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<br />The information presented here was compiled from the hydrology reports listed in <br />Table 1. <br /> <br />IV. PEAK FLOW ESTIMATING METHODS <br /> <br />Two methods were used to estimate the 100 year peak flows in the hydrology studies <br />presented in section III. The first method, a storm generated runoff calculation, uses <br />precipitation values and runoff curve numbers (CN). The second method is a <br />regression equation which is calculated from gage data at similar streams. Both <br />methods are recommended for a mid-size catchment when no stream gage data is <br />available. There are other methods available, this section considers which methods <br />are most appropriate for the Gregory Gulch catchment <br /> <br />A. Available Methods <br /> <br />There are three levels of complexity for estimating peak flows: a single value for peak <br />flow, a hydrograph, or a network model which produces multiple hydrographs. A <br />network model correlates the hydrographs from different sub-basins within the <br />catchment. <br /> <br />In the first level, there are many different methods of estimating a single peak flow <br />value. Popular methods can be categorized as statistical, empirical, or a flow balance. <br />Statistical methods include: <br /> <br />-frequency analysis of existing data; <br />-regression equation developed from frequency analysis on similar basins; <br />-transposition, adjusting data from an upstream or downstream site, or from a <br />similar stream; <br />-and flood index, relating a flood event to the mean annual flow (a frequency <br />of 2.33 years). <br /> <br />A correlation coefficient and standard error are commonly calculated for regression <br />equations to indicate how well the prediction curve matches existing data. The most <br />popular empirical method is the Rational Method. The Runoff Curve Number Method <br />is a popular form of a runoff/retention flow balance. Statistical methods are based on <br />stream flow records, while the Rational Method and Runoff Curve Number Method are <br />associated with precipitation records. <br /> <br />Unit hydrographs constitute the second level of complexity. They can be constructed <br />from existing data, or from synthetic methods such as the Snyder, the SCS Unit, or <br />the SCS Triangular. A curve number method is often employed in developing the lag <br />time for the SCS Unit Hydrograph. Information on storm intensities and durations are <br />required to construct a hydrograph, making the unit hydrograph a precipitation based <br />method. <br /> <br />10 <br />