Experiments in Wedge-Shaped Deep Sea Sedimentary Deposits Part I - Documentation of Flow - JSR, 2009

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Journal of Sedimentary Research, 2009, v. 79, 593–607

Research Article

DOI: 10.2110/jsr.2009.064

EXPERIMENTS ON WEDGE-SHAPED DEEP SEA SEDIMENTARY DEPOSITS IN MINIBASINS AND/OR ONCHANNEL LEVEES EMPLACED BY TURBIDITY CURRENTS. PART I. DOCUMENTATION OF THE FLOW

OCTAVIO E. SEQUEIROS,*1 BENOIT SPINEWINE,11,2 MARCELO H. GARCIA,1 RICK T. BEAUBOUEF,3{ TAO SUN,3 AND

GARY PARKER4

1Ven Te Chow Hydrosystems Laboratory, Department of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign, 205 North Mathews Avenue,

Urbana, Illinois 61801, U.S.A.

e-mail: [email protected] 2Fonds National de Recherche Scientifique, Rue d’Egmont 5, B-1000 Bruxelles, Belgium

3ExxonMobil Exploration Co., Houston, Texas 77252, U.S.A.

4Department of Civil and Environmental Engineering and Department of Geology, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, U.S.A.

ABSTRACT: Decelerating turbidity currents commonly emplace sedimentary wedges. Here ‘‘sedimentary wedge’’ is used as ageneric term for a sediment deposit, the thickness of which gradually decreases in the downdip direction. Examples of sedimentary wedges relevant to the research reported here include a) deposits in submarine minibasins, b) deposits on zones of lower slopes of stepped profiles, and c) deposits on the levees of submarine channels. In the present work, a genericconfiguration is used to study the flows that emplace sedimentary wedges. These flows consisted of a succession of sustainedsaline density underflows, which were used as surrogates for turbidity currents driven by fine-grained material (mud) that doesnot easily settle out. Although the flow naturally decelerated in the downstream direction, deceleration was ensured by thepresence of a barrier to the flow at the downstream end of the study reach. The density underflows carried a load of lightweightplastic particles, from which the depositional wedge was constructed. The experiments were not designed to model any specificfield configuration. This notwithstanding, the experimental configuration provides an analog for a) decelerating flows intoconfined minibasins, as well as b) levee-constructing overflows from submarine channels. This paper documents the nature of

the flows that emplaced the wedge. The sedimentary wedge itself is documented in a companion paper.

INTRODUCTION

Deep-sea turbidity currents flowing into net-depositional environments

often emplace wedge-shaped sedimentary deposits, i.e., deposits which

thin in the downdip direction. Two examples of settings for such deposits

are those in submarine minibasins (e.g., Winker 1996; Badalini et al. 2000;

Beaubouef and Friedman 2000) and those in zones of lower slope on

stepped profiles (e.g., Prather and Pirmez 2003; Prather 2003). These

deposits have often been described as ‘‘aprons’’ (Prather et al. 1998;

O’Byrne et al. 1999; Beaubouef and Friedmann 2000; Booth et al. 2002;

Prather 2000, 2003; Adeogba et al. 2005). A third example consists of theconstructional levees of submarine channels. (e.g., Pirmez 1994;

Nakajima and Satoh 2001). The goal of the present research is the use

of laboratory experiments to provide insight into the nature of the flows

that emplace such deposits. It should be kept in mind that the flows

described herein carried a load of sediment and emplaced wedge-shaped

sedimentary deposits. The deposits themselves are documented and

interpreted in a companion paper, Spinewine et al. (2009).

Minibasins, for example, frequently enclose layered deposits of

sediment dominated by clay–silt or sand emplaced by turbidity currents.

The initial stages by which turbidity currents arrive to and deposit in a

minibasin can by summarized as follows: (1) the current flows down a

relatively steep and laterally confined canyon, (2) the current debouches

into a minibasin, a topographic depression of the seafloor which is usually

wider than the canyon and has a milder bed slope, (3) the current is

ponded and deposits sediment because of a substantial diminishment of

its sediment transport capacity at the entrance and within the minibasin

itself, and (4) eventually the current overflows at the downstream end of the basin. This overflow might occur during a single event, but it more

likely requires many events to substantially fill the basin (Badalini et al.

2000) This process is called fill-and-spill (Winker 1996). The result of the

above process is the emplacement, within any given basin, of a wedge-

shaped deposit that thins in the downstream direction (Beaubouef and

Friedman 2000; Toniolo et al. 2006a).

Levees bounding submarine channels also consist of layered deposits of

mud and sand, but with mud usually dominating (e.g., Pirmez 1994;

Nakajima and Satoh 2001; Migeon et al. 2004). These levees are

constructed by a process known as flow stripping (e.g., Migeon et al.

2004), according to which the channelized turbidity current overflows its

channel. These overflow currents are net-depositional over a wide range

of conditions, and so emplace wedge-shaped deposits on the levees that

act to steepen them over time (e.g., Migeon et al. 2000). As opposed to

* Present Address: Shell International Exploration and Production B.V.,

Kessler Park 1, Rijswijk 2288 GS, The Netherlands

{ Present Address: Hess Corporation, Houston, Texas 77002, U.S.A.

1 Present Address: Department of Civil and Environmental Engineering, Uni-

versite catholique de Louvain, Place du Levant 1, 1348 Louvain-la-Neuve, Belgium

Copyright E 2009, SEPM (Society for Sedimentary Geology) 1527-1404/09/079-593/$03.00



minibasins, whereby the depositional process is limited by the cutting of achannel due to overflow at the downstream end, deposition on levees is

limited by channel avulsion. One mechanism for driving this avulsionconsists of steepening the levees to the point that the overflow becomes

net erosional (e.g., Fildani et al. 2006).

The experiments reported here were guided by the conceptualframework outlined in Figure 1. A sustained sediment-laden turbiditycurrent flows into a zone of net deposition. The flow entering this zone

represents an overflow from an upstream basin or a leveed channel. This

flow attains a critical densimetric Froude number Frd 5 1 (see

description of Frd below) at the point of overflow and is supercritical(Frd . 1) farther downstream. The barrier at the downstream end of the

reach, however, acts to force a hydraulic jump to subcritical flow, forwhich Frd , 1. As the flow proceeds downstream toward the barrier,

deposition from the current emplaces a sedimentary wedge. This wedgecan both prograde and aggrade in the downstream direction. This

framework is not intended to be a precise model of any specific field-scaleflow. It is, rather, intended to be of sufficient generality to capture thesalient features of many (but not all) wedge-shaped submarine deposits.

