2009-05-19_REVISION - M1977285 (31)

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Section 2 Topaz Mine To determine the channel capacity, CDM performed a Manning's equation analysis. Flow depth and velocity were calculated using the following equations for the 100- year peak discharges, cross section geometry (rectangular), and Manning's n values listed in Table 2-2. Q=VA A = (b + zh)h V =1.49 _(b+zh)h 2/3 Sul n b+2h 1+z2 where: Q = discharge in cfs V = velocity in feet per second (ft/sec) z :1 = side slope A = flow area in square feet (W) b = bottom width of the cross section in feet (ft) h = flow depth in ft S = channel slope in feet/feet (ft/ft) n = Manning's roughness coefficient (unitless) • Table 2-2 Innut Values for Channalc I and 9 CanaA#w Anal-1. Channel 100- r Q S n z b cfs ft/ft ft/ft ft 1 5.79 0.025 0.035 3 3 2 4.25 0.037 0.035 3 3 The calculated channel depths and velocities of Channels 1 and 2 are shown in Table 2-3. CDM recommends adding 2-feet of freeboard to the calculated flow depth to account for potential channel overtopping. Given rectangular channels with bottom widths of 3 feet each, a design depth of 2.5 feet is recommended for Channels 1 and 2. Table 2.3 Calculation Summary fnr Channalc 1 and 9 Channel h Freeboard esi n De th V ft ft ft ft/sec 1 0.43 2 2.43 3.16 2 0.32 2 2.32 3.30 To determine the appropriate channel lining (e.g., vegetated or riprap) for Channels 1 and 2, CDM calculated flow velocities using information from the Urban Drainage and Flood Control District (UDFCD) Drainage Criteria manual (UDFCD 2005). UDFCD recommends flow velocity be less than 5 feet per second (ft/sec) for grass- lined channels and velocities less than 12 ft/sec for riprap channels (UDFCD 2005). Based on UDFCD remaunenda*ins, a Mamdnes n value of 0.035 was selected for design calculations to check the capacity of grass-lined channels. Based on these 2-3 0484986 - Denis Walk Order 1 - Sunday Mims Drainage StudyWWporllfi ml RepodVFkml Sunday Mines Drainage Report 052208.doc