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Geoff Niggeler <br /> September 4, 2018 <br /> Page 2 <br /> the governing 100-year average recurrence interval (ARI) rainstorm (24-hour duration) under both <br /> existing and projected life of mine (LOM) conditions. However, the primary 1 Dam SDA does not <br /> have sufficient capacity to store seepage and runoff from either a 100-year ARI snowmelt event <br /> or a 3-hour duration summer PMP storm. Henderson has selected the criteria of detaining the <br /> 100-year ARI snowmelt under life of mine conditions, approximately 68 acre-feet, by raising the <br /> Seep Road to a minimum elevation of 8591.9 feet (without freeboard allowance). <br /> Design Basis <br /> Henderson has contracted with Wheeler to provide a civil design for construction of the SDA Seep <br /> Road raise of approximately 2.5 feet. The current storage capacity of this area is 27.6 acre-feet <br /> at a low point roadway elevation of 8589.1 feet. The average elevation of the top of the roadway <br /> is approximately 8590 feet,with a storage capacity of 40 acre-feet. The planned raise of nominally <br /> 2.5 feet will result in a top of Seep Road elevation of 8592.5 feet to the provide adequate available <br /> storage capacity to accommodate the 100-year snowmelt event under life of mine conditions with <br /> about 0.5 feet of freeboard. The storage capacity at an elevation of 8592.0 is approximately 70 <br /> acre-feet, which also provides about 2.5 times more emergency storage space for upset <br /> conditions. <br /> The raise is expected to consist of compacted soil placed on top of and on the downstream side <br /> of the existing Seep Road. The nominal side slope of the Seep Road raise is two horizontal feet <br /> for every one vertical foot. The average existing Seep Road width of 35 feet will be preserved or <br /> extended with the raise as needed for wider road sections. The raise will tie into the existing road <br /> at the north end of the SDA as the road climbs up the left abutment of 1 Dam. The raise will <br /> transition down to the existing seep road surface on the south end of the SDA. The transition will <br /> occur approximately 150 feet north of the intersection with the right abutment roadway of 1 Dam. <br /> At this location, there is an existing roadside berm that will be rebuilt to continue the impoundment <br /> up to the design elevation. The roadside berm is anticipated to extend approximately 250 feet and <br /> will tie into the edge of the right abutment road. <br /> Existing gravel road base material will be stripped up to a depth of approximately 2.5 feet below <br /> the existing seep road surface and 0.5 feet of depth will be stripped on the downstream slope and <br /> toe for topsoil removal. Subgrade preparation is required prior to fill placement. The embankment <br /> fill materials for the raise will come from onsite borrow sources. Approximately 15,000 bank cubic <br /> yards of fill is estimated to complete the project, of which 2,000 cubic yards is estimated as <br />