Forestry-Related Landslides and Their Impact on Neighbors

by John DiGregoria

On November 18, 1996, a landslide originating in a 164-acre clearcut moved swiftly down Hubbard Creek, Oregon, into a residential house killing four people. Unfortunately, this tragedy need not have happened. It could have been prevented when Champion International applied to cut this acreage ten years earlier. At that time, the Oregon Department of Forestry noted that areas within this harvest unit had the potential to slide into Hubbard Creek creating a debris flow hazard to downstream property owners. However, with no legal recourse to stop the harvest, the Department of Forestry approved the application and the land was cleared. Immediately after the four deaths in Hubbard Creek, they began to study and reevaluate the policies governing forest practices on unstable slopes.

Prior to this event, in the early 1990s, the Washington State Department of Natural Resources (DNR) adopted a watershed-analyses model to determine and mitigate hazards associated with forest practices. A major component of this model is preventing landslides and rockfalls. In practice however, road construction and clearcutting are often allowed to occur on unstable slopes. A drive through Whatcom County on a clear day attests to this sad fact by the sight of numerous landslides in the midst of recent clearcuts and along logging roads.

Clearly, mismanagement in both the past and present combine as cumulative effects at the landscape level. As we harvest areas for the second and third times, we further degrade our watersheds. The question is, why do we allow land-use activities that increase risks to public safety and neighboring property?

Naturally Occurring Landslides in Whatcom County
Granted, there is a pre-existing natural tendency for our slopes and foothills of the Cascade mountains to fail. This is due to the geology and climate in the area.

The geology of the Cascade foothills of western Whatcom County consists predominantly of Chuckanut formation, interspersed with Huntington formation and bands of serpentine over bedrock. These three rock formations have been folded and shaped by time and have the potential to slide when saturated with water and/or shaken by earthquakes. Therefore, during wet winters, both forested and naturally burned areas have an increased potential for slope failure.

In addition, the winters of western Washington can be quite variable with periods of dry spells intermixed with periods of rain and snow. Both high intensity storms and rain-on-snow events can quickly saturate soils causing increased subsurface and surface water flow. Recent studies by the Oregon Department of Forestry show that too much water on unstable slopes can trigger landslides.

A number of different types of landslides can occur in our area. In this essay the term landslide refers to large-scale movements of liquefied earth and woody debris that move rapidly downhill. These debris flows pick up rocks, wood, and soil as they scour a groove in the hillside. For example, a debris flow in Cape Creek, Oregon started as 250 cubic yards of volcanic rock sliding down a twenty-degree slope. Over the course of one-half mile it grew to 250,000 cubic yards. According to Don Easterbrook, a professor of geology at Western Washington University, “[m]uch of the area of the Cascade foothills…are potentially hazardous and have produced most of the larger landslides in Whatcom County.”

Forest Practices Increase Slope-Failure Potential
Once a watershed analysis by the private property owner has been conducted for a given watershed, prescriptions are written, typically by DNR and the property owner (read private timber interests), with the intention of eliminating or reducing slope failure due to road building and timber harvesting. Since many harvest areas are second and third generation plantations, existing roads are often repaired with the justification that it is an “existing road.” What ends up happening is that many times the prescriptions allow for the repair of roads which weren’t well-placed to begin with: they were quickly degraded by unstable slopes and the harvest of trees on steep terrain the first time around.

Road construction on steep slopes utilizes methods that increase the potential for slide occurrence. Specifically, cutting the toe of an uphill slope to form a road and then using this material to fill the downhill slope creates two problems. Over steepening the bottom of the uphill slope and the top of the downhill slope creates landslide conditions. If a culvert clogs above the road or washes out the bottom of a fill-slope, then a section of the road will probably move downhill. Cutslopes also expose subsurface flow changing the hydrology of an area. The hydrologic impacts from roads directly affect adjacent cleared hillsides.

