2019-11-20_REVISION - M2004044 (4)

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Ineffective flow areas are also known as"backwater"areas. These areas flood when the water surface elevation increases,but do not actively convey flows downstream. Ineffective flow areas have essentially no flow velocity. Ineffective flow areas are typically found upstream and downstream of bridges and culverts as flow contracts and expands through the opening. Because they do not convey flow downstream, these areas can be filled or altered without impacting flood water surface elevations. Stockpiles in other locations, which are in active flow areas, are oriented to be parallel to the direction of flow to minimize impacts to the flood water surface elevations. Since the stockpiles are located in areas where alterations are not expected to impact the flood water surface elevations,their orientation and length are not restricted such as the stockpiles in active flow areas. A schematic figure demonstrating the ineffective flow areas around a bridge is presented below. Cross Section Contracting Flow Cross Section i n --_dross Section � l Ineffective Flow Area (Typical All 0/ ` '•, Shaded Expanding Flow \'' Areas) \. Cross section - - -- tQ Additional mining and reclamation procedures will be used within the regulatory floodplain to mitigate impacts from potential flood flows. Flood season is considered to be April 1 through September 30. The southern and eastern slopes of each area will either be maintained at 31-1:1 V during the flood season during mining or concurrently reclaimed at 31-1:1 V with reclamation backfill. This restriction only applies for areas within 400 feet of the South Platte River in accordance with DRMS policy. If flood waters reach the mining Areas prior to complete reclamation,the 31-1:1 V slopes will allow controlled flow into the Areas while reducing the potential for head cutting and capture of the South Platte River. A Floodplain Use Application has been issued by Adams County for this project. Adams County is the regulating authority on the flood permit; however, Adams County may request review and comment from Urban Drainage and Flood Control District(UD&FCD). Comments and revisions from the County or the UDFCD will be incorporated into the final Floodplain Use Permit Each aspect of the mining operation is listed in the following table with associated disturbed area. The table illustrates a point in time when the mining disturbance could be at its maximum. At the proposed Tucson South Resource site, it is assumed that the mining disturbance will be at its greatest when the East Area mining is nearly complete. That will be a time when the slurry wall is installed and,topsoil replacement and initial seeding will be completed for the West Area. Aggregate Industries Tucson South Resource Permit#M-2004-044 112 Permit Amendment Page D-6