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I I I I I I I I I I I I I I I I I I I UTILE DRY CREEK FLOOD CONTROL IMPROVEMENTS ENGLEWOOD, COLORADO PROJECT DESCRIPTION Master planning studies conducted in 1974 and 1981 estimated that the City of Englewood would suffer approximately $14 million in damages from a 100-year flood along Little Dry Creek. The extensive floodplain (Figure 1) was also a severe constraint for the City's plans for redevelopment and revitalization of the central downtown area. McLaughlin Water Engineers, Ltd. was retained to design flood control improvements along Little Dry Creek through the City of Englewood. In the course of the 8-year effort of design and construction a number of challenging constraints and problems were overcome by unique solutions. Downtown Conduit Improvements A major constraint in the downtown area was the existence of a box culvert 3,500 feet long with poor entrance conditions and a twisting alignment with numerous sharp bends. Hydraulic calculations indicated that flow conditions within the conduit were extremely unstable, fluctuating between subcritical and supercritical regimes. At bends in the conduit, supercritical shock waves striking the ceiling created pressure flow conditions upstream that threatened to reduce the capacity of the conduit To confirm the hydraulic calculations and proposed designs for an extension of the conduit and an improved inlet structure, a model of the conduit was constructed at the Engineering Research Center at Colorado State University in Fort Collins. Colorado. Tests conducted with the model verified the hydraulic calculations and enabled refmement of the designs for conduit improvements to optimize its capacity, Conduit improvements were constructed in two phases. During the winter of 1985 a 400 foot long segment of 12 feet high by 24 feet wide reinforced concrete conduit was completed at a cost of $725,000. In the summer of 1987 a 630 foot extension of the box was completed within 60 days at a cost of $600,000 under a fast track schedule to meet surrounding development plans, OfT-Channel Storage Facility Comparison of the 3,650 cfs capacity of the improved conduit with the 4,400 cfs 100-year flow entering Englewood upstream confirmed the need for a storage facility that would reduce the peak flow to a magnitude within the capacity of the box culvert, A storage facility located at the upstream end of the project would also economize the sizing of channel and bridge improvements along the 0.7 mile reach downstream to the downtown conduit entrance,