Bottom Velocity Research Articles

Resolution analysis of the inverse problem for geoacoustic experiment design

This work explores effects of experiment geometry and array configuration on the resolving power of a continuous ocean bottom geoacoustic inverse problem in a shallow water environment. The uncertainty and resolution of this problem, in which ocean bottom P-wave velocities as a function of depth are estimated from noisy acoustic pressure waveforms received on vertical and horizontal line arrays in the water, can be investigated before the experiment is conducted, allowing one to improve or optimize the problem parameters to best configure an experiment during its planning phase. In this work, the resolution results of complete synthetic geoacoustic inversions at varying geometries and array configurations are compared with resolution results at various candidate seabottom profiles, initially using standard techniques of linearized inverse theory. Singular value decomposition is used to interpret the tie between geometry and regularization in the inverse problem, which directly affects the resolution. Then, additional comparisons and analysis address the nonlinearity of the problem, which causes a dependence of the resolution results on the bottom profile being solved for—which is unknown, and Monte Carlo analysis is used to show where the linearity approximation breaks down in the resolution results. [Work partially funded by ONR.]

A numerical simulation of metallic cylindrical sandwich shells subjected to air blast loading

The dynamic response of cylindrical sandwich shells with aluminum foam cores subjected to air blast loading was investigated numerically in this paper. According to KNR theory, the nonlinear compressibility of the air and finite shock conditions were taken into account in the finite element model. Numerical simulation results show that the compression strain, which plays a key role on energy absorption, increases approximately linearly with normalized impulse, and reduces with increasing relative density or the ratio of face-sheet thickness and core thickness. An increase of the impulse will delay the equalization of top and bottom face-sheet velocities of sandwich shell, but there is a maximum value in the studied bound. A limited study of weight optimization was carried out for sandwich shells with respect to the respective geometric parameters, including face-sheet thickness, core thickness and core relative density. These numerical results are of worth to theoretical prediction and engineering application of cellular metal sandwich structures.

Surf zone cross‐shore boundary layer velocity asymmetry and skewness: An experimental study on a mobile bed

An experimental study on a physical model of a beach in a two‐dimensional wave flume was designed to investigate velocity nonlinearities in the wave boundary layer. The cross‐shore velocity was measured in the surf zone along a vertical profile every 3mm from free‐stream elevation down to the still bed level. The skewness and the asymmetry of the phase averaged velocity were computed at each elevation. Observations indicate that the free‐stream asymmetry transforms into bottom velocity skewness. A linear experimental relation between free‐stream asymmetry to skewness ratio and bottom skewness to free‐stream skewness ratio is established. A theoretical linear relationship is discussed, which predicts the phase lead of the bottom velocity. This phase lead is also determined by Fourier analyzing the velocity time series. The first two Fourier components yield the same phase lead at the bed that is found to be about 30° and nearly constant over all the experiments made.

Influence of emergent macrophyte (Phragmites australis) density on water turbulence and erosion of organic-rich sediment

The effects of the emergent macrophyte Phragmites australis on water turbulence, bottom shear velocity and water turbidity were experimentally studied with the stem densities 0 stems/m2-100 stems/m2 and using organic-rich sediment. It is found that the maximum root-mean-square (rms) velocity and shear velocity decrease linearly with increasing stem density. By affecting the height of peak turbulence values, emergent plants can thus lower the frequency of sediment erosion events. Additionally, the slope of the rms turbulence increment decreases with increasing stem density. Thus, also the duration of strong turbulence is affected by the density of emergent macrophytes. The critical shear velocity for sediment resuspension is 0.0035 m/s-0.0055 m/s and not affected by the stem density. In the absence of macrophytes, water flow causes a 6.3-fold increment in turbidity, but with macrophytes turbidity values remain lower. The experiments suggest that even relatively low densities of Phragmites can have substantial effects on water turbulence and consequently on water quality.