Before proceeding, it is useful to define the densimetric Froude numberof a turbidity current, or for that matter any density underflow. Let U

denote the layer-averaged velocity of the current, h denote the thicknessof the current, g denote gravitational acceleration and F e denote the layer-averaged excess density of the current, given as

F e ~ Drra ð1Þ

In the above equation, ra is the density of the ambient water and Dr isgiven as

Dr~ r f { ra ð2Þ

where r f is the layer-averaged density of the current. The densimetric

Froude number Frd is then defined as

Frd ~ U

ffiffiffiffiffiffiffiffiffiffi gh

Dr

r f r

ð3Þ

No single experimental configuration is sufficient to describe allpossible wedge-shaped deposits emplaced by turbidity currents. Forexample, although features analogous to the downstream barrier shown

in Figure 1 may be present in the case of channel levees (e.g., the levee of

an adjacent channel), a barrier does not need to be present. The barrier is,however, necessary to describe many minibasins which have not yetevolved an outlet, and so has been included in the present configuration.

Likewise, the flow into depositional zones on stepped profiles and

minibasins need not have an overflow point where the flow reaches adensimetric Froude number of unity (e.g., Toniolo et al. 2006b). This isparticularly true when the depositional zone is fed by an incised channel.

In the case of spillover from one minibasin to another across a ridgewhich is either not yet channelized or poorly channelized, however,

Froude-critical flow can be expected at the overflow crest (e.g., Toniolo et

al. 2006b; Lamb et al. 2006). Likewise, overflow from a channelized,Froude-subcritical turbidity current onto a high levee can be expected to

be Froude-critical at the levee crest (Pirmez and Imran 2003).

The flows described here are sustained rather than pulse-like. Pulse-likeflows can likely contribute to the emplacement of wedge-shaped deposits

(Badalini et al. 2000). Justification for the role of sustained flows inemplacing deposits in minibasins and on levees is given in, e.g., Lamb etal. (2004) and Pirmez and Imran (2003).

The present paper is structured as follows. First the experimental setup

and procedure of the tests are explained. The evolution of the flow and itsmost important patterns are then described in detail. Finally a section

dedicated to computing and analyzing the densimetric Froude numberand its implications serves as an introduction to the companion paper

(Spinewine et al. 2009), where the wedge-shaped deposits emplaced by theflows are described in detail.

EXPERIMENTAL SETUP

As noted above, field turbidity currents were modeled in terms of sediment-laden saline currents, with the salt modeling a fine mud and the

sediment modeling material that can move both as bedload and suspendedload. The experiments were carried out at the University of Illinois Ven Te

Chow Hydrosystems Laboratory in a flume 15 m long, 0.45 m wide, and1.4 m deep, and with a bottom slope equal to 5% (Figure 2). The tank was

initially filled with fresh water. Saline water was fed into the flume from a4 m3 mixing tank by means of a pump. Suspended sediment was added tothe flow from a feeding silo before it entered the study reach. The discharge

rate was measured by a magnetic flowmeter (McCrometerH Serial

No. 96061675) having a capacity of up to 20 l/s. A diffuser ensured thatthe flow was injected uniformly along the flume width at its upstream end.A gate located just downstream of the diffuser and the sediment feeder

guided the mixture to the bottom of the flume. At the downstream end of the study reach was an obstruction of variable height and a 45 u adverse

slope located 9 m downstream of the diffuser. The barrier helped promotethe emplacement of the wedge-shaped deposit described in Figure 1. An

inset damping tank was located 6 m downstream of the obstruction. Theflow spilled over the invert of the damping tank and was then pumped out

to prevent reflection. A supply of fresh water from the top of the dampingtank in conjunction with an overflow device guaranteed the maintenance of

a constant level of fresh water in the flume.

Several rakes of siphons located at different positions along the channel

were used to obtain flow samples at various elevations. These samples

were used to compute salinity profiles. Figure 3 shows a typical rake of siphons and its characteristic dimensions. The siphons were built withaluminum tubes of 5 mm external diameter and 3 mm internal diameter.

The collected samples were weighed and dried in an oven to measure thesalt content. A PanametricsH transducer echosounder (C306, 2.25 MHz/

0.5 inches in diameter, 15 mW) mounted on a sliding platform above theflume was used to measure profiles of bed elevation along the channel

centerline after the tests. Vertical profiles of streamwise velocity weretaken with a SontekH 10 MHz ADV probe (acoustic Doppler velocim-eter). The ADV was connected to a VelmexH vertical positioning electric

slide. The ADV and its electric arm were mounted on the aforementioned

horizontal sliding platform, so as to allow accurate vertical displacements(, 0.03 mm). At the end of the tests, the three-dimensional bedbathymetry was measured with the help of a laser scanning system

composed of a NikonH D200 digital camera and a LasirisH laser (40 mW

power, 660 nm wavelength). The experiments were also recorded from thesidewalls using four CanonH DM-GL2 digital video cameras. Dye wasperiodically added to the current to improve visualization and

understanding of flow patterns. Water temperature both in the flumeand in the mixing tank were measured before each test.

FIG. 1.—Schematic diagram illustrating the modeled configuration for densityunderflows as they emplace wedge-shaped sedimentary deposits.

594 O.E. SEQUEIROS ET AL. J S R



The dense underflows modeled here transported sediment and emplaceda wedge-shaped sedimentary deposit, which then affected the flow patterns.Characteristic diameters of the model sediment, including geometric meansize d g (206 mm), geometricstandard deviations g , andthe sizes d 16, d 50, andd 84 are shown in Table 1. The material was a cohesionless plastic with aspecific gravity of 1.53, i.e., significantly lighter than silica. The median size

was approximately 210 mm, but particles as fine as 100 mm and as coarse as250 mm were found in the mixture. The grain size distribution of thesediment is shown in Figure 4. More details about the deposits can befound in the companion paper, Spinewine et al. (2009).

