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Flood frequency analysis and discussion of non-stationarity of the Lower Rhine flooding regime (AD 1350–2011) : using discharge data, water level measurements, and historical records
Accurate estimates of the recurrence time of extreme floods are essential to assess flood safety in flood-prone regions, such as the Lower Rhine in The Netherlands. Measured discharge records have a limited length and are, in general, poorly representing extremes, which results in considerable uncertainties when used for flood frequency analysis. In this paper, it is shown how alternative discharge monitoring stations along the Rhine, measurements of water levels, and historical records can be used to increase data availability. Although pre-processing and the conversion of data types into discharge estimates introduces extra uncertainty, the added value of this data in flood frequency analysis is considerable, because extending record length by including slightly less-precise data results in much better constrained estimates for the discharges and recurrence intervals of extreme events. Based on results obtained with the Generalised Extreme Value (GEV) distribution, it was concluded that large floods of the last century are presumably rarer than previously considered using shorter data series. Moreover, the combined effect of climatic and anthropogenic-induced non-stationarities of the flooding regime is more easily recognised in extended records. It is shown that non-stationarities have a significant effect on the outcomes of flood frequency analysis using both short and long input data series. Effects on outcomes of dominant multi-decadal variability are, however, largely subdued in the longer 240-year series.
Wave run-up and overtopping reduction by block revetments with enhanced roughness
Block revetments that are placed in special patterns, in a way that lower (standard) sets are surrounded by sets of higher (protruding) units, have been studied by means of physical model tests. This paper focuses on prediction formulae for the wave run-up and wave overtopping reduction by means of special pattern block revetments. This reduction is significant and leads to lower crest levels of dikes. Reducing or limiting crest levels without reducing safety is often a preferred solution in areas where dikes form an integral part of the existing area and where raising dikes will lead to social drawbacks or a negative impact on the landscape.
System reliability with correlated components : Accuracy of the Equivalent Planes method
Computing system reliability when system components are correlated presents a challenge because it usually requires solving multi-fold integrals numerically, which is generally infeasible due to the computational cost. In Dutch flood defense reliability modeling, an efficient method for computing the failure probability of a system of correlated components -referred to here as the Equivalent Planes method- was developed and has been applied in national flood risk analysis. The accuracy of the method has never been thoroughly tested, and the method is absent in the literature; this paper addresses both of these shortcomings. The method is described in detail, including an in-depth discussion about the source of error. A suite of system configurations were defined to test the error in the Equivalent Planes method, with a focus on extreme cases to capture the upper bound of the error. The ‘exact’ system reliability was computed analytically for the special case of equi-correlated components, and otherwise using Monte-Carlo directional sampling. We found that errors in the system failure probability estimates were low for a wide range of system configurations, and became more substantial for large systems with highly-correlated components. In the most extreme cases we tested, the error remained within three times the true failure probability. We provided an example of how one can determine if such error is tolerable in their particular application. We also show the computational advantage of using the Equivalent Planes method; large systems with small failure probabilities which take over 17 h for Monte Carlo directional sampling were computed with the Equivalent Planes in less than one second.
Overview of challenges and achievements in the climate adaptation of cities and in the Climate Proof Cities program
Editorial to a Special Issue that was incited by the end of Climate Proof Cities (CPC), a major scientific research program in The Netherlands that dealt with many of the issues related to climate adaptation in urban areas. The program has contributed to the knowledge on assessing vulnerability of cities, on adaptation options and their effectiveness, and on governance of adaptation. Important features are the role of green infrastructures in combination with available water, improved building designs and collaboration between urban planners and water managers. Nonetheless, in spite of this effort and many other national and international efforts, research in these fields is still in its infancy, and much remains to be done.
Communicating climate (change) uncertainties : simulation games as boundary objects
Climate science is characterized by large uncertainties about the direction, extent and time frame of climate change. Communicating these uncertainties is important for decision making on robust adaptation strategies, but proves to be a challenge for scientists particularly because of the complexity of uncertainties that are part of natural variability and of human induced climate change. The aim of this paper is to assess the role of a simulation game, as intermediate, to the communication of climate change uncertainties to water managers. In three workshops with water managers, the simulation game ‘Sustainable Delta’ was played to test the influence of the game on their understanding of climate change uncertainty using ex ante and ex post surveys. In each workshop an experimental- and control group were given different assignments to measure the influence of the game. The results show that although the differences between groups were not statistically significant, a change in their understanding of uncertainties was observed. The paper concludes that the learning effect of the game is inconclusive, but that the game does fosters a broader understanding of the concept climate change uncertainty. In doing so, simulation games are a promising approach to support the communication of climate change uncertainties meaningfully and support the process of adaptation to an uncertain future.
Evaluation of a tidal flat sediment nourishment as estuarine management measure
The tidal flats in the Eastern Scheldt tidal basin (The Netherlands) are eroding as a result of the construction of the storm surge barrier. These intertidal areas are important foraging grounds for birds and therefore it is important to mitigate the negative effects of erosion. As a pilot, a small part (20 ha) of the Galgeplaat tidal flat in the middle of the Eastern Scheldt was nourished with 130,000 m3 of sediment in 2008. This paper investigates this tidal flat nourishment using a large set of morphological and ecological monitoring data from 2008 to 2012. The nourishment proved effective in mitigating the negative effects of tidal flat erosion. The elevated tidal flat provides a foraging area for birds that is available for a longer period. The nourished area was relatively stable, giving time for the recovery of benthic macrofauna such that birds can benefit from the longer exposure time. Therefore, we conclude that sediment nourishments are an effective management measure to counteract the negative consequences of tidal flat erosion in the Eastern Scheldt, and have potential for other estuaries worldwide.
