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• Therefore, because all lateral berms proposed as part of the mining and reclamation plan are of adequate width, they will not require any additional protection or stabilization beyond what is currently being proposed. Flow Diversions As discussed in Section III, the currently proposed reclamation plan may create additional diversion of flows from the main channel to the south floodplain. The area of concern is shown on Figure 7. The 100-year flood boundary shown on the figure represents the boundary between the main channel floodplain and the area of shallow flooding to the south of the line. It is assumed that the line is based on the 100-yeaz water surface profile of the main channel relative to the existing ground surface elevati~~ns (which is the standazd method of determining a floodplain boundary). Therefore, in order to prevent spill of flood flows from the main channel floodplain, it would be necessary to maintain the pre-mining ground surface elevations along the existing floodplain boundary. An alternative to this would be to raise the proposed ground surface el~vatioos in the azea between the top of the proposed pit slope and the main channel to ele~~ations that would correspond with the existing ground surface elevations along the existing floodplain boundary (this would essentially be slow-profile berm running from ilorth 51" Street to a point near the South Branch Irrigation Ditch). Since the floodplain mode] assumes that some spill of flows will occur across the existing shallow flooding avea, it is not critical that a berm or levee be designed that would be guazantied to block al:. 100- • year flows. Note that this is the same area that requires adjustment of the pitside bank in order to achieve the recommended 250 foot riverside berm top width (see discus;~ion above concerning pitside bank protection}. VII. Conclusions and Recommendations • The proposed Western Mobile, Inc Lyons Pit-Rockin' WP sand and grave;l mining and reclamation plan will not have any adverse impacts on the 100-year floodplain of the St. Vrain River. • The proposed reclamation plan will provide some reduction of flood flows downstream of the permit boundary by providing storage and detention of flood flows in the reclaimed "open valley" pits. • The proposed mining and reclamation plan will not create or exacerbate any existing flooding conditions for adjacent and downstream properties. • The proposed mining and reclamation plan will not create any rise in 10(l-year flood water surface profiles provided that the mining and reclamation pltm is modified as recommended to prevent diversion of flows from the main to the south floodplain. •