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I I I I I I I I I I I I I ~ 10 ~ Because the summer thunderstorm is of small areal extent, runoff was assumed to corne only from the tributary over which the storm was centered. Adjacent tributaries also contribute runoff, but this did not appreciably affect the peak discharge due to shape of the basins, smaller storm intensities and time of concentration of the individual basins. Figure 2 summarizes the runoff from a storm occurring over the .various tributaries below the Tarn with a base flow from the Tarn of 90 cfs. The flow in the Blue River is derived from the tributary runoff plus base flow which gives the highest peak runoff. It was found that, except for the SPF runoff from Swan River, Lehman Gulch was the controlling flow. RUNOFF INVESTIGATION The MIT catchment Model (MITCAT) was used to derive runoff from synthetic storms for the four flood frequencies. This model is a general purpose simulation model of a watershed which was de- veloped at the Massachusetts Institute of TeChnology. The model is formulated in a problem-oriented language with a flexible command structure offering many input/output and computation options. It uses a Kinematic wave function, one of the simplest and most useful approaches available to the hydrologist today. A basin is modeled as a synthesized network of overland flow planes, stream reaches, and reservoirs. By segmenting the basin, the various hydrologic features can be analyzed as they vary at different points in space and time. I Input requirements for the MITCAT Model include Technical Paper 40 precipitation reduced to half-hour intensities, overland "n" values which are used to model runoff prior to flow in the channel, channel "n' values, and trapizoidal channel shapes which are used to model the existing channel. Because the flood producing event is a thunderstorm of small areal distribution and high intensity the greatest amount of rainfall was placed in the first half-hour of the synthetic storm. Soil moisture condi- tion II was assumed in the calculation of SCS soil curve numbers. I I The design storms were assumed to occur during late summer. During this portion of the year a base flow remains in the Blue River derived from a snowmelt and groundwater discharge. The average daily flow for the month of June was added to the flood hydrograph derived from the MITCAT techniques. I I I ~ Leonard Rice Consulting Water Engineers. Inc.