Kala Point

Do Trees Destabilize Bluffs and Steep Slopes?
by Steve Minta

Trees and other vegetation stabilize coastal bluffs and steep slopes. This is supported by overwhelming evidence from studies and models, with vegetation contributing a high degree of lasting stability. It is certainly the case—and particularly true—for Kala Point bluffs, slopes, and inland soils.

In addition to trees, understory vegetation and forest litter work together to stabilize bluffs. Foliage and forest litter intercept rain, evaporating and absorbing some of it while acting as a sponge to delay runoff and reduce runoff velocity. The root mats of trees and shrubs tend to break up most soils, further improving infiltration and moisture holding capacity. Deep roots tend to improve the rates of percolation of water from upper soil horizons into lower substrates toward groundwater. Finally, tree roots act as deep wicks drawing water up to be released by transpiration.

Trees provide structural support to soil. Roots are like reinforcing steel in concrete, increasing the cohesive strength within a soil horizon and the shear strength between soil layers. Several layers of plants (trees, shrubs, ground cover) multiply these benefits in resisting erosional stresses. A site with a wide variety of vegetation of various ages is usually stable because of the diversity of root types and depths.


Several Kala Point residents asked me to provide background on the notion that "trees can destabilize bluffs" because they heard it was asserted by a Puget Sound geoscientist, Donald W. Tubbs. In addition, a coastal geologist with Washington's Shorelands Program, Hugh Shipman, has mentioned, but not supported, the notion in a regional publication.

Here is what I tracked down regarding the origin of this idea:

1. Neither the assertion, nor even a hint of it, can be found on Tubb's web site or in any of his writings that I could find, including his dissertation. In all the literature I've reviewed, I cannot find a basis for the assertion except regarding soil genesis (formation) over bedrock, which occurs over centuries and millennia and does not apply here. In addition, there are unique conditions elsewhere in which heavy trees on specific types of thin-mantled slopes may actually decrease slope stability under some circumstances, but this is very uncommon, and this is not the case for Kala Point.


2. However, there is a magazine article in which Tubbs is mentioned:

"The weakened bluff at Rolling Bay is now at great risk for more slides. Tubbs suggests that the remaining large trees be removed to eliminate the potential damage they could cause as projectiles; Dave Montgomery vehemently disagrees, arguing that removing them would further destabilize the cliff."

     — Bell, Brenda. 1999. The liquid earth.
           Atlantic Monthly 283(1):58-72.

Note: David R. Montgomery is a geomorphologist and professor at the University of Washington. His research focuses on hillslope, fluvial, and tectonic geomorphology. His work has integrated forestry, hydrology, erosion, and salmon management.


3. The only other mention I can find is in this WA DOE agency publication:

Shipman, H., 2001. Coastal Landsliding on Puget Sound: A review of landslides occurring between 1996 and 1999, Publication #01-06-019, Shorelands and Environmental Assistance Program, Washington Department of Ecology, Olympia.

Since it's a large pdf file, I excerpted the relevant sections, below. Note that Shipman seems to bring it up as a possibility under certain conditions, perhaps for full balance and coverage. He says it is controversial, but provides no references. Oddly, throughout the remainder of the document, vegetation as bluff stabilizing is noted repeatedly. Also note that this document is not peer-reviewed, nor is it published in any kind of journal; it is a report. I cannot find mention of this "controversial" possibility elsewhere in Shipman's other writings.


4. In a closely-related peer-reviewed publication from the USGS, Shipman has an excellent review chapter that should have mentioned this possibility, if it was truly a consideration, but he did not, instead promoting vegetation as one way of stabilizing bluffs:

Shipman, H. 2004. Coastal bluffs and sea cliffs on Puget Sound, Washington. Pages 81-94 in M. A. Hampton and G. B. Griggs, eds. Formation, evolution, and stability of coastal cliffs—status and trends. U.S. Geological Survey Professional Paper 1693.

This is worth downloading for the photos, or you can read the text on this web site.

