Marine Pollution Bulletin 95 (2015) 72–80

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Marine Pollution Bulletin

journal homepage: www.elsevier .com/locate /marpolbul

Field experimental observations of highly graded sediment plumes

http://dx.doi.org/10.1016/j.marpolbul.2015.04.0410025-326X/� 2015 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +45 45251392.E-mail addresses: jahj@mek.dtu.dk (J.H. Jensen), sis@dhigroup.com (S. Saremi),

c.jimenez@cyi.ac.cy (C. Jimenez), l.hadjioannou@enaliaphysis.org.cy (L. Hadjioannou).

Jacob Hjelmager Jensen a,⇑, Sina Saremi b, Carlos Jimenez c,d, Louis Hadjioannou d

a Technical University of Denmark, Nils Koppels Allé, 2800 Kgs. Lyngby, Denmarkb DHI Water & Environment, Agern Allé 11, 2970 Hørsholm, Denmarkc Energy, Environment and Water Research Centre of the Cyprus Institute, Cyprusd Enalia Physis Environmental Research Centre, Nicosia, Cyprus

a r t i c l e i n f o

Article history:Received 16 December 2014Revised 16 April 2015Accepted 18 April 2015Available online 29 April 2015

Keywords:DumpingPoly-disperse plumeField observationsEnvironmental impactsFootageMulti-fractional sediments

a b s t r a c t

A field experiment in the waters off the south-eastern coast of Cyprus was carried out to study near-fieldformation of sediment plumes from dumping. Different loads of sediment were poured into calm andlimpid waters one at the time from just above the sea surface. The associated plumes, gravitating towardsthe seafloor, were filmed simultaneously by four divers situated at different depths in the water column,and facing the plume at different angles. The processes were captured using GoPro-Hero-series cameras.The high-quality underwater footage from near-surface, mid-depth and near-bed positions gives uniqueinsight into the dynamics of the descending plume and near-field dispersion processes, and enables goodunderstanding of flow and sediment transport processes involved from-release-to-deposition of the loadin a non-scaled environment. The high resolution images and footages are available through the linkprovided herein. Observations support the development of a detailed multi-fractional sediment plumemodel.

� 2015 Elsevier Ltd. All rights reserved.

1. Introduction

Dredging plays a vital role in the development of marine infras-tructure and thus on economic growth. Overflow spill, dumping ofdredge spoils, and re-handling and disturbance of fine seabedmaterials are, however, an unavoidable part of dredging works.The release of fine sediments in high concentrations can, in combi-nation with even weak ambient currents, result in sedimentsadvected into far-field waters. Predicting the sedimentation andexcursions of sediment plumes and justifying that they maintainsufficient clearance to sensitive areas, is a common challenge anda primary concern in the environmental impacts assessment(EIA) for operations involving spillage of fines. If not managed, irre-versible damage on the marine habitats may occur (Joyce (1979),Erftemeijer et al. (2012) and Ware et al. (2010)).

Today dredge plume modelling has consolidated itself as a keytool for predicting potential impacts to inform the EIA process;however, with the intensification of marine infrastructure develop-ments and the increased pressure on the marine space, the demandfor dredge plume model accuracy is increasing. The projectedfuture limited marine space is the focus of FP7-Ocean-2011 on

‘‘Multi-use offshore platforms’’ (EU research funding), and herethe potential for multi-purpose activities within the attractive sec-tors of the ocean are addressed. This includes managing dredgingoperations to a level that allow infrastructural developments toco-exist with, e.g., aqua-cultural farming.

Dredge plume modelling, however, remains very challenging.This is partly due to a lack of understanding of many physical sed-iment-related processes occurring in the near field of the dredgingand dumping sites. The interactions between solid phases,between water and solid phases, between settling and turbulenceand the presence of air bubbles in plumes are examples of topicssubject to intense research. A remarkably large variety of govern-ing equations used to describe sediment/water mixtures can befound in the literature (Jiang et al., 1997; Shakibaeinia and Jin,2012; Nguyen et al., 2012 and Saremi, 2014), reflecting that a com-mon perception of the near-field system has not yet convergedwithin the research community. For instance, it is common in sometypes of mathematical models to treat the sediment–water mix-ture as a fluid with varying density; the density corresponding tothe local fractions of water and sediment. This assumes that sedi-ment is fully dissolved locally (analogous to that of brine-solutionsor thermals), and as a consequence, a buoyancy force appears inthe momentum equations. Whereas this method seems robustfor predicting the overall characteristics of initial and entrainmentstages of plunging fine mono-sized sediments in moderate-to-high

