2012-11-13_REVISION - C1981019 (2)

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COAL COMPANY •per S- raoavcsiosi 1024 2017', and follow the model to Structure No. 32. A grass lined channel will suffice for this section of ditch. • In Taylor Ditch, rock needs to be moved from the middle of the channel to the sides in some areas. In other areas (near the bottom) riprap is sparse in the middle of the channel, and little to no vegetation has grown in the channel. Response: The rock in Taylor Ditch was moved from the middle of the channel to the sides as noted above. The entire Taylor Ditch was originally modeled as a single channel segment with rip -rap lining. It has been re- evaluated and modeled as a vegetative lined channel. It was found to have acceptable peak velocity (4.15 fps) for a vegetative lined channel. Refer to the attached SEDCAD output "Taylor Ditch 1024 2017 ". A grass lined channel will suffice for this section of ditch. • In Taylor Tributary Ditch, there are areas with sparse riprap in the middle of the channel. However, this ditch is well vegetated. Response: As noted in the Division's inspection report dated October 16, 2012, Taylor Tributary Ditch contains riprap areas and has established vegetation. That being said, Colowyo believes that Taylor Tributary Ditch is very stable and erosion is and has not been an issue. The ditch is not entirely void of riprap, but it is, as stated, sparse in some locations. Colowyo has taken several different approaches with SEDCAD to determine the output velocities if the ditch is vegetated or riprap. Within SEDCAD the structure in question is Structure #15. This structure was modeled as a vegetated lined channel (smooth brome) for a 100 year, 24 hour event and results indicated a velocity of 12.6 fps. SEDCAD does not output a velocity when using a riprap lined channel; however, assuming a Manning N of 0.035 for rock, at velocity of 10.9 fps is indicated for a riprap lined channel. Colowyo also modeled Structure #15 as a 10 year, 24 hour event. As stated in Colowyo's PAP, "Although portions of Taylor Tributary Ditch could be designed for the 10 year, 24 hour event, the uppermost portions that overlie out -of -pit spoil areas and the lower most reach which has a watershed larger than 1 square mile after the confluence with East Taylor Ditch require design for the 100 year event. For simplicity, the entire Taylor Tributary Ditch has been designed for the 100 year, 24 hour event." Colowyo believes that Structure #15 is a perfect location to model for a 10 year, 24 hour event as this area is not located on a valley fill and is not A Western Fuels - Colorado, LIE mining property