The Santiago Dam, located in Orange County, California, faced a critical need to upgrade its spillway to meet modern flood safety standards. The existing structure, built in 1933, could handle up to 46,000 cubic feet per second (cfs), but new requirements called for a capacity of 65,000 cfs to ensure the dam’s safety during extreme flood events. To address this challenge, Verdantas Flow Labs (formerly Alden Research Laboratory) was engaged by GEI Consultants to conduct a physical hydraulic model study of the proposed replacement spillway. A 1:30 scale model was constructed to simulate a wide range of flow conditions, allowing the team to verify computational fluid dynamics (CFD) predictions, develop a precise stage-discharge rating curve, and evaluate flow patterns, standing wave formation, and the risk of wall overtopping. The study also measured velocities and pressures to assess the potential for concrete abrasion and structural loading.

Through a series of design iterations, the team focused on optimizing the stilling basin, which is essential for dissipating energy and protecting downstream areas. Six different modifications were tested; adjusting the basin’s floor elevation, width, and length to achieve the best balance of performance and construction cost. The final design featured a 100-foot-wide, 200-foot-long stilling basin with a floor elevation of 645 feet. This configuration minimized downstream velocities and pressures, reducing the risk of erosion and structural wear.

The physical model confirmed that the proposed spillway, with a crest elevation of 811.9 feet, could safely pass the required 65,000 cfs without overtopping the dam. No significant runup or overtopping was observed on the spillway or chute walls under design flows, and only minor, intermittent splashing occurred at the highest flow rates, prompting recommendations for additional erosion protection.

The close agreement between the CFD and physical model results provided confidence in the final design, supporting regulatory approval and advancing the project to final design. The modifications not only improved hydraulic performance but also reduced construction costs, delivering a safer, more resilient, and cost-effective solution for the Santiago Dam and the surrounding community.