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MAY-19-04 14:24 FROM-WEISS AND VAN SCOYK 970 879 6U58 T-286 P.029/033 F-562 <br />Of parlicular concem is the laelc of documentation presented to satisfy Section <br />26.163(c)(1), Standards for Floodways, that states; <br />"Ericroachmenrs, including fill, ftebV constracceion, <br />sitbsrantial imProvements nnd orher developrrients are prohibiteil, <br />zrnless cerlication by a regislered professiortal engineer is provided <br />demonsrrnting thut encroctcytrne?2FS shall not res2tlt in any increase in <br />flood levels dterirtg the occxerrence of the bczse flood discharge. " <br />Examining Attact-unent D oF Mr. Browning's memorandum, significant fill has <br />apparently been placecl wiUiin the flooclway. Additiflnally, not only the boat chutes, but <br />each of the habitat structures described in the 404 permic appl.ication represent fill williin <br />the floodway. Conducting an a.nalysis that meets the current standard of practice within <br />the engineering community requires a quantitative, not qualitativc, assessmenl of a <br />proposed projecc. This type of analysis is typically conducted prior ta the onsec of <br />constniction. In addition, requirements set forth in the National Flood Insurance <br />1'rogram dictate that anaiysis For fill within a iloodway be conducced in aecordance wit11 <br />standard engineering practice. <br />As a member of the Larimer Couniy Flood Review Board, stanciard engineering <br />practice ror Larimer County requires a numeric analysis compaiing FL-MA ilood plain <br />dclineations and waier surface elevations to those resulling from a proposed projecl. NIr. <br />Lacy's qualitative analysis based on ", ..numerous IIEC-RAS models of similar stntctures <br />in similar rivers..." falls significantly shoR of demonstrating an analysis chat mects the <br />typical requirement of standard of cngineering practice. <br />Preliminary examinaiion of the only quantitative hydraulic analysis provided <br />indicates that there is, in fact, an adverse impac[ on the 100-year water surface elevation <br />due eo the constructed boak chute structures. Compari.ng the ITEC-RAS resulcs ohtained <br />by the CWCB staff for the effective, corrected effcctive and proposed, or in this casE as <br />built, conditions indicate that the boat chute structures result in a rise in the 100-year <br />ilood elevation in the immediate vicinity of the structures. While the CWCB analysis <br />sho«ld Ue considared preliininary until the model can be calibrated witlz a field survey of <br />actual wacer surf.ace elevations, the impact of placing •fill in the t7oodway 11as been <br />demonstrated. In addicion, it is recommended that the polential impact oF incrEases in <br />water surface elevatiou upstream of "C-Hole" on Soda Creelc be examined <br />In addition to assessing the hydraulic performance of the boat chute and habiEat <br />stntcttires immediately following construetion, an assessment of long-tenn perfonnance <br />and overall aFfect on loeal hydraulies should be conducted. As observed during the site <br />visit, and shown in Figure I, a point bar is beginning to form upstream of the "C-Hole <br />near the confluence of the Yan-ipa River and Soda Creek. Sediment deposition witliin tlle <br />floodway will serve to providz ineffective now arca wi[hin the cross section and <br />ihere:Care affect the ilow velocity and water surface elevation. Not only will the local <br />hydraulics of the Yampa River be influenced, the potzntial for impacc to Soda Creek also <br />exists. Both the boat chutes and the habital structures, construcied downstream or the <br />cliuees, stiould be analyzed in an efforc to document the long-tenn effects and impacts oF <br />4