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Extreme highwater pool elevations in Pactola Reservoir, South Dakota in 2015 and 2019 resulted in massive shoreline erosion along the southern banks. This shoreline erosion occurred despite the geologic material being highly fractured rock with fracture dip angles approximately parallel to that of the subject hillside. The shale and siltstone in the upper 30 meters of the geology had weathered into silt and clay, leaving a matrix of 70% to 90% rock and 10% to 30% fine-grained soil. When highwater occurred, the silty portion of the shore materials eroded, while the remaining small amounts of highly plastic clay were insufficient to bind the weak and thinly bedded rock together, and rock slipped along the 15 to 30-degree fracture dip angle into the water. The shore erosion was arrested by a line of pine trees, and the erosion patterns show the effectiveness of tree roots to resist erosion in highly fractured rock. The erosion case history is presented with descriptions of several individual root systems that were effective in shore stabilization, as well as descriptions of several individual tree root systems that were ineffective in shore stabilization and the trees perished in the highwater evens and toppled. 

One of the principal side effects of wildland fire extreme events is mass soil erosion event of the soil and slopes denuded by the fire. These soil erosion events may be devastating and extreme in their own rights, damaging critical infrastructure downslope or downstream of the fire burn scars. While there are many variables influencing the severity of post fire erosion, the amounts of soil erosion are largely dependent on the water content of the soil at the time of the fire coupled with the fire intensity. Fires that are lower intensity or soils that are “wet” at time of burning have significantly less damage to root structures of grasses and other plants while showing lessened soil erosion potential. Fires that are higher intensity or on dry soils have higher damages to root structures and increased soil erosion potential. In this laboratory study, a single clayey sand soil material common to the ground surface across the Black Hills of South Dakota and Wyoming is studied for erosion potential after burning in a controlled container burn. This material is varied by initial water content and burned at a soil surface temperature of 800 Celsius for 75 minutes, a temperature-time continuum consistent with severe wildland fire. Burning is used rather than kilns to preserve the same atmospheric conditions as in the field fire event. A laboratory soil-erosion device is then used to measure soil erosion potential across a range of fluid velocities and soil slopes. The results of this study show that the initial water content of the soil at the time of fire is a key parameter in understanding soil erosion potential post-fire. While not a complete study on time-temperature-water content across many soil types, this pilot research shows promise for future models and mapping tools. These future tools will enable planners to target resources for post wildfire erosion mitigation based on surficial soil water content at the time of the fire. 

The foundation systems of mega-flora (i.e. very tall or large trees), have long been used as an analogy for modern shallow and deep foundations. Terzaghi referenced trees as the model for footings and pilings. However, the topology, form, materials, distribution, and function of the natural foundation system have very little in common with the shallow and deep foundation systems that geotechnical engineers design and construct. These natural foundation systems are resilient, robust, and adaptable; ideal templates for a new generation of anthropogenic foundation systems and new understanding of soil-structure interaction. In an effort to further biomimetic geotechnics, this paper will present a review of the actual topology, form, materials, distribution, and function of mega-flora foundations, highlighting key differences with man-made foundation systems, materials and designs. This paper will dispel common myths about these natural structures, giving engineers insights into their performance under complex and extreme loads. Several key species will be highlighted, with unique aspects of each species’ foundation system highlighted. Field measurements and observations of several natural foundation systems are included in the paper to highlight recent findings about these remarkable systems.