GARBERVILLE >> Since Memorial Day weekend, the Highway 101 off ramp at the south end of Garberville had been closed to traffic as Caltrans road maintenance crews and contractors worked to replace a deteriorated storm drain culvert that caused a sinkhole near the ramp.
With the Summer Arts and Music Festival bringing the season's first surge of summer festival traffic this past weekend, Caltrans tackled the off ramp portion of the project first, before moving on to replace more of the deteriorated culvert extending under the four lanes of 101.
As of press time on Friday, May 30, Caltrans expected to complete this phase of the repairs and re-open the off ramp by the end of the workday on Friday.
After the weekend, crews planned to return to complete replacement of the culvert under all four lanes of Highway 101, which will require temporary lane closures to insure worker safety. Caltrans did not have an estimate of how much time this would take by press time.
According to Caltrans public information officer Scott Burger, Caltrans's Garberville maintenance workers discovered a small sink in the area on May 5. A partial video inspection of the storm drain was made two days later, and a full inspection took place on May 13.
On Thursday, May 22, the crew examining the culvert system noticed the sinkhole was growing and closed the off ramp.
Since then workers have been on site every day, digging out and replacing the aged and deteriorated 18" culvert and replacing it with a new 24" culvert.
In the meantime, a different Caltrans electrical crew was on the south side of Garberville last week replacing electrical conduit, a project unrelated to the sinkhole, Burger said.
The sinkhole was due to failure of the old line, which was slowly deteriorating over time. Leaking water from the culvert, full of dirt particles and other solids, scoured the line, further damaging it.
Once the repairs are complete, under normal use the new storm drain system is projected to last between 25-50 years, said Burger.
At Garberville Sanitary District's board of directors meeting last Tuesday, May 27, board member Doug Bryan informally reported that he had been to the sinkhole site and discussed the situation with the Caltrans crew.
Water used to backwash the filtration system at the existing drinking water treatment plant on the Hurlbutt property east of the on ramp is released through a small ditch on the hillside above the highway. It enters Caltrans' culvert as part of the hillside drainage, and along with storm drain discharge it is ultimately released into the South Fork Eel River.
During the summer GSD typically backwashes its filters every five days discharging about 1,500 gallons of water each time, GSD operations manager Ron Copenhafer told the board.
Burger told the Redwood Times in a telephone interview on Wednesday, May 28, that Caltrans had been in communication with GSD, and that he was unaware of any direct connection between the district's backwash and Caltrans' storm drains.
This was confirmed on Friday by GSD general manager Ralph Emerson, who had discussed the drainage with Caltrans officials who told him their backwash was "absolutely not a cause" of the sink.
Emerson said he had also spoken with Garberville residents involved in the construction of the original Garberville Water Company, who told him they made an agreement to discharge backwash water into the state's culvert 40 years ago when the town's then privately-owned water system was built.
Releasing backwash water into a river is not allowed by current state water quality regulations, however. Shortly after last Tuesday's GSD board meeting, a citizen filed a complaint with the North Coast Regional Water Quality Control Board (RWQCB).
Engineer Lisa Bernard, the district regulatory person for RWQCB, told the Redwood Times, also by phone on May 28, that her agency was aware of GSD's backwash discharge. GSD applied for a temporary exemption to the regulation in October, 2013, but due to RWQCB staff shortages and paperwork back-up, the application was not processed and approved until a few weeks ago.
The exemption was granted with the understanding that once GSD's new drinking water treatment is completed, the district will no longer discharge any backwash into the river. "They're in the middle of a construction project that will completely take care of the problem. You can't move any faster than that," Bernard said. [See related story about the GSD meeting and water treatment project in this issue.]
GSD's filtration system uses special sand and other filter media to remove particulate matter from the raw river water as the first step in making the water potable, general manager Ralph Emerson explained. From the filtration tank the water moves into the chlorination chamber for disinfection, and from there into the distribution system that brings it to Garberville households and businesses.
During the backwash cycle, treated water is pumped backward into the filtration tanks to clean out the media. From the treatment plant's present location on the Hurlbutt property, backwash is released into a ditch only a few inches wide and a few inches deep following the natural drainage on the hillside to the inlet to the Caltrans culvert, said Emerson.
The backwash water contains silt removed from the filter media and a small amount of chlorine residue that remains in potable water. When chlorinated water comes into contact with vegetation along the side of the ditch, the vegetation absorbs the chlorine. By the time this water has drained down the hillside, through the culvert under the highway, and then down the hillside into the river, solids and chlorine have been removed by natural processes, Emerson added.
When the new treatment plant is up and running, instead of being discharged, the backwash water will be returned to the chlorination chamber, disinfected again, and will enter the distribution system, so there will be no need to discharge it.
"It's a high priority once the new plant is completed never to discharge backwash again," Emerson said. "We will do everything to stay in compliance and to protect the river and our environment."