J.H. Jensen et al. / Marine Pollution Bulletin 95 (2015) 72–80 73

concentrations, the method is weak in capturing plumes contain-ing multi-fractional sediments (increasingly weak with increas-ingly graded material) and plumes that contain (larger)sediments that pass into the swarm-phase (see Eq. (1)). In Wanget al. (2014) a near-field model was presented for the settling ofmulti-fractional plumes. The model enables a better descriptionof sediment processes by distinguishing water and sediment frac-tions, and, by parametrization, including several inter-phase cou-plings. Mathematical models that fully resolve the settling andthe flow around each sediment grain, is preferable; however, suchmodels, although existing at research level, are too time exhaust-ing for near-field plume modelling (let alone plumes on marinedredging scale). Far-field plumes, on the other hand, tend to be lessconcentrated and often passive (non-buoyant), which reduces thecomplexity. To take advantage of the reduced complexity, separatemodels are typically applied for plumes in the near and far fields.Although the limitations in the description of the far-field plumeprocesses can be accommodated well by using the so-called ‘‘sce-nario approach modelling’’ (Pedersen et al. (2011)), the accuracyof the far-field simulations rely on the near-field processes throughits boundary conditions, and unless accurate, boundary conditionsdebilitates the far-field modelling (Jensen and Saremi (2014)).

Valuable knowledge of processes governing the near-fieldplume formation has been obtained through experiments. In gen-eral, experiments have both (i) supported numerical models as abaseline for validating their skill and (ii) been used to study theplume behavior phenomenologically, and thereby as a lever forimproving models. Valuable laboratory sediment plume experi-ments include that of Jiang et al. (1997), Ruggaber (2000),Winterwerp (2002), and Zhao et al. (2014). In Ruggaber (2000),comprehensive experiments was carried out to study the near-fieldplume behavior for different water contents of the load and forloads inserted into the water with different initial momentums.Because experiments are often conducted in confined basins withlimited vertical and horizontal dimensions, lateral boundaryeffects often hampers the free movement of the plume.Moreover, the limited water depth available in laboratories pre-cludes the study of medium-to-deep water plume behavior dueto the complexities of scaling down sediment sizes. The impactof the bed on the plume-induced pressure field is also an (over-looked) issue of scaling. An estimate of the available water columnover which the plume settles freely without being impacted by thepresence of the bed is given in Appendix A.

In this paper, field experimental observations conducted innear-shore waters off the southern coast of Cyprus, inside thebay of Ayia Napa, March 2014, are presented. The overall purposeof the field experiments is to:

� Provide qualitative understanding of near-field plume evolutionand its dispersion in calm waters and to study phenomenolog-ically, in field conditions, the settling processes, and, particu-larly, the settling of loads of highly graded sediments.� Provide and release high quality footage. This is done by video

recording (i) the plume formation simultaneously from differ-ent positions and angles in the water column in limpid-calmwaters and (ii) using sediments with different sizes, colorsand fall-out depths to enhance the appearance of processes.The recording is carried out with Go-Pro cameras using highframe-rate and high resolution settings.

The Footage is made available so that the dynamics of the sys-tem and the segregation of the various sediment constituents inthe plume can be fully appreciated. Field observations of plumeshave previously been reported in, e.g., Wolanski et al. (1992).Field observations are typically based on acoustic measurementsof the plume providing snapshots of concentrations in vertical

cross-sections and profiles, which convey plume processes less rig-orously. It should be mentioned that satellite imagery analysis,despite only being able to reveal sub-surface plume formations,are becoming a valuable tool in monitoring far-field (passive)plume excursions (Min et al. (2012)), and can, if used in combina-tion with regional plume modelling, provide an alternative instru-ment for quantifying and separating natural and project-inducedturbidities (Chen et al. (2010)).

The observations carried out in the present field experimentsupports a sophisticated multi-fractional sediment plume modelfor the near-field, presently being developed as part of largeresearch programs within dredging and sediment spill, but the foo-tage may have a broader appeal, owing to its scholastic merits, andthe resolution obtained.

2. Materials and methods

The field campaign undertaken is described in the following;including a description of the collected sediment, the sedimentloads and the weather and hydrographic conditions prevailing atthe site during the survey.

