FLOOD11631 (2) - Laserfiche WebLink

3.6 Peak r'lows The final task remaining in the hydrologic analysis was to consider all of the factors discussed above and synthesize them. A discharge profile for each of the three major tributaries, from the furthest gage upstream all the way to the confluence with the Rio Grande, was needed. Because of the pattern of stream gages in the county, the most difficult part of this task was the determination of flows from the middle of the county west (downstream) to the Rio Grande. For a very short time (lpss than one year) there were stream gages at the mouth of ~ach of the three major tributary streams. These gages were not maintained long enough to provide meaningful dat~. Z'he gages that do provide such long, term data are mostly clustered in and near the Sangre de Cristo foothills. Determining discharge profiles was difficult for other reasons as well. Downstream of foothills locations like F'ort Gar].and, San Luis, and Garcia average flows are sic~nificantly reduced. Factors contributing to these reductions in a~rerage annual flows as one travels from east to west are valley storage, greater losses due to infiltration into sandier soils, reservoir storage, irrigation diversions from streams that are some of the mos~ overappropriated in Colotado, inadvertent detention by highways, roads, andl agricultural activity, and significantly reduced precipitation. The area around Alamosa, the eastezn gart of Conejos County, and the western part of Costilla County has the lowest average annual precipitation of any area in Colorado according to the Nat.ional Weather Service. As a result of these factors, the floodplains in the dowmstream portion of each of the three major basins are not as obvious in the field as the floodplains farther upstream. A detailed calcula~ion of that part of each stream's discharge profile would require reviewing reservoir records and irrigation records and performing hydrologic routings to determine losses due to valley storage and inadvertent detention by highways, roads, agricultural features and so on. Such an analysis was beyond the scope of this study. For that reason, it was decided that the peak flow determined at the gage that was trie farthest downstream on each of the three major streams should be used from the location of that gage downstream to the stream's confluence with the Rio Grande. In view of the above discussion about flow attenuation, that is a very conservative assumption. The analysis conducted for TM-1 and for the SCS study of Rito Seco were both plotted on one graph, showing drainage area on the independent axis and the ratio of the calculated peak 100-year flow to drainage area (cubic feet per second per square mil~e) -26-