Problems in decision tree testing

5.3 Problems in decision tree testing

  During testing of the decision tree in the field, it has been realized that some factors are not directly convincing for some areas. In some cases, those factors required more explanation to identify the accurate situation. The major factors which were needed to modify the initial decision tree are listed in Table 5.1.

  Table 5.1: Conflicting factors in initial decision tree Clarifying factors

  Code in DT

  Present factors

  Needed additional information

  Presence of Rocky cliff

  

  Potential factor is needed to identify the cliffs

  Presence of discontinuitycrack is also important

  Vegetation anomalies

  

  To visualize vegetation anomalies, specific identifying factor is required

  

  It is needed to check whether vegetation anomalies is existed on the track of rockfall or covering entire area.

  Evidence of fallen rocks

  

  Important to know if fallen rocks are from recent events or not.

  Does the unit have concave

  

  Deposition formation and materials should be

  sections or hummocky parts?

  checked.

  Are there landforms that

  

  In some situation, it is very difficult for non-

  might be formed by landslides

  experts to understand if landforms pattern might

  be old landslides.

  

  Inventory is required to understand this.

  Does the channel show signs

  

  This is difficult to evaluate by non-experts.

  of large changes of discharge

  On the other hand, the classification of different level of susceptibility in the decision tree (in initial tree it was termed as hazard) was not systematic. There were three level of susceptibility as low, moderate and high. It was realized that based on the assessed factors, the classification was not systematic for different types of hazards. For example, in which cases the susceptibility level would be high or in what way the previous events are included to determine the different level of susceptibility. It was decided that the evidence of past activity, either from historical records, local knowledge or through field evidence, is sufficient to classify the unit as high susceptible. It is assumed that these sources of evidence would indicate activity in the last decades, and that similar type of activity is likely to occur, which makes the unit not suitable for development, without the construction of mitigation measures.

  Another problem in the initial decision tree is that there was no linkage for assessing susceptibility between different hazards in the same unit. It was found in the field that there were many units which were susceptible to multi-hazards such as rockfall flood or debris slides debris flow etc. During the field work, it was done separately when it was found the unit was susceptible to multi-hazards as shown in case example-3 (Figure 5.5). Besides, there was no consideration of runout flow path assessment from debris flow or rockfall for assessing the susceptibility.

  By using the decision tree, the assessment of susceptibility of multi-hazards is intended to be made for homogenous units. The decision tree assessment results should give the same results for the entire unit. However, in some cases it was found that different parts of the units didn’t have the same level of susceptibility, for example the border area of the unit, close to steep slopes might be undercut where this doesn’t apply to the entire unit. Therefore, the delineation of unit boundaries plays an important role to assess the hazards. Hence, considering the above mentioned problems, it was needed to modify the initial decision tree which could provide more systematic approach for assessing the susceptibility of multi- hazards.