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Mr. Jack Haines February 13, 1976 Page Four DRAINAGE & FLOOD CONTROL _ Other than the South Platte River there are no features on the site which are expected to cause a drainage problem. The entire upper area above the river within the property is flat agricultural land with no defined dra°nageways. The site is adjacent to the South Platte River and lies partially within the existing 100 year flood plain of the South Platte. South Platte River Various reports are available which summarize and discuss the existing flood conditions along the South Platte River. The site is immediately downstream of the Henderson Gauge which has a maximum discharge of 33,000 cfs recorded in 1973. The flow is larger than the expected future 100 year discharge at this site of approximately 20,000 cfs with Chatfield Dam completed. A study performed by the Colorado ;dater Conservation Board of the flood of 1973 delineated the flooded area on aerial photographs taken the day following the storm. Using this delineation as being conservative for the future 100 year floodplain, with minor correction made at the :Cilgroe- Atchison site area on the basis of reports by people present during the 1973 storm, the floodplain area at the site is delineated in Figure 4. . Some of the flooding in the vice qty of the property was caused by the lack of an adequate outlet at the existing lakes. Internal drainage facilities constructed as part of the reclamation plan should provide for an outfall for flood waters collected in the~~ lakes, and flooding in the area will be reduced. Future P4ining Operations It is planned gravel mining operations will be .x tended into the floodplain area in the future, and mining will take place adjacent to the river channel. For water .~sality consider- ations, mining should not take place in the active channel of the river and should be so situated thr' t mining operations do not cause an adverse impact on the quality of the river.