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I I I I I I I I I I I I I I I I I I I 14 remaining channel after a hypothetical encroachment of land is allowed on each side of the channel which will not cause more than a nominal rise in the water surface over the natural 100- year flood. The allowed nominal rise in water surface used in these computations is approximately one half foot. Where possible, the hypothetical encroachments are allowed only so far as they remove an equal conveyance of floodwater from each side of the channel. The backwater contributes to extreme inundation of one struc- ture west of Sheridan. The remainder of the text of this Section presents descriptions of flooding characteristics to supplement the FHAD Sheets. The descriptions are keyed to the sheets as well as to geograph- ical locations. Figures 5 and 6 show the channels at selected points. The confluence of North Dry Gulch and Dry Gulch near Harlan Street has been completely filled in, with the drainage courses being generally obliterated. The gulches are replaced by culverts of indeterminate sizes, locations, and conditions, but the pipes are without question inadequate to convey the 100-year flood. It is at this location that the flood potential of Dry Gulch materializes, since the filling near Harlan Street creates a backwater condition west of Harlan Street. Dry Gulch from Lakewood Gulch to Harlan Street (Sheets 12,13,14) Dry Gulch in this reach is a deep, steep banked natural drain- ageway with a small, relatively constant low flow. Throughout the majority of its length, the channel has sufficient capacity to convey the 100-year flow, although it is constricted at the Perry Street and Sheridan Boulevard crossings. The box culvert under Perry Street is not adequate and causes overtopping of the roadway. However, no flooding of structures would result despite a water depth of 24 feet upstream of Perry Street. The culvert under Sheridan Boulevard causes a backup of flow, but conveys the 100-year flood without overtopping the street. While the Dry Gulch channel in this reach is generally capable of carrying the 100-year flow, it should be noted that increased development in the upstream watershed is creating flood flows in excess of those natural floods which have historically formed the channel. As a consequence, it is likely that erosion of the channel bed and banks will continue; this may result in dam- age to roadways, railroad beds, and bridges in the reach. Flow velocities in the lOO-year flood will exceed l3 feet per second in some areas. Since velocities near ten feet per second can cause erosion damage, the analysis confirms that erosion will continue to be a concern in the natural channel.