Assessing Climate Change Impacts, Sea Level Rise and Storm Surge Risk in Port Cities (2008)

This study illustrates a methodology to assess economic impacts of climate change at city scale, focusing on sea level rise and storm surge.It is based on a statistical analysis of past storm surges in the studied city, matched to a geographical-information analysis of the population and asset exposure in the city, for various sea levels and storm surge characteristics.An assessment of direct losses in case of storm surge (i.e. of the damages to buildings and building content) can then be computed and the corresponding indirect lossesin the form of production and job losses, reconstruction duration, amongst other losesdeduced, allowing a risk analysis of the effectiveness of coastal flood protections, including risk changes due to climate change and sea level rise.This methodology is applied in the city of Copenhagen, capital of Denmark, which is potentially vulnerable to the effects of variability in sea level, as a low lying city.The analysis concludes that Copenhagen is not highly vulnerable to coastal flooding.In the absence of protection, however, the total losses (direct and indirect) caused by the current 120-yr storm surge event, at 150 cm above normal sea level, would reach EUR 3 billion.In the absence of protection, moreover, future sea level rise would significantly increase flood risks beyond this level.For instance, with 25 cm of mean sea level rise (SLR) total losses caused by a future 120-yr event would rise from EUR 3 billion to EUR 4 billion, to EUR 5 billion with 50 cm of mean SLR, and to almost EUR 8 billion with 100 cm of SLR.As extreme sea level events (referred to as "storm surge events") are not particularly high in Copenhagen, the city is relatively easy to protect with dykes and sea walls and the residual risk is low: while annual mean losses can reach several billions of Euros with protection of less than 1m, they decrease very rapidly with protection height, with less than 100,000 Euros per year for 180 cm.Given that the construction cost of a coastal flood protection system for Copenhagen is estimated to be a few hundred million Euros, it is a rational decision to protect the city with very high dikes, which is actually the case in most locations in the city.However, in some locations and in the most pessimistic IPCC scenarios, additional investment will be required by 2030.And because flood exposure will increase regardless of the protection level, the consequence of protection failure will increase with sea level rise.Faultless maintenance will, therefore, become even more crucial than today.Copenhagen"s high protection level and low risk to sea level rise and storm surge events is not widespread across world cities.To assess the economic cost of climate change and sea level rise in other cities, the methodology is applied on a virtual citybased on Copenhagen without its current levels of protectionand provides additional general insights into the impact of sea level rise on coastal cities.It is found that the increase in risk due to climate change is likely to be unacceptable in many cities, calling for an upgrade of flood protection in response to sea level rise.Even with upgraded defences which maintain unchanged the flood probability, the mean annual loss, i.e. the flood risk, increases because of the enlargement of the area subject to flooding.Flood probabilities, therefore, will have to be decreased to maintain the current level of risk.Moreover, while flood probability can be reduced to extremely low levels with appropriate protective measures, sea level rise will always increase the consequences of defence failure or overtopping by affecting increasingly large areas of the city.These results call for the introduction of adaptation in long-term planning to consistently take into account all dimensions of climate change.Mitigation policies can also reduce the potential impacts of climate change on coastal cities by limiting the pace of future sea level rise.