2.1. Site description

The experiments were conducted inside the small cove of AyiaNapa, off the southeast coast of Cyprus (at coordinates:34�58055.7500N, 34�101.1400E), which is an area normally well-pro-tected from wave action and tidal currents. The water depth wasapproximately constant at 10 m (±1 m) over the study area, whichcovered a 150 m � 150 m parcel of open sea. The water at the sitewas clear during the campaign period with a visibility of more than50 m, and obtaining footage of plumes on water depths exceeding10 m is possible here with skilled divers. The visibility is an impor-tant facet of the experiments, as any turbidity will impair andsmear the images of the plume. The water column was fully mixedwith temperature and salinity attaining uniform values over thewater depth.

In general, the Mediterranean Sea has weak tidal currents, butwind-induced currents can become strong. Both wind, waves andocean currents remained weak during the three-day field cam-paign (see also Section 2.4).

2.2. Collected plume material

In total, four types of natural sediments were collected; partlyfrom local beaches and partly from the seafloor at the site of theexperiment. Samples of the collected sediment types were sievedand analyzed for granular properties; including the mean grainsize, d50, and the grain sizes d16 and d84. The latter two grain sizescorrespond to the diameters of which, respectively, 16% and 84% ofthe material is finer, and are used to derive the standard deviationof the sediment type. The characteristics of the four types aresummarized in Table 1.

In Bush et al. (2003) the so-called plume fall-out depth wasderived for the case of non-graded sediments. The fall-out depthis the depth below the water surface (or the level of release), atwhich the plume loses its buoyancy. Below this depth the descentis determined by the fall velocity of the isolated grain, ws, and nolonger by the ‘‘buoyancy’’ of the cloud. The phases before and afterthe fall-out depth are referred to as the entrainment and swarmphases, respectively. Bush et al. (2003) described the fall-outdepth, Zf, by using the following fit:

Zf ¼ a1d50

ffiffiffiffiQp

wsd50

� �5=6

; Q ¼ qs � qq

gcV ; a1 ¼ 11� 2 ð1Þ

Table 1Properties and fall-out depth of sand types utilized to compose loads (see Table 2).

Type Description Size, d50 (mm) Standard

deviationffiffiffiffiffiffid84d16

q Relative densityqs/q

Color Fall-out depth,Zf (Eq. (1)) (m)

1 Fine well-sorted beach sand (mostly quartz) 0.19 1.6 2.5 Yellowish 14–212 Coarser well-sorted beach sand (igneous rock) 1.49 1.5 2.8 Black/greyish 4–73 Coarse well-sorted beach sand (igneous rock) 2.35 1.5 2.8 Blackish 3–64 Seafloor sediments (quartz and limestone) 0.34 1.7 2.4 White 8–13

74 J.H. Jensen et al. / Marine Pollution Bulletin 95 (2015) 72–80

where the coefficients are obtained empirically. Moreover, qs, c, q, gand V are the mean grain size, the sediment density, the concentra-tion, the water density, the acceleration of gravity and the initialvolume of the plume (i.e. the load). The functional dependency onthe parameters will not be discussed here; rather the functions(in the light of being fitted) were used as rule-of-thumb to obtainvalues for bulk plume parameters and for the pre-design of loads.The fall-out depths, calculated from Eq. (1), are given in Table 1.The given range in Zf reflects the different initial plume concentra-tions used (see Table 2).

Upon comparing, the four types are seen to have relatively largedifferences in their size, density and color. The sizes of the fourtypes were selected, so that the heavier sediment types (i.e. Type2 and Type 3) entered the swarm phase before reaching the sea-floor (or more precisely before having plunged 66% of the watercolumn), whereas the lighter types (Type 1 and Type 4) remainedin the entrainment phase throughout the water column.

2.3. Plume experiments

The plume experiments were initiated by emptying loads ofsediment mixtures from 30 l buckets. In total, 10 experiments wereprepared methodically, with each bucket being half-filled andemptied by two persons; one tilting the bucket and one stirringit continuously. The continuous stirring ensured that the load(i.e. 15 l) remained fully homogeneous. The release was carriedout from a diving pontoon rigged to the stern of the campaign boat,and sediment was poured into the sea from just above the watersurface (tip of the rim of the inclined bucket made contact withthe water surface). The load release period has not been extracted,but can be derived from the footage. Subsequent experiments werestarted only after the turbidity of the water retained its naturalambience. The payload of the buckets included both unmixed sed-iment types (single-types) as well as payloads composed by a mix-ture of the different sediment types (multi-types). The payloadscomposed by mixed types were included to enhance the perfor-mance of the segregation process when released. Moreover, loadswith different water contents were prepared including a load of

Table 2Specification of single-type and multi-type sediment loads and the approximate weight o

Loads 1 2 3

Concentration of sediment types in loads(% solid volume)

30% ofType 1

20% ofType 1

DrainedType 1

Weight of water (kg) 10.5 12.0 �0Weight of Type 1 (kg) 11.3 7.5 22.9Weight of Type 2 (kg) – –Weight of Type 3 (kg) – –Weight of Type 4 (kg) – –

completely drained sediments. A sketch of the experiment isshown in Fig. 1 and details of the loads are outlined in Table 2.

