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<br />AMENDMENT TO <br />HYDROLOGY ADDENDUM <br />MONTROSE FLOODPLAIN STUDY <br /> <br />A comparison was made between the Mont.!~ose Floodplain Study. and the <br />Gar'field and Mesa Counties, Colorado, F'lood Insurance Study Hydrology (FIS), <br />(Novemeber 1975 Corps of Engineers), This comparison indicated the effect <br />of snowmelt on the runoff frequency ana'lysis contained in the Hydrology <br />Addendum to the former study. <br /> <br />The Montrose area drainage basins were correlated with the snowmelt <br /> <br />relationships presented in the FIS study. The correlation was based on basin <br /> <br />area and e1evat'ion characteristics, yieldin<) 100 year snowmelt flood quantities <br /> <br /> <br />for a unit area. Snowmelt flood quantities were determined from the data for <br /> <br />other frequency events, as shown on the attached figure from the FIS report. <br /> <br /> <br />The base snowmelt curve shown on this figure was used to define snowmelt <br /> <br />curves for the three Montrose area drainage basins. The Alkali Creek basin <br /> <br />was selected to defi ne the base curve because of s imil arity of ba~;i n character- <br /> <br />istics to the Montrose area basins. <br /> <br />Also shown on the figure is the rainfall/runoff data derived for the <br /> <br />Montrose Floodplain Study. As can be seen from the figure, the impact of <br />snowmelt flooding is confined to the lower frl2quency storms. For both Dry <br /> <br />Ced,~r Creek and Montrose Arroyo, snowmelt fl oodi ng does not exceed ra i nfa 11 <br /> <br />dedved flooding for greater than the 10 year frequency event. Fo!O the <br /> <br />Cedar Creek basin, the effect of snowmelt is more pronounced; i.e., snowmelt <br /> <br />flooding exceeds rainfall runoff for flood frequencies lower than the 100 <br /> <br />yealO recurrence interval. <br />