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SECT! ON V I DESIGN CRITEFiIA The magn Itude of the size of the Big Dry Cre",k drainage bas In, the relatively undeveloped nature of the basin, the e>:lstence of continued low flows from Irrigation releases throughout '" subst",ntlal portion of the year, and the relative Infrequency of large magnitude storm discharges typical of many streams, combine to make the design crltHla for Big Dry Creek some- what different than for basins that produce smaller runoff quantities In areas already substantially urbanized. While basic hydrologic, hydraulic, sociologic, economic, and other related principles are stili vIable, certain subtle differences exist concerning constraints, as well as opportunitIes that must be consIdered to achieve an effective preliminary design. With the expanse of the Big Dry Creek flood plaIn, the approach to desIgn must consider multiple use, Including large scale use by people for recreation and for pedestrian - bike travel, and must consld.er preservation of the natural flora and fauna In certain areas. This section lists and describes the crIteria used in the preparation of this prelImInary design, Including those plans which were considered and dIscarded. During Phase A of this study, It was determined that a "non-structural" approach was the most desIrable manner by which t,o approach the PrelimInary Design. The partIcular alternate that was chosen Is a flood plain approach with structural Improvements (channels, blrdges etc.) limited to those areas where for fInancial and for other criteria it Is desirable to protect appurtenant features and/or channelize the flood waters. It Is Important that the desIgn criterIa be well conceived so as to maximize the benefits to be obtained and to Insure that the major draln-age system will operate as I n tended. As Iterated In previous reports, the major drainage sy,;tem will operate whether or not It Is planned or designed, and whether or not urban development Is wisely located In respect to It. In the first phase of this study, much of the criteria for this report was established In that a concept was determined and a design return frequency (100 year's) was chosen. The criteria presented following constItute ," refinement In the crIteria used to determine the concept and return frequency. The design criterIa utIlIzed for thIs prelIminary design are the)se contained In the Denver Re- gional Council of Governments Urban Storm Oralnag'" Criteria Manual, Volumnes One and Two. Specific planning and design criteria applicable to the Big Dry Creek study are: I. Natural Channels, Natural waterways will be used wherever feasible. a. Channel and overbank capacity must b,,, adequate for design runoff . V I-I