! david mohrig, experiments on the relative mobility of muddy sub aqueous and

8/6/2019 ! David Mohrig, Experiments on the Relative Mobility of Muddy Sub Aqueous And

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ELSEVIER Marine Geology 154 (1999) 117129

Experiments on the relative mobility of muddy subaqueous andsubaerial debris flows, and their capacity to remobilize

antecedent deposits

David Mohrig a,1, Anders Elverhi b, Gary Parker a,*

a St. Anthony Falls Laboratory, University of Minnesota, Mississippi River at 3rd Avenue, Minneapolis S.E., MN 55414, USAbDepartment of Geology, University of Oslo, P.O. Box 1047, Blindern N-0316, Oslo, Norway

Received 4 March 1997; accepted 6 November 1997

Abstract

Subaqueous debris flows share many similarities with their subaerial counterparts, but also differ in striking ways. The

present treatment is devoted to an experimental comparison of paired subaqueous and subaerial debris flows. The debris

slurry consisted of 39% water, 25% kaolin, 24% silt and 12% sand by weight. The debris flows ran down a rectangular

channel with a length of 10 m and a width of 0.20 m. In some of the runs, the channel bottom was hard (i.e., a

fixed, rough inerodible surface) and in others it was soft (i.e., an antecedent debris-flow deposit over which the debris

flow was allowed to run). On the hard bottom, the subaqueous debris flows ran farther downslope than their subaerial

counterparts with the same rheology. Their deposits also displayed extensional features not seen in the subaerial case, andwere much thinner than that which would be expected based on yield strength. On the soft bottom, the subaerial debris

flows extensively remobilized antecedent debris-flow deposits, whereas the corresponding subaqueous debris flows ran

over antecedent debris-flow deposits with no detectable remobilization. The reason for these differences is hypothesized to

be the incorporation of a thin layer of ambient water underneath a subaqueous debris flow as its head hydroplanes slightly

above the bed. The lubricating layer appears to prevent the transmission of shear stress between the two layers. 1999

Elsevier Science B.V. All rights reserved.

Keywords: debris flows; hydroplaning; modeling

1. Introduction

The development of acoustic techniques for map-ping the sea floor and imaging the subsurface hasled to a dramatically expanded view of the seascape.In particular, it has led to the identification of nu-

1 Present address: Exxon Production Research Co., P.O. Box

2189, Houston, TX 77252-2189, USA. Corresponding author. E-mail: [email protected]

merous deposits of debris slides and flows in sub-marine environments such as the continental slope.The volume of debris comprising the deposit of anindividual submarine slide or flow ranges up to thou-sands of cubic km (Hampton et al., 1996). In the caseof the Amazon Fan, significant amounts of sedimentmay be deposited by massive, infrequent debris flows

(Damuth et al., 1988). In the case of high-latitudedeep-sea fans such as the Bear Island Trough MouthFan between Norway and Svalbard, smaller but more

0025-3227/99/$ see front matter 1999 Elsevier Science B.V. All rights reserved.

P I I : S 0 0 2 5 - 3 2 2 7 ( 9 8 ) 0 0 1 0 7 - 8


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118 D. Mohrig et al. / Marine Geology 154 (1999) 117129

frequent flows have created fans that are built almostentirely from stacked debris lobes (Laberg and Vor-ren, 1995). Submarine debris flows likely play an im-portant role in determining the limiting slope of the

leading edge of clinoforms, as well as the morphologyof the continental slope itself (Steckler et al., 1999).

Interpretation of submarine debris-flow depositsresulting from slope failures is hampered by thepaucity of information concerning their dynamics.The events themselves are almost impossible to mea-sure directly in the field (e.g., Norem et al., 1990),and until recently there have been very few studiesof them under controlled conditions in the laboratory(Hampton, 1972). This lack of information hindersthe development and evaluation of the physicallybased predictive numerical models necessary to un-derstand submarine debris-flow deposits.

