Validating Post-Construction Drainage Networks in Hybrid Power Systems using as-Built Flow Accumulation and SPI Profiling

Abstract

Utility-scale solar infrastructure in tropical, high-relief regions faces extreme hydrological risks due to soil compaction and high-intensity rainfall.This study evaluates the hydrological resilience of a 16.2 MWp hybrid power station in Baomahun, Sierra Leone, by validating engineered drainage designs against as-built topographic data. Utilizing a 31-year rainfall dataset, we integrated as-built surveys with D8 flow accumulation algorithms and HEC-RAS modeling. The Stream Power Index (SPI) was utilized to quantify erosive energy, while Manning’s equations were applied to determine velocity exceedance across the finalized dendritic network. Findings indicate that bulk earthworks resulted in a 16.2% increase in the weighted Curve Number (CN 91.8) and a 66.3% reduction in surface retention (S). Consequently, peak discharge (Qp) during a 100-year storm event (220.02 mm) increased by 38% to 122.78 m3/s , with peak velocities reaching 3.8 m/s—a 3.1x exceedance of the soil’s non-erodible threshold. However, spatial validation confirms that 55% of site load is successfully consolidated into a southwestern outlet, maintaining an infrastructure safety buffer of >35 meters for all critical assets. The integration of as-built accumulation mapping and SPI profiling demonstrates that while construction significantly alters hydraulic loads, a deterministic dendritic network can effectively decouple high-value assets from hydrological stress. This study offers a scalable, "bankable" framework for certifying site stability in erodible tropical terrains.