The plume was recorded at near-surface, mid-depth and near-bed positions using GoPro HD Hero cameras 1, 2 and 3, and unlessotherwise stated, images were produced in 1920 � 1080 resolutionand with a frame-rate between 12 and 30 fps. The free position(still photography) was done using Canon Powershot G12 seriescamera producing (10 MP, 5� Optical Image Stabilized Zoom and2.8 Inch Vari-Angle LCD). Divers operating the cameras were situ-ated within 5 m from the vertical centerline of the plume (asdefined by the sediment drop-off point/line).

2.4. Hydraulic conditions during field campaign

To demonstrate the benign currents that prevailed during thecampaign period, an experiment with a load containing a mixtureof two distinct sediment fractions (Load 4, containing Type 2 andType 3 sediments – see Table 2) with different fall velocities wasemptied into the sea, and the deposition of the sediments on theseafloor was observed. Both sediment types were black in colorand relatively coarse; the latter to ensure small fall-out heights,so that the plume entered the swarm phase within the top 66%of the water column, i.e. prior to the depth, at which the seabedwill affect a plume in the entrainment stage (Table 1). As shownin Fig. 2, the footprint of the deposition was homogenous andnearly circular, indicating that currents were indeed benign. Dueto the difference in their fall velocity and thus in their retentiontime, even a weak current (or a current >10 cm/s) would be ableto segregate the two distinct sediment fractions, break up theaxial-symmetry of the descending plume and cause a visibly elon-gated deposition pattern. The calm waters were confirmed by thecaptain of the boat, who had no problems maintaining a fixedposition.

3. Results and discussions

Before proceeding to the presentation of the plumes generatedfrom multi-type loads, observations of the plume evolution

f each type.

4 5/6 7 8 9 10

10% ofType 1

10% ofType 1

10% ofType 1

10% ofType 1

10% ofType 2

10% ofType 2

10% ofType 2

10% ofType 3

20% ofType 3

10% ofType 3

10% ofType 3

10% ofType 4

12.0 12.0 12.0 12.0 10.5 12.0– 3.8 – 3.8 3.8 3.84.2 – – 4.2 4.2 –4.2 – 8.4 – 4.2 4.2– 3.6 – – – –

10m

Near-surface

Near-bed

Free

Mid-depth

Drop-off centerline

Fig. 1. Sketch of field experiment and location of cameras (divers).

J.H. Jensen et al. / Marine Pollution Bulletin 95 (2015) 72–80 75

following the release of the single-type loads (Load 1 and Load 2)are described. The plumes generated from the single-type loads(of the lighter sediment) remains buoyant, and as such display sim-ilar characteristics as that of buoyancy-driven thermal or brine-so-lution plumes. These two tests provide a reference for theremaining experiments, and, in particular, they illustrate how themulti-fractional or dewatered loads affect the settling of the lightersediments (the reference cases). To summarize observations and todistinguish the role of individual tests, the experimental log is pro-vided in Appendix B. In the context of sediment plumes, the word‘‘buoyancy’’ is understood as the bulk motion of a cluster of waterand sediment (which is faster than the settling of isolated sedi-ments), where the bulk motion is generated from the collectiveeffect of the individual sediments.

3.1. Observations of descending single-type loads

3.1.1. Initial plume formationIn the period immediately after releasing the load, and during

the period where sediments were sourced through the sea surface,the initial high concentration of the load caused it to surge rapidlydownwards. This formed the sediment plume, which initially wascharacterized by a pronounced three-dimensionality. The initialplume formation was captured by the divers located near thewater surface using the high frame-rate camera settings (framerate for these shots was set to 30 fps). In Fig. 3 four snapshots,taken immediately after the release of Load 2, are presented. Thesnapshots were taken over a period of less than 4 s, and showthe rapid descent and formation of the plume. The time lapse

Fig. 2. Circular footprint on rippled seabed from deposition following release of abi-disperse mixture of black sediments (Load 4). Taken by the (free) camera.

between the first-and-second snapshots as well as the second-and-third snapshot was 750 ms. The time lapse between thethird-and-fourth snapshots was 2 s.

