Flood Recurrence Interval: Mississippi River, KeokukThe following table presents peak disc...

Flood Recurrence Interval: Mississippi River, Keokuk

The following table presents peak discharge data for the Mississippi River at Keokuk, Iowa, from 1943 to 1992. This data set ends the year prior to the 1993 Mississippi River flood. We will use these data to estimate the recurrence interval for the 1993 flood event.

1. Complete the table to the right by calculating the remaining recurrence intervals (RIs) for the flood events of 1965, 1969, 1976, and 1972, using the formula RI = (N + 1)/rank where N = number of readings (49) and rank = order of readings (largest = 1; smallest = 49). For example, the RI for 1960 is ([49+1]/3) = 16.7.

2. Plot the discharge versus recurrence interval on the grid provided. Draw a straight line through the plotted points with RI values of 2 or more, and estimate the size of 100-year and 500-year floods for this gauging station.

3. Compare the predicted flood discharge against the discharge of the 1993 Mississippi River flood, 12,630 m³/s (446,000 ft³/s). What is the recurrence interval for the 1993 flood?