Prevention and mitigation

Date: 9/27/2010

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.


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.