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Artificial Reefs could decrease the impact of tsunami in coastal cities of Japan

By Jhofree Aponte

University of Kansas

GEOL 391- Special Studies in Geology

Independent Study Paper

July 24th, 2015.

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Table of Contents

Introduction.............................................................................................................................................2

Tsunami: Definition, Causes, and Run-up................................................................................................3

Impact of Tsunami...................................................................................................................................6

Reef Impact on Tsunami Wave Energy: The Hallberg Isopycnal Model..................................................9

Discussion on Model Results.................................................................................................................12

Earthquake History for Japan since 1990...............................................................................................13

Proposed Locations of Artificial Reefs in Japan......................................................................................17

Planning of an Artificial Reef..................................................................................................................23

Construction and Cost of an Artificial Reef............................................................................................28

Pros and Cons of Artificial Reefs............................................................................................................30

Benefits from Artificial Reefs.............................................................................................................30

Negative aspects about Artificial Reefs..............................................................................................31

Conclusions............................................................................................................................................32

Sources..................................................................................................................................................33

Introduction

Tsunamis are seismic sea waves capable of destroying coastal cities, usually caused by sudden

displacements of tectonic layers in the seabed at subduction zones by thrust faults, but can also be caused

by transform or normal faults, or even meteor impacts (Neil, 2010). Tsunami’s impact can be measured

by how much it affects people who live on affected coastal cities. The impact not only affects

infrastructures that later cost billions of United States Dollars (USD) to rebuild, but most importantly

affect people’s life, and the lucky people who survive are affected physically, emotionally and mentally

(Maddern, 2005). Most common prevention and way to decrease the impact from tsunamis on populated

coastal areas, has been the construction of seawalls ranging from 3 to 5 meters tall (Onishi, 2011). On

March 11th, 2011, Japan was greatly affected by a magnitude 9 earthquake that produced a tsunami, which

delivered up to 37 meters tall waves, overpowering seawalls on the coastal cities in Japan and clamming

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the life of about 16,000 people (Kingston, 2011) along with over $235 billion in structural damage

(Kennedy, 2011). Recently, Japan has proposed to build their seawalls even higher as a counter measure

for future tsunamis (Atherton, 2015), but other methods like implementation of artificial reefs (Kunkel,

2006) have been studied with equations and simulations in order to present a more environmental friendly

and efficient solution that would help decrease the impact of tsunamis in coastal cities.

Tsunami: Definition, Causes, and Run-up

For centuries tsunamis were called “tidal waves”, but due to studies in plate tectonics, and

geophysics, it is well-known now that they are not related to tides of the Earth. The word Tsunami comes

from Japanese meaning (tsu) harbor and (namis) wave, but it is usually referred as seismic sea wave

(Sverdrup, 2013).

Tsunamis can be generated by several methods. The most common cause is by a sudden

displacement of the ocean floor due to a thrust or normal fault in the seabed. Other causes are by

volcanoes, submarine landslides, or cosmic bodies crushing into earth, especially in the ocean. When a

tsunami is generated by a submarine earthquake due to a fault, it usually starts on a plate boundary,

particularly a subduction zone, where the denser oceanic plate moves constantly and very slowly beneath

a less dense oceanic plate. Here the plates rub against each other to build up enough stress and strain until

eventually these forces become too great and abruptly release themselves by snapping back on each other

and creating an earthquake. The abrupt movement of the plates and earthquake causes the overlaying

water of the ocean to mimic the behavior, creating a wave that would then propagate in all directions as

seen on (Figure 1). On Figure 1, the snapping of the plates at a subduction zone creates a column of water

that will turn into a wave which would propagate and carry a massive amount of water throughout the

ocean in all directions. Tsunamis occur all over the world, but they are mainly originated and more

intensified over the Pacific Ocean where subduction zones occur.

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Figure 1. Horizontally exaggerated and not to scale example of a tsunami when is generated. As

the Oceanic Place on the left drops, the column of water on top of it also drops. As the Oceanic

Plate on the right rises, the column of water on top of it also rises, causing a wave that will

propagate in the ocean in all directions (Neil, 2010).

The displacement of water generated by the earthquake could average about 4 kilometers in depth

depending on the depth at which the epicenter is located, and the water wave propagates in all directions

as a combination of longitudinal and traverse waves with a period in the range of 10 minutes to 2 hours

and wavelengths over 500 kilometers. The velocity of the wave (V) is determined by the squared root of

the product between the depth (d) or submarine topography, also known as bathymetry, and the gravity

(g) in that location of the ocean in which it travels:

V=√g ∙ d Eq. 1

The way in which a tsunami loses energy as it propagates is inversely related to its wavelength and depth.

