Breakwater draft study: Sea floor soft
Extending the rubble mound breakwater is likely not an option for the South Portage Cove harbor expansion project, according to the preliminary results of a recently conducted geotechnical survey.
Installation of a metal wave barrier will probably be pursued instead, Haines Borough officials said this week.
Based on a draft report released by PND Engineers, soft clay deposits and other sediments on the sea floor would make constructing a rubble mound breakwater “a challenging engineering endeavor” due to “submarine slope stability failure and inadequate bearing capacity.”
Harbormaster Phil Benner called the preliminary results “unfortunate.” “Our preferred method would be to put in a rubble mound breakwater, but that doesn’t seem possible given the geotech at the site. So we’ll have to look at a wave barrier or wave attenuator or something along those lines.”
A PND geologist and geotechnical engineer oversaw the exploration program conducted in September and October 2012. Using a drill rig staged over the water from a landing craft, 12 holes were drilled at depths ranging from 61.5 to 131.5 feet below the existing mudline.
“None of the holes hit bedrock of any type. They went down 130 feet and none of them hit resistance,” Benner said.
Borough manager Mark Earnest said PND Engineers is still performing laboratory testing of some of the sediments, which could affect the firm’s final recommendation. Because a rubble mound breakwater presents so many advantages over a metal wave barrier, the borough wants to be sure of the rubble mound’s utter impossibility before taking it off the table.
“We don’t want to rule it out if there’s a reasonable possibility. Basically the jury is still out on that decision, but at this point in time it’s looking more likely that we’ll end up with a wave barrier than a rubble mound,” Earnest said.
Benefits of a rubble mound breakwater include low maintenance, durability over time, and decreased wave refraction. A metal wave barrier requires maintenance, needs to be replaced every 20 to 30 years, and rebounds wave energy rather than absorbing it. “A wave barrier will refract the waves and cause the entrance of the harbor on a bad day to maybe be even worse,” Benner said.
Benner pointed out, however, that a metal barrier rising vertically out of the water also provides wind protection, a benefit not afforded by a rubble mound. Earnest said a metal barrier also allows for more maneuvering area inside the harbor.
PND Engineers also recommended in the draft report against the installation of wick drains, a means of building a rubble breakwater in soft soils, “based on the variable soil types encountered and the likelihood they would be difficult to install without being damaged and therefore would not perform as anticipated.”
The Haines Borough spent about $650,000 on the geotech survey and report, Earnest said. Other previously conducted geotech studies, including one by the Army Corps of Engineers, have reached similar conclusions about the unlikelihood of the sea floor sustaining a rubble mound.
“I think when three studies have been done and all three of them – the Corps of Engineers, the state of Alaska, and the borough’s engineering firm – all say the same thing, it would not be advisable... If we put a rubble mound in and it started collapsing, we’d run into a lot bigger problems,” Benner said.
Earnest said the PND survey was “absolutely essential” to the harbor expansion project, and that the earlier geotech studies were “very limited,” as they didn’t drill as deep, had fewer sample holes, and weren’t performed in precisely the same area.
Representatives from PND Engineers will travel to Haines in April to present a final draft of the study with the completed lab results, Earnest said. The firm will also conduct a bathometric survey of the area in April.
Though Benner said it is unfortunate a rubble mound breakwater probably won’t work out, he sees the development as an opportunity rather than an obstacle.
“I think the borough’s going to be open-minded. I think it’s an opportunity for us to think outside of the box and come up with what’s the best plan,” Benner said.
Early design concepts included both the rubble mound and wave barrier breakwater options, so the findings do not stall or otherwise derail the project, Earnest said.
In addition to the extension of the existing breakwater, the borough also is planning construction of south rubble mound breakwater built from land.