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Risk-based target reliability indices for quay walls
Design codes and standards rely on generalised target reliability indices. It is unclear, however, whether these indices are applicable to the specific risk-profile of marine structures. In this study, target reliability indices for quay walls were derived from various risk acceptance criteria, such as economic optimisation, individual risk (IR), societal risk (SR), the life quality index (LQI) and the social and environmental repercussion index (SERI). Important stochastic design variables in quay wall design, such as retaining height, soil strength and material properties, are largely time-independent, whereas other design variables are time-dependent. The extent to which a reliability problem is time variant affects the present value of future failure costs and the associated reliability optimum. A method was therefore developed to determine the influence of time-independent variables on the development of failure probability over time. This method can also be used to evaluate target reliability indices of other civil and geotechnical structures. The target reliability indices obtained for quay walls depend on failure consequences and marginal costs of safety investments. The results were used to elaborate the reliability framework of ISO 2394, and associated reliability levels are proposed for various consequence classes. The insights acquired were used to evaluate the acceptable probability of failure for different types of quay walls.
Combined effects of climate change and dam construction on riverine ecosystems
River morphology and riparian vegetation continuously adapt to changing discharge conditions, which makes it a challenge to distinguish long-term development driven by natural discharge variation from the impacts of flow alteration due to climate change and due to dams. The aim of this study was to investigate how such flow alterations affect bio-geomorphological processes and habitat suitability of several fluvial plant and animal species. This is done with a numerical model representing dynamic interactions between morphodynamic processes and riparian vegetation coupled to habitat suitability models of fluvial species. We compared a control run with natural flow regime to altered flow for two scenarios with different dam operating regimes, two scenarios with climate change, and for combinations of dams and climate change. Results show that flow stabilization leads to incision, acute reduced seedling recruitment and decline of riparian vegetation. Climate change generates a gradual response, where high flow extremes counteract an otherwise reduced seedling recruitment of pioneer vegetation, while drying reduces riparian vegetation recruitment and causes vegetation shifts towards lower elevations on the floodplain. Modelled habitat availability for facilitated plant and animal species declines most when the synchronicity between critical life history events and habitat requirements is disrupted by altered flow conditions, with opposite effects for different species. Dynamic interactions between bio-geo-morphological processes with somewhat different characteristic timescales create non-linear and adaptive behaviour of morphology, habitat patterns and facilitated species habitat. This implies that only models that include bio-geomorphological feedbacks can forecast impacts of multiple flow alteration pressures, whereas addition of single-pressure regime effects is overly simplistic.
Methods to assess bubble screens applied to mitigate salt intrusion through locks
Salt intrudes into the freshwater system via shipping locks located at the sea every time these open the lock gate. With increasing lock size and increasing shipping traffic intensity, this issue requires attention. The application of bubble screens along the lock’s entrance is one of the available mitigating measures. The effect of such a screen is often expressed as a factor reducing the speed of the lock exchange process: the so-called salt transmission factor, which may reach 0.25, depending on the bubble screen design and operation. The effect on the salt intrusion during successive lockages may be enhanced further when the duration of the doors being open is minimized. This paper presents various methods to determine the effect of bubble screens on salt intrusion, a discussion on the assessment of its design with scale model tests or numerical computations as well as the effect of the salt intrusion on the inland water system.
Time-dependent reliability in flood protection decision making in The Netherlands
Since 2017 Dutch flood protection standards are defined as target flood probabilities that all primary flood defences have to comply with by 2050. Explicitly accounting for uncertainties in probability distributions of load and resistance is an integral part of estimating the actual flood probability. Based on such estimates, many flood defences will be reinforced in the coming years, for design lifetimes that are generally 25-100 years. Therefore it is important that we correctly take into account time-dependence of both load and resistance during the lifetime. Loads are typically uncorrelated from year to year, whereas strength parameters exhibit significant correlation over time. This correlation over time of strength parameters can significantly reduce the failure rate and increase the lifetime reliability of a flood protection structure. In this paper we show the implications of time-dependent reliability for a set of illustrative cases. We consider the effect of different degrees of temporal dependence on reliability, lifetime and relative cost savings. The cases show that for common configurations, the inclusion of time-dependent effects, especially the correlation in time of strength variables, can increase the lifetime of a flood protection structure by up to 50%.
