife, ScotlaU.K.

T. J. TolhurstSediment Ecology Research Group, St. Andrews Univ., Fife, ScotlaU.K.

D. M. PatersonSediment Ecology Research Group, St. Andrews Univ., Fife, ScotlaU.K.

S. E. HagertheyCenter for Marine Science, Univ. of North Carolina, Wilmington, NC

Introduction

The treatment of cohesive sediment transport remains a recuproblem in water-related engineering disciplines, and yet it isimportant element that has played a key role in many engineeprojects. Aspects of cohesive sediment movement importanengineering projects include streambank and riverbank stabiresidual onshore sediment transport and salt-marsh integscouring around bridge piers, and navigation and water quaamong others. The prediction of short- and long-term erosion~anddeposition! rates in the marine environment is also of significanto the coastal engineer, especially if one considers that projesea-level rise over the next 60 years may cost upward of $million per annumin the United States due to coastal erosioalone~Heinz 2000!. At present, no general analytical theory focohesive sediment resuspension is available, and empiricabased field and laboratory experiments are needed. The mainficulty in characterizing resuspension stems from the fact tcohesive sediment transport is governed not only by hydronamic forces~e.g., drag, lift! and by electrochemical forces~e.g.,van der Waals bonding, Coulombic repulsion, etc.!, but also—aswe shall show—by biological forces.

The conceptual understanding of the dynamics of natural smentary systems is advancing rapidly, not simply because of tenological innovation but also due to modern integration of studthat cross disciplinary boundaries~e.g., Nowell and Hollister1985; Nowell et al. 1987; Daborn 1991; Black et al. 1998; Dy2000; Huntley 2001!. Multidisciplinary studies lead to a betterand more realistic, understanding of natural systems and tinherent physical and biological complexity~Black and Paterson1996!. The natural processes surrounding the erosional behaof cohesive sediments are an excellent example. Early stuconcentrated on describing the erosional properties of a wcharacterized clay material, such as kaolin, under defined labtory conditions~e.g., Kandiah 1974!. However, this research doenot translate easily into the natural environment, not least becaclay is only one component in natural sediments but also duethe inherent association of clays with organic matter~Mayer et al.1985!. More recently, however, other controlling factors such

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the biomass and activity of sediment-inhabiting organisms, raing from bacteria to worms and shrimps, have been cited asportant regulatory controls on natural sediment stability~Paterson1997!. In some quarters, this has given rise to an argument vakin to the ‘‘nature versus nurture’’ discussion in which manhardened advocates support one or the other camp. The quebecomes ‘‘Which is more important in governing the erosion ptential of sediments: The physical nature of the system~nature! orthe biological overlay on the physical environment~nurture!’’?We believe an argument framed in this way is misguided, alreapolarizing the debate into ‘‘physics versus biology.’’ Rather, thsystem should be considered as a whole and not as the sumconflicting forces. There is no natural sediment deposit that isinhabited by biota~Riding and Awrawik 2000!; even sedimentsgreater than 500 m beneath the ocean floor support a ‘‘deep bioof bacteria, not just surviving but actively metabolising withithat inhospitable region~Parkes et al. 2000!. The real problem isfirst to recognize that natural sedimentsare biologically active,and to recognize also that natural sediment systems are inherevariable both temporally and spatially. Once this is fully undestood then any number of multivariate approaches may be useinvestigate the processes and factors that combine to producevariability. This short review, which focuses principally on thmuddy estuarine/fluvial environment but is also relevant tonumber of sandy environments, highlights the problems and chlenges in determining the erosion potential of natural sedimen

Erosion potential, rather than erodibility, is the term useherein to describe the susceptibility of a mud bed to erosithrough interfacial fluid shear. Thus, a high erosion potential cresponds to a less stable matrix with a lower critical entrainmestress (tocrit

) and greater erosion rate~e!.

