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.