The emergence of floodplain vegetation enhances the local drag and affects the velocity distribution of the overbank flow in a two‐stage channel. Numerous analytical models (e.g., the modified SKM ...models) have been established to simulate the transverse velocity distributions of overbank flows with vegetation. However, the dimensionless eddy viscosity (λ) was simplified to be a constant in these models when quantifying transverse momentum exchange. In this paper, we develop a novel analytical model to simulate the depth average velocity profiles of overbank flows with floodplain vegetation. Considering a micro‐hexahedral element on a vegetated‐bed area for a two‐stage rectangular channel, the governing equation of the vegetated overbank flow is derived. An improved eddy viscosity model that considers the transverse variation of the λ is employed to quantify the transverse momentum transfer, and a linear empirical function simplifies the secondary current. The simulated depth average velocities agreed well with the published measurements. In comparison with existing models, the proposed model shows better prediction accuracy, particularly in the shearing layer domain. More importantly, for the optimized model parameters, we found that the dimensionless eddy viscosity plays the most influential role when simulating depth‐averaged velocities in the shearing layer domain, whereas the drag force dominates for vegetated floodplains. This study provides a convenient method for predicting the flow velocity in overbank floods with vegetation.
Hydraulic parameters associated with overbank flow in a two‐stage channel (Reference to Shiono and Knight (1991)).
The Sanriku ocean‐bottom seismometer system uses an optical fiber cable to guarantee real‐time observations at the seafloor. A dark fiber connected to a Distributed Acoustic Sensing (DAS) ...interrogator converted the cable in an array of 19,000 seismic sensors. We use these measurements to constrain the velocity structure under a section of the cable. Our analysis relies on 24 hr of ambient seismic field recordings. We obtain a high‐resolution 2‐D shear‐wave velocity profile by inverting multimode dispersion curves extracted from frequency‐wave number analysis. We also produce a reflection image from autocorrelations of ambient seismic field, highlighting strong impedance contrasts at the interface between the sedimentary layers and the basement. In addition, earthquake wavefield analysis and modeling help to further constrain the sediment properties under the cable. Our results show for the first time that ocean‐bottom DAS can produce detailed images of the subsurface, opening new opportunities for cost‐effective ocean‐bottom imaging in the future.
Plain Language Summary
Distributed Acoustic Sensing (DAS) is a relatively new measurement method that has the potential to convert existing fiber optic communication infrastructure into arrays of thousands of seismic sensors. In this research, we connected a DAS to a cable that was originally installed at the bottom of the ocean to sustain a seismic and tsunami observatory in the Sanriku Region. We show that this new type of measurement can provide reliable information to image and explore the shallow subsurface under this fiber cable. This is the first time such analysis is performed in an oceanic environment, and our methods could be readily exportable to other fiber‐optic cables that are the backbones of our modern telecommunication.
Key Points
Ocean‐bottom Distributed Acoustic Sensing is used to image shallow VS structure
Rayleigh wave phase velocity dispersion curves are extracted from frequency‐wave number analysis
Reflection image is obtained from autocorrelations of ambient seismic field
The current research aims to predict the velocity distribution and discharge rates in rivers based on the entropy concept using only one surface velocity measurement. In this direction, first, the ...uncrewed aerial vehicle (UAV)‐based image acquisition technique was applied to collect the surface velocity distribution along two European rivers, the Sajó, and the Freiberger Mulde Rivers. Seven cross sections were chosen for the analysis. At each cross section, first, the entropic parameter Φ(M) was calibrated based on the maximum and mean velocity magnitudes, derived from Acoustic Doppler Current Profilers, respectively, showing a trend for all cross sections with a range of 0.6 < Φ(M) < 0.75. Next, the maximum surface velocity provided by the UAV was implemented as a single velocity input. Finally, the bathymetry data, herein collected by UAV, were considered as the input for the entropy approach. In this way, the entropy iterative method allowed estimating the mean flow velocity by identifying the location (dip) of maximum velocities across the river site and inferring the 2D velocity distribution. The results highlighted that the entropy approach can accurately predict the velocity distribution and discharge rates with a percentage error lower than 13%.
