Development of biaxial evaluation matrix combining clean coast index (CCI) and Z-score: a case study of Philippine coastal sites

Abstract Marine plastic pollution poses a critical threat to coastal ecosystems. However, its effective assessment remains limited by the availability of consistent, scalable metrics. The Clean Coast Index (CCI) has been widely applied to quantify the absolute density of coastal litter. However, using only CCI does not account for context-specific coastal anomalies that may warrant targeted management. This paper introduces a novel biaxial evaluation matrix combining the CCI and its standardized Z-score to assess both absolute and relative coastal cleanliness. The matrix was applied to existing datasets and newly collected litter data from coastal sites in the Philippines, with Z-scores calculated by standardizing the CCI values across different units (locations, area types, and survey dates). The results demonstrated that the matrix enables the identification of salient locations requiring specific attention, such as relatively polluted zones in otherwise clean regions and chronic hotspots masked within uniformly dirty areas. This dual-axis approach enhances detection sensitivity, supports nuanced environmental assessments, and facilitates more strategic resource allocation and management. While promising, however, the matrix’s interpretability is sensitive to standardization group selection and assumes data normality. Future research should focus on longer-term monitoring and integration with additional environmental indicators to further validate its applicability.