SimCenter | Understanding Tsunamis and Their Effects, Aug. 30, 2017


Welcome. Today is Wednesday, August
30th, 2017 and this is the Natural Hazards Engineering 101 webinar series.
It is intended to provide a common knowledge base for the Nheri Community
for each of the primary natural hazards in the Nheri Program. Webinars in this
series provide an introduction to fundamental concepts and an overview of
the experimental and simulation based research. Webinars will also provide an
introduction to numerical methods and computational tools used in natural
hazards engineering research. For more information, visit the Nheri website at
Designsafe-ci.org, where you can find links to the SimCenter and Nheri
Learning Center. Today’s webinar is coordinated by the Natural Hazards
Engineering Research Infrastructures Simulation and Computational Modeling
Center. This webinar is supported by the National Science Foundation under Awards
1612843, and 1520817. Any statements in this webinar are those of the presenter and do not
represent the views of the National Science Foundation. We are pleased to
have Professor Yeh from Oregon State University providing the overview of
tsunami engineering. Professor Yeh has academic and industry experience
having worked in the area of hydrodynamics related to electrical
power plants before returning to academia. His primary research interest
is in the hydrodynamic of tsunamis, focusing on controlled
laboratory experiments and theoretical developments of Nonlinear Long Wave
Theory. I invite Professor Yeh to begin this Introduction to Tsunamis
Triggered by Co-seismic Fault Rupture. Professor Yeh?
Thank you for joining this webinar. We appreciate your time. The
title of my talk is “Understanding Tsunamis and Their Effects.” Okay that’s fine. Okay, this photograph is
taken 11 months after the tsunami attack on the town Tomioka. It’s very close to Fukushima Dai-ichi Nuclear Power Plant. Even after one year, there’s
no recovery or nothing because all the people are evacuated. So you can
see the effects of that tsunami directly even after one year. You can see that all the pavement was ripped out and the seawall was broken,
especially on the lee side of the sea walls. And so those are the effects of the
tsunamis. This is a fishing port facility. What I’m going to do today is I’m
going to try to characterize tsunamis with simple models. Of course the tsunami is
very complex but simple models can at least give you some qualitative
understanding. Of course I cannot cover all of those for just one hour, but
I’m going to try to choose maybe two or three topics that might
benefit your effort for the modeling for the tsunamis and the evaluation of the
tsunami effects on the man-made structures. The first thing. Let’s try to
talk about some very dry facts. We really need to understand the scale of
the Earth when we talk about those large natural hazards. So, that’s what I’m
going to try to explain first. Everybody knows about this, but we need
to recognize, we need to have a real feeling about this.
So, the radius of the earth is about 6,400 kilometers. Okay and then half of the
radius is the core, inside of the Earth. And then also,
we have a mantle layer which is about 3,000 kilometers. Well, if you add
3,400 that 6,400 kilometers. It’s all of the Earth. So what what I’m trying to say
here is the Earth’s crust is 120 kilometer thick, atmosphere is 80 kilometer thick,
oceans 4 kilometer thick. So, the ocean is a very, very thin layer of the water, right.
That’s the radius and then it is also quite important to recognize that
the horizontal scale of the Earth, actually on the surface, is much, much
greater than the vertical scale, and so I think we’ve got some kind of feeling
about this scale of the Earth. Now, if I try to look at the more regional scale,
you know there is a — we need to recognize that the depth of the oceans is 4,000 meters deep.