Key Points
Applying the uncrewed aerial vehicle‐based image acquisition technique to provide the maximum surface velocity
Estimating the river discharge based on the Entropy concept relying on only maximum surface velocity
Presenting cross‐sectional velocity distribution calculated by the Entropy approach through European rivers, Sajó, and Freiberger Mulde
Abstract
It has been hypothesized that submesoscale flows play an important role in the vertical transport of climatically important tracers, due to their strong associated vertical velocities. ...However, the multi-scale, non-linear, and Lagrangian nature of transport makes it challenging to attribute proportions of the tracer fluxes to certain processes, scales, regions, or features. Here we show that criteria based on the surface vorticity and strain joint probability distribution function (JPDF) effectively decomposes the surface velocity field into distinguishable flow regions, and different flow features, like fronts or eddies, are contained in different flow regions. The JPDF has a distinct shape and approximately parses the flow into different scales, as stronger velocity gradients are usually associated with smaller scales. Conditioning the vertical tracer transport on the vorticity-strain JPDF can therefore help to attribute the transport to different types of flows and scales. Applied to a set of idealized Antarctic Circumpolar Current simulations that vary only in horizontal resolution, this diagnostic approach demonstrates that small-scale strain dominated regions that are generally associated with submesoscale fronts, despite their minuscule spatial footprint, play an outsized role in exchanging tracers across the mixed layer base and are an important contributor to the large-scale tracer budgets. Resolving these flows not only adds extra flux at the small scales, but also enhances the flux due to the larger-scale flows.
The Xiaojiang fault zone system (XJFS) is located in the southeastern Tibet with high seismicity. In this study, we invert Rayleigh and Love wave dispersion curves obtained from three dense seismic ...arrays jointly for high-resolution 3-D crustal average shear wave velocity and radial anisotropy models simultaneously in XJFS. Our model reveals that the upper crust and mid-lower crust generally exhibit negative and positive radial anisotropy, respectively, implying that the deformation pattern is depth-dependent. To the east of the Lvzhijiang Fault, most of the low velocity zones in the mid-crust correspond to the positive radial anisotropy (Vsh > Vsv); the channelized weak zone seems to be continuous across the Red River Fault at depth of 20 km, probably within a thin layer. West of the Lvzhijiang Fault, where it is inferred to be the inner zone of the Permian Emeishan Large Igneous Province, the high velocity zone and positive radial anisotropy in the mid-lower crust can be attributed to the underplating and intruded magmatic rocks. In the upper crust, the high and low velocity zones are mostly belt-shape and follow the north-south direction parallel to the regional major faults; the lateral variation of the radial anisotropy spatially corresponds to the activity of different segments of the Xiaojiang fault zone. Our models depict detailed geometry of the channelized weak zone in the mid-lower crust and provide new insight into the role of major faults in regional tectonics.
•High-precision crustal Vs and radial anisotropy models in XJFS (SE Tibet) are derived.•Our models depict detailed geometry of the channelized weak zone in mid-lower crust.•Radial anisotropy is spatially associated with activity of different segments of XJF.
Sound velocity errors often lead to distortion of seafloor topography in multibeam bathymetric data processing. The current method of sound velocity profile (SVP) inversion has the problems of small ...application scope of the inversed equivalent SVP (ESVP) and terrain distortion calculated by inversed SVP under the condition of water depth consistency constraint in overlapping line area. In this article, we propose a method of SVP inversion using the empirical orthogonal function (EOF) and differential evolution (DE) algorithm. First, EOF is used to decompose the SVP matrix. Then, the water depth discrepancy between the check line and the main survey line is used as the objective function instead of the discrepancy of overlapping line area, and DE is used to find the optimal time coefficient. Finally, the SVP is reconstructed to correct the seabed topography. The method was verified by experiments in shallow and mid-deep water, and the mean absolute and standard deviation of the corrected bathymetric discrepancy values are about 50% of those before correction.
