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above the repositories would not affect the stability or integrity of the repositories. However, rockfalls above the repositories could lead to the need for maintenance and cleanup of repositor runoff control channels. The toes of the repositories are set back from the valley rim and will nc be disturbed by the effects ofnliff retreat for well over 1,000 years. The mesa top area around the repositories has a relatively very small catchment area. Drainage modifications have reduced surface runoff so that only rainfall on the disposal cell will contribu to the surface hydrologic processes in the repository area. Erosion will be controlled by riprap and no significant erosion of the repositories is expected during the next one thousand years. All the repositories are sited in stable locations which will not be affected by lateral rim reheat. The repositories are not located within the 100-yeaz flood plain of any streams. The potential fc flooding from watershed runoff has been minimized by the construction of diversion channels designed to convey the Probable Maximum Flood ("PMF"). Flooding from upstream areas along the mesa is another geomorphic consideration. This concern has been addressed by mapping the extent of the watersheds that could affect the repositories and using this information in designing drainage diversion channels sized to convey the PMF. The PMF used is based upon a Probable Maximum Precipitation (PMP) value of 8.3 inches for the one-hour localized event. Use of the local PMP is appropriate for watersheds less than ten square miles in area and maximizes the storm intensity. The design calculations for the channels have been reviewed by CDPHE and are acceptable. The channel roughness coefficients selected arc approptiate based upon NRC guidance and engineeting judgement. Using the U.S. Army Corps of Engineers computer program model HEC-1, the flow routing is appropriate for the existing conditions. These calculations together with the design assumptions and PMP selected are adequate to assure long-term protection of [he repositories from flooding. 4.4 Seismic and Tectonic Characteristics The Uravan region fs in an area which has experienced a relatively low level of seismic activity for as long as first-hand records exist, the last 125 years. The more frequent and larger earthquakes in the region have occurred in the intermountain seismic Zone which generally coincides with the Wasatch Mountain range in Utah. This zone of major seismic activity is located about 170 miles to the west of Uravan. Because of its distance from the site, the intermountain seismic zone will not have a major impact at Uravan. Relevant fault systems are associated with the western flank of the Uncompahgre Uplift, 10 to 12 miles to the northeast of the site, resulting in a peak site acceleration of 0,3g, h.5 Surface Water The San Miguel River is the primary drainage in the project area. Headwaters of the San Miguel originate in the San Juan Mountains from which the river flows northwest to its confluence with the Dolores River about four mites downstream from Uravan. The San Miguel is perennial with pronounced seasonal fluctuations. This steam flow pattern is characteristic of rivers whose flow is derived primarily from snow melt runoff. Peak discharge on the San Miguel generally occurs in late spring or early summer. The San Miguel exhibits a seasonal flow pattern typical of streams in the region. Major flows Umerco-Uravan DA February t i, 200o tz ^~ £i/60 39t/d ClUf1N dbOU J3110U L0ZLV990L6 9U tT L00Z/St/90