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j I I I I I I I I I I I I II I WWE Wright Water Engineers, Inc. DENVER OFFICE 2490 West 26th Ave" Suite SSA Denver, Colorado 80211 (3031480.1700 TULSA OFFICE 707 South Houston, Suite 302 Tulsa, Oklahoma 74127 (918) 584.7136 GLENWOOD SPRINGS OFFICE 818 Colorado Avenue P,O. Box 219 Glenwood Springs, Colorado 81602 (303) 945.7755 Denver Direct Line: 893-1608 Mr. Ben Urbonas February 27, 1986 Page 2 February 27, 1986 A box culvert having a l5-year flood capacity is designed for Van Bibber Creek through the high value Arvada Plaza Shopping Center. With the pro- posed upstream detention facil ities, this box culvert will convey the 100-year flood 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. Mr. Ben Urbonas Urban Drainage & Flood Control District 2480 W. 26th Avenue, Suite l56-B Denver, CO 80211 Re: Agreement No. 83-1.6 Dear Mr. Urbonas: The project as 1 aid out in this report be implemented without further delay. result in exposed flood hazard. The cost of constructing and implementing the project, in terms of 1985 dollars, 1S approximately $33,000,000. This includes right-of-way, bridges, channel s, detention, dam rehabil itation and all other costs in- cluding an allowance of 30 percent for contingencies and engineering. The resulting benefits, measured in terms of reduced flood damages, exceed the project cost by a factor of two. is sound and practical. It should To wait a long period of time will The report entitled "Major Drainageway Planning Lower Ralston/Van Bibber/ Leyden Creeks, Phase B Report" is attached. The engi neeri ng process i n- cluded regular meetings with the project sponsors and interested citizens. These meetings resulted in each portion of the streams receiving careful consideration during each step of the work from a variety of view points. I I I I I I , I 1'1 1 ~ I ,j I 1 The flood control project presented in this report incl udes 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 modification. The design call s for a major rehabil itation of the Leyden Lake embankment and spillway to reduce the risk to the downstream urban area from potential dam failure. The dam is classified as high hazard. 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 fl ood. The mult i -use/mul t i-purpose rel ates to thei r use as soccer fields for recreational purposes. A structural European channel is planned for 2.3 mil es of the creek, most of which is in Ralston Creek. The European channel is designed for carry- ing the 10-year flood, however, because of normal freeboard and optimized depth of the channel bottom, the 10-year designed channel will carry most of the 100-year flood when running bank full. A low flow channel simil ar to the existing one will be included. An important aspect of the project is floodplain regulation and management. For instance, until the project is completed, construction in the flood- pl~'n must be subject to the floodplain regulation ordinances of Arvada or Jefferson County. An earl y warn i ng system is recommended. Moni tor i ng and inspection of Leyden Lake should be performed regularly. We have appreciated your cooperation and guidance. Very truly yours, WRIGHT WATER ENGINEERS, INC. By It." :>>-~..,~iJt- Kenneth R. Wrlgh~ P~--' Project Manager and Chief Engineer By ~ff~~~~ Project Engineer MWG/KRW: ard