INFLUENCE OF COUPLING EROSION AND HYDROLOGY ON THE LONG-TERM PERFORMANCE OF ENGINEERED SURFACE BARRIERS
Smith, Crystal Lynn
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The main goal of this study was evaluate design strategies that couple erosion and hydrology for barriers of low level radioactive waste (LLRW) disposal facilities. This objective was met by conducting long-term (1000 yr) parametric simulations with the SIBERIA landform evolution model and the SVFLUX hydrologic model. The landform evolution modeling considered four main factors affecting fluvial erosion: (1) climate, (2) soil, (3) vegetation, and (4) topography. Several scenarios were evaluated for semi-arid and humid sites. The topography of the Grand Junction Uranium Mill Tailings Disposal Site in Grand Junction, CO was used as a realistic starting point. Modifications were made to the topography, soil surface layer, cover type, and vegetation. The topographic changes included a modified cover with a central high point and more balanced slope lengths with uniform slopes, and modified cover with terraced, concave, and natural side slopes. Three types of surface layers were evaluated: rip-rap, topsoil, and topsoil mixed with gravel (gravel admixture). Conventional resistive barriers and water balance barriers with a capillary break were evaluated with materials data. Simulations were conducted with and without vegetation with native plants for each climate. Hydraulic modeling was conducted using a one-dimensional profile for semi-arid and humid climates. Simulations were conducted using normal and wettest year precipitation data. Rip-rap, topsoil, and gravel admixture surface layers over resistive and water balance barriers were used in simulations to evaluate cumulative percolation into the waste. Climate and use of vegetation produced significant differences in maximum erosion depths. The semi-arid climate had approximately 4 m greater maximum erosion depth than the humid climate for simulations with a rip-rap or gravel admixture surface. A topsoil surface in the semi-arid climate had approximately 2.5 m greater maximum erosion than the humid climate. Vegetation decreased the amount of erosion in the semi-arid climate by 1.5 m and 4 m in the humid climate. Vegetation also increased the amount of evapotranspiration that occurred, decreasing percolation into the waste. The resistive barrier produced less erosion than the water balance barrier in the semi-arid climate. Neither allowed percolation into the waste. Percolation was high for the water balance cover in the humid climate and non-existent for the resistive cover. Both covers performed identically in the landform evolution predictions for the humid climate. Short slopes, slopes with a low grade, and slopes with small grade differences at the nickpoint were found to decrease erosion. The humid climate had the least erosion when terraced slopes were utilized. Due to higher erosion rates in the semi-arid climate, natural and concave slopes that promote deposition produced the least erosion depth. Overall, the rip-rap surface layer prevented the most erosion over any type of topography, climate, or cover type. However, the difference in soil texture between the rip-rap surface and finer soil layer beneath caused water to become trapped between the layers and caused more percolation into the waste. In contrast, the gravel admixture surface had slightly greater erosion, but prevented percolation in typical year simulations for both climates.