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<br />M-2000-158 TR Memo 3 Mav 17, 2001 <br />riprap thickness at the ends of an armored reach (DMG, 1998, page D-3 and Figure D-7, copies attached). The <br />Applicant must commit [o following the Division's guidance for end protection or provide an alternate end <br />protection design for the Division's review. <br />The TR application includes no commitment to install riverbank armor, but describes the possible installation of <br />riprap along the riverbank if required by any of the permits issued for the proposed mine or at the option of the <br />Applicant, The Division's published In-Stream Aggregate Extraction and Reclamation Guidance Document <br />recommends that riverbank armor be installed with due consideration of the potential for offsite impacts that <br />resultant channelization of the stream caused by the armoring may cause (DMG, 1998, page D-3, copy attached). <br />Armoring of a riverbank is generally installed through a zone that is likely to experience lateral migration of the <br />stream, typically the outside bank of a curve. Review of the S&H Mine pre-mining maps indicates that there are <br />several such locations within the mine reach. In particular, the Division is concerned with the eastward bend in <br />the river at the proposed Meadow Lake, the location where the river curves from a due north flow path to a <br />northwest Flow path at Sharkey's Lake, and the bend in the river west of the north-south axis of Longhorn Lake. <br />Riprap Armoring through these bends in the river be required prior to mining within 400 feet of the riverbank. <br />The reach length of the armoring to be required at the river bends may be determined by quantitative analysis of <br />scour potential during a 100-year flood, or by inspection of the extent of existing cut banks. The Division is also <br />concerned with the identification of any straight reaches of river channel where mining will encroach within 400 <br />feet of the river bank. Armoring of these reaches may also be required prior to mining within 400 feet of the river <br />bank unless the applicant can provide sufficient rationale and demonstration that such bank areas are currently <br />stable and will remain so during a 100-year flood event. It is suggested that a thorough inspection of the current <br />riverbank condition by the Applicant and submittal of a map that identifies all potentially unstable areas {to be <br />stabilized prior to mining within 400 feet of the river bank) is an effective procedure for determining the extent of <br />pre-mining riverbank armor that will be required. Such a map will be subject to Division approval. <br />Riprap Specifrcations and Drawings <br />The TR specifies that all areas to be riprapped will have a minimum DSU equal to 24 inches and will be placed to a <br />minimum thickness of Dsox 1.5. The Division has evaluated the adequacy of the proposed DSO specification and <br />has concluded that the maximum water velocities that can be anticipated at the S&H Mine during a 100-year <br />Flood will not lift and move ordinary riprap with a 24 inch DSO. Hence, the DSO specification proposed by the <br />applicant is acceptable. The Division's analysis of [he Dso specification was completed using riprap design <br />software produced by West Consultants, lnc., Carlsbad, California. A summary of the analysis will be included in <br />the permit application file. Although the Applicant has provided an acceptable DSU specification for the riprap, <br />information on the gradation of the riprap was not included in the TR. The Division's published guidance <br />includes a suggested gradation specification for 24-inch Ds,fiprap (DMG, 1998, table D-l, copy attached). The <br />Applicant may either commit to using the suggested riprap gradation specification or provide an altemate <br />gradation proposal with technical justitcation for the Division's review. <br />As stated above, the Applicant's proposed riprap thickness for both the pit slope and riverbank armor is Dso I.5 <br />or 3 feet. The Division's published guidance recommends riprap thickness of DSO 2.0 for pit slope armor and a <br />variable thickness of Dsdc 1.75 to Dsox 3.0 for riverbank armor (DMG, 1998, Figure D-5, copy attached). The <br />Applicant will need to either commit to using the suggested riprap thickness specification or provide an altemate <br />thickness proposal with technical justification. Also, the Applicant's drawing of proposed pit slope armor shows <br />[he base of the riprap keyed in at ten feet below [he anticipated water line or I S feet below the ground surface. It <br />is our understanding [hat the pit excavations will range from 45-60' in depth. Also, the designs for riverbank <br />armoring do not show that the armoring is keyed in. Therefore, the cross-sectional designs as presented are not <br />