So, you know it is difficult to make a greater than 4,000 meter wave because the entire depth is 4,000 meters and then we
have a continental rise and the continental slope those are sort of a
range of that, and the variety of the width in the continental shelf. Typically,
about 80 kilometers, sometimes much, much, very, very narrow; sometimes much
wider. For instance, the East Coast. It’s much wider than the West Coast of the
United States and then we have coastal plains. So, those are the
scales we need to recognize when we talk about tsunamis. So another
introduction. Let me just quickly talk about, you know, the origin of the tsunamis. You know first things is, we need to
focus on the seismic fault displacement for the large earthquake. That’s one
thing that we’re gonna focus on today. Of course, tsunamis are also generated by
large landslides and the volcanic eruptions. And then very rarely, you know,
tsunamis can be created by a meteorite area impact. Important part for this is landslides
and volcanic eruptions create much, much shorter tsunamis, compared to seismic
faults. Let me just explain. Volcanic eruptions for the typical
cases — well, I shouldn’t say typical — the representative event was at Krakatau,
1883. And then that creates a 40 meter wave, lots of people died, and
indeed that creates the huge tsunamis and the island dimension is
approximately five kilometers in diameter. So generated wave is just like that, like
a five kilometer. Kilometer is the order of the magnitude of the generated waves.
How about a landslide? Volcanic landslides and pyroclastic flow — this is
similar to, that they have both volcanic eruptions. Famous one was 1793, Mount Mayu, Japan, that also creates pyroclastic flow, and the picture of the pyroclastic
flow is on the left-hand side, and again that the special scale of this event is about,
you know, 4 kilometers wide, several kilometers. And then, the landslides. This
is a huge event for the 1958 Lituya Bay, Alaska. You can see that the splash-up
goes up to 524 meters high. It’s probably the largest tsunami measured, so
far. But, the scale of that event is smaller. It’s about 800 meters by 900 meters. So,
it’s, you know, about order of the kilometers. So, those are the tsunamis
generated by a landslide, some eruptions and those kind of things. But if
we talk about Co-seismic fault displacement, which is much, much larger.
The typical source area is 50 by 100 kilometers. Those are typical. What typical
means, like a significant tsunami is created by like a magnitude high 7s and low 8s.
There are exceptions, of course. The 2004 Indian Ocean tsunami’s
magnitude is greater than 9. The source area is 1,000 kilometers by 150
kilometers. The 2011 East Japan Tsunami is 450 kilometers by 150 kilometers. It’s
much, much larger than those caused by volcanic eruptions or landslides. This is
important to recognize that those scale. So, what’s the implication of the large
source area? Hundreds of the kilometers in the ocean. So, typical of a significant
tsunami is length scale of the wave is about hundred kilometers and ocean depth,
typical ocean depth… four kilometers. So, ratio of the water
depth to the wave length is 0.04, which is really small. Same thing for the 2004
and 2011 tsunamis, huge case. The ratio of the depth to the wave length is
really small. So that means this wave is qualified as long waves, or shallow water
waves. So, at the same time the tsunami amplitude in the open ocean is only a
few meters. So, if you compare with the depth, your amplitude ratio of the
amplitude to the depth is really small. So, all the tsunami’s propagating in the
open ocean can be approximated by linear long water, long waves theory, or shallow
water wave theories. Okay, now let’s just try to feel what kind of, you know,
lengths here I’m talking about. This is 2004 Indian Ocean Tsunami and I’m trying
to indicate the area of Chennai area for here. And I plotted bathymetry, you know the width of
the continental shelf there is 150 kilometers and then suddenly decreased
about 335 hundred meter deep. And then we do know that the wave length of the
2004 Indian Ocean Tsunami which is about 450 kilometers which it will be
satellite. So if I try to plot 450 kilometers just like that. So, it’s much,
much longer than the width of the continental shelf.
Now this sketch here is distorted. If I try to sketch one-on-one scale but
[cross key] on the horizontal scale is the same, then you’re gonna get like that. It’s
very, very flat. So that’s this tsunami for that. So if I say that this
is not a typical tsunami, this sketch is not the typical tsunami. Although I said
so typical, sometimes this is correct, sometimes not. But this is not not the,
you cannot fix the idea this is the tsunami wave form. Alright? So, let’s talk
about a run-up inundation path first. There is a variety of the tsunami forms
approaching the shore which is quite important to model the tsunami effects
on the shore and on the shore means on the human beings, right. And so we do need
to know what kind of tsunami forms is approaching. If you’re going to do some kind of laboratory study for that structure and
infrastructure for tsunami hazards, we need to know what can the waveform it is.