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Excerpts from Shipman (2001)

pp. 15-16:

Factors in slides

A variety of factors influence the distribution, occurrence, and timing of landslides, including slope steepness, slope materials, hydrologic conditions, and others [Varnes, 1978; Jochim and others, 1988]. Landslides result when the stresses acting on a slope (driving forces) exceed the resistance of the slope to downward movement (resisting strength). The primary driving force is gravity and its effectiveness depends directly on slope geometry and loading. Resistance to earth movement depends primarily on the properties of the geological materials, hydrology, and the presence of additional strengthening elements, such as retaining walls or tree roots. Slides can be triggered whenever one of these factors changes sufficiently to result in unstable conditions. Erosion by wave action can steepen a slope, causing a slide. Heavy rain can saturate soils, reducing their internal strength. Earthquake shaking can also weaken soils or place additional loads on a slope.

Many landslides, particularly in developed areas, are aggravated by human actions. Shannon & Wilson [2000] found that 84% of the landslides inventoried in Seattle were influenced, at least in part, by human activities. The most common contributors are the directing of runoff onto a steep slope or the failure of an existing drainage system on or above a slide-prone slope. Other situations include excavation and undermining of slopes, placement of fill material on slopes, failures of retaining walls, and clearing of vegetation.

pp. 18-20:

Hydrology

Hydrological factors affect both the overall stability of a slope and the timing and occurrence of landslides. As discussed previously, the presence of impermeable barriers to downward movement of groundwater can lead to zones which are particularly susceptible to landsliding. This commonly happens along the interface between weathered colluvium and underlying unweathered soil and between well-drained glacial outwash and underlying fine grained silts and clays. Again, the contrast in permeability between the two units may be much more important than the predicted or measured (in the laboratory) strength of the individual units.

When wet weather cause groundwater levels to rise, slope stability is compromised in several ways. As pore pressures increase in the sediments, the strength of the units is reduced, and slope failure can occur along the resulting zone of weakness. Increased groundwater levels also increases seepage and flow rates, which can result in erosion of the lower slope, saturation of soil below the seepage zone [Tubbs, 1974], and in some cases, erosion of the seep zone itself, undercutting the upper slope. Groundwater blowouts [Shannon and Wilson, 2000] occur when rapid erosion occurs of sediments near the bluff face as a result of anomalously high pore pressures. Finally, runoff saturates surface soils and colluvial materials on the slope, increasing their weight and further loading the slope.

Surface water enters soils, the shallow water table, and deep groundwater zones by a number of means. Deep groundwater is generally influenced by prolonged periods of precipitation over relatively large areas. Shallow groundwater levels respond more directly to rainfall events and may be extremely susceptible to modifications in natural drainage, wetlands, or vegetation cover.

The saturation of surface soils depends on soil properties and vegetation cover and responds quickly to precipitation, but can be exacerbated by directed surface runoff (natural or artificial) and groundwater seepage (particularly on the face of steep slopes where springs occur).

Tubbs [1974] found that 70% of the landslides in Seattle in early 1972 occurred on one of three days in which precipitation exceeded 1.75 inches in 24 hours. 90% occurred when 24-hour rainfall exceeded 1 inch. Shannon and Wilson [2000] notes that geologists in the region have relied on a rule of thumb that predicts significant sliding when daily precipitation exceeds 2 inches or when two day precipitation exceeds 3 inches. Intense rain is more likely to trigger landslides when soils are already saturated, explaining why landslide events occur more frequently after several days of heavy rains and why slides often occur later in the winter [Tubbs, 1974; Chleborad, 2000].

Human activities can directly impact both runoff and the rate of infiltration of stormwater into soils and the groundwater, thereby influencing slope stability. Human modifications to hydrology may increase or decrease slope stability, depending on the local geologic conditions and the combination of activities involved. Vegetation modification (clearing, landscaping) and development directly affect runoff volumes. Stormwater controls can greatly modify the location and volume of infiltration within a developed or developing area. For example, a road paralleling the top of a steep slope can intercept large volumes of surface runoff and shallow groundwater, redirecting this flow to concentrated locations at culvert crossings, and possibly modifying the stability of the slope.

