Dear Listers,
Some info I picked up from the internet on PV.
In 1999 & 2002, earthquakes recorded were >7 with little
or no tsunami in some parts of Efate Is and 6.6m elsewhere.
In 1999 a 40cm run-up height was recorded in PV.
Pls also see (you may have to try accessing it later) for info
on detailed bathymetry. The channels are running in a
different direction.
http://dev.sopac.org.fj/VirLib/ER0110.pdf
Regards,
Parminder
----- Original Message -----
From: "Palmer, Steve" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Monday, March 14, 2011 6:25 PM
Subject: Re: Coastal vegetation, harbour depth and community preparedness as factors in reducing
tsunami risk
On the point about water depth and underwater obstacles, there would
need to be detailed modelling of the water flows. For waves in water,
the longer the wavelength, the faster they travel. As a wave moves into
shallower water, it moves more slowly, but the amplitude increases until
it breaks down in turbulence. There is reflection, refraction and
interference effects from obstacles or shapes in the seabed. This can
result in significant local enhancements to the wave amplitude. In
particular deep water channels to harbours may concentrate the water
flow - the well-known phenomenon of "bore" waves travelling up rivers is
a result of concentration of the normal tidal flow into estuaries.
Sea-bed excavation close to the coasts would probably make things worse
- the aim must be to dissipate the energy of the wave through turbulent
break-down before it reaches significant areas.
Coastal wetlands will give an increased area for turbulent dissipation
of tsunamis or storm surges. Tree belts will also enhance the resistance
to flows - provided the trees remain anchored.
I hear that the time between the earthquake and tsunami impact on the
mainland coast was 6 minutes, but others may have a better figure.
Regards
Steve
-----Original Message-----
From: Natural hazards and disasters
[mailto:[log in to unmask]] On Behalf Of Kelly
Sent: 13 March 2011 23:19
To: [log in to unmask]
Subject: Re: Coastal vegetation, harbour depth and community
preparedness as factors in reducing tsunami risk
On the point about vegetation limiting the impact of (storm or seismic)
surges, experiences in Sri Lanka and elsewhere suggest that water flows
more easily around trees, particularly mature trees, than around
buildings. However, a dense wall of vegetation in a surge-impacted area
would likely be subject to the same level of building-destructive forces
of a surge, and may not fare much better than the de-facto walls of
buildings.
If we seek to engineer our environment, whether using concrete or
wetlands, we probably need to run the numbers to ensure what we expect
is what we are going to get.
Regards,
Kelly
-----Original Message-----
From: Natural hazards and disasters
[mailto:[log in to unmask]] On Behalf Of Ben
Wisner
Sent: Sunday, March 13, 2011 10:25 PM
To: [log in to unmask]
Subject: Coastal vegetation, harbour depth and community preparedness as
factors in reducing tsunami risk
Parminder's observations are very interesting.
First, however, it's important to say that we all share the shock and
grief felt by our Japanese friends and colleagues in the wake of such a
tragic catastrophe. They, and the whole world, will also be looking for
answers to questions and lessons that can lead to better protection in
the future.
1. Regarding COASTAL WETLANDS and forests, there is a lot of evidence
that they do help to dissipate the energy of tsunami waves, of course
coastal wetlands also provide other valuable ecosystem services. The
fundamental question is whether in a highly urbanised and industrialised
country like Japan, there is the political will and financial resources
to rebuild the affected towns further inland so as to leave room for new
plantings and a regeneration of coastal wetlands. In the long run, can
Japan come up with the political and financial capital incrementally to
shift settlement in high risk tsunami zones inland?
This is not just a challenge to Japanese politicians and policy makers,
but to other countries that have a good deal of coastal exposure of
critical infrastructure and industry, not to mention population in the
face of sea level rise. While the time scale involved in tsunami and
sea level rise are orders of magnitude different, the political will and
economic and social cost of moving historically established settlement
and investment to higher ground are similar.