Two sets of experiments were run, corresponding to two different flow

and boundary conditions. A total of 24 tests, or flow events, were run forthe first set, and 33 tests for the second set. The two sets differed in termsof flow and sediment discharge, initial concentration, and obstructionheight. In the case of the first set, the flow discharge at the nozzle was 3 l/

s, the sediment feed rate was 640 g/min, the salinity concentration at themixing tank was 38 g/l, and the obstruction height was 32.5 cm. Theaverage temperature of the incoming dense flow was 13.2u C, with astandard deviation of 0.8u C. The average temperature of the ambientwater in the flume was 16.1u C, with a standard deviation of 2.4u C. Themaximum fractional density excess due to temperature for the first set of

tests can be estimated as (ri 2 ra)/ra < 9 3 1024, where ri is the

density of the incoming water (with the salt excluded) computed for atemperature of 12.6u C and ra is the density of the ambient waterestimated for a temperature of 18.5u C. This is negligible compared to thefractional density excess due to salinity, which was equal to 0.027.

For the second set of experiments the flow discharge at the nozzle wasset to 2 l/s, the sediment feed rate was 161 g/min, the salinity

concentration at the mixing tank was 49 g/l, and the obstruction heightwas 41.9 cm. The average temperature of the incoming dense flow was13.9u C with a standard deviation of 0.6u C. The average temperature of the ambient water in the flume was 16.0u C with a standard deviation of 1.7u C. The maximum fractional density excess due to temperature for thesecond set of tests was < 7 3 1024. Again, this is value is not significantcompared to the fractional density excess due to salinity, which was 0.034.

Insofar as stratification due to temperature was two orders of magnitudesmaller than that due to salinity, temperature effects are considerednegligible in this study. Table 2 summarizes the characteristics of bothsets of experiments. In that table, H b denotes the height of the barrierforming the downstream end of the model minibasin, Q0 is the upstreamdischarge of saline water, QS 0 is the upstream sediment feed rate, C 0 is the

concentration of salt at the inflow point, Dr/r is the fractional densityexcess of the inflowing saline water relative to the ambient fresh water in

the flume, T inflow is the temperature of the inflowing saline water, andT flume is the temperature of the ambient fresh water in the flume. Also

given in the table is the duration of each test, or flow event, which was20 minutes for set 1 and 30 minutes for set 2.

The mechanism of flow release is schematized in Figure 5. The current

emanated from a diffuser located just above the bottom of the flume. Thediffuser released a dense flow of uniform concentration along the wholewidth of the flume. The location where the sediment was incorporated

into the dense flow is also shown in Figure 5.

EXPERIMENTAL PROCEDURE

At the beginning of each set, an antecedent sediment-covered bed with

a roughly constant slope near 6% was emplaced over an inerodible bedwith a slope of 5%. This was done by depositing sediment from severalturbidity currents and then smoothing the deposit by reworking it with

several saline flows. These flows were run without the obstruction inplace. The resulting depth of coverage of the rigid bed ranged from about

6 cm upstream to about 2 cm downstream.

The flow of each test in a set was allowed to run over the depositcreated by the previous flows. The final deposit of each set was thus the

cumulated deposit of all the tests in the set. As outlined below, theexperimental procedure adopted here resulted in the emplacement of a

sedimentary wedge that thinned in the downdip direction.

The focus of the experiments reported here is on sustained flows. Thatis, the object of study was the dynamics of the current body rather than

the head. Because each set consisted of many repeated tests (24 for set 1and 33 for set 2), care was taken to make sure that the deposit in the

model minibasin was not reworked by each head. This was achieved inthe following way. A very low, sediment-free saline flow was introduced

into the flume until saline water was barely overflowing the lip of theobstruction. The procedure ensured that the water in the flume was

everywhere fresh except within the minibasin itself. After this conditionwas achieved, the flow was increased to the design discharge, sediment

feed was started, and the test itself was commenced. The head of thecurrent thus impinged upon the saline water at the upstream end of the

minibasin before it had a chance to rework the deposit within.

In order to reach a quasi-steady state flow, the head of the current mustreach the downstream end of the study zone and be reflected backward in

FIG. 2.—Sketch of the flume and instrumentation. The length units are in centimeters.

WEDGE-SHAPED SEDIMENTARY DEPOSITS AND BEDFORMS EMPLACED BY TURBIDITY CURRENTS, PART 1 595J S R



the form of an upstream-migrating bore. This process sets up a subzone

of ponded flow. The role of flow setup in the morphodynamics of

sedimentary wedge emplacement is dominant when the duration of thecurrent is less than the setup time, i.e., the time required for the upstream-

migrating bore to stabilize as an internal hydraulic jump. It remains

important when the duration of the current is longer than, but of the same

order of magnitude as, the setup time. It becomes negligible when the

duration of the current is long compared to the setup time (Toniolo et al.

2006b). At a laboratory scale similar to the one reported here, Lamb et al.

(2004) estimated that the current must be sustained for at least 30 seconds

beyond the initial reflection of the bore. Shorter durations fall within the

realm of pulsed density currents. In the present work all tests lasted

between 20 and 30 minutes, and can thus be unambiguously classified as

continuous flows. It is of interest to note that 30 minutes corresponds to

the field time scale estimated by Lamb et al. (2004) to be necessary for

sustained flow to be achieved in much larger field-scale minibasins in the

Gulf of Mexico.

Figure 6 describes the process of slow pre-filling of the basin with saline

water, which was carried out before the discharge was raised to the design

value. The flow introduced was sediment-free, and the discharge was low

enough (, 0.1 l/s) to ensure that neither the head nor the body was able

to entrain bed sediment as either bedload or suspended load. At time t1 asmall and slowly-moving current head has not yet arrived at the barrier.

At time t2 the current front has hit the barrier. At times t3 and t4 the dense

flow has been reflected by the obstruction and an upstream migrating

bore has been created. At time t5 the ponded flow has reached the top of

the barrier, while the bore continues to move upstream. At time t6 a weak

overflow over the barrier has commenced. Finally at t7 a quasi-steady

state pre-filling condition has been achieved, with the bore stabilized as an

internal hydraulic jump, thus indicating Froude-supercritical conditions

upstream and Froude-subcritical conditions downstream.

