City of Seaside Wastewater Treatment Plant Shoreline Stabilization Project
Necanicum River Estuary
Seaside, Oregon
This project was constructed along the eroding banks of the Necanicum River Estuary on the northern border of the city’s wastewater treatment plant in Seaside, Oregon. It consisted of stabilizing 870 feet of shoreline, an 18 feet tall cliff at the western (downstream) end that consisted entirely of highly erodible sand. The erosion threatened the City of Seaside’s wastewater treatment plant built in the 1940s on the sand dunes. The plant is built on a peninsula with the river flowing along its eastern boundary and in its estuary along the plant’s northern boundary. Nearby, to the west, lies the Pacific Ocean which, during large storms, is known to send large waves rolling into the estuary and along the plant’s northern shoreline. One of the significant challenges facing this project was to construct a live vegetated stabilizing structure in the saline environment of the estuary.
The Necanicum River Estuary is a volatile and dynamic environment. In the winter of 1997/1998 during an El Nino year, the river moved 1,000 feet south towards Seaside, eating away much of the sand dunes that lie between the river and the treatment plant. It was mentioned by local residents that during large storms, waves break along the plant’s eroding shoreline sending spray 20 feet into the air. This project was constructed to protect the treatment plant from the ongoing erosion and in addition, at the behest of the permitting agencies, to enhance the habitat available to aquatic and terrestrial animals within the estuary.
The project was divided into two phases, an emergency repair project protecting the 
area directly in front of one of the plant’s buildings and a more complex project along the remaining 670 feet of eroding shoreline. The second phase included bank stabilization, revegetation and fish habitat structures. The emergency project was constructed in January 2014 along 200 feet of 18 foot tall eroding bank. When construction began, the eroding bank had retreated to within 8 feet of the plant’s perimeter fence. This project consisted of constructing 3 quarried rock deflectors with a rock revetment where the erosion was closest to the plant. A logging slash brush barrier was constructed between the deflectors to capture sand and provide an additional layer of protection along the bank.
The second phase of the project was constructed January – March 2015 along 670 feet of 4 to 12 foot tall eroding bank located upstream of the emergency project. This phase consisted of constructing rock and live willow brushlayer lifts, ten feet wide, and stepped up to the top of the bank, along the entire length of the eroding shore. 40 logs with rootwads were placed underneath the lifts with the rootwads extending 10 feet further into the estuary. Between the logs with their rootwads a logging slash brush barrier was constructed to provide additional protection to the shoreline. The logs will create fish habitat within the estuary and also provide protection to the shoreline. In addition to stabilizing the riverbank, the project also included an extensive revegetation plan. The goal of the revegetation plan was to restore the areas disturbed during construction and revegetate the shore pine forest located at the top of bank, much of which had been lost as the eroding riverbank retreated into the forest. These areas were replanted with a variety of native species including coastal grasses, 90 shore pines, 70 evergreen huckleberry plants, 80 western sword ferns, 100 salal plants, 40 salmon berry plants, and 20 kinnikinnick plants.
Emergency Project Construction
Project site before construction of the emergency project and full project.
Before construction of the emergency project, the eroding bank had retreated to within 8 feet of the treatment plant’s perimeter fence.
Typical riverbank erosion at the emergency project site. The highly erodible, sandy banks ranged between 12 to 18 feet tall.
Construction of the emergency project. 4 – 6 ton boulders were used to construct boulder wing deflectors and rock revetment.
February 2014: Emergency project just after construction.
Full Project Construction
Once the emergency project was completed we began to formulate a design that would stabilize the remaining 670 feet of eroding sand riverbank. As part of the challenge that we faced in this volatile environment, both the state and federal agencies requested we include a complete revegetation in our design and that we build in very significant fish habitat using large logs with their rootwads attached. The challenge to accomplish this was made greater by the fact that we were working in a saline environment.
Typical erosion along the length of the project. The eroding riverbank ranged from 4 to 12 feet tall. Much of the once robust shore pine forest was lost as the riverbank eroded and the trees fell onto the beach.
The fallen shore pines were cleared from the beach prior to construction. They were set aside to be used later as a brush barrier, one part of the shoreline protection design.
After the fallen trees were cleared, the beach was regraded and surveyed before construction of the bioengineered structure.
A toe trench was excavated to water level and 2 – 4 ton boulders were placed in it.
25 to 30 foot long logs with their rootwads attached were placed perpendicular to the bank and toe trench. The logs were placed below grade underneath the brush barrier and rock and willow lifts. A total of 40 logs were placed, one every 15 feet along the length of the project.
The first rock platform was constructed on top of the toe trench rock and logs using 2 – 4 ton boulders .
After the first rock platform was constructed, river run gravel was poured on top of the rock platform to provide a growing medium for the live willow branches.
Live willow branches were laid down on top of the river rock and boulder platform. Vertical willow branches were placed against the bank behind the rock and live willow lifts.
A layer of river rock was poured on top of the willow branches to provide them with a growing medium and to protect them from abrasion when the next layer of boulders was placed on top of them.
The layers of boulders, live willow branches, and river run gravel was repeated until the structure was flush with the top of bank.
A newly completed section of the rock and willow lifts.
A 10 foot wide by 4 foot deep brush barrier was constructed between the logs in front of the rock and willow lifts. We used logging slash for this, provided by a local timber harvest company. The brush barrier was included in the design as an additional layer of protection to prevent scour from reaching the toe of the rock and willow lifts.
The crew assembles the irrigation system. The rock and willow lifts and the revegetation area will be irrigated for two growing seasons depending on local weather conditions.
The newly constructed irrigation system running for the first time.
The newly planted shore pines. The shore pines were planted in areas where the existing coastal pine forest was sparse.
Three months after construction the willows show tremendous growth. Photograph taken May 2015.
The area on top of the lifts has become a popular walking path for locals. Photograph taken 8 months after construction.
The rootwads create scour pools underneath them to provide fish habitat at high tide.
3 months after construction.
8 months after construction.