A closer look at the interface of the plume in this immature anddynamic, stage of the plume formation reveals the emergence ofsmaller internal trails (see Fig. 3). The rugosity of the plume inter-face is an indication of the length scales of these internal trails. Thelength scales of the trails are seen to change from snapshot tosnapshot (which can also be appreciated in the footage). It is clearthat the trails (or rugosity of the plume surface) grow in size withtime. It can be inferred, that this growth is a consequence of a bat-tle, taking place between the different trails in their initial plungedownwards, and their continuous dilution. The faster of these con-vective structures eventually dominates locally, and takes over thedownward penetration. The surface of the plume rapidly becomesless rugose in this engulfing process, and the well-known morecoherent mushroom shape starts to form. The entrainment ofwater into the plume increases with time during this stage, and,as the mushroom-shape develops, the entrainment process orga-nizes itself on the back of the plume.

Understanding the initial stage of the plume formation from apoint of view of system stability seems appropriate here. For statis-tically homogenous mixtures, similar emergent structures wereobserved and studied analytically using perturbation methods by,e.g., Batchelor and Janse Van Renseburg (1986). In particular,Batchelor and Janse Van Renseburg (1986) observed sediment clus-ters plunging within such homogeneous mixtures. In the presentcase, the system is quite different, being perturbed by a highlybuoyant blob (i.e. the load), and trails continues to grow through-out the settling process as the blob dilutes. Nevertheless, the pro-cess of selective amplification seems to be at play again, and onecoherent trail eventually emerges. The emergence of the coherenttrail (the mushroom) marks the onset of the next stage in the des-cent of the plume.

3.1.2. Entrainment stageThe sediment plume developing from the release of Load 8 is

shown in Fig. 4. These snapshots show typical plume formationsduring the entrainment stage. Both the mushroom and the so-called trailing stem (Ruggaber (2000)), lagging behind the mainplume, are clearly observed. Whereas the trailing stem displays ahigher degree of three-dimensionality being directly exposed tothe lee-wake turbulence, the mushroom part of the plume retainsnearly axial-symmetry and, from a simple imagery analysis, thesize (i.e., its width) is seen to grow nearly linearly (as also com-monly accepted).

In this stage, the plume displaces the ambient water as it grav-itates downward, generating an upward (compensating) returnflow field around it. The water separates on the back of the plume,and is sucked into the core of the plume by the induced pressuredifferences. This entrainment of water causes a rapid expansionof the plume, and, as part of this process, the plume dilutes. Thedilution of the plume reduces its density, and causes its decelera-tion. From an environmental perspective, this stage of the plumedescent is critical, because (i) the plume increases in size, (ii) sed-iments in the trailing stem become detached from the buoyantmain cloud, and (iii) the retention time of the sediments increases(the retention time is the period over which, the released sedimentremain in suspension). In other words, the sediments become morepassive, and during this stage, the sediment is more susceptible tothe ambient flow field, being the agent for feeding far-field plumes.

For the pure Type 1 sediment release (Load 2), which does notenter the swarm phase over the water depth, D, the plume frontwas observed to reach the seabed in approximately 30–40 s fromthe onset of release. This period obviously sets a lower estimateof the retention time.

Fig. 3. Initial development of plume (Load 2).

Fig. 4. Plume in the entrainment stage (Load 8). A and B are taken by the (free) camera.

76 J.H. Jensen et al. / Marine Pollution Bulletin 95 (2015) 72–80

3.1.3. Seabed impactThe fall-out depth for the different sediment types used in the

experiment is both smaller and larger than the water depth(Table 1). This implies that the plumes, containing a mixture ofsediment types, will impact, and therefore deposit, on the seabeddifferently and the deposition footprint of the fractions revealedby the black and yellow colors of the sediments. For Type 2 andType 3 sediments, which are characterized by a relative smallfall-out depth (i.e. Zf < 2/3 D), the deposition process is not affectedby the buoyancy of the plume, and sediment settles-out as shownin Fig. 2. The observed deposition process and patterns for sedi-ments of Type 1 and partly of Type 4 is different. In this case, thetypes have a relatively large fall-out depth (i.e. Zf > D), and theplume remains buoyant throughout the water column. When theplume encounters the seafloor, it deflects, and continues movinghorizontally as a dense bottom current driven by its momentum(Nakatsuji et al. (1990)). The movement of the plume, in this stage,is radial, and often a ring-shaped plume is formed. The ring-shapedplume slowly dilutes as the radius increases, and the sedimentsettles out. The area of deposition is thus larger (but less concen-trated) than that associated with Type 2 and Type 3 sediments.An example of such a ring-shaped plume was captured by thedivers for Load 1 and Load 2 (Fig. 5).