A number of factors affect landslides in clearcuts, including root strength, increased soil saturation and surface runoff, and the creation of ruts and gullies during “yarding.” Root strength declines rapidly after a harvest with the lowest root strength occurring four to ten years after a harvest. During this period of low root strength, increased soil saturation in yarding ruts (created by dragging cut logs along the ground to remove them from a site) can channel surface runoff starting a debris flow. With no trees immediately downhill this flow will continue until the slope mellows or it runs into an obstacle large enough to stop it. This combination of roads and clearcuts on unstable terrain leads to increased damage to neighboring property.

Property Damage in Whatcom County
By looking at the past we can foresee future landslides originating on private timberlands and impacting adjacent property. This impact includes damage to personal property, public facilities, and the public’s aquatic resources.

Damage to private property can include burying land under piles of debris as well as the destruction of buildings and vehicles. This exact scenario occurred in 1979, when a landslide originating on a Georgia Pacific logging road moved down Sygitowicz Creek damaging a couple of outbuildings, two new cars and a commercial fishing boat. Georgia Pacific settled out of court with the property owners. Will a similar fate befall nearby Terhorst Creek (a watershed with a history of landslide activity)? Crown Pacific–who recently purchased the majority of Trillium timberlands in the county–plans to clearcut thirty-three acres in the headwaters above Terhorst Creek gorge. The fact that below the gorge are residences and farms, a public road and the south fork of the Nooksack River makes one question the wisdom behind this project.

Surprisingly enough, damage to public roads caused by clearcutting is not uncommon throughout the county. A 1989 Department of Transportation report states that since 1962, “the Boulder Creek bridge…has been buried by flood debris on at least eleven occasions.” This study found “an eighteen fold increase in areas of landsliding along a 2.5 mile stretch of the main channel…due to” the combined effects of “the local geology, hydrology and timber harvest activities….” It further states that the “probability of a new landslide forming in the next 2 years is 74 percent, and increases to 97 percent for the next five years.” Interestingly enough, using our tax dollars, the Department of Transportation is required to conduct routine maintenance and to repair any damage to Boulder Creek bridge. Besides repair costs, when landslides destroy this bridge, large amounts of debris flow into the nearby north fork of the Nooksack River threatening wildlife habitats.

We have learned the hard way that when landslides deposit debris in aquatic systems they directly impact fish habitat. The initial flow can move into the river channel and scour the banks before stopping. The flow can also dam the stream creating the potential for future problems when the dam breaks. Landslide deposits increase the instream sediment load and can smother salmon eggs and alevin. Landslides that stop short of salmon-bearing streams still have the potential to cause sediment-loading into downstream habitat during prolonged wet periods. With native salmon declining throughout the region we must reduce the risk of damage from landslides to existing fish habitat.

Minimizing The Risks?
Shortly after the tragic landslide at Hubbard Creek, Oregon, the state of Oregon acted to protect public safety and high-use public roads. A recent study by the Oregon Department of Forestry conducted during two high intensity rainstorms showed, “on very steep slopes, three out of four sites that had been clearcut 0 to 9 years prior to these storms experienced a higher rate of landslides than sites that had not been cut in the past 100 years.” Nevertheless, the state of Oregon has not yet acted on minimizing timber harvesting-related impacts to aquatic resources or private property.

Similarly, Whatcom County and Washington state both allow road construction and land clearing in areas that increase the risk of property damage to downslope neighbors. With the current county council, there are no expectations that new rules or regulations are forthcoming that would protect downslope property from upslope land-use activities.

Public concerns such as personal safety, listing and protecting wild salmon, and damage to property from activities on adjacent property should take precedence over land clearing and road building on unstable terrain. At this point, however, the public does not have legal recourse to prevent damages. Do people have to die before our state changes its policies? The risks associated with landslides in the Cascade foothills can be reduced if we stop forest practices on unstable terrain.

Crown Pacific, as mentioned above, has recently purchased the majority of Trillium timberlands in the county. Crucial is the fact that most of the remaining old growth timber on these lands is found in steep gorges and other unstable terrain. As we welcome Crown Pacific into our community, we have to wonder whether this liquidation forestry corporation will manage their lands to reduce the landslide risks to neighboring property or will they continue to maximize their bottom line?
John DiGregoria, an ecologist, was living in Bellingham at the time this was written.

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