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I I I I I I I I I I I I I I I I I I I VI-1 SECT! ON VI DESCRIPTION OF PROPOSED PROJECT A structural European channel is to be constructed in 2.3 miles of the creek bottom, most of which is in Ralston Creek. The European channel is generally designed for carrying the 10-year flood, however, because of normal freeboard and optimi zed depth of the channel bottom, the 10-year design channel will carry most of the 100-year flood when running bank full and without any allowance for debris plugging. A low flow channel similar to the one proposed for grass lined sections is included here. The pre 1 imi nary des i gn of the Ral ston, Van Bi bber and Leyden Creeks flood control project is described in this section of the report. The flood control project presented in this report includes 12.7 miles of creek channel, 9 street bridges requiring improvements to pass the 100-year flood, 10 bridges to be improved to pass the 10-year flood, and 7 bridges which require no modifications. A box cul vert is des i gned for Van Bi bber Creek through the hi gh val ue Arvada Pl aza Shoppi ng Center. With the upstream Oak Street detention facil iti es, the box cul vert wi 11 convey the laO-year f1 ood on Van Bibber Creek of 1,300 cfs. Without the detention storage, the 100-year flood would be 2,600 cfs at the Arvada Plaza Shopping Center. This would result in residual flooding because the capacity of the box culverts would be ex- ceeded. The design calls for a major rehabilitation of the Leyden Lake embankment and spillway to reduce the risk to the downstream urban area from potential dam failure. The dam is presently classified as moderate hazard, but is being treated as a high hazard structure as a result of identified inade- quaci es. Multi-use/multi-purpose detention storage facilities have been designed for Van Bibber Creek to store up to 187 acre-feet of flood waters during a 100- year flood. These facilities are known as the Oak Street detention facili- ties. The multi-use/multi-purpose relates to their use as soccer fields for recreational purposes. To control erosion and limit velocities of flood waters, 16 drop structures are proposed. The drop structures would be permanent, probably of concrete, grouted rock or grouted rubble. Gabions shoul d not be used because of the tendency for erosion of the wire with subsequent corrosion and 1 ater fail ure of the baskets. Ni ne drops are on Ralston Creek and 7 drops are on Van Bibber Creek. Of the 12.8 miles of creek channel involved, 8.1 miles will remain unim- proved. Here, the 1 ands adj acent to the channel wi 11 be allowed to flood as they have historically. The floodplain and floodway should be strictly regul ated by Arvada or Jefferson County and floodpl ain management methods will be practical, including flood warning, education, flood insurance, flood proofing and other techniques. The preliminary design includes four canal crossing structures which are part of the improvements. These are for the Manhart Ditch, the Reno-Juchem Ditch, Farmers Highline Canal and the Croke Canal in the study area. There are 1.9 miles of channel which are to be shaped into an attractive channel with grass lining and a low flow channel which simulates the exist- i ng low flow section. Thi s wi 11 enhance the reestab 1 i shment of ri pari an plants and wildlife. It is recommended that constructi on generally begi n at the lower end of each creek and proceed upstream to reduce the flood 1 i abil ity of the gov- ernmental agencies. Lower Ralston should be accomplished first. It is prudent to not improve capacities upstream first because this may cause higher peak flows to occur downstream of where a "bottleneck" now exists.