At greater depths the wavelength would be greater, but the energy loss would be little. As the tsunami

approaches the coast the shallow depths decreases the speed, wavelength, and the energy, but increases

the height of the wave (Figure 2). On figure 2, the wavelength of the wave decreases and the height of the

wave increases as consequence of a shallower depth. This is because the amplitude of the wave, which

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expands vertically up and down, is increased when the horizontal wavelength is reduced. A typical

tsunami wave can range between 2 to 10 meters in height, depending on the bathymetry, when it reaches

the shore, but in the open sea it can barely reach 1 meter and it is virtually undetectable by ships or buoys.

The energy of a tsunami is not based on a single wave; it is rather based on a sequence of waves that

oscillate like regular beach waves, but with long periods and low frequency. Besides the fact that the

original wave carries a powerful blow of a massive column of ocean water, the wave succession is what

make a tsunami more powerful, because each wave demolishes structures and then adds the debris to the

next wave (Neil, 2010).

Figure 2. Tsunami changes in wavelength and height as it approaches the coast. In Deep Ocean

the wavelengths are bigger and the wave heights are shorter than on shallow waters (Neil, 2010).

The energy of the wave is commonly expressed and measured in terms of its run-up, which is the

difference in distance between the regular sea level conditions and how far the water reaches horizontally

and vertically during a tsunami. This relationship is due to the fact that the stronger the wave, the further

inland it will reach, causing more damage and impact on coastal populated areas. Therefore, run-up has

become one of the most important characteristics to be found and estimated on a tsunami, because it helps

to predict and calculate the damage or impact of a possible future tsunami (Neil, 2010).

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According to several simulations and experiments starting with scientists Hall and Watts in the

1950’s until now, the run-up can be estimated by using:

Rd

=2.831√cotβ ( Hd )

5 /4

Eq. 2

Here, (R) equals the run-up distance, (d) the depth, (H) the height of the wave, and (β) the angle of the

slope (Figure 3). According to Figure 3, and the equation above; as the slope angle decreases, the run-up

decreases; as the depth decreases the run-up increases (Neil, 2010).

Figure 3. Diagram showing the run-up (LR) in relation to the height of the wave (H), depth of the

basin (d), angle of the beach shore (β), and elevation of the terrain (R) (Neil, 2010).

Impact of Tsunami

The impact of a tsunami can be measure by its intensity over human populations that live near or

on coastal areas. The intensity is usually a term used to measure how earthquakes affect people and their

structures, but can also be used to approximate the impact of a natural disaster like a tsunami over dense

populated areas.

On March 11th, 2011, a magnitude 9 earthquake, located some 80 miles east of Sendai, Japan, hit

the eastern coasts of Japan followed by a devastating tsunami that killed nearly 16,000 people and

displaced nearly half a million people from their homes (Kingston, 2011). The tsunami, according to

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geologists, was caused by a sudden rupture or thrust fault in the subduction zone of the Japan Trench, and

the impact on coastal cities of Japan was later intensified by the nuclear meltdown in a nuclear reactor in

Fukushima, which was greatly affected by the earthquake and tsunami (Pletcher, 2015). According to

aftermath reports from the United Nation’s Intergovernmental Oceanographic Commission, the tsunami

waves reached up to 37 meters high in some cities like Koborinai, Japan, which only had seawalls of

about 5 meters high (Vergano, 2011).

Figure 4. Tsunami heights on Left and most affected areas on Right for the tsunami that hit Japan

in 2011 (Left retrieved from

http://jasper-cs373-wc.appspot.com/crisis/tohoku_earthquake_and_tsunami/, Right retrieved from

http://www.emsc-csem.org/Earthquake/196/Mw-9-0-off-the-Pacific-coast-of-Tohoku-JapanA-

Earthquake-A-on-March-11th-2011-at-05-46-UTC)

As seen on Figure 4, the tsunami created by the magnitude 9.0 earthquake on the coast of

Tohoku, Japan, made waves of different heights along the coast of Japan. The waves of greater heights

occurred on coasts closer to the epicenter of the earthquake.

The World Bank stated that the estimated cost to rebuild and restore most of the infrastructure in

Japan was about $235 billion, and also Japan’s economy was going to be affected by the lack and

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disruption in production of electronics and automotive, which play an essential role in Japan’s economy

(Kennedy, 2011). On figure 5, we can only see a glimpse of the devastation that a tsunami caused on

Onagawa, Japan back in 2011. Here we can see that there is a vehicle on the 3rd floor of a building,

suggesting that the tsunami that hit this town, was at least 20 meters high. Tsunamis can not only damage

and destroy the infrastructure in cities that would later cost billions of dollars to rebuild (Figure 5), but

also and most importantly the loss of life.