For plumes containing a mixture of the sediment types, a segre-gation of the black and yellow sediments can be observed in thedeposits on the seafloor (e.g. Load 8, Load 9 and Load 10). Theheavier constituents (black sediment) of the plume settle first,whereas lighter constituent (yellow sediments) remains in suspen-sion for prolonged time. The associated deposition pattern is circu-lar with the increasing fine material being deposited withincreasing distance from the origo of the initial seabed impact.The well-known double-peaked deposition, reported in the

experiments by Nakatsuji et al. (1990), is too weak to be observed(in the experiments with mixed sediments) as also furtherexplained in Section 3.3.

3.2. Plume formation with dewatered loads

The sediment plumes forming from dumping of dewatered mix-tures (i.e., Load 3) are very different from those of the watered mix-tures. The plume formation from release of Load 3 is presented inFig. 6. While lumps of coherent sediment were seen to plunge fasttowards the seafloor, air initially locked between the sediment wasseen to return to the water surface. For this type of load, the char-acteristic mushroom-shape did not evolve (at least not over thewater depths considered). This variant was treated in Ruggaber(2000) and Zhao et al. (2014). The case is of particular relevanceto plumes generated from the disposal of sediment spoils releasedfrom a split-hull barge. In this case, the loading of spoils is carriedout, while an excess watery mixture overflows, and the fullyloaded barge therefore often contains a partially dewatered orslurry-like load.

The observed evolution of the lumps and the characteristics ofthe plume as shown in Fig. 6 have a remarkable resemblance tothe plume snapshots displayed in Zhao et al. (2014) where smallparcels of dry glass beads were released into the water column.Although only the case of completely drained loads was investi-gated in Zhao et al. (2014), the present experiment shows thatlump-formation can take place for dewatered cases as well.

Although observed plume processes, following the release ofwatered payloads, are very different from the plume processesassociated with dewatered payloads, the overall retention timeswere comparable. The lumps surged relatively fast towards thebed, however, the air initially locked between the sediment, but

Fig. 5. Radial dispersion of plume upon impact with seafloor (Load 1). Images are color enhanced to accentuate the plume. (For interpretation of the references to color in thisfigure legend, the reader is referred to the web version of this article.)

Fig. 6. Sediment plume with drained load (Load 3). Note the air bubbles at the back of the plume and the disorganized surge of the plume front.

J.H. Jensen et al. / Marine Pollution Bulletin 95 (2015) 72–80 77

eventually returning to the surface, dispersed finer fractions in theupper part of the plume relatively more. The air-induced disper-sion as well as the lack of buoyancy of the non-lumped sedimentdid not materialize in the desired reduction of the overall retentiontime.

3.3. Plume formation with multi-type loads

The experiments with sediment of mixed types (Load 5, Load 6,Load 8, Load 9 and Load 10) were carried out to study in detail theprocess of sediment segregation occurring during the descent ofthe loads. At some point after release of a given mixed load, sedi-ments of Type 2 and Type 3 were observed shooting out fromthe front of the plume. At this stage, the plume front was takenover by the coarser constituents of the load. The segregation ofthe distinct sediment types was, however never completed. Thefine sediments (Type 1), now comprising the upper part of theplume, were advected downwards with the interstitial flowinduced by the drag of the heavy particles. The main downwardadvection of the fines took place inside, at least one, but moreoften, several corridors emerging within the core of the plume.The return flow generated from the displacement of water by thevolume of the gravitating plume was, however, seen to erode theperimeter of the upper part of the plume. The return flow thuscaused an upward transport of fines along the skirt of the plume.To capture these segregation processes, the divers used high reso-lution settings (i.e. 4096 � 2160 pixels at 12 fps).

