Research Description
Most coastal cities have been experiencing unprecedented urbanization-induced flood risk, climatic events, and
haphazard anthropogenic activities, jeopardizing residents' lives and building environments. Despite mounting
flood-related studies, analyzing the correlation between the spatiotemporal dynamics of Built-up Expansion
patterns (BE) and flood risk remains unknown and holds divergent perspectives. In this context, the coastal city
of Alexandria, Egypt, characterized by multiple urban patterns and experiencing heavy rainfall annually, was
selected as a testbed. Our method defined the spatiotemporal rates of BE from 1995 to 2023, quantified flood risk
spatially, and finally investigated the correlation between BE and flood risk through spatial and statistical
analysis. Our results show the built-up area occupied 30.32 % of the total city area till 2023, and the infilling
pattern dominated the BE growth by 45.21 % of the total built-up area, followed by leapfrogging and edge
expansion by 33.25 % and 21.55 %, respectively. The unplanned-infilling pattern is predominantly highlycorrelated with the flood-vulnerable peaks (correlation coefficient (rk) = 0.975, p-value < 0.05) and lowers
dramatically towards planned-infilling regions with flood protections. Meanwhile, a spatial mismatch exists
between high-risk peaks and leapfrogging and edge expansion (rk = 0.118 and 0.662, respectively, with a pvalue
< 0.01), indicating that controlling the built-up amount is inadequate for mitigating flood risk. Porositybased
urban configuration and spatial distribution of built-up patches in harmony with nature-based solutions
are recommended for shaping flood-resilient and effective urban planning.
