Terrence J. Toy

Accelerated erosion: Process, problems, and prognosis

Soil erosion may well be the worlds most serious environmental problem. A variety of human activities accelerates the rate of this geomorphie process by altering the natural characteristics of a site. The problems arising from accelerated erosion and subsequent deposition provide numerous research opportunities to both physical and social scientists whose cooperation will be necessary in our search for solutions. Factors of demography, economics, geography, and historical inertia to change suggest that man-induced erosion is likely to continue into the future unless this trend is abated by timely and forceful action.

Hillslope form and climate

The relationship between climate and hillslope form was investigated at 29 sites along two traverses: one from Berea, Kentucky, to Pioche, Nevada; the other from Glendive, Montana, to Ruidoso, New Mexico. A rigorous site-selection procedure was used to minimize the importance of nonclimatic factors and to ensure the inclusion of hillslopes in a variety of climatic regions. Five hillslope variables were selected for comparison with nine climate variables. Each hillslope and climate variable has a theoretical or conceptual basis for use in this study. The results support the hypothesis that hillslopes reflect regional climatic variations. Fifty-nine percent of the variation in the rate of curvature of the convex segment was explained by the climate variables. Forty-three percent of the variation in the slope of the straight segment was accounted for on the basis of climate. Thirty-seven percent of the variation in the slope of a regression line fitted through the entire profile was explained by variations in climate. Twenty-six percent of the variation in the slope length ratio was explained by the climate variables. It was concluded that hillslopes in arid areas tend to be shorter, steeper, and have smaller radii of curvature of the convex segment than those in humid areas. Examination of the residuals from regression suggests that the site-selection procedure was successful in minimizing the geologic influence on hillslope form while in no way ignoring its importance. The statistical significance of the relationships indicates that there exists a state of quasi-equilibrium on the hillslopes. Lastly, evidence is provided for the existence of quantifiable relationships between geomorphological processes and hillslope forms. While this has been frequently asserted and is the basis of climatic geomorphology, it has rarely been tested.

Changing Surface-Mine Reclamation Practices in Minas Gerais, Brazil

Changing reclamation practices for surface mines in Minas Gerais, Brazil, were examined through visits to several mines and interviews of reclamation specialists, environmental consultants, and regulatory-agency personnel. The quality of reclamation improved consiaderably in the past 13 years due to changes in legal requirements, corporate attitudes, new reaclamation practices, and better application of selected practices. Most experienced reclamation specialists developed techniques well-suited to the specific environmental conditions at their mine. New challenges await as reclamation goals and requirements become more rigorous. On the horizon is long-term, land-use planning as part of mine-closure plans, increased environmental monitoring and mine inspections, and possibly reclamation bonding. On the ground, adoption of topographic-reconstruction practices, complete analyses and use of available topsoil, development of dependable seed sources for wider varieties of plant species, and techniques to insure uniform seed distribution would facilitate continued improvement in reclamation quality. A national reclamation organization would promote reclamation-technology transfer.

Geomorphology of Surface-Mined Lands in the Western United States

The Western United States is a repository for bountiful reserves of minerals and fossil fuels. Often, these resources are located relatively near the surface, and hence, can be extracted very economically using surface-mining techniques. Few human activities so thoroughly alter the local physical environment as these mining methodologies. Protective vegetation cover is removed. Soil profiles are disrupted. The geologic materials overlying the resources are nearly homogenized. Steep hillslopes of “spoils” are produced. Drainage networks and stream channels are obliterated. Consequently, the geomorphic processes operating on these disturbed lands usually differ significantly in magnitude, frequency, and work performed, when compared to adjacent, undisturbed areas.

