Think Hazard - French Guiana

In the area you have selected landslide susceptibility is classified as medium 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 infrequent hazard phenomenon. Based on this information, planning decisions such as project siting, project design, and construction methods, should take into account 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

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
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
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
IMPACTS: Carefully consider the impact of landslides on planned project infrastructure, such as the potential for total or partial damage and loss of life. Also consider reduced utility of the infrastructure if threatened landslides result in closure, and/or if landslides result in greater operational and maintenance costs. More information
Even small landslides can cause considerable disruption, injury or fatalities. It is frequently forgotten that landslides often affect project infrastructure, rather than the project site itself, causing disruption, economic losses, and sometimes loss of life.

Landslides are particularly common on new roads or railway lines on steep slopes. Thus, take great care when cutting slopes to create a road bench to ensure that the newly created slope is stable. Simple mitigation measures, such as gabian baskets, can greatly increase the stability of a cut slope.

In many cases the road bench is created by cutting or blasting a rockslope. In this case, be careful to ensure that rock blocks have not been left in a state that could allow instability to develop. After blasting or cutting, inspect the slope for signs of instability, and remove blocks that may fall.

Be very careful to manage water appropriately as in many cases, instability arises from poor management of surface water. Ensure that drains and culverts are continuous - i.e. that they:

Do not end without a suitable discharge point;

Do not feed water into, or concentrate water on, the slope (which can greatly reduce stability);

Discharge into a channel that is both secure from erosion and has the capacity to cope with the additional flow, even in heavy rainfall.

Where drainage lines cross the line of the road or railway, ensure that there is a culvert in place that can cope with the flow.

Sometimes it is necessary to create a road or railway line through use of cut and fill measures. Such fill slopes can be very prone to failure if inappropriately designed. Ensure that the fill:

Has proper levels of compaction;

Is appropriately retained;

Has drainage to prevent high water pressures from developing.

After construction, regularly inspect completed roads and railway lines, and take prompt action if distress is developing.

Other infrastructure can be affected by landslides, with serious consequences. Thus, for example, water or gas pipelines are easily damaged by landslides. Ensure that appropriate measures are in place to guarantee their security, and monitor them for signs of landslides. Electricity infrastructure can also be affected. Examples include small scale project hydroelectric plants, which are often affected by debris flows, and pylons that may be struck by, or built upon moving, landslides. Plan the location of such infrastructure carefully.

In some areas, early warning of heavy rainfall can be available that may help to identify if a potentially hazardous event may occur. Monitor local and national weather forecasts for periods of heavy rainfall.

Where landslides do pose a risk, put in place a contingency plan to deal with the consequences of an event, and ensure that all staff are aware of its implementation.

Carefully consider the consequences of a landslide for local people and properties. Ensure that the likelihood that the project could trigger landslides with high consequences for local stakeholders is acceptably low.

Weblinks: An example of a landslide early warning system: http://hkss.cedd.gov.hk/hkss/eng/landslip_top.aspx
TECHNICAL EXPERTISE: Engage a qualified local or international geotechnical engineer to ensure landslide susceptibility in your project area is integrated in project siting, design and implementation. More information
Difficult or hazardous sites often require professional advice to provide guidance. Landslides can be complex problems that require a high level of skill to understand and to plan for. This is best undertaken by someone professionally trained with appropriate experience. In many jurisdictions this will be a chartered engineer or ground engineering professional.

It is commonly the case that landslide problems arise because the structure was located in an inappropriate place. Thus, professional advice on the potential hazards can greatly reduce the risk of damage or loss of a structure and its occupants;

Sometimes the hazard can be associated with slopes located away from the project site. Professional advice can provide guidance on the risk in such situations. Do not assume that the hazard lies only within the boundaries of the site.

The best advice in terms of siting may be obtained from a geomorphologist or engineering geologist with local experience.

Landslide problems can often be mitigated with appropriate engineering design. This is best provided by a qualified geotechnical engineer.