Figure 5. Tsunami disaster in Onagawa, Japan, 2011. The tsunami swept this man’s car on top of

a three story building (Kingston, 2011).

People who die during tsunami, not only get killed by the vast force of the tsunami waves rushing

in from the sea, most of them get killed or injured by the debris of structures, cars, houses, that move

along with the tsunami as it moves further inland. People who manage to survive the tsunami are usually

left with scars not only physically, but also mentally. Reports done after the tsunami that struck the Indian

Ocean on December 26th, 2004; show that many survivors who were lucky enough to not lose a limb, or

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major injuries, developed health issues and chronic diseases due to being exposed to rotting corpses and

mixture of debris and chemicals in the affected area. The most affected were people of low income who

lost everything and tried to go back to claim whatever they could find; children who lost their parents and

wondered around looking for them; and rescue volunteers, survivors and workers who took the risk of

getting sick in search for more survivors. Besides physical issues and after witnessing the destruction and

corpses of the approximately 250,000 people who perished during the tsunami along with the loss of their

family, friends, home, and belongings, most survivors developed psychological and post-traumatic stress

disorders (PTSD), which unfortunately many will more likely still experience them for years to come

(Maddern, 2005).

Reef Impact on Tsunami Wave Energy: The Hallberg Isopycnal Model

Previous studies indicate that the energy of wind-driven waves can be reduced to up to 80% when

they go across reefs. Empirical evidence shows that indeed coral reefs could diminish the energy of a

tsunami, but also if the reef exhibit either natural or man-made gaps in it, it could funnel the energy

through the gap and result in a greater run-up (Kunkel 2006).

The proximity of coral reefs to the source of the tsunami has proven to serve as little protection as

seen on Indonesia during the 2004 Indian Ocean tsunami, where between the presence or absence of

corals in the area resulted in little to no significant difference. Also, reefs are currently degrading by

anthropogenic pressures that result in bleaching or destruction of them, which weaken them and increase

the chances for them to collapse during severe storms and other natural factors. It is estimated that at least

60% of reefs could be dead by the year 2030 if the current environmental conditions stay constant. As

reefs age and die, they become weaker and fragile, being less successful to stand strong and exert enough

drag against waves. According to field studies reefs have a higher drag coefficient than of sand, which is

between 0.03 and 0.1 (Kunkel 2006).

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Important parameters are tsunami wavelength and amplitude, reef geometry and health. A

nonlinear shallow water model has been used to understand how reefs would diminish the energy of

tsunamis, called Hallberg Isopycnal Model. This model first demonstrated its ability to simulate

scenarios, when it was used to reproduce global tides and the 2004 Ocean Indian tsunami (Kunkel 2006).

The model used an idealized coral reef topography as observed in typical barrier reefs, separated

by a 4m deep lagoon extending about 100m as it slopes up to the shore, with a back-reef of about 20m

wide, crest varying from 60 to 500m and 0 to 4m deep, fore-reef slope of 40 degrees to a depth of 85m

with then decreasing slope down to 10 degrees towards the ocean floor as seen on Figure 6 (Kunkel

2006).

Figure 6. Typical Reef Model. Figure shows the characteristics of a Reef (Kunkel 2006).

The model tests scenarios in which parameters like lagoon width, reef depth, drag coefficient, and

amplitude of the wave are changed to analyze the run-up of the wave, which is the elevation of the

maximum distance inland of a wave as it reaches the shore. After running the simulations by changing the

parameters, the model delivered the following results:

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Figure 7. Reef Modeling Run-up Results by changing parameters like Lagoon Depth, Reef

Depth, Drag Coefficient, and Incident Amplitude. Circled value are the 1m amplitude, 100km

wavelength wave on a 200m wide and 2m deep reef located 1000m offshore (Kunkel 2006).

According to the results, a lagoon behind the reef starts to be efficient after having a width over

1,000m, a less deeper reef is more efficient than a deeper one, the healthier or higher drag coefficient on

the reef the better is to reduce the run-up, and the lower the incident amplitude the better will the reef be

able to handle the wave and reduce the run-up (Kunkel 2006).

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Discussion on Model Results

0 0.5 1 1.5 2 2.5 3 3.5 4 4.50

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

f(x) = 0.124421052631579 x + 0.0405964912280702R² = 0.992167519635646

f(x) = 0.146105263157895 x + 0.144315789473684R² = 0.991755209620295

f(x) = 0.157473684210526 x + 0.287754385964912R² = 0.909062943425656

Run-up versus Reef Width and Depth

Width 60mLinear (Width 60m)Width 200mLinear (Width 200m)Width 500mLinear (Width 500m)

Reef Depth (m)

Run-

up (r

elati

ve to

no

reef

)

Figure 8. Regression Analysis between Run-up versus width and depth of a reef relative to no

reef (Aponte 2015).

