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I I I I I I I I I I I I I I I I I I I Reach III A (Wads\\Orth Boulevard to 900-fept Upstream of Kipling Street Sta. 449+00 to Sta. 559+00) Vi-8 The add it ion of a 50-foot bridge span to the ex i st i ng K i pI ing Street bridge and the construction of 1800-feet of rock-lined channel upstream and down- stream of the bridge \\Ould pI iminate the overtopping of Kipl ing Street hy the laO-year flood. A rock-lined channel transition from the proposed CSDH channel improvements downstream to the bridge, with a concrete-lined channel through the bridge, would be required to lower the 100-year flood water surface elevation to the existing condition immediately upstream of the bridge, thereby mitigating the increased flooding depths created by the CSDH channel improvements. It is recommended that the CSDH recognize the need for the transition channel through the bridge as being a part of the 1-76 construction improvements. The lOO-year fl oodpl ain is not contained within the improved channel between Stations 540+00 and 549+00, and Stations 555+00 and 559+00. The 1 imits of the lOO-year floodpl ain for the improved channel conditions should be regu- I ated in this area after the channel improvements have bef'n constructed. Channel improvements in this reach by the Colorado State Division of High- ways (CSDH) both existing and proposed, significantly alter the existing 100-year floodplain. At the time of publication of this report information was not avail able from the CSDH to del ineate the lOO-year floodpl ain and water surface profi I I" after the channel improvements are constructed. How- ever, using normal depth calcul ations and a typical CSDH channel section a water surface profile was plotted and is shown on the prel iminary design plan and profile drawings (sheets 4, 5, 6 & 7). It was determined that the channel improvements will contain the lOO-year flood discharge, except for the stretch of channel upstreilTl of Sheridan Boulevard to the 1-76 crossing. In this reach, the left bank of the channel is not high enough to contain the lOO-year flood. When the information becomes available, it is recom- mended that standard backwater computations be performed for the CSDH chan- nel improvements to determine the 100-year floodplain and water surface pro- file for the improved condition. This new floodpl ain should then be evaluated, approved, and regulated by the appropriate governmental entities. The Clear Creek channel between Tennyson and Lowell will be left in its natural condition. The 100-year flood discharge through this natural chan- nel sect ion wi 11 be greater after the CSDH channel improvements have been constructed upstream. Any changes to the existing 100-year floodplain should be determined by the CSDH and evaluated by the regulating agencies. Again, additional flood damage mitigating measures should be considered as being a part of the 1-76 construction improvements. Reach II B (1400-feet UpstreilTl of the 0 & RGW Railroad Bridge to Wadsworth Boulevard Sta. 272+30 to Sta. 449+00) Lowell Boulevard bridge is impacted by the CSDH upstream channel improve- ments in a similar manner as Tennyson Street. Unless other improvements are provided to mitigate the increases caused by the CSDH construction, the laO-year water surface elevation immediately upstream of the bridge will be increased by approximately 3 feet above the existing lOO-year flood level. An additional IOO-foot long bridge span, with 520-feet of rock-lined channel improvements downstream to the proposed CSDH channel, would be required to maintain the 100-year flood water surface at the same level as the existing channe 1 condit ions. Th i s improvement should be recogn i zed by the CSDH as being a part of the 1-76 construction improvements. It is recommended that during final design the possibility of lowering the channel, preserving the existing bridge, and relocating the Fisher Ditch diversion structure upstream be evaluated, since costs of this alternative may make it viable. The CSDH channel improvements upstream of Tennyson Street increase the 1.00- year flood discharge at the Tennyson Street bridge. Unlf'ss other improve- ments are provided to mitigate this increase, these CSDH improvements will raise the lOO-vear water surface elevatinn above the existing laD-year flood . . level by approx imate I y one foot, iml'led i ate I y upstream of the bridge. The 100-year floodplain north of the proposed 1-76 highway embankment will be altered after construction of the CSDH channel improvements. More water will be directed into the main channel of Clear Creek allowing less water to flow overland across Tennyson Street and Lowell Boulevard. It is recom- mended that computations be made by the CSDH to determine the lOO-year