Friday, April 1, 2016

Trees, Views and Slopes

Western Washington is a land of trees. Trees that grow tall fairly fast. As such trees may impact views. Trees can also play a big role in slope stability. Slopes stable in a forest condition can become unstable as more water reaches the soil or roots that provide soil cohesion (root cohesion) die. Trees also provide a natural backdrop that is highly valued. They also provide habitat.

Tall fast growing trees can also create conflict., westseattleblog and  (crosscut) all cover a story of tree removal from a slope in West Seattle. In this case, trees on Seattle Public Parks land were cut without permission in order to maintain or create a view from some homes to the Seattle downtown skyline.

Based on the images, most of the tress cut were big leaf maple. The images also suggest that at least a portion of the tree cutting was at sites where the maples had been previously cut.

2007 (King County)
Blue line outlines approximate area where cutting took place
derived from articles, but may be less area was cut


The 2007 images suggest that tree removal was a well established practice on part of the slope to the east. The 2007 images also indicate tree cutting was beginning to expand to the northeast of the homes at that time. Note there appears to have been trees removed between the two 2007 images.


1936 aerial (King County)

There were a lot less trees on the slope in 1939.

Many of the steep slopes in the city have a long history of being unstable and hence the large swaths of hillsides within the urban areas of thick forest. Unbuildable, or at least expensive to build, land is a characteristic of may areas of Seattle. Much of this land has be over time acquired as park land.

The LiDAR imagery of the slope indicates likely large landslide scars in the vicinity of the site as well as secondary slide and erosion features.

LiDAR (King County)

Obviously unpermitted tree removal from a public park is a potential trouble for the parties involved. These hill slope areas will get more attention as trails are built and the value of urban open spaces with habitat become more valued by the public.

The value of trees on this particular slope from a slope stability perspective is not entirely clear without further assessment. Overall, the LiDAR suggests some deeper failure mechanism. It should also be noted that this slope is relatively close to the Seattle Fault zone.

Based on the articles and the policy issues and now public/political scrutiny, this view enhancement project will be rather expensive.

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