Surface Water Hydrology of the Proposed Low-Level Radioactive Waste Isolation Site, Hudspeth County, Texas

This report describes the surface-water hydrology at the proposed low-level radioactive waste isolation site in Hudspeth County, Texas. The objective of these investigations was to evaluate the flooding potential at the site based on computer simulation of runoff from observed and hypothetical rain events. Analytic techniques and assumptions used in this study are based on recommendations of federal and state regulatory agencies regarding flood insurance and dam safety criteria. Published topographic maps, aerial photographs, and site surveys were used for delineating drainage basins and surface-water pathways on the study area. Surface-water runoff volumes were calculated for rain events monitored at the site during the study period. Hydrologic computer models were employed to determine correlation of rainfall to surface-water runoff. These computer models were calibrated using rainfall and stream-flow data measured at the site. Flood profiles were calculated for 100-yr and probable maximum rain events, which were estimated from historical data. The following conclusions regarding the flooding potential at the study area were drawn on the basis of these studies: (1) Computer simulation indicates that floods resulting from hypothetical 100-yr and probable maximum precipitation events are contained within existing channels in the study area, leaving large interchannel areas unflooded. Some overland sheet flow is encountered over the flat area, but the velocities of flow are very small. (2) Rainfall events recorded during the 1988-1989 period were short and localized. The response of runoff to rainfall is rapid and the duration of the peak water flow after rainfall is relatively short. (3) Flow velocities range from 3 to 13 ft/sec (0.9 to 4 m/sec) in channels and are lower over flat areas. Maximum depth of flow due to a 100-yr flood in the better-defined channel on the central part of the study area is about 5 ft (1.5 m).