The Trail Maintenance Team for the Sonoma Overlook Trail does a variety of tasks, but one of the most important things we do is manage the flow of water across the trails. Essentially, what we try to do is to move the water off the trail as soon as it comes on. The best way that this happens is in sheet flow from the upper side of the trail to the lower side, but that is rarely the situation we can accomplish.
One challenge is that water often comes onto the trail not in a gentle sheeting flow, but in what is essentially a tiny creek. Another is that it isn’t always an option to have the water sheet off the downhill side. We often need to allow it to flow down part of the trail before directing it off in a waterbar (a channel we cut in the trail to force the water off the trail).
There are also other structures we build to manage water. Perhaps chief among them is the armored swale. An armored swale is essentially a deep channel you cut in the trail that you “armor” with rocks so it doesn’t erode as the water flows over it and off the trail. Another strategy we use to encourage sheet flow off the trail is cut the “berm” or the very edge of the trail that can build up in a way that keeps water on the trail instead of allowing it to sheet flow off of it.
All of these structures require ongoing maintenance, and the more subtle structures require more maintenance than the more dramatic ones.
Over the last few days (one of them during the recent rainstorm) I was out renewing the structures in place and cutting berm to better enable water flow off the trail. I also deepened a few waterbars and performed other tasks to better channel water from the trail. I will again be out there tomorrow afternoon during the rainstorm with a shovel, to see what needs fixing. It’s part of what we stewards do.
Today, the second day of an “atmospheric river” storm, I hiked the Montini and Overlook properties to check out how well the water was getting off the trails. More water was running off than I recall ever seeing before, although I had missed the October storm which by all accounts was a gangbuster.
You know it’s a major storm when the waterfall on the Holstein Hill trail on the Montini Preserve is running as it was today (see photo).
Most of the trail structures put in place to get water off the trail were working as intended, although a few need to be cleaned out or enhanced. The properties are so rocky, however, that in places water seeps from the hill above and sheets over the trail. In other words, there’s just no getting around having water on the trail, so be careful.
Today Jess 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.
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 Jess 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!
Since rain was predicted for today (and it came!) I headed out earlier in the week with a mattock to clear trail drainage channels, which had become clogged with rocks, leaves and miscellaneous debris since spring. Water must be guided off our trails immediately or else it will erode the trail and harm it, especially over time. Plus we don’t want the soil that gets eroded to end up in our waterways.
I also cut new ones where I thought they may be needed. Some of this work is obvious, but I know that additional work will be needed once we actually have flowing water on the trails and can see the trouble spots, where water is pooling or running down the trail.
This is just one task that the volunteer stewards group does throughout the year to keep our trails safe and well-maintained.