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• Figure 2a shows Intensity-Duration-Frequency (IDF) curves for the mine site <br /> area that can be used to determine runoff from any of the three sub-basins using <br /> the Rational Method. Two curves are provided, one for annual series (all <br /> seasons) and a second for partial duration series (May through October). <br /> Figure 2b presents the NOAA data and calculations used to develop the IDF <br /> curves. <br /> Figure 2c through 2e present the Rational Method calculations of peak discharge <br /> and runoff volume for each sub-basin and for return periods of 2 through 100 <br /> year storm events. The calculated time of concentration for each sub-basin is <br /> about 5 minutes of less. Therefore the intensity values from Figure 2a for 5 <br /> minutes were used. <br /> • <br /> • p�q�r <br /> 10 <br /> Ana 23.M9 <br /> TR d= <br />