2016-03-25_REVISION - C1981012 (2)

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Response to DRMS (MR -14) 1. DRMS: In Exhibit 15 - Subpart H of the submittal there are three tables of inputs for the universal soils loss equation (one table for each outfall). The C factor in these tables is called "cropping management factor" rather than "cover management factor". Most textbooks describing the methodology for the RUSLE use the term "cover management factor". Please explain. Lorencito: Technical Note #50 (revised) was used in conjunction with SedCad to derive "C" factors. In that Note, the "C" factor is defined as a cropping management factor. However, a quick literature review of the "C" factor determined that DRMS is correct, it is more commonly referred to as the "cover management factor" and therefore has been revised. Please see revised pages 4-9. 2. DRMS: Please elaborate on how the existing Exhibit 15 was used to develop the C values in the universal soils loss equation analyses. Where does the value of 0.003 originate? Was vegetation sampling data used to develop this value? If not, please explain why. Lorencito: The 0.003 value was derived using SedCad, by using the total cover as presented in Table 10. Table 10 values make assumptions based on a vegetation study (reference areas) and professional judgment. However, the value that should have been used for pre -mining is 0.06 for the Oak Pinyon vegetation type. Additionally, upon closer inspection of aerial photos from Google Earth, the percent cover for existing conditions is closer to 60-80% for the current configuration, which would change the C factors used. When available, data from a vegetation study is used for determining percent cover and/or other factors. However, it is not necessary or required to obtain a vegetation study for estimating percent cover. It is common practice to use the best engineering/professional judgment to make that determination. Please see revised "C" factors on pages 4-9. 3. DRMS: Please elaborate on how the LS values were determined. The best way to do this is to provide topographical maps showing the locations of where lengths and slopes were calculated. If maps are provided, some examples would suffice, a map for every analysis is not necessary. Lorencito: Please see the attached figure and description in the Subpart H write up. Google earth data was used for the existing configuration, and a hard copy of a topo map was used for the pre -mining condition. The methodology was the same for each configuration. A "LS" derivation is attached, but is intended for review purposes only. Additional Changes: The Subpart H demonstration has been revised to further explain the derivation of the "LS" factor as well as the "C" factor, and the entire document has been page numbered. Attached is revised Exhibit 15.