Okay, this picture is taken in the 2011 Japan tsunami in Fukushima and
tsunami form looks like a bore. The bore a steep truncated very long wave. You can
imagine that the wave has a step formation, and the front of the step is
broken. So which is something like this in here. This is called bore. So this is
the one case and people sort of look at this kind of wave form time to
time on the Discovery Channel and those kind of things. And then there are some
kind of variations. This is a formation of the the Undular Bore. You know Undular Bore
instead of having a [time variant front], you have a series of small waves behind
that. This picture was taken in 2004, Thailand. You can see the boat in here, so you
can see the scale on that. And inside this wave looks like dry [dirt] so
looks like a leading depression wave followed by this Undular Bore is
rushing onto the shore. This is a picture for 2011 Japan case and different
locations in the same time frame. The series of a wave is coming. So this is
sort of an Undular Bore starting to break in front of the Undular Bores. If it’s a very,
very rare case, this is sort of a sketch. There’s no photographs for the event
in 1946 Aleutian event. This looks like a wave breaking directly
onto the shore and the completely destroyed reinforced concrete
structure which is the lighthouse. This is very, very rare occasion. Usually
reinforced concrete structure is very strong and can withstand against a tsunami, but this is the exception. I can
show you more exceptions later on which we found at a 2011 Japan tsunami case.
Now last formation is just like this. This is a picture in 1983. Japan Sea … Tube Japan Sea Tsunami, I guess. Yeah. You can look like a this is gradual
flooding. It’s not breaking waves coming rushing to this, so you’re gonna get the
variety of the forms, wave forms approaching to the shore. So which is not
you know you cannot uniquely identify the tsunami form in every
location. And if you consider you know the variety of tsunami, this is
something else on this. It depends on several factors. You know one of the
factors is that if the leading waves depression, the depression means put, putting
the water or leading elevation wave which is the pushing the water. We’re going to
talk about this effect later. Very quickly, you know for the next time, the
next series of slides. And then also bathymetry is quite important. And the
broad continental shelf versus narrow continental shelf. Take a look at this
sketch. If you have a very, very long slope — okay if this is, you know, try to
represent continental shelf or slope — if this this length scale, L sub-b is much
greater than L sub-0, then the incident wave tends to break in the early stage because
it takes a long time to propagate to the shore. On the other hand, if the
wave length is very, very long and the continental shelf is steep and narrow,
then there is no time for this wave to deform. So, you might be able to have —
tends to have a gradual flooding for this case. So, it’s just simply sort of a
character, right. And then you know this wave it might be amplified because of
a steep slope. You might be able to get a strong wave reflection to have
amplifications. So, you know characterization of the wave form has a —
depending on lots of factors, bathymetry and also incoming wave forms.
In order to know this, let’s try to understand the generation first. Okay, I’m
going to try to find the wave forms, right? How wave form changes. You know to
understand the wave form, you have to understand how tsunamis are
generated at the subduction zone. As most people know, learn from the Discovery
Channel and those kind of stuff, how tsunamis are generated, this is a
typical case, not always true. I mean I shouldn’t say typical, this is a simplified case.
You know, oceanic plate is subducting underneath of the continental plate,
and you’ve got to bring this one down. So, gradually deformed down and lifting this.
This process takes a long, long time, storing the energy. And then suddenly the
stored energy is released, and just pushing up creates the tsunami coming
down the shore. So, that’s the deformations. I’d like to
emphasize that this is sort of a very simplified tsunami form based upon
the displacement of the sea floor. If you’re gonna get the bouncing back, this
is a lifting up, right. At the same time, near the land
you’re gonna get the subsidence, right. And that gives you a lift, the negative
displacement here. So, that determines if the incoming wave is
a leading depression wave or a leading elevation wave. Let me explain. Okay, for
that, if you have a subduction zone for this. Let me just go — subducting like
this, uplift and subsidence. The subduction of the 2004 Indian Ocean
Tsunami. This is Sumatra subducting this direction. Indeed, we predicted uplift in this side, east side. Subsidence in the east side.