After the minibasin was pre-filled, the discharge was abruptly increased

to the design value, i.e., 3.0 l/s for all tests in set 1 and 2.0 l/s for all tests

in set 2. Sediment feed was also commenced at this time. In the upstream

supercritical reach the flow adjusted quickly to the changed conditions.

When the head crossed the jump into the subcritical region, adownstream-traveling internal wave was generated at the interface. When

it reached the barrier, part of the flow was reflected backward as a bore

and part of it overflowed the obstruction. The upstream-migrating bore

so generated had a smaller amplitude, and also was faster than the one

generated during pre-filling. The time evolution to reach quasi-steady

state for the design discharge is outlined in Figure 7. This quasi-steady

state condition was then maintained for the duration of each test. The

FIG. 3.— Illustration of the rake of siphons used to sample salinityconcentration.

FIG. 4.— Grain size distribution of the model sediment, which consisted of plastic particles with a specific gravity of 1.53.

TABLE 1.— Characteristics of sediment mixture.

Sediment type Specific gravity d g [mm] d 50 [mm] d 84 [mm] d 16 [mm] s g

plastic 1.53 206 210 237 182 1.15




internal hydraulic jump migrated upstream as the flow adjusted from pre-

filling conditions to test conditions.

The design flows for each test were chosen so that both the head andthe body of the flow were competent to move sediment, mostly as bedload

but with some suspended load as well. The design flows were so longcompared to the time of transit of the head to the obstruction (20– 30 minutes as opposed to a few tens of seconds) that the morphody-

namics was governed by the quasi-steady flow of the body. In addition,the head impinged on the ponded saline water created by pre-filling,which precluded significant reworking of the deposit in the ponded zone

by the head.

During each test the density current was drained from the bottom of the damping tank at the downstream end of the flume while clear water

was supplied at the top of the damping tank so as to reduce pollution of the ambient water above the bottom current with salt water (Fig. 2).After each test the salty water remaining in the flume was drained outvery slowly and replaced with fresh water to set up the initial conditions

for the next test. The obstruction was slightly leaky, so that the salinewater could be allowed to completely drain out of the ponded zone as wellbefore pre-filling for the next test.

RESULTS: DESCRIPTION

The hydrodynamic quasi-steady state of the flow conditions created bythe tests of sets 1 and 2 are described here. It should be noted that theconditions did not correspond to morphodynamic steady state, because

the flow and bed slowly co-evolved from test to test. This co-evolution is

described in more detail in the companion paper, Spinewine et al. (2009).

As outlined in Table 2, the imposed conditions of the tests of set 1 and

set 2 differed from each other. These differences notwithstanding, the

tests of the two sets showed many similarities, which are documentedbelow in the context of set 1.

The flows of each test reached a quasi-steady state quickly, i.e., in a

matter of tens of seconds. This state is documented in Figure 8. For better

visualization the image is split in two, one showing a mosaic for the first5 meters in the upper part, and one for the remaining 5 meters in the

lower part of the figure. The dense flow was dyed red, and so appears

darker in the image than the fresh ambient water above it. Immediatelydownstream of the injection point the flow was momentum-driven rather

than density-driven, and all the supplied sediment was swept away by a

bottom jet. The momentum of the jet was dissipated in about 0.5 meters,beyond which the flow acquired the characteristics of a typical bottom-

hugging density current.

The presence of an internal hydraulic jump in the upper image of

Figure 8 documents the condition of supercritical flow upstream andsubcritical flow downstream. The interface between saline and fresh water

upstream of the jump was diffuse and turbulent, indicating the

entrainment of ambient water expected of supercritical flow (Garcia

1993; Garcia and Parker 1989). The same interface downstream of the jump was sharper and free of eddies, and showed little mixing between the

saline and fresh water.

Figure 8 also serves to illustrate the geometry of the sedimentary wedge

emplaced by the flows. The deposit shown therein, which represents theaccumulation from the first four tests in set 1, clearly tapers from

upstream to downstream, so taking the shape of a wedge. Bedmorphodynamics are described in more detail in the companion paper,

Spinewine et al. (2009).

Active transport of sediment, mostly as bedload but with some

suspension, was observed upstream of the hydraulic jump. This transportwas accompanied by the presence of the downstream-migrating bedforms

shown in the upper panel of Figure 8, which pertains to set 1. The

experiments in set 2 displayed similar features. Supercritical flow passing

over an erodible bed tends to convert the initially flat bed into a train of smoothly shaped bedforms known as antidunes. Figure 9 documents the

fact that the undulation of the interface between the saline flow and the

fresh water was in phase with that of the bed. This phasing is an indicator

of supercritical conditions both in open-channel flows (Kennedy 1963;Engelund and Fredsøe 1982; Yagishita and Taira 1989; Grant 1997;

Carling 1999; Alexander et al. 2001; Carling and Shvidchenko 2002) and

in density currents (Hand 1974). Both the sediment transport rate and the

bedforms gradually petered out downstream of the hydraulic jump as the

flow decelerated. The bedforms are described in more detail in thecompanion paper, Spinewine et al. (2009).

The hydraulic jumps apparent in the experiments did not appear to be

as sharp as those in many river flows. Measurements presented below

indicate that the density excess of the flow compared to the ambient waterabove declined smoothly in the vertical direction, so that the fluid near

the bottom of the supercritical flow was denser than the flow higher up.

As a result, the flow toward the bottom possessed more momentum that

the flow higher up, allowing the lower flow to form a bottom jetpenetrating into the zone of ponded flow. This bottom jet is illustrated in

TABLE 2.— Summary of experiments.

Set Tests H b [cm] Q0 [l/s] QS0 [g/min] C 0 [g/l] Dr/r [-] T inflow [C] T flume [C] Duration of each test [min]

1 24 32.5 3.0 640 38 0.027 13.2 16.1 202 33 41.9 2.0 161 49 0.034 13.9 16.0 30

FIG. 5.—Setup for releasing dense underflows by means of a diffuser, showing atop view (above) and a lateral view (below).




