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.