From Theoretical to Natural Sediments

Properties of Natural Sediments

Fine clastic sediments arise principally through the weatheringpreexisting continental rocks. Of concern to this discussionsiliciclastic silt particles, and more extensively weathered clminerals, that form a major component of riverbank, intertidmudflat and deltaic sediments, as well as some low energy senvironments. Rivers to the ocean annually transport somebillion metric tons of sediment (2.531013kg). By the time grainfragments come into contact with the estuarine environment, thare inevitably flocculated and are also physically associated wdissolved and detrital organic matter as well as suspended baria. van Leussen~1988! notes that virtually all sediments in estuaries possess a surface coating comprised of organic materialmetal hydroxides. Once deposited, the sediments form a compmulticomponent system that is both biologically active anchemically reactive. Estuarine mud consists of closely spacedand clay grains embedded within an amorphous matrix of detrand bonded organic material. Usually they are associated witvariety of benthic biological communities and display stee

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physical and biogeochemical gradients in surface layers~Patersonand Hagerthey 2001!. Often they contain a relatively high percentage of sand as well as localized anoxic regions. Clearly,collective properties of muddy sediments are remote from thof the constituent mineral grains. It is this complexity that prcludes definition of a general analytical theory for cohesive sement resuspension.

Mechanics of Cohesive Sediment Erosion

Moving water exerts a force on a sediment bed that, in appromate terms, can be estimated as the product of a shear stressto ,and the bed’s surface area exposed to this stress. In the caseparticle or patch of cohesive mud, the force is counteracted bysubmerged weight of the particle, frictional interlocking of graaggregates, and cohesion. Erosion can thus be thought ofsee-saw type interplay between a hydrodynamic driving forwhich usually is turbulent, and a resisting force, which domnantly is attributable to cohesion. It is generally accepted withthe field that a certain critical shear stress~tocrit

, or tractive forcein engineering terms, also sometimes expressed as a critical svelocity! must be exceeded forsignificantentrainment and resuspension to occur. Thereafter erosion proceeds at a rate~e! propor-tional to the shear stress, and can be either constant throughor decreasing through time depending on bed structure. For nrally formed cohesive mud, the critical entrainment condition rlates to the entrainment of the very surface sediment layers, tcally a few hundred to a few thousand microns thick, wheresolids volume concentration is fractionally greater than thatquired for a particle-supported matrix~;300–400 kgm23! ~Tay-lor and Paterson 1998!. Ensuing erosion will excavate deeper layers and storm conditions, in particular, may remove 5–10 cmsediment from tidal flats or considerably more in unvegetatedsemivegetated fluvial environments~e.g., Couperthwaite et al.1998!. Representative values oftocrit

for estuarine tidal mudflatsrange between 0.02 and 2.0 N m22.

Kuijper et al.~1989! review the various erosion rate formulasThe two most commonly used expressions used in numermodels are those developed at the University of CaliforniaDavis, by E. Partheniades and coworkers~McAnally and Mehta2001!

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z ~sometimes referred to as the bed shear strength and dents!; e f is termed the floc erosion rate; andb anda5exponent andrate coefficient, respectively. This formulation is usually applito freshly deposited beds in the process of initial, self-weigconsolidation. Eq.~2! is used for consolidated or mechanicalemplaced beds where the bed properties are comparativelyform over the uppermost few centimeters

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where eM5rate coefficient andd the exponent. Values fore inestuarine environments typically range from1023– 1024 kg m22 s21. The erosion rate and rate coefficients atypically obtained from flume experiments in which time-seriessuspended sediment concentration are recorded. Strictly, the

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duction in bed stress through the experiment as erosion proceshould be accounted for, but this is only amenable through turblence modeling or direct measurement.