Figure 10 for the case of set 2. Figure 10 also shows that the jet detaches

from the bed farther downstream, when its momentum dissipates and the

current starts to flow over a bottom layer of denser water ponded behind

the barrier. While the bottom jet has an analogy in open-channel

hydraulic jumps (Rajaratnam 1967; Wu and Rajaratnam 1995) and two-

layer flows (Rajaratnam and Powley 1990; Holland et al. 2002), the

detachment is likely unique to continuously stratified density flows. The

flow in the ponded zone re-accelerated as it overflowed the barrier, where

presumably it attained a critical Froude number near unity. Further

documentation of this is presented below.

FIG. 6.—Photographs illustrating the pre-filling of the zone updip of the barrier for test 8 (set 1) with a low-discharge saline underflow before commencing the maintest. The purpose of a slow pre-filling is to minimize the influence of the current head. As the head of this low-discharge flow hits the downstream obstruction a weak borepropagates upstream. The sequence from top to bottom corresponds to time intervals of 20 seconds. Distances are in meters.




As discussed previously, the free surface in the flume and damping tank

is maintained constant throughout each test by a combination of

regulating devices located in the downstream damping tank. Since no

ambient fresh water is supplied at the upstream end of the flume, the

constraint of a constant free-surface level has a direct consequence, in that

any continuous entrainment of fresh water into the saline current must be

balanced by a supply of fresh water from the downstream tank,

generating a mild upstream backflow in the ambient region of the flow.

In the case of field-scale turbidity currents, the thickness of the ambient

fluid is two or more orders of magnitude larger than the current

thickness, and the backflow is minimized or negligible. It must be kept in

mind, however, that the backflow for our laboratory flows may not benegligible, due to the relatively high rates of entrainment of ambient water

into the saline current in the supercritical reach and the fact that the

ambient fluid thickness is far from infinitely large. As will be explained

further, however, the backflow provides a direct method to quantify

water entrainment in the upstream reaches.

Bed slope tended to increase from test to test but did not vary greatly

between the sets. As documented in Table 2, however, inflow discharge

was lower, barrier height was higher, and excess density of the inflowing

saline water was greater in set 2 than in set 1. The combination of these

factors led to a hydraulic jump, the upstream point of which was located

farther upstream in set 2 than in set 1. More specifically, the upstream end

of the hydraulic jump was located between 2.6 and 3.5 m downstream of

the flow entrance point in set 1; the corresponding values for set 2 were

2.0 and 3.2 m. In addition, the thickness of the flow over the top of the

barrier was 10 cm in the tests of set 1, and 7 cm in those of set 2. The

combination of the different barrier heights and the different overflowthicknesses cause the interface in the ponded zone of the tests of set 2 tobe about 6 cm higher than those in set 1.

RESULTS: FLOW MEASUREMENTS

Each experiment was long enough to measure one or two velocityprofiles in the vertical with the ADV. The flow pattern did not varygreatly from test to test in the region of subcritical flow. Somewhat more,but still muted, variation was observed in the region of supercritical flow.Late in each set, however, more significant variation was observed toward

the upstream end of the region of supercritical flow. This was due to anabrupt change in bedform type, as documented in Spinewine et al. (2009).

The general flow pattern can be visualized by plotting together all of

the measured profiles for a given set, as shown in Figure 11 for the case of set 1. Although the profiles are from different tests, in the aggregate they

provide a reasonable picture of the flow patterns for a given set. Theapproximate positions of the interface between fresh water and saltywater and of the free surface are also shown. It will be noted that thebottoms of several of the profiles are above the illustrated bed at thebeginning of the set but below the bed at the end of the last run of the set.This is not because near-bed velocities could not be obtained, but ratherbecause, for simplicity, only two bed profiles are plotted in the figure.These two profiles illustrate that a) the deposits were wedge-shaped andb) the bed aggraded and steepened from test to test. The deposits aredescribed in the companion paper, Spinewine et al. (2009).

FIG. 7.—The four photos document the process by which quasi-steady conditions were achieved after increasing the flow to the design discharge. The images show therapid updip migration of a weak bore. The approximate position of the bore is indicated with arrows. Images pertain to test 8 of set 1. The sequence from top to bottomcorresponds to time increments of 20 seconds. Distances are in meters.




The characteristics of the velocity profiles of Figure 11 are as follows.Toward the upstream end of the hydraulic jump, as well as upstream of it,the streamwise flow velocity has a single peak. This peak velocity is

attenuated as the flow enters the ponded zone and eventually splits into

two relatively weak velocity maxima in the vertical. The lower peak isassociated with a bottom jet, and the upper peak may be associated withan uplifting of the flow as it tries to avoid the relatively dense, slow

ponded fluid. In addition, the upper peak may be influenced by the high-

FIG. 8.—Illustration of quasi-steady state conditions, as achieved for test 4 of set 1. The upper panel illustrates conditions in the upstream portion of the study reach,and the lower panel illustrates them in the downstream portion. Distances are in meters.

FIG

. 9.—The image, which is from test 6 of set1, illustrates supercritical flow that is in phasewith the bedforms. The arrow indicatesflow direction.

FIG. 10.—The image illustrates flow detach-ment from the bed within a zone of subcriticalflow around x 5 6 m. Images pertain to test 11of set 2. Distances are in m.




velocity flow overtopping the barrier at the downstream end. Note that

the velocity profiles of Figure 11 illustrate the weak backflow in the

ambient water discussed in the previous section. This backflow is moving

upstream to supply entrainment water. At any section, the point of zero

velocity provides a reasonable quantification of the location of the

interface between the saline water and the ambient fresh water.

The corresponding structure for salinity concentration in the case of set

1 is seen in Figure 12, which is constructed in the same way as Figure 11.

In Figure 12, the profiles toward the upstream end of the flume show a

continuous decline in salt concentration in the vertical direction. Toward

the downstream end in the ponded zone, however, the lower portions of

the profiles show only weak variation in the vertical, whereas

concentration declines more strongly in the vertical higher up. This

strong variation is a signal of the interface itself.

Downstream of the lip of the barrier, the overflow formed a

plume of saline water that impinged upon the bed. This caused some

mixing of salt water, and may have slightly polluted the ambient fresh

water with salt. The effect must have been weak, as evidenced by the near-

zero salt concentrations in the ambient water well above the interface in

Figure 12.

