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A rapid environmental risk assessment of the Kakhovka Dam breach during the Ukraine conflict
No full textAssessing habitat and biodiversity loss in active conflict zones is a major challenge1. Independent scientific evidence on wartime impacts is essential to inform the environmental priorities of reconstruction and recovery plans. Such plans are typically developed during the conflict resolution period2. However, evidence to shape biodiversity recovery plans has historically emerged many years after resolution and reconstruction2. The combination of remote sensing capabilities, the availability of global environmental data and impact zone modelling now enable rapid assessments of wartime environmental impacts. Here, we describe an early scientific assessment of the potential environmental impact caused by the Kakhovka Dam breach during the ongoing conflict in Ukraine, and highlight the need to build on this initial evidence to inform biodiversity recovery plans in the region
Tsunami ping-pong: generating the whole tsunami event
No full textConsiderable progress has been made over the past two decades in modelling representative tsunami waves in the laboratory. Developments such as the pneumatic tsunami simulator, TS, (Chandler et al, 2021), large wave paddles (Schimmels et al, 2016) and pump driven systems (Goseberg et al, 2013) have allowed full incident tsunami time series to be reproduced at large enough scales to provide reliable physical modelling data. These developments have enabled new insights into the time-varying influence of tsunami waves on run-up (McGovern et al, 2018), buildings (Foster et al, 2017), coastal structures (McGovern et al, 2022) and in particular the scour of sediments around these structures (McGovern et al, 2019). These advances are changing the way engineers and others design and plan for tsunami. However we are still only modelling ‘half the story’.
In all existing published studies, the experiments simulated the effect of a single incident wave. We have so far ignored two significant elements of real tsunami events. Firstly, the rundown or return flow and secondly, the effect of multiple waves within a wave train. Both these aspects are difficult to model physically, particularly as the return flow and subsequent waves require a representative topography downstream of a test section. In most facilities there is not enough flume left to create this overland topography. In addition, changing this bathymetry to represent different scenarios such as a coastal plain or a small foreshore and an inland cliff, requires significant modelling effort and cost.
To solve these challenges we propose the use of two TSs facing each other with the test section in between. This concept is shown in Figure 1. This extended abstract presents data from initial trials of the dual TS system conducted at HR Wallingford in 2023 during a three-day test window as part of the MAKEWAVES collaboration. The importance of simulating the return flow from a tsunami is demonstrated through its influence on scour around a rectangular building
Numerical modeling of the influence of scour and scour protection on monopile dynamic behavior
No full textScour of seabed sediments can occur around offshore foundations. For monopile-supported offshore wind turbine structures, the reduction in foundation stiffness due to scour presents certain operational challenges. In cases where scour causes the natural frequency of the structure to become, for example, close to the range of rotor loading frequencies then—due to the increased risk of fatigue damage—turbine support structures are at risk of reduced operation, or even premature decommissioning. In practice, scour protection and/or remediation systems are typically used to mitigate the development of scour. As well as preventing further erosion, scour remediation systems may have a restorative effect on the stiffness of the foundation. This paper describes a one-dimensional (1D) finite-element model for the analysis of natural frequencies for monopile-supported turbine support structures with active scour process around the foundation. The model also incorporates procedures to model the influence of a rock fill scour remediation system on the foundation stiffness. The numerical model is calibrated and validated by comparison with a set of previously described reduced-scale model tests conducted in a flume. The calibrated 1D model is applied to a field case study at a UK offshore wind farm site
