by MEREDITH M. GRIFFITH

Sounder contributor

Silt-choked Cayou Lagoon should be breathing easier next year, released from the stranglehold of the 1970s-era rock fill and bridge that have reduced tidal interchange, caused bank erosion and suffocated native species.

Public Works project engineer Colin Huntemer is working with a consultant to complete the design for a replacement bridge by December 2014 with plans to begin construction in April 2015.

With the nuts and bolts in place, “We’re looking for feedback on the aesthetic elements,” explained Huntemer at a July 10 public meeting on Orcas.

The proposed replacement bridge will be 80 feet long and 28 feet wide, with 10-foot traffic lanes and four-foot pedestrian lanes on each side. The bridge will be two and a half feet higher than the existing bridge, designed to clear the 100-year flood water elevation by three feet.

A $2,284,000 federal Bridge Replacement Advisory Committee grant will cover 80 percent of essential construction, with a 20% match by the county Public Works department.

The existing bridge, built in the 1970s, has just a 52-foot span and is 24.5 feet wide. An earlier bridge, its predecessor, was 60 feet long and only 15 feet wide.

Huntemer said the community can help determine the placement and angle of the bridge, and details like guardrail design and decorative concrete finishes and whether the bridge will include pedestrian lookouts and informational plaques.

The grant will pay for basic construction only, so any aesthetic enhancements will need to be locally funded, he said.

Construction is expected to take four to six months, and traffic will be kept flowing along Channel Road using either a detour bridge or a staged construction approach, depending on the exact location chosen. The angle of the new bridge will also determine the exact curve of the road on each end, and how the neighboring properties will be affected.

“The three property owners directly west of the bridge have demonstrated a willingness to work with us to discuss a road realignment to address the bluff erosion,” public works engineer Rachel Dietzman told the Sounder. “Areas we expand to outside of the existing roadway and/or easement will need to be acquired through a right of way process.”

According to an environmental assessment conducted in 2005 as part of the Deer Harbor Estuary Habitat Restoration Project, the 17 feet of rock fill placed on each side of the lagoon’s mouth in the 1970s has partly blocked the tidal interchange of oxygen-rich seawater and lagoon water, causing oxygen levels to drop. The rock also prevents tidal scouring, allowing silt to pile up and suffocate bottom-dwelling species like oysters, flatfish and eelgrass.

The tide rushing in through the smaller opening also causes greater erosion of the lagoon’s west bank and more silt, estimated to be up to 10 or 15 feet deep.

With a larger opening and phased silt removal, Huntemer said the project will restore tidal hydrology, decreasing tidal velocities through the lagoon’s mouth and reducing pocket erosion. He said the accumulated estuary silt is expected to flush out at a rate of about two inches per year, helping restore habitat for native species.

The bridge span was determined after consideration of construction costs and recommendations by three environmental groups that have studied the estuary.

The assessment in 2005 suggested a 90-foot bridge; and People for Puget Sound advised 90 to 120 feet. Huntemer said the most thorough study was conducted by the Puget Sound Nearshore Ecosystem Restoration Project, which recommended an 80-foot span.

Huntemer also said a 90-foot bridge would require a three-and-a-half-foot thick slab of concrete, but the shorter 80-foot span would require only three feet of concrete, reducing the expense. He said the 80-foot length is expected to allow 95 percent fish passage.

The bridge designers have met regularly with the Deer Harbor Plan Review committee and consulted with the Spring Point Association, local property owners and other members of the public.

“We want community-supported decisions,” Huntemer said.

To learn more about the proposed bridge, go here and then select Current Projects. Huntemer is available at 370-0500 and Dietzman at 370-0500.