A close-up view of the variation of streamwise velocity and salt

concentration is given in Figure 13. The profiles pertain to set 1. Three

profiles of velocity and three of concentration are shown therein. Each

velocity profile can be considered to be paired to a concentration profile

that was taken nearby. Thus the velocity profile at x 5 2.64 m

(downstream of the inlet) and the concentration profile at x 5 2.57 m

constitute a pair that characterize the flow upstream of the hydraulic

jump. The velocity (concentration) profiles at x 5 6.25 m (x 5 5.97 m)

constitute a pair in the subcritical ponded zone near the detachment point

shown in Figure 10. Finally, the velocity (concentration) profiles at

x 5 8.00 m (7.45 m) constitute a pair in the ponded zone about a meter

upstream of the barrier.

FIG. 11.—Velocity profiles for set 1 are shown at an early stage of development of the wedge-shaped deposit. Bed profiles at A) the beginning of the set and B) the endof the set are shown for reference.

FIG. 12.—Concentration profiles for set 1 are shown at an early stage of development of the wedge-shaped deposit.




The concentration profiles document a dramatic thickening of the flow

as it traverses the ponded zone. The velocity profiles likewise document a

substantial deceleration of the flow, with a concomitant tendency for the

peak flow velocity to migrate upward toward the level of the barrier

overflow. The supercritical profiles show the characteristics expected

from the work of Garcia (1993). The subcritical profiles are more

complicated than those in Garcia (1993), largely due to the influence of

the downstream flow over the barrier.

Velocity and concentration profiles corresponding to Figures 11, 12,

and 13 were also generated for set 2. The three relevant figures are not

shown here because they show patterns that are very similar to those of

set 1.

It should be noted that, apart from the weak backflow of ambient

water above the interface, negative velocities appeared in two other areas.

The more manifest one was located in the subcritical region downstream

near the barrier, i.e., between 6 and 9 meters from the inlet for set 1 and

between 4 and 9 meters for set 2. This backflow of relatively dense saline

water was sandwiched in the vertical between two other strata within

which the flow moved downstream, thus giving rise to the double-peak

velocity distributions of Figures 11 and 13. The second area of negative

velocities was less obvious and more unsteady in nature. It was found

within the hydraulic jump above the dense bottom jet. The sketch of

Figure 14 synthesizes the overall flow pattern in the minibasin for both

sets 1 and 2, including backflow and recirculation areas. Most of the

FIG. 13.— A) Typical velocity profiles for set 1 are illustrated at three locations: in the zone of supercritical flow (x 5 2.64 m), in the zone of subcritical flow near thedetachment point (x 5 6.25 m), and in the zone of subcritical zone near the downstream barrier (x 5 8.00 m). B) Corresponding salinity concentration profiles are

illustrated at x 5 2.57 m, 5.97 m, and 7.45 m.

FIG. 14.—Sketch of flow patterns in thedepositional zone, including backflows andrecirculation cells. Vertical distances and recir-culation cells are exaggerated for the sakeof clarity.




entrainment of ambient water appeared to take place in the supercritical

region due to strong mixing. The detachment of the bottom jet was

always observed downstream of the hydraulic jump.

FROUDE ANALYSIS

Two layer-averaged parameters, i.e., streamwise flow velocity U and

salinity concentration C (here measured in g/l), as well as the layer

thickness h itself, can be determined with appropriately defined moments

(Ellison and Turner 1959):

Uh ~

ð hi

0

udy ð4Þ

UCh ~

ð hi

0

ucdy ð5Þ

U 2h ~

ð hi

0

u2dy ð6Þ

In the above equations the parameter UCh represents the salinity

transport rate, which is constrained to be constant along the streamwise

direction because no salt is lost or added along the minibasin. This

constant is specified by the inlet flow discharge per unit width and salt

concentration. In the above equations, y denotes the coordinate upward

normal from the bed, and hi is the distance from the bed at which u and c

can be approximated as zero. The layer-averaged values for U , C , and h

used to estimate the densimetric Froude number as expressed by

Equation 3 are computed as

U ~ U 2h

Uh ~

Ð hi

0 u2dy

Ð hi

0 udy

ð7Þ

h ~ Uhð Þ2

U 2h ~

Ð hi

0 udy

2

Ð hi

0 u2dy

ð8Þ

C ~ UCh

Uh ~

Ð hi

0 ucdyÐ hi

0 udy

ð9Þ

A moment-defined parameter that is of more relevance to the present

problem is the layer-averaged excess density difference F e, defined in

Equation 1. This parameter is related to the local excess density difference f e (averaged over turbulence) according to the relation

F e ~ Dr

ra

~ UF eh

Uh ~

Ð hi

0 uf edyÐ hi

0 udy

ð10Þ

The relation between fractional excess density and salt concentration in

grams per liter is not necessarily linear. An empirical curve developed to

convert layer-averaged salt concentration C into layer-averaged excess

density F e, however, showed a linear relationship over the ranges of

experimental parameters studied herein, as shown in Figure 15. Using

this conversion, the densimetric Froude number was then computed

according to Equation 3. It should be noted that the layer-averaged

density of the current r f can be calculated from Equations 2 and 10.

The variation of the densimetric Froude number along the domain forboth experimental sets is shown in Figure 16. Both sets show a

supercritical region (Frd . 1) followed by a subcritical region

(Frd , 1) farther downstream. Just upstream of the barrier overflow

the Froude number increases rapidly downstream, again reaching a value

near unity.

As mentioned above, for the experiments in set 2 the hydraulic jump

was located farther upstream than in set 1, so constraining the

supercritical region to a smaller domain upstream. In the subcritical

zone, the Froude number dropped to a minimum of around 0.2 in a

region upstream of the barrier.

The analysis of the evolution of the depositional wedge evident in

Figures 8 and 11 is treated in the companion paper, Spinewine et al.

(2009). In order to better understand the behavior of the densimetric

Froude number indicated in Figure 16, however, it is necessary to

FIG. 15.—Linear relationship between salinity concentration and density excessover the investigated range of salinity.

