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Spur dikes (SD) are installed along the outer bank of a river to control river bank erosion. The first SD is more susceptible to failure due to toe scouring. A protective spur dike (PSD) is an effective measure to reduce the scouring around the nose of the first SD. In this study, experiments were conducted to determine the optimal location and height of the PSD to minimize scour depth at the nose of the first SD. The results indicate that the maximum scour depth depends on the height and distance of the PSD from the first SD. The best performance was observed when the ratio of PSD height to flow depth (HP/y) was 1.25, and the ratio of the PSD distance from the first SD to flow depth (XP/y) was 1.25. Scour depth at the nose of the first and second SDs was reduced by 54% and 32%, respectively. These findings suggest that PSDs can significantly reduce scour depth, thereby enhancing the effectiveness and stability of SDs. KEYWORDS river engineering scour flow separation permeable spur dike protective spur dike

Seismicity and ground deformation measurements show how a recent segmented dyke intrusion in the Bárðarbunga volcanic system in Iceland grew laterally for 45 kilometres over 14 days; dyke opening and seismicity were focused at the most distal segment, where lateral dyke growth with segment barrier breaking by pressure build-up occurred. Dyke growth in Iceland's largest volcanic system A recent segmented dyke intrusion (a sheet of magma tracing its way through and across the surrounding rocks) is reshaping the landscape in the Bárðarbunga volcanic system in Iceland. Freysteinn Sigmundsson and co-authors have used seismicity and ground deformation mapped by global positioning system (GPS) and satellite radar images to show that the dyke grew laterally for 45 km during a 14-day period in August 2014. Dyke opening and seismicity were focused at the most distal segment, where lateral dyke growth with segment barrier breaking by pressure build-up occurred. Dyke growth was slowed by an effusive fissure eruption near the end of the dyke. Crust at many divergent plate boundaries forms primarily by the injection of vertical sheet-like dykes, some tens of kilometres long 1 . Previous models of rifting events indicate either lateral dyke growth away from a feeding source, with propagation rates decreasing as the dyke lengthens 2 , 3 , 4 , or magma flowing vertically into dykes from an underlying source 5 , 6 , with the role of topography on the evolution of lateral dykes not clear. Here we show how a recent segmented dyke intrusion in the Bárðarbunga volcanic system grew laterally for more than 45 kilometres at a variable rate, with topography influencing the direction of propagation. Barriers at the ends of each segment were overcome by the build-up of pressure in the dyke end; then a new segment formed and dyke lengthening temporarily peaked. The dyke evolution, which occurred primarily over 14 days, was revealed by propagating seismicity, ground deformation mapped by Global Positioning System (GPS), interferometric analysis of satellite radar images (InSAR), and graben formation. The strike of the dyke segments varies from an initially radial direction away from the Bárðarbunga caldera, towards alignment with that expected from regional stress at the distal end. A model minimizing the combined strain and gravitational potential energy explains the propagation path. Dyke opening and seismicity focused at the most distal segment at any given time, and were simultaneous with magma source deflation and slow collapse at the Bárðarbunga caldera, accompanied by a series of magnitude M  > 5 earthquakes. Dyke growth was slowed down by an effusive fissure eruption near the end of the dyke. Lateral dyke growth with segment barrier breaking by pressure build-up in the dyke distal end explains how focused upwelling of magma under central volcanoes is effectively redistributed over long distances to create new upper crust at divergent plate boundaries.

In many estuarine areas around the world, the safety of human societies depends on the functioning of embankments (dikes) that provide protection against river floods and storm tides. Vegetation on land-side slopes protects these embankments from erosion by heavy rains or overtopping waves. We carried out a field experiment to investigate the effect of plant species diversity on soil loss through erosion on a simulated dike. The experiment included four diversity treatments (1, 2, 4, and 8 species). In the third year of the experiment, we measured net annual soil loss by measuring erosion losses every 2 weeks. We show that loss of plant species diversity reduces erosion resistance on these slopes: net annual soil loss increased twofold when diversity declines fourfold. The different plant species had strongly diverging effects on soil erosion, both in the single-species and in the multi-species plots. Analysis of the dynamics of the individual species revealed that the main mechanism explaining the strong effects of plant species diversity on soil erosion is the com pensation or insurance effect, that is, the capacity of diverse communities to supply species to take over the functions of species that went extinct as a consequence of fluctuating environmental conditions. We conclude that the protection and restoration of diverse plant communities on embankments and other vegetated slopes are essential to minimize soil erosion, and can contribute to greater safety in the most densely populated areas of the world.

