Final answer:
To determine the Santa Clara River's flood frequency, rank the largest discharge events, calculate the recurrence interval using the equation Recurrence Interval = (N+1) / M, plot the data on a flood frequency graph, and understand the probabilities associated with recurrence intervals.
Step-by-step explanation:
To determine the flood frequency or recurrence interval of the Santa Clara River, the process begins by analyzing historical discharge data from a USGS stream gauge. One must first assign ranks to each flood event with the highest discharge being ranked 1, and so on, down to the smallest ranked 20. The recurrence interval is then computed using the equation Recurrence Interval = (N+1) / M, where N is the number of years of record and M is the rank of the individual flood.
Next, one will create a flood frequency graph to visualize the relationship between discharge amounts and their corresponding recurrence intervals. The 2009 flood event, with a discharge of 21,200 cfs, is particularly noteworthy and its recurrence interval should be plotted on the graph as a point of comparison.
To understand flood recurrence intervals as probabilities, one can relate them to dice roll chances. A 3-year flood, for example, has a 1-in-3 chance of occurring in any given year. This does not guarantee that such floods will happen at regular, three-year intervals; several could happen close together or be many years apart.