Investigation and Redesign of the Highway Stormwater Management System to Mitigate Gully Erosion, and Enhance Drainage Infrastructure Resilience; Case in Gokwe Town, Zimbabwe

This study evaluates the impact of land use changes on gully development and redesigning a highway stormwater management section in Gokwe Town, Zimbabwe, aiming to address gully erosion and enhance drainage infrastructure resilience, crucial for urbanizing watersheds and around highways. Using GIS processes, topographical profiles were derived; geotechnical lab investigations, and site hydrological assessments were conducted in evaluating the drainage system. Previous interventions failed to manage drainage properly to stop further erosion. The natural terrain slope, varying from 7.3% to 7.5% between 1985 and 2023, combined with highly cohesionless silty Kalahari sands (∅=21.5°, c=8kN/m2, permeability of 3.31x10-5m/s), indicated high infiltration and low surface runoff under minimal human activity, increasing subsurface water drainage risks and soil collapse on the slope face. Urbanization increased runoff peak discharge (7.47m³/s) as in 2023 due to ground consolidation, contributing to significant gully erosion. A trapezoidal channel network was designed to efficiently collect rainwater, reducing infiltration with additional drains. The redesign directed runoff to a detention pond, then down a slope drain to a gentler slope with finer soil particles. SWMM software modelling showed the system increased the collected runoff by 32%, reducing infiltration, erosive runoff on the soil and preventing soil collapse in sloping areas. Keywords: Gully erosion, Stormwater management, Drainage, Kalahari sands, infiltration