Journal of Robotics, Networking and Artificial Life, Vol. 4, No. 3 (December 2017) 234–238___________________________________________________________________________________________________________Experiments of Floatable UAV for Wave Dissipating Block InspectionKeisuke Watanabe
Dept. of Navigation and Ocean Engineering, Tokai University,
3-20-1 ORido Shimizuku, Shizuoka, Shizuoka, Japan
E-mail: keisukejapan@gmail.com
Koshi Utsunomiya

Dept. of Navigation and Ocean Engineering, Tokai University,
3-20-1 ORido Shimizuku, Shizuoka, Shizuoka, Japan
E-mail: bebop0911@gmail.com
Kazuho Mitsumura

Dept. of Navigation and Ocean Engineering, Tokai University,
3-20-1 ORido Shimizuku, Shizuoka, Shizuoka, Japan
E-mail: kazuho.rc@gmail.com
Shun Takasaki
Dept. of Navigation and Ocean Engineering, Tokai University,
3-20-1 ORido Shimizuku, Shizuoka, Shizuoka, Japan
E-mail: 4bkw1105@tokai-u.ac.jp

Abstract
In this paper, we present our modification experience of making a UAV watertight and floatable. Then we present
several experiments on taking pictures considering ocean monitoring in the sea near our campus. The watertightexperiments are done from our school’s pier. In these experiments, we repeated to start flying the UAV from thesea surface and fall it down into the water several times. Using this UAV, we tested several experiments to takepictures of wave dissipating blocks at MIHO Island where beach line erosion is critical and many blocks arearranged to prevent waves from washing the shore. We can see the shoreline very clearly from more than 200m
height as a macro image and also we can inspect the blocks looking around its stacks as well as approaching near
the sea surface where waves break and wave splash washes the UAV.
Keywords: Floatable UAV, environmental monitoring, coastal erosion, offshore structure monitoringsuch as coastal line monitoring where beach erosion issevere, inspection of offshore structures like wind
power stations or coral reef observation etc. Usually theUAV flies high above the water surface, however, it is

1. Introduction
A UAV is an effective low cost photographingapparatus for ocean environment monitoring1,2. Taking
images is essential for these environment monitoring

Copyright © 2017, the Authors. Published by Atlantis Press.
This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).
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Journal of Robotics, Networking and Artificial Life, Vol. 4, No. 3 (December 2017) 234–238___________________________________________________________________________________________________________possible that the UAV happens to fall down into thewater due to operator’s control mistake or malfunctionof itself. In addition, in some operations, it is desirable
for the UAV to be able to float on the water surface tosave energy consumption. In those cases, we need tomake the UAV watertight and floatable.
As shown in Fig.1, while the MIHO Island inShizuoka prefecture is designated as a World Heritage
Site, rapid and massive beach erosion of the island is
one of the critical issues3,4. It is said that UNESCO is
warning if the wave dissipating blocks will not be
removed, they will cancel the WHS designation because
the blocks damage the beauty of the landscape of thecoastline of the island. However, the wave dissipating
blocks are essential to keep shore sand from being
washed away by waves or currents. There is a very
important road Route 150 that faces the shore and themost important role of these blocks is protecting theroad from caving or depression due to road foundation
washing away.
So it is very important for us to monitor theshoreline as well as the condition of those blocks widely
viewed for some long term constantly. Using watertight
UAVs are one of the possible solutions for this problem.

Fig.1 Coastal shore erosion and wave dissipating blocks

Fig.2 Electrical component in a watertight container

2. UAV modification for watertight
To realize the UAV monitoring system, we firstly needto examine how to make it watertight and floatable. The
electrical components that are vulnerable to water are
GPS module, wireless communication system, motor
drivers, batteries and motors for propellers. We packedthose systems inside a watertight container as shown in
Fig.2.
The watertight container is made of polypropylene.
We chose polypropylene because it’s light and no radio
wave is shielded. The lid has a silicon rubber sealer andcan be attached tightly to the box itself.

Fig.3 Diving of the UAV drone into the water

Fig.4 The UAV sunk in the tank235

Journal of Robotics, Networking and Artificial Life, Vol. 4, No. 3 (December 2017) 234–238___________________________________________________________________________________________________________We packed and arranged the electrical components
inside the container and only the wires of thrusters comeout of it through a hole which is sealed with siliconsealant. A float made of foamed polystyrene is attachedto each arm to have enough buoyancy.
We tested it both in our tank and in the sea to check
whether this method is available or not. At first, we
experienced some troubles, for example, one of the wire
connectors of motors inside the container is detachedbecause of the impact of falling down to the water
surface. In this case, the UAV capsized due to the flying
force unbalance. In one case, the lid was fallen off due
to the impact force by the water surface, so we modified
the attaching apparatus using mechanical hooks to makeit tighter than ever.
From Fig. 3 to Fig. 5 show an experiment of retakeoff of the UAV after diving into the water. As thefloat of each arm has enough buoyancy, once the UAV
dives into the water, it can come back to float on thewater surface immediately. Through these experiences
we confirmed the UAV to be able to use on the water
surface.
As the motors for the propeller are brushless motors
and if we wash those motors with fresh water soon afterwe use them, they can be used even if we put the UAV
into seawater as shown in Fig.6 and Fig.7.

