On October 28 Thompson Bros. Excavating (TBE) will begin the breaching of the summer levee located along the Tillamook and Trask rivers. Sediment control during breaching, and on all phases of the project, has been and continues to be of paramount importance.
For example, prior to the storms of October 13-17, a coffer dam was constructed at the east end of the project area where a gated spillway is under construction in the last opening of a newly constructed set back levee. The purpose of this emergency action was to confine storm surge within the project area and away from private lands adjoining Highway 101.
When the highest tides of the year overtopped the summer levees, about 6 small breaches and two larger ones resulted in storm waters entering the project. These breaches brought sediment into the interior project area. One of the benefits of the completed project is the collection of even more sediment in this area during flooding events. This area is subsided by 2-3 feet due to the existing levees that have for decades disconnected the interior project lands from the floodplain. Removal of these legacy levees will not only reconnect 521 acres to the flood plain, but will also reopen about 14 miles of ancient channels that will provide critical over wintering habit for juvenile coho salmon. Substantial flood reduction benefits over a 3,000 acre area will also result from the levee breaching.
Throughout the storm the project was continually monitored by the contractor and project managers until the waters subsided. The breaches were then repaired to allow completion of the final interior work prior to the final large scale breaching that will begin on October 28th. The final breaching will start at the furthest point away from the Hall Slough and work eastward along the Trask River, finalizing the breaching at the new levee.
TBE will be using two excavators along with two dozers to complete this work. With two, two-man crews working, TBE anticipates the levee breaching being completed in seven days. Work will begin at low tide and continue as the tide moves landward at the high tide current. Starting at low tide allows TBE to monitor any turbidity that may occur. A project monitor will also be present with turbidity monitoring equipment throughout the breaching. The project team expects some turbidity and sediment to occur during breaching. TBE’s strategy of working their way out of the project and stopping while the tides become higher in elevation allows turbidity to be accumulated back on to the project.
Along the Trask River there are a total of five interior tidal channels to be connected to the Trask River. TBE will be placing a floating silt curtain at the connection points. Placing the silt curtain will help confine turbidity that may occur in the Trask River at the time of connection. TBE will remove the curtains as they continue to remove the summer levee. TBE and the project monitor will be taking pictures throughout the breaching.
Despite a few challenges, project work for 2016 is drawing to a close. The recent storm, which included extreme high tides and storm surge, caused some overtopping of the remnant summer levees as well as a few breaches. In anticipation of the storm, a temporary coffer dam was constructed at the East end of the project area to close off the opening in the middle levee where the new gated spillway is under construction. This action by Thompson Bros. Excavating, Inc. (TBE) confined storm waters to the interior project area and off of adjoining private lands. SFC project managers and TBE actively monitored the site during the storm.
While one TBE crew is busy with final shaping of the south levee, other crews are grass seeding and installing coir matting and fabric over the sides and top of the north and middle levees as well as over the recently completed repairs to the Hall Slough Levee. The middle levee gated spillway is formed and rebar tied in place. The first concrete pour was completed on October 21. The final concrete pour is slated for October 26. The six floodgates will be hung on the spillway as soon as concrete tests demonstrate the desired strength; likely in a week following the final pour.
Excavation of contaminated soil at the former mill sites is also nearly complete. The discovery of 300 cubic yards of asbestos roofing caused some delays while specially-trained crews were brought in to wrap and dispose of the material at a licensed facility. Despite the excavation and hauling of additional solid waste mixed with soils contaminated with decades-old machine lubricants, TBE completed the placement of soils in the containment cell last weekend. There are still several weeks of installing the vent pipes, welding a geotextile membrane and then capping the cell with 18 inches of topsoil. Using funds provided by the City of Tillamook, TBE will also construct a gravel parking lot on top of the east side of the cell for public recreational access to the newly restored wetland and Hoquarten Slough. Hydro seeding of exposed topsoil will complete the cell’s construction.
