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<br />The third level of complexity is the network model. A computer is usually required at <br />this level to organize calculations. The models develop hydrographs and route flow <br />for each sub-basin in the system. The networks also account for storage within the <br />system. Commonly employed programs are the Corps of Engineers program, HEC 1, <br />the USDA Soil Conservation Service program, TR 20, and the Environmental Protection <br />Agency program, SWMM, which also models water quality information. All three <br />models are event oriented and do not monitor antecedent moisture conditions. <br /> <br />B. Snow Melt vs. Rainfall Events <br /> <br />Flood events in mountain catchments can often be classified as either a storm based, <br />or a snow melt instigated event. A review of the flood history of a particular <br />catchment indicates whether the catchment has a propensity to either event. Flood <br />information can also indicate the cause, storm based or snow melt, which produces <br />the highest peaks. A frequency analysis was carried out by the Corp of Engineers on <br />Clear Creek. The results from the Silverthorne station show that snow melt generated <br />floods, except in the case of very low frequency events, are more severe than storm <br />based floods (Figure 4). <br /> <br />The historical flood events for the Gregory Gulch catchment have been listed in Table <br />3. The table shows thirteen events, but one flood event on May 7, 1898 should be <br />disregarded because it was instigated by manmade, not natural, causes. Out of the <br />remaining twelve events, ten floods occurred in July and August. These dates support <br />the assumption that storm based events are the primary cause of flooding in the <br />Gregory Gulch catchment. <br /> <br />C. Evaluation of Methods for the Gregory Gulch Catchment <br /> <br />Part of this scope of work is to determine the 100-year peak flow at the entrance to <br />the Gregory Gulch flume. Several methods have been presented above for estimating <br />peak flows. Some of the methods have been applied in previous studies. Which <br />method of estimating peak flow is most appropriate for the Gregory Gulch catchment? <br />From the information gathered, an appropriate flow estimate method can be selected <br />by process of elimination. <br /> <br />Data <br />The first elimination is apparent. The lack of data in the Gregory Gulch catchment <br />removes the statistical methods of frequency analysis and flood-index, from the list <br />of flow estimate options for the Gregory Gulch catchment. The method of <br />transposition must also be eliminated because the nearest gage stations to the <br />catchment are on Clear Creek, and the drainage basins for these stations are <br />significantly different. <br /> <br />11 <br />