Question

non-exceedance probability of a flood event is 98.42%. what is the magnitude of the flood if the mean and standard deviation of the floods are 400 m3/s and 200 m3/s respectively.

Answer 1

solution:-

• non exceedance probability = confidence
• i.e. for probability of 0.9842 you can find the z value from the normal distribution table .
• download the table and note how to find the z value for a given probability in the following two step.

1. 1st find the probability value in the table i.e. 0.9842 and see the corresponding row for the z value which is 2.1.
2. and then see the corresponding column for 0.9842 and add the top most z value which is 0.05 to the previous z value.

• hence here z value for 0.9842 probability = 2.15

​​​​​​​

where

• x = magnitude of flood
• mu = mean
• sigma = standard deviation

​​​​​​​

x = 830 m^3/s

​​​​​​​hence magnitude of flood is 830 m^3/s