My last project in geomorphology involved creating a landslide hazard assessment map of Lake Oswego, OR. I was given a LIDAR image and topographic map of the area and had to color code the LIDAR image to indicate landslide risk. This was by no means a comprehensive project and a lot more work needs to be done in order to better understand the hazards of the area. So, Lake Oswego is basically a suburb of Portland (although, us Portlanders will never claim it as such). It’s about 5 or 6 miles south of here and is considered the land of the rich. The part of LO that I was looking at happens to sit right next to Tryon State Park, which was also included in the report. So, here’s the LIDAR image with my color coding. Red stands for high risk, yellow for moderate and green for low:
The color coding decision was fairly straightforward and basic. If an area has seen landslides in the past, then it was colored red. Yellow means that it hasn’t seen any landslides or visible movement, but due to external forces, could see movement in the future. Yellow areas also shared the same lithology as those marked in red. Originally I had a couple areas marked in green, but decided to forego it. As you can probably guess, this is a very generalized map of the area. In an ideal world many more factors would have been taken into consideration: surface soil depth, water run-off from houses, future development projects, joint width in the basalt , etc… However, as a two week project, this was the best that could be done. And I apologize that I can’t get the LIDAR image bigger when you click on it. This project has been plagued by software issues from the get go. If you want a larger view of the area, click here.
The geology of the area is fairly standard: basalt and more basalt. If you’re lucky maybe some fluvial deposits the closer to the Willamette you get. For the most part, the area rests upon Waverly Hills basalt from the Eocene. The closer you get to the river (and consequently old landslides) the more diverse the basalt units become. If you take a look at the LIDAR image (all this is coded in red) you’ll see an old landslide toe that juts out into the Willamette River. This toe comes in at about .6 km wide and is at least that long. I can’t speculate on where it came from because any traces of its path have been wiped away by housing developments. I am willing to conjecture though that it came from the hills that sit at the ridge line. Just northwest of the toe, still along the banks of the Willamette, is a ridge that has some smaller scarps tucked into it. And on the other side of the ridge is an area that has probably seen some landslides, but for which the evidence isn’t readily visible. I say that because I can’t make heads or tails of it. Maybe it’s my untrained eyes, but the terrain looks “scooped” out. But I think I see some scarps on the west and northwest facing sides, also. As I had mentioned earlier, this area is fairly diverse as far as basalt units go. If you look at the topographic map, you can see about three other units from the middle Miocene. I can’t help but wonder if this hasn’t contributed to the instability of the area.
So, what was my conclusion at the end of all this? Well, to paraphrase a fellow geology undergrad, “a bunch of rich people decided to build million dollar homes on some pretty unstable terrain”. And most of these homes do not have landslide insurance. In fact most insurance companies classify this as “an act of God” and don’t cover landslides. Even more interesting is that Lake Oswego is no stranger to landslides, but people still continue to build in some pretty risky areas.
And here’s the Google Earth image of the area: