Designating landslide hazard zones and providing hazard maps

Needs Address

The needs to prevent or reduce disaster risks, in the context of increased landslide risks associated with new housing developments, and changes in rainfall patterns due to climate change. 

Adaptation effects

By specifying identifying where landslide disasters are likely to occur, hazard maps can be provided, and improvements can be promoted in warning and evacuation systems. 

Overview and Features

• The Act on Sediment Disaster Countermeasures for Sediment Disaster Prone Areas (hereinafter referred to as the "Sediment Disaster Countermeasures Act") was developed by the Ministry of Land, Infrastructure, Transport and Tourism. Its purpose is to promote public awareness about risks, improve warning and evacuation systems, limit new housing developments, and promote non-structural measures such as encouraging the relocation of existing homes away from hazard zones. The Act does so by designating areas where there is a risk of occurrence of landslides. 
• The Sediment Disaster Countermeasures Act imposes the following responsibilities upon the national and prefectural governments of Japan. 

1. Formulation of basic guidelines on landslide disaster prevention:
   The Ministry of Land, Infrastructure, Transport and Tourism and creates implementation guidelines for basic studies, such as specifying the guidelines for designating landslide hazard zones. 
2. Implementation of basic studies:
   Each prefecture is to carry out basic studies in accordance with the Ministry's basic guidelines on landslide disaster prevention. For streams and slopes with risk of landslide disasters, they are to study the areas' terrain, and geology, and land use, such as the presences of facilities in the area. 
3. Designation of landslide hazard zones: 
   Based on the basic research, prefectures are to designate area as having the risk of landslide. 

• Landslide hazard classification and selection criteria
  Based on the results of basic studies prefectures select landslide hazard zones (yellow zones) according to certain criteria. Landslide hazards are classified into three types (slope collapse, debris flow, and landslip) as shown in the table below along with designation criteria. Among the hazard zones, areas with an especially high level of danger are designated as special landslide hazard areas (red zones). 

  Table: Classification table of landslide hazard zones, with selection criteria

  Steep slope collapse hazard zone
  What is a slope collapse? Phenomenon in which an entire slope suddenly collapses, in an earthquake, or when soil resistance weakens due to becoming waterlogged from heavy rain or a long period of rainfall	Criteria A. Area with a slope having a gradient of 30 degrees or greater and height of 5 meters or greater B. Area within a horizontal distance of less than 10 meters from top of steep C. Area within two times the height of a steep slope from the lower end of the steep slope (50 meters maximum if over 50 meters)
  Debris flow hazard streams
  What is a mudflow? Phenomenon in which a large amount of soil and rock flows with water and can strike like a tsunami, triggered by intense torrential rain.	Criteria Areas with a gradient of 2 degrees or greater, downstream from the head of the alluvial fan of a stream known to have the risk of a mudflow.
  Landslip hazard zones
  What is a landslip? Phenomenon that occurs due to intense or long period of rainfall, or due to a large-scale earthquake, when ground water saturates a soil layer and the layer above it slips downward Identification criteria	Criteria A. Landslip zone (areas that have landslipped or have the risk of landslip) B. Area equidistant to the length of the landslip land mass from the lower edge of the landslip area (250 meters maximum if this exceeds 250 meters)

  Source: http://www.city.hokota.lg.jp/bousai/contents/4.html

  Table: Classifications based on danger level of landslide hazard

  Zone name	Description
  Landslide hazard zone (yellow zone)	In areas with the threat of death or injury to residents in the event of a landslide disaster (steep slope collapse, landslip, mudflow), systems must be in place to issue danger alerts and to conduct emergency evacuations.
  Special landslide hazard zone (Red Zone)	In areas with a significant threat of death or injury to residents due to building collapse in the event of a landslide disaster, a permit system is instituted for development activity, and structural regulations are applied to buildings.

• The Sediment Disaster Countermeasures Act also stipulates the requirement of having warning and evacuation systems, and in the case of facilities being used by persons requiring assistance (e.g., elderly persons, persons with disabilities, infants, etc.), municipalities are required to convey information to the facilities about the landslide hazard, in order to smoothly facilitate notification and evacuation. Legislation also requires the provision of hazard maps showing landslide hazard zones and evacuation shelters. 

Considerations

It can be suggested that for persons who are not skilled at reading maps, aerial photographs could be used instead of a map (see example below) in order to ensure that they are aware of the hazard zones. If that approach is taken, there is a need to be able to obtain aerial photographs and to have basic technology to plot locations on photos. 

Co-benefit, suitability for developing countries

Same

Information Resources

Website of Hokota City, Tochigi Prefecture (in Japanese)
http://www.city.hokota.lg.jp/bousai/contents/4.html