Using the Hallberg Isopycnal Model results from figure 7, the Regression analysis (R2) is closest

to 1 for Reef Width of 500m, followed by 200m then 60m. This means that there is a better relationship

between the Run-up and Reef Width of 500m as the Depth of the reef increases. The 500m width reef

also displays the lowest values of run-up relative to no reef, suggesting that a wider shallower reef will

dissipate the energy of tsunami better than a thinner deeper reef or no reef.

Building artificial reefs could be a costly project, but cheap compared to the millions of lives and

infrastructures it could help save worldwide. To minimize the cost of making them, wider shallower reefs

could be place along coastlines where tsunami waves average to be high and devastating, while less deep

and thinner reefs could be placed along coastlines that are hit by moderate to low tsunami waves.

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To suggest where artificial reefs can be created would require knowing where the next

earthquakes will occur, but unfortunately earthquakes are still unpredictable and can occur anywhere

along the subduction zone near Japan. A way, perhaps, to estimate where will they occur next or are most

likely to occur, is to study which plates are moving faster through subduction beneath other plates near

Japan and to review where have most of the earthquakes occurred in the past since the faster movement of

the plates would increase the frequency of the earthquakes, as well as the amount of energy released when

the slabs relaxes the built up momentum.

Earthquake History for Japan since 1990

Figure 9. Map of Japan showing the strongest earthquakes since 1990 (Retrieved from Google Earth using data from USGS on June 25, 2015).

Japan experiences approximately 1,500 earthquakes a year, mostly originated from the

convergent margin between the Pacific and Okhotsk Plate, located on the east coast of Japan (Hijino

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2014). This has forced Japan to equip itself with earthquake resistant buildings that can stand up to a

magnitude 8 or 9 in the Richter scale, and with seawalls along the coast of Major cities that can counter

the destructive force of the tsunami created by these earthquakes (Onishi 2011). Unfortunately, these

seawalls have only proven to stand tsunami originated from under magnitude 8 earthquakes, and are

vulnerable to those originated from magnitude 8 or above. Therefore, studying where have most of the

magnitude 8 and above earthquakes have occurred can greatly help propose a possible location for further

protection against future tsunami.

According to Figure 9, created with Google Earth using data from USGS, it is clear that most of

the high magnitude earthquakes have occurred on the east coast of Japan. Other data for earthquakes of

lower magnitude also support this.

Using data from USGS about all the magnitude 6 and above earthquake epicenters that have

occurred in Japan since 1990, and focusing on the earthquakes that have occurred on the east coast of

Japan between the Pacific and Okhotsk Plate, the following observations were made:

257 of 529 were Magnitude 6 earthquakes.

26 of 51 were Magnitude 7 earthquakes.

Only 4 Magnitude 8 and 1 magnitude 9 earthquakes have occurred in Japan since 1990

and all have occurred on the east coast of Japan between the Pacific and Okhotsk Plates.

Total of 585 Earthquakes of Magnitude 6 and above, from those 288 have occurred on the east

coast of Japan between the Pacific and the Okhotsk Plates. Meaning that about 49% of all the earthquakes

that occurred in and around Japan have epicenters on the East coast between the Pacific and the Okhotsk

Plates. Breaking down that percentage by magnitude, 48.5% of all magnitude 6 earthquakes, 51% of all

magnitude 7 earthquakes, 100% of all magnitude 8 and 9 earthquakes have happened between the Pacific

and Okhotsk Plates.

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Taking a closer look on the areas where most of the epicenters have been for the earthquakes on

the east coast of Japan, we can see that:

121 out of 529 magnitude 6, 15 out of 26 magnitude 7, and the only magnitude 9

earthquake are east from the coast of Honshu, Japan.

103 out of 529 magnitude 6, 5 of 26 magnitude 7, and 3 out of 4 magnitude 8 earthquakes

are east from the Kuril Islands, Japan.

This analysis suggests that building artificial reefs along coastlines that are facing the Kuril

Islands and along the coast of Honshu would greatly protect the main Island of Japan against a

devastating tsunami. Japan is well protected by seawalls on highly populated areas against low to average

wave high tsunami, but it is still vulnerable against Major tsunamis usually created by magnitude 8 and

above earthquakes that occur along the subduction zone between the Pacific and Okhotsk Plates.

Using “Cones of Impact”, like described on the following figure 10, we can estimate the area of

common impact by the majority of the tsunamis over magnitude 6 that has hit Japan since the year 1990.

