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I I I I I I I I I I I I I I I I I I I 19 The laO-year flood plain averages 100 feet in width, the channel velocities average 5 feet per second, and overbank velocities average 2 feet per second. profile is also shown where it can be distinguished on the profile drawings. Future basin development and existing channel conditions were used to define the elements of the flood hazard area. This report is intended for use by local officials and other interested parties in planning the use and regula- tion of the flood hazard areas. The second reach extends from Section 33-34 line to the upstream limits. In- cluded in this reach are the existing and projected filings of the Ken Caryl Ranch Plains subdivision. At the upper end the drainageway has been replaced by Continental Divide Road. The roadway will be overtopped without failure during major flows. No other drainage problems are known to exist. The 100- year flood plain averages 80 feet in width, the channel velocities average 2 feet per second, and overbank velocities average 3 feet per second. Floodway The floodway of each drainageway was defined using Jefferson County's defini- tion for "high hazard area". The criteria is lOa-year flood depth greater than or equal to 1.0 foot and/or lOa-year flood velocity greater than or equal to 3.0 feet per second. Arapahoe County had no specific criteria for floodway and agreed to go along with the Jefferson County criteria for continuity. The contract suggested the floodway criteria of encroachment to produce a 0.5 foot rise in lOa-year flood profile. For a typical stretch of Massey Draw, the floodway was defined using both criteria. The Jefferson County criteria proved more restrictive so it was used. In addition, no encroachment was permitted into the defined channel, and engineering judgment was used to smooth the limits in the final definition. The lOa-year floodway was not defined wherever the lOa-year flood hazard area is confined within a well defined channel. Floodway information is keyed to cross section reference points and summarized on Tabl€$ IV-l through IV-5. In the overland flow areas immediately downstream of roadway crossings and other controls which are overtopped, a warning note is also given. It explains that encroachment into the flood hazard area, and fill elevations should also consider the overland flow created by the upstream control. Summary The lOa-year flood plain and flood profile and the lOa-year floodway have been defined for the study area and are documented in this report. The 10 year flood ~ Leonard Rice Consulting Water Engineers, Inc.