second and third cycles resulted in little additional reduction. Van Klaveren (1987) suggested that critical
shear strength of soil might be half of its initial value after one FT cycle. Edwards and Burney (1987)
used a laboratory rainfall simulator to determine that FT of a bare soil increased sediment loss by 90%,
and that this loss increased significantly when overland flow was added. Rill erosion laboratory
experiments of Van Klaveren and McCool (1998) revealed slightly higher erodibility of thawed soils after
a single FT cycle compared to that of an unfrozen soil. Edwards et al. (1995) conducted similar
laboratory tests, except that four diurnal cycles of freezethaw were performed prior to a final 12-hr
freezing cycle. Erosion of this cycled and initially frozen soil produced a mean sediment yield 25%
greater than a similar soil that had never been frozen. This overview of work to date suggests that the
effects of FT on soil erosion vary, and that a quantitative parametric understanding does not yet exist.
The objective of the experiments described and analyzed in this paper was to isolate and quantify
the effects of FT on soil erosion and rill development in bare soil. Frost-susceptible silt was used in the
experiments to obtain an upper bound on the effects caused by a single FT cycle. Two identical soil bins
were prepared for each experiment, one to remain unfrozen as a control (C), the other to be completely
frozen and thawed (FT). This parallel approach allowed measured differences in soil loss to be directly
attributed to the FT process and related to the other controlled parameters. Three series of experiments
were performed, each within a specific soil moisture range. Each series included six experiments at three
nominal flow rates and two slopes. Soil characterization for each soil moisture condition measured
physical changes induced by FT. The results of the experiments were closely related to experimental
conditions, imposing a need for tightly regulated soil moisture, bulk density, applied flow, and slope
throughout each series. We performed replications of several experiments with differing variability in
conditions to quantify the probable uncertainty and reliability of results.
Description of Experiments
We used frost-susceptible Hanover silt for all experiments to obtain an upper bound for FT
effects on soil structure and subsequent overland-flow-induced erosion. Hanover silt is a low-plasticity,
inorganic, clayey-silt classified as ML in the Unified Soil Classification System, with a specific gravity of
2.72, and a liquid limit of 28% (Shoop and Gatto, 1992). This soil is composed of 82% silt- and clay-
sized particles by weight and 18% fine sand, with about 66% in the grain size range from 0.1 to 0.01 mm.
During soil preparation the water content was adjusted into the appropriate range and periodically
checked using either a Vitel Hydra or a Delta-T Theta probe. A resistivity sensor and thermistor were
secured to the bottom of the FT bin prior to adding soil, and used to monitor the frozen or thawed state of
the soil bottom. The FT and C bins were prepared with equal soil density by adding increments of equal
weight that were compacted to the same thickness until a volume of about 38 L (79 cm long, 37 cm wide,
13 cm deep) was filled. Three groundwater wells were placed along one side of the longitudinal
centerline of each bin, and the finished soil surface was flush with the tops of these wells. A metal plate,
8 cm wide by 1.5 cm thick, was tamped its full thickness into the soil between the flow inlet and the
invert of the downstream weir. This imposed rectangular rill, with a compacted soil bottom, was oriented
along the longitudinal centerline.
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