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Figure 10. Magnitude 6 and above earthquakes along the east coast of Japan showing a “cone of

impact” to estimate the most affected areas by tsunami (Retrieved from Google Earth using data

from USGS on June 25, 2015).

On figure 9 and 10, Magnitude 7, 8 and 9 earthquakes are displayed and shown happening on the

subduction zone between the Pacific Plate and the Okhotsk Plate. Other earthquakes of magnitude 6 and

below, not shown on figure, occur in abundance along the subduction zones of the Philippine Plate and

the Pacific Plate, and the Philippine Plate and the Eurasian Plate, but these cause relatively small tsunami

waves for which Japan have already developed structures like seawalls to prevent their damage onto their

coastal cities.

Using GPS Plate sensors located in several cities along divergent, convergent and transform plate

boundary zones around the World, it has been recorded that tectonic plates move at different rates, some

faster than others. The Okhotsk Plate; for example, is moving southeast and the Pacific Plate is moving

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northwest, which makes their speed on collision even greater than the Pacific Plate moving towards the

Philippine Plate. The faster rate would make the subduction area to be more active, creating more

frequent earthquakes and releasing more energy when the slabs snap after the excessive built up pressure

from the collision (Sella 2002). This aims to the proposal that an artificial reef location can well be

located on the east coast of Japan where the tectonic plates are more active due to subduction and along

the coast of cities with high population density.

Proposed Locations of Artificial Reefs in Japan

Japan has been building artificial reefs since the 1650’s, mainly to attract fish and increase the

growth of aquatic vegetation. The methods used to build these were mainly by using bamboo structures

and by dumping rocks to the bottom of the sea. The locations of these were dictated by fishermen, who

used the reefs to increase their capture rate and keep the fish abundance constant. During the last 100

years, the techniques in building artificial reefs have improved going from rocks and bamboo to concrete

blocks and non-toxic metal frames and structures. Since the 1950’s Japan has been creating and planning

projects to keep building artificial reefs to improve fishing as demand increases due to the increase of

population over the years (Nakamae 1990). All the previous artificial reefs built by Japan so far have been

focused to attract marine life to its coast, but these are not shallow enough and were not created aiming

towards decreasing the impact of tsunami.

The east coast of Japan varies in depth from location to location, but on average it is about 20

meters deep about 3 kilometers from the beach shore, which means that Japan has a low angle and low

depth on its coasts. According to Figure 3 and Equation 2, having a low angle and low depth along the

coast greatly increases the run-up and impact of the tsunami; therefore, placing it about 3 kilometers away

from the beach shore could protect the populated areas behind the artificial reef . To propose a location 3

kilometers from the beach it is also important to take in account the trading routes on the coast of Japan,

in order to ensure that vessels and navigation is not harm by artificial reefs. Using a Global Shipping Lane

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Network data published by Sean Gorman, who is a researcher and practitioner in the field of data science

with specialty in location based analytics and is currently the Chief Strategist for ESRI’s DC

Development Center, we can approximate better a possible location for artificial reefs along the coast of

Japan Major cities (Figure 11).

Figure 11. Global Shipping Network around Japan. Most common routes used by commercial

and private vessels around Japan (Retrieved from Google Earth using data from Sean Gorman

from GeoCommons.com on July 8, 2015).

Based by population density, Japan counts with 8 Major cities located on the east coast, these are:

Yokohama Kawasaki Tokyo Mito

Sendai Iwaki Kushiro Kitami

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Also secondary cities like:

Yokosuka Miura Awa Kamogawa

Choshi Kamisu Hitachinaka Hitachi

Ishinomaki Shiogama Tagajo Soma

Minamisoma Miyagi Hachinohe Shimokita

Asahi Futaba Watari Kamiiso

Hakodale Futami Yamakoshi Tomakomai

Horoizumi Nemuro Menashi Kitaibaraki

Takahagi

Followed by medium to small cities like:

Muroran Noboribetsu Hokuto Katsuura

Date Misawa Kuji Miyako

Kamaishi Ofunato Otsuchi Rikuzentakata

Kesennuma Minamisanriku Oshica Tomioka

Hirono Naka Kashima Sanbu

Chosei Kamogawa Tateyama Minamiboso

Awa Futtsu Kimitsu Kisarazu

Sodega Ichihara Chiba Hanamigawa

Narahino Funabashi Ichikawa Minato

Koto Edogawa Hodogaya Konan

Kanazawa Natori Iwanuma Oarai

From these the most affected by the 2011 tsunami were:

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Sendai Chiba Ichihara Oarai

Natori Iwaki Tokyo Iwanuma

Kesennuma Mito Miyako

All these cities are in danger of being struck by a major tsunami like the one that occurred on

March 11th, 2011. Therefore, to suggest a plan to build artificial reefs along the coast near to them would

be an impractical economic challenge for the near future due to the amount of cities that need to be

protected and that are equally vulnerable if another major earthquake of magnitude 8 or above would

happen again soon. A viable solution would be to build a few near cities that have already suffered the

devastation and are in a recovery phase, rebuilding and looking for a new beginning, in order to give them

a piece of mind that if it happens again, at least they will have an extra way to diminish or avoid further

damage like the one caused by the Tohoku earthquake in 2011.