The interaction between a turbulent boundary layer flow and compliant surfaces is investigated experimentally. Three viscoelastic coatings with different material stiffnesses are used to identify the ...general surface response to the turbulent flow conditions. For the softest coating, the global force measurements show two obvious regimes of interaction with an indicated transition at $U_b/C_t\sim 3.5$, where $U_b$ is the bulk flow velocity and $C_t$ is the coating shear velocity. The one-way coupled regime shows friction values comparable to those of the rigid wall, while the two-way coupled regime indicate a significant increase in fluid friction. Within the one-way coupled regime for $U_b/C_t>1.2$, the flow measurements show a low level of two-way coupling represented by the change of the velocity profile as well as the increase in the Reynolds stresses in the near-wall region. This is supported by the surface deformation measurements. Initially, the turbulent flow structures induce only an imprint on the coating surface, while a change in surface response occurs when the surface wave propagation velocity $c_w$ equals the shear wave velocity of the coating $C_t$ (i.e. $c_w/C_t\sim 1$). Above $U_b/C_t>1.2$, a growth in wavelength is observed with increasing flow velocity, most probably due to the surface wave formation generated downstream the pressure features of the flow. The surface response is stable and correlates with the high-intensity turbulent pressure fluctuations in the turbulent boundary layer, with a wave propagation velocity $c_w\sim 0.7\unicode{x2013}0.8$ $U_b$. Within the two-way coupled regime, additional fluid motions and a downward shift in the logarithmic region of the velocity profile are observed due to substantial surface deformation and confirm the frictional drag increase. Another type of surface response is initiated by phase-lag instability in combination with surface undulations that start to protrude the viscous sublayer, where the propagation velocity of surface wave trains is $c_w\sim 0.17\unicode{x2013}0.18$ $U_b$.
The locations of ultralow‐velocity zones (ULVZs) at the core‐mantle boundary (CMB) have been linked to a variety of features including hot spot volcanoes and large low‐velocity province (LLVP) ...boundaries, yet only a small portion of the CMB region has been probed for ULVZ existence. Here we present a new map of lower mantle heterogeneity locations using a global collection of highly anomalous SPdKS recordings based on a dataset of more than 58,000 radial component seismograms, which sample 56.9% of the CMB by surface area. The inference of heterogeneity location using the SPdKS seismic phase is challenging due to source‐versus receiver‐side ambiguity. Due to this ambiguity, we conducted an inversion using the principle of parsimony. The inversion is conducted using a genetic algorithm which is repeated several thousand times in order to construct heterogeneity probability maps. This analysis reveals that at probabilities ≥0.5, 0.25, and 0.125 up to 1.3%, 8.2%, or 19.7% of the CMB may contain ULVZ‐like heterogeneities. These heterogeneities exist in all lower mantle settings, including both high‐ and low‐velocity regions. Additionally, we present evidence that the Samoan ULVZ may be twice as large as previously estimated, and also present evidence for the existence of additional mega‐sized ULVZs, such as a newly discovered ULVZ located to the east of the Philippines. We provide new evidence for the ULVZ east of the Philippines through an analysis of ScP records.
Plain Language Summary
Past deep‐Earth studies have identified thin zones of reduced seismic velocities located atop the core‐mantle boundary. These zones, referred to as ultralow‐velocity zones or ULVZs, have been related to several important features of the Earth's interior such as mantle plumes, continent‐sized low‐velocity provinces, and large igneous provinces. Nonetheless, we do not know what ULVZs are made of, whether they only exist in specific regions of the core‐mantle boundary (CMB), how they link to other mantle properties, or whether all ULVZs are the same. We have assembled a large dataset of seismic waveforms that are sensitive to deep mantle heterogeneity and have identified the waveforms with features indicative of heterogeneous mantle properties, such as ULVZ presence, and search for the simplest distribution of lower mantle heterogeneities. We find that as much as 19.7% of the CMB area must have anomalous seismic properties to explain our dataset. We also use a type of seismic wave reflected off the CMB to verify our initial results in a newly discovered heterogeneous region that lies on the CMB to the east of the Philippines. Our new heterogeneity map provides greater coverage than previous maps and suggests that several mega‐sized ULVZs may exist.
Key Points
We inverted for ULVZ location from highly anomalous SPdKS data using the principle of parsimony
As much as 19.7% of the CMB by surface area may contain ULVZ‐like heterogeneity
Several mega‐ULVZs may exist in the deep Earth
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