Biological Influence on Sediment Erosion

The biology and ecology of soft sediment bottoms has a lonhistory extending back farther than the engineering interestsediment transport. Yet only recently has there been any substtial recognition of the linkages or quantification of the interplabetween the two in terms of the mediation of sediment transpoAlthough physical forces undoubtedly overwhelm most biologicinfluence during storms and floods, during quiescent periodsspectrum of biotic effects rises in importance. These can broadbe classified as either contributing to sediment stability~biostabi-lization! or factoring against it~biodestabilization! ~Table 1!.Here, it is of most interest to describe two major aspects of bioinfluence~1! increased cohesion and adhesion through secretof biopolymers by inhabitant microbes, and~2! destabilization ofsurface layers through macrofaunal bioturbation.

Biostabilization

The interface region between sediment and water or air is an aof extreme microbial activity. Bacteria are a ubiquitous component of fine aquatic sediments and occur in abundances upabout 109– 1012 cells g21 dry weight mud. They are tenaciouslyadhesive and can survive adequetly, for example, attachedrocks in fast flowing streams or on the hull surfaces of ships. Itlikely, in fact, that historical laboratory studies on ‘‘cleaned’’ sediments have unwittingly included bacterial influence to a greaterless extent. Bacterial adhesiveness arises from the secretionextracellular polymeric substances~EPS! which coats grains andbridges interstitial pores to form a cohesive network. EPS isflexible, viscoelastic material when hydrated, and organic-rich agregates are known as able to absorb turbulent energy far meffectively than clean aggregates~Jenkinson et al. 1991!. EPS insediments enhances the existing network of mineral cohesiveterparticle forces to the extent that erosion potential is measuradecreased. The importance of EPS in modifying sediment hydralic properties has been shown by a number of studies. Dade et~1996! for instance, determined a 60% increase in critical shestress over control sediments for microbially bound marine clayIn a separate study, they showed that the erosion potential ofine sand was increased four-fold by the presence of either a pexopolymer alone or EPS generated duringin situ growth of thebacteriumAlteromonas atlantica~Dade et al. 1990!. We have ob-tained similar results using isolated bacterial polymer~xanthangum! in our laboratory on both sand and mud~Tolhurst et al.2001!.

Although the bacteria are, in numerical terms, the dominamicrobial genera in mud, a second microbial group—termed tmicrophytobenthos—also contribute to microbial binding ofgrains. The microphytobenthos comprises a number of photoatotrophic groups of microscopic algae~i.e., they can satisfy theirorganic requirements from inorganic materials using light energ!but are dominated by diatoms, euglenids, and cyanobacteBenthic diatoms, in particular, also secrete EPS and their larsize~characteristically 10–200mm! in comparison to bacteria af-fords a greater capacity to modulate particle-particle bonds. Intconnecting sheets and strands of EPS associated with diatomsforming macromolecular bridges~Fig. 1! have been reported from

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a wide range of estuarine environments~Paterson 1995!. Diatomsrequire light and are therefore restricted to the sediment-air asediment-water interface of estuary sediments. This dictates ttheir influence is mostly upontocrit

rather than erosion rate. Aconsiderable number of studies to date, on both sands and mhave documented increases intocrit

related to increased microphy-tobenthic biomass and also to direct measures of EPS~Paterson1997!. The entrainment threshold-grain diameter function~theShields function! used by engineers to assess critical entrainmestresses for particle diameters above about 20mm usually fallsbelow actual values from the field~Fig. 2!. While this may not besurprising, it does highlight the limited utility of the Shields function for predicting transportation of natural sediments. The

Table 1. Summary of Stabilizing and Destabilizing Biotic Influenceon Natural Cohesive Sediment

STABILIZING DESTABILIZING

EPS secretion: Extracellularpolymeric substances bybacteria and microphytobenthosenhances cohesion, promotesflocculation and hence deposition

Blistering: Trapping ofoxygen bubbles in biofilmsincreases buoyancy of thebiofilm to such an extentthat it pulls away fromthe sediment

Sediment compaction:Burrowingmacrofauna increasesediment densityand hence stability

