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Influence of oblique wave attack on wave overtopping at caisson breakwaters with sea and swell conditions
The amount of wave overtopping at coastal structures such as vertical caisson breakwaters is strongly dependent on the angle of wave attack. The reducing effects of oblique waves on wave overtopping compared to perpendicular wave attack has been studied by means of three-dimensional wave basin tests. In these physical model tests the caisson breakwater has been exposed to wave conditions with wave angles between perpendicular and very oblique waves. Short-crested and long-crested waves have been tested and analysed. Also, crossing bimodal conditions have been studied with directional wind waves approaching the breakwater under a different angle than the simultaneous unidirectional swell conditions. Vertical caisson breakwaters with and without a recurved parapet (also referred to as a bullnose or a recurved wave return wall) have been tested. The measurements showed the large influence of oblique waves on wave overtopping. Also, the influence of a recurved parapet can be large although the influence reduces for larger wave angles. Guidelines have been proposed to account for the influence of oblique waves on wave overtopping at vertical caisson breakwaters with and without a recurved parapet, as well as for crossing bimodal conditions with simultaneous sea and swell conditions from different directions.
Dyke failures in the Province of Groningen (Netherlands) associated with the 1717 Christmas flood : a reconstruction based on geoscientific field data and numerical simulations
The 1717 Christmas flood is one of the most catastrophic storm surges the Frisian coast (Netherlands and Germany) has ever experienced. With more than 13,700 casualties it is the last severe storm surge with a death toll of this order. At the same time, little is known about the hydrodynamic conditions and the morphological effects associated with this storm surge. In this study, 41 potential dyke failures in the Province of Groningen (Netherlands) associated with the 1717 Christmas flood were systematically reconstructed and mapped by using historical maps and literature and by analysing the recent topography in search of typical pothole structures and sediment fans. The dimensions of the sediment fans as derived from the topography show a good accordance with the dimensions documented by vibracore profiles, direct push tests and electrical resistivity tomography data taken at three fieldwork sites. Moreover, the fan dimensions closely agree with the dimensions as simulated using a process-based morphodynamic numerical model for one of the three sites, the village of Wierhuizen. Consequently, the recent topography is still indicative for the locations and dimensions of dyke failures and sediment fans associated with the 1717 Christmas flood. Considering the large number of detected dyke failures (41) and the large dimensions of the potholes and particularly of the sediment fans up to a few hundred metres wide and up to 0.7 m thick, this study proves significant morphological effects of the 1717 Christmas flood on the mainland of the Province of Groningen.
Large-scale sensitivities of groundwater and surface water to groundwater withdrawal
Increasing population, economic growth and changes in diet have dramatically increased the demand for food and water over the last decades. To meet increasing demands, irrigated agriculture has expanded into semi-arid areas with limited precipitation and surface water availability. This has greatly intensified the dependence of irrigated crops on groundwater withdrawal and caused a steady increase of non-renewable groundwater use, i.e. groundwater taken out of aquifer storage that will not be replenished in human time scales. One of the effects of groundwater pumping is the reduction in streamflow through capture of groundwater recharge, with detrimental effects on aquatic ecosystems. The degree to which groundwater withdrawal affects streamflow or groundwater storage depends on the nature of the groundwater-surface water interaction (GWSI). So far, analytical solutions that have been derived to calculate the impact of groundwater on streamflow depletion involve single wells and streams and do not allow the GWSI to shift from connected to disconnected, i.e. from a situation with two-way interaction to one with a one-way interaction between groundwater and surface water. Including this shift and also analyse the effects of many wells, requires numerical groundwater models that are expensive to setup. Here, we introduce a simple conceptual analytical framework that allows to estimate to what extent groundwater withdrawal affects groundwater heads and streamflow. It allows for a shift in GWSI, calculates at which critical withdrawal rate such a shift is expected and when it is likely to occur after withdrawal commences. It also provides estimates of streamflow depletion and which part of the groundwater withdrawal comes out of groundwater storage and which parts from a reduction in streamflow. After a local sensitivity analysis, the framework is used to provide global maps of critical withdrawal rates and timing, the areas where current withdrawal exceeds critical limits, and maps of groundwater depletion and streamflow depletion rates that result from groundwater withdrawal. The resulting global depletion rates are similar to those obtained from global hydrological models and satellites. The analytical framework is particularly useful for performing first-order sensitivity studies and for supporting hydroeconomic models that require simple relationships between groundwater withdrawal rates and the evolution of pumping costs and environmental externalities.
Infiltration capacity of rain gardens using full-scale test method : effect of infiltration system on groundwater levels in Bergen, Norway
The rain gardens at Bryggen in Bergen, Western Norway, is designed to collect, retain, and infiltrate surface rainfall runoff water, recharge the groundwater, and replenish soil moisture. The hydraulic infiltration capacity of the Sustainable Drainage System (SuDS), here rain gardens, has been tested with small-scale and full-scale infiltration tests. Results show that infiltration capacity meets the requirement and is more than suffcient for infiltration in a cold climate. The results from small-scale test, 245–404 mm/h, shows lower infiltration rates than the full-scale infiltration test, with 510–1600 mm/h. As predicted, an immediate response of the full-scale infiltration test is shown on the groundwater monitoring in the wells located closest to the infiltration point (<30 m), with a ca. 2 days delayed response in the wells further away (75–100 m). Results show that there is sucient capacity for a larger drainage area to be connected to the infiltration systems. This study contributes to the understanding of the dynamics of infiltration systems such as how a rain garden interacts with local, urban water cycle, both in the hydrological and hydrogeological aspects. The results from this study show that infiltration systems help to protect and preserve the organic rich cultural layers below, as well as help with testing and evaluating of the efficiency, i.e. SuDS may have multiple functions, not only storm water retention. The functionality is tested with water volumes of 40 m3 (600 L/min for 2 h and 10 min), comparable to a flash flood, which give an evaluation of the infiltration capacity of the system.
