Risk based design of flood defence systems:

A preliminary analysis for the New Orleans Metropolitan Area

S.N. JonkmanM. Kok

M. Van LeddenJ.K. Vrijling

Background

Flooding of New Orleans due to hurricane KatrinaImprovements to hurricane protection system

Question: “How safe is safe enough?”Risk informed decision-making

Objective: “Apply economic optimization to New Orleans metropolitan area” Simplified and indicative approachStudy was part of “Dutch Perspective for Coastal Louisiana” by Netherlands Water Partnership

Economic optimization

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Safety standards in the Netherlands

Applied after 1953 flood disaster in the Netherlands(Indirect) basis for safety standardsCurrent safety standards (derived in 1960’s) are expected to be outdated, due to growth of economy and population

Proposed flood protection system

Flood protection system proposed in Dutch perspective

Leading to different bowls / dike rings

Approach

Data is needed: Flood Damage Safety level: Flooding probability Investment costs and relationship with flooding

probability

Full scale risk analysis is preferred (IPET or FLORIS in the NL)

Here: Simplified approach is used Indicative estimates (‘order of magnitude’) Limited number of safety levels: 1/100, 1/1000,

1/5000, 1/10.000, …. Focussed on hurricane flooding (no rainfall and river

flooding)

Flood Damage

Based on evaluations of damage (IPET, Kok et al.): north NO (central part) $15 billion north NO: (eastern part) $ 10 billion south NO: $ 5 billion

(pre Katrina damages)

Risk to life and other types of consequences are not taken into account

Risk (=prob. x damage) determined as net present value

Real discount rate of 2% is used

Safety levels

Flooding probability is assumed to be equal to the probability of exceedance of design conditions (water level, waves)

Post Katrina safety level: 1/100 per year

Investments: Investments relate to levee improvement and

structures, e.g. floodgates Costs are in the range of $ 20 - $ 60 million / km

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Results: Central part of New Orleans

Optimal safety level: 1/5000

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Results: Eastern part

Optimal safety level: 1/1000

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South NO

Optimal safety level: 1/1000

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Sensitivity analysis: central part

flood probability Base case 1/5000 Flood Damage

- 50 % lower - 100% higher

1/1000 1/5000

Net Discount rate - 50% lower - 100% higher

1/5000 1/5000

Investment costs - 50% lower - 50% higher

1/5000 1/5000

Conclusions

It is possible to apply economic optimization to NO (and other areas)

Higher safety levels than 1/100 per year are obtained for densely populated areas

Obtained safety level for central NO is 1/5000 per year

With recent (higher) cost estimates a somewhat lower safety level (e.g. 1/1000 per year) could be obtained

Application of optimization leads to differentiation in protection levels

Recommendations

Similar analysis based on better estimates (costs, damage, safety levels and flooding probabilities)

Also for river flooding and rainfall Apply other perspective in decision-making, esp.

Risk to life Integrate such analyses in the decision-making

process with stakeholders

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