Zoek binnen publicaties
Laboratory investigation of the Bruun Rule and beach response to sea level rise
Rising sea levels are expected to cause widespread coastal recession over the course of the next century. In this work, new insight into the response of sandy beaches to sea level rise is obtained through a series of comprehensive experiments using monochromatic and random waves in medium scale laboratory wave flumes. Beach profile development from initially planar profiles, and a 2/3 power law profile, exposed to wave conditions that formed barred or bermed profiles and subsequent profile evolution following rises in water level and the same wave conditions are presented. Experiments assess profile response to a step-change in water level as well as the influence of sediment deposition above the still water level (e.g. overwash). A continuity based profile translation model (PTM) is applied to both idealised and measured shoreface profiles, and is used to predict overwash and deposition volumes above the shoreline. Quantitative agreement with the Bruun Rule (and variants of it) is found for measured shoreline recession for both barred and bermed beach profiles. There is some variability between the profiles at equilibrium at the two different water levels. Under these idealised conditions, deviations between the original Bruun Rule, the modification by Rosati et al. (2013) and the PTM model predictions are of the order of 15% and all these model predictions are within 30% of the observed shoreline recession. Measurements of the recession of individual contour responses, such as the shoreline, may be subject to local profile variability; therefore, a measure of the mean recession of the profile is also obtained by averaging the recession of discrete contours throughout the active profile. The mean recession only requires conservation of volume, not conservation of profile shape, to be consistent with the Bruun Rule concept, and is found to be in better agreement with all three model predictions than the recession measured at the shoreline.
41. Dresdner Wasserbaukolloquium 2018 : Wasserbauwerke im Bestand - Sanierung, Umbau, Ersatzneubau und Rückbau (Dresden, 08. und 09. März 2018)
Risk-based safety standards and safety assessment tools in the Netherlands
The Netherlands has established new safety standards for flood defences since 2017. This paper describes the definition and format of the new standards, as well as the tools that have been developed to carry out the assessments in practice. The assessment itself is a layered approach from coarse to fine, including the possibility to work with a conventional semi-probabilistic approach (with partial safety factors), or to opt for full probabilistic analysis. After describing the main features of the assessment, two examples are given for the failure modes of wave overtopping and slope stability. We conclude by pointing out the main differences between the old and the new assessment, and the advantages and opportunities of the new approach.
Nonhydrostatic and surfbeat model predictions of extreme wave run-up in fringing reef environments
The accurate prediction of extreme wave run-up is important for effective coastal engineering design and coastal hazard management. While run-up processes on open sandy coasts have been reasonably well-studied, very few studies have focused on understanding and predicting wave run-up at coral reef-fronted coastlines. This paper applies the short-wave resolving, Nonhydrostatic (XB-NH) and short-wave averaged, Surfbeat (XB-SB) modes of the XBeach numerical model to validate run-up using data from two 1D (alongshore uniform) fringing-reef profiles without roughness elements, with two objectives: i) to provide insight into the physical processes governing run-up in such environments; and ii) to evaluate the performance of both modes in accurately predicting run-up over a wide range of conditions. XBeach was calibrated by optimizing the maximum wave steepness parameter (maxbrsteep) in XB-NH and the dissipation coefficient (alpha) in XB-SB) using the first dataset; and then applied to the second dataset for validation. XB-NH and XB-SB predictions of extreme wave run-up (Rmax and R2%) and its components, infragravity- and sea-swell band swash (SIG and SSS) and shoreline setup (n>), were compared to observations. XB-NH more accurately simulated wave transformation but under-predicted shoreline setup due to its exclusion of parameterized wave-roller dynamics. XB-SB under-predicted sea-swell band swash but overestimated shoreline setup due to an over-prediction of wave heights on the reef flat. Run-up (swash) spectra were dominated by infragravity motions, allowing the short-wave (but not wave group) averaged model (XB-SB) to perform comparably well to its more complete, short-wave resolving (XB-NH) counterpart. Despite their respective limitations, both modes were able to accurately predict Rmax and R2%.
