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M-2000-158 TR Memo 3 Mav 17 2001 riprap thickness at the ends of an armored reach (DMG, 1998, page D-3 and Figure 0.7, copies attached). The Applicant must commit to following the Division's guidance for end protection or provide an altemate end protection design for the Division's review. The TR application includes no commitment to install riverbank armor, but describes the possible installation of riprap along the riverbank if required by any of the permits issued for the proposed mine or at the option of the Applicant, The Division's published In-Stream Aggregate Extraction and Reclamation Guidance Document recommends that riverbank armor be installed with due consideration of[he potential for ofl'site impacts that resultant channelization ofthe stream caused by the armoring may cause (DMG, 1998, page D-3, copy attached). Armoring of a riverbank is general ly installed through a zone that is likely to experience laterel migration of the stream, typically the outside bank of a curve. Review of the SBtH Mine pre-mining maps indicates that there are several such locations within the mine reach. In particular, the Division is concemed with the eastward bend in the river at the proposed Meadow Lake, the location where the river curves from a due north flow path to a northwest Flow path at Sharkey's Lake, and the bend in the river west of the north-south axis of Longhorn Lake. Rpprap Armoring through these bends in the river be required prior to mining within 400 feet of the riverbank. The reach length of the armoring to be required at the river bends may be determined by quantitative analysis of scour potential during a 100-year flood, or by inspection of the extent of existing cut banks. The Division is also concerned with the identification of any straight reaches of river channel where mining will encroach within 400 feet of the river bank. Armoring of these reaches may also be required prior to mining within 400 feet of the river bank unless the applicant can provide sufficient rationale and demonstration that such bank areas are currently stable and will remain so during a 100.year flood event. It is suggested that a thorough inspection of the current riverbank condition by the Applicant and submittal of a map that identifies all potentially unstable areas (to be stabilized prior to mining within 400 feet of the river bank) is an effective procedure for determining the extent of pre-mining riverbank armor that will be required. Such a map will be subject to Division approval. Riprap Specifrcalions and Drawings The TR specifies that all areas to be riprapped will have a minimum D~ equal to 24 inches and will be placed to a minimum thickness of D,~x 1.5. The Division has evaluated the adequacy of the 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 Dw, Hence, the Dso specification proposed by the applicant is acceptable. The Division's analysis of the D~ 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 tile. Although the Applicant has provided an acceptable D,o 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 D~iprap (DMG, 1998, table D-1, copy attached). The Applicant may either commit to using the suggested riprap gradation specification or provide an altemate 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 D,~ I.5 or 3 feet. The Division's published guidance recommends riprap thickness of D,~c 2.0 for pit slope armor and a variable thickness of D,~x 1.75 to Dix 3.0 for riverbank armor (DMG, 1998, Figure Q5, copy attached). The Applicant wilt 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~he-riprap-keyed in-at fen feet Itelow-the anticipated water-line or-t-5-feet-beiaw-the ground~urface. 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