2024-05-17_REVISION - M1977493 (4)

Home Browse Search

G-5 Flood Control A critical component in the operation of the Climax facilities is flood protection. Climax has dedicated significant resources to the protection of the facilities from extreme flood events. Protection of the TSF's from extreme precipitation events is in accordance with regulatory requirements and FMI corporate standards. G-5.1 Background In 2005, W. W. Wheeler and Associates (Wheeler) completed a site-wide hydrological assessment of the Climax Mine and evaluated its ability to safely handle the flood generated by the PMP event(Wheeler,2005). As part of that study,Applied Weather Associates performed a hydro-meteorological analysis and developed a site-specific PMP design storm for the Climax site (Applied Weather Associates, 2005). The 2005 hydrology study indicated that, under the configuration at that time,the water storage facilities and TSF water pools at Climax were capable of passing the site-specific PMP in accordance with DRMS and Colorado Division of Water Resources requirements. Although no significant impacts to the drainage basins and hydrologic regime have occurred at Climax due to mine production, the water storage capacity and dam crest elevations of the TSFs change due to tailing deposition. In order to evaluate the implications of mine operation and closure on the system's flood handling capabilities, the hydrology model has recently been revised to model the expected site conditions for the current operating scenarios with a new flood decant riser on Mayflower TSF (Wheeler, 2019). This study included the use of a new extreme precipitation development tool known as the Colorado-New Mexico Regional Extreme Precipitation Study (CONM-REPS Tool). This study confirmed that the flood protection facilities at the Climax Mine were capable of passing the new PMP generated flood and the results were used to develop the flow capacity design criteria for the new decant riser on Mayflower TSF. Over the last several years, the State of Colorado Dam Safety Branch of the State Engineer's Office (State Dam Safety) has updated the tools and methods recommended to be used to perform hydrologic analysis of water storage dams in the state. These updated tools and methods for water storage dams in Colorado are described in Guidelines for Hydrological Modeling and Flood Analysis (DWR, 2022) will be used to update the design storm flood hydrology for the Climax Mine site. The procedures to be used in the flood study update are summarized below. G-5.2 Drainage Basins The Tenmile Creek drainage basin area is generally typical of high mountain basins in the Rocky Mountains. The slopes of the basin are generally steep and elevations range from a maximum of nearly 14,000 feet along the eastern boundary,to a minimum of about 10,600 feet at the crest of 5 Dam. The drainage basin is divided into several sub-basins to model flood impacts on the three primary TSFs and Clinton Reservoir. Figure G-01 shows the configuration of the various sub-basins and summarizes the drainage areas. The mean basin elevation for the areas tributary to the three primary TSFs are 11,200 feet for the Robinson TSF, 11,440 feet for Tenmile TSF, and 11,520 feet for Mayflower TSF. G-5.3 Soils and Infiltration The approach now recommended by DWR in Colorado for estimating infiltration losses from direct rainfall is the Colorado State University— Soil Moisture Accounting (CSU-SMA) approach which can simulate three distinct types of storm runoff: infiltration-excess, saturation-excess, and subsurface runoff. The CSU-SMA approach, which also employs the Linear Reservoir baseflow method, is used in conjunction with the Clark Unit Hydrograph method (for rainfall to runoff transformation) to more accurately reproduce the runoff response mechanisms that are observed in the diverse terrain across Colorado. The method requires that various raster datasets describing other pertinent soil properties:%sand,%clay,% organic matter, and depth to restrictive layer be acquired from the DWR Dam Safety website for the Climax Mine watershed. These soil property raster files are then used as inputs to the CSU SMApython2 script (a program for automating ArcGIS calculations) provided by the DWR to characterize each sub basin property statistically, generating the minimum, maximum, average, range, and standard deviation of each sub-basin Exhibit G G-19 May 2024