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I : r I I I I ' I I I I I I I I I I I I I 19 The extensive flooding on North Dry Gulch is due primarily to the fact that there is no channel or other facility to con- vey the flow, even though the anticipated flooding is only two to three feet deep. The projected damage to the many commercial buildings gives this area an extremely high damage potential. and the runoff does not reach Colfax Gulch. The grading of Colfax thus serves to protect Colfax Gulch to a certain extent. In general, the Colfax Gulch flooding is less than one foot in depth. As a consequence, potential damages may not necessarily be severe. On the other hand, since the reach along Colfax Gulch is not subject to frequent large scale flooding, the aware- ness of the damage potential may be lacking. Colfax Gulch from Pierce Street to Balsam Street (Sheets 19, 20) Colfax Gulch derives its name from the fact that it inundates Colfax Avenue throughout the majority of its length. It is not actually a gulch, since no defined channel is visible at present, and since there is no evidence that such a channel ever existed. The drainageway, prior to urbanization, was probably a small tributary to North Dry Gulch or actually a portion of the North Dry Gulch valley. ,'Jhen Colfax Avenue was constructed, it became a ridge between North Dry Gulch and Colfax Gulch. Thus, the flows in the two channels were artifically separated by the street so that they could not meander as they had in the past. Flow can now pass from one drainageway to the other only when the water backs up on one side of the street to a sufficient depth to overtop the crown. The hydraulic analysis indicates the following flow transfers across Colfax: southward at Balsam and Ammons; northward at Allison, Zephyr, and Yarrow: northward at Vance; southward at Pierce. In storms of less than five year frequency, the North Dry Gulch flow does not overtop Colfax at Balsam and Ammons, The structures affected by flooding in the Colfax Gulch channel are predominantly commercial west of Teller, and predominantly residential between Teller and Pierce. The area has experienced drainage problems in the past, but it is likely that they were attributable to local runoff rather than from the flows in North Dry Gulch. The flooding illustrated on the FHAD Sheets would occur only when the overflow from North Dry Gulch at Balsam and Ammons is added to the local runoff.