Fig. 7 shows snapshots of plumes with mixed sediments. Thedynamics of the fine sediments are nicely visualized in the mixedplume footage, and it is clear, that the finer constituents of theplume are governed by the flow induced by the settling coarsersediments. The plumes shown in the upper snapshots are capturedby the diver located at mid-depth following the release of Load 9.The lower snapshot is captured by the mid-depth diver, and showsthe plume following the release of Load 10. The heavy (black)

sediments in Load 9 are seen to shoot out from the bottom of theplume (Fig. 7A) as detailed above, and a bowl-shaped front(Fig. 7B) is formed with a trailing tail consisting of mostly fine (yel-low) sediments (Type 1). The plume generated from the release ofLoad 10 (Fig. 7C) clearly shows the impact of the segregation on thedistribution of fines, i.e. how the finer (yellow) fractions (Type 1)are dragged down in the interior of the plume inside one main cor-ridor (see Fig. 7).

The initial volume of the 15 l loads is significantly increased bythe dispersion and dilution associated with the entrainment pro-cess, and the diameter of the plume at the time of the snapshotsshown in Fig. 7 (approximately at mid-depth) is 3.0–3.5 m. Thecapacity of typical split-hull barges are in the range of 600–5000 m3. The volume of a load dumped from a barge is thus easilya factor 105 larger, and the associated size and potential range ofsuch a plume can be envisaged.

Fig. 8 depicts snapshots of the plume following the release ofLoad 8. The process of finer sediment being dragged by the leadingcoarser fraction towards the seafloor is captured.

The footprints of the black and yellow sediment depositionscontained in Load 10 can be compared to that of Load 7, as canLoad 9 be compared to that of Load 4; the latter is shown inFig. 2. The sets of loads have the same amount of black sediments,but Load 7 and Load 4 did not contain the fine yellow sediments.The final diameter of the deposition of black sediments fromLoad 9 and Load 10 was only slightly larger than that associatedwith Load 4 and Load 7, respectively, indicating that the presenceof finer sediments is not significantly dispersing or affecting thecoarser sediments while descending. On the other hand, the finersediments are affected by the coarser sediments as also discussedabove. The finer sediments are stretched out, and thus diluted, bythe coarse sediments, and to a degree such that the plume loses itsintegrity. The deposition thus occurs without the radial movement.The radial movement was observed with loads containing the finesediments only (i.e., single-type load), and its occurrence is

Fig. 7. Plume from release of Load 9 in (A and B) and Load 10 (C). Note the black sediment shooting out through the plume front (A), the corridors of yellow sediment draggeddownwards (B), and the yellow sediment being dragged down by the coarser sediments inside one main internal corridor (C). (For interpretation of the references to color inthis figure legend, the reader is referred to the web version of this article.)

Fig. 8. Plume from release of Load 8: Mid-depth plume formation (A), and plume prior to seabed impact (B). Taken by the (free) camera.

78 J.H. Jensen et al. / Marine Pollution Bulletin 95 (2015) 72–80

pre-conditioned by buoyancy (i.e. the plume needs to be in thestage of entrainment). Once the heavier sediments are settled,the remaining dilute plume displays little buoyancy. The impactof the coarse material on the descent of the fines became evident,when clocking the settling time: As mentioned earlier, the front ofthe plume initiated by the single-type load (containing fines only),reached the seafloor after 30–40 s, whereas the fine part of themixed load, i.e. the load where the fine sediments are mixed upwith Type 2 and Type 3 sediments, started to reach the seaflooronly after more than 60 s. Although the dragging, in isolation, isa mechanism that reduces the retention time of fines, it cannotcompensate for the destruction of the buoyancy, and the loitering

of fine sediments. The overall retention time of the fines, in plumesof mixed loads, were found to increase compared to that of single-type loads.

4. Conclusion

Descending plumes in open-calm-limpid and relatively deepwaters have been studied in the field experimentally, and high res-olution high frame-rate footage from four positions in the watercolumn has been obtained simultaneously. The full underwaterfootage is available through the link provided in ‘‘References’’.The footage provides an unprecedented visual documentation of

Table 3Observation log.