Understanding bimodal seas, shingle beach response and flood risk at Hayling Island, UK
No full textBeach management through nourishment and annual recycling has been applied for over 37 years to manage flood and coastal erosion risk to 1700 properties at Eastoke, Hayling Island, UK. This form of natural flood risk management has proved successful in avoiding the annual flooding that blighted this area previously. However, there have been unexpected storm events that have caused beach erosion and localised flooding. Using long-term nearshore wave datasets and state-of-the-art statistical methods, new multi-variate extreme wave conditions were derived and applied to assess beach performance. Eastoke beach was found to meet its original design criteria of a 0.5% AEP standard of protection for unimodal wave conditions. However, it experiences greater rollback erosion, wave overwash and therefore flood inundation under certain (but not all) bimodal wave conditions, causing uncertainty around the future standard of protection. Communicating these inconsistencies to practitioners and a non-specialist audience is challenging as we tend to oversimplify, despite every beach and storm being unique. Better understanding of mixed beaches, both in situ and through parametric and numerical models, would reduce uncertainty to ensure communities are resilient to climate change
Numerical study on the stiffening properties of scour protection around monopiles for Offshore Wind Turbines
No full textAn active marine seabed causing scour around monopile foundations has to be addressed as a geological hazard. These structures are often protected with rock armour to prevent foundation failure. However, scour protection also increases the confining pressure and embedment length, providing additional stiffness to the soil–pile system. This study focuses on the stiffening effect of scour protection to optimise foundation design. A parametric analysis of the dimensions and materials of scour protection is carried out with more than 100 simulations to assess the stiffening effect of scour protection. Therefore, small-diameter and large-diameter monopiles are investigated. Numerical analysis with the Finite Elements method is conducted to estimate the natural frequencies and the static capacity through moment and lateral load () curves and the Load Utilisation method. These methods, which are new to the study of scour protection, are proposed for the quantification and assessment of scour protection in foundation design. The results show that rock fill restores initial foundation conditions independently of the pile dimensions. While for small-diameter monopiles, scour protection fulfils its double purpose of preventing scour and providing stiffness to the foundation, for large-diameter monopiles the contribution to the stiffness is limited and should only be considered for heavy rock armour and significant scour protection heights. The parametric analysis indicates that a thicker and heavier scour protection increases the static capacity by 10%, whereas the width or a densification through sand accretion have negligible effects (). The () curves and the Load Utilisation method have shown to be effective in assessing the static capacity of monopiles supporting Offshore Wind Turbines
A new formula for pressure transmission inside rubble mound breakwaters protecting land reclamations
No full textRecent land reclamation projects where maximization of land use is crucial have resulted in the appearance of types of coastal defenses that have not been extensively studied. Rubble mound breakwaters with open filters or geotextiles are increasingly used in reclamation projects. Due to placement of landfill material (e.g. dredged sand with silt) close to the rubble mound, these structures are potentially vulnerable to internal erosion and material loss through the coarser breakwater layers, a process which is often characterized as suffusion. Several past studies have highlighted these issues (Cantelmo et al 2011; Polidoro et al 2015). Incipient of sediment motion in the landfill area is highly dependent of the pressure gradients developing and reliable prediction of pressures is of great importance. Some available methods (Burcharth et al 1999, Troch 2002, Muttray and Oumeraci (2005), Vanneste and Troch 2012, Tomasicchio et al 2020) are reliable for generic rubble mound breakwaters, but have weaknesses with respect to breakwaters protecting land reclamation areas, where a reflective interface is formed. These weaknesses are discussed in Dimakopoulos et al (2023).
