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Quantifying Errors in Long-term Coastal Erosion and Inundation Hazard Assessments

Abstract : As assessments of coastal vulnerability to erosion and inundation become more widespread due to the impacts of climate change, the methods used to make these assessments come under further scrutiny. Within a French inter-ministerial program called "Impacts of climate change, adaptation, and costs in France," the "National risks, insurance, and climate change" (GT RNACC) working group developed a strategy to evaluate coastal risks at the regional scale in France. However, this study identified numerous sources of significant errors in the estimation of areas affected by coastal erosion and inundation due to sea level rise. Thus, the goal of this study is to assess these errors associated with the estimation of shoreline change and inundation due to sea level rise from three perspectives: variability of sea level rise predictions, variability of methods applied to estimate long term shoreline change (and identify potentially eroded areas), and quality of the topographic data used. Three independent analyses investigate the errors associated with each of these factors. The study site is an approximately 20-km stretch of the French Mediterranean coast in the Languedoc-Roussillon region. The coastal zone is a low-lying area, composed mostly of narrow, sandy beaches, which are in some regions backed by short dunes (generally less than 5m height), homes, and public infrastructure. This region is currently exposed to coastal erosion and inundation hazards with during extreme storms, and sea level rise is predicted to aggravate these coastal hazards in the future. Sea level rise observations from local tide gauges range from 2 to 3 mm/year, while sea level reconstructions in the Mediterranean and regional model projections suggest less than 2.5 mm/year of sea level rise. At the global scale, sea level rise projections from the IPCC fourth assessment report are between 2 and 6 mm/year, although more recent studies addressing the increasing rate of polar ice cap melt have indicated rates of more than 10 mm/year. Therefore, four scenarios of sea level rise are selected for the comparative analysis: 0m, 0.5m, 1m, and 1.5m by 2100. To estimate the impact of sea level rise on the coastal zone, the Bruun Rule is most commonly applied method. The Bruun Rule assumes that a beach profile maintains a constant equilibrium form that adjusts to sea level rise with a vertical and landward translation. The formula assumes that sea level rise causes only erosion and that the sediment budget is closed between the berm and the offshore closure depth, neglecting the effects of the local geology or other physical forcings. Several studies have questioned the validity of the assumptions in this formula. Therefore, this study explores applying a variety of methods commonly used to estimate long term shoreline change, including the Bruun Rule, the extrapolation of historical trends, the extrapolation of historical trends with an adjustment to take into account sea level rise, the use of a fixed erosion rate, and a qualitative analysis of impacted zones by coastal scientists with knowledge of the regional coastal zone. Finally, the third source of errors addressed in this study is the quality of the topographic data near the coastal zone. Identifying coastal areas impacted by erosion and inundation requires topographic data for delimiting zones, particularly when considering vertical inundation levels. Studies at the regional and national scale are often limited by the quality of available data, thus this study uses three independent digital terrain models, with varying horizontal and vertical resolution, to investigate the impacts on hazard mapping. The digital terrain model BD Alti from the French IGN (Institut Geographique National) is publicly available at the national scale, but has 50 m horizontal resolution, and increases in increments of 1 m (step changes) in the vertical. In addition, two privately available datasets provide significantly higher resolution. The digital terrain models were produced from aerial surveys with an interferometric synthetic aperature radar (from Intermap Technologies) and aerial Lidar (light detecting and ranging) surveys (data provided by the DREAL LR, Direction Regionale de l'Environnement, de l'Amenagement et du Logement in Languedoc-Roussillon). The resultant horizontal resolution of these two digital terrain models is 5 m and 2 m, respectively, and the vertical errors are reported to be within 15 to 20 cm. This study quantifies three sources of errors in coastal erosion and inundation hazard assessments, with the intent of identifying and minimizing the primary sources of error for future regional hazard assessments in France.
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Contributeur : Marissa Yates <>
Soumis le : jeudi 10 février 2011 - 11:22:45
Dernière modification le : vendredi 4 mai 2018 - 11:02:01

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  • HAL Id : hal-00564846, version 1

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Marissa Yates, Gonéri Le Cozannet, Nicole Lenotre. Quantifying Errors in Long-term Coastal Erosion and Inundation Hazard Assessments. Journal of Coastal Research, Coastal Education and Research Foundation, 2011, SI 64, pp.230-264. ⟨hal-00564846⟩

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