2013-10-30_REVISION - M1977310 (3)

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JD -6 Mine — Drainage Design Plan 8 The Maximum Permissible Velocity of 7 ft/sec is applicable to earthen channels to prevent excessive erosion and scour. Velocities of 7 to 13 ft/sec are acceptable for rubble -lined channels, 20 ft/sec for concrete channels, 12 to 15 ft/sec for smooth and rough asphalt channels, respectively, and 30 ft/sec for gunite (sprayed concrete) sections. At the JD -6 Mine, channels that are excavated into bedrock are similar to rubble -lined or concrete channels, and higher permissible velocities may be acceptable in specific sections or reaches of the channel. Erosion and sedimentation have less effect on fisheries and macroinvertebrates in drainages at the toe of Monogram Mesa than at other locations, because surface water is intermittent or ephemeral and there are no perennial streams (or "live streams ") as receiving waters. 2.3.3 Culverts Values for hydraulic headwater, tailwater, barrel flow rate and barrel velocity were determined using methods and parameters provided in the Federal Highway Administration (FHWA) document HEC 22 (FHWA 2009) and modeled in the FHWA software program HY -8 Version 7.2 (FHWA, 2012). Although these design calculations indicate that smaller culverts could provide the carrying capacity required in some sub - basins, all culverts installed by Cotter will be 18 -inch diameter minimum to reduce the potential of plugging with wind -blown debris and animal denning activities. 2.3.4 Retention Pond and Spillway The stormwater retention pond has been evaluated to determine its ability to contain the 10 -year 24 -hour runoff volume with a minimum of 1.0 ft of freeboard, with freeboard reduced to 0.5 ft while passing the 100 yr 24 -hour event. A broad crested weir, as described in HEC 22, chapter 8 (FHWA, 2009), was used to establish spillway design parameters and flow through the pond and spillway was evaluated using HydroCAD& 2.3.5 Riprap / Granular Bedding / Gravel Mulch Channel protection requirements, riprap sizing and riprap grading specifications for channels were evaluated using HEC 15 (FHWA, 2005). Riprap sizing for the spillway was evaluated using the U.S. Bureau of Reclamation Simplified Design Guidelines for Riprap Subjected to Overtopping Flow (Frizzell, et al., 1997). The channel analysis was conducted for the major stormwater conveyance structures (diversion ditches) using the depths, widths, and side slope ratios established for each feature. The analysis determined that most channels at the JD -6 site do not require riprap, granular bedding, or gravel mulch (Section 5.3). Where riprap is required, the riprap installed in channels will meet the criteria of HEC -15 (FHWA, 2005), including but not limited to: • The ratio of sizes of 13100/1350 and D50/D20 shall fall between 3.0 and 1.52. • Placed thickness should equal the diameter of the largest rock size in the gradation. • Material shall be durable and angular but not slabby (length:thickness < 3). • Riprap shall extend 1.0 ft above design flow level (i.e, 1 -ft freeboard). All riprap will be installed with a granular (Type I) bedding or filter fabric below Type 11 bedding to protect underlying soils per Urban Drainage and Flood Control District specification for gradation and thickness ( UDFCD, 2011) as shown in Attachment 1. For drop structures, the UDFCD (2011) specifications allow a combination of filter fabric and Type 11 bedding as an acceptable alternative to a 2 -layer filter (bedding). 2 The largest rip rap size (DI00) shall be no larger than 3 times the median particle size (D50) and no smaller than 1.5 times the median particle size (D50). Similarly, median particle size (1)5o) shall be no more than 3 times and no less than 1.5 times the Dzo size. This specification assures an appropriate size gradation for erosion control. 4148A.131028 Whetstone Associates •