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Flatirons, Inc. — Surveying & Engineering <br />The Hydraflow Express Extension for AutoCAD Civil 3D 2013 was used to design the <br />spillway. The top of the emergency spillway is designed to handle twice the 100 -year peak <br />- flow and not exceed 0.5' in depth across the top of the emergency spillway. Riprap <br />protection will be added to the top of the emergency spillway and the exterior slope of the <br />TSF Spillway Chute channel from the top of the emergency spillway to the toe of the slope. <br />The riprap placed on the emergency spillway and both diversion channels will be underlain <br />by non -woven geotextile fabric. <br />The Hydraflow Express Extension for AutoCAD Civil 3D 2013 was used to model the weir <br />condition of the of the check dams in the South TSF Contact channel . <br />3.0 DRAINAGE BASINS AND SUB - BASINS <br />Maior Basin Description <br />The property is located in the California Gulch Basin approximately 1 -mile upstream of the <br />confluence with the Arkansas River. The area has a history of mining and milling activity <br />dating back to the 1800s. The surface runoff directed around and through the site will flow <br />towards the south until it reaches the California Gulch channel, where it will flow toward <br />the west and eventually to the Arkansas River. <br />The Union Mill property is not located in a Flood Hazard Area according to the FEMA <br />Food Insurance Rate Map; Community Panel No. 080282 -0004 A, dated March 1, 1998. <br />Basin Descriptions <br />The onsite drainage is defined by two basins. The onsite basins are designated A and B in <br />this report. The offsite drainage was divided between three basins in the original Drainage <br />Report and labeled OS1, OS2, and OS3. However, after the DRMS review, Basin OS1 was <br />combined with Basin OS2, and Basin OS 1 was eliminated from the Drainage Report. The <br />flows Basins OS2 and OiS3 join near the northeast corner of the property and flow through <br />Basin B in a southeasterly direction. <br />Basin A encompasses 3.96 acres and is located in the northwest portion of the site and <br />includes the milling facilities and TSF. All runoff from Basin A is directed to the TSF <br />where it will be retained onsite. Basin A encompasses the impacted areas of the property <br />and is assigned a RCN of 91 which estimates the runoff from an industrial site. There is no <br />offsite runoff conveyed through the basin, and all runoff from the basin will be directed to <br />the TSF and retained onsite. <br />Union Mill 7 March 5, 2014 <br />