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• Basin characteristics. The contributing drainage basin was delineated using ArcMap 10.7 software (ESRI, 2019) on 2016 digital elevation model (DEM)tiles.This basin includes modifications to the area based on proposed project changes (i.e., the culvert modifications under Old Hwy 91), resulting in a 4.4-acre basin. The current outflow point to the basin is a 10- foot diameter CMP culvert that runs underneath Old Highway 91 and into the local drainage channel. Climax proposes to construct a seepage cutoff wall around the culvert intake, retaining the containment pond behind the wall. Below is the stage-capacity table for the containment pond. Interpolated to integer elevation values: Elevation-Area-CapacityTable Water Surface Area' Storage Elevation Capacity (feet) (sq.ft.) (acres) (ac-ft) 10,354.2 0 0.00 0.000 10,355.0 61 0.00 0.002 10,356.0 259 0.01 0.005 10,357.0 696 0.02 0.018 10,358.0 1,436 0.03 0.041 10,359.0 2,299 0.05 0.080 10,360.0 3,182 0.07 0.147 10,361.0 4,148 0.10 0.231 10,362.0H5,094 0.12 0.338 10363.0 0.17 0.468 10,364.0 0.22 0.662 10,365.0 0.25 0.894 10,366.0 0.29 1.150 All other value:are inte-oolated'rom this table. "Value:aoa.e 10365 0 are e.traaolated 'From GIS contour shapefile • Precipitation. Precipitation estimates for this study were developed using MetPortal 2.2.0 (MetPortal, 2019). MetPortal guidance recommends a point precipitation frequency estimate for basins smaller than 50 square miles. Only two storm types to govern peak runoff behavior were considered due to the small size of the basin: 1. 100-year Local Storm, 2-hour duration, 1.41-inches precipitation with a "Synthetic West"temporal distribution; and 2. 100-year Mesoscale with Embedded Convection (MEC) Storm, 6-hour duration, 1.69- inches precipitation with a "Synthetic West" temporal distribution. Longer duration storms were not considered for this study as they result in smaller peak inflows into the basin.The seasonality of the storm was considered to determine if the precipitation would fall as rain or snow. From the MetPortal guidance, both the 2-hour and 6-hour storms are most likely to occur in July or August(MetPortal, 2019). It was conservatively assumed the precipitation would fall as rainfall since summer temperatures at Climax are typically above freezing.