2010-01-21_PERMIT FILE - M2009082 (11)

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Page 8 of II Mr. Jared Ebert January 20, 2010 Adequacy Review #2 Comments and Responses: The Division has several concerns about the berms of unexcavated native material ground between several of the mining cells. These berms may erode and down cut during a flood. The potential for berms around gravel pits to be damaged during flooding is discussed in detail in the 1987 Urban Drainage and Flood Control District (UDFCD) Publication "Technical Review Guidelines for Gravel Mining Activities within or Adjacent to 100-year Floodplains." Strips of native ground or constructed fill between the stream and the gravel pit, and generally aligned with the flow direction of the stream are referred to in the UDFCD document as riverside berms. Strips of native ground or constructed fill between adjacent pits and generally perpendicular to the stream are referred to as lateral berms. Both riverside berms and lateral berms are proposed in the Parsons Mine application; both these berms may be prone to erosion during a flood event. The proposed inflow/outflow structures are intended to mitigate possible slope failure during flood events for these berms. The Parsons Mine is outside of the political boundaries of the UDFCD, but the technical floodplain factors that led to the formation of the UDFCD guidance still apply and were used to review the adequacy of the flood mitigation structures proposed for the site. There are lateral berms between mining cells 5A and 513, 4C and 4B, 4C and 4A, and 4A and 4B. The minimum top widths between these berms is 130 feet according to the proposed mining and reclamation plan maps. UDFCD states that these berms can be left in a reclaimed but unimproved condition if the top widths for lateral berms is 250 feet. The current proposed plan calls for several inflow/outflow structures and culverts to be installed to mitigate erosion damage of these lateral berms. Please demonstrate how the proposed mitigation structures will limit the differential head between each of the above mentioned mine cells, to ensure the lateral berms will not erode. Inflow/outflow structures have been sized in accordance with the 1987 UDFCD Publication "Technical Review Guidelines for Gravel Mining Activities within or Adjacent to 100 year Floodplains. " Although this publication was intended to be applicable to the floodplain of the South Platte River within UDFCD boundaries, the use of these methodologies for sizing inflow/outflow structures is considered to be conservative for the Parsons Mine. Mining cell 4A is not close enough to the river for an inflow/outflow structure to be required. However, to equalize water surface elevations between 4A and 4B, an inflow/outflow structure connecting these two cells will be provided. During major storm events, the inflow/outflow structures will allow the water surface in the cells to rise as the river stage rises so that the cells are filled when overbank flooding begins. 2. Page SE8 of the Stability Analysis states a HECRAS model was used to analyze the rate of flow for the various inflow/outflow structures during different stages of a flood event. In order to verify the adequacy of the design of the inflow/outflow