Urban Stormwater Diversion

Wallace Group partnered with the City of Pacific Grove to design an innovative urban stormwater diversion project aimed at protecting the Pacific Grove Area of Special Biological Significance (ASBS) in Monterey Bay. After successfully securing funding from a Proposition 1 Grant, our team developed detailed Construction Documents to bring this project to life.

The system captures dry weather flows and stormwater runoff from the 85th percentile storm—preventing pollutants from reaching the sensitive ASBS. Rather than flowing into the bay, the stormwater is stored in an underground reservoir where it may be gradually released into the sewer system. It is then treated at the Monterey One Water Regional Wastewater Treatment Plant and applied for beneficial reuse.

In another part of the watershed, runoff is directed to a stormwater lift station that pumps directly to Monterey One Water’s facilities. To support this, Wallace Group conducted a comprehensive hydrology analysis to estimate runoff volumes across average, dry, and wet years. This data informed the design of key infrastructure, including:

• A new underground reservoir
• Stormwater pump station and forcemain
• Upgraded storm and sewer mains
• Diversion structures and metering facilities

When initial construction bids exceeded the available grant funding, our team stepped in to value engineer the project. We reimagined the design, meeting the grant requirements, while reducing costs—ultimately consolidating the system into a single underground storage tank and revising the pump station design.