In the river system, the deployment of impermeable dikes often leads to significant morphological changes, including scouring due to strong momentum exchanges between the dike field and the main flow. This presents a challenge in managing riverbank erosion effectively. This study aims to analyze the impact of alternative length layouts of dikes on flow dynamics in an open channel, with the focus on minimizing the scour responsible factors around dikes. Employing a Computational Fluid Dynamic approach, this research investigates flow behavior around a series of emerged and a protective dike with varying lengths. The study utilizes the Reynolds Stress Model (RSM) to capture detailed flow characteristics such as velocity, turbulence, and recirculation eye displacement both upstream and downstream of the protective dike. The results showed that employing a protective dike with 0.5 L -0.6 L (where L represents the dike length) can significantly reduce depth averaged velocity and turbulent kinetic energy near the first dike head by 35% and 41%, respectively. The study recommends constructing protective dikes with optimized dimensions to mitigate the adverse impacts of momentum exchanges, thereby enhancing riverbank protection and reducing erosion risks.

Natural hazard prediction and efficient crust exploration require dense seismic observations both in time and space. Seismological techniques provide ground-motion data, whose accuracy depends on sensor characteristics and spatial distribution. Here we demonstrate that dynamic strain determination is possible with conventional fibre-optic cables deployed for telecommunication. Extending recently distributed acoustic sensing (DAS) studies, we present high resolution spatially un-aliased broadband strain data. We recorded seismic signals from natural and man-made sources with 4-m spacing along a 15-km-long fibre-optic cable layout on Reykjanes Peninsula, SW-Iceland. We identify with unprecedented resolution structural features such as normal faults and volcanic dykes in the Reykjanes Oblique Rift, allowing us to infer new dynamic fault processes. Conventional seismometer recordings, acquired simultaneously, validate the spectral amplitude DAS response between 0.1 and 100 Hz bandwidth. We suggest that the networks of fibre-optic telecommunication lines worldwide could be used as seismometers opening a new window for Earth hazard assessment and exploration. Imaging the internal structure of faults remains challenging using conventional seismometers. Here, the authors use fibre-optic cables used for telecommunications to obtain strain data and identify faults and volcanic dykes in Iceland and suggest that fibre-optic cables could be used for hazard assessment.

Recent flood dynamics of the Mekong Delta have raised concerns about an increased flood risk downstream in the Vietnamese Mekong Delta. Accelerated high dike building on the floodplains of the upper delta to allow triple cropping of rice has been linked to higher river water levels in the downstream city of Can Tho. This paper assesses the hydraulic impacts of upstream dike construction on the flood hazard downstream in the Vietnamese Mekong Delta. We combined the existing one-dimensional (1-D) Mekong Delta hydrodynamic model with a quasi-two-dimensional (2-D) approach. First we calibrated and validated the model using flood data from 2011 and 2013. We then applied the model to explore the downstream water dynamics under various scenarios of high dike construction in An Giang Province and the Long Xuyen Quadrangle. Calculations of water balances allowed us to trace the propagation and distribution of flood volumes over the delta under the different scenarios. Model results indicate that extensive construction of high dikes on the upstream floodplains has had limited effect on peak river water levels downstream in Can Tho. Instead, the model shows that the impacts of dike construction, in terms of peak river water levels, are concentrated and amplified in the upstream reaches of the delta. According to our water balance analysis, river water levels in Can Tho have remained relatively stable, as greater volumes of floodwater have been diverted away from the Long Xuyen Quadrangle than the retention volume lost due to dike construction. Our findings expand on previous work on the impacts of water control infrastructure on flood risk and floodwater regimes across the delta.

Canal dikes in low‐lying polders, as well as in other regions worldwide, are critical infrastructure for flood protection and water management. The subsurface water conditions can cause dike failures during excessive rainfall and prolonged periods of drought. There is a lack of multi‐year monitoring of subsurface water conditions in canal dikes and an insufficient understanding of their geohydrological behavior. This study provides and analyses a novel multiyear data set of soil moisture and hydraulic heads (from February 2020 until March 2023) from a monitoring network covering various canal dikes with different characteristics in the western Netherlands. The data, including two extremely dry summers, highlight the impact of meteorological variations on the subsurface water conditions. Non‐hydrostatic hydraulic head levels were observed during droughts that can be detrimental to dike stability and that are often not accounted for in safety assessments for drought situations. The effectiveness of various meteorological drought indicators applied to subsurface water conditions was evaluated: the precipitation deficit is the most reliable measure and outperforms the standardized drought indicators (SPEI and SPI). The drought recovery of dikes was analyzed to understand seasonal transitions and the sequence of different failure mechanisms, during dry and wet situations. This analysis also reveals differences between meteorological, soil moisture, and groundwater droughts, highlighting soil's storage capacity after drought and the limitations of meteorological drought indicators as proxies for soil moisture and groundwater. The insights from this study enhance assessments, inspection procedures and the identification of weak spots of dikes and other earthworks of infrastructure. Key Points Novel multiyear observations of soil moisture and hydraulic heads from various canal dikes reveal geohydrological behavior The precipitation deficit emerges as the most reliable meteorological drought indicator and can be used as an indicator of dike safety The drought recovery lasted 4.5 months in 2022, which is important for the transition between seasons and different failure mechanisms

