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i ~ <br />mitigation measures will be employed. Lesser setbacks may be used if armoring or spillways are <br />installed to protect the setback (DMG, 1998). On smaller rivers, setbacks of 100 feet or less may be <br />sufficient. The Line Camp Pit application specifies a minimum setback of approximately 200 feet at <br />the southeast corner of the south pit, with most of the pit areas being setback from the riverbank <br />considerably more than 200 feet. The Division analysed these proposed setbacks and determined that <br />they are adequate, with the basis for this determination being the shallow depth and low velocity of <br />flooding in and around the excavated areas during a 100-year flood, as detailed below. <br />The 100-year flood on the Dolores River has been rigorously described in a document prepared by <br />the Army Corps of Engineers (USAGE, 1978). Water surface elevations of the 100-year flood <br />through the Line Camp Pit area are illustrated on plates included in the Army Corps' study. Depth <br />of water in the channel and adjacent flooded areas can be determined from the profiles. The Division <br />of Minerals and Geology has completed an evaluation of 100-year flood depths at five critical <br />locations in the vicinity of the Line Camp Pit. These locations are marked and numbered on the <br />attached map, which was modified from a map included with the permit application. <br />Location No. 1 is at the north or upstream end of the proposed excavations. At this location, the <br />base flood elevation is 7,225 feet and the mining setback from the riverbank is approximately 570 <br />feet. The floodwater depth is one foot, at a point 200 feet in from the riverbank, and on a line from <br />Location No. 1 to the closest point on the riverbank. At Location No. 1, the floodwater depth will <br />have tapered to zero. Water from the 100-year flood is not likely to spill into the pit at this location. <br />Location No. 2 is the northernmost or most upstream location where the 100-year floodplain and the <br />proposed pit clearly intersect. Floodwater can be expected to begin spilling into the pit at this point. <br />Since the floodplain boundary is at this location, the depth of the floodwater will have tapered to <br />zero. <br />Location No. 3 is at the north end of the northeast lobe of the proposed south pit. This point is well <br />within the floodplain, and it can be expected that floodwater flowing approximately north to south <br />will be spilling into the pit at this location. The depth of floodwater at this point will be <br />approximately one foot and the distance from this point to the riverbank is approximately 400 feet. <br />Location No. 4 is to the south and east of Location No. 3 and is closer to the riverbank. Intuitively, <br />based on its location at the comer of a proposed pit in the floodplain in the direct Gne of anticipated <br />flood flows, this would appear to be the most likely point where the river channel could be redirected <br />into the pits. However, the flood depths at Location No. 4 taper to near zero; at the most, the depths <br />at this location are substantially less than one foot. <br />The final location evaluated, Location No. 5, is at the southeast comer of the proposed south pit. <br />Since it is at slightly less than 200 feet, Location No. 5 is the closest approach of the proposed <br />excavations to the riverbank. The base flood elevation at this point is 7,217.5 feet, and the depth of <br />floodwater will be less than 1.5 feet. <br />The Army Corps of i~pgineers stud~eslimates that the velocity of flow d ng a 100-year event on <br />-- the Dolores and West Dolores Rivers would average approximatel 7 t feet per second in the <br />channel, and 4 feet per second in the over bank areas (USAGE, 1978). ven that the 100-year flood <br />2 <br />