Design rainfall

Question 1

Design storm approach

- using single events

Answer 1

Observed rainfall
↓
Statistical analysis to obtain design storm with return period TS
↓
Rainfall-runoff model
↓
Design flood with return period TQ = TS

Question 2

Derived flood frequency approach - using

continuous rainfall

Answer 2

Observed rainfall (or synthetic
rainfall, see Chap. 2.5)
↓
Rainfall-runoff model
↓
Statistical analysis of simulated flood series to obtain design flood with return period TQ ≠ TS

Question 3

Design storm approach – statistical analysis:

Answer 3

 Rainfall is characterised by rainfall depth P or intensity PI and
rainfall duration D  statistical analysis in two steps:

 Possibilities for sampling and estimation of return periods T:

1) Annual maximum series (AMS): one event per year
2) Partial duration series (PDS): events above threshold

Results of statistics to derive design storms: Depth-Duration-Frequency (DDF) and Intensity-Duration-Frequency

Question 4

Selection of relevant duration D for design:

Answer 4

 Basic condition is decrease of intensity with increasing duration

 Critical duration Dm just after time of concentration Tc

 Try several storms with durations around Tc of the catchment

Question 5

Constant rainfall intensity:

Answer 5

 Most simple assumption

 Not valid for longer durations

Question 6

Design hyetograph – variable intensity:

Answer 6

a) Analysis of observed storm events and deriving typical shapes (e.g. Chow et al., 1988);
b) Theoretical construction using IDF-curves considering maximum intensities from all durations (ATV, 1999)
c) Simple geometric profiles like triangles or double exponential profiles