Related to the final point about clogged rivers, there are three impacts
which may occur: 1) the flow of debris into the rivers and streams has
likely resulted in clogging (this happened in Sri Lanka, but on a relatively
small scale, but still enough to lead to local flooding when the rains
came), 2) damage to embankments and flow management structures may result in
out-of-bank flows during the rainy season, but also tidal surges up-stream
and tidal damming of rivers during flood periods and 3) the transport of
sediment up-stream by the waves, which has narrowed and reduced channel
depth and thus reduced drainage capacity.
Immediately, clearing debris is a necessary part of the first phase of the
recovery process, and drainage systems (in towns as well as larger
river-based systems) should be part of these efforts.
Assessing the need for repairs to flood protection and river channeling
systems would also be priority in the overall damage assessment process.
Rebuilding flood (and sea wave) protection structures will likely take time.
This process would profit from a stronger eco-system based approach, with
somewhat less reliance on built structures and more on using eco-systems
such as wet lands to reduce the risk from flooding where feasible.
Regards,
Kelly
-----Original Message-----
From: Natural hazards and disasters
[mailto:[log in to unmask]] On Behalf Of COPE, David
Sent: Sunday, March 27, 2011 12:24 AM
To: [log in to unmask]
Subject: Tsunami Run-up, River Channels, etc.
Dear list members
I'm not an expert on natural disasters, but obviously they impinge on our
work - and I am expecting increased parliamentary interest over the next
months.
What's more, having been in Tokyo at the time of the earthquake (attending a
meeting to discuss their creating an office similar to ours to serve their
parliament), I have something of a personal interest.
So, I wanted to share some ideas with you experts - and seek your views.
First, I am hoping that there has been some really good aerial/satellite
photography of the extent of the March 11th tsunami run-up for the affected
areas on the north east coast of Japan. Presumably, the debris and
destruction will pinpoint the extent of the run-up from the coastline very
clearly.
Those of you who have visited Japan will know of the incredible amount of
coastal engineering intended to resist tsunamis (although I don't think any
was designed for a 10-15m impact, as tragically indicated at the Fukushima
power plants).
The extent of run-up at particular points could perhaps be correlated with
the level of anti-tsunami protection installed prior to the event? It
strikes me that information on this could be really useful for informing
anti-tsunami defences around the world.
It appears that some anti-tsunami defences (sea gates) were not activated in
time, or failed to activate - perhaps some of you saw the harrowing video of
the fire department team who were manually attempting to shut such a gate
when the tsunami struck, leading to the deaths of most of the team.
Second, I wanted to raise a concern regarding 'knock-on' effects. Again,
the videos from Japan showed clearly how the tsunami has dumped presumably
millions of tonnes of debris in river channels, as the water retreated.
Obviously, and rightly, attention is focussed on humanitarian relief,
temporary shelters, etc. But, I am worried about those clogged river
channels.
Again, any of you who has been to Japan will have seen how the rivers are
channeled, embanked, etc. - and all with good reason, because their
gradients are usually very steep, certainly compared with UK rivers.
In a few months (starting June), the rainy season will occur in Japan
(precipitation charts for Sendai are readily available on the web).
And it sure does rain!
I am just wondering about the implications of that regarding rising river
levels confronting debris-clogged channels downstream - and causing a second
natural disaster, maybe even impacting temporary rehousing, (which
presumably will be put on higher ground above the devastated areas).
I'll pass on any good advice to my contacts in Japan.
All the best.
Professor David Cope
Director
Parliamentary Office of Science and Technology
Houses of Parliament
Westminster, London SW1P 3JA
UK
tel: +44 (0)20 7219 2840 (PA)
fax: +44 (0)20 7219 2849
email: [log in to unmask]
POST's Web Site: www.parliament.uk/post
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