2. Regarding THE DEPTH OF HARBOURS, I too would love to see comments
from specialists. From first principles, Parminder's observations make
sense to me. If deeper waters and underwater obstacles also mitigate
the size and force of tsunami waves, then should Japan not undertake
large scale habour excavation and build artificial reefs, etc? What
would the environmental cost be? The financial cost? The effectiveness
of such measures? Certainly a country that can afford to move the top
of a mountain into Osaka bay to provide the landfill for the Kansai
International airport is no stranger to large-scale geo-engineering.
3. Finally, turning to the COMMUNITY SCALE, I just found a very
interesting overview of progress in earthquake science, policy and
practice in Japan since Kobe (1995). It's by an official in the
government's "Science and Technology Foresight Center"
(http://www.nistep.go.jp/achiev/ftx/eng/stfc/stt019e/qr19pdf/STTqr1907.p
df) and was published in 2006. It includes an overview of public
awareness surveys as well as technical advances.
He says that only 122 of Japans 991 coastal municipalities (12%) had
tsunami hazard maps, and those that existed showed schools, etc. but few
showed evacuation routes (p. 102). That was in 2005-2006, and the
author says that the Central Disaster Management Council had recommended
over five years (approximately 2006-2011) more tsunami hazard mapping to
include all coastal municipalities and "tsunami management measure" to
be taken. I wonder if this goal was achieved? Where there tsunami
hazard maps and sign-posted evacuation routes in the affected towns on
the northeast coast of Honshu? Had there been drills? Much attention
had been directed at the coastal areas to the South of Tokyo and the
tsunami that might follow a forecast, possible Tokai, Nankai or
Tonnankai earthquake. I am aware of a great deal of public awareness
raising, drills and plans in the Nagoya-Osaka area. Had a similar level
of community-based preparation, back up by good maps and models, taken
place in the coastal areas to the North of Sendai?
The tsunami warning system does seems to have worked. I am not sure how
long residents had to escape. If they were aware of the risk and had
practiced evacuation, the escape routes were sign posted (as many are in
Japanese and English), then even ten minutes should have been sufficient
for most people to get to higher ground or to shelter vertically
(something that people should have worked out in advance as part of a
personal protection plan).
From the point of view of local risk governance and decentralisation --
something that is the focus of Views from the Front Lines 2011 survey
being conducted at this present moment by the Global Network of Civil
Society Organisations for Disaster Reduction
(http://www.globalnetwork-dr.org/)-- the availability of appropriate
hazard maps at the municipality level is extremely critical, as is
community participation in detailed planning.
All the best to you all,
BEN
Dr. Ben Wisner
Aon-Benfield UCL Hazard Research Centre, University College London
-----Original Message-----
>From: Parminder Singh <[log in to unmask]>
>Sent: Mar 12, 2011 3:58 AM
>To: [log in to unmask]
>Subject: 8.9 tsunami
>
>Dear Listers,
>
>This tsunami in terms of strength is lesser than the I-O '04 tsunami
that struck Acheh.
>Although the no. of casulities are running into a thousand or so at
>this moment, in Acheh it reached around 200,000 + at the end of the
>day. The infrastructure affected was different betweeen but you could
>see alot of debris like loose pieces of wood etc in both of them but I
couldn't see where they came from in Japan. I saw vast agricultural land
that was swept by the tsunami.
>
>What can be learnt from these 2 tsunamis is that adequate warning which
>the Japanese had compared with the Indonesians which resulted in far
>lesser loss of lives but the devasations were about the same excluding
the nuclear reactors.
>
>I consulted an eminent professor from Japan on possible mitigation
>systems not too long ago and one stands out is coastal forests which
>can break the energy of these waves. I don't know how many tsunami
>proned countries have this form of defence already in place. Another
>one I picked up from a personal observation is the deep water in Port
>Vila, Vanuatu Is. Many large e/qs had occured in the past but the
run-ups less devastating than other places in the area. Reasons possibly
I think is due to the unusual depth of the harbour >100m and the
presence of channels and humps on sea-bed could help to break the waves.
>Perhaps someone out there can give their comment.
>
>Best Regards,
>
>Parminder Singh
>Researcher
>Malaysia
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