2014-02-07_REVISION - M1979094HR (5)

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JD -7 Mine — Drainage Design Plan 15 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 -7 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 for the three existing culverts at the site (CV1, CV2 and CV3, Figure 11), using methods and parameters provided in the Federal Highway Administration (FWHA) document HEC 22 (FWHA 2009) and modeled in the FHWA software program HY -8 Version 7.2 (FWHA, 2012). The results of the culvert capacity analysis for the current condition are provided in Section 5.6. For the planned condition, the area draining to each of these culverts will be the same or smaller, so it was not necessary to re- analyze culvert designs for the planned condition. This drainage design plan does not contain recommended locations for any new culverts. However, if the need for new culverts is identified in the future, any new culverts installed by Cotter will be designed with adequate capacity to convey the 100 -year 24 -hour storm and will be 18 -inch diameter minimum to reduce the potential of plugging with wind -blown debris and animal denning activities. 2.3.4 Retention Ponds and Spillways The stormwater retention ponds have been evaluated to determine their ability to contain the 10 -year 24- hour runoff volume while passing the 100 yr 24 -hour event with a minimum of 1.0 ft of freeboard. For the proposed stormwater retention pond at the toe of the expanded open pit waste rock pile, spillway design parameters were established according to HEC 22, chapter 8 (FHWA, 2009) design specifications for broad crested weirs. Flow rates and velocities over the spillway were evaluated in HydroCAD using the characteristics of a trapezoidal weir. 2.3. S Riprap / Granular Bedding / Gravel Mulch Channel protection requirements, riprap sizing and rip rap grading specifications were evaluated based on the channel velocities calculated using WinXS Pro software and spillway velocities calculated using HydroCAD. Based on the channel velocity and scour, the HEC 15 guidance (FHWA, 2005) was followed to determine the requirements for bed material size. The analysis was conducted for the major stormwater conveyance structures (diversion ditches) using the depths, widths, and side slope ratios established for each feature. HEC -15 (FHWA, 2005) gives the following design recommendation for rip rap lined channels: • The ratio of sizes of D100/D50 and D50/D20 shall fall between 3.0 and 1.5Z. • Placed thickness should not be less than 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 x flow depth above flow level. 2 The largest rip rap size (13100) 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 (1350) shall be no more than 3 times and no less than 1.5 times the D20 size. This specification assures an appropriate size gradation for erosion control. 4149A.140207 Whetstone Associates