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

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.

As-built Validation, Flow Accumulation, Stream Power Index (SPI), Utility-Scale Solar (USF), Hybrid Power Systems

1. USDA-NRCS (2004). National Engineering Handbook, Part 630, Chapter 10: Estimation of Direct Runoff from Storm Rainfall. Washington, D.C. (The foundational standard for your SCS-CN 91.8 calculation). [Google Scholar] [Crossref]

2. Cook, L. M., & McCuen, R. H. (2013). "Hydrologic Response of Solar Farms." Journal of Hydrologic Engineering, 18(5), 536-541. (The primary study on how PV arrays alter runoff timing). [Google Scholar] [Crossref]

3. O’Callaghan, J. F., & Mark, D. M. (1984). "The extraction of drainage networks from digital elevation data." Computer Vision, Graphics, and Image Processing, 28(3), 323-344. (Fundamental for your D8 flow accumulation methodology). [Google Scholar] [Crossref]

4. Moore, I. D., & Wilson, J. P. (1992). "Length-slope factors for the Revised Universal Soil Loss Equation: Simplified method of estimation." Theoretical and Applied Climatology, 46(1), 13-18. (Standard reference for the SPI profiling in Figure 18). [Google Scholar] [Crossref]

5. Gash, J. H., & Shuttleworth, W. J. (2007). "Tropical Hydrology." The Geographical Journal. (Provides the ecohydrological context for high-intensity rainfall in West Africa). [Google Scholar] [Crossref]

6. Al-Hamdan, O. Z., et al. (2015). "Modelling concentrated flow erosion processes on steep slopes." Hydrological Processes, 29(4), 577-589. (Supports your analysis of the Critical Scour Zone at the SW outlet). [Google Scholar] [Crossref]

7. earing, M. A., et al. (2017). "Impacts of Climate Change on Surface Runoff and Soil Erosion in Tropical Regions." Earth-Science Reviews. (Contextualizes your 100-year storm event analysis). [Google Scholar] [Crossref]

8. Roure, B., et al. (2020). "Effect of soil compaction on the peak flow and runoff volume in utility-scale solar installations." Journal of Renewable and Sustainable Energy. (Crucial for justifying the CN increase from 79 to 91.8). [Google Scholar] [Crossref]

9. Armstrong, A., et al. (2014). "Solar park microclimate and vegetation management: Effects of shading and ground cover." Environmental Research Letters. (Relevant to your recommendations for inter-row grass seeding). [Google Scholar] [Crossref]

10. Tarboton, D. G. (1997). "A new method for the determination of flow directions and upslope areas in grid digital elevation models." Water Resources Research. (An advanced alternative to D8 that strengthens your mapping methodology). [Google Scholar] [Crossref]

11. Sierra Leone Meteorological Agency (SLMA) (2022). Annual Rainfall Climatology and Extreme Weather Events Report. Freetown. (The source for your 31-year rainfall dataset). [Google Scholar] [Crossref]

12. Zevenbergen, L. W., & Thorne, C. R. (1987). "Quantitative analysis of land surface topography." Earth Surface Processes and Landforms. (Supports the topographic leveling validation in your report). [Google Scholar] [Crossref]

13. Kibria, M. G., et al. (2021). "Impact of Utility-Scale Solar PV Plants on Local Hydrology and Soil Health." Current Sustainable/Renewable Energy Reports. (Discusses the long-term impacts of solar footprints). [Google Scholar] [Crossref]

14. Wolka, K., et al. (2018). "Impact of soil and water conservation measures on catchment runoff in tropical regions." Journal of Environmental Management. (Validates your use of check dams and rip-rap). [Google Scholar] [Crossref]

15. Wubnehe, A. T., et al. (2023). "Predicting Soil Erosion Vulnerability in West African Catchments using SPI and TWI." Geocarto International. (Directly supports your use of the Stream Power Index to predict gully formation). [Google Scholar] [Crossref]