Pelletization: Theformationof faeces and psuedo-faeces can enhanceerodibility

Increased drainage: Burrowand channel formationpromotes dewatering

Grazing: Organismsfeedingon intertidal flats causesphysical disturbance andresuspensionof the sediment andreduces the stabilizinginfluence ofmicrophytobenthos

Network effects: Filamentous biotaramify through the sediment matrixbinding sediment particles together

Burrow cleaning: Somebenthic fauna cleantubes they inhabit inthe sediment giving riseto a localizedbenthic flux

Flow effects: Plants and animal tubesform dense fields that induceskimming flow in the overlyingwater, protecting the sediment bedfrom erosion

Boundary layer effects:Burrows, tubes and track-ing of the sediment surfaceincrease bed roughness andhence near bed turbulenceso enhancing erosion

Biofiltration and biodeposition:Feeding by organisms removessediment from suspension andleads to depositionSediment armouring:Organisms cover thesediment surface, protecting it fromerosionBoundary layer effects: Smoothing ofthe sediment surface reduces interfacestress by decreasing bed roughness andnear bed turbulence

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observations have often been expressed in terms of abiostabili-zation index, SB5@tocrit

~measured)]/@tocrit~Shields)] ~e.g., Man-

zenrieder 1983!. The denominator need not necessarily be theShields critical entrainment stress, and some workers use wintetime ~i.e., minimum biological influence! values or laboratory de-termined values on sterilized sediment.SB typically ranges up tosix or seven for heavily colonized sediment~Tolhurst et al. 1999!.A series of elegant field experiments on both sand and mud in

Fig. 1. Scanning electron micrograph of the surface layers of aestuarine mud showing a benthic diatom~L shape! embedded withinsheets and strands of EPS. The diatom is about 20mm long. Courtesyof Black ~1991!.

Fig. 2. Compilation of threshold entrainment data from the literaturewhere microbial influence is pronounced. The solid line is the threshold bed shear stress for motion of quartz grains in seawater of 10°and salinity 35, and is based upon Shields’ original concepts as prposed by Soulsby~1999!.

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volving removal~albeit probably not complete! of the microbialcomponent using selected biocides has demonstrated andfirmed the importance of microbiota in natural sediments@seeBlack ~1997! and references therein for a review#. Recent work inour laboratory reveals that fine-grained sediments that wcleaned of organic matter/microbes and then reconstituted tosame water content lost their structural integrity and became fl~Tolhurst et al. 2001!. Extraordinary though it may seem, thesstudies show clearly the presence of microorganisms is esseto the continued existence of tidal mudflats, which would othewise be eroded by even intermediate tides. Biology thus beggeomorphology. The study of Daborn~1991! and others, is anexcellent example showing that the intertidal zone of the BayFundy is, in fact, only cohesive as a consequence of its microbpopulation.

Biological processes are intrinsically linked to temperaturand biological influence can at times completely dominate phycal aspects of mud transport. In regions where the phase ofwater coincides with afternoon periods~as in parts of N.W. Eu-rope!, or in riverine situations during low summer flow, extensivcolonization of expansive areas of subaerially exposed sedimis frequently observed~Fig. 3!, sometimes to the extent that diatom biofilms completely obscure the sediment surface~Fig. 4!. Inthis situation, traditional sediment transport approaches are reddant, as the boundary character may be changed from routransitional to smooth and a flow velocity sufficient to peel thmat from the surface is necessary prior to entrainment ofsediment proper.