The experimental research on paired subaqueousand subaerial debris flows reported here is intendedto help fill this gap. Insofar as subaerial debris flowshave been subjected to relatively intense laboratoryand field investigation (e.g., Takahashi, 1991), theyserve as a familiar baseline from which to evalu-ate the subaqueous case. The dynamics of subaerialdebris flows moving over a dry, rough bed are con-trolled almost entirely by the downslope pull ofgravity and the rheology of the debris slurry, with

the ambient fluid (air) playing a negligible role. Inthe subaqueous case, however, the ambient fluid iswater, which has a density of the same order of mag-nitude as the debris slurry. This allows a richer andmore interesting range of behavior (Mohrig et al.,1998). In particular, as the head of the debris flowpushes water out of its path, it generates a dynamicpressure at its leading edge. If this dynamic pressureis sufficient, it causes the head to hydroplane, i.e.,uplift and detach slightly from the bed. This permitsthe intrusion of a thin layer of lubricating water be-

tween the debris slurry and the bed. This in turn cancause a dramatic reduction in resistance to flow nearthe head, leading to higher head velocities and=orlonger run-out distances. Some of the differences be-tween the dynamics of the head of subaqueous andsubaerial debris flows can be seen in Fig. 1.

While results for both subaqueous and subaerialdebris flows are reported in Mohrig et al. (1998),the two cases are not systematically paired. Suchpaired experiments were used in the present study

Fig. 1. Head of a subaerial (a) and subaqueous (b) debris flow.

The subaerial flow fills the crenelations of the bed up to the

leading edge, indicating that it is not hydroplaning. The subaque-

ous head is clearly hydroplaning; the large black particles are

neutrally buoyant markers for particle imaging velocimetry.

to examine the differences in run-out dynamics anddeposit architecture associated with the two flowconfigurations. The experimental results can be usedto develop and test methods for the numerical simu-

lation of debris flows (Huang and Garcia, 1999) thatcan then be applied to the field.

2. Experimental design

The experiments were conducted in the FishTank, a glass-walled tank 10 m long, 3 m highand 0.6 m wide. Suspended within the tank is arectangular channel with an inner width of 0.2 m

and transparent vertical walls. The channel is seg-

mented with a break in slope, the upper and lowerslope angles being 6 and 1, respectively. The slopebreak was located 5.7 m downslope of the head-tankgate from which the debris slurry was released. Thebed of the channel was roughened to prevent basalslippage of the debris flows. In the subaqueous con-figuration, the tank was filled with fresh water before

commencing a run, leaving a very short subaerialreach (


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D. Mohrig et al. / Marine Geology 154 (1999) 117129 119

Table 1

Data for experimental debris flows

Run Bed type d s y V Qd td xh h6(t=m3) (kg m1 s1) (Pa) (l) (l=s) (s) (m) (mm)

1a hard 1.6 0.035 49 34.5 * * 4.94 35

2a hard 1.6 0.035 49 31.6 8.0 4.0 5.07 31

3a hard 1.6 0.023 36 30.6 9.1 3.5 6.29 23

4a hard 1.6 0.019 33 36.8 8.5 4.5 7.61 21

5a soft 1.6 0.019 33 33.8 8.9 4.0 8.70C 25*

6a soft 1.6 0.019 33 34.4 8.0 4.5 8.70C 24*

1w hard 1.6 0.035 49 33.7 * * 7.46 18

2w hard 1.6 0.035 39 32.0 9.1 3.5 7.36 18

3w hard 1.6 0.023 36 29.1 9.1 3.5 7.72 16

4w soft 1.6 0.035 49 21 * * 7.27 6.5*

5w soft 1.6 0.019 33 32 6.1 5.0 7.40 *16

Notes. An asterisk marks runs in which a debris flow overran an antecedent deposit. The listed values of h6 for those runs are equal

to the final thickness minus the antecedent thickness. For Runs 4w and 5w, these are good estimates of the thickness of the overriding

deposit due to the absence of remobilization. Listed values for Runs 5a and 6a are considerable underestimates of the thickness of the

deposit formed by the overriding run due to extensive remobilization.

In each experiment, approximately 30 l of debrisslurry was placed in the head tank. The head gate was

opened suddenly to form a slot with a height of 20 mmand width of 170 mm from which the slurry flowed.The volume discharge of slurry into the channel wasmeasured with a sonic profiler that recorded the timevariation of the free surface of the debris in the headtank. The total volume of debris V released, the aver-

age peak volume discharge of slurry Qd, and the totallength of time td for the head tank to drain are given

in Table 1 for each of the 11 runs.The motion of each debris flow was recorded by

eight fixed video cameras, each mounted in front ofone of the eight panels of glass comprising the frontside of the Fish Tank. In the earliest runs, a singlevideo camera attached to a moving carriage was usedinstead. Videotaping continued until after the flowhad come to rest and formed a deposit. Analysis of

the tapes allowed the determination of such parame-

ters as head velocity, head and body flow thicknessand deposit thickness. Deposit thickness was alsomeasured by inserting a meter stick into the deposit.