Fig.5 The UAV re-takeoff from the water

Fig.6 UAV watertight experiment in the sea

3. Monitoring of wave dissipating blocks
After we confirmed our watertight method is available,
we took several experimental pictures of the wave
dissipating blocks at MIHO Island.
Fig.8 shows one area of those blocks from the heightof around 100m. From this picture, we can see theblocks captured some sand that is put by using many
dumper trucks to fill the eroded shore to recover.
However, it also shows at the edge of the lined blocks,
there seems another erosion started as shown in theupper left of Fig.8. We can see this situation from awider range picture. Figure 9 shows that shore erosion
proceeds in between two block areas. As we can see
from the picture, the shore line vanished clearly
between the two block areas. From the different colorsof the water, we can see the sand flow in the sea. Fromthe lower part of the Fig.9, we can notice a kind ofvortex flow which surrounds the edge of blocks where

Fig.7 UAV floating experiment on the sea surface

Erosion started area

Fig.8 Sand captured by the blocks

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Journal of Robotics, Networking and Artificial Life, Vol. 4, No. 3 (December 2017) 234–238___________________________________________________________________________________________________________water color is rather green than blue compared to theleft side of the picture. This implies a mountain ofblocks can keep some sand around its area, however, theeffect of accumulation is quite limited. For more than 20years, hundreds of dumper trucks transported sand to fill
the shore and put tons of sand into water, the shore line
has not recovered yet. From the pictures taken this time,
we confirmed our feeling that sand put into water neverrecover shoreline, instead, waves wash away sand. This
picture indicates if we monitor the same point for a longterm and trace changes of the landscape, we canestimate the amount of washed away sand as well as
find more effective arrangements of the blocks tocapture sand put by the dumper trucks every day.
Figure 10 shows the closer look of the blocks where
the island of blocks are sagged. From the picture, wecan find the sagging downward of the blocks. Those
blocks are not old but newly put into the shore. The
reason why this area is sagged downward is not clear,
however, it is possible that several blocks underwater
are collapsed. To find the reason, we inspected theblocks further by taking many pictures.
As shown in Fig.11, we found a broken block thatlooks newly put judging from its surface condition that
there are few seaweeds on the surface of the block. The
blocks are designed to collapse slowly absorbing thewave force to prevent sand from being washed away.
However, as we can find several broken blocks likethese, where the collapsing speed of a block looks too
fast, and the reason of sagging downward shown in theFig.10 seems due to this kind of collapse. We need toinvestigate further to find the exact reason of this kindof collapse of a block.
Furthermore, we found the evidence of wave
abrasion of the block as shown in Fig.12. This isbecause the water around the mountain of blocks
contain sand as we can see in Fig.9. From Fig.12, weguess sand mixed water abrades the block faster than weexpected compared to normal sea water.
As we mentioned above, one advantage of thefloating UAV is that we can observe these structuresfrom far away to very close to them. The point where
we took picture of Fig.12 is the opposite side of theshore. At this sea side of the blocks, it is very dangerous
and difficult for us to approach there by a boat and keep
the boat there to examine the target block because thewave is breaking and swell is strong. In addition, as
there splashes salt water of waves, it is secure for us toFig.9 Erosion proceeds between block areas

Fig.10 Downward sagging area of the blocks

Fig.11 Broken blocks which are relatively newFig.12 Wave sand abraded block237

Journal of Robotics, Networking and Artificial Life, Vol. 4, No. 3 (December 2017) 234–238___________________________________________________________________________________________________________make the UAV watertight and floatable. Thanks to thisadvantage we newly found the possibility that sandmixed water abrades the block faster than it is designed,
comparing between the picture taken from far (Fig.9)
and near (Fig.12).
4. Estimation of lost sand amount

AThe purpose of arranging wave dissipating blocks along
the coast of Miho peninsula is not only breaking waves
but also catching sand flowing from a big river, Abekawa river, which supplies massive sand frommountains. In addition, for more than 15 years, thegovernment puts massive sand from shore using
hundreds of trucks to fill up the shore but in vain.
As we see in the Fig.9, we cannot say the blocks
worked well but massive sand had been lost. We tried toestimate the amount from the picture taken by our UAV
shown in Fig.13.
The distance from the point A to B is 117.8m. Usingthis distance, we can estimate the each side of the red
triangle as 133.5m, 471.2m, 549.7m respectively. So wecan estimate the area as 27322.9 m2. The average water
depth can be estimated as 3m, so we estimated the lost
sand amount around 81968.8 m3. If we use the specificweight of sand as 2.0, then this amount equals to163,938 tf. This means almost 16,000 10-ton trucks.
Although we have to investigate more precisely how
much amount of sand is losing every year, especially bymeasuring the water depth profile further, we have justlearnt to be able to start this kind of estimation using ourUAV.

BFig.13 Lost sand estimation from the image

Acknowledgement
This research is supported by KAKANHI, Grant-in-Aidfor Challenging Exploratory Research, JSPS
JP26630230.
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5. Conclusion
In this paper, we modified the UAV to be able to use on
the sea. We packed the electrical components into awatertight container and checked the brushless motors
of propeller can be used with no special fabrication if
we maintenance them correctly. Using this watertightand floatable UAV, we tried monitoring of wave
dissipating blocks at MIHO shore. We found out several
new things related to collapse of blocks and shore line
erosion from observation using this UAV. This
investigation is new because approaching the offshoreside of the blocks had been difficult and expensive.
Using a floating UAV, we can access such kind of
dangerous zones easily with low cost. We’d like toinvestigate further to sophisticate this new apparatus.

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