Question

The maximum annual instantaneous flows from a river are listed below. Plot the flood frequency curve on lognormal probability paper by the empirical method. Determine the flood having a probability of exceedance of 0.5. Also determine the flood having a return

a) 100-year flood
b) 50-year flood
c) 2-year flood
d) 25-year flood

Answer 1

To plot the flood frequency curve on lognormal probability paper, calculate the recurrence interval for each flood event, plot the discharge values and recurrence intervals on a graph, connect the data points with a best-fit line, and determine the discharge values for specific return periods.

Step-by-step explanation:

To plot the flood frequency curve on lognormal probability paper by the empirical method, follow these steps:

1. Calculate the recurrence interval for each flood event by using the formula: Recurrence Interval = (n + 1) / m, where n is the total number of data points (20 in this case) and m is the rank of the flood event.
2. Plot the discharge values on the x-axis and the calculated recurrence intervals on the y-axis on lognormal probability paper.
3. Connect the data points with a best-fit line.
4. Find the discharge value corresponding to a probability of exceedance of 0.5 on the best-fit line.
5. For the return periods, use the best-fit line to determine the discharge values corresponding to the desired return periods (100-year flood, 50-year flood, 2-year flood, and 25-year flood).