Pikes/Pines | Even on Capitol Hill, the land slides

Those of us who have resided in Capitol Hill for some time may turn a blind eye to our risk for natural disaster. Rainier? Lahars, dangerous slurries of pyroclastic debris won’t reach our hill. Tsunamis? Similarly, they’re unlikely to reach us. Flooding? We’ve no rivers to swell during storms. Earthquakes? We’ll have the big one sure, “but not in my lifetime.” Landslides? Surely engineers have ensured our safety…right?

SPU Landslide Map

A map of landslides in past decades on Capitol Hill. Credit: Seattle Public Utilities.

 

Blissfully ignoring the risk of natural disasters isn’t a unique trait. People worldwide live atop unstable cliffs, in flood prone valleys, or near major fault lines out of necessity or for beauty. Of course, the tragedies of natural disasters aren’t diminished with or without foreknowledge of risks.

Most of the aforementioned threats are unlikely to directly take lives or damage property on Capitol Hill. However, we are not a fortress atop the hill. The slopes leading to our cultivated crown in particular, have a consistent history of landslides.

As citizens curious about local natural history, let’s explore why we are at risk. Unsurprisingly, the reasons are not simple, evidenced debate whirling around Oso. Ultimately in Seattle they relate to glaciers.

During the last glacial period, a wall of glacial ice advanced on the Puget trough, creating a dam. The trapped water formed a lake.  Doing what most lakes do, it laid down sediment, creating a sizeable clay deposit. Today we call it the Lawton Formation, and atop it rests glacial till. This simplistic version of convoluted geologic history serves a major point, we have unstable ground, we have impermeable clay.

Clay being largely impermeable, water tends to pool on clay instead of moving through it. Our landslides often occur exactly because of the Lawton formation. This slide of groundwater on slick clay sheds glacial till quite well, along with roads, houses, and whatever else rests on it.

Then we come in. The grade is altered, creating new faults. Hills are denuded of trees, which hold slopes and mitigate flooding. Barriers to natural water flow diverts it toward unforeseen consequences. People understandably want views and build on cliffs, changing the loads on hills. Generally things more even more unstable. West Capitol Hill, Interlaken, North Capitol Hill. Slides every decade going back in our modern record. I won’t tally the slides in Hill history — that would take too long.

All this said, landslides are part of the natural history of the Pacific Northwest. The hills didn’t begin crumbling once people of European descent landed Schooner Exact. The ground moves with or without us, we’ve just exacerbated inherent tendencies. According to recent research out of University of Washington, those tendencies are even greater than previously thought. Turns out, there’s much greater danger of landslides with major movement of the Seattle fault, with 10,000 buildings in the city in trouble spots (including schools). The risk of damage from earthquake triggered landslides is not news to anyone, we just didn’t have a adequate data. Thankfully, we don’t sit right above the fault; unfortunately we’ll never be in enough control to completely reduce the risk of earthquakes or landslides.

Alstadt et al.. BSA Seismological Society of America

Areas of Seattle with heightened risk of landslide during an earthquake. Credit: Alstadt et al.. BSA Seismological Society of America

So landslides happen, afterwards the natural world continues on. The slide settles. Nitrogen fixing plants, who don’t mind soil denuded of nutrients, (they find their own via mutualistic bacterium), fertilize the soil and make way for others. Animals quickly follow. Before long, things are building back. Mount St. Helens is one example of how quickly things return.

Ultimately, we take disasters similarly. We pick ourselves up, help our neighbors, mourn our losses, and rebuild, just hopefully not in the same spot and with more insight. Unlike plants and animals that re-establish in the wake of disaster, we often have more options.

Previously in Pikes/Pines

One thought on “Pikes/Pines | Even on Capitol Hill, the land slides

  1. Pingback: Pikes/Pines | Don’t smoke these Capitol Hill weeds | CHS Capitol Hill Seattle

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