On individual residential lots, concentrated drainage from driveways and roofs is often directed to the adjacent slope. If well designed and maintained, such drains may improve stability by reducing infiltration above the bluff, but if poorly designed or if failure occurs, such drains can greatly exacerbate problems by leading to concentrated infiltration on the slope itself or serious surface erosion. On-site septic systems and drainfields, which predominate along a bulk of Puget Sound's residential shoreline, can lead to increased infiltration, as can poorly planned or maintained irrigation systems.

Loading

Any process that adds weight to the top of a potentially unstable slope can increase the risk of sliding. The placement of fill on or adjacent to a steep slope can lead to slides, as can the natural deposition of material from landsliding from farther upslope. On a mid-slope bench, the sliding of material off of the upper scarp may help drive the movement of material across the bench, to the point where it slides down the lower slope towards the shoreline. Although less common, the construction of heavy structures on a slope, the stacking of clearing debris and logs, or the storage of heavy construction equipment near the edge of a slope can lead to failures.

Water itself can add a surcharge to a slope. Runoff or groundwater seepage can saturate loose surficial soils and colluvium, leading to failure as the material's mass increases. A common scenario on residential property involves the dumping of yard waste over the edge of the slope. When these materials become saturated during heavy rains, they often slide, occasionally triggering larger slides of soil and vegetation downslope. Loads can also be imparted to a slope when wind stress causes large trees to shift [6] or by earthquake shaking (see Appendix).

     [6] The movement of large trees by wind is also suggested, at least by some, to cause loosening of soils and increased infiltration, also leading to slides. Although often cited as a cause of slides and a reason for removing large trees, it is unclear how significant wind stress actually is. In some cases, removal of vegetation can in itself reduce slope stability by decreasing root strength or modifying hydrologic conditions, suggesting that decisions to remove vegetation need to carefully considered and are likely to be highly situation-dependent.

Vegetation

Most of the steep slopes surrounding Puget Sound are heavily forested, or were so prior to human settlement. Even where slopes themselves did not support heavy vegetation due to unfavorable geologic materials or due to rapid erosion, the upland areas above the slopes which directly affect shallow groundwater recharge were forested. The influence of vegetation on slope stability is poorly understood and loudly debated. The debate is complicated by additional considerations that are only indirectly related to slope stability, but that strongly influence opinions. Vegetation removal enhances views from shoreline property, a major consideration for property owners; large trees are often perceived as a hazard (during windstorms) or an obstacle to landscaping (shadowing and leaf litter); and extensive root systems obstruct drainfields, drain systems, and property improvements. On the other hand, vegetation along the shoreline can help stabilize steep slopes and is a critical ecological resource that provides habitat by contributing shade, woody debris, and other organic material to the beach and to the aquatic environment [Thom and others, 1994].

The primary influence of vegetation on shoreline bluffs appears to be on hydrologic characteristics - since it affects infiltration and surface runoff. Vegetation protects soils and steep slopes from surface erosion and decreases the rate and volume of infiltration of rainfall into the soil. Vegetation is an important mechanism for removing water from soils by transpiration - this may be particularly relevant for conifers that continue to transpire during winter months when precipitation is high and slides more likely. These factors may be as important for forested areas well above the slopes as they are on the slope itself, due to their impact on shallow groundwater recharge.

On the slopes, vegetation can add strength directly through the development of extensive root systems and the ability of larger trees to buttress the slope. Mature root systems not only bind weathered soils together, but can anchor these soils to underlying geologic materials. On some Puget Sound bluffs, gradual soil creep and small slides have led to the development of dense webs of woody material near the toe of the slope that act as natural bulkheads against wave action and may provide support to the slope itself.

Vegetation can also destabilize slopes. Vegetation growth increases weathering of soils and root action can, particularly in compact units like glacial till, loosen natural fractures and joints in the material, leading to failure. Movement of trees by wind stress may loosen soils, enhancing infiltration, and in some cases, may impart significant loads to the slope itself that may trigger failure. Regardless of their role in stabilizing or weakening slopes, trees can become lethal projectiles when a slope does fail, endangering structures that are not adequately set away from the toe of the slope. As a consequence, each site warrants a complete, but individual analysis.

pp. 75-76:

Controlling and Preventing Landslides

It is outside the scope of this report to undertake a comprehensive review of the methods commonly utilized to stabilize steep slopes and to control landslides. A wide variety of techniques can be applied , ranging from drainage improvements to large-scale reengineering of the slope through grading and the construction of retaining structures [Jochim and others, 1988; Macdonald and Witek, 1994].

Common measures employed to address unstable slopes on Puget Sound include bulkheads to protect slopes from toe erosion, drainage improvements aimed at reducing surface runoff and shallow groundwater from affecting the slope, and removal of trees to reduce stress on soils during high winds (the latter is a particularly controversial solution, in part because trees also provide significant stabilizing benefits).

These are only the most common approaches found and they generally reflect a combination of relatively low cost and simplicity, familiarity with the technique (by both property owners and by contractors and engineers), and in the case of bulkheading and vegetation removal, ancillary benefits in the forms of improved beach access and expanded views, respectively. Many other approaches are also used, and as property values increase and property owners are more prepared to spend large sums of money, there has been an increased use of upslope retaining walls, deep vertical and horizontal drainage systems, reinforced soil embankments, soil nailing, and extensive slope regrading and engineering.

Failures of slope engineering measures are not rare on Puget Sound, although no data are available to document the number or type of problems that occur. Many occur simply because the basic measures employed on a site did not adequately address the character of the landslide danger. Many bulkheads built at the toe of slopes, ostensibly to protect bluffs from erosion and sliding, are buried or damaged when slopes fail above them. The problem may be that on these steep coastal slopes, toe erosion over a period of many decades or centuries may have set the stage for sliding, but that the actual trigger for a slope failure is related instead to soil saturation and pore pressures, possibly combined with weathering processes. On sites where deep-seated failures occurred at or below beach level, bulkheads and seawalls may have simple ridden along with the slide. Many of the most notable landslides that occurred during the last several years on Puget Sound affected properties on which toe protection had been present for decades.

In recent years we have seen the failure or partial failure of numerous multiple-tiered retaining walls on shoreline bluffs. These may reflect inadequacies in engineering design, failure of contractors to accurately carry out engineering plans, or incomplete understanding of geological conditions. Unfortunately, some of these larger retaining walls appear to have escaped rigorous review by local building officials. Another source of problems reflects a long tradition of property owners implementing their own creativity in solving erosion problems on waterfront lots - without the benefit of solid engineering guidance. I have seen some remarkably high walls constructed of decorative, interlocking blocks of the kind now readily found in home centers, apparently without the benefit of any additional reinforcement of the slope itself nor adequate drainage or backfill. Such structures do very little to strengthen a slope, may actually precipitate failure by impeding free drainage, and may be hazardous when they fail.

Along Puget Sound shorelines, there is increasing emphasis on avoiding disturbances to hydrology and vegetation that might lead to landsliding, rather than structurally stabilizing slopes. This emphasis reflects several concerns: 1) poor drainage and injudicious removal or modification of vegetation are common contributors to landsliding, and avoiding problems is far less expensive than fixing them later, 2) effective stabilization may be extremely expensive and often is not justified on residential property [Kockelman, 1996; Thorsen, 1987], whereas avoidance, primarily through careful site planning and substantial setbacks, may be more appropriate, 3) much of the regulatory concern directed at shorelines is driven by the need to minimize impacts of development on neighbors and to preserve the natural shoreline environment - drainage and vegetation management are consistent with this goal, whereas structural modifications such as bulkheads and retaining walls, extensive slope re-engineering, and vegetation removal, are less likely to be.