Search inside publication
Historic storms and the hidden value of coastal wetlands for nature-based flood defence
Global change amplifies coastal flood risks and motivates a paradigm shift towards nature-based coastal defence, where engineered structures are supplemented with coastal wetlands such as saltmarshes. Although experiments and models indicate that such natural defences can attenuate storm waves, there is still limited field evidence on how much they add safety to engineered structures during severe storms. Using well-documented historic data from the 1717 and 1953 flood disasters in Northwest Europe, we show that saltmarshes can reduce both the chance and impact of the breaching of engineered defences. Historic lessons also reveal a key but unrecognized natural flood defence mechanism: saltmarshes lower flood magnitude by confining breach size when engineered defences have failed, which is shown to be highly effective even with long-term sea level rise. These findings provide new insights into the mechanisms and benefits of nature-based mitigation of flood hazards, and should stimulate the development of novel safety designs that smartly harness different natural coastal defence functions.
Climate change induced socio-economic tipping points : review and stakeholder consultation for policy relevant research
Tipping points have become a key concept in research on climate change, indicating points of abrupt transition in biophysical systems as well as transformative changes in adaptation and mitigation strategies. However, the potential existence of tipping points in socio-economic systems has remained underexplored, whereas they might be highly policy relevant. This paper describes characteristics of climate change induced socio-economic tipping points (SETPs) to guide future research on SETPs to inform climate policy. We review existing literature to create a tipping point typology and to derive the following SETP definition: a climate change induced, abrupt change of a socio-economic system, into a new, fundamentally different state. Through stakeholder consultation, we identify 22 candidate SETP examples with policy relevance for Europe. Three of these are described in higher detail to identify their tipping point characteristics (stable states, mechanisms and abrupt change): the collapse of winter sports tourism, farmland abandonment and sea-level rise-induced migration. We find that stakeholder perceptions play an important role in describing SETPs. The role of climate drivers is difficult to isolate from other drivers because of complex interplays with socio-economic factors. In some cases, the rate of change rather than the magnitude of change causes a tipping point. The clearest SETPs are found on small system scales. On a national to continental scale, SETPs are less obvious because they are difficult to separate from their associated economic substitution effects and policy response. Some proposed adaptation measures are so transformative that their implementations can be considered an SETP in terms of 'response to climate change'. Future research can focus on identification and impact analysis of tipping points using stylized models, on the exceedance of stakeholder-defined critical thresholds in the RCP/SSP space and on the macro-economic impacts of new system states.
Time integrative sampling properties of Speedisk and silicone rubber passive samplers determined by chemical analysis and in vitro bioassay testing
Compared to grab samples, passive samplers have the advantage that they sample over a longer time period and can detect lower compound concentrations in water quality monitoring campaigns. To allow the determination of time-weighted average concentrations, however, sampler uptake should remain linear in time over the entire sampling period. Therefore, the time integrative or linear uptake properties of adsorption-based Speedisks and partitioning-based silicone rubber samplers were assessed with respect to chemically analyzed single compounds and measured bioactivity in in vitro bioassays. Both sampler types were deployed in consecutive and overlapping time series in a WTTP effluent and in the river Meuse up to 105 days. Extracts were chemically analyzed for PCBs, PAHs and pesticides and tested in the Aliivibrio fischeri and DR-LUC bioassays. Speedisks showed time integrative sampling for the detected pesticides as well as for bioassay responses at both sampling locations for the entire sampling period. The silicone rubber samplers showed poor linear uptake in time for the unknown compounds causing bioassay responses. The bioassay results indicate that conversion of a bioassay response to a passive sampler extract into a time-weighted average bioactivity per liter water seems justified for Speedisks, confirming that concentrations in the samplers correspond to a single volume of sampled water for all compounds. The bioassay results also indicate that a similar conversion for silicone rubber extracts should be interpreted with caution. In principle, it is also impossible, because the concentration of each compound contributing to the bioassay response corresponds to a different sampled water volume.
Laminar-turbulent transition of a non-Newtonian fluid flow
Transition from laminar to turbulent flow of non-Newtonian fluids is investigated using velocimetry data. These data are obtained by applying particle image velocimetry to images obtained through ultrasound imaging (echography). This yielded the observation of intermittent structures (puffs and slugs) that are formed during transition. Post its observation, transition is characterized using the friction factor curves and turbulence intensity. Further, a number of models used to predict transition are assessed. This showed the Reynolds number based model by Slatter and the stability parameter based model by Hanks to be most suitable for non-Newtonian fluids with yield stress and low behaviour index.
Prediction of mean wave overtopping discharge using gradient boosting decision trees
Wave overtopping is an important design criterion for coastal structures such as dikes, breakwaters and promenades. Hence, the prediction of the expected wave overtopping discharge is an important research topic. Existing prediction tools consist of empirical overtopping formulae, machine learning techniques like neural networks, and numerical models. In this paper, an innovative machine learning method -gradient boosting decision trees- is applied to the prediction of mean wave overtopping discharges. This new machine learning model is trained using the CLASH wave overtopping database. Optimizations to its performance are realized by using feature engineering and hyperparameter tuning. The model is shown to outperform an existing neural network model by reducing the error on the prediction of the CLASH database by a factor of 2.8. The model predictions follow physically realistic trends for variations of important features, and behave regularly in regions of the input parameter space with little or no data coverage.