Variation in morphological characteristics of Japanese fluvial sculpin related to different environmental conditions in a single river system in eastern Japan

AbstractStream fish with low mobility are often restricted to a narrow range of a stream course, and their morphological characters tend to be affected substantially by the environmental conditions of their habitat. We examined the relationships between environmental conditions and morphological characters of Japanese fluvial sculpin (Cottus pollux), a small, benthic fish with low mobility, at seven tributaries of the same river system in eastern Japan. Discriminant function analysis (DFA) indicated significant morphological differences in sculpins amongst the seven tributaries, and over 80% of individuals were classified correctly in each tributary. The strong positive correlation/loadings on the first discriminant function (DF1: 49.9% of total variation) were anterior caudal peduncle depth (0.436) and caudal peduncle truss (0.387), both of which could be regarded as an attribute of caudal peduncle depth. The second discriminant function (DF2: 16.5% of total variation) had a high negative loading for distance between the posterior first dorsal fin and anterior anal fin (−0.454), which could be regarded as an attribute of body depth. Correlation analysis indicated that sculpin tended to have a lower caudal peduncle depth under faster bottom current velocities at the tributary scale. Individuals deviating substantially from the centroid of DF2 in each tributary had a poorer body condition. Our results suggest the possibility of changes in the morphological characters of Japanese fluvial sculpin in relation to environmental conditions even at the tributary scale within the same river system.

Ordering of two small parameters in the shallow water wave problem

The classical problem of irrotational long waves on the surface of a shallow layer of an ideal fluid moving under the influence of gravity as well as surface tension is considered. A systematic procedure for deriving an equation for surface elevation for a prescribed relation between the orders of the two expansion parameters, the amplitude parameter α and the long wavelength (or shallowness) parameter β, is developed. Unlike the heuristic approaches found in the literature, when modifications are made in the equation for surface elevation itself, the procedure starts from the consistently truncated asymptotic expansions for unidirectional waves, a counterpart of the Boussinesq system of equations for the surface elevation and the bottom velocity, from which the leading-order and higher order equations for the surface elevation can be obtained by iterations. The relations between the orders of the two small parameters are taken in the form β = O(αn) and α = O(βm) with n and m specified to some important particular cases. The analysis shows, in particular, that some evolution equations, proposed before as model equations in other physical contexts (such as the Gardner equation, the modified Korteweg–de Vries (KdV) equation and the so-called fifth-order KdV equation), can emerge as the leading-order equations in the asymptotic expansion for the unidirectional water waves, on equal footing with the KdV equation. The results related to the higher orders of approximation provide a set of consistent higher order model equations for unidirectional water waves which replace the KdV equation with higher order corrections in the case of non-standard ordering when the parameters α and β are not of the same order of magnitude. The shortcomings of certain models used in the literature become apparent as a result of the subsequent analysis. It is also shown that various model equations obtained by assuming a prescribed relation β = O(αn) between the orders of the two small parameters can be equivalently treated as obtained by applying transformations of variables which scale out the parameter β, in favor of α. It allows us to consider the nonlinearity-dispersion balance, epitomized by the soliton equations, as existing for any β, provided that α → 0, but leads to a prescription, in asymptotic terms, of the region of time and space where the equations are valid and so the corresponding dynamics are expected to occur.

