CCTV inspection of sewer segments: calibration of performance indicators based on experts’ opinions

Asset management is an increasing concern for wastewater utilities and companies. Indicators are developed for supporting the definition of investigation and rehabilitation programs. These indicators are mostly based on visual inspections, which provide major information. However, difficulty remains in the translation of a visual inspection survey into dysfunction indicators. Condition grade of a sewer segment may be obtained by comparison of a single score to thresholds which must be in accordance with practices or opinions of utilities’ experts. The confrontation between expert assessments of sewer segments (condition grade) and calculated scores also demonstrates the necessity of considering diagnosis imperfection when establishing thresholds. To fill this niche, an algorithm has been recently proposed (Ibrahim et al., 2007) in order to fix thresholds by minimizing a cost function. This article presents a successful application of the proposed algorithm on 30 CCTV reports of sewer segments located in the urban community of Strasbourg or in the Département du Bas-Rhin. For each CCTV, a score is calculated using observation codes converted into numerical scores and in parallel, grade is assessed by several experts (sewer managers of different French local utilities). This method has proved its interest when threshold determination is required to assess indicators. Moreover, this approach is generally applicable to numerous domains, when levels of performance need to be defined.