Value of information of combinations of proof loading and pore pressure monitoring for flood defences
Spatial variability and limited measurements often result in low reliability estimates of geotechnical failure modes of dikes (i.e., earthen flood defences). Required dike reinforcements are usually not executed within a few years after inception, which enables efforts to improve reliability estimates by reducing uncertainty. Often decision makers are unclear on whether uncertainty reduction is worth investing, and which (combination of) methods yields the highest Value of Information (VoI). This paper presents a framework to assess the VoI of two uncertainty reduction methods (proof loading and pore pressure monitoring) for a case study of a typical river dike with an insufficiently stable inner slope, using a decision tree. In all cases, a positive VoI was found for at least one strategy consisting of a proof load test, monitoring or both. The optimal strategy of proof loading and monitoring has a VoI of 4.0 M€, being a reduction in total cost of 25% compared to a conventional dike reinforcement. It was also found that sometimes proof loading enhances the VoI of pore pressure monitoring, which demonstrates the benefits of jointly considering different methods in a single decision tree. The decision framework yields insight in total cost and VoI of risk reduction strategies, which enables decision makers to determine where proof loading and/or pore pressure monitoring are efficient, leading to more efficient flood defence asset management.
NGO: samenwerken door kennisdeling
Het bestuur van de Nederlandse Geotextiel Organisatie (NGO) wordt voorgesteld. Daarnaast wordt ingegaan op de doelstellingen van de NGO en de strategie voor de komende jaren.
Beschadiging van geotextielen door vallende stenen
Vallende stenen leiden regelmatig tot beschadiging van geotextielen tijdens de constructie van waterbouwkundige werken. In de ‘Ontwerprichtlijn Geotextielen onder Steenbekledingen’ is dit dan ook een belangrijk aandachtspunt. De richtlijn is gebaseerd op geometrische criteria. Om de richtlijn te toetsen zijn valproeven uitgevoerd in het Laboratorium voor Geotechniek van de Universiteit van Gent. Bij deze valproeven bleken nog andere aspecten belangrijk. Op grond van ervaringen uit de praktijk en de hierboven genoemde valproeven is in januari 2020 een Erratum uitgebracht (CROW, 2020). In dit artikel wordt eerst het geometrisch criterium beschreven en dan de resultaten van de valproeven op één geotextiel en op twee op elkaar geplaatste geotextielen. Omdat de resultaten van de valproeven niet geheel aansluiten bij het geometrisch criterium, wordt ook aanvullende theorie ontwikkeld.
Factors determining the natural fresh-salt groundwater distribution in deltas
Most river deltas are densely populated areas with intensive agriculture. The increased shortage of fresh surface water that results from rising demands are expected to lead to increased groundwater pumping, which leads to sea water intrusion. To correctly project the future of fresh groundwater resources in deltas, knowing the current fresh‐salt groundwater distribution is a prerequisite. However, uncertainties about these distributions and their drivers are large. To understand these uncertainties, we conducted a global sensitivity analysis of a complex three‐dimensional variable‐density groundwater model of a synthetic delta, simulating the effect of the last glacial low stand and the subsequent marine transgression. The analysis is unique in its wide range of geometries, hydrogeological parameterizations and boundary conditions analyzed, making it representative for a large number of deltas worldwide. We find that the aquifer hydraulic conductivity is the most uncertain input and has a strong non‐monotonous effect on the total salt mass onshore. The calculated fresh‐salt groundwater distributions were classified into five classes and compared to real‐world case studies. We find that salinity inversions occur in deltaic systems with high representative system anisotropies as a remnant of a marine transgression. These salinity inversions were observed in half of the real‐world cases, indicating that their fresh‐salt groundwater distributions are not in a dynamic equilibrium. We conclude that it is very likely that past marine transgressions are still reflected in the current fresh‐salt groundwater distributions in deltas. This makes paleo‐groundwater modeling a prerequisite for effective simulation of present‐day groundwater salinity distributions in these systems.
Handleiding FEWS Vecht (versie 8 - 2020)
Dit rapport geeft een beschrijving van de technische achtergrondgegevens van het FEWS Vecht, alsmede een handleiding voor het gebruik. FEWS Vecht is in de huidige vorm een goede basis voor het maken van operationele, betrouwbare hoogwaterverwachtingen op de Overijsselse Vecht en de daarop uitkomende stroomgebieden. Het biedt als instrument ook een goed handvat voor de vele waterbeheerders om gezamenlijk op te trekken tijdens hoogwater.
Analyse fysische processen Waddenzee : inzet numeriek model voor analyse systeemwerking en gevoeligheidsscenario's
Voorliggende rapportage beschrijft de inzet van het nieuw ontwikkelde slibmodel voor de Waddenzee (Flexible Mesh model met D-Water Quality) om het systeembegrip te vergroten. Ook zijn enkele extreme scenario's doorgerekend om te verkennen wat de effecten op de slibdynamiek zouden zijn indien grootschalige slibvangen worden aangebracht langs de vastelandkust van de Waddenzee.