E3S Web of Conferences 415, 01021 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341501021

Segregation of water and gravel using large ball mill device laidroughness bed
Kazuki Saito1*,and Toshiyuki Horiguchi2
1National Defence Academy Civil Engineering, 2388686 Yokosuka Kanagawa, Japan
2Associate professor, National Defence Academy Civil Engineering, 2388686 Yokosuka Kanagawa, Japan

Abstract. This paper presents an experimental approach to study the phenomena of segregating water andgravel using a large ball mill device that reproduces segregation phenomena under debris flow. First, theexperiment was conducted in a ball mill device under various conditions: water and gravel mixing, bed
velocity, and laid roughness. The occurrence conditions of the segregation phenomena of water and gravel
mixing are categorized and compared with each other with and/or without roughness on the bed.
Furthermore, the movements after water and gravel mixing in the ball mill device are observed to investigate
the segregation mechanism.

1 Introduction
In recent years, heavy rains and large typhoons haveoften caused natural disasters in Japan, resulting insediment flow that exceeds the capture effect andcapacity of dams. Therefore, it is necessary to install a
Sabo dam. The open Sabo dam, a countermeasure
structure, is designed based on the premise of thesegregation phenomenon in which gravel or boulder
concentrates on the front debris flow. However, there
are many unclear points in the design information, suchas the fact that there are cases where debris flow is
flowing down without being classified.
The authors [1] of conducted experiments on straightcanals. Therefore, the classification may not be accurate
because of the limitation of the channel length,depending on how the bottom surface roughness isconsidered. It is difficult to observe the classification
phenomenon occurring at the front of a debris flow indetail.
Therefore, research has been conducted to contain an
infinite length using a ball mill device to observe thephenomena occurring in the water channel at a fixed
point under a steady state. Further, the authors [2,3] haveconducted research using a ball mill device that observes
the phenomena occurring in channels at fixed points.
We attempted to segregate the occurrence of thephenomena for two particle sizes by organizing theconcentration mechanism of large particles in a mixed
sphere. Subsequently, after confirming that the front of
the driftwood rises like a surge [4], a characteristic ofdebris flow, in the ball mill device containing only water,
the movement form of the driftwood in the mixed stateof water and driftwood was summarized. However, it
was considered that the classification phenomenon ofdriftwood occurs owing to the circulation of water in thestate where a surge was generated. However, the surge
formation conditions, motion morphology, andclassification mechanism in a mixed experiment ofwater and gravel in a ball mill device have not beenclarified.
Therefore, this study elucidates the classification
phenomenon that occurs when gravel and stone flow
down. The surge formation condition, behavior, andoccurrence condition of the classification phenomenon
are described using a mixed fluid of water and gravelusing a ball mill device. We mainly examined the effectof bottom roughness on surge and classification.

2 Outline of experiment
2.1 Ball mill device, bed roughness, gravelmodel
Fig.1 shows the equipment used in the experiment. The
experimental device is a rotated channel with a diameter,depth, and width of 235, 30, and 30 cm, respectively.
The device is rotated clockwise using an electric motor.
The bottom and side surfaces are made of stainless steel
glass, respectively, and the internal behavior can beconfirmed from the outside through the glass on the sidesurface. The ball mill device has no limitation on theflow path length, and the observation position is almost
fixed. Therefore, if these conditions are met, anequilibrium state can be created and maintained for along time.
The roughness of the bottom is such that theroughness height (k) is "3 × 3 mm acrylic square pillar",and spacing (b) is attached to the bottom of the rotatedchannel at intervals of 26 mm.

* Corresponding author: em61030@nda.ac.jp© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0(https://creativecommons.org/licenses/by/4.0/).

