Laserfiche WebLink
1. Introduction <br />A complete inventory of all surface water control structures has been made and a complete new <br />list of culverts, collection ditches and diversions has been made for the permit area. Some <br />culverts are new, others have been redesigned to account for a more accurate drainage area, and <br />others have been removed from past permit text because they no longer exist on the site. <br />This inventory has the latest topography for the site and all designs east of 2700 Road show the <br />existing reclaimed topography, while the area west of 2700 Road shows the proposed postmining <br />topography. <br />Map 2.05.3(3)-1, Surface Water Hydrology shows the current features of the surface water <br />hydrology including sediment ponds, collection ditches and diversions, and culverts. This map <br />also shows alternate sediment control features such as straw wattles and small are exemptions. <br />Map 2.05.3(3)-1-2, Post Mine Surface Water Hydrology does not show sediment ponds, <br />collection ditches and diversions, and culverts since they will not be post mining structures. <br />However, this map does show the drainage basins for ditches and culverts east of 2700 Road. <br />The drainage basins that were used to design sediment ponds, collection ditches and diversions, <br />and culverts west of 2700 Road are shown in appropriate attachments of the PAP. <br />2. Parameters for Designs <br />Temporary structures have been designed for the 10 year 24 hour event, using a Type II storm. <br />Permanent structures have been designed using a 100 year 24 hour event, Type II storm. <br />Historical SCS Curve Numbers for the New Horizon Mine, as shown on Table 2.05.3(3)-1 were <br />used for designs conducted prior to October 1, 2015. After October 1, 2015, the SCS Curve <br />Numbers shown in Table 2.05.3(3)-lA have been used for designs. All times of concentration <br />and peak flows were determined from the SCS method using Carlson Software. <br />The basic design for all ditches assumes trapezoidal cross-section with 2H:1 V sideslopes, <br />constructed of earth with some vegetation, resulting in a manning's n of 0.0275. If portions of <br />ditches along hillsides encounter durable rock, the widths may be reduced since no erosion will <br />occur on the bottom. See applicable ditch designs for dimensions of individual ditches. The table <br />below shows the rainfall for important frequency events at the mine site. <br />Design 24 Hour Event <br />Rainfall (inches) <br />10 <br />2.0 <br />25 <br />2.4 <br />100 <br />3.0 <br />Attachment 2.05.3(3)-1 Page 5 March 2016 (TR -70) <br />