Managing flood risk underground in Santa Clara County

Addressing a long-standing flood risk

Llagas Creek, one of the tributaries of the Pajaro River, drains a 104-square-mile watershed in southern Santa Clara County, where flooding has historically affected homes, businesses, and agricultural land. The multi-phase Upper Llagas Creek Flood Protection Project is designed to reduce flood risk for approximately 1,000 homes, 500 businesses, and more than 1,300 acres of agricultural land while improving creek habitat and preserving natural flows.

Phase 2A of the Upper Llagas Creek Flood Protection Project advanced this work by constructing an underground bypass tunnel and reinforced-concrete box culverts that redirect peak storm flows away from downtown Morgan Hill. Constructing these facilities within a constrained commercial corridor and residential area required Mott MacDonald and the project team to accommodate existing utilities, active roadways, adjacent businesses, and limited work areas while maintaining safe public access.

Creating capacity below the surface

At the heart of Phase 2A is an innovative tunnel-based flood control system that rethinks conventional approaches. Rather than relying solely on channel widening, the project implements a dual conveyance strategy using a large underground bypass tunnel and reinforced concrete culverts while allowing everyday flows to continue through the existing creek corridor. The tunnel alignment beneath Nob Hill and the surrounding urban area demanded careful planning of excavation, staging, temporary works, quality assurance, schedule interfaces and construction monitoring. This dual conveyance strategy increases flood capacity without expanding the surface footprint, preserving both the urban environment and ecological function.

Managing complexity through collaboration

As prime consultant, Mott MacDonald led a team providing Valley Water with pre-construction and construction management services for Phase 2A. Our responsibilities encompassed construction administration, inspection, materials sampling, scheduling, change order management, progress payment processing, claims support, safety, and stormwater pollution prevention plan reviews, labor compliance, and general office engineering support. Together, these services provided the field oversight, quality assurance and schedule controls needed to manage risk and maintain progress across interconnected tunnel, culvert, utility and surface-work activities.

We helped deliver a critical component of the flood protection system within an active, space-constrained urban environment. The underground bypass tunnel and associated infrastructure were constructed beneath active roadways, around existing utilities and alongside operating businesses, and homes. Numerous utility relocations and third-party interfaces required early conflict identification, coordinated work windows and closely managed sequencing. Working closely with Valley Water, the City of Morgan Hill, federal partners, regulatory agencies, utility owners, and the contractor, we helped resolve interfaces before they affected field production and kept construction activities safely integrated within the downtown corridor.

Limited right-of-way, utility conflicts and the need to protect local businesses and residents shaped the construction approach. Our team supported phased work areas, traffic control and access planning so residents, customers, emergency services and deliveries could continue to move through the corridor. By aligning tunneling, utility relocations, surface restoration and stakeholder notifications, the team optimized available space, reduced disruption and protected schedule continuity. Proactive risk management and collaborative decision-making helped the bypass tunnel advance as the hydraulic backbone of the broader program, enabling future phases to operate as an integrated system and strengthening long-term flood resilience for Morgan Hill.

Strengthening community resilience

Today, Phase 2A strengthens flood resilience in Morgan Hill by diverting peak storm flows underground while maintaining the natural function of the creek. As part of the broader program to provide 100-year flood protection for urban areas, the project helps safeguard homes, businesses, and critical infrastructure while creating a foundation for future phases of the Upper Llagas Creek Flood Protection Project.

Recognized by the American Society of Civil Engineers (ASCE) San Francisco Section as an Outstanding Flood Management Project, Phase 2A demonstrates how innovative engineering, coordinated project delivery and sustained community focus can create infrastructure that helps protects people, supports local businesses and strengthens long-term resilience.