2019-07-09_REVISION - M2004044 (30)

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1 Jared Ebert July 8, 2019 Page 6 of 18 A hydraulic model was prepared in support of the Adams County Floodplain Use Permit Application. The hydraulic modeling shows that flows within the East and West Cells varies from approximately 6,890 cfs to 12,750 cfs. The hydraulic modeling shows that flow velocities in within the East and West Cells vary from 1.3 fps to 2.3 fps. Flows and velocities vary greatly in the vicinity of the South Cell due to the contraction of the flow under the Highway 7 bridge. The flow rates in the are of the South Cell vary from 110 cfs to 7,450 cfs. The velocities, as calculated by the model, in the South Cell vary from 1.3 fps to 1.9 fps but is expected be near 0 fps in much of the cell because of the contraction effects of the Highway 7 bridge. b. The Applicant shall provide the flood elevations to be expected under a "worst case" flooding scenario and specific mitigation measures to be implemented to minimize the potential for any offsite impacts. An exhibit from the Floodplain Use Permit Application is attached to this response letter. This exhibit shows pre-project water surface elevations from the hydraulic model. There are multiple scenarios that were considered for the hydraulic modeling, all of which have differing results. The "worst case" scenario that was considered was stockpiling the materials for each phase at the same time. This scenario is considered very unlikely because it is neither practical or economical to stockpile material in this fashion because concurrent reclamation is planned. Depending on how mining progresses, excavated non- commercial materials will be placed directly into reclamation slopes or stored in the pit bottom instead of at the surface. The hydraulic modeling results of this "worst case" showed that a maximum increase in water surface elevation of 0.09 feet may occur at cross section 106553. Other increases in water surface elevation may be expected generally north of the Highway 7 bridge. Increases in water surface elevation are generally confined to areas of the river where there is only aggregate mining or open space. Increases in water surface elevation are allowed under Adams County regulations and per National Flood Insurance Program regulations. c. Riverside berms are proposed in the amendment, which may be prone to erosion during a flood event. The potential for"berms" around gravel pits to be damaged during flooding is discussed in detail in the 2013 Urban Drainage and Flood Control District (UDFCD) Publication "Technical Review Guidelines for Gravel Mining &Water Storage 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 river are referred to in the UDFCD document as riverside berms. Engineered inflow and outflow structures are intended to mitigate possible slope failure during flood events for these berms. Please include a design for the protection of these berms. The guidelines for protective measures are presented in Table 2.1 of the Technical Review Guidelines for Gravel Mining & Water Storage Activities (2013). The proposed mine is located along a master planned section of the South Platte River. The master plan shows riverbank protection for select sections of the river bank along the west bank. Riverbank protection has been installed according to UDFCD representatives. According to the table, the minimum top width for a master planned area with riverbank protection is 200 feet from the river bank. The planned limit of mining is set no less than 200 feet from the riverbank. Therefore, no inflow or outflow structures are proposed in excess of the guidelines. Aggregate Industries—WCR,Inc 1687 Cole Blvd Suite 300 Golden,CO 80401