Subaerial and subaqueous debris flows were re-leased into the channel under two different bottomconditions. The hard bottom consisted of the rough,inerodible rubber matting on the bottom of the chan-

nel. The soft bottom consisted of the deposit of anantecedent debris flow resting on top of the rubbermatting. The antecedent deposit was generated in all

cases by a subaerial debris flow. Before running asubaqueous debris flow over an antecedent subaerialdeposit, the tank was filled with water sufficientlyslowly as to prevent any disturbance of the deposit.These two configurations allowed for a characteriza-tion of flow dynamics over a rigid bed versus thoseover a deformable bed. The latter case specificallyaddressed the question as to whether or not a de-

bris flow can increase its mass by mobilizing anantecedent deposit.

These two bed types have markedly different sur-face textures. The rubber matting that provides thebasal boundary for the hard-bed cases is crenelatedinto rectangular ridges and grooves. The width of eachridge or groove is 6.4 mm, and the elevation differ-ence between them is 3.2 mm. In the soft-bed cases,the surface of the antecedent subaerial debris flowwas smooth. The antecedent deposit was not allowedto consolidate or segregate to any substantial degree.

In the soft-bed cases, a freeze-coring device wasused to determine the extent to which the antecedentdeposit was remobilized by the overriding debris flow.A water-soluble dye was added to the slurry of theoverriding event in order to render the two depositsdistinct in the cores. The corer consisted of a wedge-shaped hollow aluminum container, which was filledwith a mixture of methanol and solid CO2 (dry ice)and inserted into the deposit from above. The corerwas held in place for approximately 2 min in order to


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freeze the debris to its sidewalls (Wright, 1991). Theinevitable smearing of the deposit at the debris=corerinterface was easily removed, revealing an essentiallyundisturbed record of deposit stratigraphy.

3. Debris properties

All results presented here are for flows of a singleslurry type. The grain-size distribution, water contentand mineralogy of this debris slurry were selected toapproximate the composition of debris-flow depositsfrom both the Bear Island Trough Mouth Fan and theNorth Sea Fan in the North Atlantic Ocean (LabergandVorren, 1995; King et al., 1996). These well-datedsubmarine fans are constructed predominantly froma multitude of stacked debris-flow deposits. In bothcases, the primary source for the debris flows has beenidentified as deformation till delivered by continentalglaciers extending to the shelf break during periodsof maximum glaciation and low stand in sea level.

39% of the weight of the slurry used in the ex-periments consisted of water, the rest was sediment.The sediment itself consisted of 40% kaolin, 40%silt and 20% sand by dry weight. The resulting slurryhad a density of 1.6 t=m3. The kaolin had a reportedmedian size of 13 m. A grain-size analysis of the

mix of sand and silt revealed the following respectivegrain sizes in m for D5, D10, D16, D50, D84, D90and D95: 7, 12, 18, 57, 350, 500 and 700. The siltand sand grains were quartz.

The hydraulic conductivity K of the sedimentmixture was estimated from tables given in Freezeand Cherry (1979) summarizing measured values fornatural soils. The value of K was inferred to benear 1 104 mm=s. The associated measure forpermeability kis 108 mm2.

In lieu of a more rigorous analysis (e.g., Iverson,

1997), the rheological properties of the debris slurryare described using a simple viscoplastic (Bingham)model. That is, the two parameters describing therheology are taken to be a yield strength y anda Newtonian viscosity s. Estimates of y and swere calculated by means of experiments on slurryflow in half-pipe channels using a method suggested

by Johnson (1970), and outlined in more detail inAppendix A. This channel had a length of 5 mand an inner diameter of 41 mm, and had the same

roughness as that of the channel in the Fish Tank.Estimates for y were also determined using thethickness of subaerial deposits on the segment ofchannel with a slope of 6. The small amounts of dye

needed to distinguish the degree of remobilizationof an antecedent deposit unfortunately had a notice-able effect on debris rheology. Dye-free slurry wascomparatively sticky, yielding values for y and sof 49 Pa and 0.035 kg m1 s1. The addition of theleast amount of dye used in the experiments gave themore runny values for y and s of 36 Pa and 0.023kg m1 s1; the corresponding values for a largeramount of dye were yet more runny (i.e., 33 Pa and

0.019 kg m1 s1). The calculated values of y ands for the slurry used in each experiment are givenin Table 1. Here, the above three slurries are referredto as the sticky slurry, the medium slurry and therunny slurry, respectively, in order to distinguishtheir rheologies.

