An investigation of bergmounds as analogs to erosion control factors on protective barriers

Included in several of the final disposal strategies proposed in the Interim Hanford Waste Management Plan (DOE-RL 1986a) is design of a protective barrier to isolate the underlying waste sites from the environment. The conceptual protective barrier design requires a fine-grained sediment to retain precipitation near the top of the barrier where evapotranspiration can recycle the moisture back into the atmosphere. The design incorporates gravel into the topsoil as one way to reduce its erosion. Information is needed to determine the optimal ratio of gravel to topsoil needed to reduce erosion without significantly reducing evapotranspiration, and its effect on erosion. Bergmounds are mounds with a gravelly surface that were formed about 13,000 years ago and represent natural analogs to the topsoil portion of the protective barrier. The primary goal of this study was to identify characteristics of bergmounds and the effects of these characteristics, especially the gravelly surface, on the amount and rate of erosion. A secondary goal was to apply a technique normally used to estimate vegetation cover to measure percent gravel cover, and to compare this technique with particle size distribution based on weight percent. Four bergmounds were investigated for this study, two in a windy site and two in a more sheltered site. Each bergmound was sampled in eight locations. Two methods were used to estimate the amount of surface gravel: the ocular point-intercept method which estimates the percent gravel cover, and sieved samples of the surface sediments which measure the percent gravel by weight. Holes were dug at each bergmound's eight sampling sites to examine and sample the subsurface sediments