Some of the summer levees along the Wilson River were recently removed just prior to the recent storm in a location that is not hydrologically connected to the remaining project area. As a result of recent storms and extreme tides, this breached area is already showing promising response to tidal exchanges, resulting in widening and deepening of the reconnected channels. Although the south end of the bay tends to be dominated by freshwater during this time of year, as expected the return of brackish water in the spring and summer will bring about a change in vegetative cover to more of a saltwater marsh over much of the project area.
Within the remainder of the 521-acre portion of the project that will soon become subject to daily tidal inundation, TBE crews are finishing the removal of old fence posts, utility poles, and other man-made remnants. Prior to the final breaching of the remaining summer levees along the Trask and Wilson rivers, crews have to complete a “punch list” that includes final interconnections of ancient sloughs, removal of a few legacy tide gates and final filling of the remaining drainage ditches. TBE expects to begin final breaching this week. This will coincide well with a series of tides that are expected to remain below 7 feet elevation at high tide. TBE will work three crews day and night breaching the levees down to the final design height of 8 feet Mean Sea Level. As levees are removed, crews will also reconnect existing and ancient channels to the river and bay at depths that result in a positive gradient for runoff. Work will not occur in the water but the equipment will operate only when tide levels are below the work area. TBE expects the breaching to take four to five days.
The removal of these levees is the project element that is expected to provide the most flood reduction. Work for 2016 is expected to be completed by mid-November. The remaining work scheduled for midsummer 2017 will be summarized in the next project update.
The Southern Flow Corridor Project (SFC) is intended to provide flood level reduction benefits by removing man-made impediments to flood flow and to permanently restore and protect tidal wetland habitats. This is being accomplished by extensive removal of existing levees and fill around the Trask and Wilson rivers and the smaller sloughs. This project will provide a substantial public benefit through the reduction of life safety risk from floods, reduce flood damage to property, improve freshwater and estuarine water quality, and enhance the quality of the habitat for native fish and wildlife species including the federally-listed Oregon Coast coho salmon.
The SFC project is removing approximately 234,000 cubic yards (cy) of fill including about 35,000 cy from an area along Hoquarten Slough that once housed several mills. The property was undeveloped until the 1920’s when the Tillamook Spruce Veneer Company opened a veneer mill on the east end of the property near Douglas Street and Front Avenue. The mill was built on pilings in the low-lying, marshy area. Logs were transported to the mill from Hoquarten Slough and into a narrow inlet on the north side of the mill. The mill included saws, dry kilns, a woodworking house, a boiler house, a machine shop, and an oil house. It is believed that the mill was powered by steam generated from wood waste from the mill.
In 1944, the mill was abandoned and a new mill (referred to as the west mill), operated by Aberdeen Plywood Company, was constructed west of the original mill. A log pond was created in the low-lying area west of the mill and the Hoquarten Slough inlet was no longer used for log delivery to the mill. The entire mill was also built on pilings and had a lathe room, a filing room (for saw and knife sharpening), a clipper room (for trimming veneer) and a power room. A second log pond was created west of the existing pond in the mid- to late-1950’s. The west mill reportedly operated on diesel-powered equipment.
The green veneer was shipped from the new mill to the company’s plywood plant in Tacoma, Washington for further processing. There is no evidence that plywood manufacturing was ever performed on the subject property.
The mill closed in the mid-1960’s and the log ponds were drained. Fill material, possibly from the areas around the former mill buildings, was placed on the southeast corner of the west log pond around the same time as the closure of the mill. The fill material was sampled during an environmental site assessment in 2014.
Some of the fill materials are contaminated by petroleum byproducts as identified in recent investigations by Anderson Geological. The physical composition of the fill material varies from mineral soils (sand, silt and clay) to wood waste (sawdust, wood chips, logs) and construction waste composed primarily of large pieces of concrete.
To fulfill the flood-control requirements for the Southern Flow Corridor (SFC) project, the ground surface across the entire 5 acre mill property is being excavated to a common elevation of approximately 8 feet above mean sea level. Soils that are planned for removal are referred to as “take” soils. The soils that will be left exposed after the excavation are referred to as the “leave” soils.