Trains of wing dams (spur dikes) are used in river engineering for navigation in many rivers, such as the Mississippi River. These structures increase water level due to added resistance, and thus increase flood stage. The redistribution of bed sediment associated with constriction scour in the central channel zone and deposition along the banks between wing dams, however, may result in a compensating decrease in water stage. The net effect of wing dams on flood stage is determined by a balance between these two effects. We apply a high‐fidelity 3D numerical model (LES rather than RANS or shallow water approach) to investigate the flow, water level and sediment transport in wing dam fields. We study both fully emergent (tops of wing dams protrude above water surface) and submerged (tops of wing dams below water surface) fields. Our results for a simplified configuration show that (a) the additional resistance of wing dams does indeed increase water stage, but (b) much of this increase is reduced via in‐channel redistribution of bed sediment, including non‐local contraction scour in the main channel. In all cases studied here, when the bed is fully erodible, wing dams increase depth in the central channel region between the wing dam field, promoting navigability there. We provide a direct upscale our results to an Upper Reach of the Lower Mississippi River (URLMR) using distorted Froude scaling, but outline numerous caveats which motivate future studies. Plain Language Summary The research was motivated by a debate that has been raging for decades since the Great Flood of 1993 on the Mississippi River (https://www.usgs.gov/centers/cm‐water/science/great‐flood‐1993). The Mississippi River is lined with thousands of wing dams. These are supposed to maintain navigation depth at low flow by constricting the main flow of the river. They are, however, designed to be overtopped during floods, so mitigate their effect on flood stage. Some argue that wing dams have substantially increased flood stage, whereas others have argued that they have a negligible effect on flood stage. We have obtained what we believe is the first in‐depth look at this problem via a numerical model. We find that wing dams do increase flood stage, but that this is substantially (not completely) mitigated by central scour and side deposition induced by the wing dams. Key Points High fidelity numerical models offer powerful tools to evaluate the morphodynamics of navigable rivers with fields of wing dams (spur dikes, groins) Fields of wing dams installed to maintain river navigation add resistance and thus raise water stage These same fields induce constriction scour that acts to compensate added resistance and lower water stage

The emplacement of dikes and sills plays a crucial role in crustal mechanics. The parameter used to describe their resistance to propagation, fracture energy, remains controversial. Here, we show how different stress biaxiality levels experienced by dikes can directly affect the micromechanisms of crack propagation in rocks, consequently impacting fracture energy. We performed controlled tensile crack propagation experiments under opposite stress biaxialities. We connect fracture energy variations monitored through a compliance‐based method to crack microstructures observed on post‐mortem specimens. Microscopy techniques showed that biaxial tension generates intricate microstructures driven by topological instabilities, such as deflections and branches, to circumnavigate tougher grains. This yields a higher fracture energy, that we attribute to front roughening and bridging mechanisms due to front fragmentation. Bridging toughening is gradual and increase with crack size. This hints at the existence of a scale dependency of fracture energy of dikes also experiencing biaxial tension. Plain Language Summary Dikes and sills play a key role in volcanic systems by storing and helping magma move beneath the Earth's surface, affecting volcanic eruptions and the formation of igneous rocks. However, the resistance of rocks to the propagation of these cracks, known as fracture energy, varies widely and is not well understood. This study investigates how stress conditions, specifically stress biaxiality, influence crack growth in rocks and their overall resistance. We conducted lab experiments under two opposite stress conditions: tension in both principal directions and a mix of compression parallel and tension perpendicularly to the crack, mimicking the stresses that dikes experience. Using advanced imaging techniques, we discovered that under biaxial tension, cracks form intricate patterns, like zigzags and branches, to bypass stronger grains. This surprisingly increases the energy required to break the rocks. This is due to the creation of larger surfaces and to the fragmentation of the crack front trapping intact rock patches called bridges, which resist crack opening. As their number increases with crack size, fracture energy gradually increases. This finding suggests that the fracture energy of dikes under biaxial tension may also depend on the length of the crack, providing new insights into volcanic processes. Key Points Biaxial tension enhances crack deflections, branching, and bridging formation around tougher grains Despite circumnavigating tougher grains, tests under biaxial tension yields higher fracture energy, predominantly attributed to bridges In geological settings, bridging occurrence coincides with scale‐dependent toughness of fluid‐driven cracks experiencing biaxial tension