Mad River Bluffs Riverbank Stabilization and Revegetation Project
McKinleyville, California
In the summer of 2008 BioEngineering Associates, Inc. (BE) was contacted by Chris Whitworth, civil engineer with Humboldt County Public Works Department. Mr. Whitworth described an erosion problem that his agency needed to solve along 1,300 feet of 23 to 40 foot tall vertical bank along the Mad River Estuary near McKinleyville,California. There had been severe bank retreat over the last several years and homes near the top of the bank had lost 50 feet of their back yards. The Public Works Department had commissioned a probability study which indicated a 50% chance that the erosion would reach the homes within two years and a 90% chance it would get there within 10 years. Those findings qualified the site for emergency status which helped considerably with funding and expediting the permits.
The design to stabilize the eroding riverbank included constructing a rock platform to the height of the high tide. This was done so that the live willow bioengineering structures would be constructed above high tide. The live willow branches would not survive being inundated daily with brackish water. Next, a series of coir wrapped soil and live willow brushlayer lifts were constructed on top of the rock platform. The coir and willow lifts were separated by a series of boulder wing deflectors. Redwood logs were anchored beneath the deflectors to provide fish habitat.
The work was started on September 2nd and against all expectations completed before the end of October, well within the allotted permit time frame and within the budget. This was the first major bioengineering work in Humboldt County and it has been very well received. The City of McKinleyville has installed a bench overlooking the bioengineered structures and there are plans to construct an educational walking path at the site.
The following pictures show the work from the start to seven years later. Below the project pictures is a time lapse movie filmed throughout the duration of the project.
The project consisted of of stabilizing 1,300 feet of a 23-40 foot high vertical stream bank that was eroding, threatening homes and the road into the neighborhood.
To begin, a road was excavated from the meadow down to the toe of the vertical bank. A rock platform was constructed at the base of the vertical bank.
The view of the project site from across the river. The rock platform has just been completed. Note the proximity of the homes at the top of bank to the edge of the eroding bank.
A floating silt curtain was used to contain sediment and silt that was stirred up during construction of the project.
A dense mass of live willow branches were laid on a coir-wrapped soil layer. The branches were then covered with local river run gravel and soil.
The next layer of soil was brought in with a front end loader and was wrapped in coir, a coconut fiber net and blanket that is strong, long lasting and bio-degradable.
The construction crew pinned down the coir forming the coir wrapped soil layer.
In the fore ground, one of the boulder wing deflectors separating the 60 to 80 foot long coir and willow lifts. The boulder wing deflectors were constructed using 3 – 6 ton rock. Note that the floating silt curtain is containing the project generated sediment.
The project site one year after construction.
This photograph was taken one year after construction. The willows are thriving and preventing further erosion of the riverbank.
The project seven years after construction. Since completion of the project, no further erosion of the riverbank has occurred.
Read a testimonial from a property owner on the Mad River Bluffs.
Watch a time-lapse construction video of the project (6:23)