E3S Web of Conferences 415, 01021 (2023)DFHM8

https://doi.org/10.1051/e3sconf/202341501021

モデルＡ
Model A（小）

モデルＢ
Model B
（中）モデルＣ
Model C（大）ΦΦ== 88 mmmm

ΦΦ== 15mm
15 mm

ΦΦ== 20mm
20 mm

Direction of
回転方向
rotation

2350 mm mm
直径：2350

Fig. 2. Gravel model
Table 1. Experimental caseWidth

水路幅：30030 mm mm

Bottom roughDepth
水路深：300
300 mm mm
Fig. 1. Ball mill device

(a)

Water

GravelBottom velocity

Mix(A,B,C)
1.5 L

0.0 m/s
1.0 m/s
2.0 m/s
3.0 m/s
4.0 m/s
5.0 m/s

No18.5 L
Withv = 1.0 m/s(b) v = 5.0 m/s
Fig. 3. Movement form in the mixture between water and gravel (no roughness)(a)v = 1.0 m/s(b)v = 2.0 m/s(c)v = 3.0 m/s(d)v = 4.0 m/s

Fig. 4. Movement form in the mixture between water and gravel (with roughness)

Fig.2 shows three types of gravel models (hereafterreferred to as models A–C). These materials are
solidified coal ash, and the average particle sizes of A,
B, and C are 8, 15, and 20 mm, respectively.

increased in increments of 1.0 m/s. In addition, we
compared two cases: one with and one without bottomroughness. Subsequently, the presence or absence of asurge was determined, and the motion form wasobserved.
In the water and gravel mixing experiment, thevolume of water and gravel was 18.5 and 1.5 L,
respectively.

2.2 Experimental procedure
In the mixed experiment of water and gravel, thebottom speed was set between 1.0 and 5.0 m/s and2

E3S Web of Conferences 415, 01021 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341501021

Table 2. Stage wave formation classification, motion morphology, and classification in each experiment
Bottom velocity v (m/s)

GravelmodelMix(A,B,C)

Roughness hed1.0

2.0Surge

MovemetClassification

Surge

No×Ⅰ

○with×Ⅰ○※1 Surge○ : Surge, × : No surge

3.0

Mov
emet

Classification

Surge

×

Ⅰ

○

○Ⅱ○

4.0

MovemetClassification

Surge

○Ⅰ○

○

Ⅲ△

※2 Classification○ : Classification occurrence△ : Difficult to distinguish× : Vaporization

Classification

Surge

○Ⅱ△

○Ⅳ×Movemet

Classification

○Ⅱ△

○

Ⅳ×Ⅰ

Water gravel separation stateⅡWater gravel mixed state

Ⅲ

Water gravel mixed state and tip vaporization state

ⅣVaporization state

Small particle size
circulation area

3 Result

5.0Movemet

Large particle size
circulation area

3.1 Movement
Fig.3 and 4 show the movement results for the cases
without and with bottom roughness in the water andgravel experiment.
When v = 1.0 m/s, as shown in Fig.3(a), the gravel
mass is located behind the water and becomes a gravelseparated state. At this time, the gravel moves
individually in the mass, and the maximum particle size(green) is concentrated forward. At v = 5.0 m/s in Fig.4(b)., the gravel mixes uniformly with water and becomesa gravel-mixed state.
Fig.4 (a) shows the results at v = 1.0 m/s, the same
motion mode as that shown in Fig.3 (a) (water/gravel
separation state). At this time, the gravel collides withthe roughness of the bottom surface, and the larger theparticle size, the easier it is to collect in the front. Fig.4(b) shows the results at v = 2.0 m/s when water andgravel are mixed uniformly. Similarly, in the case of v =3.0 m/s, as shown in Fig.4 (c), water and gravel are
mixed. However, as the bottom velocity increases, thegravel at the tip strongly collides with the roughness ofthe bottom and evaporates as it jumps up, making theclassification difficult. Fig.4 (d) shows that the bottom
velocity is considerably high at v = 4.0 m/s, and thewater thinly spreads as it is pulled because of theroughness of the bottom. Consequently, the gravel
jumps violently and vaporizes with almost no influenceof water.