FIG. 16.— Densimetric Froude number as a function of streamwise distance,with data shown for both sets of experiments. Squares indicate measurements

before the appearance of cyclic steps (c.s.) in the supercritical region, and circlesrefer to data after cyclic steps have formed.




describe briefly the morphodynamics of the sediment bed. Starting from anearly constant initial slope of 0.06, the bed aggraded by deposition of sediment in the supercritical reach up to the hydraulic jump and in a shortreach within the region of the jump. Beyond this region the flow velocity

declined so much that the flow lost most of its capacity to move sediment.The thin sediment deposit visible in Figure 8 just upstream of the barrierconsists of the antecedent sediment laid down before the barrier wasinstalled and the experiments commenced, plus only a thin cap from the

experiments themselves. As is shown in the companion paper, Spinewineet al. (2009), this flow pattern resulted in the emplacement of adepositional wedge with a topset upstream of the jump, a rather longforeset in the jump region, and a thin bottomset near the barrier.

A remarkable feature of the experiments was the appearance of thesediment waves that Kostic and Parker (2006) and Fildani et al. (2006)have identified as cyclic steps created by turbidity currents. Cyclic stepsare rhythmic, upstream-migrating sediment waves bounded by hydraulic

jumps (e.g., Taki and Parker 2005). They can occur in bedrock or alluvialrivers as well as on the floor of the deep sea (e.g., Fildani et al. 2006).Cyclic steps appeared late in the experiments of both sets 1 and 2, near the

upstream end of the deposit. Measurements before and after theappearance of cyclic steps are discriminated in Figure 16. Note that

subcritical flow occurs locally within cyclic steps due to the formation of hydraulic jumps. Two cyclic steps and the intervening hydraulic jump areshown in Figure 17. More information concerning cyclic steps is given in

the companion paper, Spinewine et al. (2009).

DISCUSSION AND CAVEATS

The estimation of flow discharge by means of Equation 4 may besomewhat inaccurate for two reasons. In the supercritical reach thevelocity distribution is affected near the bed by a three-dimensionalpattern of bedforms. In addition, the flow patterns in the subcritical reachwere subject to fluctuations and recirculation cells that had a componentwith a characteristic time that was long compared to the time window

over which the velocity was measured with the ADV. As a result thedouble-peak velocity distribution shown in Figures 11 and 13 may notrepresent true long-term averages. An alternative way to compute thedischarge of the flow involves the use of the backflow of ambient waterabove the visible interface of the saline current. This backflow isgenerated by the replacement water added at the downstream end, whichbalanced the loss of ambient water to the underflow by entrainment. This

backflow tended to be relatively steady, and was not affected bybedforms. The following relationship describes overall flow continuity:

Qi ~ Q0 z Qbfi ð11Þ

where Qi represents the discharge of dense flow below the interface at anycross section i in the minibasin, Q0 is the discharge of dense fluid at the

inlet, and Qbfi is the backflow discharge of ambient water at the same

cross section above the interface as depicted in Figure 14. Note that Qi

and Q0 are defined positive when pointing downstream, whereas thebackflow Qbfi is defined positive when pointing upstream.

The inlet discharge could be accurately measured from the flowmeter

described above, and Qbfi could be estimated from the velocity profilesabove the interface with better accuracy than by a direct estimation of thedensity-current discharge below the interface using velocity profiles.

Figure 18 shows the estimated discharge of the density current along the

flume employing both methods for set 2.It is seen that the flow discharges per unit width estimated directly fromthe velocity profiles of the dense underflow show considerably more

scatter than those computed based on backflow measurements andEquation 11. The former measurements bracket the latter measurements,however, suggesting that the latter estimates may be more accurate and

useful. Indeed, it has been found from other studies that evaluations frombackflow profiles provide a consistent way to evaluate layer-averagedforward discharge per unit width (e.g., Garcia 1993). Calculations of

forward discharge per unit width from backflow profiles record a) asteady downstream increase in current discharge in the supercritical

stretch due to water entrainment from above and b) a rather constantdischarge in the subcritical reach associated with negligible entrainmentacross the interface (Fig. 18).

FIG. 17.—The figure illustrates two cyclicsteps and a hydraulic jump, as observed in test 30of set 2. Arrows indicate flow direction andposition of the jump.

FIG. 18.—Flow discharge of the density underflow as a function of downstreamdistance. The discharges were computed using data on flow velocity for set 2 basedon profiles for the density underflow and the backflow, as described in the text.




An issue that deserves special attention is the effect of morphodynamicchange of the bed on the flow itself. The repeat of 24 sediment-ladensaline underflows in set 1 and 33 flows in set 2 caused a gradualsteepening of the depositional wedge. This process is described in moredetail in the companion paper, Spinewine et al. (2009). It must be notedhere, however, that the flow itself evolved accordingly from test to test.

Figure 19, for example, documents the downstream migration of thehydraulic jump in terms of velocity profiles. All three velocity profileswere taken within 0.15 m of each other in the streamwise direction, and

all pertain to set 2. The test number and Froude number obtained fromthe profiles are, respectively, (4, 0.25), (19, 0.34), and (28, 0.89). Thedifferent Froude numbers might be thought to represent a fundamentalchange in the character of the flow from test to test. Rather than showsuch a change, however, they simply document the fact that the positionof the hydraulic jump moved downstream as the bed aggraded andsteepened.

The forces governing internal hydraulic jumps in density currents andstandard hydraulic jumps in rivers are the same, (pressure, gravity, andfriction), but in the former case some of these forces are reduced inmagnitude while others are increased by the presence of an overlying fluidmass of non-negligible density (Komar 1971). Yih and Guha (1955),experimenting with water hydraulic jumps on horizontal bottoms flowingunder an immiscible less dense liquid, found that hydraulic jumps indensity currents are lower than their otherwise identical counterparts in

rivers because of the lowered density contrast. Internal jumps found innature are usually composed of miscible fluids throughout, so that mixingand entrainment result in continuously varying vertical density gradients.Wood (1967), studying internal hydraulic jumps in salt solutions, foundthat for the same Froude number, jumps in density currents whereentrainment is possible were greater in height than are jumps under air.This may indicate that for some conditions of high entrainment thevolume increase brought about by entrainment offsets the decrease in thesize of the jump due to the lowered density contrast.