Observations of the Vertical Structure of Tidal Currents in Two Inlets

Lippmann, T. C., J. Irish, and Hunt, J., 2013. Observations of the vertical structure of tidal currents in two inlets. In: Conley, D.C., Masselink, G., Russell, P.E. and O’Hare, T.J. (eds.), Proceedings 12 th International Coastal Symposium (Plymouth, England), Journal of Coastal Research, Special Issue No. 65, pp. 2029-2034, ISSN 0749-0208. Observations of the vertical structure of broad band tidal currents were obtained at two energetic inlets. Each experiment took place over a 4 week period, the first at Hampton Inlet in southeastern New Hampshire, USA, in the Fall of 2011, and the second at New River Inlet in southern North Carolina, USA, in the spring of 2012. The temporal variation and vertical structure of the currents were observed at each site with 600 kHz and 1200 kHz RDI Acoustic Doppler Current Profilers (ADCP) deployed on low-profile bottom tripods in 7.5 and 12.5 m water depths near the entrance to Hampton Inlet, and in 8 and 9 m water depth within and outside New River Inlet, respectively. In addition, a Nortek Aquapro ADCP was mounted on a jetted pipe in about 2.5 m water depth on the flank of the each inlet channel. Flows within the Hampton/Seabrook Inlet were dominated by semi-diurnal tides ranging 2.5 - 4 m in elevation, with velocities exceeding 2.5 m/s. Flows within New River inlet were also semi-diurnal with tides ranging about 1 – 1.5 m in elevation and with velocities exceeding 1.5 m/s. Vertical variation in the flow structure at the dominant tidal frequency are examined as a function of location within and near the inlet. Outside the inlet, velocities vary strongly over the vertical, with a nearly linear decay from the surface to near the bottom. The coherence between the upper most velocity bin and the successively vertically separated bins drops off quickly with depth, with as much as 50% coherence decay over the water column. The phase relative to the uppermost velocity bin shifts over depth, with as much as 40 deg phase lag over the vertical, with bottom velocities leading the surface. Offshore, rotary coefficients indicate a stable ellipse orientation with rotational directions consistent over the vertical. At Hampton, the shallower ADCP, but still outside the inlet, shows a rotational structure that changes sign in the vertical indicating a sense of rotation at the bottom that is opposite to that at the surface. Within the inlet, the flow is more aligned with the channel, the decay in amplitude over the vertical is diminished, the coherence and phase structure is nearly uniform, and the rotary coefficients indicate no sense of rotation in the flow. The observations are qualitatively consistent with behavior described by Prandle (1982) for shallow water tidal flows.

Quasi-2D sediment transport model combined with Bagnold-type bed load transport

ABSTRACT Rahman, S., Mano, A. and Udo, K., 2013. Quasi-2D Sediment Transport Model Combined with Bagnold-type Bed Load Transport. Suspended load and bed load sediment transport are important component for the sediment transport in surf zone. The purpose of this study is to obtain a good model for sediment transport in surf zone by combining a quasi-2D sediment transport model and the Bagnold-type sediment transport model. The quasi-2D sediment transport model is used to simulate the suspended load transport, while the Bagnold-type for the bedload transport. A quasi-2D numerical wave model called Funwave was expanded to accommodate the sediment transport model. The model is validated by the published data for sediment transport in a wave flume. Two mode of morphological change is compared to evaluate the influence of wave-current (mode A) and instantaneous bottom velocity (mode B) in the third and forth velocity moment of Bagnold-type sediment transport model. Four sets of bed load transport parameters are...

浅水流の仮定を用いない水深積分モデルによる種々な小規模河床形態の統一的解析法の構築

The objective of this study is to develop a numerical computation method based on depth integrated model for sand waves varying with hydraulic conditions. To calculate depth scale flow structures with sand dunes, we developed the general bottom velocity computation method based on a depth integrated model, in which bottom velocity and pressure acting on bed surface are calculated without taking the assumption of the shallow water flow, such as the hydrostatic pressure distribution. The flow separation behind the dune crest, which has been found to play an important role of the dune formation, is evaluated by the production terms of the depth integrated vorticity equations. Non-equilibrium sediment transport is calculated by using momentum equation of the sediment motion with the relaxation length of the particle movement. We applied the model to experimental results on sand dunes in a narrow channel and discussed the performance of the model.

