Landslide Hazard level: Low ?

Landslides involve the downslope movement of rock, soil, and debris under the influence of gravity, triggered by factors such as heavy rainfall or seismic activity. The susceptibility to landslides is determined by terrain slope, geological composition, soil type, vegetation cover, and land use patterns. Landslides can range from slow soil creep to rapid debris flows, with impacts varying from localized slope failures to catastrophic mass movements affecting entire communities.

In the area you have selected landslide susceptibility is classified as low according to the information that is currently available. This means that this area has rainfall patterns, terrain slope, geology, soil, land cover and (potentially) earthquakes that make localized landslides an uncommon hazard phenomenon. Based on this information, the impact of landslides may be considered in different phases of the project. Risk studies, planning decisions such as project siting, project design, and construction methods, may want to consider the potential for landslides. Further detailed information should be obtained to better understand the level of landslide susceptibility in your project area.

Recommendations

• GOVERNMENT EXPERTISE: Contact the governmental organizations responsible for management of landslides in the project country (e.g. ministry of environment, national geological survey and/or local authorities) to obtain more detailed information on areas previously affected by landslides and areas considered to be highly susceptible. More information
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  Responsibility for landslide management is undertaken by a range of organisations, with significant variations between jurisdictions. It is important to identify the key organisations to contact.

  In many cases, national or state geological surveys maintain databases of landslides, which can include maps of known landslides, case studies of significant landslide events and a timeline of when major landslides have occurred. This provides important information about the local conditions (such as slope angle, geological materials and land use) in which landslides have occurred. This information can be used to see whether sites with similar conditions to those at the project site have suffered from landslides, or whether the project site (and its adjacent slopes) may have been subject to instability in the past.

  The data can also be used to consider the conditions under which landslides have been triggered. For example, what is the level of rainfall intensity and duration that led to landslides, and/or what was the nature of earthquake shaking that led to landslides occurring? In such cases, determine how large the landslides were, and examine how far they travelled.

  It is important to note that many such datasets are not complete because of the complex nature of collecting, curating and portraying landslide information. Thus, the absence of known landslides in similar conditions to those at the sites under consideration does not necessarily indicate that the sites will not be subject to landslides.

  More detailed information is often held by local government organisations. The format of these data can be variable – in some cases a detailed map of landslides might be available, whilst in others there might be a portfolio of case studies. This can provide insight into the problems that might be encountered at the sites under consideration, and of the processes that lead to landslides occurring.

  National and local government officials in planning and/or engineering departments, often have strong knowledge of specific local or site conditions (for example the types of materials and or slope gradients) that have been associated with landslide problems in the past. Seek their insight and advice where possible.

  However, every site is different, such that landslides can occur in areas that have not be prone to instability before. Thus, remember that this type of desk study does not remove the need to understand the likelihood of slope instability at a particular site.

  Weblinks: The USGS provide useful information on signs that might indicate that instability is developing: http://landslides.usgs.gov/learn/prepare.php

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• DO NOT INCREASE HAZARD: Carefully consider the sites of all planned project infrastructure to ensure that landslide hazard will not be increased significantly through project construction, for example because of inappropriate excavation, slope loading, vegetation removal, and interference with natural waterways and/or existing drainage systems. More information
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  Landslide hazard can substantially increase during the construction phase of a project. Temporary works can frequently increase the likelihood of slope failures. Common examples include the following:

  • Temporary drainage works are often inadequate, and often result in either the blocking of natural water courses or the concentration of water into specific locations, increasing the likelihood of erosion or slope failure.

  • Trenches are often dug that are too deep or too poorly supported. Trench collapse is a frequent source of infrastructure damage, and can be extremely hazardous to those working within them. Never allow worker to enter an unsupported trench that is deeper than their waist.

  • Excavations require careful design, especially with respect to support for the cut faces. Failure of the walls of deep excavations is common where design is inadequate or planned support is not provided.

  • Loading of slopes must be carefully designed. Temporary stores of soil and rock can fail catastrophically, whilst the loads placed on slopes by temporary storage of material can also cause induce failure in the underlying material, especially where weak and/or saturated.

  Cuts into slopes that are awaiting permanent reinforcement can also be vulnerable to failure.

  Particular care is needed to ensure that works are able to withstand high intensity rainfall or events, or the effects of seismic shaking. In designing works, the timing of the procedures in relation to annual weather cycles needs to be considered. Thus, for example, different levels of mitigation may be required if the works are planned in the wet season. Monitor local and national weather forecasts to ascertain if wet weather might affect the site. In such cases, inspect temporary works to ensure that they can withstand the likely conditions, and take action where appropriate before the weather arrives.

  Vegetation removal can also result in changing the ground and surface water conditions, which may be important in triggering landslides. Clear vegetation only where necessary.

  During construction, it is important to have rigorous monitoring to determine whether slope distress is being observed. Ensure that monitoring is undertaken by suitably qualified personnel, and that communication channel are clear should problems be identified. In cases in which the consequences of slope instability are particularly serious (for the project or for the adjacent population, infrastructure and/or buildings), consider monitoring via rigorous techniques such as regular surveying or automatic data collection.

  If signs of slope instability are observed, rapidly commission a more detailed review by an established expert. If necessary move quickly to stabilise the slope. It is generally cheaper and less problematic to address slope problems before they fully develop than it is to deal with a failure after it has fully developed.

  If instability does develop be careful to ensure that those who might be affected are informed.

  Weblinks: Hong Kong GEO Guidance on the management of slopes during temporary works: http://www.cedd.gov.hk/eng/publications/guidance_notes/doc/TGN40.pdf

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• REGULATIONS: Ensure that the project obeys existing (if any) landslide zoning regulations. More information
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  Landslide zoning can be undertaken by a range of institutions, including national government, state level authorities and local governments, as well as private organisations. Determine which authority, if any, is responsible for landslide zonation in your area. In general the local planning department will be aware of the relevant regulations, although they may not undertake the zonation itself.

  Landslide zoning can be enshrined within legislation or it can be advisory. It is important to determine which is the case. Seek professional advice if necessary, and ensure that where zonation is prescriptive the recommendations are implemented.

  Landslide zoning most commonly takes into consideration slope angle and the type of material present. Other key factors can include the elevation of the site; its vegetation; previous land use; and its proximity to a slope. In some cases, zonation may include experience from previous events that, for example, has indicated that landslides can have a high degree of mobility.

  Remember that a site does not have to lie on a slope to be affected by a landslide. In many cases buildings on the valley floor are affected by landslides from adjacent slopes.

  In some cases it may be appropriate to undertake a landslide zonation for the project. If so, seek advice as to the most appropriate methodology to follow. Data for the zonation can be obtained from the local authority, geological survey, mapping agencies, satellite data providers, etc.

  Remember that a landslide zonation is not foolproof, and in particular may not take into consideration local site conditions. It does not remove the need for proper site investigation.

  Weblinks: Organization of American States guidance on landslide hazard zonation: http://www.oas.org/dsd/publications/unit/oea66e/ch10.htm

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