2001-05-16_REVISION - M2000158 - Laserfiche WebLink

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Memo to Erica Crosby 4 May 15, 2001 Application, the Division will conduct a thorough inspection of the current riverbank condition for the reach that runs throughout the extent of the proposed operation. The purpose of the inspection will be to determine the extent and location(s) of pre-mining riverbank armoring that will be required. Riprap Specifrcatioi:s and Drawings The TR specifies that all areas to be riprapped will have a minimum Dso equal to 24 inches and will be placed to a minimum thickness of D50x 1.5. The Division has evaluated the adequacy of [he proposed DSO specification and has concluded that the maximum water velocities that can be anticipated at the S&H Mine during a 100-year flood will not lift and move ordinary riprap with a 24 inch DSO. Hence, the D;O specification proposed by the applicant is acceptable. The Division's analysis of the Dso specification was completed using riprap design software produced by West Consultants, Inc., Carlsbad, California. A summary of the analysis will be included in the permit application file. Although the Applicant has provided an acceptable DsO specification for the riprap, information on the gradation of the riprap was not included in the TR. The Division's published guidance includes a suggested gradation specification for 24-inch DSO riprap (DMG, 1998, table D-1, copy attached). The Applicant may either commit to using the suggested riprap gradation specification or provide an alternate gradation proposal with technical justification for the Division's review. As stated above, the Applicant's proposed riprap thickness for both the pit slope and riverbank armor is DSOx l.5 or 3 feet. The Division's published guidance recommends riprap thickness of Dsox2.0 for pi[ slope armor and a variable thickness of D50x 1.75 to DSOx 3.0 for riverbank armor (DMG, 1998, Figure D-5, copy attached). The Applicant will need to either commit to using the suggested riprap thickness specification or provide an altemate thickness proposal with technical justification. Also, the Applicant's drawing of proposed pit slope armor shows the base of the riprap keyed in at ten feet below [he anticipated water line or 15 feet below the ground surface. It is our understanding that the pit excavations will range from 45-60' in depth. Also, the designs for riverbank armoring do not show that the armoring is keyed in. Therefore, the cross-sectional designs as presented are not acceptable. Figure D-5 of the Division's guidance suggests that the base of the riprap should be keyed in to at least three feet below the river invert elevation. The Applicant will need to either commit to following the design specifications provided in the Division's attached guidance document or provide a technical justification for their proposal. The Division's specification for the key depth results in little practical difference in the base elevation of the pit slope riprap in this instance. This is because the invert of the river appears to be about 12 feet below the original ground surface in the area to be mined. The riverbank armor installation plan depicted in the TR shows riprap ending at two fee[ below the rivers normal water level. The Division's guidance recommends that riverbank armor be keyed five feet below the river bed or, alternatively, extension of a thicker (doubled) layer of riprap into the channel bed (DMG, 1998, Figures D-5 and D-8, copies attached). The Applicant will need to commit to one of these toe protection options or provide technical justification for their proposed design.