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Flatirons, Inc. — Surveying & Engineering <br />Basin B is an onsite basin located to the east of Basin A, and south and west of the roads <br />that serve the mill buildings. Water flowing through the basin is kept separated from Basin <br />A. The basin is covered by mature trees with a poor understory, and portions of two gravel <br />roads traverse the basin. Offsite Basins OS2 and OS3 are conveyed through the basin. <br />Basin B is not impacted by the milling facilities. The basin contains 4.08 acres and has a <br />weighted RCN of 79. <br />Basin OS2 is 13.17 acres and situated to the north of the mill site. Runoff from the basin is <br />captured by the ditch on the upper road to the mill building and conveyed to the east where <br />it will be combined with Basin B and Basin OS3. There are no improvements associated <br />with the mill in the basin. A mature forest with an understory in poor condition covers the <br />entire area. Basin OS2 was assigned a RCN of 77. <br />Basin OS3 is located at the eastern portion of the property. The area is covered with <br />mature forest with a poor understory. A natural channel captures the runoff from the basin. <br />The storm water is then combined with Basin OS2, and directed to Basin B. Basin OS3 <br />will be left undisturbed. Basin OS3 contains 1.47 acres and was assigned a RCN of 77. <br />Sub -Basin Description <br />The three delineated sub - basins in the report are basins designated al, a2, and bl. The sub- <br />basins are part of the larger basins and were established to calculate peak 100 -year flows <br />for specific areas within the larger basins. The peak flows and runoff volume are then used <br />to calculate the cross section of open channels and sedimentation volumes. <br />Basin al was created to calculate the peak flow rates and runoff volume for the area on the <br />outside western slope of the TSF. The runoff volume from the basin was used to establish <br />the sedimentation required for the contact area. The basin covers 0.24 acres and was <br />modeled as grass open space in fair condition. The basin was assigned a RCN of 79. <br />Basin a2 was created to calculate the 100 -year peak flows for runoff being directed by a <br />berm to the TSF at the northwest corner of the TSF. The north one -third of the basin is <br />covered by the mill and is impervious. The southern two- thirds is the lower flat surface <br />between the mill and TSF on the south side of the mill. The impervious area was assigned <br />a RCN of 98, and the lower flat surface was modeled as gravel road for soil group C with <br />and RCN of 89. The 0.29 acre basin has a composite RCN was 92. <br />Basin b was created to calculate the peak flow rates and runoff volume for the area on the <br />outside slope of the southern and eastern sides of the TSF. The runoff volume from the <br />Union Mill 8 March 5, 2014 <br />