This work presents a methodology for modifying existing formulas for estimating internal pressures in rubble mound breakwaters in the presence of reflective surfaces. The methodology follows a relatively simple concept and is applied to correct predictions of the formula proposed by Burcharth et al (1999). The corrected formula is compared against numerical modelling predictions published in Dimakopoulos et al (2023). Comparison and further verification of the formula will be performed following additional numerical (CFD) model simulations and experimental data
Modelling reservoir sedimentation management options
No full textWhile dams serve many beneficial purposes, their construction brings various socio-environmental impacts. Some of those are related to interrupted flow of sediment downstream of the dam and sedimentation in the reservoir as well as upstream. To minimize these impacts, several sediment management approaches exist. The most suitable approach is to be selected based on the operational constraints and objectives of sediment management. Modelling the physical process of sedimentation in the reservoir, as well as upstream and downstream, can help in assessment of each approach and aid the selection process. In this paper, we describe the application of a sedimentation model to assess the upstream and downstream impacts, as well as impacts on the dam and reservoir itself, for the proposed 45 m tall Mpatamanga dam on the Shire River in Malawi. The model used is a long term one-dimensional model RESSASS, developed specifically for the purpose of reservoir sedimentation. It covers 22 km from the reservoir tail to the main dam, three tributaries, and the regulating reservoir and dam some 7 km downstream from the main dam. It is linked to the downstream model that covers several ecologically important areas, water intakes for irrigation, and another dam. Several scenarios were tested, exploring the impacts of different sediment inflow scenarios, to account for uncertainty in sediment yield estimations, and the impacts of different sediment management scenarios. The results show that all sand and most of finer sediment will deposit in the reservoir, causing erosion of existing sand deposits downstream. In absence of any sediment management strategy, after some 30 years there could be noticeable raise in bed and water levels upstream of the reservoir, mainly sand deposits, with impacts on the infrastructure, including power production of an upstream dam. Fine sediment deposits would reach the level of the bottom outlet, complexifying its safe operation. Management options considered include two main approaches: reduction of sand inflow on the main Shire River and tributaries, and operating at lower level during the wet period of the year to facilitate sand movement further into the reservoir. The model predicts that the water level drawdown scenarios are more efficient in mitigating the upstream impacts and improve releases from the reservoir, although sand is still not passing downstream in the first 30 years of operation
A review of scour impacting monopiles for offshore wind
No full textMonopiles are important structures in offshore wind engineering, primarily serving as foundations for wind turbines. When installed on a sandy seabed, local scour around monopiles can reduce their capacity, and is a significant hazard to be addressed in the design process. This paper provides a review of research related to local scour around monopiles, focusing on the following aspects: (1) identifying flow structures related to local scour, (2) synthesizing existing data on equilibrium scour depth and scour time-scale, (3) summarizing technology for observing local scour through laboratory experiments, numerical simulations and field observations, and (4) outlining the existing challenges and research opportunities
Design and use of a novel test apparatus for retention of fine dredged fill
No full textThis paper will discuss the problem of retaining fine sand in reclamations protected by rubble mounds with geotextile filters. It formed part of a wider study that will be presented at this conference by Dimakopoulos et al., where the hydraulic gradient at the reclamation / landfill (see Figure 1) will affect retention of the sand. In particular, we will describe the design and application of a novel test device to identify sand retention in experiments on an example sand sample and geotextile subject to reversing heads. It also directly follows on from the work by Cantelmo et al 2011 and Polidoro et al 2015, where hydraulic gradients in the core were examined during the assessment of the extent / suitability of the retention method. The paper will summarise numerical modelling to derive appropriate hydraulic gradients (Dimakopoulos et al.), the design of the test device using relatively easily obtained materials, and the use in a forensic failure study
Lessons for dam safety in the UK from the landslide-generated waves incident in the Apporo dam reservoir, Japan
No full textWe report and analyse the damage caused by landslide-generated waves in the Apporo reservoir (Japan) and take lessons for dam safety in the UK. The incident occurred in September 2018 following an M6.6 earthquake and typhoon Jebi. Apporo dam is a trapezoidal Cemented Sand and Gravel dam with a height of 47.2m. The simultaneous occurrence of the earthquake and the typhoon triggered thousands of landslides. Through field surveys, we identified several landslides on the banks of the reservoir at a close distance to the dam, causing a runup height of 5.3m at the shore. Visible damage, confirmed by site engineers, indicated that the waves damaged the reservoir bank revetments. Here, we model the landslide using Plaxis 3D, replicate the landslide-generated waves applying empirical equations, and discuss the lessons for dam safety in the UK. Using GIS data on elevation, rainfall, and seismicity, we identified the UK regions most susceptible to landslides. Region 3, the highest risk area, contains 252 large reservoirs, indicating the need to include landslide-generated wave risks in assessments of potential failure modes. We discuss prediction capabilities that can be applied for hazard and risk assessment of UK reservoirs regarding landslide-generated waves and propose a four-step methodology for such assessments