Relative sea-level rise and compound flooding in Mediterranean coastal cities: an evidence synthesis and adaptation framework for Alexandria and Egypt’s Nile Delta

Abstract

Coastal cities face increasing climate risks as rising mean and extreme sea levels interact with dense exposure, critical infrastructure, land subsidence, and drainage limitations. This study presents a structured evidence synthesis of coastal urban risk, focusing on Alexandria and Egypt’s Mediterranean coast. Using the IPCC AR6 risk framework and a semi-systematic PRISMA-guided approach, it integrates evidence on relative sea-level rise, extreme sea levels, compound coastal–pluvial flooding, elevation uncertainty, vertical land motion, exposure, and adaptation pathways. The study develops an integrative relative-sea-level and compound-risk adaptation framework linking ocean-driven sea-level rise, local subsidence, terrain uncertainty, compound flooding, and adaptive pathways into a decision-oriented structure. The synthesis shows that coastal risk in Egypt is governed by relative-sea-level rather than oceanic sea-level rise alone, as subsidence can accelerate threshold exceedance and increase flood frequency. For Alexandria, evidence indicates a rising sea-level baseline, increasing extreme high-water-level risks, and vulnerability to compound flooding driven by rainfall, coastal water levels, and drainage constraints. Exposure estimates are highly sensitive to elevation data and vertical land-motion treatment, highlighting the need for high-resolution terrain data and subsidence-aware assessments. The evidence supports context-specific hybrid adaptation portfolios combining selective structural protection, sediment-based and nature-based measures, drainage improvements, and adaptive pathways under uncertainty. The study concludes that climate-resilient coastal planning in Egypt should move beyond static historical assumptions toward relative-sea-level-based, compound-risk-informed, and pathway-oriented adaptation.