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I I I I I I I I I I I I I I I I I I I As this study was being conducted, homes were being constructed adjacent to the channel below 112th Avenue. The developer has allotted a 30 foot right-of-way which extends southeast for approximately 600 feet where it empties into the first of two deten- tion ponds. A six foot reinforced concrete pipe with an overflow channel above the pipe is being proposed for this stretch by the developer, which, if properly constructed, will check the erosion problems currently plaguing the channel. The ponds as they now exist are capable only of adequately detaining the 2-year runoff. Inadequate spill ways subject these ponds to potential erosion, exposing the down- stream reaches to additional flood hazards. Below the ponds, the land use remains primarily agricultural. Two irrigation ditches traverse the basin and, since no formal separation structures exist, intercept a majority of the silt-laden runoff. A residence near Riverdale Road is subjected to flooding from the 100-year runoff. All traces of the historic channel have been eradicated by cultivation and by the upstream interception of runoff by the irrigation ditches. A detailed discussion of the existing flood hazards on Direct Flow Area 0054 is included in the Flood Hazard Area Delineation Report. Summary of the Phase B Plan The flood control plan selected by the Sponsors is summarized narratively in the following paragraphs and illustrated schematically on the drawings, Sheets 1 through 8. The directive memorandum is included in Appendix A for reference, but the selected plan is essentially that recommended by the Consultants in the Phase A analysis. Deviations from the Consultants' recommended plan have been noted in Chapter IV under the sub- heading, "Selection of the Recommended Plan". During the Phase B study, modifications to the directive memorandum were considered more feasible. After being mutually agreed upon by the Sponsors, the changes were incorporated in the Phase B report and are addressed in this section. East of Holly Street in Adams County, implementation of the selected plan would involve two phases. The initial phase would require the construction of a two-year grass channel and would prevent the damage caused by the relatively minor but frequent floods and greatly reduce the impact of the major flood events. The final phase would consist of enlarging the 2-year channel to 100-year capacity, and would be constructed when the adjacent property, which at present is predominently agricultural, is developed. Two tributaries that flow into the channel are also earmarked for minor drainageway improvements. 1-2 Above Holly Street, which is within the corporate City limits of Thornton, channel improvements are designed entirely to accommodate the 100-year event. North of 112th Avenue, a detention facility coupled with storm sewer improvements are proposed. Between 112th Avenue and Holly Street, the plan takes advantage of existing right-of-way to mitigate potential flood hazards. This includes the installation of a major underground conduit with an overflow channel, both of which drain into an existing pond. The pond itself is to be enlarged and improved to better detain and more safely convey storm runoff. Implementing the plan will provide for improved water quality and reduced public inconvenience as well. Figure 1-1 provides a geographical reference for the locations of the various proposed improvements. For the purposes of this study, the 0054-1 channel has been divided into four reaches: Reach 1 Confluence with the South Platte River to Riverdale Road, Adams County Riverdale Road to Holly Street, Adams County Holly Street to East 112th Avenue, City of Thornton East 112th Avenue to the Upstream Study Limits, City of Thornton Reach 2 Reach 3 Reach 4 In Reach 1, the implementation of the recommended plan would transpire in es- sentially two stages. The initial stage calls for a 2-year grass-lined channel preliminarily designed without the benefit of increased detention storage proposed for upstream. These upstream improvements cannot be assumed to have been fully implemented at the time of the initial phase of chll.nnel construction. The predominantly agricultural land use in Reach 1 does not substantiate a larger channel due to the relatively low amount of probable flood damage. But the absence of an established drainageway through this Reach necessi tates the construction of a formal channel to convey storm waters and alleviate much of the damages that are sustained during the higher frequency storms. Regulation of the residual floodplain in Reach 1 gives rise to the final ph(lse of the plan. It is flood- plain regulation which would prevent the unwise development in the floodplain or would require adequate channelization through any future development to safely convey flows through the reach without increasing the probable flood damage. A floodway adequate to convey the 100-year flood with increased upstream detention then becomes the final stage of the plan, with its construction a condition of the development of adjacent lands. A flow separation structure is recommended where the improved channel is to cross the Lower Clear Creek Canal, precluding the entry of unwanted storm waters into the canal during the deSign flood. Right-of-way acquisition and an access trail, which is to be constructed alongside the improved channel to provide for economical and regular maintenance, are both necessary parts of the plan.