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I I I I I I I I I I I I I I I I I I I golf course by small diameter pipes, but larger floods will be conveyed by the fairways. A small lake has been constructed, Sheet 31, on the golf course and is used for irrigation. The normal water surface is about 1 or 2 feet below the confining embankment, so no appreciable detention is achieved. Downstream from the lake, another fairway of the golf course has been constructed. Townhomes line the edges of the fairway, but the first floor elevations, checked by field surveys, are above the estimated 10o-year water surface elevations. A grass-lined channel section has been constructed behind single family residences, Sheets 31, 32 and 33, which has capacity sufficient for conveying the 10o-year event. Double elliptical 66" x 42" concrete culverts placed beneath Otis Drive, Lamar, and Chase Streets are inadequate and floodwaters flow over the streets. From Chase Drive to Lake Arbor, the channel becomes substantially more shallow, yet is capable of conveying the 10o-year flood with only minor overflow occurring before the channel terminates in Lake Arbor. Lake Arbor, a natural lake used for irrigation, collects flow from Tributaries "B" and "C" and behaves essentially as a detention facility, retaining storm flows until the water surface elevation of the lake exceeds the spillway height. The spillway, coupled with the storage capabilities of the lake, regulate the flow rate such that 450 cfs is the maximum outflow over the spillway for the 100-year storm. The lake's banks to the north of the spillway structure are insufficient in depth so that they are overtopped and subjected to shallow flooding before the maximum spillway elevation is reached. This overflow is quickly funneled towards the trapezoidal concrete channel which extends from the spillway outlet to its confluence with the main stem of Little Dry Creek. As it approaches West 80th Avenue, the concrete channel turns abruptly to the south, Sheet 34. The 3 feet by 7 feet concrete culvert under 80th Avenue is sized to handle only the 10-year storm, approximately 235 cubic feet per second, without flowing over the road. Flows larger than this pond behind 80th Avenue before flowing over the -28- low point of the street which is about 500 feet east of the culvert. From here, shallow overland flow essentially follows Chase Circle before meeting up with Little Dry Creek's main stem approximately 650 feet south of 80th Avenue. As a result of this excess runoff coming over the road, the multi-family housing complex south of 80th Avenue is subjected to possible flooding from storms of greater than the 10-year magnitude. Tributary "C" The upper stretch of Tributary "C", Sheet 35, is completely developed with single family residences, erasing any traces of the historic channel. An underground conduit begins above Allison Court and runs to just east of Webster Street, collecting runoff as it follows the centerline of Pomona Drive. The conduit is three feet in diameter and only 45 cfs can be accommodated in the storm sewer. The excess flow ponds to a depth of about four feet behind Wadsworth Boulevard before continuing through the storm sewer to Webster Street. While the backwater allows the 100-year event to pass through the culvert without flowing over Wadsworth Boulevard, several homes immediately west of Wadsworth are subjected to flooding. As the tributary flow proceeds beyond Webster Street to Lake Arbor, Sheets 36 and 37, it follows Pomona Drive and essentially remains confined to the channel. A slight backwater effect occurs at the two crossings at Vance and Lamar Streets. The double 3.5' x 5.5' arch CMP and the double 34" x 55" elliptical CMP at Vance and Lamar Streets, respectively, accommodate the 10-year events, while larger quantities flow over the streets.