In order to estimate where and how an artificial reef can be built, let’s focus on one city, Miyako.

Miyako, is located at coordinates 39°38’29” N, 141°57’25” E on the east coast of Japan, Iwate Prefecture,

and is one of the cities that got most of the damage from the tsunami back in 2011. Miyako, had 7.2-meter

high seawalls when the tsunami stroke the city, but it was not enough to protect the city (Figure 13). As

seen on Picture 12, most of the houses and buildings close to the coast disappeared.

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Figure 12. Satellite images of Miyako, Japan taken before and after the tsunami created by the

9.0 magnitude Tohoku earthquake. This is the Northeastern side of the city of Miyaku and the

first to be impacted by the tsunami in that area (Retrieved from Google Earth on July 20th, 2015).

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Figure 13. Tsunami wave going over 7.2 meter high seawall at Miyako, Japan on March 11th,

2011 (Kurtenbach 2015).

Miyako has an inlet from the ocean that leads to the coast of the city as seen on Figure 14.

Therefore, placing a 500 meters wide artificial reef across the inlet should provide enough protection for

the wave to dissipate enough to not damage or go over the already in-place 7.2 meter seawalls on

Miyako’s coast. The reef can be about 2 meters deep, which according to the Hallberg Isopycnal Model

and with a 500 meter width should decrease the energy and run-up of the wave for about 70%. At 2 meter

depth, light vessels can still navigate over the reef without getting exposed to any harm, but heavier

usually commercial vessels could use the alternate passages on the sides of the artificial reef to avoid

getting damaged or getting stuck in it (Figure 14). The location of this reef was chosen by using the depth

values provided by Google Earth where the depths measured between 15 to 20 meters and right before the

depth on the seabed increases rapidly into a deeper slope.

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Figure 14. Miyako’s suggested Artificial Reef location. Showing affected area from picture 12

and suggested routes for heavy commercial vessels (Retrieved from Google Earth on July 20th,

2015).

Planning of an Artificial Reef

The planning for this reef would require civil engineers, oceanographers, engineering geologists,

as other scientists experts on building marine structures to come up with the right combination of sand,

rock and materials to make an stable reef able to sustain hurricanes, severe storms, underwater currents,

longshore currents, waves, and of course tsunami. Also, permits, assembly area, financing, labor, possible

conflicts of interest, and transportation must be carefully considered before beginning the project. The

composition of the materials will depend on the location weather pattern, soil composition, and

bathymetry (Yip 1998). In general, the composition will be similar to of an underwater Breakwater, made

of sand and topped with a large pile of rocks. The material deposited to build these underwater

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breakwaters should add up from 12 to 17 meters in high. To make this underwater breakwater into a

productive artificial reef, non-toxic materials like the ones shown on Figure 15, can be placed on top of

the underwater breakwater.

Figure 15. Comparison of various materials that have been used in the development of

underwater artificial reefs in the United States, but that could be used for reefs in Japan. Although

used historically, material types in shaded rows are currently prohibited for use as artificial reeds

by the U.S Army Corps of Engineers due to past negative performance (Broughton 2012).

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From Figure 15, the most promising for our proposal are Concrete Designed Structures, because

offer long-term stability and can be shaped to the environment where they are meant to be put on. These

structures can be 1 meter tall “Reef Balls” along with some “Meshwork Direct Current Reefs”.

Reef Balls are dome-like symmetrical concrete structures of different sizes, but generally

measuring about 1 meter tall with a 2 meter width base in diameter and with holes measuring about 20 to

30 centimeters in diameter (Figure 16). Due to their shape, weight, and holes in it, it offers stability and

endurance to the reef whenever it is struck by hurricanes, storms or tsunami waves (Lance 2010).

Figure 16. Cluster of reef balls in Curacao. Reef modules sponsored by Reef Ball Foundation to

promote and protect natural reef systems, more than half a million of these have been placed in

oceans (Lane 2010).

