Flood frequency analysis (FFA)

Question 1

Definition

Answer 1

Statistical assessment of floods is done based on their return period T. Is the average elapsed time T in years between occurrences of an event (or its characteristic) which exceeds a certain magnitude X > x.

Tflood = 1/Pe = 1/(1-Pne)

Question 2

Procedure

Answer 2

1. Selection of sample (e.g. annual series)
2. Plausibility of data (homogeneity, consistency)
3. Selection of probability distribution F(x)
4. Estimation of parameters
5. Test for goodness of fit, confidence intervals
6. Application for design

Question 3

Recommended distribution functions

Answer 3

Generalised extreme value (GEV) → 3 par → shape B, D, C

Question 4

Estimation of parameters for probability distributions:

Answer 4

Probability weighted moments/ L-moments (NEW):

• Using relation between L-moments and parameters
• Exponent r at probability (see Eq. 6.4) → more robust against outliers

Question 5

Extreme Value

Answer 5

1. Extreme value distribution type I (EI, Gumbel):
   Simple, often used PD with 2 parameters c-location, d-scale
2. Generalised extreme value distribution (GEV):
   - 3 parameters: a-shape, c-location, d-scale, very flexible shape
   - Contains as special cases EI, EII, EIII