Uplift in the west side, subsidence in the east side. In 2011 East Tsunami,
subduction is from east to west. So, displacement indeed creates the uplift in
the east side and the subsidence in the west side. Just, you know,
supports this hypothesis, right. So this is sort of a typical
pattern for the tsunami generation in the subduction zone. So
let me ask, as a precursor of tsunami, the sea always recedes to a
considerable distance. Well, if I think about this, that might be true, but I’ll
say, not always. Let me explain to you. Can we predict when and where a tsunami
forms the leading depression wave or receding waves? Maybe.
Okay, let’s take a look at this. If this rupture occurs offshore — okay, just take a look this. This is…ocean here. This is the coastal line,
continental shelf… continental sloping here.
So, if rupture occurs in the offshore, you can have this kind of pattern, right. Uplift
in the offshore region, subsidence in the near shore regions. So in this case,
you’re gonna get the leading depression waves coming to the coastal lines and
the leading elevation wave is going outward. In this case, all of the co-seismic
displacement takes place in the offshore. So for that, you’re gonna have,
for this is a typical case, I mean for that, this is a representative
case for the Indian Ocean tsunamis, uplift in this side, subsidence
in this side. So for this case, if you examine the tidal gauge, tidal gauge
measuring at the Thailand is indeed shows that leading depression waves and
tidal gauge shown in India shows the leading elevation wave. This
complies with this model. All right? So, for that in the leading
depression wave, you show us that huge bore, bore formations. Because if you
have a leading depression wave, wave front tends to become steeper and
steeper much easier. So that’s why for this, creating bore formations. And on
the other hand, on the picture it’s flooding. It’s a narrow continental shelf here.
It’s a very broad continental shelf here. If it’s broad, wave tends to
break, creates the bore formations. If it’s narrow, then leading elevation wave
does not have time to deform, so simply gives you flooding. Now, if the earthquake happens and the
displacement occurs near the shore.. So, for this case, this is the rupture
area, all right, and then this is the deformation pattern for the black
line. The red line is the wave relative to the shore elevations. Note that if this
kind of displacement happens, the shoreline also subsides at the same time, right.
So, water displacement is relative to the shore location is just like
that, right. So, for this case, it tends to have leading elevation waves
…by emphasizing on that. Whenever we have a
displacement on the sea floor, and at the same time, the shoreline is also
displaced, so the amount of the displacement of the ocean and the amount
of the displacement at the land, at the shoreline must be the same. So, there is no
discontinuity in the elevation instantaneously. That’s important. So, this
is the subsidence. It’s located at the Nicobar Island 2004 Indian Ocean Tsunami.
Lighthouse is submerged. It’s you know after the event, it’s submerged. This
is another thing in Shikotan Island 1994. This area is Shikotan Island. This subsided
area, elevated area. So initial wave is elevation. Sorry.
So, initial wave relative to this is elevation, right. Slight subsidence in
here. And then actually the tide gauge can show that the quantities
will show the subsidence, the 70 centimeters subsidence for this case,
from the wave gauge. Now, the last one is if the rupture
really happened near the coastal line and including the inland. This is February 2010 Chile Tsunami. In this case, this is the displacement.
So, this is the shoreline. The shoreline is displaced up to here. So, you’re gonna get
the coastal uplift in Chile, right. So, the initial waves
clearly in and leading elevation waves and so that, in this case, coastal line
uplift. At the same time, sea surface at the shore also pushes up at the same amount.