It should be noted that all experiments were con-ducted using fresh rather than sea water. The differ-

ence between the two can have a noticeable effect onclay rheology, especially in the case of active clays.Kaolin, on the other hand, is a relatively inactiveclay.

4. Experimental results: hard runs

Eleven experiments were conducted, each in oneof four configurations. Four (1a, 2a, 3a and 4a) werehard-bed subaerial experiments; two (5a and 6a)were soft-bed subaerial; three (1w, 2w and 3w)were hard-bed subaqueous; and two (4w and 5w)were soft-bed subaqueous. Within each configura-tion, all the experiments were repeats of each otherexcept for the inadvertent variation of slurry rheol-

ogy due to the dye. Measured parameters listed in

Table 1 include the volume V of slurry released, theaverage peak slurry discharge Qd, the duration ofrelease td, the run-out distance measured from thegate of the head tank to the leading edge xh of thehead deposit, and the average deposit thickness h6on the reach with a slope of 6.

As might be expected, the run-out distance for

the four hard subaerial runs correlated inverselywith the stickiness of slurry rheology, with the stickyslurry (highest values of y and s) running out the


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Fig. 2. Elevation profiles of the deposits for the paired hard-bed

runs. The profiles are for, in order from top to bottom, Runs 1a

and 1w, 2a and 2w, and 3a and 3w. In the diagram, x denotes

a streamwise coordinate and y denotes elevation relative to an

arbitrarily chosen datum for each run.

shortest distance (Figs. 2 and 3; Table 1). SubaerialRuns 1a and 2a, which used the sticky rheology,ran out 4.94 and 5.07 m, respectively. These valuesindicate a considerable degree of repeatability. Theaverage thickness of debris h6 on the 6 slope was 35and 31 mm, respectively. Subaerial Run 3a used the

medium rheology, yielding a longer run-out distancexh of 6.29 m and a lower h6 value of 23 mm.

Subaerial Run 4a used the runny rheology; the valuesof xh and ht were 7.61 m and 21 mm, respectively.Of the four hard subaerial runs, the first two did notpropagate beyond the break in slope from 6 to 1 atthe point located 5.7 m downstream of the head gate.

The three hard subaqueous runs showed a differ-ent behavior that was rheology-independent to firstorder. Runs 1w and 2w were repeats using the same

sticky rheology; the respective values for xh of 7.46

and 7.36 m and the values h6 of 18 mm for bothcases indicate excellent repeatability. In the case ofRun 3w, which used the medium rheology, xh in-creased only slightly to 7.72 m, and h6 decreasedonly slightly to 16 mm.

The runs were conducted as subaerialsubaqueous pairs with the same rheology, the pairs

being Runs 1a and 1w, Runs 2a and 2w and Runs 3aand 3w. The first two pairs used the sticky rheology,and the last pair used the medium rheology. The

subaerial Run 4a using the runny rheology, whichhad the longest run-out distance of any of the hardruns, has no subaqueous counterpart in Table 1. Thedeposit shapes of the paired runs are shown in Fig. 2,

where they are given as elevation profiles, and inFig. 3, where they are given as deposit-thicknessprofiles. It can be seen in each pair that the subaque-ous run propagated well beyond the break in slope,whereas the subaerial run either did not reach it orpropagated only slightly beyond it. Evidently somemechanism was acting to increase the mobility ofthe subaqueous debris flows as compared with theirsubaerial counterparts with the same rheology.

The nature of that mechanism can be at least par-tially inferred from Figs. 2 and 3. The deposits of thesubaqueous hard runs differ markedly from thoseof their subaerial counterparts. In every subaqueouscase, there was a zone of necking, or very thin de-posit, between the thicker head and upslope deposits,as the head ran out ahead of the body. In the case ofRun 2w, the head actually detached from the body.Further evidence of this tendency was provided bythe observation of tension cracks in the deposits ofthe subaqueous hard runs in the vicinity of thenecking, a feature never observed in the subaerialhard runs.

