Tuesday, December 11, 2018

Growing Gully Threatens Paugussett Trail at Indian Well

This gully keeps advancing up hill
The Paugussett Trail crosses a tiny stream at Indian Well State Park that has managed to create a rather large gully. Canyon might be a better word. It swallows large trees. It's might be thirty feet deep in places and far too wide for a pedestrian bridge. The unstable sides of the sandy gully are impossible to walk up or down. In other words, it's not something a trail can get across.

Overview: Stream erosion from Tahmore Place and gully advancement up the hill
The Paugussett doesn't need to cross the gully just yet, but the gully head is advancing up the hill and is now within fifty feet of the bridge. Based on the location of a previous trail route, it can be inferred that the crossing used to be about 100 feet downstream, but was moved up the hill to escape the growing chasm. No idea when that was. The northern approach to the bridge needed to be rerouted in 2018 because a fifteen-foot drop at the head of the gully was too close to the trail. The trail was one large storm away from collapsing into the gully. The original southern approach was apparently abandoned some years ago, but can still be seen on the LIDAR aerials. The current southern approach is in jeopardy as well, but there is not an easy reroute due to the topography.

The head of the gully is now near the property line between Indian Well State Park and the Shelton Land Conservation Trust (the red line in the aerial below). The bridge is on Land Trust property. 

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Why is this stream eroding so badly? Obviously it's a steep, sandy hill, so it's highly prone to erosion, but compared to the many other small streams descending the river bank, this one seems to be suddenly eroding at a very fast pace. The most likely explanation is that the stream handles too much stormwater runoff from subdivisions built on top of the river bank. All the water from streets, roofs, and driveways goes right down the stream whenever there's a storm.  The hillside consists mostly of sand that was at the bottom of a glacial pond. It's not the normal Connecticut glacial till. The consistency is closer to sugar, and it washes out badly during big storms.

Head of the gully, where a waterfall drops about 15 feet into a narrow trench
The head of the gully is only about five feet wide, but it's maybe fifteen feet deep. It's a narrow trench.  A waterfall plunges down the face of the trench, and undermines the sandy bottom. Eventually the walls of the trench collapse and the gully advances up the hill and widens. 

Where does all the washed out sand go? A lot of it piles up on Indian Well Road. Here's a photo of the gully taken in 2007 from Indian Well Road right after a major storm:

Bottom of the gully at Indian Well Road

And here is some of the sand that had to be scraped off of the road with backhoes:

Sand scraped off of the road
That's a lot of sand. Some of it is presumably making its way into Lake Housatonic, where it would be filling in the lake created by the Derby-Shelton dam. At some point after the 2007 storm, some check dams were installed at the foot of the gully to stop sand from washing onto the road, but they don't stop the gully from advancing up the hill. 

Check dams near Indian Well Road in 2018
So the gully just keeps on growing, getting wider, deeper, and heading up the hillside. 

It's just a matter of time before it reaches the Paugussett Bridge. And then what? Assuming no action is taken to stabilize the slope, we'll need to move the bridge up the hill again. And over time we'll need to do it again and again, and the trail route will need to go up the hill closer to the houses. Eventually the gully will either hit bedrock (hopefully) and stop growing, or it will go all the way up to the houses on the hill, at which point it's not clear how the Paugussett will get across.

Paugussett Bridge

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