Collision between large particles andlow surface roughness

Fig. 5. Velocity distribution during flow

and gravel. In addition, with bottom roughness, at v =2.0 m/s, it is in the same category as v ≧ 4.0 m/s with noroughness, and it can be observed that the roughness has
the same effect as no roughness at low velocity. Further,it can be observed that when v ≧ 3.0 m/s, the vaporizedstate gradually starts from the tip, and with large particle
size, the entire area vaporizes. Regarding theclassification, the classification occurs at v ≤ 2.0 m/s,
regardless of the presence or absence of bottomroughness. From v ≧ 3.0 m/s, when there is no bottomroughness, water and gravel mix with each other as thebottom velocity increases, making the classificationdifficult. However, with the bottom surface roughness,
the fluid gradually vaporizes and cannot be classified.
Therefore, it can be inferred that classification is likely
to occur in the low-speed region.
3.3 Classification method and bed roughness
From the above results, when the gravel classification
mechanism is summarized, the same phenomenon asthat pointed out in previous reports 2) and 3) on theeffects of water contamination and bottom roughness isconfirmed, as shown in Fig. 5.When gravel withdifferent particle sizes is mixed, large gravel particles
rise to the upper layer because of the Brazilian nut effectwhile climbing backward on the bottom surface.
Therefore, as it travels forward along with the forward
flow in the upper layer of small particles, gravel isformed forward and classified in the circulation regionof large particles.
The classification associated with this circulation
becomes smooth when water is mixed with gravel. In
addition, when the bottom surface roughness isapplicable, the collision force between the roughness
and the large grain-sized gravel triggers the Brazil nut
phenomenon such that the riverbed surface causes ahigher degree of classification.

3.2 Movement category and classification
Table 2 lists the wave formation and motion
morphology classifications in the mixed experiment ofwater and gravel. First, the surge formation
classification is in complete agreement with theexperimental results of water, as shown in Table 2. In
addition, when a surge is not formed, the water andgravel are separated and are in equilibrium, regardlessof the presence or absence of bottom roughness. On theother hand, at v = 3.0 m/s with no roughness, a surge isformed, and water and gravel are in a separate
equilibrium state. At v ≧ 4.0 m/s with no surface
roughness, a surge is formed, and the gravel is uniformly
diffused into the fluid, resulting in a mixed state of water3

E3S Web of Conferences 415, 01021 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341501021

Occurrence of debris flow due to rainfall, etc.
2) Water gravel mixed state, classification

2) Water precedent / remaining gravel3) Complete advance of water, fan-shaped groundOccurrence section
Flow section
Sedimentation section

Sweep section

Fig. 6. Expected movement in a real riverIn this experiment, the bottom surface is incontinuous motion, but this is not the case in the actual
river; therefore, the circulation motion in thisexperiment causes a flow velocity difference such thatthe lower and upper layer flow velocities in the actual
river are lower and higher, respectively. Considering
this point, Fig. 6 shows the phenomenon observed in thisexperiment compared to that in an actual river. That is,1) The multiphase flow in which water, earth, andstone are mixed in the upstream area becomes a rapid
flow owing to the steep slope and flows down to a state
where water, sediment, and gravel are uniformly mixed,
as shown in Fig.3(b). Thus, gravel with a large diametercollects at the tip owing to the mechanism shown inFig.5.
2) When the slope becomes gentle downstream, thespeed of the entire earth and stone flow decreases. In thisexperiment, this state corresponds to that with a low
bottom surface speed, as shown in Fig.4(a). Further, as
the bottom surface friction received by the earth andstone is predominant, the earth and stone begin to settle
and show a tendency to remain. Even at this time, thespeed difference between the upper- and lower-layers
results in the collection of large gravel particles at thehead.

2) When water and gravel were mixed, classification
was observed under all conditions up to 5 m/s withoutbottom surface roughness; however, when bottom
surface roughness was applied, the entire fluid did notvaporize at v ≤ 3.0 m/s. The classification occurred in
the range of 1–2 m/s.
In this experiment, only one case was considered forthe bottom surface roughness. However, as it isnecessary to examine the effect of finer bottom surface
roughness, it will be addressed in future work.
Furthermore, we conducted experiments on the mixing
ratio of water and gravel only for the cases of 18.5 and
1.5 L. In our future work, experiments will be conducted
at different concentrations.

References
1.

2.

3.

4 Conclusions
This study experimentally investigated the surge
formation conditions, motion morphology, andclassification occurrence mechanism of the water andgravel mixture by using a rotating cylindricalexperimental device. At this time, we considered theformation process of a surge, the morphology of motion,and the classification phenomenon owing to theinfluence of the presence or absence of bottomroughness. The results obtained are summarized asfollows.
1) In the experiment with water and gravel mixture,
there were two types of motion: water/gravel-separated
state and water/gravel-mixed state, with no bottomroughness. On the other hand, with the bottom surface
roughness, there were four types of motion: water/gravel
separated state, water/gravel mixed state, water/gravel
mixed state + tip vaporization state, and vaporization
state.

4.

5.

4

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