BORN-AGAIN GULLY: THE REESTABLISHMENT OF THE MILLER BROOK GULLY, NORTHERN VERMONT

Glacial sediments influence many surficial processes in the northeastern United States, especially landsliding and gulling. Such mass wasting is particularly common in river valleys where slopes are steep and lake clays are common. At the Miller Brook gully the glacial sediments, from the bottom up, are 1) till with a clay matrix 2) sands and gravel from near glacial runoff 3) glaciolacustrine inter-bedded clays and fine sands and 4) capping sands and gravels. Such common stratigraphies and the associated mass wasting can have significant impacts on land use in glaciated landscapes.

The exact date of gully initiation is unknown. Time series aerial photographs from the 1940s to the 1970s suggest that the gully became active in the late 1960s or early 1970s. The gully eroded due to a hydraulic conductivity difference between the sands and gravels (high) and the overlying glaciolacustrine sediments (low). Such a difference in hydraulic conductivity focused groundwater in the sands and gravels and formed a piping network that surfaced on the adjacent hillslope. In several places the pipe’s roof collapsed and provided windows to the stratigraphy and the pipe. By 1998 the gully expanded to 50 m x 8 m x 2.5 m. The landowner wanted the erosion to stop and in the summer of 2001 the north bank was intentionally collapsed to fill in the gully. Since the piping network was not affected, new roof collapses were observed in the fall of 2001.

Since 2001, the gully has continued to expand; however, the gully axis is at a different orientation and is eroding naturally deposited glacial sediments, not just the fill. An August 2004 survey of the gully suggests an additional 830 m³ of erosion, approximately equal to the original volume of the gully. The erosion rate since 2001 (~280 m³/y) is almost an order of magnitude higher than the estimated erosion rate from initiation until 2001 (~30 m³/y). Using the volume of an adjacent stable gully that bottoms on till as an analog, we estimate that the active gully has at least several more decades before it stabilizes. This study suggests that in order to stabilize gullies in glacial sediments, one must correctly identify the erosion processes; otherwise the resulting erosion rates could be as much as an order of magnitude greater than the natural erosion rates.