The evidence for the subaqueous hard runs is

consistent with hydroplaning of the head, as de-scribed in Mohrig et al. (1998). This hydroplaningwas clearly observed in the videotapes of the sub-aqueous runs, but was not observed in the subaerialruns.

5. Experimental results: soft runs

All the antecedent deposits for the soft runswere generated subaerially using the sticky rheology.

Both subaerial soft Runs 5a and 6a used the runnyrheology. Both runs behaved similarly, bulking byremobilizing large amounts of antecedent materialfrom below. This allowed the generation of highhead velocities and long run-out distances. As notedin Table 1, the run-out distances xh were in excess of8.7 m (i.e., the highest values of all the runs reported

here).An example of the behavior of a subaerial soft

run, Run 6a, is given in Fig. 4. The thickness profile


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Fig. 3. (a) Deposit-thickness profiles for Runs 1a and 1w. (b) Deposit-thickness profiles for Runs 2a and 2w. (c) Deposit-thickness

profiles for Runs 3a and 3w.

of the antecedent deposit is shown in the figure

as Deposit 1. The thickness profile of the depositresulting after Run 6a is shown as Deposits 1 C2. Cores of the final deposit were taken at the fivelocations marked with vertical lines in the figure.The length of the vertical line denotes the extent ofthe deposit with the color of the overriding material,measured at the channel centerline. It can be seen

that at least some overriding material worked its waynearly to the hard bed over the first 4 m of deposit,indicating extensive remobilization of the antecedent

material. Even downslope of the slope break, it can

be seen that large amounts of antecedent materialwere displaced.This behavior can be seen more dramatically in

the images of the cores in Fig. 5, which were taken1.79 m, 4.88 m and 7.71 m downstream of thehead gate. The darker material is the overridingmaterial and the lighter material is the antecedent

material. Not only is extensive remobilization of theantecedent material apparent, but in addition somemixing of the two is evident.


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Fig. 4. Deposit-thickness profiles for the antecedent (Deposit 1) and antecedent-plus-overriding (Deposits 1 C 2) deposits of Run 6a. The

vertical bars represent the extent of the overriding material at the centerline of each of five cores.

The two subaqueous soft Runs 4w and 5w fol-lowed the same protocol as the subaerial soft runs,with the Run 4w having the sticky rheology andRun 5w having the runny rheology. The profiles ofdeposit thickness for Run 5w are shown in Fig. 6,with Deposit 1 denoting the antecedent deposit and

Deposits 1 C 2 denoting the final deposit afterthe overriding run. The six vertical lines denotethe position of six cores; their extent denotes theportion consisting of overriding material along the

Fig. 5. A view of three of the cores from Run 6a. Extensive remobilization of the antecedent deposit is apparent.

channel centerline. In all cases but one, the bottomof the vertical line is nearly flush with the positionof the top of the antecedent deposit, suggestingthat the overriding deposit draped itself over theantecedent deposit with no mixing, remobilization,or deformation due to loading.

Three of the cores for this case are shown inFig. 7; they are located 1.67 m, 5.04 m and 7.48 mdownstream of the head gate. The interface betweenthe lighter-colored antecedent material and darker-


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Fig. 6. Deposit-thickness profiles for the antecedent (Deposit 1) and overriding (Deposits 1 C 2) deposits of Run 5w. The vertical bars

represent the extent of the overriding material at the centerline of each of six cores.

colored overriding material is quite sharp; the slightblurring was observed to be caused by moleculardiffusion of the dye. The cores argue strongly foran almost complete lack of remobilization of the an-tecedent deposit. This conclusion was confirmed byan analysis of videotapes taken during the runs. The

Fig. 7. A view of three of the cores from Run 5w. No remobilization of the antecedent deposit is evident.

videotapes showed that the overriding head did re-mobilize small amounts of antecedent material overthe very short subaerial reach between the gate andthe water surface in the Fish Tank, but once sub-merged, simply passed over the antecedent depositwithout mobilizing it.


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Fig. 8. (a) Front velocity as a function of distance downslope for Runs 2a6a. Velocity data for Run 5a are incomplete because the flow

outran the video camera. (b) Front velocity as a function of distance downslope for Runs 1w5w.