The leave surface will include areas of contamination that slightly exceed ecological risk concentrations. The leave surface will be covered with a veneer of topsoil with the expectation that periodic inundation of the area from high water levels in Hoquarten Slough will result in the deposition of additional sediment and the establishment of wetland vegetation, further stabilizing the leave surface. Areas where the leave surface is over-excavated due to grossly-contaminated soil or landfill debris will be backfilled with clean soil and covered with clean topsoil.
It is not the intent of this project to remove all soil and groundwater that exceeds cleanup levels. Only the contaminated material that must be removed to allow the completion of the SFC project, material that would result in higher exposure point concentrations than currently exist, and materials containing garbage and/or free product contaminants, will be removed. Material that would result in higher exposure point concentrations than currently exist will be placed onsite in a containment cell. It is estimated that approximately 900 cy of materials containing garbage and/or free product contaminants will require disposal at an approved solid waste disposal facility.
Oversight and monitoring for this phase of the project is being performed by Erik Anderson, R.G., a professional geologist registered in the state of Oregon, under a plan approved by the Oregon Department of Environmental Quality.
The estimated volume of in-place soil to be excavated is 35,800 cy. This includes:
-13,500 cy of clean soil/fill and 1,500 cy of waste concrete in which no contaminants have been detected or are present below ecological screening levels and Clean Fill Criteria. Metals are present at or below natural background concentrations. These materials are classified as Type 1 and are being used to construct a setback levee designed to keep the daily tide off the farmland adjoin the southern project area. That levee is presently about 60% completed.
-5,000 cy of soil/fill in which organic contaminants have been detected above aquatic ecological screening levels but below Clean Fill Criteria. Metals are present at or below natural background concentrations. These materials are classified as Type 2.
-5,700 cy of mineral soil and organic soils/wood waste in which organic contaminants have been detected above terrestrial ecological screening levels and Clean Fill Criteria. Metals are present above natural background concentrations. These materials are classified as Type 3.
-900 cy of oil-saturated soil and garbage, classified as Type 2 or Type 3, is being disposed offsite at a Subtitle D landfill.
It is estimated that approximately 10,700 cy of Types 2 and 3 contaminated soil and fill will be removed in this area and placed in an on-site containment cell that is currently being constructed along Front Street on city and county property west of Douglas Avenue. Once completed and capped, a gravel parking area will be constructed on top of the cell to facilitate public recreational access to Hoquarten Slough and the newly created wetland area.
The mill sites, location of contaminants, containment cell and progress of work at site through September 2, 2016 is shown on the enclosed attachment.
Despite summer rainfall, the project is on schedule. Work on the north setback levee is proceeding well with completion of this element expected by the end of August. This is one of three setback levees designed to keep daily tides off adjoining farm land that is outside the project area. The approximate 2 ½ miles of these levees are specially designed with a clay-type soil core. Once fully dried and compacted, the core will prevent water infiltration through the levee that might otherwise cause erosion and levee failure. The levee core will be capped with a granular material, also available on site, and graded to a 1/5 slope to minimize erosion during overtopping flood events. The three setback levees are in locations that enable efficient flood discharge over the remainder of the overland flow corridor.
The City of Tillamook, one of the many project partners, is presently contemplating an expansion of a planned parking area on the so-called “containment cell” that is being constructed along the north side of Front Street between Douglas Avenue and an old mill pond channel located west of Cedar Avenue. The proposed revision would accommodate parking for 30 vehicles instead of the 14 contemplated in the initial design. The lot will be available for recreational access to Hoquarten Slough and the surrounding area. A decision by the City on the possible expansion is dependent on a revised bid proposal from the project contractor, Thompson Brothers Excavating, Inc.
In any event, parking atop the containment cell is a creative use for the earthen berm that will be used to encapsulate the slightly contaminated soils that remain on the project site from the two veneer mills that operated from about 1920-1965. The contaminants in the soil consist of remnant motor oil and machine lubricants used at the mills. Although the contamination levels do not pose a risk to humans, the worst of the soils will be transported to a licensed treatment facility in Hillsboro. The lesser contaminants will be encased within the cell and monitored long term. Currently those contaminants are uncontained at the site and pose a risk to the environment.
Further questions may be directed to Project Manager Paul Levesque at 503 842-1809.