Design Hydrograph and Estimation of exceptional floods

Question 1

For some problems the whole flood hydrograph is of interest (e.g. design of flood control reservoir, FCR). Basically, two possibilities exist to obtain a design hydrograph:

Answer 1

a) Rainfall runoff model (event based or continuous)

b) Analytical hydrograph

Question 2

Specific analytical hydrograph:

Answer 2

• Dimensionless mean hydrograph of direct runoff QD
• QD(t) will be normalised with peak runoff HQD(ts), and the time t is normalised with time to peak ts
• Empirical functions like Kozeny hydrograph :

Question 3

Estimation of m:

Answer 3

• Parameter m can be estimated with the least squares method comparing observed and analytical hydrographs
• A mean analytical hydrograph can be obtained by analysing many events and averaging the parameter m

Question 4

Design hydrograph:

Answer 4

a) Re-scaling of the mean Kozeny hydrograph with peak discharge HQD(T) and time to peak ts
b) Different hydrograph types for different flood types can be used

c) Alternative: multivariate flood frequency analysis considering several variables simultaneously
(e. g. peak and volume) using multidimensional PD’s or Copulas

Question 5

Estimation of design flood HQ with T»100 yr: Objective:

Answer 5

• Design of important hydraulic structures e.g. large dams
• In Germany: large return periods are requested regarding design for safety of the spillway (BHQ1=1000yr) and for stability of the dam (BHQ2=10000yr) according to DIN 19700
• Problems for estimation: small sample sizes and non-stationarity

Question 6

Estimation of design flood HQ with T»100 yr: Methods:

Answer 6

a) Extend sample size by spatial information extension (regional flood frequency analysis)
b) Using a rainfall runoff model with extreme design storm or long synthetic rainfall as input
c) Convention: prescribed PD e.g. use PIII distribution with maximum skew of cS=4 (see e.g. Kleeberg & Schumann, 2001) or by empirical scaling of lower standard quantile e.g. HQ(1000) = x * HQ(100)

Question 7

Estimation of the probably maximum flood (PMF): Definition and purpose:

Answer 7

• Maximum Flood which might occur given the most extreme meteorological and hydrological conditions  deterministic definition
• PMF is usually not used for design since uneconomical
• PMF is used for assessment of risk and required mitigation measures

Question 8

Estimation of the probably maximum flood (PMF): Methods to estimate PMF:

Answer 8

a) From probable maximum precipitation (PMP) using a simple rainfall runoff model (e.g. event + large runoff coefficient)
b) Using regional envelope curves developed from maximum observed or simulated floods