Bioturbation

The burrowing, particle sorting, tracking, and tube-building ativities ~i.e., bioturbation activities! of benthic fauna, such assnails, crustaceans, bivalves, and polychaetes also affect musediments. These mediate engineering properties of interest sas porosity and permeability over a greater vertical scale relatto EPS influence~for example, the clamMya arenariaburrows toabout 30-cm depth!. Bioturbation also modifies the bed roughnesand therefore changes the friction experienced by overlying floand consequently sediment transport~Nowell et al. 1981!. Wid-dows et al.~1998! determined a significant correlation betweeerosion rate~e! and the density ofCerastoderma edule~a bivalvemollusk! and an index of bioturbation activity ofMacoma bal-thica ~also a bivalve! on the north shore mudflats of the HumbeEstuary, U.K., together with a noticeably poor correlation ofewith measured physical sediment properties. Wood~2000!showed in a follow-up modeling study that destabilization in thcentral belt of the mudflat byMacoma increases local erosionequivalent to doubling the offshore supply of mud. Thus, macrfaunal destabilisation has potentially far-reaching engineerconsequences. The study of Kornman and de Deckere~1998! onthe Heringsplaat of the Ems Dollard estuary also links sedimtransport phenomena to biological processes though in a sowhat more complex manner. A particularly cold winter decimatthe Corophium volutator~a benthic amphipod! population. Thesubsequent reduction of grazing pressure resulted in a bloombenthic diatoms. The diatoms stabilized the sediment as descrand enhanced deposition of fine sediment leading to a dramdrop in suspended sediment concentration in the main chanBy late spring theCorophiumpopulation began to recover and thhigh abundance of food~diatoms! fuelled a population explosion.The diatom biofilm began to degrade and erode due to the grazpressure, which coupled to the burrow cleaning activity of t

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amphipods gave rise to an increase in suspended sediment ccentration in the main channel. This study highlights the linkagebetween environmental forcings, biological processes, and enneering consequences.

In reality, the situation is somewhat more complex and theare appreciably more biological interactions to consider. Filtefeeding by organisms such asMytilus edulis ~the mussel!, forinstance, can remove significant quantities of sediment from toverlying water column. This biodeposition has been proposeda significant long-term influence on sediment dynamics~e.g.,Anderson et al. 1981!. All benthic macrofaunal communities have

Fig. 3. Gray-scale representation, aerial image taken from a Compact Airborne Spectrographic Imager of surface diatom colonizatioon the intertidal mudflats of the Humber Estuary, U.K. Dense colonies are found in the central belt of the intertidal zone~darker areas!,especially flanking large drainage channels. The sea is just shownthe bottom of the image. The mudflat is 3.5–4 km wide.~Imagesupplied by Natural Environment Research Council courtesy of Start White, NEODC, Rutherford Appleton Laboratory, Chiltern, U.K.!

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annual growth patterns and therefore a seasonal bioturbationnal. It should be stressed that most organisms exhibit a numbedifferent effects upon sediment properties and processes, forample snails destabilize sediments through surface ploughinggrazing but at the same time, they secrete adhesive stabiliEPS. Generally though, bacteria and microphytobenthos tenbe sediment stabilizers and benthic fauna sediment destabiliz

Prediction Problem

Paterson~1997! discussed the desire within cohesive sedimetransport for a quantitative predictive relationship relating sement properties~or a subset thereof! to erosion potential. Whereastocrit

for noncohesive, relatively organic-free sediments maydetermined sufficiently accurately by using one of the versionsthe Shields function from knowledge of grain density and sand the fluid properties~Fig. 2!, analytical expressions for cohesive sediments have yet to be developed. Currentlytocrit

~and e!

has to be determined for mud experimentally. The notable exction is the study of Dade et al.~1992! who provided a theoreticalbasis relating the critical entrainment stress to the bed yield st~ty , a rheological property which is a measure of the degreebed structure! for grain sizes within the range 1–25mm, althoughthis relates to geometrically flat beds under simple shear flSince the emergence of a serious interest in cohesive sedimtransport in the 1950s there has been a concerted effort to defunctional relationships between major physical~soil mechanical!sediment properties and bothtocrit

and e. Dry and bulk densityhave received particular attention, however other propertiesclude mineralogy and clay fraction electrochemical propertiwater content, vane shear strength, and elastic and plastic proties~Whitehouse et al. 2000!. More recently, interest has focusseupon the sediment biochemical properties including chloropha, organic and total microbial content, and EPS content~Patersonet al. 2000!. To date, no universal relationship or predominaproperty has been discovered, although chlorophylla content isan increasingly promising candidate. This is the kernel of tpredictability problem, and it is compounded by the fact that m

Fig. 4. A close-up of extensive surface colonization of estuary muThe mud surface is completely blanketed by a biofilm preventerosion. Note the step-like structure of the surface, which indicathat strips of mud have been torn from the surface. This is an entidifferent mode of erosion from that of uncolonized mud. Intencolonizations are a feature of intertidal and riverbank surfaces dusummer months in temperate latitudes.