Artificial reef impact on macrobenthic community on south-eastern Brazil coast

Spatial variations on the macrobenthic composition and structure associated with artificial reefs (ARs) on the south-eastern Brazil coast were related to particle size and geochemical characteristics of the sediment and to bottom currents velocity in the vicinity of ARs and two control areas (CAs). The sampling periods were related to Paraiba do Sul River (PSR) flow, highest in summer months. The sediment was more heterogeneous near the ARs, composed mainly of mud and gravel and with greater abundance of shellfish compared to the CAs predominated by mud. The influence of the artificial structures on the hydrodynamics of the site was evident with significantly lower current velocity. Macrobenthos richness, abundance and diversity varied spatially, with higher mean values near the ARs in both sampling periods, reinforcing the influence of artificial structures on the associated infauna. Significant temporal differences in the concentrations of phosphorus, nitrogen and dissolved organic carbon only on the CAs suggest that the effect of the artificial structures surpasses the environmental seasonality related to the PSR flow. The artificial modules created a more heterogeneous grain size of the sediment that contributed to the enrichment of associated macrobenthos, with higher species richness and abundance due to a higher niche availability.

底面流速解析法を用いた津波による構造物周りの三次元流れと局所洗掘解析

底面流速解析法を用いた津波による構造物周りの三次元流れと局所洗掘解析

TOWARD INTEGRATED MULTI-SCALE SIMULATIONS OF FLOW AND SEDIMENT TRANSPORT IN RIVERS

An integrated multi-scale simulation method of flow and sediment transport in rivers is required for river channel designs and maintenance managements. This paper highlights the need of a new depth integrated model without the assumption of the shallow water flow for integrated simulations of 2D and 3D phenomena in rivers, indicating the applicable ranges and limitations of previous depth integrated models. The general Bottom Velocity Computation method without the shallow water assumption is proposed and its applicability for flows with 3D structures and bed variations is discussed in this paper. The unified calculation method for integrated multi-scale simulations is presented to elucidate flow and sediment transport in laboratory channel and rivers during flood. Finally, we discuss issues to be solved and the future works toward integrated multi-scale simulations of flow and sediment transport in rivers.

Numerical Simulation and Flow Field Analysis of Suspended Grid Stilling Pool Based on VOF Method

The unsteady flow RNG k ~ ε turbulence model and VOF Method are employed to numerically simulate 3-D flow field of diversion tunnel outlet stilling pool in Xinjiang dina river wuyi reservoir. The computational and experimental water surface elevation, pressure on the bottom and cross-sectional mean velocity of the suspended grid stilling pool are compared in well agreement. Suspended grid is used in stilling pool, the number of vortex and range are increased in the pool, and the size of the vortex is decreased along with the flow increase. The suspended grid position is determined at end of the vortex. In the suspended grid stilling pool water stability, flow regime is good.

DEPTH OF CLOSURE DETERMINATION IN THE VICINITY OF COASTAL STRUCTURE

This study is conducted to analyze the effect of coastal structure to depth of closure variation. Analysis on time series bathymetry data has been applied to determine location of depth of closure. The deviation of bathymetry profile changing is also considered. Furthermore, longshore variation of depth of closure is proposed. The hydrodynamic conditions are simulated using Boussinesq model derived by Peregrine (1967). This model is applied considering its applicability to observe non-linear and dispersion phenomenon while wave propagates to the shoreline. The simulation is carried out under regular wave assumption with 20% wave height in deep area is applied as representative wave. The simulation results are obtained in term of surface water level, bottom velocity in x and y direction and current velocity. The result is utilized to calculate maximum bottom velocity just outside boundary layer. To observe sediment movement along the coast, maximum shear stress is calculated under wave-current combined motion. Dimensionless Shields parameter is also assessed. The simulation results are depicted in spatial map. Furthermore, the effect of coastal structure to depth of closure variation is confirmed using hydrodynamic conditions.

Study on dynamic angle of repose for submarine pipeline with spoiler on sandy seabed

This paper investigates the dynamic angle of repose underneath the submarine pipeline. A series of tests are carried out to explore the bottom velocity in the scour hole for the case of with spoiler, and gap-ratio in unidirectional current. On that basis, the formulas of dynamic angle of repose in the scour hole are proposed using a sand deposition model. The formulas consider the bottom velocity (which is mainly influenced by the spoiler height and gap-ratio) that dominates the angle of repose. The calculation results are verified by the test data and a good agreement is obtained.

