One of California’s largest central coast range rivers, the Napa River, drains 426 square miles on its 50-mile journey from Mt. St. Helena to the San Pablo Bay. An estuary system comprises the last 17 miles of its journey.
Over the years, stream bank erosion and plant life removal have created significant river sedimentation. In fact, 70,000 to 200,000 tons of sediment enter the Napa Valley watershed every year due to development, roads and hillside vineyards. Unfortunately, the sediment deposits into stream channels and ponds, which impairs fish spawning and reduces habitat diversity and the food supply for fish.
In an effort to protect the river’s ecosystem, the state has identified the Napa River as a 303(d) body of water in the Clean Water Act—a body of water whose effluent limitations are not sufficient to meet state and federal water quality standards.
In April 2003 when work began on the $31.5 million Maxwell Bridge replacement project over the Napa River, maintaining the river water quality was key. The state’s Regional Water Quality Control Board required water discharge not exceed 10% in areas where natural turbidity is greater than 50 NTU. The project contractor, C.C. Meyer, was up for this challenge.
“By August, we had six 48-ft deep cofferdams in the middle of the Napa River that we had to dewater,” explained Shane Dees, C.C. Meyer’s project manager.
Using four 4x65 interlocking steel plates, C.C. Meyer created a series of temporary watertight enclosures that could be pumped dry to expose the river bottom. Only then could the contractor build footings, columns and drive piles 30 ft under the river to support the bridge. Once the footings and columns were completed, the cofferdams were backfilled up to the riverbed and pulled out of the river.
“During excavation, we were stirring up a lot of mud in the cofferdam,” noted Dees. “It was very difficult to pump 1,000 gallons of water a minute and try to keep it clean.”
At various times during the excavation and installation project, Dees noted that the cofferdams became so muddy that the water couldn’t be cleaned without operating up to three filtration systems.
“We didn’t have the money or the space for such a large set up,” he said. “With a lot of work, we were able to convince the state environmental agencies to allow us to pump from one cofferdam to the other and wait for the sediment to settle.”
Creating successful solution
To contain and filter the dirty river water, C.C. Meyer turned to Baker Tanks, who quickly put two filtration systems in place—one on each side of the river. The solution consisted of 15 Baker Style 20,000-gallon settlement tanks, three four-pod sand filters with submersible pumps, two Baker Roll-off boxes, carbon filters and berms to provide secondary containment.
“It was a pretty tight area and we needed to design the system to fit within the space and allow adequate room to drive 130 ft piles into the river,” recalled John Coon, Baker Tanks sales representative. “In order to meet space requirements, we had to build platforms and lift the tanks in by crane rather than have our drivers position them onto the jobsite. As a result, the decreasing elevation of each tank allowed the water to cascade from tank to tank to maintain the necessary water flow.”
Coon chose the Baker T-style tanks due to their smaller footprint and ease of modification.
“We were able to make different size valve alterations to the tanks to accommodate high and medium discharges allowing for settling and skimming of cleaner water,” he said. “We customized each tank with two 10-in. pipelines to maintain the 1,000-gallon-a-minute flow. We manifolded the tanks so we could add equipment if needed. We met C.C. Meyer’s needs and had the filtration system set up and running in two days.”
Testing the waters
To comply with California’s Regional Water Quality Control Boards regulations, upstream and downstream readings had to be taken at various intervals.
“We used a turbidity meter to conduct the readings,” said Coon. “We had to bring the turbidity levels down from 1,000 NTU’s to the natural turbidity of the river.”
Given the high turbity levels, C.C. Meyer and Baker Tanks brought in California-based ProTech General Contracting Service, Inc., to institute a multi-step suspended material clarification process.
A chemical coagulant was added to the influent water and mixed to form a dense flocculent that settled out by gravity. The flocculent was then collected in a series of settlement tanks. This was a particularly efficient method to remove suspended material, more than 90% of particles greater than 25 microns.
To meet the specific needs of the bridge replacement project, ProTech also used an aluminum chlorhydrate coagulant, which is a high-molecular-weight polymer specifically designed for water treatment systems. The coagulant was injected into the influent water at 50 ppm, using a metering pump that can maintain a system flow rate of 500 gpm running 24 hours a day. Because proper mixing of the coagulant and raw water was necessary, the coagulant was injected into the line 20 ft upstream from the filtration system. This method helped prevent violent agitation or high-speed mixing which can decrease coagulant activity. Gravity settled the Napa River sediment in clarifier tanks. As a final precaution, the water was routed through a sand filter for final polishing.
To avoid delay and extra expense involved in filtering at high tide, Baker Tanks’ Coon took it upon himself to consult the tide charts. “With careful planning, we were able to work on other areas of the job during high tide, which allowed us to churn out close to one million gallons of water a day—running 24/7 on several occasions,” Coon said.
The Maxwell Bridge replacement project is slated for completion in late 2005. Throughout the long project, Dees credited the successful collaboration between C.C. Meyers, Baker Tanks and ProTech for keeping the project on schedule and on budget.
