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I I I I I I I I I I I I I I I I I I 1-2 safety factors, normally expressed as channel freeboard. The preliminary design presented in this report utilizes the freeboard during larger floods, such as for the lOO-year flood, in such a manner that most of the larger flood is carried in the channel. In some places, as shown on the preliminary design drawings, the capacity of the channel bridges and cul- verts is exceeded by larger floods with resulting overbank flooding. A review of the drawings will show that the residual floodplain is relatively modest in the urbanized and developed areas with the exception of the far downstream reach of Ralston Creek where it was impossible to satisfactorily contain the floodplain in the channel vicinity of the trailer park at the Cl ear Creek confl uence. In some places where nei ghborhoods are part i all y inundated with shallow flooding it may be appropriate for the City of Arvada to cons ider modest channel improvements beyond those recommended. The engineers did not design these improvements because this was beyond the context of the selected plan and design frequency. HYDROLOGIC AND DESIGN CONSISTENCY The 1985 master plan and preliminary design for Lower Ralston, Van Bibber, and Leyden Creeks was performed by the engineering staff of Wright Water Engineers. The hydrologic analysis of flood potential was based upon the early 1976 work by Wright Water Engineers for Wright-McLaughlin Engineers. Wright Water Engineers for this 1986 work utilized the same computer model technique and data. The same two hydrologists involved in the 1976 work were responsible for the current flood projections and frequency analysis. This provided consistency in approach and design criteria over the decade between the studies. LEYDEN LAKE Leyden Lake is situated upstream of the City of Arvada, It is a reservoir with a normal water storage content of 1,152 acre-feet. The existing spillway is small and settlement of the embankment has occurred. These inadequacies have resulted in a restriction being imposed on the storage of the dam. The dam can be expected to be overtopped prior to the spillway flowing at full capacity. The dam is classified as a high hazard struc- ture. ACKNOWLEDGMENTS The Wright Water Engineers' 1986 preliminary design and report preparation was performed by the following individuals: Kenneth R. Wright, P.E. Mark W. Gl i dden, P. E, Dr. Leo M. Eisel, P.E. Denny Tuan, P.E. Peter F. Johnson, P.E. W. Curtis Chandler, P,E. Peter r-bros Ed Lanyon It was found during the course of the overall study that it would be feasi- ble to rehabilitate the dam and spillway, with the cost being justified in flood control benefits downstream, For this reason, we have recommended that the Leyden Lake dam be rehabil itated to meet normal safety standards of the State of Colorado. We wish to also thank the following individuals for their cooperation and 9uidance in completing this study. Ben Urbonas, UD&FCD Charles Handy, City of Arvada Patri ck Dougherty, City of Arvad a Jeannie Rossillon, Jefferson County Lynn Goedert, Van Bibber Citizens Peggy Hunnicut, Fairmont Improvement Association Leyden Lake is owned by the Farmers Highl ine Canal and Reservoir Company. Large stockholders in the company are Westminster, Arvada, Thornton, and the Adolph Coors Company with Westminster being the largest. The citizen groups involved with Wright Water Engineers planning liminary designs were the key to this successful project. submitted by the citizen groups follow. and pre- Letters