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cxAPTER z.o <br />OFF RIVER GRAVEL MINING <br />2.1 TYPES OF RIVER ALIGbU~2VT <br />The centerline alignment of the South Platte River, and possibly its <br />tributaries affects stability. This section describes three classifi- <br />cations of river alignment and how each affects the stability of the <br />South Platte River. This description may or may not apply to tribu- <br />tary streams. <br />2.1.1 Existing Unstable Alignment <br />The alignment of the South Platte River through Adams County in <br />1985 is considered unstable. Studies of aerial photos taken <br />since 1937 clearly show the river continuously shifting its <br />main channel horizontally, sometimes as much as 1,700 feet. In <br />more recent years, the river also has exhibited a tendency to <br />lower its thalweg through degradation. As it exists today, the <br />river channel is considered unstable and capable of significant <br />lateral and vertical movement. <br />As a minimum, if the gravel mining operator elects to preserve <br />the river in its current unstable alignment, the river banks <br />shall be regraded to have a 3H:1V side slope and then revege- <br />tated per section 2.2 of these Criteria. Sufficient river <br />channel width must be provided to convey the 100-year flood to <br />88th Avenue and the 10-year flood downstream of 88th Avenue as <br />described in the Master Plan. It is the intent to provide this <br />capacity by natural erosion phenomena or excavation, if neces- <br />sary. <br />2.1.2 Master Plan Alignment <br />The alignment shown in the South Platte River Phase B Master <br />Plan, prepared by Wright Water Engineers for the Urban Drainage <br />UDLFCD GRAVEL GUIDELINES 12/17/87 <br />2-1 <br />