Monday, April 7, 2014

Canyon Creek Another Path to Reducing a Geologic Hazard

The Whatcom County Council acting as the Whatcom County Flood District Board of Supervisors will be voting this evening on accepting a grant agreement with the Washington State Department of Ecology for Flood Plain Restoration Provsio funds whatcom.council/agendabills/ab2014-139.pdf. The $2 million grant will be used to widen the flood plain area on the lower reaches of Canyon Creek. The project serves two purposes: 1) improves salmon habitat on a degraded reach of the stream, and 2) reduces the risk from flooding, erosion or debris flows at the site. 

Lower Canyon Creek 2006 USDA image via Google earth

The grant will fund additional work that has already been done to reduce the geologic risk at the site. The Google earth image below from 2013 shows active work on the west bank of the creek as the levee was pulled back last summer.

The lower reach of Canyon Creek flows across the eastern side of a broad alluvial fan before entering the Nooksack River. The creek channel and flood plain of the creek were well incised down into the older fan surface leaving the impression that most of the fan was an old feature and not subject to flooding. The creek location had been stable for many decades. A large subdivision was platted on the older fan surface with roads laid out and homes were built. The apparent peaceful situation all changed in 1989.

LiDAR image of the fan with the Nooksack River just to the west of and eroding the distal portion of the fan. Deep-seated landslides are apparent on both side of the canyon upstream of the alluvial fan.

A large flood impacted the Nooksack River basin in 1989. Extensive major flooding took place on the Nooksack. Canyon Creek also flooded, but more importantly the creek eroded is west bank on the lower reach chewing hundreds of feet westward and into the platted lots of the subdivision. After the flood the eroded bank was lined with boulders to impede further erosion. Within days of completion of the project another major flood event took place and boulders were stripped away. These events destroyed 4 homes and damaged a resort property near the mouth of the creek.

Yes, there had been extensive flooding, but the flows and violence of the floods on Canyon Creek were far in excess of what would be expected by flooding. By one estimate the flood levels on Canyon Creek were estimated to have been a 1 in 10,000 year event. The problem was in the canyon above, deep-seated landslides had shifted and dammed the creek triggering huge surges of water as the dams failed.

LiDAR image of landslides impinging on both sides of the creek

In addition to the huge flow of water a huge sediment input from the slides was being deposited on the lower flood plain of the creek pushing the creek to the west. In the early mid 1990s a large rock levee was constructed in the flood plain to stop the erosion and to narrow the stream flow area such that sediment would be transported down the creek.

LiDAR of fan and levee
Heavy blue arrows indicate the Nooksack River
Thin blue lines denote potential avulsion paths of Canyon Creek

The project worked as intended during its first flood event. However, three big problems became apparent right away and a fourth even bigger problem began to be recognized.

One problem was that the flow forces were greater than the largest rocks that could be moved with construction equipment and portions of the structure were damaged in the first winter. Another problem was that funding to build the levee was not enough to extend it the entire length of the lower creek and hence the lower end of the levee was built with even smaller rocks. This levee section was breeched subjecting part of the resort buildings to flood and thick gravel deposits. The lower levee breach was a good thing because another problem was high flow surged water across the Nooksack River where the opposite bank was eroded by the water surging out of Canyon Creek.

Yet another problem was the new structure destroyed all of the salmon habitat on the lower reach of the creek and had created a fish barrier for up stream fish migration. And not just any salmon, but an endangered species of salmon. Hence, any repair work on the levee was going to require significant mitigation.

But the biggest problem may have been the conclusions of numerous geologists that the levee was not providing adequate protection to allow building on significant portions of the upper alluvial fan and furthermore that the levee actually was increasing the risk for some properties beyond the risk if nothing had been done. The biggest problem was that the even a modest flood event had damaged the levee, and if the levee was breached where that damage had taken place, the creek would then be stuck on the wrong side of the levee and flow with all its force through a wide swath of the subdivision as shown with the thin blue arrows in the above LiDAR. The potential levee failure seemed rather likely as there was no funding source to repair the levee and the repairs would be exceedingly expensive.

