The Sierra Chapter was pleased to welcome Nina Buelna, PE, from the City of Sacramento Department of Utilities (DOU) as our guest speaker for the May meeting.  Miss Buelna is the Wastewater Supervising Engineer for DOU and oversees the Wastewater Department.  In this role she is responsible for budgeting, long-range planning, asset management, Capital Improvements Project delivery, and managing staff.

Miss Buelna’s presentation focused on the Combined System and the recently constructed McKinley Vault. The Combined System captures, conveys, and treats a combination of wastewater and stormwater runoff in the same pipe and serves over 200,000 people in the Downtown, Land Park, Curtis Park, Oak Park, and East Sacramento neighborhoods. One of the challenges with the Combined System is the ability to capture and convey runoff during peak storm events.  The City has continuously invested in improvements to reduce outflows to the Sacramento River and outflows to the street that occur during peak events. Between 1995 and 2021, DOU has spent $156 million in capacity improvement projects which includes 20 million gallons of storage.  DOU now has a total of four peak storage facilities including McKinley, R Street, Med Center, and Oak Park. More peak storage facilities are planned as part of their long-term planning.

For over 90% of storm events, wastewater from the Combined System is sent to the Regional San facility in Elk Grove. Storms that are high intensity or exceed 1” of depth (>60 MGD) require the Pioneer Reservoir and Combined Wastewater Treatment Plant to operate to treat the additional volume. In rare occurrences when the runoff exceeds the capacity of the three treatment facilities (over 440 MGD), flow can be bypassed directly to the Sacramento River. This last occurred in October 2021.

The McKinley Vault was designed to provide additional storage during peak storm events to reduce street flooding and untreated releases to the Sacramento River.  The Vault was designed with a 6 million gallon capacity with a peak inflow of 60,000 GPM which was based off of the volume for a 10 year, 6-hour storm.  Once the Vault reaches capacity, it is slowly pumped out into the pipe network after the storm passes and the peak resides.

Construction began in June of 2019 and included over 60,000 cy of excavation and 9000 cy of concrete.  One of the challenges was high groundwater at the project site which required continuous pumping to keep the site dewatered during construction. The area above the vault was reconstructed with a public park. The Vault was completed on October 15, 2021 and was put to the test just nine days later when a 100-year storm event passed over the City on October 24, 2021, bringing 5.62 inches of precipitation in a 24-hour period. Although the Vault did not eliminate flooding entirely, it functioned as designed and reduced flooding significantly.

The total cost of the project was $32 million. DOU used a Guarantee Maximum Price contract which was a first for the City.  This contract format saved over $1 million in additional costs.  It is exciting to see projects like this completed that have a direct benefit for City residents.