Sinking cities : an integrated approach towards solutions
In many coastal and delta cities land subsidence exceeds absolute sea level rise up to a factor of ten. Without action, parts of Jakarta, Ho Chi Minh City, Bangkok and numerous other coastal cities will sink below sea level. Increased flooding and other widespread impacts of land subsidence result in damage totalling billions of dollars per year. A major cause of severe land subsidence is excessive groundwater extraction due to rapid urbanization and population growth. A major rethink is needed to deal with the ‘hidden’ but urgent threat of subsidence. Deltares presents a comprehensive approach to address land subsidence from the perspective of more sustainable and resilient urban development.
Real-time flood control by tree-based model predictive control including forecast uncertainty : a case study reservoir in Turkey
Optimal control of reservoirs is a challenging task due to conflicting objectives, complex system structure, and uncertainties in the system. Real time control decisions suffer from streamflow forecast uncertainty. This study aims to use Probabilistic Streamflow Forecasts (PSFs) having a lead-time up to 48 h as input for the recurrent reservoir operation problem. A related technique for decision making is multi-stage stochastic optimization using scenario trees, referred to as Tree-based Model Predictive Control (TB-MPC). Deterministic Streamflow Forecasts (DSFs) are provided by applying random perturbations on perfect data. PSFs are synthetically generated from DSFs by a new approach which explicitly presents dynamic uncertainty evolution. We assessed different variables in the generation of stochasticity and compared the results using different scenarios. The developed real-time hourly flood control was applied to a test case which had limited reservoir storage and restricted downstream condition. According to hindcasting closed-loop experiment results, TB-MPC outperforms the deterministic counterpart in terms of decreased downstream flood risk according to different independent forecast scenarios. TB-MPC was also tested considering different number of tree branches, forecast horizons, and different inflow conditions. We conclude that using synthetic PSFs in TB-MPC can provide more robust solutions against forecast uncertainty by resolution of uncertainty in trees.
Effect of density differences on the forces acting on a moored vessel while operating navigation locks
This paper investigates the effect of density differences on the forces acting on a moored vessel during lock operations, focusing on the effect of the position of the moored vessel in the lock chamber in the presence of density currents. The extensive scale-model research performed for the new sea lock in IJmuiden, The Netherlands has shown that the combination of density differences and an asymmetric layout of the moored vessel in the lock chamber may lead to high forces on the vessel that can largely exceed the allowable force limit. In particular, as a result of the density currents, forces in the transverse direction build up, pushing the vessel away from the chamber wall, during leveling and after opening the lock gate, leading to higher loads on the mooring lines. The forces caused by the density difference are the dominant forces and the performance of the leveling system cannot be assessed without taking this into account. Based on the results of the performed tests, criteria for achievable leveling times and allowable hydrodynamic forces during leveling are determined for the new sea lock of IJmuiden. Furthermore, the obtained results can be used for calibration or validation of numerical models that are valuable tools for the design of future locks.
New design guidance for underlayers and filter layers for rock armour under wave attack
Conventional underlayers for rock armoured slopes under wave attack are often designed to have a stone mass M50u that is 1/10th to 1/15th of that of the armour, M50a. But what if a designer or contractor would like to design an underlayer with small and very wide graded material to be placed directly on a geotextile or directly on sand and then acting as a filter layer for the sand? This paper first describes three situations that may exist in reality: conventional underlayers; thin underlayers on a geotextile; and finally thicker underlayers that act directly as a filter for underlying sand. Small scale physical model tests have been performed on a 1:3 rock armour layer and the size and thickness of the underlayer have been varied. The behaviour of the underlayers as well as the armour layer has been analysed. It appeared that the smallest part of the stones in the underlayers often moved or were displaced, but the overall thickness of the underlayer remained mainly unchanged up to acceptable damage conditions for the armour layer. The smallest and widest graded underlayers resulted in slightly more damage to the armour layer than those with relatively large and narrow graded material. Too small underlayers showed considerable adverse effect on the stability of the armour layer and also showed significant rock movement or displacement from the underlayer. Configurations with a larger thickness of the underlayer gave much better results than configurations with a thin underlayer. Design criteria have been developed from the model test results, taking into account the practical situations described above and are based on the grading width of the underlayer material and on its size relative to the armour layer rock size.
Proceedings of the ICE conference Coasts, Marine Structures and Breakwaters 2017 : realising the potential (5-7 September 2017, Liverpool)