Basketball and rainwater fans can celebrate together.

Over the last five years, the SFPUC has helped guide the design and oversee approvals of the City’s largest rainwater reuse system for the newly opened Chase Center, the Golden State Warrior’s new state-of-the-art sports and entertainment complex in San Francisco’s Mission Bay neighborhood. This rainwater reuse system is the outcome of achieving compliance with the SFPUC’s Stormwater Management Ordinance and the City’s Phase II MS4 permit. To comply with the Stormwater Management Ordinance, the arena complex was required to treat all stormwater on-site prior to discharge to the SFPUC separate storm sewer system and the Bay. 

The overall complex development includes the arena and two adjacent office towers, and boasts a 250,000-gallon rainwater cistern – making it San Francisco’s largest yet. Over two-thirds of the rainwater that hits the roofs and plaza areas – 303,000 square feet of impervious surface – is collected, stored, and treated to provide over 2.6 million gallons per year (MGY) of alternative water supply that is used for non-potable toilet & urinal flushing and irrigation. This means that every year, over four Olympic sized pools of clean drinking water is diverted from entering Mission Bay’s storm sewer system and reduces the demand for potable water by this same amount.

In addition to providing rainwater capture, the Chase Center will comply with the SFPUC’s Non-Potable Ordinance by capturing graywater (water from showers and tubs, sinks, and washing machines) from the adjacent office towers and HVAC condensate within an efficient 6,500-gallon cistern, leading to another 0.3 MGY reduction in potable water use. The combined reuse of these ’alternative’ rainwater, condensate, and graywater sources cut the potable supply going to non-potable water demands in half.

While notably the largest rainwater system to date, the Chase Center represents only one of the over 110 currently planned or built private and public parcel projects in San Francisco that collaborate with the SFPUC to design rainwater reuse systems to meet the City’s Stormwater Management Ordinance compliance. Of these projects, 14 are also using rainwater harvesting for compliance with the City’s Non-Potable Water Ordinance. 

To date, the three quarters of these rainwater reuse systems that are far enough along to quantify will cumulatively capture almost 37 acres of impervious surface runoff – equivalent to 10 square blocks – and will bring about 2.1 million gallons of privately maintained rainwater cistern storage into operation. This storage volume represents nearly 14 million gallons of rainwater per year that will be diverted from the City sewers while making San Francisco more resilient. As more and more of these systems come online, the SFPUC looks forward to using its local and Hetch Hetchy water supplies more efficiently while also putting rainwater to beneficial use for watering landscapes, flushing toilets, and cooling.

The SFPUC credits the achievement of a national-caliber project to multiple collaborative visioning sessions and compliance meetings with the arena’s project team during the planning and design process, along with ongoing policy and synergies coordination throughout the Agency. Through close coordination with key stakeholders, the SFPUC was able to find an efficient solution for stormwater management. As an example, one key factor that allowed for a successful project outcome was the early realization that the wet-season rainfall supply nicely matched the timing of the arena’s peak arena demands (i.e. game day flushing) during the Warrior’s full season. By designing around the project’s ‘abundance’, the project was able to harness the available resource to create an operation and maintenance friendly design that maximizes both stormwater benefits and non-potable offsets.

The Chase Center highlights the potential of SFPUC’s OneWater approach and the synergistic benefits between the Stormwater Management Ordinance and the Non-Potable Water Ordinance.