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I I I I 'I . I I II V-3 master pl an was to provide cost effective improvements which could be im- plemented. By allowing an affordable design the opportunity for implemen- tation increases. features of this area and the Colorado and Southern Railroad, an extensive embankment would be required on which to construct the road surface. This would present significant impact on the floodplain of Ralston Creek. To minimize the impacts of this embankment, the direction for the lower reach of Ralston Creek was modified. In the revised approach, the entire 100- year flows are passed south and beneath the Ralston Road extens i on. Thi s deviates from the original 10-year plan proposed in the Phase A. Byen- larging the bridges at Lamar Street and at Ralston Road and constructing a lOO-year capac ity channel between these two cross i ngs, flow can be con- tained within the channel and passed to the south of the proposed embank- ment. This results in the elimination of additional hazards north of the road. Because of new ri ght-of-way constraints downstream of the Ral ston Road crossing, a European type channel was incorporated into the design rather than the originally proposed grass lined channel. The Ralston Road extension raised the costs of flood control, but also stimulated a higher level of flood protection in this area which is advantageous. Along Van Bibber Creek the improvements are designed to convey the 100-year flood event assuming the Oak Street detention facil ities are constructed. Shoul d funds not be avail ab 1 e for the impl ementat i on of the Oak Street detention facil ity, the improvements along Van Bibber Creek downstream would have the capacity to convey the IS-year flood event. This level of protection is sufficient to provide a favorable benefit cost ratio. II In addition to the structural improvements proposed, a non-structural solu- tion must be superimposed on the entire lOO-year residual floodplain. In all cases, a vigorous non-structural floodpl ain management program should be maintained or implemented. This plan should discourage the development of flood prone properties and should promote compliance with the recommen- dations of this master plan. In addition, a flood warning system is en- couraged to provide the highest level of protection to citizens living or working within the floodplain. I I '. il I I 'I 'I I PHASE B DIRECTION In July, 1984, the Urban Drainage & Flood Control District directed Wright Water Engineers to begin the Phase B portion of this study. The direction indicated we were to proceed with the preliminary design based on the rec- ommended pl an presented in the Phase A report. No modification to the recommendations were initially proposed by the Urban Drainage & Flood Control District and the sponsoring communities of the City of Arvada and Jefferson County. DESIGN CRITERIA Introduction The basic planning and design criteria used in this report are those set forth in the Phase A Master Plan, and the Drainage Criteria Manual of the Urban Drainage & Flood Control District, except that all bridges are assumed to be free of debris. This section summarizes the criteria used in the preparation of preliminary design for the Phase B portion of the major drainageway planning for Lower Ralston, Van Bibber and Leyden Creeks. Subsequent to the Notice to Proceed, the City of Arvada embarked upon a traffic master plan. Included was an extension of Ralston Road. This traffic pl an prepared by Centenni al Engineers incl uded extending Ral ston Road from Lamar Street to Sheridan Boulevard. Because of the topographic Planning Criteria The City of Arvada and Jefferson County must continue to regul ate the floodplains within their respective boundaries. This continued regulation will prevent the creation of additional flood damages within the flood- plains. It should also prevent the reconstruction of structures damaged by floods when such reconstruction is likely to result in future flood dam- ages.