Justification of parameters and sensitivity analysis

In order to help make the quantitative risk assessment process transparent and easy to understand the choice of parameters should be clearly explained and recorded. A useful method of justifying the baseline input parameters and also defining the likely sensitivity of each one is summarised in Table 6.6. 

5% vA/ To cause migration concentration must be consistent over period of migration. Gas levels are variable and using maximum values is unrealistic as they do not occur all over site or all the time. So use mean value and check with maximum values in sensitivity analysis. Note that this approach requires a high density data set and possibly continuous monitoring which may help to identify a baseline value 

Parameter Baseline value Range for sensitivity analysis Justification 

Carbon dioxide concentration and oxygen depletion N/a N/a The critical level of methane is 1 % v/v. The critical level for carbon dioxide is 1.5% v/v and for oxygen depletion any gas must remove oxygen so that levels fall to 17% v/v or below before effects take place (CIRIA Report 149). This requires more than 1 % v/v of gas. So worse case is 1 % v/v design criteria. 

Borehole flow rate 13.6 l/h N/a Borehole flow rate is maximum recorded in the site and therefore no sensitivity analysis has been used for this parameter 

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There should be a comprehensive justification of parameters and sensitivity analysis... 

The justification of parameters and sensitivity analysis are an important part of fault tree analysis. Using fault tree analysis results to make statistical predictions about future system performance is complex and requires a great deal of experience. It is a complex and specialist field and anyone imder-taking an analysis should be fully conversant with the application of the technique and be aware of how it is applied in other fields. It is important not to become unduly reliant on the numbers remember they are only an aid to judgement. 

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