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<br />Memo to Erica Crosby 3 May I5, 200I <br />riverbank west of the S&H Mine location. The areas to be armored under the Applicant's proposal <br />appear to have been selected based on qualitative analysis of the river and Floodplain geomorphology, as <br />opposed to quantitative analysis of Flood Flows and velocities and related potential for scour. >n general, <br />it appears that the areas identified by the Applicant as requiring pit slope atmor are the most likely <br />locations for pit slope erosion during a Flood. However, unless the Applicant can furtherjustify the <br />extent of the proposed armoring through quantitative analysis, the Division will require that the <br />following locations also be armored: <br />• The south terminus of the Applicant's proposed armoring must be extended around the groin of the <br />pit slope at the southwest corner of Meadow Lake and continued over to the southeast corner of the <br />lake. <br />• The gas well pads and access road alignments for the wells in Sharkey's Lake must be armored <br />around their entire perimeters including the groins in the pit slopes where the access roads join the <br />Lakeshore. <br />• Any low spots, such as natural swales or abandoned tail water ditches, et cetera, that connect the <br />proposed gravel pits and the river must be armored were the pit slope intersects the low spot. <br />• Other areas requiring the installation of riprap armor as identified in the field by the Division and <br />Applicant during a forthcoming site inspection. <br />The pi[ slope armoring proposed in the TR as illustrated on a permit area map truncates abruptly in [he <br />various reclaimed lakes where armor is to be installed. Also, based on the language in the TR <br />pertaining to riverbank amoring, there may be a number of locations within the mine reach where <br />riprapping will end and natural riverbank will resume. The Division's published guidance for riprap <br />end protection recommends doubling the riprap thickness at [he ends of an armored reach (DMG, 1998, <br />page D-3 and Figure D-7, copies attached). The Applicant must commit to following the Division's <br />guidance for end protection or provide an alternate end protection design for the Division's review. <br />The TR application includes no commitment to install riverbank armor, but describes the possible <br />installation of riprap along the riverbank if required by any of the permits issued for the proposed mine <br />or at the option of the Applicant. The Division's published guidance recommends that riverbank armor <br />be installed with due consideration of the potential for offsite impacts that resultant channelization of <br />the stream caused by the armoring may cause (DMG, 1998, page D-3, copy attached). Armoring of a <br />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 <br />there are several such locations within the mine reach. In particular, the Division is concerned with the <br />eastward bend in the river at the proposed Meadow Lake, the location where the river curves from a due <br />north Flow path to a northwest flow path at Sharkey's Lake, and the bend in the river west of [he north- <br />south axis of Longhorn Lake. Riprap Armoring through these bends in [he river should be required <br />prior to mining within 400 feet of the riverbank. The reach length of the armoring to be required at the <br />river bends may be determined by quantitative analysis of scour potential during a 100-year Flood, or by <br />inspection of the extent of existing cut banks. Prior to Board consideration of the Regular ] 12 <br />