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. . deducted from the channel flow for similar analysis at consecutive points downstream, The water surface profile in the channel below 11 th A venue, therefore, represents the 100-year flow reduced by amounts escaping into the overbanks, banks. Generally, the maximum flow velocity desirable for a channel would be 6 feet per second to prevent erosion, and a minimum velocity of 2 feet per second to minimize sedimentation, Flow velocities in the overbank areas average 4 to 6 feet per second in the streets and stay below 4 feet per second on grassed or other non-paved areas, . . The floodway is defined as the channel and that portion of the adjacent flood plain required to carry the 100-year discharge without raising the flood profile more than an acceptable amount. It is the most hazardous portion of the flood plain, For that portion of Cherry Creek upstream from Colfax Ave- nue, the floodway has been defined based upon an equal degree of encroachment resulting in a rise in the water surface profile of 0.5 feet or less. The floodway data is displayed in Table 3, The construction of Cherry Creek Dam has provided protection for Denver against floods origina- ting upstream from the Dam. The proper operation of the reservoir is essential to providing this flood protection and it is probable that occasional releases from the reservoir will be necessary to regulate the pool elevation for flood control purposes, Reservoir releases of up to 5,000 cfs are possible and may be sustained for 5 to 10 days. While the present Cherry Creek channel downstream from the Dam could handle reservoir releases without flooding areas outside the channel, sustained flows could severely erode the sandy channel and damage structures which are not well protected, such as bridge piers or drop structures, RELEASES FROM CHERRY CREEK DAM . . For that portion of the creek between the South Platte River and 11th Avenue, the large amounts of water which leave the channel and never return make the above floodway criteria inappropriate, In this reach of the creek, the walled channel should be considered as the main floodway, Auxiliary floodways should be provided to convey the overbank flood waters through the flooded areas. Development proposals in the overbank areas should be handled on a case-by-case basis in accordance with Denver's flood plain zoning ordinance. In Reach 4, the maximum probable reservoir release of 5,000 cfs is greater than the 100-year flood and potentially more dangerous because of its longer duration, The floodway in Reach 4 should be maintain- ed to pass the maximum release from Cherry Creek Dam, rather than the 100-year flood, The water surface profile and flood outline for the 5,000 cfs . . . The flow velocities expected during the 100-year flood are provided on the Drawings. The channel velocities to be expected will reach 10 feet per second which is capable of causing severe erosion to stream . 8 ~ maximum reservoir release is included in the drawings upstream from the confluence of Goldsmith Gulch and those areas where the average flow velocities exceed- ed 6 feet per second are noted as areas where channel erosion will be more severe. The Cherry Creek channel above Goldsmith Gulch is still in a relatively natural state and the flood plain mostly free from encroachment. Because the area which it drains is comparatively small and has not experienced the development existing throughout the remainder of the Cherry Creek Basin, the 100-year flood for this Reach is not as significant as possible releases from Cherry Creek Dam, As this area is developed, Cherry Creek must be maintained to pass, at least, the maximum reservoir release of 5,000 cfs, TABLE 3A AUXILIARY FLOODWAY DATA Cross Section Left Overbank Width Disch8.l."R'e Right Overbank Width Discharge 1 2 3 4 5 6 7 8 9 10 II 12 13 1,500 2,000 1,500 2,000 1,500 2,000 970 1,750 950 1,750 640 1,750 810 1,750 600 700 500 700 270 700 460 1,100 270 400 235 1,100 150 400 185 1,100 170 400 260 1,100 Note: Table 3A above gives data on the auxiliary floodways required in the overbank areas outside the main channel to carry the IOo-year flood between the South Platte River and 11th Avenue, as discussed heretofore on page 8.