Spatial Hydrological Modeling of Flash Flood Runoff Impacting the Benghazi City: An Integration of GIS, Morphometric Analysis, and SCS-CN Methodologies

Abstract

Unmanaged spatial growth across the Benghazi coastal plain has severely altered regional hydrologic regimes, exposing urban infrastructure to un quantified flood risks originating from the peripheral Al Rajmah escarpment. This study addresses the critical research gap in ungauged semi-arid coastal watersheds by delivering a GIS based hydro morphometric assessment, integrating Benina station IDF data with the SCS-CN framework to evaluate surface runoff dynamics across eleven catchments under 5-to-100-year return periods. The analysis reveals a compound dual hydrodynamic regime governing urban inundation. Steep escarpment small basins (Basins 3-10) generate rapid flash flood surges driven by short flow paths (1.07-1.75km) and steep hydraulic gradients (ΔH up to 98m), yielding times of concentration under 30 minutes (Tc 15min in Basin 3) that instantly surcharge perimeter drainage networks. Conversely, regional wadi systems Wadi Al Qattara (Basin2, 1320.87km2) and Wadi Al Dirjati (Basin11, 193.71km2) dominate volumetric yields due to their scale and high order channel development. Predominant clay rich soils (HSG C and D), drive cumulative regional runoff volumes from 218.68x106m3 (5year) to 453.30x106m3 (100year), with Basin2 contributing over 85% (388.44x106m3). To mitigate this compound risk, an integrated engineering framework is proposed, combining upstream check dams, dry detention basins, and engineered channel armoring with culvert, real time early warning telemetry, and hydraulic buffer easements. This research provides a transferable hydro engineering blueprint for flood risk management in rapidly expanding coastal semi-arid plains.