So, in this case, you can have leading elevation waves to be different. If you
take a look at this coastal line here for Chile, you can see my arrow mark, I hope. So, if you take a look at the shoreline, here’s
uplift in here. All of those uplift. Here is the subsidence in here. So, the
uplift is red, the blue is subsidence. So all that the tsunami tide
gauge data shows initial wave to be elevation, except this guy. This must be
affected by local bathymetry and topographies. Now, so this is the pattern
we talked about this. So for this leading elevation and the leading
depression wave we talked about this, and the wave form is different —
bores, undular bores, the gradual floodings. Very rarely, we have a direct wave
breakings, or depending on the bathymetry on the continental shelf and the
sea-floor displacement pattern. And I have a…Co-seismic sea-level
displacement relative to the land is you know that gives you, that
determines the leading depression wave and the leading elevation waves, and as I said this is a very simplified model. We have to
recognize that there’s exceptions. Some incoming wave might be grossly
affected by local bathymetry and topography. So this may not be
always the case, but in general this is sort of a rule of thumb to think about this,
right. The leading depression, the leading elevation wave can be, find out you
know how the co-seismic displacement relative to the land. And I also said
that the ratio of the tsunami wave length and the length of the continental
shelf is important. If the wave length is a very long, continental shelf
is very short. Then wherever simply goes reach to the shore without too much of
deformations and vice versa. And the effects of the land uplift subsidence is
very important for the modeling, numerical modeling. We have to include this
land uplift and subsidence. Sometimes if you have a land subsidence without
taking into account in the tsunami models and people sometimes
claim that this is conservative. It may be so. But that creates lots of
uncertainties. The reason for that is wave form itself changes. So it is very
important if the land subsides and the uplift we should take into account.
The last … is the one which I just talked about. Shoreline does not
change instantaneously, because the uplift occurs simultaneously in both the
offshore sea bottom and onshore ground. So, I’m going to talk about the
propagation for this. Why the 2004 Indian Ocean Tsunami could travel across the
Indian Ocean and beyond without significant attenuation? We also observed significant
trans-oceanic tsunami effects in 2010 Chile and 2011 in Japan’s tsunamis. Let’s think about
it. If a tsunami has – okay if you have a displacement in sort of a localized
circular shape, you might be able to say this is the model for the volcanic
collapse, a landslide, a meteorite impact. The wave attenuates one over square root ‘ r ‘.
R is you know the radial distance. So it attenuates very quickly, all right. So, if you have a Krakatoa case, if you have you
know collapse of the island, the East Africa wave, by the time the wave
reaches to the other side of the Atlantic, say New York, the output you must
decay very quickly because the source is small, right. On the other hand,
if you have a co-seismic fault rupture case, the source area is very much
elongated and very long and very large. So for this kind of a source, we know
that you’re going to have a directivity for the energy propagations.
And then this directivity is depending on the ratio, how elongated it is, the
more the source of the elongation, the stronger the directivity. This is
important things to think about. The distant tsunami means, you know
that tsunami happened away from the area of interest. So, this
directivity, analytical model for directivity. Yes of course, you can
show that numerical model also. In the numerical model, that
clearly shows the directivity for this. This is a 1963 tsunami hitting
Japan and they also, we also need to recognize that the Earth is not flat,
all right. It’s you know almost spherical shape. So any directed tsunami energy
will propagate along the Great Circle so from Chile to Japan and then Alaska to
San Francisco. It’s a Great Circle and Distance Tsunami Sources. Okay, based upon this directivity and Great Circle directions, I try to identify you know
which source will affect the U.S. or West Coast. So right hand side, this
is sort of a chart which is provided by USGS for the tsunami genic faults in the
Western Pacific subduction zones. I looked around this and I can only
identify two locations for tsunami, the fault direction. And if you push this
line to the Great Circle and going to the West Coast. There’s only two locations,
nothing other place, you know, like a big big rupture for the Japan case is
not directing into the West Coast of the U.S. That does not mean there’s no effect. Of course, in 2011 case, there’s small
tsunamis aimed at the West Coast, but it’s not in the direction of the
Great Circle aligned perpendicular to the fault directions. But there are other locations. Here it is. The Alaska is the one which
creates, which points at the West Coast of the United States. So if you think
about the east end of the Aleutian subduction zone, this is directly hitting
California and this is most probably the most important source
for the distant tsunamis. This might give you a significant tsunami effects
for the West Coast of the United States and I shall mention that because of this
shoreline, you know, this elongation, the tsunami source can
be doubled because the mirror effect. It can be a mirror effect in here. So, this
is a very important source if you are interested in distant tsunami,
this is the primary candidate. Of course, there’s other sources, but this may be
of the most concern for that based upon that mechanism. Well, in summary
and we talk about length, sometimes scale matters, a subduction zone, and tsunami
generation mechanism is important. Now is also talk about leading depression
elevation waves, coastal subsidence and uplift, and then elongated source gives
you that activity and the Great Circle. Those are the stuff we talked about in summary.
