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Theme 4:
Prevention and mitigation
Theme Manager: Farrokh Nadim
Mitigation measures for risks associated with geohazards can broadly be
classified in six categories: (1) land use plans, (2) enforcement of
building codes and good construction practice, (3) early warning
systems, (4) construction of physical protection barriers, (5) network
of escape routes and "safe" places and (6) community preparedness and
awareness building.
Early warning systems and construction of physical protection barriers
have been singled out as specific tasks in the proposed ICG research.
Together with the other four categories, they form the backdrop for a
mitigation strategy.
The
results of hazard and risk mapping and analyses will be used to
formulate mitigation strategies to assist decision-making on the need
and cost-benefit of hazard mitigation works. Based on such a strategy,
protection measures can be developed and their cost-effectiveness and
environmental soundness compared.
Figure 1. Examples of
tsunami risk mitigation master plan. Left: Patong City: elevated
green-belt areas approximately 400 m inland from the beach, serving as
safe escape hills, and system of escape routes; car traffic to be banned
from escape routes. Right: Ban Nam Khem fishing village: layout
of protection dike around Ban Nam Khem with escape routes to safe high
areas (Karlsrud et al. 2006).
A
mitigation strategy would involve: (1) identification of possible
disaster triggering scenarios, and the associated hazard level; (2)
analysis of possible consequences for the different scenarios; (3)
assessment of possible measures to reduce and/or eliminate the potential
consequences of the danger; (4) recommendation of specific remedial
measure and if relevant reconstruction and rehabilitation plans; and (5)
transfer of knowledge and communication with authorities and society.
The strategy developed by ICG and NGI for the tsunami-affected areas of
Thailand after the 26th December 2004 Indian Ocean tsunami
provides a good example of what can be done.
Any
mitigation strategy needs to be adapted for different natural hazards
and different parts of the world. Especially for developing countries,
it is vital to establish and promote proper land-use planning and
construction practices to regulate human activities that increase risk
to earthquakes, landslides or tsunamis and to prevent settlement of
communities in high-risk areas. The research performed by ICG will be
part of the strategy plans and the communication programs. Ensuring that
people do not live in "high risk" zones will be included in the decision
process. As for physical protection measures, an "how to" and "do's and
don't's" guideline will be prepared, as well as recommendation for “best
practice”.
Early warning systems
Systems need to be developed to monitor both short term and long term
geohazards and their effects, and to forewarn of impending danger, in
areas where geohazards could affect life and property. To develop
reliable early warning systems, the physical processes and mechanisms
need to be understood and methods need to be developed for measuring,
modelling and predicting geohazards, for example landslides and
tsunamis.
Developing early warning systems also requires (1) setting criteria for
parameters to be monitored and threshold values; (2) developing
monitoring equipment and systems; (3) coordinating satellite radar data
with local monitoring stations; (4) planning monitoring programs for
high-risk areas; and (5) developing computer-aided decision-making tools
with e.g. mobile data mapping and retrieval, and information management
using geographical information technology (GIT), Remote sensing (RS) and
3D modelling.
In
particular, criteria will have to be established, for example, for the
rate and scale of ground movements in vulnerable locations, and links
will have to be established between ground movement, rainfall and
groundwater levels that can be used to develop a methodology for
landslide forecasting. An early warning system can also be used to
"measure" the effectiveness of landslide management strategies.
ICG
aims to develop new techniques in remote sensing for detailed
investigation and monitoring of, for example, large rock-slope
instabilities and failures (lidar, radar, remote sensing) and slope
instability following a flood, including methods and tools for high
resolution digital elevation models (DEM) analysis. Passive seismic
monitoring techniques will be adapted to monitoring of potentially
unstable rock slope sites, providing options for an early warning system
and vital information for the general understanding of rock-slope
failure, and its dynamics. A passive monitoring system was installed at
Åknes in the fall of 2005. The data are being analysed in real-time, and
will be integrated with other continuous measurements into an early
warning system.
ICG
will prepare user-guidelines for data review, alarm facility and
follow-up, telemetry links, and actions to be taken in the event of
threshold values being exceeded. Logical diagrams (flowcharts) for the
interpretation of the monitoring and early warning system will be
developed and tested before they are released for use.
Early warning systems are quite target-specific, depending on the hazard
type and the local conditions. For example, earthquake prediction (in
the strict sense) is not yet within reach, so for the foreseeable
future, developing a “warning system” for earthquakes is not a realistic
mitigation strategy. For tsunamis, however, the situation is different
and more promising, even if the short warning times are still a major
challenge. A few minute to one-hour tsunami warning would have saved
many lives in December 2004.
Figure 2. Block
diagram of a typical early warning system (DiBiagio & Kjekstad 2007).
Physical protection measures
Physical protection measures include, but are not limited to, integrated
land use planning, drainage, erosion protection, vegetation and ground
improvement techniques, barriers (earth ramparts, artificial elevated
land, anchoring systems, retaining structures), and offshore or onshore
walls to reduce the energy or the loads induced by geohazards (e.g.
landslide, rock slide, tsunami, floods).
Buildings need to be designed (and placed in locations) to withstand the
impact forces of geohazards and to provide safe dwellings for people.
Land can also be elevated to ensure that buildings are above a critical
height, for example to protect against tsunami danger.
Physical protection barriers may be used to stop or delay the impact of
the geohazards, reduce the maximum reach of its impact, or dissipate the
energy of the geohazards. On land, such barriers may include “soft”
structures in the form of dikes or embankments, or “hard” structures
like vertical concrete or stone block wall. Offshore, the structures
could be jetties, moles or breakwaters, or even submerged embankments.
Any measures need to be part of a community’s master plan and subjected
to analyses to assess and circumvent any negative environmental impact.
If a
well functioning and efficient warning system is in place, warning and
escape are probably the best way to prevent loss of life due to
geohazards. Developing functional networks of escape routes and safe
places could include a number of different measures, strongly dependent
on the local context. Area, village or city analyses should provide
maximum tolerable distance from buildings and activities to a safe
place, and assess how to achieve this maximum distance. Distances
between buildings and safe areas could be shortened by reducing the
escape routes, or by establishing new safe areas as artificial escape
hills and safe buildings that are accessible to people at large.
ICG
will contribute to the development of templates for communities to
assess and select physical protection measures. The above descriptions
are only examples of possible measures. A multitude of considerations
need to be taken into account when preparing templates that are to be
implemented in real-life cases. Local conditions are determinant in many
cases. A "how to" and "do's and don’t’s" guideline will be prepared.
A
recommendation for “best practice” for physical protection measures will
be prepared towards the end of the second five-year period of ICG.
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Figure 3. Examples of physical protection
measures against debris flows from Austria (left) and France
(right). |
References
DiBiagio, E.B.D. & Kjekstad, O. 2007. Early
Warning, Instrumentation and Monitoring Landslides. 2nd
Regional Training Course, RECLAIM II, 29th January–3rd
February 2007.
Karlsrud, K., Bungum, H., Harbitz, C.B., Løvholt, F., Vangelsten, B.V. &
Glimsdal, S. 2006. Strategy for re-construction in Thailand following
the 26 December 2004 tsunami event, in: International Conference on
Geotechnical Engineering for Disaster Mitigation & Rehabilitation,
edited by: Chu, J., Phoon, K.K. & Yong, K.
www.nat-hazards-earth-syst-sci.net/6/1/2006/ Nat. Hazards Earth Syst.
Sci., 6: 1–19.
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