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I I I I I I I I I I I I I I I I I I I I-I SECTION I INTRODUCTION traffic. In addition, improvements were made to a portion of the Van Bibber Creek channel, The moneys and effort expended to date have been used and useful, and the new bridges provide a marked improvement in flood carrying capacity, however, for the most part the past preliminary designs in general were not implemented, One of the main reasons for thi slack of action was the magnitude and impact of the recommended improvements, parti- cularly in certain residential areas along Ralston Creek where the size of the channel would require the removal of many residences. This major drainageway pl anning report for Lower Ralston/Van Bibber/Leyden Creeks represents a viable and recommended preliminary plan to reduce the flooding problems in the City of Arvada and adjacent suburban Jefferson Co unty areas. This report is the Phase B portion of the agreement between Wright Water Engineers, the Urban Drainage 8. Flood Control District, the City of Arvada, and Jefferson County. It also includes a Flood Hazard Information report for Ralston Creek above Leyden Creek. This 1986 prel iminary design took into consideration the past work. The prel iminary design contained in this 1986 report is based upon the scal ing down of many channel sizes, elimination of Ward Road Reservoir with the substitution of multi-use detention ponds, and a provision for the exten- sion of Ralston Road to the east. In addition, Leyden Lake was determined to require formal re-evaluation studies and possibly upgrading the hazard rating to high because of an embankment and spillway which are not up to current State Engineer safety standards. The study areas for which prel iminary design has been performed incl ude: Ral ston Creek from the Cl ear Creek confl uence to the Leyden Creek confl u- ence; Leyden Creek from its lower end to Leyden Lake; and Van Bibber Creek from its confl uence wi th Ral ston Creek to Indi ana Street, The study area is in Townships 2 and 3 South, Ranges 69 and 70 West, in Jefferson County, Colorado. The total basin area draining into these creeks totals 91 square miles, a total of some 58,000 acres. Flood potential and subsequent damage poten- tial is largely due to this extensive area draining into Arvada. It was for thi s reason that the local sponsors and the Urban Drainage 8. Flood Control District selected these streams for restudy. The total length of flood control channel for which designs have been pre- pared is 13 miles. Excluding the cost of Leyden Lake dam rehabilitation, the cost of the improvements amounts to $2.4 mi 11 ion per mil e. From the cost-benefit ratio standpoint, the recommended project is feasible. From the standpoint of urban systems, economics, and future flood damage poten- tial, we recommend that the project proceed with final design and construc- tion. In the past years, the Ral ston Creek drai nage basin has been stud i ed by the U. S. Army Corps of Engineers, Wright-McLaughl in Engineers, Frasi er and Gingery Engineers, and Gingery Associates. The former master plans and preliminary designs were completed and accepted, and final design was per- formed for the Ward Road Flood Detention Reservoir. The City of Arvada proceeded to design and construct a number of improvements to important bridges to alleviate flood problems along Ralston Creek and to accommodate TEN-YEAR CHANNEL DESIGN One of the decisions made by the City of Arvada and Jefferson County was to select the 10-year frequency flood for design purposes on Leyden Creek and Ralston Creek. This decision was made upon the recommendation of the con- sultants. The 10-year frequency flood criteria allowed the engineers to design channels which would convey the design floods under future basin development conditions. Normal engineering practice and criteria of the Urban Drainage 8. Flood Control District requires the inclusion of certain