Risk based design of flood defence systems: A preliminary analysis for the New Orleans Metropolitan Area S.N. Jonkman M. Kok M. Van Ledden J.K. Vrijling

Background n Flooding of New Orleans due to hurricane Katrina n Improvements to hurricane protection system n Question: “How safe is safe enough?” n Risk informed decision-making n Objective: “Apply economic optimization to New Orleans metropolitan area” n Simplified and indicative approach n Study was part of “Dutch Perspective for Coastal Louisiana” by Netherlands Water Partnership

Economic optimization Investments Risk Total Costs optimum

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Netherlands n (Indirect) basis for safety standards n Current safety standards (derived in 1960’s) are expected to be outdated, due to growth of economy and population

Proposed flood protection system n Flood protection system proposed in Dutch perspective n Leading to different bowls / dike rings

Approach Data is needed: n Flood Damage n Safety level: Flooding probability n Investment costs and relationship with flooding probability Full scale risk analysis is preferred (IPET or FLORIS in the NL) Here: Simplified approach is used n Indicative estimates (‘order of magnitude’) n Limited number of safety levels: 1/100, 1/1000, 1/5000, 1/10.000, …. n Focussed on hurricane flooding (no rainfall and river flooding)

Flood Damage Based on evaluations of damage (IPET, Kok et al.): n north NO (central part) $15 billion n north NO: (eastern part) $ 10 billion n 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 n Flooding probability is assumed to be equal to the probability of exceedance of design conditions (water level, waves) n Post Katrina safety level: 1/100 per year Investments: n Investments relate to levee improvement and structures, e.g. floodgates n Costs are in the range of $ 20 - $ 60 million / km

Results: Central part of New Orleans Optimal safety level: 1/5000

Results: Eastern part Optimal safety level: 1/1000

South NO Optimal safety level: 1/1000

Sensitivity analysis: central part

Conclusions n It is possible to apply economic optimization to NO (and other areas) n Higher safety levels than 1/100 per year are obtained for densely populated areas n Obtained safety level for central NO is 1/5000 per year n With recent (higher) cost estimates a somewhat lower safety level (e.g. 1/1000 per year) could be obtained n Application of optimization leads to differentiation in protection levels

Recommendations n Similar analysis based on better estimates (costs, damage, safety levels and flooding probabilities) n Also for river flooding and rainfall n Apply other perspective in decision-making, esp. Risk to life n Integrate such analyses in the decision-making process with stakeholders