I’m going to talk about very quickly the observation of the 2011 Japan tsunami.
What kind of effects we got for geotechnical structure issues. I’m going
to just show just the two or three cases. It’s not all and there are so much
different kinds of variety of the effects. One of those Upright (nearly-vertical) Seawalls.
Here is Kirikiri. The vertical seawalls before, this is before
and then after, and this was the seawalls destroyed and all pieces pushed to
the offshore world. So, evidently this is broken during the
drawdown and this portion is cut off. You can see the car here, automobile.
And this is flipped in this direction. So, you can see the size of this concrete
wall compared to this adult human being. So its failure is definitely created by the geotechnical issue, foundation failure.
And then I just need to sketch. Total failure by the scour in the front
and back. And then just a few kilometers away there’s another town called Hakozaki. Very similar. Still this is standing but you can see
from those two pictures, the entire village is destroyed by the overtopping the seawalls,
but in fact this seawall is still standing. There’s no damage. So there is
a sketch, as is, the existence of the flange. That makes a difference. That’s my guess, of
course, but there’s a difference for the existence of the flange is the
difference between those two cases. How about mound-type sea dikes? Okay, you can
that the front of the sea dikes has no damage at all. All the damage is in the
lee side, not only for lee side top part and a huge scour in
the top of the lee side, but front side is fine. So, if I make a sketch, this is
the damage on the other crown and the severe scour hole behind the dike.
And then some Japanese coding this laboratory experiments and this
is the water surface for steady flow. We tried to take measurements of pressure, it’s a huge negative pressure in here created
by a centrifugal force on that and a large pressure force at the top,
lee side because water is flowing from left to right. So this shows
something, the effects. How about extreme tsunami effects on the
structures? This is the 2011 East Japan tsunami on Onagawa. From time to time, I
show the sketch, of these pictures. You can actually see this quite
intriguing effect. There’s a shadow zone for the strong building and there is
some kind of gap. That gap is creating huge flow, and this this building is you
know it’s affected by rotations. So the localized effect, variability of the
local effect is significant for the tsunami run-up. This is another view for that. So
this is a failure for the concrete structure. As I said at the very beginning,
prior to this, the concrete structure was supposed to be very, very safe only
exception was 1947 Aleutian tsunami at the lighthouse. But this is just not
only one, there’s several of those. All those concrete structure felled by
rotations, but structure itself might be okay, simply flipped and
it’s also dragged by 250 meters. And similar kind of building. This is still
standing. You know this is three stories. I think this is four stories, I
believe and same location, I show you. There is one which is still standing. This fell and then there’s —
I don’t have to speculate why these things happen, but yeah looks like it just
has a very small amount of windows. This is you know all the windows are
broken. So this is flooded by water. This wasn’t. So the buoyancy force might be
different. And then just the other day, August 21st only a few days before,
I revisited Onagawa. It’s all flat and they accumulate — they
put some kind of — they raised the ground level about by 10 meters or
something like this. So it’s completely different, the scenery but no buildings.
All the buildings were removed. Just for the comparison, if I compare with this.
This picture is taken from from here from this direction, okay. So, I just want to
make a short comment. It’s very difficult to make an accurate and
quantitative predictions for extreme natural hazards. That’s one thing, and the
construction of the critical infrastructure buildings for true
provable maximum loading may be economically impractical. Even if it’s
practical, uncertainty involved is unknown or unreliable. We have to
recognize that. So, therefore, what we can do for the best, is to cleverly tweak
the design based on understanding of the physics so that the beyond-the-design-basis
events will not lead to the total disaster, but partial
protection can be retained. So that’s the sort of idea for this tsunami
design. We always need to think about this. So last thing, just very
briefly talk about quite intriguing effect on this.
