Risk-based flood protection planning under climate change and modelling uncertainty: a pre-alpine case study

Abstract

Planning authorities are faced with a range of questions when planning flood protection measures: is the existing protection adequate for current and future demands or should it be extended? How will flood patterns change in the future? How should the uncertainty pertaining to this influence the planning decision, e.g. for delaying planning or including a safety margin? Is it 10 sufficient to follow a protection criterion (e.g. to protect from the 100-year flood) or should the planning be conducted in a risk-based way? How important is it for flood protection planning to accurately estimate flood frequency (changes), costs and damages? These are questions that we address for a medium-sized pre-alpine catchment in southern Germany, using a sequential Bayesian decision making framework that quantitatively addresses the full spectrum of uncertainty. We evaluate different flood protection systems considered by local agencies in a test study catchment. Despite large uncertainties in damage, 15 cost and climate, the recommendation is robustly for the most conservative approach. This demonstrates the feasibility of making robust decisions under large uncertainty. Furthermore, by comparison to a previous study, it highlights the benefits of risk-based planning over a planning of flood protection to a prescribed return period. IntroductionTechnical flood protection measures have long life times of, on average, 80 years (Bund / Länder-Arbeitsgemeinschaft Wasser, 20 2005). The uncertainty over such a long planning horizon is large, both in terms of climatic and socio-economic development.It is thus not trivial for planning authorities to take decisions on flood protection planning that are economical while not leading to excessive losses or high adjustment costs. I, it is important to consider costs -in construction, adjustment and flood damages -over the entire measure life time.Ideally, the planning of flood protection infrastructure is performed through a risk-based approach. Thereby, potential 25 damages are considered in the decision-making process. Considering that the annual maximum discharge is the main driver for flood damages, the annual flood risk in year , t , is defined as (e.g. Merz et al., 2010a) Nat. Hazards Earth Syst. Sci. Discuss., https://doi