Selected Habitats of Slimy Sculpin in Coldwater Tributaries of the Upper Mississippi River in Minnesota

Slimy sculpin Cottus cognatus are being reintroduced into coldwater streams in the upper Midwestern United States, where they were extirpated by stream degradation during the early- to mid-1900s. Habitat use and selection by slimy sculpin were examined in nine coldwater tributaries of the Upper Mississippi River in southeastern Minnesota to determine which stream habitats are important for successful reintroduction. Most (>70%) individuals (n = 1932) used coarse substrates and vegetation, shallow water (<30 cm), slow current velocities (<20 cm/sec), and moderately embedded (15–60%) substrates. Compared to habitat availability, adults selected boulder substrate and vegetation, whereas young-of-year (YOY) selected gravel, rubble, and vegetation. YOY sculpin selected shallow water (<30 cm), whereas adults exhibited broader selection (1–60 cm). Both age groups selected the slower bottom velocities (especially <20 cm/sec). Habitat use and selection by adult fish were consistent among 3 y of surveys. Habitat suitability index values for sculpin were similar among native sculpin streams and streams where sculpin have been or may be reintroduced within the same geographic region. Slimy sculpin displayed habitat selection similar to that of other sculpin species, except for selecting much slower current velocities. Suitable physical habitats are present in coldwater streams in southeastern Minnesota to support further reintroduction of slimy sculpin. Failed reintroductions of sculpin likely are not related to lack of suitable physical habitats.

Habitat and prey availability attributes associated with juvenile and early adult pallid sturgeon occurrence in the Missouri River, USA

The pallid sturgeon Scaphirhynchus albus is a federally endangered species native to the Missouri and lower Mississippi Rivers, USA. As part of recovery efforts, over 360 000 pallid sturgeon have been stocked into the Missouri River since 1994, and a standardized, long-term monitoring program was initiated in 2003. Understanding the distribution and habitat require- ments of juvenile and early adult pallid sturgeon (fork length <720 mm, age <10 yr) is an impor- tant goal of the monitoring and recovery programs. In this study, we collected information on habi- tat characteristics and prey availability from the upper Missouri River along the Nebraska-South Dakota border and compared these attributes between capture (present) and non-capture (absent) locations (N = 59). To evaluate the relative influence of habitat and prey availability on pallid sturgeon occurrence, we examined several candidate models using an information- theoretic approach. A prey availability model had the most support and included site-specific information on Diptera and Ephemeroptera abundance. A habitat-based model showed that juve- niles and early adults were found in relatively deeper water and avoided areas where bottom velocities were greater than 1.2 m s �1 . Although not as well supported as the prey-effects model (evidence ratio = 6.4), habitat features also provided a plausible model for predicting occurrence. The models developed here could be used to evaluate pallid sturgeon habitat potential in the Mis- souri River basin and help guide future monitoring and conservation management of this endan- gered species.

Full two-dimensional rapid chute flows of simple viscoplastic granular materials with a pressure-dependent dynamic slip-velocity and their numerical simulations

We present a fully two-dimensional, novel Coulomb-viscoplastic sliding model, which includes some basic features and observed phenomena in dense granular flows like the exhibition of a yield strength and a non-zero slip velocity. The interaction of the flow with the solid boundary is modelled by a pressure and rate-dependent Coulomb-viscoplastic sliding law. The bottom boundary velocity is required for a fully two-dimensional model, whereas in classical, depth-averaged models its explicit knowledge is not needed. It is observed in experiments and in the field that in rapid flow of frictional granular material down the slopes even the lowest particle layer in contact with the bottom boundary moves with a non-zero and non-trivial velocity. Therefore, the no-slip boundary condition, which is generally accepted for simulations of ideal fluid, e.g., water, is not applicable to granular flows. The numerical treatment of the Coulomb-viscoplastic sliding model requires the set up of a novel pressure equation, which defines the pressure independent of the bottom boundary velocities. These are dynamically and automatically defined by our Coulomb-viscoplastic sliding law for a given pressure. A simple viscoplastic granular flow down an inclined channel subject to slip or no-slip at the bottom boundary is studied numerically with the marker-and-cell method. The simulation results demonstrate the substantial influence of the chosen boundary condition. The Coulomb-viscoplastic sliding law reveals completely different flow dynamics and flow depth variations of the field quantities, mainly the velocity and full dynamic pressure, and also other derived quantities, such as the bottom shear-stress, and the mean shear-rate, compared to the commonly used no-slip boundary condition. We show that for Coulomb-viscoplastic sliding law observable shearing mainly takes place close to the sliding surface in agreement with observations but in contrast to the no-slip boundary condition.

