The slope risk is defined as a measure of the probability and severity of an adverse effect to human, assets or the environment. Generally, the calculation of risk is based on the following correlation:
Risk = Hazard x Consequence
- Hazard is defined as a condition with the potential for causing an undesirable consequence. The description of hazard is normally given in term of hazard rating and the probability of their occurrence. Five hazard rating are used namely Very Low, Low, Medium, High and Very High.
- Consequences is defined as the outcome or potential outcomes arising from the occurrence of a landslide expressed qualitatively or quantitatively, in terms of loss, disadvantage or gain, damage, injury or loss of life.
In order to calculate the risk, the Hazard and Consequences need first to be determined. To calculate the hazard, Cawangan Kejuruteraan Cerun utilize the established formulae for the cut and embankment slopes. These formulae are developed using statistical approach in which the Discriminant Analysis Model is used. By putting the slope data such as slope height, slope gradient, vegetation cover types, upslope/downslope geometry, evidence of seepage, present of defects etc. in the formulae, the instability score in term of probability of failure are obtained. To translate the probability failure to hazard rating, the following table is used:
For example, if the probability of failure obtained are range between 35% to 75%, then the respective hazard rating with reference to the table fall between Low Hazard and High Hazard.
In the consequence analysis, at least the following five parameter are considered in the calculation/assessment:
- Element at Risk (EAR)
- Vulnerability of EAR
- Spatial Temporal Probability
- Landslide travel distance (Runoff distance)
- Probability of Landslide reaching EARs
Consequence is calculated using the following equation:
C - Consequence
PTL - Probability of landslide reaching the EAR
PST - Spatial Temporal Probability
V - Vulnerability
Element at Risk (EAR) includes Properties (Building, Utilities, Facilities, Plantation etc), Infrastructure (Roads), Traffic (Vehicle), People (people in building and vehicle and pedestrian). In consequence analysis, only EARs that located within the landslide travel direction are considered in the calculation. Another aspect is whether the landslide travel distance could reach the EAR or not (PTL) . If the EAR location within the landslide travel distance, the value of PTL > 0. On the other hand, if the EAR location beyond the landslide travel distance than the value PTL = 0 and thus not considered in the consequence calculation.
The spatial temporal probability (PST) is the probability that the element at risk is in the area affected by the hazard at the time of its occurrence. It is a conditional probability, and is between 0 and 1. For example, for building, the PST = 1, because the building is permanently there at all time. Whereas, for vehicle or people in vehicle, they are movable object thus their PST is equal to the proportion of time in a year when it will be in the path of the landslide.
Vulnerability is the degree of loss (or damage) to a given element, or set of elements, within the area affected by the hazard. For property, it is expressed on a scale of 0 (no loss or damage) to 1 (total loss or damage) for property. For persons, it is the probability between 0 and 1. 0 mean the affected person is unhurt while 1 the person is killed. The value of vulnerability for properties depends on many factors such as types of material used, quality and method of construction, properties height, landslide velocity and magnitude. While for people, the vulnerability om whether they are in the open space or in building or vehicle, landslide velocity and magnitude etc. Persons who are buried by a landslide mass have a high vulnerability. Death is more likely to result from asphyxia than from crushing or impact.
Based on the result of hazard and consequence discuss above, the risk for individual EAR or Specific Risk (SR) can be calculated using the correlation discuss above (Risk = Hazard x Consequence). The risk of a particular slope is taken by summing up all SR values that related to the slope and is called Total Risk (Rt).
RISK.E - Specific Risk for a particular EAR
- Ir. Roslan b. Majid (JAPK), CKC
- Ir. Elya Shuhaira bt. Shafei (JAK) CKC @elya