6. Interpretation of run-out in terms ofhydroplaning

The difference in behavior between the subaerialand subaqueous runs reflected in the deposit shapesof Figs. 2 and 3 is also evident in the measured headvelocities. Head velocity vh is given as a function of

distance from the head gate for the subaerial hardRuns 2a, 3a and 4a and subaerial soft Runs 5aand 6a in Fig. 8a. (Velocity data were not taken for

Run 1a.) In the case of the three subaerial hardruns therein, head velocity over the region with the6 slope is seen to increase monotonically as therheology progresses from sticky (Run 2a) to runny(Run 4a). Values of head velocity vh at a pointhalfway down the region with the 6 slope (i.e., 2.85m downstream of the head gate) are given in Table 2.

The indicated values ofvh are 0.69 m=s for Run 2a,0.93 m=s for Run 3a, and 1.23 m=s for Run 4a. Thissame pattern is reflected in the run-out distances. In


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Table 2

Parameters for evaluating debris flow mobility

Run a vh h6 Frd hy(t=m3) (m=s) (mm) (mm)

1a 0.001225 * 35 * 30

2a 0.001225 0.687 31 0.035 30

3a 0.001225 0.926 23 0.054 22

4a 0.001225 1.231 21 0.075 20

1w 1 0.616 18 1.90 80

2w 1 0.645 18 1.99 80

3w 1 0.605 16 1.98 59

4w 1 0.481 6.5 2.47 63

5w 1 0.625 16 2.04 54

Notes. The values of h6 for Runs 4w and 5w refer only to

the overriding deposit. An asterisk denotes a case for which the

information was not collected or could not be computed.

the cases of the two subaerial soft runs, the effectof bulking due to remobilization leads to values ofhead velocity near 1.5 m=s over the 6 slope.

The head velocities of the subaqueous runs, on theother hand, show much less dependence on rheology,as illustrated in Fig. 8b. All the hard and softsubaqueous runs are shown therein. Head velocitiesequilibrate near 0.50.7 m=s by a point 2 m down-stream of the head gate, and maintain these values

until about 1 m beyond the slope break. While the

diagram reveals some dependence ofvh on rheology,its role is clearly secondary in comparison with thesubaerial runs.

These velocity patterns are consistent with the hy-pothesis that the subaqueous runs had hydroplaningheads, whereas the subaerial runs did not. Furtherevidence for this can be found using an appropriate

criterion for hydroplaning. Mohrig et al. (1998) havefound that debris-flow heads show evidence of hy-droplaning for a densimetric Froude number Frd inexcess of about 0.3, where

Frd Dvhs

d

a 1

gh cos

(1)

and d is the density of the debris slurry, a is thedensity of the ambient fluid, h is the thickness ofslurry in the body behind the head and denotes the

slope angle. In order to evaluate these parameters,vh was computed at a point halfway down the 6slope, h was approximated by h6 (i.e., the average

thickness of the deposit over the zone with a 6slope), and a was estimated using standard valuesfor air and water. These values are given in Table 2for the subaerial hard Runs 2a, 3a and 4a and all

the subaqueous runs. The soft subaerial runs wereexcluded because remobilization of the antecedentsubstrate made it difficult to estimate h.

As seen in Table 2, the subaqueous runs displayvalues of Frd within a narrow band between 1.9 and2.5 (i.e., well in excess of the value of 0.3 previouslyfound necessary for incipient hydroplaning of thehead). All three hard subaerial runs, on the otherhand, are well below the condition for hydroplan-ing.

Using clay-free slurries with a different rheologythan those of the present work, Mohrig et al. (1998)observed debris flows with hydroplaning heads thatattained higher head velocities than non-hydroplan-ing flows. The same was not observed in the presentcase; as can be seen from Table 2 and Fig. 8, thesubaerial flows consistently attained higher head ve-locities on the 6 slope than the subaqueous flows.This notwithstanding, all of the paired hard sub-aqueous runs ran out farther than their hard sub-aerial counterparts, as shown in Figs. 2 and 3. Fig. 8and the concept of hydroplaning allow for an inter-pretation of this seeming contradiction. The hard

subaerial runs were all strongly decelerating, even onthe 6 slope. Had this slope continued beyond the 5.7m point the flows would clearly have come to restwithin a few more meters. The heads of the subaque-ous debris flows, on the other hand, attained nearlyconstant head velocities as a result of hydroplaningon the 6 slope, and evidently could have continuedmuch farther down a 6 slope. The implication is thathydroplaning can result in longer run-out distancesby suppressing deceleration of the debris flow.

