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Pathways towards democratization of hydro-environment observations and data
This White Paper grew out of the collaborative work conducted by several IAHR Technical Committees, with a focus on the Technical Committee on Experimental Methods and Instrumentation. we provide a community-based discussion of the latest technological developments in flow measurement techniques and instrumentation. We highlight key future developments that contribute to the democratization of hydro-environment observations and data, considering democratization as the transition towards more shared and affordable access to information by the scientific community, stakeholders, and society in general. A natural example of democratization can be found in the increasing application of citizen science to monitor flow and in the emergency response to natural hazards. Nevertheless, many other types of flow observations can also benefit from recent innovations to enhance their accessibility and outreach potential.
Extensive testing on PVC sewer pipes towards identifying the factors that affect their operational lifetime
Polyvinyl chloride (PVC) sewer pipes have operated for decades in a hostile environment, raising concern among sewer managers over the longevity of their drainage systems. Inspection data (CCTV and PanoramoVR ) reveals that severe defects have already surfaced, yet it is unknown if the material properties of PVC sewers have been affected. In order to address this issue, extensive testing (among others flexural and tensile tests, FT-IR, X-ray, viscosity measurements) was conducted on eight exhumed PVC sewer pipes (16–43 years old) with known defects and one brand-new for reference purposes. Visual examination during excavation revealed various failure causes, including uncontrolled handling of the pipes during construction or due to digging activities in the direct vicinity of the pipes. The test results indicate that physical ageing is extensively detected while other degradation mechanisms had minimal or no effect on the investigated pipes. However, mechanical testing on exhumed 3-layer pipes show that the incorporation of layered wall constructions is potentially a critical factor for the structural status of the pipe.
Nature-based solutions for coastal engineering and management
There is a growing scientific and engineering interest in exploring how natural processes can provide management solutions to resolve the degradation and vulnerability of coastal environments. Climate change and associated sea level rise together with drivers, such as subsidence, reduced sediment supply and coastal squeeze represent major risk factors for coastal systems sustainability. Using natural processes to deal with these risk factors presents a nontrivial challenge. This special issue focuses on nature-based solutions and state-of-the-art interventions in the coastal environment.
A generating-absorbing boundary condition for dispersive waves
The detailed modeling of free-surface waves and their interaction with bottom-mounted or floating structures requires large computational resources, which is why efficient boundary conditions with low spurious reflection are desirable. The present work presents a review of existing generating-absorbing boundary conditions (GABCs) for dispersive waves and their reflection characteristics. Hereafter, an adaptation of the classical Sommerfeld condition is proposed by using a depth-varying coefficient to improve absorption efficiency over a range of wave numbers. An analytical model is proposed to analyse the reflection characteristics for both propagating and evanescent modes, and a considerable improvement in comparison to the Sommerfeld condition is documented over a broad frequency range (reflection coefficients below 5% for nondimensional wave numbers in the range (0,10). The new boundary condition is implemented in OpenFoam (waves2Foam) and the functioning for regular, irregular, solitary, and phase-focused waves is presented.
Numerical modelling of wave overtopping at dikes using OpenFOAM
Accurate calculation of wave overtopping is important for determining the required crest height and geometry of a dike. Berms and roughness elements are widely used to reduce the average overtopping discharge at dikes while the reductive effects of berm and roughness are still not fully understood. Several empirical formulae are available to predict the overtopping rate at coastal structures. However, the extrapolation of these empirical formulae is not always applicable for complex structures (e.g. a dike that has a berm and/or roughness elements on the waterside slopes) or wave conditions that are outside the applicability of the empirical predictors. A 2D numerical model based on OpenFOAM® is set up in this study for predicting wave overtopping at dikes that have complex configurations with berms and roughness elements. The validation results show that this OpenFOAM® model is capable of reproducing the incident waves accurately and predicting the wave overtopping discharge with good accuracy. Subsequently, the numerical model is applied to study the reductive influence of a berm and protruding blocks on the mean overtopping discharge at dikes. The roughness of protruding blocks is incorporated by explicitly modelling the protrusions using refined mesh. The model shows reasonable behaviour of the reduction of wave overtopping influenced by a berm and roughness. This indicates the capabilities of the numerical model in the design and safety assessment of dikes.
Towards a particle trajectory modelling approach in support of South African search and rescue operations at sea
The ability to provide rapid decision support and more precise search area coordinates for rescuers to conduct search and rescue operations at sea are of high impact value for marine and maritime stakeholders. Search and rescue operations rely on accurate information about metocean conditions to locate objects in the ocean. These include local knowledge, operational ocean and wind forecasts and empirical drift relationships between ocean currents, ocean surface winds and the objects being searched for. To provide more accurate decision support for rescuers looking for persons or objects lost at sea, a virtual particle tracking tool was combined with an empirical Leeway drift model. The Lagrangian Ocean Search Targets (LOST) application builds on a Lagrangian ocean analysis framework which has been adapted to provide real-time estimates of the positions of objects based on operational ocean and wind forecasts. LOST incorporates the impact of ocean currents, surface winds and stochastic motion, the latter being critical in accounting for sub-grid scale processes that are not resolved in the ocean and wind forecasts. This study assesses the accuracy of LOST, demonstrating its feasibility as a decision support tool for search and rescue operations by applying it to three use cases in the South African regional ocean. These use cases are real-life scenarios that highlight the value of combining state-of-the-art ocean and wind forecasting systems with Lagrangian ocean analyses frameworks and sub-grid scale parameterisation to support global operational oceanography.
