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I I I I I I I I I I I I I I I I I I I 12 The upstream end of the well defined channel of Dry Gulch is at 15th Place and Miller Street. The flow which can reach this location is greatly restricted due t.o inadvertant detention ponding at two upstream points -- l4th Avenue and Miller Street (point D-3 on FHAD Sheet 2), and 20th Avenue and Nelson Street (point D-O). It is presumed that future basin development will eliminate the ponding area on 20th Avenue, so the hydrology in this report does not reflect lS detention storage. The ponding on 14th Avenue is considered an existing channel condition, and a reduction is made to the peak flows due to the storage. analysis. The North Dry Gulch flow reaches Colfax at Balsam (point N-2) and then diverges. A portion proceeds easterly on Colfax in North Dry Gulch; another portion crosses Colfax to the south into a channel which has been designated "Colfax Gulch" by the writers, solely for the purposes of identifica- tion. Dry Gulch crosses Colfax at Garrison Street with facilities which are adequate to convey only small flows (less than a two- year flood). The flow splits at this location, with a portion crossing Colfax to the Dry Gulch channel, and a portion flowing easterly on Colfax into the North Dry Gulch basin. East of Garrison, the Dry Gulch channel is generally well defined, and no flow splits or diversions of major significance are observed. Between Balsam and Pierce Streets, North Dry Gulch flows easterly on the north side of Colfax, and Colfax Gulch flows easterly on the south side of Colfax. It appears that the North Dry Gulch channel is the "natural" channel, while the Colfax Gulch channel has apparently been created when Colfax Avenue was constructed in such a way that the flow is prevented from flowing north to North Dry Gulch. The two channels converge near Yarrow Street, and then diverge again. They then converge at pierce Street (poinm N-6 and N-7) into the North Dry Gulch channel, which joins Dry Gulch near Harlan Street. The North Dry Gulch analysis begins at Dover Street just north of Colfax (point N-l). At this point the runoff from the North Dry Gulch basin combines wlth the diversion from Dry Gulch at Garrison and with an overflow from the Rocky Mountain Ditch. The ditch has been observed to be a factor in delivering flood flows to the channel, but the magnitude and frequency are indeter- minate, and thus the effect of the ditch is disregarded in the The computations of the flow splits described above are greatly dependent upon the grading of Colfax and the capacity of facil- ities crossing the street. Any future changes could have signifi- cant effects upon the hydrology, and thus the flooded outline designations. A detailed, technical appendix covering the hydro- logic analysis is available at the office of the Urban Drainage and Flood Control District.