27 July 2012
The amazing vanishing garden
Posted by Dave Petley
Komonews ran a slightly perplexing story yesterday about a couple living in Whidbey Island, which lies a little to the north of Seattle in Washington State, USA. In the link there is a video that shows the rather interesting retrogressve failure that is steadily eroding away their garden. With luck and a following wind you might be able to see it below:
It is not at all clear to me why this should be happening at this site at this time. The owners describe a geysir 25 feet (about 8 metres) high, which suggests very high pore pressures developed in the ground at this location. This presumably indicates a change in subterranean plumbing, but the cause is far from clear. The process now appears to be a set of retrogressive failures that are slowly but steadily devouring the garden. There is a good article on the geology of Whidbey Island here – it is formed of glacial deposits that are likely to be highly erodible. Indeed, Whidbey Island has a long history of landslide problems, suggesting that one should not be too surprised that a failure can occur at this location. It is unlikely that the owners can do much except wait for the system to stabilise naturally.
The University of British Columbia is built on many of the same glacial sediments, and has experienced many seepage-induced landslide events. most spectacularly, the 1935 “Grand Campus Washout” retrogressive event created a canyon almost 80m deep and 300m long. Unfortunately, despite a good groundwater control plan, many new campus buildings are built “sustainably”, with grey water and storm water being discharged to the soil. It will be interesting if this causes renewed instability of the campus.
[Thanks. Strangely enough from where I am typing this I can see the forested coastal slope, above Wreck Beach, on the edge of the campus. Great location. D.]