This is a pump station of Fukushima. Reinforced concrete structure. It’s not
felled by rotations. It’s not felled by the foundation. Its structure
itself was destroyed. This is very, very unusual. There are just two or three other cases we observed in the 2011 event, but
this is one of the most severe cases. Why do these things happen? I do not know,
but I can only speculate. Before and after. They are protected by sea wall. I
can call the sea dikes. Here is a piece. The sea dikes is still standing
and huge amount of the scour because of this sea wall or sea dike, incoming
tsunami here can be stopped and they accumulate the water level and then all
of a sudden is spilled over for the stored accumulated energy into this
structure. That’s my speculations. And then this sort of related to this type
of event. You try to store the energy and then after overtopping that
creates the huge release of the energy even though this is still standing. Okay that’s my last talk and last
slides, and if you have any questions, please ask me. I can try to give you some answers. Thank you, Professor Yeh, for this presentation and at this time we will
open up the question and answer session. Attendees are reminded that questions
should be submitted through the chat panel and sent directly to the moderator
and I will relay these to Professor Yeh. And so we have a couple questions, Professor. And I think the first one,
everyone is currently thinking about Houston and the devastating
effects of storm surge and flooding due to Hurricane Harvey and the question is:
can we consider hurricane storm surge similar to the tsunami hazard? It’s
yes or no. It’s not identical, of course. Both storm surge and tsunamis can
be considered as long waves. And storm surge is created by pushing the water on
the continental shelf. So, storm surge totally relies on the continental
shelf itself, and the effects of the storm surge is raising the water level
and then you’re gonna have a shorter wave created by wind forces. And tsunamis are
a little bit different. Tsunamis are long waves coming from the offshore, rushing
into the shore. Now similarity on that is a very severe case, storm surge can create
various similar surge of the water pounding onto the structure, just like
bore formations. It’s a very rare case though but I have seen such kind of
stuff in the typhoon Haiyan and I was also told by one of my colleagues that this
kind of phenomenon can be observed in the Bay of Bengal, and those kinds of areas. So, there are similarities and there are
differences. A storm surge can raise the water and the pounding of the water,
pounding by the wind waves the wave’s surface are shorter than the
tsunami’s. The distinct difference is that the effect of the tsunami can propagate
across the ocean. The tsunami can bounce this one up and very, very long
waves and then that can create an effect on Japan even though the source is in Chile.
Its 17,000 miles, 17,000 kilometers away. That is not the case for the storm
surges. Storm surges created at Houston,Texas, but there is not so much
effect in Cuba, for instance. But similar kinds of things happen for
tsunamis. Cuba is also in great danger by this effect. So, there is similarity
and differences for both cases. Okay. The next question touches on the sea
wall examples that you gave where I guess one was broken and maybe one was
not broken and the question is: could one of the reasons be that one
is in the open coast and the other is within a bay? That’s not the case. Both of
those, the one I showed you on those two events, is both are similar kinds
of locations. One of those is the Kirikiri — you’re talking about this. Oh sorry
about that. Can we go back? Here we go. This place, this is also in the
bay, and this location is also at the bay and the tsunami run-up height for this is
40 meters. It’s a little bit greater, 70 meters. Both are overtopped. So, I don’t
think this is the effect of the locations for this case. And the two
locations are very close. I think the difference in that the damage for the
sea wall here is, I believe, that the structures, the design, this has a flange
that makes it different, prevents the scour area. So that’s why this one is still standing. Another question is: what do we expect
the leading tsunami wave form for the Cascadia event along the Washington and
Oregon Coasts? Right, I think this is a very interesting question and I have to
be very careful, but the continental shelf around the Washington and the
Oregon coast is about 50 kilometers long. It’s not really too wide and then the
displaced, co-seismic displacement, must occur just like near the shore.
