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.' Mr. Jack Haines February 13, 1976 Page Four DRAINAGE & FLOOD CONTROL Other than the South Platte River there are no featu.es 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 drainageways. Tlie 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 summa ~ze 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 Q~,proximately 20,000 cfs with Chatfield Dam completed. A study performed by the Colorado !dater Conservation IIoard of the flood of 1973 delineated the flooded area on aerial photographs taken the day following the storm. Using this delineation as being cor,.,ervative,for the future 100 year floodplain, with minor correction made at the Kilgroe- Atchison site area or. .he 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 vicinity 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 these lakes, and flooding in the area will be reduced. Future Plining Operations It is plann~3 gravel mining operations will be extended into the floodplain area in the future, and mining .11 take place adjacent to the river channel. For water quality consider- ations, mining should not take place in the active channel of the river and should be so situated that the mining operations do, not cause an adverse impact on the quality of the river.