7. Effect of hydroplaning on the capacity to

remobilize antecedent deposits

It is evident from Fig. 8b and Table 2 that theheads of the soft subaqueous debris flows hy-droplaned in the same way as the hard subaqueousruns. This provides a key as to why the subaque-ous debris flows were unable to mobilize antecedentdeposits, in contradistinction to the subaerial flows.


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Here a simple Bingham rheology is again as-sumed for the sake of argument. The pattern of de-position for the subaerial debris flows indicated thatthe thickness of deposition was largely controlled by

the yield strength y. According to this hypothesis,then, the deposit thickness should correspond to theyield thickness hy, where:

hy Dy

.d a/g sin (2)

(e.g., Schwab et al., 1996). The values of hy pre-dicted from Eq. 2 on the 6 slope are compared with

the observed average values of h6 for the subaerialand subaqueous hard runs in Fig. 9. Whereas theobserved values of deposit thickness are quite closeto the predicted ones in the case of the subaerialruns, they are well below the predicted values forthe subaqueous runs. The subaqueous debris flowsevidently formed deposit thicknesses that were muchless than those associated with the yield strength.

Comparing Fig. 9 with Figs. 2 and 3, it is seenthat the hydroplaning heads of the subaqueous runstended to run out ahead of their bodies, in one case

Fig. 9. Deposit thickness hy on the 6 slope predicted from Eq. 2 compared with observed average thickness h6 for the subaerial hard

Runs 1a4a and the subaqueous hard Runs 1w3w. Runs 1w and 2w plot on top of each other.

even detaching from it. That is, hydroplaning of thehead can extend the body behind it and result in amarked decrease in the thickness of deposit belowthe value dictated by the yield strength.

This observation allows for the identification ofone reason for the difference between the soft sub-aerial and subaqueous runs. If a deposit is resting atits yield strength, the placement of more material ontop should immediately mobilize it. The antecedentsubaerial debris deposits of Runs 5a and 6a wouldthus have been remobilized even by static loading asa soft subaerial run passed over it.

The antecedent deposits of the soft subaqueousRuns 4w and 5w were deposited subaerially, andwere thus likely close to subaerial yield conditions.Once submerged in water, however, these same de-posit thicknesses would have been rendered less than40% of the yield thickness due to the buoyancy re-sulting from immersion. As can be seen from Figs. 6and 9, in the case of Run 5w even the static loadingassociated with the second (overriding) deposit re-sulted in a total deposit thickness that was still belowthe immersed yield thickness.


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The observation that the deposits of debris flowsassociated with hydroplaning heads should havethicknesses that are considerably less than the yieldthickness provides part of the explanation as to why

no remobilization was realized in the soft subaque-ous runs. There is, however, another, more com-pelling reason. The passage of the head of a movingdebris flow over an antecedent deposit should gen-erate a marked transient dynamic component to theinterfacial shear stress that would be effective in re-mobilizing even a deposit below the yield thickness.In the case of the subaqueous soft runs, however,hydroplaning would strongly suppress this by sand-wiching a thin layer of low-viscosity ambient fluidbetween two layers with higher viscosity. The differ-ence in viscosity would strongly damp the transmis-sion of shear stress from the base of the overpassingdebris flow to the top of the antecedent deposit, ac-cording to the classic mechanism of lubrication (e.g.,White, 1994).

8. Discussion and conclusions

The experiments reported here indicate that hy-droplaning of the head can cause subaqueous debrisflows to run out farther over an inerodible bed than

otherwise identical subaerial debris flows. The effectof hydroplaning, when it occurs, strongly mutes therole of debris rheology, and causes the head to runout ahead of the body. This in turn results in a thick-ness of deposit that is well below that associatedwith the yield strength.

