HAZARDOUS ENVIRONMENTS -HAZARDS CAUSED BY MASS MOVEMENTTS

Movement is mainly through the air in rocks and debris falls. Slides and flows move on the surface both downward and outward. The more rapid ones as a more hazardous. A slide is distinguished from the flow by being a solid mass moving along a single failure plane or fracture zone. Further classification is based on the nature of the material moved. Dry material can produce hazardous movement ranging from rock and debris flows and slides to slumps. Hazardous flows occur when earth or rock is mixed with water to form earthflows, mudflows and lahars.

Both landslides and slumps occur in solid rocks and in weathered material. In a landslide the plane along which movement takes place is inclined and material moves in a mass breaking up with comes to rest. In a slump the movement is rotational along a curved slip plane and results in an arcuate slip face and a relatively unbroke toe which rises towards the end of the slipped mass. Landslips occur in areas with rock types of different permeability.

Warning signs that say mass movement is likely to a tear include cracks resulting from tension in buildings and in the ground, tilted structures, bulging walls and steep slopes, especially with deposited material above them. Arcuate shaped cracks on ground above cliffs indicate the sites of future slumps. Once noticed the slopes can be closely monitored for change. 

Large moving masses like Man Tor's landslip are monitored by GPS and laser surveys to determine the amount of movement each year of a number of fixed markers on the surface of the landscape. GPS measures distance and direction very accurately. Arrows are then drawn on a map to show the direction and length of annual movements. By comparing movements with rainfall it may be possible to determine how much water the slope can hold without moving.

Rainfall and changes and soil moisture are also monitored. Measures in boreholes indicate changes in groundwater content and the pressure it exerts; the weight of additional water can trigger mass movement. Many drainage pipes have been placed in the Man Tor landslip to remove water and reduced movement.

Delicate instruments such as tiltmetres which record the amount of tilt occurring and strainmetres which record changes in the crustal strain to a very high degree of precision are used in areas with active faults

NOAA and the USGS operate a debris flow early warning system in California when heavy rain falls by warning system in California when heavy rain falls by monitoring areas where wildfires have removed the protective vegetation cover.

Landslide hazard mapping

Landslide hazard assessment maps are drawn up using factors that affect slope stability and knowledge of previous movement. Rocks are classified as having high medium or low susceptibility the landslide and the degree to which they have been affected in the past is sisimilarly classified. Maps of the routes taken by rock falls in mountain areas reduce the chances of building in their past.

In the usa geologist engineers are trained to recommend actions to mitigate for geological and geomorphological hazard in training planners help local communities to undertake what is necessary to reduce the dangers.

The degree of risk perceived varies according to distance from the site of the hazard, that amount of knowledge a person or group of people have and the length of time since hazard has last occurred.

Insurance companies have a clearly defined assessment of the risk where as a visitor to the area may have no knowledge of risk at all. Government agencies are likely to be well informed and to take agencies are likely to be well informed and to take responsibility for educating the public about any risk in the locality. Private land owners also have a responsibility to inform the public about and protect them from any risk on their land. In the USA warnings are issued during national and local weather forecasts.