Load 1 Reference case for the mixed load plumes: A single load plume (ofType 1) which develops into its full entrainment stage. Plumeremains in this stage throughout (i.e., stayed buoyant and didn’tenter the swarming phase). Pronounced ring-shaped plumeobserved after seabed impact

Load 2 Reference case for the mixed load plumes: A single load plume (asLoad 1, but with lower initial concentration). Plume onlydisplayed small changes from that of Load 1

Load 3 Coherent clusters of sediment (Type 1) settles and breaks up, butis only dissolved into its granular components slowly. Thebuoyant plume characterizing the entrainment stage is notdeveloped

Load 4 Plume forming from the load containing two heavy sedimenttypes (of Type 2 and 3), which enters the swarm phase (andwithout radial movement). Deposition was circular (indicating noambient currents)

Load 5 Plume forming from the load containing two sediment types (ofType 1 and 4), which segregate, and where the buoyancy of thefiner type of the plume was partially (and less than for Load 8,Load 9 and Load 10) diminished by the settling of the coarser type

Load 6

Load 7 A single load plume (of Type 3) which enters the swarm phase,and with a deposition footprint, which was similar to thedeposition footprint of Type 3 sediments of Load 10

Load 8 Plume forming from the load containing two sediment types (ofType 1 and 2), where the Type 2 sediment enters the swarmphase. The buoyancy of the Type 1 sediment plume was brokendown. One main corridor (in which fine sediments are pulleddown by coarser sediments) was observed

J.H. Jensen et al. / Marine Pollution Bulletin 95 (2015) 72–80 79

the plume evolution for loads of different sediment compositionand concentration.

The experiments with loads containing multi-type sedimentsshow that the coarser constituents both drag the part of the plumecomposed of the finer constituents faster towards the seabedwithin corridors, and transport fine sediment, eroded on theperimeter of the upper part of the plume, upwards with the returnflow. However, in this process the part of the plume remaining insuspension, after the precipitation of the coarse sediments, isdeprived of its buoyancy. It is thus observed that, by adding coarsermaterial to the load, the overall retention time is prolonged. Thefield experiments show that the plumes from the release of drainedloads and releases of loads containing multi-type sediments (i.e.including coarser types) are complex, and are associated with anincrease in dispersibility, when compared to the plumes of thewatered and single-type loads.

The series of experiments provide a challenging baseline for aqualitative validation of the detailed multi-fractional sedimentplume model presently being developed. The development is partof a large research initiative focusing on dredging and spill.Improved predictions from the near-field plume model can be usedto more accurately determine the amount of ‘‘passive’’ sediment,which defines the source for far-field plume models. Managingdredging operations, and predicting ecological impacts of worksinvolving fine material releases relies on accurate process-basedmodels, and the demand for accuracy will increase with the inten-sification of marine space usage.

Load 9 Load containing three types of sediment (Type 1, 2 and 3).Deposition of Type 2 is similar to that observed for Load 4. Thebuoyancy of the Type 1 sediment plume was broken down.Several smaller corridors, in which fine sediments was draggeddownward, was observed. A plume with a vertical segregation ofsediments from the multi-loads was observed

Load 10 Load containing two types of sediment (Type 1 and 3).

Acknowledgement

Marina Evriviadou, Antonis Petrou, Marinos Eliades and VasilisAntreou are acknowledged for laborious granular analysis anddelivering skillful underwater recordings.

Segregation more pronounced then in Load 8. Deposition of Type3 is same as for Load 7. Again, buoyancy of the Type 1 sedimentplume was broken down. One main corridor was observed (as inLoad 8)

Appendix A

One may assess the impacts of the bed on the free gravitation ofplumes in the entrainment stage by consulting the work of Brenner(1961), in which the speed of rigid spheres settling towards a solidhorizontal surface is derived. It was found that the settling of thesphere is hindered when its distance from the wall becomes smal-ler than the body diameter, which leads to a deceleration of thesphere close to the wall. The deceleration occurs when the pres-sure field induced around the settling sphere encounters the bed.If the implications of the bed on the near-bed rigid sphere settlingis transferred to a buoyant plume, then it can be inferred that theplume, will respond to the presence of the bed by (further) decel-eration when within roughly one-plume width from the bed.Although this seems a rough analogy, since water within the plume(although carried downwards with the settling sediment) isreplenished; i.e., the plume is permeable and ventilating, the pres-sure field induced around a sediment plume (in the entrainmentstage) is quite similar to that of the sphere. In both cases, the pres-sure decreases exponentially away from the settling mass (andover the length scale of the settling object), which for the plumecorresponds to its width, b. In the entrainment stage, the plumeis known to grow linearly with depth as it plunges; i.e.,b � a2(D � z), where a2 = 0.5 ± 0.1, D is the water depth and z isthe vertical coordinate (attached to seabed). Consequently, a buoy-ant plume will reach a critical depth, at which it no longer settlesand deforms freely. By using the empirical relation for b, the criticaldepth can be estimated to be D/(1 + a2) � 2/3 D, which implies,that the plume is unhindered over the top 66% of the water columnonly.

Appendix B

See Table 3.

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