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I I I I I I I I I I I I I I I I I I I The focal point of the recommended plan in Reach 4 is the construction of a 22 acre feet capacity detention pond just North of East 112th Avenue. The existing 42 inch culvert beneath 112th would be utilized to drain the pond with a modified inlet which would serve as part of a dual system. Specifically, the modification would restrict outflow rates to that of the Lower Pond for the 2 and 5 year events while the 10 year outflow rate would be 90 to 100 cfs, approldmate1y the capacity of the existing 42 inch culvert. Curb cuts and possibly modification of street grades will be necessary to direct spillway flow over 112th Avenue and return it to the channel to the south, See Fig. 1-3. Further upstream in Reach 4, it is reQommended that the storm sewer at the end of East 112th Way cul-de-sac be expanded to accommodate the 10-year storm. The en- largement would entail the expansion of existing inlets and the installation of a necessary unoerground culvert which tentatively is to be placed between the houses at the end of the cul-de-sac. Since the design storm is less than the 100-year event, flood insurance and regula tion of the residual 100-year floodplain are inherent aspects of this plan necessary to offset residual flood damage. A 10-year grass lined channel is recommended as an interim measure to convey runoff from the proposed storm sewer and the existing 2-year storm sewer to the detention pond proposed for above 112th Avenue. An inundation depth of 6 inches at the crown of East 112th Avenue as allowed by the Urban Storm Drainage Manual for arterial streets during the 100-year storm can be ta.ken advantage of to carry approximately 330 cfs of the storm runoff. The remainder of the 100-year flow should be conveyed through this stretch when adjacent lands are to be developed utilizing open channel and/or closed conduit. ApproDriate curb cuts will be necessary to direct street flow from 112th Avenue to the pond to the north. It should be pointed out that included in the improvement costs for Reach 4 is right-of-way acquisition costs for land that the developer has previously expressed a willingness to set aside for drainage way imDrovements. If these costs were to be left off of the estimates, a benefit/cost ratio of .89 would be realized. Furthermore, the City of Thornton retains the option of making channel improvements in Reach 4 a condition of the development of adjacent land, further reducing improvement costs that would be incurred by the City. Any improvements to the Middle and Northern Branches of DireQt Flow Area 0054- 1 are not ecomonically warranted at the present time due to the lack of potential flood damage in these Reaches. It is therefore recommended that floodplain regulation and floodplain management continue and that required flood control works such as channeli- zation, measures to limit channel velocities and to control erosion, and 2- and 100-year on site detention be required of the developers of adjacent and upstream lands in these tributary basins. Several Dotential parks and recreational opportunities exist with the construction of the recommended channel improvements. It is estimated that the bottom dimensions of the 112th Avenue pond will be 150 yards by 70 yards, making it suitable for various field sports. Additionally, although not as cost effective, an underground conduit may be constructed in place of concrete channel in Reach 3, making it possible to use the above ground right-of-way for a continuous green belt and a hiker/biker trail as is recommended in Thornton's Comprenehsive Plan, Reference Q. Finally, should the area from Riverda1e Road to the South Platte River ever become developed, the drainage right-of-way would also provide a continuous hiker/biker trail and green belt which would extend the entire length of Reach 1. Acquisition of the public right-of-way in this portion of the floodplain cannot be economically justified from a flood control standpoint at the present time, however, because the average annual flood damages are too low. 1-3