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2012-07-03_REVISION - M1984014
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2012-07-03_REVISION - M1984014
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Last modified
6/15/2021 2:25:42 PM
Creation date
7/11/2012 3:05:11 PM
Metadata
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Template:
DRMS Permit Index
Permit No
M1984014
IBM Index Class Name
REVISION
Doc Date
7/3/2012
Doc Name
RESPONSE TO ADEQUACY REVIEW #2
From
COTTER
To
DRMS
Type & Sequence
AM1
Email Name
GRM
Media Type
D
Archive
No
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JD -8 Mine — Drainage Design Plan 6 <br />2.3 Run -on / Run -off Peak Discharge Estimates <br />Peak discharge from each sub -basin area was estimated using the Natural Resources Conservation Service <br />(NRCS) graphical peak discharge method (TR55) developed by the U. S. Department of Agriculture for <br />estimating surface runoff (MRCS, 1986). <br />This method graphically generates a unit peak discharge rate (Q„) based upon the sub -basin time of <br />concentration (TO, curve number (CN) and associated initial abstraction (I precipitation (P) and rainfall <br />distribution type (type II for Colorado). Peak discharge (Q for the sub -basin area is then calculated from: <br />Qp = QuAmq <br />where: Q = Peak discharge <br />Q„ = Peak unit discharge for the appropriate time of concentration (TO <br />Note that T, includes Travel time for sheet flow (T + shallow concentrated flow (TO + <br />channel flow (T <br />A = Sub -basin area in square miles <br />q = Runoff in inches <br />2.4 Channel Design <br />Channel capacity was evaluated with a single section analysis using Manning's Equation applied to a <br />trapezoidal shape per the Colorado Department of Transportation Drainage Design Manual (CDOT 2004). <br />Total depth has been specified at 2.0 x flow depth to maintain sufficient freeboard. <br />2.5 Culvert Design <br />Values for hydraulic headwater, tailwater, barrel flow rate and barrel velocity were determined using <br />methods and parameters provided in the Federal Highway Administration (FWHA) document HEC 22 <br />(FWHA 2009) and modeled in the FHWA software program HY -8, Version 7.2 January 17, 2012. <br />Although these design calculations indicate that smaller culverts could provide the carrying capacity <br />required in some sub - basins, all culverts installed by Cotter will be 18 -inch diameter minimum to reduce <br />the potential of plugging with wind -blown debris and animal denning activities. <br />2.6 Spillway Design <br />A broad crested weir, as described in HEC 22, chapter 8 (FWHA 2009), was used to establish spillway <br />design parameters with additional weir coefficient data obtained from HDS #5, pg 38 (FWHA 2001) and <br />the Urban Storm Drainage Criteria Manual (UDFCD 2011). All retention ponds have been designed to <br />contain the 10 -year 24 -hour runoff volume with 1.0 ft of freeboard. This allows for 0.5 ft of spillway <br />headwater with 0.5 ft of remaining freeboard whilst passing the 100 yr 24 -hour event. <br />2.7 Riprap / Granular Bedding / Gravel Mulch <br />Channel protection requirements and riprap sizing were established from HEC 15 (FWHA 2005), as were <br />rip rap grading specifications. <br />Riprap will meet the criteria of HEC -15 (FWHA 2005), including but not limited to: <br />• Gradations will fall in the range of D100 /D50 and D50 /D20 between 3.0 to 1.5. <br />• Placed thickness should equal the diameter of the largest rock size in the gradation. <br />• Material shall be durable and angular but not slabby (length:thickness < 3). <br />• Riprap shall extend 1.0 x flow depth above flow level. <br />4122A.120702 Whetstone Associates • <br />
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