Building an Armored Swale

Today Jessica M. and I (both volunteer stewards) tackled a Montini trail project that I had long planned. There was a place on the Rattlesnake Cutoff Trail where water, during heavy storms, would flow across the trail. Since the initial construction of the trail didn’t take this into account, the water would pool on the trail, leading to a long trail segment that would essentially turn into a tiny lake (see photo).

The Montini Preserve was heavily mined in the early 1900s (this property and adjoining Schocken Hill, now the Sonoma Overlook Trail, essentially paved the streets of San Francisco at the time), and there is a quarry “divot” in the hill above that collects rainwater and funnels it out to the trail. Being essentially bedrock, the water has nowhere to go but out and down. 

The finished armored swale.

What the trail needed is what’s called an “armored swale,” which is essentially a channel cut into the trail that is protected by a floor of rocks to prevent erosion. I had been waiting for rain to soften the soil, as we would first need to excavate the existing rocks and in summer the ground is like concrete. It rained yesterday, so the soil was going to be about as soft as it was going to get. Also, since I knew that an “atmospheric river” was set to dump several inches of rain in a matter of a few days, now was the time to act.

So Jessica and I went out today and built it. We first needed to excavate a channel across the trail, then place stones to protect from erosion, and fill in gravel and soil around the stones. Do your worst, atmospheric river, we’re ready for you!

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