They also possess longevity made to last for centuries, and their rough surface and openings

incentivize corals to attach to the structures, reproduce and create a natural habitat for other species to

move in to them. According to Dr. Lee Harris of the Florida Institute of Technology and the CEO of the

Reef Ball Development Group Ltd, Todd Barber, Reef Balls work better than solid or rock submerged

breakwaters, because their design allows the water to create whirlpools inside them as currents pass

through them, which assists corals to be fed better. Traditional breakwaters work by making waves break,

but when the waves don’t break it can create washout, which could represent a hazard as the energy

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would then still be conserved. Due to wind tunnel testing on Reef Balls, it was discovered that they can

create whirlpools and offers a variate of angles of reflection, causing the waves to fight themselves as

they pass through them and losing energy in the process (Figure 16). The main hole on top of the Reef

Ball works also as an added stability feature that would decrease the chances of the Reef Ball to be lifted

or moved out of place (Harris 2001).

Figure 17. Reef Ball wind tunnel testing and Stability test. On top is a picture of the Reef Ball

being tested against wind aided by smoke to see the pattern of the wind as it moves through it, the

smoke and wind would represent the behavior of the water currents and waves as it hits the Reef

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Ball. On Bottom is a sketch of the flow inside the Reef Ball as the wave approaches, giving it

more stability as the current exits by the top hole of the structure (Harris 2001).

Along with Reef Balls, some Meshwork Direct Current structures can be placed specially on

places where corals struggle to survive due to today’s harsh water quality and ocean conditions (Figure

18). Since the year 1998 scientists have been testing how coral reefs can be created by using Direct

Current by a type of electrolysis. Most of the tests have been done in the bay of Tensing Pen, Jamaica,

where the current originated from solar cells is applied to a metal lattice that causes the calcium-carbonate

to settle on the metal lattice, which then is invaded by a natural settlement of corals. The corals created

this way have proven to be more resistant than other colonies, be able to prosper under bad water quality,

and even be able to recover already bleached stocks of corals in a few days (Yip 1998).

Figure 18. Meshwork Metal Lattice covered with Corals at a Jamaica AR-Test Site (Yip 1998).

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Construction and Cost of an Artificial Reef

The construction of the artificial reef can be done by dredging material like sand and rocks from a

nearby source and depositing it on the desired area. According to the National Oceanic and Atmospheric

Administration (NOAA), dredging is “the act of removing material from the bottom of bodies of water”,

and is mainly used to replenish beaches with sand, or mine soil from lakes or ocean to then be transported

and deposited on areas where that material is being needed.

The first building stage would be to build a Bermbreakwater (Figure 19), which is a type of

breakwater characterized by its rocky material towards the seaside of the structure, but of low height to

keep it at least 3 to 4 meters underwater (Tutuarima and d’Angremond 1998). The second stage would be

to place the Reef Balls on top of the Bermbreakwater in a staggered arrangement with about 50

centimeters from each other (Figure 20).

Figure 19. Bermbreakwater modified to fit a submerged 500 meters width breakwater (Tutuarima

and d’Angremond 1998).

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Figure 20. Reef Balls Staggered Formation showing approximate distance between them (Aponte

2015).

The artificial reef for Miyako extends about 5,700 meters and is about 500 meters wide, which in

a staggered arrangement and each reef ball base measuring about 2 meters, totals about 231 rows of 2,280

reef balls each row for a grand total of about 526,680 reef balls. According to Reef Ball Development

Group Ltd, back in 2001, the cost to produce and place 150 reef balls was about $1.5 Million USD, which

would mean that to place 526,680 reef balls would cost about $5.3 Billion USD (Harris 2001). According

to W.H. Tutuarima and K. d’Angremond on their Cost Comparison of Breakwater Types Report, the cost

for a bermbreakwater of about 1,500 meters in length and 36 meters wide is approximately 270 Million

Dutch Guilders (DGL), which converts to $150.8 Million USD using a conversion of $0.56 USD per 1

DGL (Tutuarima and d’Angremond 1998). Since our proposed breakwater has approximately 5,700

meters in length and 500 meters wide, then the grand total cost for the bermbreakwater would be of about

$7.96 Billion USD. Adding the cost of the Bermbreakwater and the Reef Balls, the approximate cost for

the project in Miyako could cost $13.26 Billion USD.

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Pros and Cons of Artificial Reefs

Benefits from Artificial Reefs

Reefs can certainly stimulate the marine life to grow and develop in areas where have gone

scarce. Japan is a country where the cuisine is mainly based on fish and other marine species; therefore,

stimulating the growth of marine species would help balance the excess fishing already occurring in the

area. They can increase the coral population, which has been decreasing over the years due to bleaching

and the global warming; attract tourist to the area and diving, which would certainly help Japan economy;

greatly decrease the cost of rebuilding lost structures to tsunami and the loss of life; and can create as

estuary for marine species (Lombardo 2014).