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sediment properties vary on a number of spatial scales anddepth beneath the sediment surface.

Various approaches can be adopted to parameterize thelogical influence. One option is to parameterize the various blogical effects through commonly used terms in sediment traport formulas~e.g., the bed roughness length,zo! e.g. Sauriauet al. ~1999!, or simply use Manzenrider’s biostabilization indeconcept~which can be either positive or negative in value!. Thiscould be used as an indicator of seasonal changes in sedimstability within numerical transport codes. Alternatively, it is usally possible to establish the form of theto2e relationship fromflume experiments, and consequently it should also be possibexpress the erosion coefficient in terms of sediment bio-physchemical properties. Of necessity this must first involve labotory experimentation, where there is control over the independvariable, rather than direct field experimentation where dircause-effect relationships are difficult to discern. To some degthis approach has been adopted with respect to bed shear strebed density, and yield strength. Galliani et al.~1996! for example,have expressedeM in Eq. ~2! in terms of a consolidation periodT @eM}(Tn)21#, and Ravisangar et al.~2001! found eM

50.0032e0.44* pHfor kaolin clays. A different alternative, follow-

ing the philosophy of Willows et al.~1998!, is to model the sus-pension time-series during erosion and represent the biotic inence as a set of additional model parameters. Finally, simmultivariate correlation and regression analysis~e.g., stepwiseelimination! have been used in the past to produce multiparamepredictive expressions for the critical entrainment stress andsion rate. Various researchers~e.g., Amos et al. 1998; Underwoodand Paterson 1993; Tolhurst 1999! have expressedtocrit

in termsof two or more of water content, chlorophylla content, and bulkor dry density.

The multivariate approach in particular offers a better prospfor prediction of sediment erosion at the seabed in estuaries, sit treats cohesive sediment transport as a multifaceted problThe advantages to a multivariate approach are twofold. First,dination methods~e.g., stepwise elimination, principal componeanalysis, or canonical correlation analysis! can be used to examine the erosion potential among samples with similar and dissilar properties. Ordination techniques, for example, are usedgroup together sites with similar properties~e.g., grain size, or-ganic matter, or chlorophylla!. Erosion measurements can thebe compared both within and among groups. This essentiallythe approach of Widdows et al.~1998!. Second, multivariatemodels can be developed to predict erosion rates. Soil scien~who face similar problems! have adopted this approach, in paticular, however they are conspicuously much farther ahead tus in this respect. Pioneering work to predict soil transport winitiated by Walter H. Wischmeier in the late 1950s. His groudetermined an empirical Soil Erodibility Factor~K!, which is afunction of soil texture, including the amount of fine sandaddition to the usual sand, silt, and clay percentage used toscribe soil texture, organic matter, structure, and permeabilitythe soil profile. This is then used to determine long-term averaannual rates of erosion on field slopes through the Revised Uversal Soil Loss~RUSL! equation~Renard et al. 1997!. An analo-gous approach in the scientific engineering community woulda considerable advance, particularly if data could be gathefrom the world’s major estuaries and rivers. However, the RUequation was derived from data from about 11,000 plot-yearsresearch from 47 locations in 24 states of the United Statesachieve a comparable database in cohesive sediment researc

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take many years. The matter is further complicated by scale. Mtivariate methods are only of use if the measurement of bed perties is on the scale of the erosion process. For example, inent entrainment is a surface phenomenon and consequdensity, aggregate dimensions, chlorophyll, etc. should be msured in the top-most millimeter at most. That said, prolongerosion will scour deeper layers and so sediment property msurements will need to extend down several centimeters or mA solution in this example is fine-~micron! scale property mea-surement in the surface millimeter and then millimeter-scale msurement at depth.

