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2011-11-28_REVISION - M2005050
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2011-11-28_REVISION - M2005050
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Last modified
6/16/2021 6:05:23 PM
Creation date
12/8/2011 2:04:42 PM
Metadata
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Template:
DRMS Permit Index
Permit No
M2005050
IBM Index Class Name
REVISION
Doc Date
11/28/2011
Doc Name
Response to Division Comments TR-02
From
Rimrock Exploration and Development inc
To
DRMS
Type & Sequence
TR2
Email Name
GRM
Media Type
D
Archive
No
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Response to Division M- 2005 -050 TRO2 Review Letter of September 26, 2011 <br />Attachment 4 - Surface Water Management and Controls Analysis <br />Division Comment 5) (Attachment 4 - Comment 1): The introduction on the first page of <br />Attachment 4 states the calculations for the pond /weir sizing is performed using TR -55. <br />However, the calculations on this page clearly use the rational method, not TR -55. The <br />rational method is used for peak flow estimates and it appears, detention basin sizing. The <br />Division requirement for stormwater control is based on a 24 -hour design storm event (10- <br />and 100 -year recurrence intervals). The rational method does not utilize 24 -hour <br />precipitation; it is solely based on the estimated storm intensity for the related sub basin <br />time of concentration. Furthermore, runoff volume calculations using the rational method <br />are unreliable and again do not account for a 24 -hour design storm. Please resubmit the <br />peak flow and volume calculations using the 24 -hour design events and TR -55 <br />methodology as alluded to in the introduction. <br />Response - In discussion with the Division's hydrologist in 2008 at the time drainage <br />calculations were originally performed (pond volume and flow calcs) it was agreed <br />that the TR -55 method was more suited for rural areas, but that given the higher <br />volumes obtained using the rational method, it was a more conservative approach <br />and therefore acceptable. The site calculations have been re -run using the <br />requested TR -55 and TR -20 method and the results are included. As a further note, <br />while peak 100yr flows were used in the weir and riprap sizing calculations, running <br />a time vs. volume analysis shows that at the time of discharge from the ponds, the <br />flows will be significantly less - dropping to about 3.5 cfs for the lower pond and 1 <br />cfs at the upper. <br />Division Comment 6) (Attachment 4 - Comment 2): The introduction also states the hydrologic <br />soil group (HSG) is "D ". This is inconsistent with the Soil Survey Map (Exhibit T, Figure 3) <br />submitted in the original EPP in 2009. The soil map unit on Figure 3 is primarily <br />108/Wrayha stony clay loam, which according to TR -55 is HSG "C ", In conjunction with the <br />response to Comment 1, please provide rationale for each HSG and SCS /NRCS curve <br />number used for the TR -55 analysis. Please keep in mind the vegetation and origin of the <br />material when selecting curve numbers, especially for unvegetated waste rock, topsoil, <br />and ore piles. <br />Response - Having previously researched the site characteristics in 2008, I neglected to <br />check the 2009 EPP for the current soil map characteristics. Upon cross - checking <br />this with the NCRS WebSoil service, the 2009 EPP is correct that the site is currently <br />classified as 108/Wrayha stony clay loam. However, Websoil also places this in the <br />hydrologic soil group (HSG) 'D.' My copy of TR -55 does not have any supplements <br />to contradict this. Therefore, as HSG class 'D' soils are more subject to runoff 1 feel <br />it appropriate to retain that in calculations. Per your request `supporting' <br />documentation is being submitted (including a copy of a current soil map). <br />Page 2 of 2 <br />
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