2008-02-22_APPLICATION CORRESPONDENCE - C2008086 (39)

Home Browse Search

CHAPTERFOUR Environmental Consequences and Mitigation slopes of the Book Cliffs in the northeastern one-third of the project area. In numerous places along the steeper slopes, colluvium boulders up to 5 feet in diameter were observed that had obviously fallen from the steeper slope uphill. Furthermore, the exposed resistant sandstone beds comprising the cliff forming rocks of the Book Cliffs are fractured such that large blocks rest above the steeply sloping to near vertical terrain at higher elevations. Weathering and freeze-thaw action occurring seasonally could potentially free a large block of this bedrock producing a rockfall. Accordingly, the risk of rockfall in much of the mine area is considered high. Landslide Hazards A relatively small landslide is located along the east-northeast permit boundary. The landslide does not appear to be active as there is no fresh head scarp, closed depressions, or pressure ridges; however, sudden changes in existing conditions such as changes in ground water conditions, slope cuts, or additional loading could reactivate this landslide. The landslide appears to be younger than the predominant pediment deposit but is sufficiently old to have developed a defined drainage at the base. In addition to the existing landslide, areas as potentially unstable slopes (PUS) comprise steep slopes that are stable in their existing condition and present moderate to high risk of future landslides or other slope instabilities. Modifications to slope grade, loading, storm run-off, or ground water conditions could promote conditions where risks associated with landsliding are increased. Accelerated Erosion Due to the loose sandy composition of the steeper slopes in the northeast and the high weathering susceptibility of the Mancos Shale, the risk of accelerated erosion is moderate to high. Additionally, there are three zones in the project area where evidence of accelerated erosion is distinctly visible. Both of these areas are marked by dendritic drainage patterns unlike the pervasive parallel drainage patterns that feed the larger streams such as Salt Wash to the southeast. Furthermore, each of the zones of rapid or accelerated erosion is marked by steep headward erosion scars and appears to be advancing upstream towards the Book Cliffs. Other Geologic Hazards Soil material derived from the Mancos Shale and the Mesaverde Group may contain clays that, on wetting, can swell causing damage to structures. Old small earthen dams are scattered across the property. The area behind (upstream) these dams may contain soft soils with significant organic material that, on loading, may prove susceptible to collapse and/or differential settlement. Mesaverde Group and Mancos Shale bedrock may contain radioactive minerals that, on decay, may produce radon gas. The presence of radon gas in structures has been identified as a potential health risk. The evaluation of risk due to the natural occurrence of radon gas at this stage of investigation is beyond the scope of this EIS. Earthquake risk in the project area is considered low. The property is located in Seismic Zone 1 characterized by earthquakes of Modified Mercalli Intensity VI or smaller, and minor damage. No active faults have been identified in the project area that would require consideration of surface rupture. 4-73 DBMS 628