So instead of a repair job a new  path was forged. Buying out properties on the fan including the existing resort and removing the buildings and ultimately moving the levee back. In all the resort was removed and 28 building lots were purchased. This effort was in part helped by the fact that the County regulations made it very difficult to obtain a building permit as there were not very many geologists that would challenge the County's view that the properties in question were with a severe geologic hazard area. (I do know of one lot with a new home that I deemed unsafe, but another geologist concluded otherwise and the County accepted the report. I ten to be fairly conservative regarding geology hazards and utilize a higher standard than what was at that time used by Whatcom County). 

The moving back of the levee and the purchase of properties in the highest risk zone including the resort with its log cabins has been a big step in reducing the future risks at this site. There are still homes that in the event of a very big landslide and dam burst will be at risk, but the efforts are a big step in the right direction.

A bit on the geology. The two deep-seated large landslides are associated with a fault contact within metamorphic rocks and like involved highly sheared and altered bedrock.

Mount Baker 1:100,000 Quad (USGS)
Ql= landslide,
note the extension fault trace the length of the landslides on both sides of the creek

Area where slides are impacting creek.
The forest road on the east side slope of the canyon has dropped several feet as the slide continues to move

While the hazard has been reduced, ongoing monitoring of the landslides in the steep narrow canyon is warranted as this pinch point could become a really big problem if the slides have significant rapid movement.


Ryan M. Ferris said...

Thanks for this detailed history and analysis. Completely new to those of us who have been here less than 20 years... Is this a prelude to completing the construction on that subdivision? They are spending a chunk of change. What guarantees that the concerns of the state and county are more than slope stability and salmon?

Dan McShane said...

As noted a fair number of lots were purchased so the buildout will be less than the original subdivision. Portions still will not be greatly challenged to get a home permit due to the still existing hazard. County code says that the hazard should be defined as the "maximum credible event"; the scale of that event may be subject to a degree of geologic interpretation. Some areas on the fan are outside what I interpret to be the maximum crdeible event hazard area.
One thing I did not go into was that a breech of the old levee not only increased the risk to some of the homes, but it also increased the risk to the State Highway located on the lwoer end of the fan and above the river. There has been some discussion of moving the highway, but that cost is very high as it also would entail a new bridge over the Nooksack and a new bridge over Canyon Creek.

Ryan M. Ferris said...

All I can say is I feel like this is Oso redux; only its 1999 and nobody is reading the geological reports. I could never fault myself for being too cynical this month by suggesting that perhaps the funding from the State is designed to create false sense of security in an area we may be digging bodies out of the landslide muck ten years from now.

As a kid, I learned to grow distrustful of Q100 or Q1000 stats. These were probabilities the state of California used to trot out to describe a drought, rainslide, storm (Q100 = 1 in 100 years, etc.) I used to think if a Q100 happened twice in my childhood, how meaningful was that statistic?

So in this case, (as you say) the "pinch point could become a really big problem if the slides have significant rapid movement." Or a seismic event plus a really wet winter or steep snow pack and avalanche with earth movement or a highway disappears.

As the North Cascades grow, we need a very responsible way of assessing risk and our response. An Oso disaster is costly in so many ways and mitigation efforts in this case and in Oso appear to have been irrelevant. Building a dense skyline in downtown Bellingham appears much more logical after each such event.

wynneforplants said...

A '100 yr' event (Q100) actually means there's 1 chance in 100 that such a flood/landslide/etc could happen in any given year. A '1000 year' event (q1000) means there's 1 chance in 1000 each year. Makes a huge difference in what we perceive the odds are! Language matters.

The Wizard said...

Be sure to download and use my new 1954 Oso Landslise GIS database and other Oso Landslide GIS databases at
I offer some perspective based on my observations, but feel free to make your own.
E. Martinez