The same mechanism of hydroplaning acts to sup-press the remobilization of an antecedent debris de-posit by an overpassing subaqueous debris flow. Be-cause the thickness of an antecedent deposit is likelyto be well below the value associated with the yield

strength, the addition of a deposit on top may be insuf-ficient to mobilize it statically. In addition, dynamicmobilization due to strong transient shear stresses as-sociated with the passage of the head is suppresseddue to the incorporation of a lubricating layer of waterbetween the overpassing and antecedent debris. Thislubricating layer inhibits the transmission of shearstress between the two debris layers.

Neither the results reported here nor the earlierresults of Mohrig et al. (1998) conclusively demon-

strate that the considerably greater run-outs typicallyobserved in the field for subaqueous as compared tosubaerial debris flows (e.g., fig. 19 of Hampton et al.,1996) are due to hydroplaning. By the same token,

they do not rule out the possibility that subaqueousdebris flows in both the laboratory and the field canin some cases remobilize antecedent deposits. Thesereservations notwithstanding, hydroplaning offers arheology-independent mechanism for both greaterrun-out distance and suppression of remobilizationin the subaqueous environment that is both physi-cally well founded and appealing in its simplicity.In addition, the experimental evidence that subaque-ous debris flows can leave deposits that are muchthinner than that associated with their yield strengthopens room for reinterpretation of the emplacementmechanism of some field deposits.

Acknowledgements

This research was funded as part of the MarineGeology and Geophysics program (STRATAFORMproject N=N00014-93-1-0300) of the U.S. Office ofNaval Research. Additional funding was provided bythe Research Council of Norway (European Mast IIIproject ENAM II). The method to determine debris

rheology using two half-channels was introduced tothe authors by K.X. Whipple.

Appendix A

This appendix is devoted to a brief description of the use

of a half-channel to determine debris slurry rheology. Consider

the steady, equilibrium subaerial flow of debris slurry down a

half-channel of circular cross-section with radius R, a horizontal

free surface, and a streamwise slope angle . The inside wall

is roughened to prevent sliding at the boundary. The boundary

shear stress b

associated with equilibrium slurry flow is given

by:

b D12 Rdg sin (A1)

A fairly general expression for non-Newtonian rheology of

the debris slurry can be written as:

D ysgn

@u

@y

C

@u@y1

@u

@y(A2)

where denotes shear stress, y denotes yield strength, u denotes

flow velocity, y denotes normal distance from the boundary, is

a coefficient and is an exponent. For the case of a Bingham


13/13


fluid, for example, is equal to unity and is the fluid viscosity.

In general, the steady, equilibrium flow satisfying the above

rheology possesses a plug zone of uniform velocity Up for

r < ry , where r D R y denotes radial distance from the

channel center and ry is given by:

ry D Ry

b(A3)

A shear zone prevails over the layer ry < r < R, where the

flow velocity u.r/ varies from Up to 0. Plug velocity Up is given

by the relation:

Up D R

dRg sin

2

1=

1 C

1

ry

R

.1C/=(A4)

and the velocity variation in the shear layer is given by:

u.r/

UpD 1

r ry

R ry

.1C/=(A5)

The flow discharge through the half-channel is given by:

Q1=2 D R2Up

1

2

1 C 2.1 Ory/Ory

1 C 3.1 Ory/

2

(A6)

where Ory D ry=R.

The experiments are best performed with two half-channels.

The value of ry, and thus y from Eq. A1 and Eq. A3, is

determined photographically with the larger of the two channels.

This value can be checked against the value obtained for a flat,

sloping surface. Once the yield stress has been estimated, the

determination of a value of Q1=2 for each of the two channels

specifies two constraints on the two remaining unknowns and

using Eq. A6 reduced with Eq. A4. The larger the difference

in diameter the more accurately the two parameters can bediscriminated. The analysis can also be performed with a single

channel, in which case Up is also determined photographically

and and are evaluated from Eqs. A4 and A6. Here a

simplified version of the single-channel method was employed,

according to which Up and ry are evaluated photographically and

is set equal to unity (assumed Bingham rheology). This allows

for the estimation of via Eq. A6 reduced with Eq. A4.

The above methodology is predicated on the assumption that

there is minimal vertical segregation of the material during a flow

event. The effect of vertical segregation can be evaluated as fol-

lows. The surface velocity profile is determined photographically

using markers, and then integrated to determine the discharge

Q1=2 that would result if the flow reflected this same profile

everywhere without vertical segregation. If vertical segregation isnegligible, the value so determined should be equal to the one

determined by Eq. A6.

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