Morphodynamic evolution of a fringing sandy shoal : from tidal levees to sea level rise
Intertidal shoals are vital components of estuaries. Tides, waves, and sediment supply shape the profile of estuarine shoals. Ensuring their sustainability requires an understanding of how such systems will react to sea level rise (SLR). In contrast to mudflats, sandy shoals have drawn limited attention in research. Inspired by a channel‐shoal system in the Western Scheldt Estuary (Netherlands), this research investigates governing processes of the long‐term morphodynamic evolution of intertidal estuarine sandy shoals across different timescales. We apply a high‐resolution process‐based numerical model (Delft3D) to generate a channel‐shoal system in equilibrium and expose the equilibrium profile to variations in wave forcing and SLR. Combined tidal action and wave forcing initiate ridge formation at the seaward shoal edge, which slowly propagates landward until a linear equilibrium profile develops within 200 years. Model simulations in which forcing conditions have been varied to reproduce observations show that the bed is most dynamic near the channel‐shoal interface. A decrease/increase in wave forcing causes the formation/erosion of small tidal levees at the shoal edge, which shows good resemblance to observed features. The profile recovers when regular wave forcing applies again. Sandy shoals accrete in response to SLR with a long (decades) bed‐level adaptation lag eventually leading to intertidal area loss. This lag depends on the forcing conditions and is lowest near the channel and gradually increases landward. Adding mud makes the shoal more resilient to SLR. Our study suggests that processes near the channel‐shoal interface are crucial to understanding the long‐term morphodynamic development of sandy shoals.
Applying transfer function-noise modelling to characterize soil moisture dynamics : a data-driven approach using remote sensing data
The increasing availability of remotely sensed soil moisture data offers new opportunities for data-driven modelling approaches as alternatives for process-based modelling. This study presents the applicability of transfer function-noise (TFN) modelling for predicting unsaturated zone conditions. The TFN models are calibrated using SMAP L3 Enhanced surface soil moisture data. We found that soil moisture conditions are accurately represented by TFN models when exponential functions are used to define impulse-response functions. A sensitivity analysis showed the importance of using a calibrated period which is representative of the hydrological conditions for which the TFN model will be applied. The IR function parameters provide valuable information on water system characteristics, such as the total response and the response times of soil moisture to precipitation and evapotranspiration. Finally, we encourage exploring the possibilities of TFN soil moisture modelling, as predicting soil moisture conditions is promising for operational settings.
How human activities affect the fine sediment distribution in the Dutch Coastal Zone seabed
The fine sediment distribution in the seabed is an important indicator for the ecological functioning of shallow coastal seas. In this paper, we investigate the processes and conditions that determine the fine sediment distribution in the Dutch coastal zone surficial seabed, while also assessing the response of the system to human interventions. An extensive sediment dataset, collected in the Dutch coastal zone from 2006 to 2014, is presented. These data are used to map the distribution of fines in the seabed of the DCZ at unique spatiotemporal scales. For the entire Dutch coastal zone, the distribution of fines generally agrees well with previous studies. The recent extension of the Port of Rotterdam, the Maasvlakte 2 reclamation, was found to locally change the distribution of fines. In the sand mining pit and directly south of the reclamation, fines percentages in the seabed increased by more than 10%. We developed a conceptual framework to analyse the distribution of fines and how it is affected by human interventions. Three components are distinguished within this framework: (1) sources of fines; (2) transport pathways; and (3) accumulation potential. These components are determined both qualitatively and quantitatively, based on high-resolution bathymetric and hydrodynamic model data. The distinction between the three components makes it possible to unravel the contributions of different human interventions to the changes in the fines distribution. In the case of Maasvlakte 2, the local increase of fines percentage in the seabed could thus be attributed to a temporary additional source of fines and enhanced accumulation potential. The high spatiotemporal resolution of the new sediment dataset proved crucial to enable development and testing of the framework to evaluate the impact of (large) engineering works on the spatial distribution of fines.
Governance of climate adaptation, which mode? : an exploration of stakeholder viewpoints on how to organize adaptation
There are many normative answers on the question how to realize climate adaptation, ranging from pleas for the government to play a decisive role, to calls for refraining from action and relying upon spontaneous adaptation of both government and non-governmental actors. In this article, we present a Q methodological study, aimed at investigating the governance preferences among non-governmental actors in the Netherlands and the “narratives” they use to motivate these preferences. Our empirical results underline the fact that the question “how to organize adaptation”, is a controversial one. The results resemble the various positions in the current academic debate about the governance of adaptation, and add important insights and nuances to it. Many respondents feel that the current climate adaptation policy is too non-committal. The dominant viewpoint underscores a need for more rules and norms and the possibility to sanction organizations that do not adapt. Minority viewpoints show an urge to stimulate and support self-organization of partners, as well as a need for more action. However, financial and regulatory preconditions are needed to stimulate actors in order to see to the necessary investments. Policy-makers have to invest in mixing their policy instruments. Clearly, most nongovernmental actors are in favor of the government setting a framework with rules and norms for climate adaption. However, the viewpoints show that this is not sufficient. The government should facilitate networks, joint efforts and create the financial and regulatory preconditions to remove current barriers blocking adaptation measures.
Experimental study on the influence of berms and roughness on wave overtopping at rock-armoured dikes
The average overtopping discharge is an important parameter for the design and reinforcement of dikes. Rock armour on the waterside slopes and berms of dikes is widely used to reduce the wave overtopping discharge by introducing slope roughness and dissipation of energy in the permeable armour layer. However, methods for estimating the influence of a rock berm and roughness of rock armour at dikes on the average overtopping discharge still need to be developed and/or validated. Therefore, this study aims to develop empirical equations to quantify the reductive influence of rock armour on wave overtopping at dikes. Empirical equations for estimating the effects of rock berms and roughness are derived based on the analysis of experimental data from new physical model tests. The influence of roughness of the rock armour applied on parts of waterside slopes is estimated by introducing the location weighting coefficients. Results show that the newly derived equations to predict the average overtopping discharge at dikes lead to a significantly better performance within the tested ranges compared to existing empirical equations.