So the leading wave is very likely in the elevation waves, leading elevation
waves. I wonder if I can show this one. If I show this one, it’s probably
better. Here it is. Very likely for the Cascadia case in the Oregon and the
Washington coast, it’s just like this. Because we do know the coastal line is
subsided, by one to two meters for the period tsunami studies, or the
geological evidence. So in that case, leading wave must be
an elevation wave and I do know some of the numerical simulations, they also
show that the leading wave is elevation waves. But of course, since we do not have
any history and observation of the wave itself, there’s some kind of
uncertainties. Well from the simple-minded mechanism which I
explained to you today, shows that, indicates that the leading wave ought to
be elevation. A follow-up question to that is: what is the maximum probable
run-up distance for tsunami in the West Coast of the United States? Okay, distance
all depends on the slope of course. If it’s very, very flat this depends on
the period. So, as far as the West Coast is concerned, you know most of that
area is sort of steep terrain, so distance wise it’s sort of uncertain. In order of, if
I penetrate one kilometer, that’s sort of unusual. They might penetrate
around the rivers for the much longer distance, but as far as height is
concerned, of course I do not know exactly, the exact number, but most of the
numerical simulations show that order of the ten meters. And if we return to our
focus to Hurricane Harvey, what lessons can the Nheri Community learn
from this event in its approach to assessment or preparedness of the
tsunami hazard on the built environment? In both cases, Hurricane Harvey
he behaves completely different from expectations. You know he hit the Texas coast and came back to the Gulf
and hit again. This is not the way we understood for the behavior of the
hurricanes. But, that’s the way it is. If you think about the extreme natural hazards,
some unexpected things can happen. If you really think about the
critical infrastructure and critical buildings, we need to consider such kind
of stuff. However, we might not be able to design for everything just like that. So,
you know we have to make some kind of design guideline, try to make a design,
you know, code to make something. That’s fine. Even though you’re gonna get the critical structures, you’re gonna have,
you have to design code, but at the same time, the designer always has to think about
something beyond, and you might be able to make some small changes, you know
small, not not really obvious, but some very clever changes, design
changes, may be able to reduce when the beyond-design-basis event happens. And
this is the case for the Houston event, I guess. They have huge
precipitations, right. I mean rainfalls is unbelievable, and none of those drainage
system was designed, so that’s what happened in the flooding, such kind of
stuff. So, for that whenever you gotta try to make a drainage system, you have to
make some kind of design for flood and those kind of stuff. But you can always think
about what if something beyond things happen, what can we do? You might be able
to make some different kind of drainage system. You might
be able to get just one system to relieve a little bit. I mean those are
just examples, and so same thing for the tsunamis. Tsunami is sort of
unpredictable and now when these things happen, rare event and high
impact event. So, you simply cannot design for that for everything, you just make
small changes so that the structural infrastructure can be resilient for
those kinds of events. For instance, the sea wall can be overtopped, you know. Try to
design the see wall can be overtopped. But, you know, try not to make a total
destructions. Try to make it for a partial failure. Another thing is
you know one of the buildings, try to make it some kind of failure so that
they can…maintain standing even though there’s a partial
failure. Those kind of ideas are necessary and not just for the design
of the building itself, you may want to think about a regional design to cope
with this kind of extreme natural hazards. That’s my opinion.
Okay, well we are at the conclusion of today’s Natural Hazards Engineering 101
webinar. On behalf of the attendees, thank you Professor Yeh for taking the time to
share this Introduction to the Tsunami Hazard. To the attendees, thank you for
your participation and your questions. The Nheri SimCenter is continuing its
focus on Hydrodynamic Hazard Engineering with additional NHE 101 webinars. The
next webinar will focus on methods for modeling coastal waves with Professor
Lynett from USC. Register for upcoming webinars
on the Designsafe-ci website and check your email inbox for registration links
from [email protected] If you aren’t receiving these emails, check your
DesignSafe account settings to make sure that you receive those
announcements. Thank you for attending today’s forum. you

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