2003-10-17_REVISION - M1989120 - Laserfiche WebLink

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Memo to Kate Pickford 2 October 17, 2003 Floodplain Issues Permit No. M-1989-120 The primary potential problem with the original plan is that there is only one side channel spillway intended to be installed. With riverside berms longer than 1300 feet for pits in the general area of the Platte Valley Pit, two side channel spillways are necessary.' This issue was raised in the adequacy review for the original application in 1990, but was not properly followed up and it appears that the plan to install only one side channel spillway was accepted by default. A second item that may be problematic was not addressed in the original design. Mining will occur or has occurred approximately 200 feet from the edge of an outside bend in the river as part of excavation of mining phase 1. Armoring of this bend in the river is likely to be necessary. The updated flood management plans for this pit must include the following details that were not in the original plan. • Maps illustrating locations where flood management structures will be installed. • Determination of the 10-yeaz floodwater surface elevation and survey results showing the 10- year flood will not enter the pit, or, alternatively, plans for earthwork necessary to exclude the 10-yeaz flood. • Gradation specifications for riprap with a description of the methods used to develop the specification. • Plans for installation of riprap including a description of bedding and of how the riprap will be keyed at the base of the armor layer. Extending riprap vertically downwazd beyond the toe of the slope at least 5 feet below the channel thalweg is recommended unless it is demonstrated that an alternate design will be effective. Cross sections illustrating riprap installation plans must be provided for each location where riprap is to be installed. • Crest widths along with justification for the selected crest width must be provided for the lateral berms. Cross sections illustrating the proposed concrete rubble cutoff walls must provided. cc: Cazl Mount, DMG (via email) "Urban Drainage and Flood Control District, "Technical Review Guidelines for Gravel Mining Activities within or Adjacent to 100-year Floodplain," December 1987.