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<br />0018Jl <br /> <br />FLOOD HYDROLOGY <br /> <br />Past Floods <br /> <br />Hydrology for the Rio Grande has been developed by the COE for <br />Alamosa, by the Federal Insurance Administration for Monte Vista and <br />Del Norte, and by the Soil Conservation Service for South Fork. All <br />agencies used recorded stream gage data for their analysis. The <br />hydrologic information is taken from previous reports including the <br />Flood Insurance Study (Ref. 1) and the Damage Survey Team Report (Ref. <br />2). <br /> <br />The hydrologic data for the flood impacted reach of the Rio Grande is <br />presented in the following table: <br /> <br />Ta b 1 e 1 <br /> <br />RIO GRANDE FLOOD FREQUENCIES AND DISCHARGES <br /> <br /> Drainage Flood Frequency <br />Location Area 10-year 50-year 100-year 500-yea r <br />Alamosa 1710 4,600 9,000 10,900 18,000 <br />Monte Vista 1590 9,320 11,850 12,880 15,150 <br />South Fork 1164 7,360 10,620 12,230 16,530 <br /> <br />In compa ri ng the recorded peak di scharges with the fl ood frequency <br />i nformat i on, it appears that the magnitude of the flood ranged from <br />about one in a 30-year event above Del Norte to about a one in 10-year <br />event in Alamosa. The reduction in magnitude is due in part to <br />diversions by irrigation ditches and natural storage in the valley <br />floodplain. <br /> <br />There are about 50 points of diversion for irrigation ditches on the <br />Rio Grande between South Fork and Alamosa. It has been estimated that <br />approximately 2,500 cfs can be diverted from the river through those <br />ditches. For fl oods of low magnitude, say below the 20-year return <br />interval, these irrigation diversions have a significant impact on <br />reducing the flood peak downstream. However, for a 100-year or <br />greater magnitude flood, these diversions will have a minimal impact <br />on reducing flood losses. No formal agreements for operation of the <br />ditches for flood control purposes are known to exist between the <br />private ditch companies and the towns. <br /> <br />Fl oodi ng on the Ri 0 Grande can result from snowmelt runoff, general <br />rains, cloudburst storms, or a combination of those conditions. The <br />runoff from snowmelt occurs during the period from late May to early <br />July. This is the time of the year that the river is most <br /> <br />-9- <br />