Turpentine Run, the only perennial stream on St. Thomas, experiences significant flooding due to intense rainfall, steep topography, and shallow soils. The Nadir neighborhood and surrounding area have endured flooding from Hurricane Maria, tropical storms, and other major rainfall events over the past 50 years. DCCM provided hydraulic modeling, analysis, and design support for improvements intended to protect the community from a 25-year storm event.

The team conducted field investigations and gathered local flood-history information, including observed high-water marks. HEC-HMS was used to model watershed runoff for 25- and 100-year storm events, while a two-dimensional HEC-RAS model evaluated existing and proposed hydraulic conditions. The design focused on safely routing approximately 8,000 cubic feet per second through a constrained corridor between a roadway and the neighborhood, beneath a bridge, through dense vegetation, and ultimately to the sea. The analysis also informed streambed and bank protection, riprap sizing, and the evaluation of sea-level rise impacts.

DCCM performed flood-frequency and hydrologic-hazard analyses to evaluate water-surface elevations, uncertainty, and residual risk under existing and proposed conditions. Multiple design alternatives were modeled and compared with computational fluid dynamics results to optimize performance. The findings supported USACE’s risk analysis and development of a Risk-Informed Design for the project.