This project is an ongoing project since 1998. Derek Eaton GE '05 gradaute from Skidmore College, collected more data and is in the process of writing a manuscript for Northeastern Geology and Environmental Science. Please come back for a final analysis, until then enjoy our preliminary data.
This project is funded in part by a Vermont Geological Survey grand to Eaton.
Near Stowe, Vermont, we have been monitoring an active gully/alluvial fan system
since 1998. Our first survey of the gully, shown above had a volume of ~800
m3. The alluvial fan had a volume of ~ 1000 m3. These measurements were within
our survey error. In the summer of 2001, the property owner had one gully wall
removed in hopes of stopping the deposition on the alluvial fan. Read below
to see if the solution will work.
HISTORY
The Miller Brook Gully is located on an abandoned terrace riser of Miller Brook. The hillslope has been extensively logged in the past (see the aerial photos below 1942, 1963, and 1978) and started to be logged again in the fall of 2001. The road used to lie at the base of the riser, but was moved sometime between 1963 and 1978.
There is an established gully less than 100 m to the south of the active gully. This gully bottoms on till, and there is usually a small trickle of water running down the channel.
The stratigraphy is common for previously glaciated terrains. From the bottom up is: basal till, sands and gravels (from near ice runoff), and at the top is glacial lake silts and fine sands. The gully started to form in the sands and gravels that have high hydraulic conductivities. Erosion in the sands and gravels focused groundwater into the void and accelerated sapping erosion. Continuous erosion formed a pipe. Localized roof collapses formed open windows to the surface. We used a dye tracer (below) and timed the transport to determine the geometry of the pipe.
In 1998 we timed the dye from the gully headwall to the pipe outlet and we recorded the time as the dye passed each of the windows. The time/distance graph was linear suggesting that the pipe had simple geometry from the headwall to the outlet. The alluvial fan (below) exhibited several generations of fan head trenching in response to the changing hydraulic conditions caused by wet and dry periods.
The gully walls were eroding by different processes. The south wall was at the angle of repose and slumping of sediment was the major erosional process. Piezometers (10 feet deep) were dry during the wet spring of 1998, likely because the adjacent gully to the south lowered the groundwater table. The north bank was near vertical. The piezometers on this side had water, and water was seeping out of the wall. The north wall was failing by toppling blocks caused by high water pore pressures.
In the summer of 2001, the north bank was removed, but the south bank and piping network were still intact. By the fall of 2001 the roof of the pipe again collapsed and formed windows to the surface. By the winter 2002 (a preliminary map is shown below) the windows exposed the pipe bottom, almost 2 meters below the surface. The approximate old gully margin is shown by bold outline. The "new" gully is not forming in the same location as the "remediated" gully.
The most recent survey in August 2004 (preliminary map below), shows the reestablishment of the gully system. The current gully is forming in undisturbed material probably because the sands and gravels, overlain by glacial lake silts and fine sands, are still intact and can reestablish the piping network. As you might be able to discern, the solution only partly worked, because they failed to understand the processes responsible for the gully initiation and growth.
Check back in May 2005 to see the final analysis of this project.