Experimental study on drowning accident risk in river and side ditch
Sosuke Oka, Takaaki Okamoto, Keiichi Toda, Yasufumi Touma
Climate change increases the frequency of torrential rains and drowning accident frequently occurs. Therefore, it is very important to study the hydrodynamic force exerted on the humanbody in flood water. In this study, two kinds of the flume experiments were conducted using a small-scale model of the human body. First, we measured the drag force exerted on thehuman body in flood water. Second, we examined the drowning accident risk in a narrow side ditch. Flume experiments were conducted by changing the flume width (In real scale,
B=1.6m, 0.8m, 0.4m). Drag force exerted on the human body model was measured by a Force gauge. The results can be used as primary assessment to define the drowning risk in a riveror a channel after torrential rain.
Fig.1 shows the drag force measurement set-up. Hydrodynamic force on the human body model (1/10
scale) in x-direction was measured by a force gauge. Sampling rate is 10Hz and sampling time is 60s.
Considering falling down in the flood water, the posture of the human body was changed (standing andsitting). Human body model heights are k1=15cm (real scale: 1.5m, 12 year child) for standing position and
k2=7cm (real scale: 0.7m) for sitting position. To investigate the effect of clothing, we put cotton clothes on
the human body model. Bulk mean velocity Um and water depth H were changed (In real scale, flow depth
H=0.35, 0.50, 0.70m and flow velocity Um= 1.0, 1.5, 2.0m/s). When H=0.7m, the leg was submerged forstanding position and the whole body was submerged for sitting position. Table 1 shows the hydrauliccondition.
Second, flume experiments were conducted by changing the flume width (In real scale, B=1.6m, 0.8m,
0.4m), as shown in Fig.2. Drag force exerted on the human body model (1/4 scale) was measured by a force
gauge. Human body model heights are k1=30cm (real scale: 1.2m, 8 year child) for standing position and
k2=21cm (real scale: 0.84m) for sitting position.

Fig.1(a) Posture change of human body model (1/10 scale),(b) Drag force measurement

Fig.2(a) Change of flume width, (b) Posture change ofhuman body model (1/4 scale)

FD = 0.5 ρC DU c2 Ax = S = μ(Mg − Fb )Fb = ρgVo

Fig.3 Drag force exerted on human body (effectof posture change)

Fig.3 compares the values of the drag force exerted on thehuman body (1/10 scale) in a standing position and in asitting position (unclothed case). The drag force values
were converted to the force values in real scale. The
experiment data showed that the drag force in a sitting
position is 2-3 times larger than that in a standing position.
This is because the projected area normal to the incoming
flow increases in a sitting position, considering the falling
down in flood water.
Fig.4 compares the values of the drag force exerted on thehuman body (1/10 scale) for unclothed and clothed cases. Fig.4 Drag force exerted on human
The flow depth is H=0.7m in real scale. In a standing body (effect of clothing)position, the drag force exerted on the human bodyincreases by 57% for clothed case compared with that for
unclothed case.

Fig.5 Critical condition for drowning
accident (6 year old, 12 year old, adult)

Fig.6 Drag force exerted on human body(effect of flume width)

Fig.7 Backwater rise in narrowside dicth

Fig.5 shows the critical velocity Uc of 12-year child (M=46kg) for drowning accident in a wide channel. The critical velocity Uc
for 8 year child (M=46kg) was also calculated using scale ratios.
For the same water depth, the incipient velocity in a sitting position is smaller than that in a standing position. There are two
reasons. First, the drag force exerted on the human body increases in a sitting position compared with that in a standing position.
Second, the submerged volume of human body becomes larger in a sitting position and consequently, the frictional resistance S
decreases.
Fig.6 compares the values of the drag force exerted on the human body (1/4 scale) in a wide channel and a narrow channel. The
experiment data showed that the drag force in the narrow channel is 2 times larger than that in the wide channel.
For a narrow channel, the backwater rises in front of the human body (Fig. 7) and the hydrostatic pressure is exerted on the
body. The buoyancy force acting on the body increases due to the backwater rise.