Towards spatially smart abatement of human pharmaceuticals in surface waters: Defining impact of sewage treatment plants on susceptible functions
For human pharmaceuticals, sewage treatment plants (STPs) are a major point of entry to surface waters. The receiving waters provide vital functions. Modeling the impact of STPs on susceptible functions of the surface water system allows for a spatially smart implementation of abatement options at, or in the service area of, STPs. This study was performed on a nation-wide scale for the Netherlands. Point source emissions included were 345 Dutch STPs and nine rivers from neighboring countries. The Dutch surface waters were represented by 2511 surface water units. Modeling was performed for two extreme discharge conditions. Monitoring data of 7 locations along the rivers Rhine and Meuse fall mostly within the range of modeled concentrations. Half of the abstracted volumes of raw water for drinking water production, and a quarter of the Natura 2000 areas (European Union nature protection areas) hosted by the surface waters, are influenced by STPs at low discharge. The vast majority of the total impact of all Dutch STPs during both discharge conditions can be attributed to only 19% of the STPs with regard to the drinking water function, and to 39% of the STPs with regard to the Natura 2000 function. Attributing water treatment technologies to STPs as one of the possible measures to improve water quality and protect susceptible functions can be done in a spatially smart and cost-effective way, using consumption-based detailed hydrological and water quality modeling.
Microplastic contamination in brown shrimp (Crangon crangon, Linnaeus 1758) from coastal waters of the Southern North Sea and Channel area
This study assessed the capability of Crangon crangon (L.), an ecologically and commercially important crustacean, of consuming plastics as an opportunistic feeder. We therefore determined the microplastic content of shrimp in shallow water habitats of the Channel area and Southern part of the North Sea. Synthetic fibers ranging from 200 μm up to 1000 μm size were detected in 63% of the assessed shrimp and an average value of 0.68 ± 0.55 microplastics/g w. w. (1.23 ± 0.99 microplastics/shrimp) was obtained for shrimp in the sampled area. The assessment revealed no spatial patterns in plastic ingestion, but temporal differences were reported. The microplastic uptake was significantly higher in October compared to March. The results suggest that microplastics >20 μm are not able to translocate into the tissues.
A staged geogenetic approach to underwater archaeological prospection in the Port of Rotterdam (Yangtzehaven, Maasvlakte, The Netherlands): A geological and palaeoenvironmental case study for local mapping of Mesolithic lowland landscapes
This study presents the geogenetic approach to detect presently drowned archaeological sites in the transgressive palaeoenvironment of the Holocene Rhine-Meuse delta. A staged and practical approach is advocated in which subsurface archaeological predictions are based on geological mapping and palaeoenvironmental reconstruction of the underwater location. The study area is located in the Maasvlakte harbour extension of the Port of Rotterdam, formerly a part of the southern North Sea. Prior to construction works, it was suggested that dredging of the new harbour (Yangtzehaven) would disturb the subsurface stratigraphy to around −21 m below present mean sea level, a zone which is known to contain archaeological remains. The staged approach makes use of geological data starting from a conceptual model that indicates the depths of layers that could be rich in Upper Palaeolithic/early Mesolithic artefacts. This initial model is used to determine the strategy of the subsequent phases of investigation, such as whether to proceed with dredging as part of the engineering work, down to 17 m water depth, to remove the upper (younger) sands and thereby improve the opportunities for underwater survey of fluvio-deltaic layers of Mesolithic age. Following the development of the initial site model, a full-area investigation was carried out using geophysics and coring, the latter providing material for palaeoenvironmental analysis. This allowed the reconstruction of the long-drowned former landscape, which included inland dune areas and local drainage systems and provided the physiographic context for the geoprospection of Mesolithic archeology. This predictive modeling identified two areas in the harbour for detailed investigation, again employing geophysics and coring at higher resolutions, allowing fine tuning of the palaeolandscape models at the localities of presumed highest archeological potential. Cores from one of the selected areas, an inland dune area within the Early Holocene wetland region, yielded in-situ evidence of Mesolithic occupation of this site in what is now the southern North Sea. These finds and the palaeolandscape context created with the data from the prospection phases were critical in the decision to undertake an underwater archaeological excavation using a large, boat-mounted grab sampling system. This paper provides an account of the geological and palaeoenvironmental work undertaken in the prospective phases leading up to the discovery of the site, highlighting the importance of the staged geogenetic approach for informing sampling strategies and securing high-quality information on landscape contexts, which in turn, informed archaeological decision-making and geoprospection strategies. Such an approach has wider generic application for palaeolandscape reconstruction and mapping at regional scales.
Numerical analysis of the interaction of irregular waves with two dimensional permeable coastal structures
This paper will address the validation and application of a volume of fluid method for coastal structures under the influence of normal incident irregular wave fields. Several physical processes will be addressed as part of the validation process, namely: (i)wave reflection from permeable and impermeable structures, (ii) wave transformation over a small shoal, (iii) wave damping inside of a permeable structure, (iv) the resulting wave induced internal setup and (v)wave induced forces. The numerical model will be validated against a multiple of experimental data sets for two dimensional coastal problems. The impact of air-relief gaps on the modelled wave induced pressures on a crest wall is analysed for the two dimensional layout of the structure. This is analysed to study the importance of the cushion effect from the incompressible air phase. Subsequent to the validation of the numerical model the internal setup in permeable coastal structures is given separate attention. A combination of an analytical prediction of the magnitude of the internal setup and numerical results is used to derive numerically based empirical formulae for the magnitude and the time scale of the internal setup. The formulae include the effects of wave height, wave period, material properties of the coastal structure and the dimensions of the structure. The numerical model is based on the OpenFoam® CFD-toolbox.