Salt Marshes for Flood Protection

Salt marshes for fl ood protection fl Salt marshes for Salt marshes for fl ood protection Long-term adaptation by combining functions in fl ood defences Long-term adaptation by combining functions in fl ood defences combining functions in fl by Long-term adaptation Jantsje M. van Loon-Steensma Jantsje M. Jantsje M. van Loon-Steensma Proefschrift Jantsje omslag.indd Alle pagina's 20-8-2014 14:37:40 Salt marshes for flood protection Long-term adaptation by combining functions in flood defences Jantsje M. van Loon-Steensma Thesis committee Promotors Prof. Dr P. Vellinga Professor of Climate Change, Water and Flood Protection Wageningen University Prof. Dr M.J.F. Stive Professor of Coastal Engineering Delft University of Technology Other members Dr T.J. Bouma, Royal Institute for Marine Research (NIOZ), Yerseke Prof. Dr S.N. Jonkman, Delft University of Technology Dr I. Möller, University of Cambridge, United Kingdom Prof. Dr A. van den Brink, Wageningen University This research was conducted under the auspices of the Graduate School for Socio-Economic and Natural Sciences of the Environment (SENSE) Salt marshes for flood protection Long-term adaptation by combining functions in flood defences Jantsje M. van Loon-Steensma Thesis submitted in fulfilment of the requirements for the degree of doctor at Wageningen University by the authority of the Rector Magnificus Prof. Dr M.J. Kropff, in the presence of the Thesis Committee appointed by the Academic Board to be defended in public on Wednesday 8 October 2014 at 11 a.m. in the Aula. Jantsje M. van Loon-Steensma Salt marshes for flood protection; Long-term adaptation by combining functions in flood defences 200 pages. PhD thesis, Wageningen University, Wageningen, NL(2014) With references, summaries in Dutch and English ISBN 978-94-6257-099-3 Contents Chapter 1 Long-term adaptation by combining functions in flood defences: Context, 7 meaning and implications Chapter 2 Green adaptation by innovative dike concepts along the Dutch Wadden Sea 19 coast; A systematic design and evaluation of innovative dike concepts Chapter 3 Salt marshes to adapt the flood defences along the Dutch Wadden Sea 47 coast; What is the potential for integrating salt marshes in the flood defence zone? Chapter 4 Trade-offs between biodiversity and flood protection services of coastal salt 67 marshes Chapter 5 Development of salt marshes for coastal defence; three case studies in the 79 Wadden region Chapter 6 Further quantification of the ecological value of stabilized and restored salt 125 marshes; Vegetation of two recently restored Wadden Sea salt marshes compared to established salt marshes Chapter 7 The effect of vegetation characteristics on wave damping 139 Chapter 8 Synthesis 157 References 171 Summary 183 Samenvatting 187 Curriculum Vitae 193 Dankwoord 197 Chapter 1 1 Long-term adaptation by combining functions in flood defences: Context, meaning and implications 1.1 Evolution of flood protection in the Netherlands In 2008, the second Delta Committee recommended that the Netherlands increase its flood protection level and seek flexible and integrated climate-change adaptation measures (Deltacommissie, 2008). This initiated a quest for new flood protection concepts that could meet these requirements. The main concerns underlying the Committee’s recommendation were the effects of an accelerated sea level rise and changes in regional precipitation patterns due to global climate change (IPCC, 2007), ramifications of land subsidence, and increasing residual risk (i.e. potential damages caused by a dike breach) due to demographic (more people) and economic (increasing capital) trends. Since the flood disaster of 1953, the Netherlands has implemented a risk-based flood protection strategy using so-called ‘dike rings’, based mainly on the work of Van Dantzig (1956). That approach forms the basis of Dutch flood protection, and is still being refined and extended (e.g. Speijker et al., 2000; Jonkman et al., 2003; Jonkman et al., 2011; Brekelmans et al., 2012). By law, the dike rings must protect the encircled hinterland against river floods and storm surges of a severity that could be statistically expected at a frequency varying from once in 1,250 years to once in 10,000 years, depending on the region and the related values at risk (Ministerie van Verkeer en Waterstaat, 2007a). Dutch law prescribes not only dike design requirements, but also their regular assessment and management. When the concept of a risk-based flood protection system was conceived (Van Dantzig, 1956), anthropogenic-induced climate change and its far-reaching effects were only starting to receive attention (Revelle & Suess, 1957). In the 1990s, however, it became evident that climate change alters hydraulic boundary conditions and introduces