20 January 2019
Landslides on the White Bluffs in Hanford Reach National Monument
Geologist Bruce Bjornstad has posted a really interesting video onto Youtube showing the development of landslides on the White Bluffs in Hanford Reach National Monument in Washington State. The video, which should be visible below, combines beautifully shot drone footage with a commentary on how these landslides have developed.
The image below provides a still from the video – these are very spectacular features:-
In the video Bruce explains the landslide mechanism, in which irrigation water moves down through the soil before hitting an impermeable layer and spreading laterally. This moves to the bluffs on the edge of the river, driving up pore water pressure and thus triggering failure. The resultant landslides have been developing with time. The locations in which water is exiting the slope can be seen in the Google Earth imagery in the video as vegetation growth has been enabled.
The resulting landslides are causing a serious change in the morphology of the bluffs in this protected area, as the still below shows:-
This is a beautiful example of the ways in which poor water management can inadvertently trigger slope problems away from the location in which the irrigation is being applied. Of course the long-term implications are damage to the bluffs (which may well have implications for the ecosystem in this location), increased sediment supply to the river (which may have downstream implications) and of course loss of land at the top of the slope. Providing excess irrigation is presumably inefficient as well, so this is an unfortunate situation.
Thanks to Bruce Bjornstad for putting together the video and for highlighting it to me.