Artificial reefs could protect areas from hostile weather, improve fish abundance and variety, and

guard species from predators. Overtime submerged structures used as artificial reefs, could decrease the

demand for already over-stressed natural coral reefs. Not only, some reefs created by man have been used

to please divers, but nowadays even surfers are enjoying them when they catch waves that break off from

these artificial reefs like the ones created at Cable Station Beach and Narrowneck in Australia, which at

first was made intended to reduce coastal erosion and increase the abundance and variety of marine life in

the area. The goal for foundations, like Reef Ball, interested in building artificial reefs, is to protect the

reef systems and rehabilitate reef ecosystems. Reef Ball Foundation plans to introduce reef modules to

stimulate the reef habitat of marine species (Lane 2010).

Artificial reefs play an important role as a replacement for natural habitats that have been lost.

These reefs offer shelter and hiding spots for vulnerable species that are usually preyed in the open ocean.

Artificial reefs almost instantly attract bait fish, larger fish, and sand-dwelling species, making the reef fill

to capacity within 3 to 5 years (Yip 1998).

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According to the Hallberg Isopycnal Model results from figures 6 and 7, a 2 meter deep and 500

meters wide artificial reef could help reduce up to 70% the energy and run-up of tsunami waves, making

the tsunami easier to counter by the already existing seawalls protecting the coastal cities of Japan,

ultimately saving lives and avoiding the destruction of infrastructures as it happened due to the tsunami

created by the Tohoku Earthquake back in March 11th, 2011 (Kunkel 2006).

Negative aspects about Artificial Reefs

The materials used to build the reefs could be contaminated or toxic to the marine environment.

Excessive amount of reefs along the coast will affect the free navigation routes and complicate the trading

by sea. Building reefs in one area and not on other areas would divert and intensify the energy towards

areas not well protected (Lombardo 2014).

Even though artificial reefs can be made to create potential habitat for a variety of marine species,

these habitats not always result in happy places for the species to live in. Some reefs could present a

hazard to already existing natural environments can get disrupted during the construction of artificial

reefs and can even displace species that already occur naturally in that area. They could also make these

areas vulnerable to overfishing, since they usually tend to unnaturally concentrate fish. Some projects,

according to Surfrider Foundation, have failed to deliver the desired outcomes and are now being planned

to be removed, and other projects have been misused as an inexpensive place where to dump trash, which

have introduced pollutants and other toxins to the marine environment (Lane 2010).

Some recycled materials and sunken ships that have been deposited in the bottom of the ocean

have been found to contain some toxic materials like Polychlorinated biphenols and heavy metals, which

according to environmentalist, the overuse of contaminated artificial reef materials could lead to toxins to

be passed on into humans through the consumption of contaminated fish that reside in that area. Also,

poor design and mounting of the reef could lead to disaster if the material deposited to build the reefs is

misplaced or unstable to the point that could collapse or be moved out of place by a storm or tsunami,

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which could represent a potential danger for navigation of vessels in the area and could pollute the ocean

with unintended debris (Yip 1998).

Also, the cost will be high of approximately $13 Billion USD per city in comparison to the

planned $6.59 Billion that Japan is willing to invest per city to make their walls bigger , especially

considering that Japan already has seawalls in-place to counter average tsunami, but in contrast to the

$235 Billion USD already spent in recovering from the 2011 tsunami and the almost 16,000 people who

lost their lives, investing and building artificial reefs seem like a small price to pay for more security and

peace of mind.

Conclusion

To reduce the impact of tsunamis on populated coastal areas, the most reasonable solution would

be to move away from coastal areas usually affected by tsunamis. Unfortunately for people in highly

populated countries like Japan, this is not an option since Japan has only an area of about 377,944 km2

and holds about 127 million people, which makes it one of the most densely populated per kilometer

squared country in the world. Another fact is that Japan’s main economy and cuisine depends on fishing

(Hijino, 2014). Therefore, coastal cities like the ones in Japan, after 2011 disaster, are planning to build

their seawalls higher (Atherton, 2015), which would indeed reduce the impact of future tsunamis, but the

building of higher walls could present a high cost estimated to be about $6.59 billion USD per city, and

may cause a negative environmental impact on coastal areas in Japan (Kurtenbach 2015). Other more

environmental friendly strategies can be taken to diminish the impact, like implementing natural barriers

like artificial reefs by dredging environment safe, non-toxic, material and depositing it about 3km away

from the coast or where depths reach 20 meters deep in a way that does not entirely affect marine trading

routes and navigation, which if planned properly, could help build broader beaches, decrease the energy

of the tsunami waves by energy dissipation by about 70%, and even provide a natural environment for

marine and coastal species to live in.

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