Hydrologists use records of past river discharge levels over long periods to predict future flood likelihoods and return periods. They rank discharge levels and calculate recurrence intervals using a formula, which allows them to plot interval against discharge on a graph. This graph's line of best fit can then predict when the next flood of a particular magnitude might occur, known as its return period. The document asks the reader to use this process and graph to determine return periods for example discharges from the East Fork of the Carson River based on recordings from 1997 and 1955 over a 200 year period.

1. Flood Prediction and
Flood Return periods
Year 12 geography GCE

2. Flood Prediction?
Hydrologists try to forecast the likelihood of future flood events using past
records. The further back the records of the floods go, the more accurate the
prediction.
Records of river discharge are ranked over the longest period available, from the
highest peak discharge to the lowest peak discharge. The following formula is
then used to calculate the recurrence intervals

3. When the recurrence interval is plotted against discharge as a scatter graph on
semi –logarithmic graph, it is possible to use the line of best fit to predict when
the next flood of a particular magnitude might occur. This is called the flood
return period.

4. Look at the graph below. What would the flood return period be for a discharge
10,000 and 5,000 cumecs?

5. Using the formula, calculate flood return period for the discharge recorded in
1997 and 1955 for the East Fork of the Carson River. The recording period was
200 years.