Observations of the Surf-Zone Turbulent Dissipation Rate

Abstract The contributions of surface (breaking wave) boundary layer (SBL) and bottom (velocity shear) boundary layer (BBL) processes to surf-zone turbulence is studied here. The turbulent dissipation rate ε, estimated on a 160-m-long cross-shore instrumented array, was an order of magnitude larger within the surf zone relative to seaward of the surf zone. The observed ε covaried across the array with changing incident wave height, tide level, and alongshore current. The cross-shore-integrated depth times ε was correlated with, but was only 1% of, the incident wave energy flux, indicating that surf-zone water-column turbulence is driven directly (turbulence injected by wave breaking) or indirectly (by forcing alongshore currents) by waves and that the bulk of ε occurs in the upper water column. This small fraction is consistent with laboratory studies. The surf-zone-scaled (or Froude-scaled) ε is similar to previous field observations, albeit somewhat smaller than laboratory observations. A breaking-wave ε scaling is applicable in the midwater column at certain locations, indicating a vertical diffusion of turbulence and ε balance. However, observations at different cross-shore locations do not collapse, which is consistent with a cross-shore lag between wave energy gradients and the surface turbulence flux. With strong alongshore currents, a BBL-scaled ε indicates that shear production is a significant turbulence source within the surf zone, particularly in the lower water column. Similarly for large currents at one location, the dissipation to shear production ratio approaches one. Both dissipation scalings depend upon wave energy flux gradients. The ratio of BBL to SBL ε has complex dependencies but is larger for a deeper part of the surf zone and more obliquely incident waves.

Site classification and Vs30 estimation of free-field TSMIP stations using the logging data of EGDT

The Engineering Geological Database for TSMIP (EGDT), the Taiwan Strong Motion Instrumentation Program, has been under construction by the National Center for Research on Earthquake Engineering and the Central Weather Bureau in Taiwan since 2000. Site characterization, comprising surface investigations and logging measurements, was carried out throughout the project. We provide a set of specifications and a description to help users understand the subject matter of the database. EGDT contains 469 surveyed stations, 439 of which were drilled and the logging measurements completed. Of these, 385 had logging data reaching at least 30m, and we used these to examine and determine the most accurate extrapolation of Vs30 (the average S-wave velocity of the top 30m of strata) for the other 54 stations with velocity profiles less than 30m. The chosen method assumed that the bottom velocity is identical from the actual depth of the hole to a distance of 30m, that is, the Bottom Constant Velocity (BCV) method. In order to utilize other existing boreholes which have only N values but no velocities in the future, the empirical S-wave velocity equations for seven different regions and the whole of Taiwan were evaluated by a multivariable analysis. Henceforth, for those existing boreholes which have an N profile less than 30m, the S-wave velocity profile can first be calculated by empirical S-wave velocity equations, and then Vs30 can be estimated by reliable extrapolation. Some other studies of site classifications of TSMIP stations were compared with our results to demonstrate the necessity of reclassification. Ultimately, the Vs30 values of the 439 drilled free-field TSMIP stations were derived and the new site classification was achieved according to the Vs30-based provisions of the National Earthquake Hazards Reduction Program.