Frequency Analysis

Question 1

What is frequency analysis about?

Answer 1

Is assigning the probability that a flow event will occur

Question 2

What is a statistical trial?

Answer 2

process or activity in which one outcome from a set of possible outcomes occurs

Question 3

What is an elementary outcome?

Answer 3

each different outcome is known as an “elementary outcome”

Question 4

What is an event?

Answer 4

is a subset of the sample place, can be a group of the elementary outcomes

Question 5

If the thing we are studying is random what happens to the elementary outcome?

Answer 5

Each elementary outcome has equal probability of occurrence

Question 6

If the thing we’re studying is independent what happens to each elementary outcome?

Answer 6

Each elementary outcome is not influenced by a previous elementary outcome

Question 7

If the thing we are studying is independent and random how do we calculate the probability?

Answer 7

do 1/the amount of potential elementary outcomes

Question 8

How do we define probabilities if the event is independent?

Answer 8

It’s the outcomes that give that event over all total outcomes

Question 9

How do you build a flood frequency analysis?

Answer 9

Get a hydrograph that spans (hopefully) 30 years, and pull out the peak discharge for each year.

Question 10

How can we have 1:50 year rainfall but a 1:100 year flood?

Answer 10

You would have snowmelt contribute to flood and also rainfall would go into river faster because of more overland flow as ground is frozen.

Question 11

In flood frequency analysis can yu extrapolate your flood return graph? Whats the limits on this?

Answer 11

Yes you can, but you can’t extrapolate for more than 2x the number of sample years

Question 12

What is risk and reliability?

Answer 12

risk- Risk is the probability of at least one flood in a certain number (n) years.
reliability- is the probability of no floods in a certain number (n) years

Question 13

What are the equations for reliability? What are the equations for risk?

Answer 13

For reliability, the equations are (1-1/T)^n
where t is return period and n is years
(1-P)^n where P= 1/return period

For risk:
1-reliability
or 1 - ( (1-1/T)^n)

Question 14

What does a flow duration curve do?

Answer 14

Reports how often a river will exceed a certain flow rate

Question 15

What are the axis of a flow duration curve?

Answer 15

Here we plot the flow rate over the percent exceedance (which is cumulative frequency of getting that flow rate)

Question 16

What are flow duration curves useful for in calcuating?

Answer 16

calculating Q95, Q50, Q10, this is the probability the flow will be at or higher than the value we get 95% of time etc.

Question 17

What are two things we need to watch out for in frequency analysis?

Answer 17

Step changes in your time series, probably caused by development (reservoirs, weirs, or even landuse change).
Gradual changes, possibly caused by development or climate change.
(as this stops independence of events)