uncertainty in extrapolations of historic patterns of sea-level rise, wind direction and strengths, rainfall and 7 Chapter 1 river discharge. This was recognized as having considerable short-term impact on Dutch flood safety. Even though the basic idea of differentiated risks for different areas of the country (calculated using cost-benefit analyses) remains equally applicable under a scenario with climate change, greater safety margins and investment levels are now considered to be prudent and new dike designs are thought to be desirable. Hence, a programme on future flood safety (Waterveiligheid 21e Eeuw) was established in the 1990s to reconsider Dutch flood protection policy. This led to a comprehensive set of flood protection studies (e.g. Klijn et al., 2004). The second Delta Committee recommended at least a tenfold increase in the flood safety level while also emphasizing the need for development of flood protection along with climate change and ecological processes. The challenge posed by these combined requirements led to intensified research on flood protection, especially from an interdisciplinary perspective (e.g. the Knowledge for Climate Research Programme, Delta Programme). The research presented in this thesis was carried out against this backdrop. It combines hydraulic, ecological, geographical and economic aspects in a search for new discoveries and new insights on the role of salt marshes in flood protection. Central in this research is the idea to combine flood protection with other functions in the flood defence zone to increase the flood safety level and to adapt to the effects of climate change. The idea to combine the flood protection function with nature and landscape values is especially explored for the Wadden region. 1.2 Robust, multifunctional flood defences Triggered by the recommendation of the second Delta Committee, interest in innovative flood protection techniques grew and a number of over-dimensioned dike designs were introduced. The Delta Committee introduced the ‘Delta dike’ (Deltacommissie, 2008), simultaneously with the ‘Unbreachable dike’ (Silva & Van Velzen, 2008) also called the ‘Broad dike’ (Vellinga, 2008). Table 1.1 and Figure 1.1, respectively, present details about these concepts and the relationships between them. 8 Chapter 1 Table 1.1: Key dike designs and related terms (in italics) in the Netherlands (for the relations between the various concepts see Figure 1.1) (Van Loon-Steensma & Vellinga, 2014). Traditional dike Can withstand statistically prescribed extreme water levels, wave heights and wave overtopping. Rijkswaterstaat (2007) applies a design that anticipates foreseen changes and uncertainties with respect to subsidence and climate change over a specific planning horizon (50 years, or 100 years for dikes in built areas), and that reserves a zone to allow for dike reinforcements in the future. This is called 'robust design' as the dike is designed slightly over- dimensioned according to the actual requirements at the time of construction. prescribed extreme water The Dutch Water Law specifies a safety level in terms of expected flooding levels frequency. This varies from once in 1,250 years (0.08% annual probability) in the riverine area, to once in 10,000 years (0.01% annual probability) in the province of North Holland. Over-dimensioned dike Can withstand more extreme situations than prescribed (in terms of water levels, wave heights and wave overtopping). Delta dike Has practically zero probability of failure due to sudden or uncontrollable failure (Deltacommissie, 2008). Enhanced safety can be achieved by inner constructions (such as sheets and walls) or by heightening the dike. However, increased strength is more effectively realized by enlarging the inner berm (e.g. Klijn & Bos, 2010; Knoeff & Ellen, 2011). Unbreachable dike Owing to its increased width, has 100 times less probability of failure due to (synonyms: Broad dike, erosion by overflowing, piping, or macro-instability on the landward side than by Vellinga, 2008; traditional dikes (Silva & Van Velzen, 2008). However, the Unbreachable Climate proof dike, by dike does not exclude overflow or wave overtopping which may lead to Hartog et al., 2009) damage. Robust dike Remains functioning without failure under a wide range of conditions, does not collapse during overtopping and reduces a flood disaster to a shallow flooding event. The Robust dike design includes the Unbreachable dike and Delta dike as subsets. robustness The ability of a system to continue to function despite disturbances, where the magnitude of the disturbance is variable and uncertain (e.g. De Bruijn et al., 2008; Hall & Solomatine, 2008; Haasnoot et al., 2011; Mens et al., 2011). Multifunctional dike Intentionally combines

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