Safety and reliability - safe societies in a changing world : proceedings of ESREL 2018 (June 17-21, 2018, Trondheim, Norway)
Transient groundwater travel time distributions and age-ranked storage-discharge relationships of three lowland catchments
The contribution of groundwater to streams is controlled by temporally and spatially variable groundwater flow paths with distinctive travel times. The aggregated average Travel Time Distribution (TTD) of all these flow paths functions as a catchment characteristic. Currently, research on TTDs is expanding towards dynamic TTDs and building on this we present dynamic backward Travel Time Distributions and Residence Time Distributions (RTDs) using forward particle tracking on a high-resolution spatially distributed groundwater flow model (25*25m). We show that the dynamic backward TTDs of three Dutch catchments are determined by the interplay between the activation of shallow short flow paths and the intensification of fluxes through all flow paths when groundwater levels rise. In addition, the preference for young water in our lowland catchments appears strongly controlled by drainage density. Variations in catchment mixing with time and between catchments were analysed using dynamic StorAge Selection (SAS) functions. This showed the effect of differences in geology and topography on the shape of the SAS-functions. Additionally, the variability of SAS-functions in time was shown to depend on the extent to which new flow paths can be activated. Time-varying SAS-functions are required for computation of dynamic TTDs and this research showed realistic values for the variability in the SAS-functions of lowland catchments. The step towards dynamic TTDs is crucial for understanding the temporal and spatial behaviour of streams, their chemical composition and their ecological value.
New data from terp excavations on sea-level index points and salt marsh sedimentation rates in the eastern part of the Dutch Wadden Sea
This paper presents new geological data from two terp excavations at Englum and Ezinge, in the Dutch province of Groningen, and compares them to similar data from the western part of Friesland, in particular from the terp of Wijnaldum-Tjitsma. This terp is situated at a salt marsh ridge of the same height and thickness as Englum and Ezinge, although habitation started 650 years later at Wijnaldum. The measured levels of the tidalflat/ salt-marsh boundary underneath these terps make it possible to reconstruct palaeo-Mean High Water (MHW) levels. These sea-level index points show that palaeo-MHW in the Groningen part of the Wadden Sea was at the upper limit of the range of palaeo-MHW that has been reconstructed for the Dutch Wadden Sea on the basis of data from its western part. The deviating levels indicate that there are differences between regions of the Wadden Sea; this has earlier been established for the German section of the Wadden Sea. In the eastern part of the Dutch Wadden Sea, MHW nowadays is considerably higher than in the western part of the Wadden Sea; the data suggest that this may have been the case already in the 1st millennium BC. Salt marsh levels under dated terp layers make it possible to establish the rate of sedimentation of the developing salt marsh, at 23–91 cm per century for the pioneer zone and low marsh. This rate of development slowed to 4–5 cm per century for the middle marsh and 3–4 cm per century for the high salt marsh.
PCR-GLOBWB 2 : a 5 arcmin global hydrological and water resources model
We present PCR-GLOBWB 2, a global hydrology and water resources model. Compared to previous versions of PCR-GLOBWB, this version fully integrates water use. Sector-specific water demand, groundwater and surface water withdrawal, water consumption, and return flows are dynamically calculated at every time step and interact directly with the simulated hydrology. PCR-GLOBWB 2 has been fully rewritten in Python and PCRaster Python and has a modular structure, allowing easier replacement, maintenance, and development of model components. PCR-GLOBWB 2 has been implemented at 5 arcmin resolution, but a version parameterized at 30 arcmin resolution is also available. Both versions are available as open-source codes on https://github. com/UU-Hydro/PCR-GLOBWB_model. PCR-GLOBWB 2 has its own routines for groundwater dynamics and surface water routing. These relatively simple routines can alternatively be replaced by dynamically coupling PCR-GLOBWB 2 to a global two-layer groundwater model and 1-D–2-D hydrodynamic models. Here, we describe the main components of the model, compare results of the 30 and 5 arcmin versions, and evaluate their model performance using Global Runoff Data Centre discharge data. Results show that model performance of the 5 arcmin version is notably better than that of the 30 arcmin version. Furthermore, we compare simulated time series of total water storage (TWS) of the 5 arcmin model with those observed with GRACE, showing similar negative trends in areas of prevalent groundwater depletion. Also, we find that simulated total water withdrawal matches reasonably well with reported water withdrawal from AQUASTAT, while water withdrawal by source and sector provide mixed results.
Proceedings of the 5th IAHR Europe Congress : new challenges in hydraulic research and engineering (Trento, 12-14 June 2018)
A scale model study assessing the performance of a bubble screen mitigating salinity driven lock exchange
Bubble screens are applied at shipping locks between salt and fresh water bodies, in order to reduce salt intrusion as a result of the locking process. An important challenge is to optimize the design or use of the bubble screen such that the required salt intrusion reduction is met at a minimum use of energy. This abstract shows model measurements, performed recently at Deltares, which provide data of high spatial resolution. The goal of these experiments is to assess the difference in performance as a result of varying the bubble screen’s air flow rate. As these differences may be subtle, high accuracy measurements are required.