2001-05-16_REVISION - M2000158 - Laserfiche WebLink

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• • iii iiuiiiini m STATE OF COLORADO DIVISION OF MINERALS AND GEOLOGY Depanmem of Natural Resources 1 313 Sherman 51 ,Room 215 Denver, Colorado 80203 Phone: 1303) 866-356: FAa: 1303)832-8106 DATE: May 16, ?001 DIVISION OF MINERALS GEOLOGY RECLAMATION MINING•SA FE T7 TO: Erica Crosby Bill Owens Governor Greg E Watcher FROM: Allen Sorenson & Tom Schreiner Eneeul.re Dtrector Michael B. Long Drv~sion Director CC: •^I Carl Mount, DMG RE: Riverbank and Pit Slope Armoring Proposal, Technical Revision to S&H Mine Section 112 Reclamation Permit Application, Platte Valley Sand and Gravel, File No. M-2000-158 The Applicant has provided a technical revision (TR) to further address concerns with erosion potential through the gravel pit mining cells during a large flood event. The TR includes an illustration of locations that the Applicant has determined to be sensitive areas with the potential for excessive erosion. The TR includes a proposal and designs to stabilize certain of these potentially sensitive areas by armoring through the placement of riprap. The TR also includes cos[ estimates for the placement of riprap and a description of the timing of riprap installation relative to the sequence of excavation in the gravel pits. The Division's review of the TR is broken out into four sections in this memorandum as follows: • The determination of locations that may be subject to erosion during a large flood. • The specification and drawings that will govern the sizing and placement of riprap. • The estimated cost to purchase and take delivery of riprap that meets specification, and to install [he riprap in accordance with designs and drawings approved by the Division. • The timetable for riprap installation, in particular the installation of the riverbank riprap. As a reminder, the purpose of the flood control/mitigation plan is to minimize the potential for excessive scouring and erosion of pit slopes and of berms protecting critical stmctures such as gas pipelines, and oY offsite impacts due to sedimentation or realignment of the existing river channel. Riprap Locations In order to prevent excessive erosion. areas that may be subject to scour during a flood should be armored. To identify areas of scour, the depth and velocity of floodwaters should be estimated and the gradation of the soil and subsoil (sand and gravel) may be examined along with vegetation characteristics to determine if the erosive power of [he flood will cut into the vegetation and soil. Placement of larger and heavier material (riprap), that cannot be lifted and moved by t7oodwater, will prevent erosion and down cutting in locations identified to be prone to scour. There are a number of