The healing of disturbed hillslopes by gully gravure

Results of accelerated erosion on certain constructed surfaces in southeastern Arizona appear similar to those described by Bryan (1940) as gully gravure. Twenty cross-section excavations in eight rills incised into silt-rich lacustrine and fluvial deposits reveal partial filling of the rills by debris derived from overlying fluvial sand, gravel, and cobbles. Interstices of the coarse material gradually fill with fine-grained erosion products, decreasing permeability of the fill and deflecting subsequent runoff to the margins of the fill. Rills and rill fillings thus increase in width with time, and complete veneering of the surface by coarse debris ultimately may occur. Through incision, filling, lateral planation, and armoring, channels of the dissected surface heal and the new hillslope approaches an equilibrium condition. Natural hillslopes in the area with similar geologic conditions have inclinations of 16°-22°, have generally unbroken veneers of coarse debris, and appear subject to the same erosional processes identified at constructed hillslopes. Field measurements and the revised universal soil loss equation (RUSLE) were used to estimate soil loss on natural and constructed hillslopes. Results suggest that erosion may be almost three orders of magnitude greater on steep, non-veneered, nonvegetated constructed surfaces than on natural hillslopes. Attributes of stable natural hillslopes provide criteria for the design of stable constructed hillslopes.

Prediction by regression and intrarange data scatter in surface-process studies

Modeling is a major component of contemporary earth science, and regression analysis occupies a central position in the parameterization, calibration, and validation of geomorphic and hydrologic models. Although this methodology can be used in many ways, we are primarily concerned with the prediction of values for one variable from another variable. Examination of the literature reveals considerable inconsistency in the presentation of the results of regression analysis and the occurrence of patterns in the scatter of data points about the regression line. Both circumstances confound utilization and evaluation of the models. Statisticians are well aware of various problems associated with the use of regression analysis and offer improved practices; often, however, their guidelines are not followed. After a review of the aforementioned circumstances and until standard criteria for model evaluation become established, we recommend, as a minimum, inclusion of scatter diagrams, the standard error of the estimate, and sample size in reporting the results of regression analyses for most surface-process studies.

RECLAMATION REVISITED: MINAS GERAIS, BRAZIL

The last systematic evaluation of surface-mine reclamation in Brazil was completed in 1986. Since that time, there have been significant changes in reclamation laws and reclamation practices. The purpose of this investigation was to examine the evolution ofreclamation during the past 13 years based upon mine visits and interviews with reclamation personnel at the mines, with environmental consulting companies, and with the regulatory authority. Overall, reclamation quality has improved substantially since 1986. Nearly all mines work to reclaim the lands that they disturb. Environmental analyses and reclamation planniog is much more thorough than in the past. At most mines, geotechnical studies have largely eliminated the occurrence of mass-movements. Hillslope-gradient reduction and terracing provide stable platforms for reclamation. Revegetation is a twoor three-stage process, rather than the nearly exclusive use of rapid-growing, introduced, vegetation species as in the past. Nevertheless, there is a wide range in reclamation quality, from excellent on a world-class scale, to minimal, apparently intended to temporarily appease the regulatory authorities. Reclamation quality will continue to improve as laws, regulations, and their enforcement become simplified but more stringent, postreclamation land use receives more attention, and reclamation specialists gain

Assessment of hillslope designs using RUSLE at the Coeur Rochester Mine, Lovelock, NV

The Coeur Rochester Mine is a mountain-top, open-pit, heap-leach operation engaged in the production of silver and gold. The Revised Universal Soil Loss Equation (RUSLE) was used to assess the erosion potential of various reclamation-hillslope designs at this mine. Soil-loss estimates were computed for reclaimed rock-disposal and leach-pad sites as well as natural, undisturbed, sites. Factor inputs reflected the expected post-reclamation hillslope characteristics. The estimated average annual soil loss from the natural hillslopes ranged from 0.16-1.77 T/ha (0.07-0.79 t/ac). The estimated soil loss from the various designs for hillslopes developed on rock-disposal sites ranged from 0.22-3.41 T/ha/yr (0.10-1.52 t/ac/yr). The estimated soil loss from the various designs for hillslopes developed on leach-pad sites ranged from 0.04-0.22 T/ha/yr (0.02-0.10 t/ac/yr). The low soil-loss rates are the result of low R- and C-factor values coupled with a low to moderate K-factor values. These estimates were carefully considered in the selection of appropriate hillslope designs. RUSLE is a powerful tool for assessing the erosion potential of various hillslope designs during the reclamation-planning process.