Implications for Hydraulic Engineering

An enhanced and physically more realistic level of predictabifor cohesive sediment erosion has two major implications forgineering studies. The first isknowledge transfer; that is the adop-tion of procedures, methods, protocols, technology, etc., intoengineering field, which may be of use in practical concerns. Tsecond concernsmodelingof cohesive sediment erosion. Eqs.~1!and ~2! form the algorithms used to represent the flux bottoboundary condition in engineering transport models. Mostmerical models explicitly containtocrit

and thus a greater abilityeither to measure this directly or to ascertain it from a subsesediment properties constitutes a major advance. The benthic~e! is perhaps of greater relevance to engineers, especially wtocrit

is frequently exceeded by environmental flows, as it is vartion in e which is linked to larger scale geomorphologic develoment within a particular system. However,e is directly contingentupon tocrit

and consequently quantitative errors here may progate through the computation of net flux~with potentially disas-trous results whered.1, e.g., Fukada and Lick 1980; Gailanet al. 1991!. Although operational models for cohesive sedimetransport in estuaries are available~e.g., TELEMAC, DELFT3D,MIKE3MT, and DIVAST!, these are based largely upon simpfied cmpirical expressions for the bottom boundary conditioBoth Willis and Crookshank~1997! and Uncles and Stephen~2001!, for example, simply parameterizetocrit

through measuresof sediment bulk density. Clearly, improvements can nowmade to the existing empirical equations. A note of caution hoever; while second-generation multiple variable parameterizatwill undoubtedly improve the utility of numerical sediment tranport models, the engineer will for some time be faced with tlarge degree of spatial variability intocrit

ande in both fluvial andestuarine systems. Field measurements may still be requireelse a numerical sensitivity analysis to establish the importancvariability. Further, biology is inherently linked to temperatuand so muddy sediment systems can have a strong season~aswell as interannual! component. Accounting for temporal varability is an additional challenge.

Future Prospects—‘‘Hydro-Bio-Sedimentology’’

There is perhaps a fundamental gulf between environmentalence~biology! and engineering with a deeply embedded perction that it is the nature of scientists to identify problems andengineers to solve them. This historical situation, coupled tvirtually mutually exclusive literature, has done little to fostercross-disciplinary exchange of the relative strengths of each,yet each possesses characteristic strengths that are relevant

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other field. Manson et al.~1999! note that environmental sciencecurricula would be improved by incorporating some fundamentaof environmental engineering and training. Of course, the coverse is also true. The creation by the IAHR in 1996 of an ‘‘echydraulics’’ forum testifies to the increasing recognition of thimportance of biological phenomena within the engineering frternity. Ecohydraulics has been described as a ‘‘nascent field cre-ated by necessity.’’ Nonetheless, few new engineering textbookmake reference to biological processes~aside from sewage/wastewater treatment issues!. Contemporary research fora, for themost part, gloss over this topic~e.g., Mehta and McAnally 2000!and, even now, review articles and papers are published thatnore this area. True interdisciplinarity and convergence is yetcome. Recognition of the linkages and the interplay betwephysical ~hydrodynamic-atmospheric! and biological processeswithin cohesive sediment deposits will form the cornerstonefuture advances in this area. Furthermore, biology affects mother aspects of cohesive sediment transport~dampening of tur-bulence, aggregate dynamics, sediment deposition, boundroughness, sediment porosity! and consequently, there is an enormous new research area that awaits our attention.

Acknowledgments

This work was supported through the following European anUnited Kingdom grants: MAS3-CT98-0166-CLIMEROD,MAS3-CT97-0158-BIOPTIS, and A/S/2000/00513-BENBONERC.

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