Course Unit 11

Course Unit 11

Handling Uncertainty H.P. Nachtnebel

Dept. of Water-Atmosphere-Environment Univ. of Natural Resources

and Life Sciences

hans_peter.nachtnebel@boku.ac.at

Sources of Uncertainty

 Data gaps, limited sample size

 Measurement errors

 Stochastic component in nature (climate, hydrology,…..)

 Modelling errors (limited scope, simplified equations, unknown boundaries and initial conditions, parameters)

 Changing preferences in our society

 Unknown development of technologies….

Quantifying Uncertainties

 Graphical display of a time series

Course unit 11: Handling Uncertainty H.P. Nachtnebel

Quantifying Uncertainties

 Graphical display of a time series

 Histogram of a data set

H(Q)

Q [m³/s]

Quantifying Uncertainties

 Graphical display of a time series

 Histogram of a data set

 Fitting a pdf to a data set

model and data uncertainty

parameter estimation (moments, likelihood ?)

Gewässer: Donau Pegel: Kienstock (bei Krems) Beobachtungszeitraum: 1976 - 1999

0 10 20 30 40 50 60 70 80 90 100

250 500 750 1000 1250 1500 1750 2000 2250 2500 2750 3000 3250 3500 3750 4000 4250

Q [m³/s]

[%]

Unterschreitungswahrscheinlichkeit Ueberschreitungswahrscheinlichkeit

v x c s x, ,

Course unit 11: Handling Uncertainty H.P. Nachtnebel

How to handle uncertainty and risk ?

QIN f(QIN)

HP f(HP)

Water Resources Planning and DM: Unit 10 Uncertainty and Risk H.P.

Nachtnebel

Handling Uncertainties

 All the outcomes of a project, the rankings are subjected to uncertainty: they have a range, a pdf

Handling Uncertainties

 Sensitivity analysis

 Simulation

Simulation

 For each variable a pdf is assumed

Interest rate Probability

Investment costs Probability

Course unit 11: Handling Uncertainty H.P. Nachtnebel

Simulation

 For each variable a pdf is assumed

 Hundreds of simulations

 A pdf for the efficiency criterion is obtained

1 BCR Probability

Simulation

 For each variable a pdf is assumed

 Hundreds of simulations

 A pdf for the efficiency criterion is obtained

1 BCR Probability

Failure probability

The project will not be economically justified f(BCR)

Course unit 11: Handling Uncertainty H.P. Nachtnebel

Handling Uncertainties

 Sensitivity analysis

 Simulation:

We can estimate the failure probability

 We could prespecify a certain minimum probability of success.

E.g. I would accept a project with a 90 % probability of sucess:

P_S >0.9

F

S f x dx P

P ^ ^{( )} ¹

1



 ¹

0

) (x dx f

P_F

Example: Water Shortage









0

*)) ( ( ) (

*)

(Q f QD D QD Q dQD

R

t Q(t)

Q* Required amount Water deficits QD_i

QD (m³) Damage D (€)









 ⁱ

i

t

t

i Q Q t dt

QD

1

)) ( ( ^*

Example: Water Shortage

 Objetive: reduce the risk

Manage the demand (improve water use efficiency) Manage the supply (increase the supply rate by

building/enlarging a reservoir)

Storage capacity S

Costs C Costs C

Water Use Efficiency WUE

Example: Water Shortage

 What is the decision criterion ?

 ^{R ( Q *) C ( S )} 

Min 

 ^{R ( Q *) C ( WUE )} 

Min 

Course unit 11: Handling Uncertainty H.P. Nachtnebel

What happens after building a levee ?

f (Q)

Q

Potential D (Q)

Q X*

old new

PDFs and Risk curves

Risk curves

Flood probability f(Q)

Q

Damage potential D(Q)

Q

X*(T)

Damage potential D(Q) Prob(Damage>D)

1/T*

Risc curve

Increased damage potential

Design level X*

Course unit 11: Handling Uncertainty H.P. Nachtnebel

Consequences

 The expected damages may be larger after flood implementation of flood protection measures

 Land management and development strategies are required

 Safety of levees ?

Comparison of different risk curves

• Comparison of two hazards with quite different consequences

• A1 very low probability of occurrence but extreme consequences

• A2 high probability of occurrence but lower consequences

• E.g. A1 nuclear power station and A2 thermal power station

Cumulative probability

Damages

A₁ and A₂

A1 has a low mean value but is highly skewed A2 has a higher mean but an upper limit

1

0.5

Which alternative would you prefer ?

Course unit 11: Handling Uncertainty H.P. Nachtnebel

Comparing risks

 Two alternatives, A1 and A2, exhibit annual net benefits (k€) with a certain probability. The outcomes are A_ik

 Which one should be chosen ?

Comparing two Uncertain Outcomes

 Possible Decision Criteria Max { w_i NB_ik}

Max {Max(NB_ik)}

Max {Min (NB_ik)}

Course unit 11: Handling Uncertainty H.P. Nachtnebel

Decision techniques

 Bernoulli criterion: choose the one where K₁ is better:

K₁ = max {K_1,i} = max { w_k A_ik} K_1,1 = 4 333 k€/a

K_1,2 = 4 266 k€/a

Decision techniques

 Neumann-Morgenstern criterion: try to avoid losses or take a risk averse position

 K₃ = max{K_3,i} = max{min(A_ik) for w_k >p₀}

 Choose A₂ because the worst outcome is 3 600 k€/a which is better than the outcome of A₁

 Is a useful criterion for public investments, safe decision

Course unit 11: Handling Uncertainty H.P. Nachtnebel

Comparing two Uncertain Outcomes

Two alternatives with uncertain net benefits Which alternative is better

A1 A2

w1=33% 6 200 4 900

w2=33% 4 100 4 300

w3=33% 2 500 3 600

Perception of risk

Course unit 11: Handling Uncertainty H.P. Nachtnebel

Comparing two Uncertain Alternatives

Damages Social Costs

Comparing two Uncertain Alternatives

 The mean value of A1 < A2

 There is high probability that the damages vom A1 are smaller than from A2

 But it may happen that the damages from A1 are >>>

than from A2

 Being risk adverse select A2

Course unit 11: Handling Uncertainty H.P. Nachtnebel

Summary and Conclusions

 Uncertainty is essential in DM

 All our decisions are subjected to uncertainty

 Several performance indicators were defined failure rate, reliability, risk

 Several strategies for risk management were discussed Max average

Min losses (risk averse) Max max (risk prone)