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i-. I j- 1 , I t l 1 te l 1 L L 1 I_e II. !)escription of Flood Improvement Projects St Vrain Creek to Twin Peaks Mall 1\. As mentioned previously, the City of Longment has authorized TST Consulting Engineers to provide the engineering design of the lowenrost sectjon of channel improverrents shown on Figure 2 and the profiles in Figures 3 and 4. Folla-ling a thorough investigation of lX>ssible channel routing alternatives and various channel and/or culvert designs, TST has selected a design \'lhich includes approximately 3318 linear feet of double concrete box culverts with an inlet structure, and 1312 linear feet of OTJen grass-lined channel including two channel gradient drop structures. The construct ion drawi ngs for these- improvements accaTIPany thi s suhnittal. Construction \'1i 11 begin thi s spring. The selected routing alterrlative is illustrated on the Project Layout 1>,1ap (Figure 2). This routing design will eliminate any upstream flC\v contributions from passing beneath State Highway 119, "nich should allE~viate some of the flooding problems downstream of SH 119 (the floodpl,:dn study for this reach is detailed in the FIS report for the City of Longmont - FEMA, 1983). However, these PIDPOsed floodplain amendrrents will affect the charmel upstream of SH1l9 only;' the exi sting channel downstream of SH119 to trle confluence wi th the St. Vrain Creek is inadequate to convey eVl~n the local i zed runoff proouced along this reach. The new Dry Creek No.1 outlet to the St. Vrain will be . located at a roint approximately 4,300 linear river feet upstream of the exi st i ng confluence. B. Twi n Peaks I''lall Construction on this section, designed by Charles H. Sain Associ ates, has been completed to the City of Longmont' s fi nal approval. The improvements consi st of approximately 1400 linear feet or concrete-lined ooen channel (bottom::: 30', side sloTJes = 1.5:1), including one double 6' x 12' ccrlcrete box culvert section reneath an access road crossing. Construction plans for this section are available from the City of Longmont Engineering Division. A public road crossing at Hover Hoad, designed and financed by the City of Longmcnt, is presently under construction. C. Upstream of Hover Hoad A 2,830 linear-foot section inrnediately upstream of the Hover Road crossing is presently under design by Rocky Mountain Consultants, Inc. The developer of this site is planning to create a parkway strip along the grass-lined channel (bottom = 18', side slopes = 10:1).