The relationship between coastal sea level variability in South Africa and the Agulhas Current
Sea level observations suggest that western boundary currents, such as the Gulf Stream, contribute to coastal sea-level variability and rise. Since the influence of the Agulhas Current on coastal sea-level variability and rise has not yet been quantified, this study considers how variations in the transport and position of this current affect sea-level variability along the South African coast. This was achieved by analysing timescales of variability in the time series of an Agulhas Current transport proxy and absolute dynamic topography at the core position, at the ACT array, comparing these to monthly mean sea level records of East London (33°S, 28°E). The timescales embedded in each of the datasets were separated through the Empirical Mode Decomposition (EMD) method. Monthly gridded satellite altimetry sea level anomalies were also utilised to extend the analysis to other tide gauge locations on the east and south coast of South Africa. The results suggest that, on timescales ranging from a few months to decades, coastal sea-level is associated with the Agulhas Current through absolute dynamic topography variations in the Agulhas Current core. Absolute dynamic topography variations in the Agulhas Current core are found to be in response to displacements of the current both near and offshore from the coast during eddies and meandering events. The study suggests that the Agulhas Current variations should be taken into account for better coastal planning, management and engineering.
Effects of swimming behaviour and oceanography on sea turtle hatchling dispersal at the intersection of two ocean current systems
The knowledge gap on the early life-history of sea turtles during the “lost years” continues to hinder research and conservation of this critical life stage when mortality rates are the highest. An oceanic model was used in combination with a Lagrangian particle tracking framework to simulate and identify potential post-hatchling dispersal trajectories of loggerhead and leatherback turtles in the South Western Indian Ocean. The study aimed to investigate the effect of hatchling swimming behaviour on hatchling dispersal and survival probability. To our knowledge, this study provides the first estimate of neonate sea turtle dispersal in the SWIO, by combining a particle tracking model with in situ hatchling behavioural data. The model revealed that most virtual hatchlings are transported south-westward in the Agulhas Current with three distinct final locations after a year-long simulation (each zone comprising on average > 20% of the total amount of particles): the Agulhas Return, the SE Atlantic and the Southern Ocean zones. However, because loggerhead hatchlings are less strong swimmers compared to leatherbacks, they can be advected northward in the near-shore coastal current. Simulations revealed that initial active swimming (frenzy) as well as variability in oceanic conditions strongly influenced dispersal of virtual hatchlings. Furthermore, variability in oceanic conditions dispersed virtual hatchlings into different areas where threats, like fisheries bycatch, might also influence their survival. Lastly, the results of this study have potentially broad implications for climate change if turtles adapt by nesting earlier/later during the nesting season or further south which may influence hatchling locomotor performance and ultimately survival at early life stages.
Impact of hydraulic model resolution and loss of life model modification on flood fatality risk estimation : case study of the Bommelerwaard, The Netherlands
Flood simulations are important for flood (fatality) risk assessment. This article provides insight into the sensitivity of flood fatality risks to the model resolution of flood simulations and to several uncertain parameters in the loss of life model used. A case study is conducted for river flooding in a polder in the Netherlands (the Bommelerwaard) where the Dutch approach for loss of life estimation is applied. Flood models with resolutions of 100, 25, and 5 m are considered. Results show locally increased mortality rates in higher resolution simulations nearby structures including road embankments, dikes, and culverts. This causes a larger maximum individual risk value (annual probability of death for a person due to flooding) which has consequences for safety standards based on the individual risk criterion. Mortality rate in the breach zone is also affected by representations of buildings as solid objects versus as roughness elements. Furthermore, changes in the loss of life estimation approach via alternative ways of including people's behaviour, building characteristics, and age of the population, have a significant impact on flood fatality risk. Results from this study can be used to support future risk assessments and decision making with respect to safety standards.
Woods versus waves: wave attenuation through non-uniform forests under extreme condition
Worldwide, communities are facing increasing flood risk, due to more frequent and intense hazards and rising exposure through more people living along coastlines and in flood plains. Nature-based Solutions (NbS), such as mangroves, and riparian forests, offer huge potential for adaptation and risk reduction. The capacity of trees and forests to attenuate waves and mitigate storm damages receives massive attention, especially after extreme storm events. However, application of forests in flood mitigation strategies remains limited to date, due to lack of real-scale measurements on the performance under extreme conditions. Experiments executed in a large-scale flume with a willow forest to dissipate waves show that trees are hardly damaged and strongly reduce wave and run-up heights, even when maximum wave heights are up to 2.5 meters. It was observed for the first time that the surface area of the tree canopy is most relevant for wave attenuation, but that the very flexible leaves hardly add to effectiveness. Overall, the study shows that forests can play a significant role in reducing wave heights and run-up under extreme conditions. Currently, this potential is hardly used but may result in considerable cost savings in levee designs.