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111-3 tracks then east along the north side of the railroad tracks into Clear Creek. The ditch or swale capacity is inadequate to carry even the regularly recurring storms and significant street overtopping and flow splitting occurs. A portion of the Hidden lake Planning Basin, south of 64th Avenue between Sheridan Blvd. and Tennyson Street, is actually tributary to Clear Creek by way of a ditch or drainage swale along the west side of Tennyson Street. However, the storm flows greater than about 60 cfs will flow along the north side of the railroad tracks and combine with the flows of the rest of the planning basin. Since the area is not normally tributary to the main floodways of the planning basin, the area was analyzed separately, while still being con- sidered part of the Hidden lake Planning Basin. 2. Bates lake Planning Basin The Bates lake Planning Basin contains 0.74 sq. mi. (474 acres) (see Figures 11-2 and III-I) and lies mostly within the City of Arvada. The main features of the basin are Bates lake Park and the railroad and Sheridan Blvd. embankments, which serve to essentially isolate the area from combining with other storm runoff. Approximately one-third of the upper reaches of the basin are tributary to Bates lake Park by storm sewer and overland flow. The Bates lake Park was established as a storm runoff detention pond by the City of Arvada in 1971 and is presently in its final grading phase of construction. The outflow from the lake is controlled by a 21" RCP storm sewer which serves to throttle the flows from the several larger diameter storm sewers entering the lake. The outfall line follows south to 60th Avenue, increasing in size to pick up local storm runoff. From 60th Avenue south, the outfall line is increased to 42" RCP and continues south across the railroad tracks and into Ralston Creek near the