An innovative, marina-area storm drain design for the city of Benicia, Calif., not only affords flood protection for residents and their property, but also provides a higher level of water quality control into San Francisco Bay and the nearby Carquinez Strait. Benicia, population 28,000, is located about 30 miles northeast of San Francisco. The city was established more than 150 years ago. Named one of Money Magazine’s "Best Places to Live," the city is also home to Benicia Capitol State Park, the site of California’s third seat of government (1853-1854)—the only pre-Sacramento capitol that survives. Benicia’s marina, with 350 boat slips, is used year-round and is an important waterfront attraction.

The city’s downtown and lower eastside drainage system was constructed throughout the 1970s. During construction of the marina, the main storm drain outfall originally located in this area was relocated approximately 1,000 feet south, draining directly into the Carquinez Strait. As a cost savings measure during the recessions of the 1970s, the pipe material was changed from reinforced concrete to corrugated metal, and the flap gate structure at the outfall was eliminated. Fortunately during that period there were no significant flooding events until the El Nino storms in 1997 and 1998.

A storm drain pipe installed in the 1970s was obstructed and prevented the drainage system from acting as a flood-control feature.

But after 20 years of storms and saltwater immersion, the corrugated metal pipe degraded and was as weak as tinfoil in some areas. And because there was no flap gate, large levels of bay sediment migrated into the pipe—95 percent of the existing storm drainpipe was obstructed and prevented the drainage system from acting as a flood-control feature. These deficiencies were discovered during heavy storms in 1997 and 1998.

The city had no choice but to use large emergency pumps at a temporary pump station to pump stormwater directly into the marina. But this was not a viable long-term solution. A hydraulic computer model analysis also revealed that the last half-mile of the storm drain system had no hydraulic benefit during a severe storm. There wouldn’t be enough water deposited by rains to push the stormwater out against the tide.

A photo taken at extremely low tide shows the poor condition of the original storm drain outlet into the Carquinez Strait.

So the city began a detailed engineering and environmental analysis and investigated 10 engineering solutions, ranging from tunneling through the degraded 66-inch corrugated metal pipe to building a large 70,000-gallon-per-minute pump station.

The storm drain design project had to satisfy the following five key areas:

1. Economic—little or no disruption to local businesses; no increase in dredging costs; minimal operations and maintenance effort; designed/constructed/operated with little or no new revenue.
2. Environmental—little or no impact to cultural resources (historic or archeological), plants, or animals; minimal impact on traffic during construction; no increase in fecal coliform levels already present in surrounding waters; adaptable to future stormwater quality regulations.
3. Public Health & Safety—eliminate or reduce upstream flooding; be a permanent facility; reliable; little or no impact to beneficial use of surrounding waters.
4. Social—don’t distract from surrounding aesthetics and don’t diminish the quality of life for residents or businesses.
5. Technological—simple to maintain/operate; can be permitted by regulatory agencies; able to accommodate rainstorms.

Paving the way for design
Before the design and subsequent construction could be implemented, regulatory and public input was needed. Environmental documents were prepared and the city worked closely with numerous state and federal agencies, including the Regional Water Quality Control Board, U.S. Army Corps of Engineers, San Francisco Bay Conservation and Development Commission, and the Solano County Department of Resource Management Division of Environmental Health Services.

The city also held a series of town hall meetings, public hearings, workshops, and neighborhood meetings not only to impart information properly, but also to receive input, recommendations, and opinions that would help shape the design.

Easements also had to be acquired. The city, for example, owns the marina but has leased it for 60 years to a third-party private operator. In a rather unique scenario, the city was unable to negotiate a storm drain easement and was forced to file an eminent domain action against a piece of property it already owned—technically a condemnation action of a lease agreement to get temporary and permanent permission. The city and the marina operator eventually came to terms and the easement was granted.

Implementing the design
The design not only had to be consistent with the approved Environmental Impact Report, it had to be "design transparent"—the city didn’t want the infrastructure to visually conflict with the marina.

A new overflow system provides flood protection by discharging excess stormwater through 48-inch reinforced concrete pipes directly into the marina.

Some key design elements included the following:

• Convert the temporary and interim pump station into a valve vault for the new pump station.
• Construct a new dry weather/first flush flow diversion pump station with two, 48-inch reinforced concrete discharge pipes into the marina.
• Install two stormwater treatment units—CDS stormceptors—upstream of the pump station to remove unwanted material in the stormwater. The city, for instance, has a lot of baseball fields and it was quickly ascertained that pumps don’t like passing softballs, floating bottles, leaves, oils, and other debris. The treatment units are situated underground and in line with the storm drain system.
• Construct an outfall structure that uses concrete texturing so it looks like riprap surrounds it. An overlook was built on top of the outfall so people can stand over the top of it and look out over the marina. The overlook also provides shadow: If someone is on the opposite side, they can’t see the outfall structure since it is cast in shadow.
• Build a landscaped redwood patio deck over the overflow control structure instead of leaving a 25-foot-square blank slab of concrete.
• Place oil-absorbent pillows in the drainage system just upstream of the pump station to pick up oils not initially captured by the treatment units. About twice the size of a king-size pillow, they are made of absorbent material, float on the water, and last for about 90 days.

The new outfall structure uses concrete texturing on wingwalls to give the appearance of riprap.

Storm drain design accomplishments
Benicia plans to budget permanently for maintenance of the stormwater drain system. The city is now able to divert all dry weather flows around and away from the marina via small pumps and discharge these into nearby wetlands for treatment. In winter, during heavy storm events, the overflow system in place (regulated by weirs within the pump station) affords flood protection by discharging stormwater through the dual, 48-inch reinforced concrete pipes directly into the marina. If the water never reaches a set elevation to overflow, it remains in the drainage system until pumped into the wetlands. There is a higher level of water quality control into San Francisco Bay and the Carquinez Strait, and adjacent wetlands are thriving on the fresh water from the storm drain system.

Michael Throne, P.E., is Benicia, Calif.’s city engineer. He can be contacted at michael.throne@ci.benicia.ca.us. Joubin Pakpour, P.E., is founder and president of Pakpour Consulting Group, Inc. (www.pcgengr.com), a municipal civil engineering design and construction management firm located in Pleasanton, Calif. He can be contacted at jpakpour@pcgengr.com.