Ellison and Turner (1959), Wilkinson and Wood (1971), and Wood andSimpson (1984) also investigated the effects of entrainment by experi-menting with saline and thermal density flows. The ambient liquid waspure water, so that entrainment was possible and hence density reductionsoccurred in the density currents. It was found that the rate of entrainmentis a function of the Froude number; i.e., the higher Frd, the greater the

rate of entrainment. At low Frd, the flow attains a state of equilibrium in

which the gravitational force acting on the flow is balanced by the drag onthe interface due to entrainment together with the drag at the base of the

flow.

Internal hydraulic jumps tend to be more diffuse than open-channel

hydraulic jumps. More specifically, the former show more internal

structure than the latter, and develop more gradually in the downstreamdirection. The reason for this can be seen in terms of excess density

profiles. In the case of a hydraulic jump in an open channel, the density

difference associated with the flow is the difference between water and theair above. Unless the current is entraining copious amounts of air, thedensity profile in the vertical shows a sharp break between a constant

value for water and a constant, much lower value for air. In the case of adensity underflow, however, excess density shows a smoother and more

gradual decline in the upward normal direction. The result is a jump thatevolves more gradually in the streamwise direction. The pressure

difference between sections prior to the occurrence of the jump and afterit decreases because the ambient fluid is denser. The friction forces grow

as the jump length increases, and the drag comes not only from the

bottom but also from the interface. Table 3 shows results from previousexperiments on open-channel jumps as well as internal hydraulic jumps.The densimetric Froude number at the end of the jump and at the

beginning of it are indicated as Frd2 and Frd1 respectively, the ratiobetween the corresponding flow thicknesses is h2/h1, and the ratio

between the jump length and the initial flow thickness is L j /h1. Allparameters are similar in all experiments except for the jump length in theopen-channel cases, which is less than half the minimum length found in

internal jumps. Open-channel data are taken from Bradley and Peterka(1957), and are the open-channel equivalent to the present study (set 1),

because both Froude number and bottom slope are approximately the

same. Results due to Hand (1974) and Garcia (1993) on internal hydraulic jumps pertai n to a somewhat different jump configuration, butnevertheless show a similar tendency as they present a longer L j than

the open-channel case.

CONCLUSIONS

A succession of continuous saline underflows together with acontinuous supply of lightweight plastic particles were introduced into

FIG. 19.— Velocity profiles in the transition zone between supercritical and subcritical flow are illustrated for several tests, and at several locations: A) test 4,x 5 3.5 m, Frd 5 0.25; B) test 19, x 5 3.65 m, Frd 5 0.34; C) and test 28, x 5 3.6 m, Frd 5 0.89.




a depositional zone defined at the downstream end by a barrier. These

underflows emplaced a wedge-shaped sedimentary deposit that declined

in thickness downdip. The flows emanating from the upstream end of the

study zone were in the supercritical flow regime. The barrier forced an

internal hydraulic jump to subcritical flow. Two sets of experiments were

performed, one involving 24 continuous flows and one involving 33 such

flows. Each flow lasted for 20–30 minutes. Successive flows in a given set

were allowed to run over the deposits emplaced by the previous flows.

The essential findings of the present paper can be summarized as

follows.

N The saline underflow underwent a clear transition from the

supercritical to the subcritical flow regime as it approached the

barrier, so confirming the formation of an internal hydraulic jump.

N The hydraulic jump gradually migrated downstream. This was caused

by sediment deposition, which was biased toward the proximal end of

the study reach. This bias caused the bed to aggrade and steepen over

time.

N Significant entrainment of ambient water across the flow interface was

observed only in the supercritical region of the flow.

N The hydraulic jump was rather more diffuse than one associated with

open channel flow. The flow within and downstream of the jump

showed a structure that was more complex than expected, with two

peaks in the upward-normal profile of streamwise velocity.N One aspect of this structure was the tendency for the formation of a

dense bottom jet as the flow entered the jump region. This bottom jet

decelerated and detached farther downstream. The detachment

process appeared to be closely related to the rather complex flow

structure in the ponded zone itself.

N Especially near the barrier itself, the flow velocity showed a reversal

between these peaks, so that a zone existed in the ponded zone of the

minibasin where a weak flow was directed upslope, forming a

secondary flow recirculation cell within the minibasin.

N This pattern of flow and sediment transport observed in the

experiments created conditions appropriate for the emplacement of

a sedimentary wedge. The sedimentary structures themselves are

described in more detail in the companion paper, Spinewine et al.

(2009).

ACKNOWLEDGMENTS

Funding for this work from ExxonMobil Exploration Co. as part of theStratigraphy Tripod Project is gratefully acknowledged. The authors alsothank Enrica Viparelli, Eric Anders, Mariano Cantero, Andy Waratuke,Rocio Luz Fernandez, and Martino Salvaro for their assistance during theexperiments and helpful discussions. The manuscript was greatly improved byreviews by Brad E. Prather, David Mohrig, and Carlos Pirmez.

NOTATIONS

c local salinity concentration function of x and yC 0 concentration of salt at the inflow pointC layer-averaged salinity concentration

d i sediment size i % of which is finer by weight (d 50 is the mediansize)

d g geometric mean size f e local excess density difference of the currentF e

layer-averaged excess density of the currentFrd densimetric Froude number g acceleration of gravityh layer-averaged thickness of the currenthi distance from the bed to the visible interface between the saline

and fresh waterH b height of the barrier forming the downstream end of the model

minibasinL j length of hydraulic jumpQ0 discharge of dense fluid at the inletQi discharge of dense flow below the interface at any cross section i Qbfi backflow discharge of ambient water at any cross section i QS 0 upstream sediment feed rateT flume temperature of the ambient fresh water in the flumeT inflow temperature of the inflowing saline water

u local downstream velocity of the current function of x and yU layer-averaged downstream velocity of the currentx downstream from the inlet coordinate y upward normal from the bed coordinatera density of the ambient fluidri density of the incoming waterr f layer-averaged density of the currentDr difference between the layer-averaged density of the current and

rasg geometric standard deviation

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Received 27 March 2008; accepted 4 February 2009.


Experiments in Wedge-Shaped Deep Sea Sedimentary Deposits Part I - Documentation of Flow - JSR, 2009

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