WSP Water Solutions

Hydrologic and Hydraulic Modeling 

Hurricane Helene I-40 Pigeon River Gorge Emergency Repair Design-Build North Carolina Department of Transportation (NCDOT) | North Carolina ■ Emergency response to catastrophic flooding ■ Advanced hydrologic and hydraulic modeling ■ Integrated wall and drainage design Hurricane Helene caused catastrophic flooding and embankment failure along Interstate 40 in North Carolina’s Pigeon River Gorge, destroying miles of roadway. WSP rapidly mobilized to provide urgent hydrologic and hydraulic engineering services for NCDOT, including drainage, wall design, river corridor planning, and construction sequencing to restore resilience and functionality. To characterize flow frequency and magnitude, the team performed stream gauge analysis using HEC SSP per Bulletin 17C. Hydraulic modeling employed SRH-2D within the Surface-Water Modeling Software interface. Analyses evaluated water surface elevations, shear stresses, and velocity distributions to inform emergency repair design. WSP’s insights enabled NCDOT to address drainage challenges, created a 50-foot-tall wall design and established river corridor constraints. Our efforts ensured safe reconstruction in a high-risk area.

Ashokan Reservoir Rehabilitation New York City Department of Environmental Protection | Ulster County, New York

Lake Lenape Dam Rehabilitation Atlantic County, New Jersey | Hamilton Township, New Jersey

■ Hydraulic Engineering Center (HEC) River Analysis System (RAS) modeling ■ 3-D CFD modeling ■ Design and alternatives development The Ashokan Reservoir supplies about 40% of New York City’s daily drinking water. Built in 1915, its aging infrastructure (including the Dividing Weir, Dividing Weir Bridge, and Ashokan Spillway) requires rehabilitation to meet modern Probable Maximum Flood (PMF) standards and ensure reliable service for the next century. As a major subconsultant, WSP provides planning, permitting, design and construction support. Our most impactful contribution is advanced hydraulic modeling, which includes sub-basin-wide 2-D HEC-RAS, 3-D CFD using FLOW-3D and dam breach/inundation analyses. These tools guide design alternatives addressing inadequate spillway capacity, flow obstructions at the Dividing Weir Bridge and downstream flood risks. WSP’s phased approach integrates physical modeling, 3-D laser scanning, and 3-D CAD to optimize constructability and minimize disruptions. The project results in resilient, low-maintenance infrastructure that preserves historical features while protecting a vital water supply for generations.

■ Hydraulic design for spillway replacement ■ Upstream flood reduction study ■ 2-D HEC-RAS and CFD modeling

Lake Lenape Dam, a high-hazard earthen embankment built in the late 1800s, serves a 200-square-mile drainage area and 300-acre lake. Classified as unsafe due to an undersized spillway and aging infrastructure, WSP was contracted to design remedial improvements that meet current safety standards. Using HEC-HMS, WSP modeled the Probable Maximum Flood to assess inflow and applied 2D HEC-RAS for reservoir routing and downstream impacts. FLOW-3D hydraulic modeling evaluated the new labyrinth spillway and adjacent powerhouse as a low- level outlet and construction bypass. These analyses informed alternatives and an upstream flood reduction study. WSP delivered the final design and bid package, including plans, specifications, cost estimates, and schedule, while supporting permitting and regulatory coordination. Key elements include a 124-ft labyrinth spillway, rockfill and masonry overflow, low-level outlets, embankment grouting, steel slide gate, fish ladder relocation and safety/access upgrades.

Page 35