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I I I I I I I I I I Boulevard to the intersection of the gulch with Florida Avenue, ten-year flows are again confined in the gulch, and overflows spill down Florida Avenue and Florida Drive to Wadsworth Boule- vard (Sheet 18). These flows do not reach any residential units. The construction of new drainage facilities in the Sanderson and North Sanderson Gulch Basins has greatly reduced the flood damage potential along these streams. On Sanderson Gulch, with few exceptions, lOa-year flows are confined to the established flood channel. Culverts were generally designed for 10-year capacity with overflows confined to small areas and spilling back to the gulch within a short distance. On North Sanderson Gulch, existing development limits the practical size of a drainage channel, and capacity usually approaches a la-year flow design. Overflows are directed down streets and may attain significant depths and velocities. Above the gulch intersection, all flows exceeding the capacity of a small storm sewer system spill directly down Florida Avenue (Sheets 18 and 19). These flows may cause some shallow flooding to homes bordering this street. At Garrison Street, lOa-year flows spill across the roadway and flood some open space areas before funneling onto West Florida Avenue (Sheet 19). Flood Velocities I I I I I I I I I Average veloticites during a 100-year event are shown for typi- cal channel cross sections on the flood profile drawings, Sheets PI to P8. Typical velocities in improved channels range from 5 to 8 feet per second. Grasslined channels can generally withstand these flow velocities with only minor erosion damage. Average flow velocities in open space areas where the flood- plain expands over fairly wide areas are generally in the range of 2 to 5 feet per second. - SU1~RY - This report presents the probable floodplain limits that can be expected during a storm producing runoff that approaches the magnitude of the lOa-year event. These limits are shown on the Flood Hazard Area Delineation Mapping, Sheets 4 to 19 , for areas along Sanderson and North Sanderson Gulches. The floodplain limits presume planned, future development in the basin, existing floodplain conditions and no blockage at the existing culvert and bridge structures. Even with these assumptions, floods of greater magnitude can and will occur in which the limits of flooding will exceed those shown in this report. (8)