A range of techniques are available to mitigate dangerous slopes. These can include:-

Support structures;

Mechanisms to deflect a landslide;

Strengthening of rock slopes;

Anchors and soil nails;

Barriers.

A suitably qualified engineer will be able to advise on appropriate approaches given the nature of the hazard, the level of risk and the available resource.

A suitably qualified engineer can also advise on the management of water on the site.

If there is a need for a design solution, ensure that it is correctly implemented. Engage the engineer through the construction phase, and monitor the site after construction to ensure that it remains safe.

Weblinks: Hong KONG GEO guidance on the assessment of natural slopes for landslide hazards: http://www.cedd.gov.hk/eng/publications/guidance_notes/doc/TGN22.pdf
TECHNICAL EXPERTISE: Contact local or international staff that have experience working in the project area to understand how they sought to reduce landslide risk in past projects (see ‘more information’ and further resources on this page). More information
In most areas, the types of landslides that are likely to occur at a given site have also affected previous developments. Thus, those involved in previous projects can provide insight into: The types of landslide hazard that might be present;

The ways that such hazards have been identified and assessed in previous cases;

The impacts of such hazards on earlier projects;

Ways in which hazards have been mitigated, such as effective structures and control measures;

Appropriate mechanisms adopted to monitor for ongoing landslide hazards.

Examples in which landslide mitigation or management did not go to plan.

Where possible, build up a portfolio of examples of appropriate approaches, and ensure that those responsible for project design are aware of these case studies. Try to learn from the experience of others, especially where measures were not successful. In most cases these failures stem from either a failure to understand the ground conditions properly or from designs that were not sufficiently robust.

In some cases, no similar examples will be available. In this case it may be appropriate to seek examples from similar projects and/or similar environments from beyond the project area.

Document the approaches adopted in your project, and their success or failure, together with lessons learned. Where possible make these available to other projects so that experience can be shared.

Weblinks: JRC report on lessons learnt from landslide disasters in Europe: http://esdac.jrc.ec.europa.eu/ESDB_Archive/eusoils_docs/other/eur20558EN.pdf
REGULATIONS: Ensure that the project obeys existing (if any) landslide zoning regulations. More information

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
FURTHER INFORMATION: Find out if the exact project location is in a hazardous zone, e.g. by collecting local landslide hazard information either from maps or by interviewing local governmental organizations. More information
Local experience can provide invaluable insights into the potential landslide threats at any given location. Important information can be gained from people living close to the site, local practitioners and those working for government agencies.

Be particularly aware of the possibility that a site on a slope might be underlain by an old landslide that could reactivate. Such existing landslides can be reactivated by construction, by changes in surface and/or groundwater, by extreme weather, or by an earthquake.

Existing landslides can also indicate that there is potential instability upslope from the site that might affect the location.

Ask local people to describe when and how landslides have been observed, for example during:

Short duration, high intensity rainstorms;

Prolonged periods of wet weather;

An intense earthquake.

Such information can provide significant insights into the likelihood of a landslide recurring at a given site, and the likely conditions in which instability can occur.

Maps can also provide information about landslide hazard. For example, some geological maps include the distribution of known landslides, so determine whether such maps are available, and if so examine them carefully. Incorporate appropriate information into the design of the project.

Consult also with local authorities as to whether the site is known to have landslide problems. Appropriate bodies might include:-

Local planning departments;

The appropriate regional or national geological survey;

Local engineering organisations.

Other organisations may on occasions also hold information. For example, if there is a local university with academics interested in landslides then they may be able to provide guidance on the safety of the site.

In most cases, appropriate actions at an early stage can avoid more serious problems during the lifetime of the structure.

Weblinks: UK Government guidance on planning in areas subject to slope instability: http://planningguidance.communities.gov.uk/blog/guidance/land-stability/land-stability-guidance/

High	Low
Medium	Very low

Further resources

For further information the following resources could be consulted:

French Guiana

Landslide Hazard level: Medium ?

Recommendations

Further resources

Landslide Hazard level: Medium