Evaluating coastal urban underground space for ecological and marine-pollution suitability using AHP–TOPSIS

An integrated Analytic Hierarchy Process–TOPSIS framework is developed to evaluate the ecological and marine-pollution suitability of urban underground space (UUS) projects in coastal cities. The AHP–TOPSIS hybrid approach is particularly suitable for coastal infrastructure evaluation because it integrates structured expert judgement with transparent aggregation of distance-to-ideal performance, enabling balanced consideration of ecological, marine, technical, and social objectives in complex planning environments. A structured hierarchy of 10 criteria, grouped into Environmental/Marine, Economic/Technical, and Social/Governance dimensions, is weighted based on judgements from 50 experts in planning, underground engineering, coastal and marine environments, and infrastructure finance. Environmental/Marine factors receive the highest aggregate weight (0.53), ahead of Economic/Technical (0.25) and Social/Governance (0.22), with energy efficiency, air quality, stormwater management, marine impact, and lifecycle cost emerging as the most influential criteria. Five representative UUS alternatives—underground transit hub, subsurface retail complex, underground green park, multi-level underground parking, and subterranean mixed-use complex, are assessed using TOPSIS. The underground green park and transit hub have the highest closeness coefficients (0.72 and 0.65), while retail and parking options systematically underperform due to weaker Environmental/Marine and social profiles. Sensitivity and scenario analyses confirm the robustness of the rankings to substantial variations in criteria weights, supporting the prioritisation of ecologically oriented and transit-supportive UUS concepts in coastal planning. The framework provides operational guidance for coastal planning authorities seeking to align investment in underground infrastructure with ecological resilience and long-term marine pollution mitigation objectives.