Surface Water Hydrology ofProposed Low-Level Radioactive Waste Isolation Site

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The purpose of surface-water hydrology studies at the proposed low-level radioactive waste isolation site in Hudspeth County, Texas, was to define the flooding potential as interpreted from the applicable regulatory requirements. Federal Emergency Management Agency Report 37 (FEMA, 1985), which details guidelines and specifications for flood insurance studies, and other published reports (Texas Department of Water Resources, 1979a-c) were used as the primary source to develop the scope of field reconnaissance and hydrologic evaluation of the study area. The scope of this project included:

(1) Delineation of drainage basins and identification of drainage divides and potential surface-water pathways on and near the study area. (2) Collection of rainfall and surface runoff data from the study area. (3) Development and evaluation of a hydrologic model to determine the extent of flooding at the site due to actual and potential storms. (4) Definition of a 100-year floodplain for the study area by determining depth, velocity, and extent of surface runoff resulting from a hypothetical 100-year rainstorm.

The approach adopted to meet these objectives consisted of estimating soil properties, monitoring rainfall and surface-water runoff rates, matching simulated flows to observed data on surface-water runoff, and predicting flow characteristics based on calibrated computer models.

Two methods were used to evaluate the flooding potential. The first method treated the study area as an active alluvial fan: this analysis indicates that the study area lies within a 100-year floodplain. This method excluded the topographic features and the hydraulic storage in the existing channels. The second method incorporated computer models to determine flood elevations for a 100-year flood: this analysis indicates that runoff from such an event would be mostly contained within the existing channels, leaving large sections in the middle of the study area uninundated.


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