Actual Rivers Research Articles

On the Migrating Speed of Free Alternate Bars

AbstractFree alternate bars exhibit wave properties. However, there has been a significant lack of research regarding these wave properties, including the migration speed. In this study, we quantified the migration speed M for bars and identified the dominant variable exerting control over M. Experiments were performed in a laboratory flume under controlled steady state conditions to ensure the development of alternate bars; in addition, the bed level and flow depth were continuously measured. We derive a hyperbolic partial differential equation (HPDE) for the bed level by assuming that the bed level is a continuous function and the advection velocity of the function equals M. Then, to verify the HPDE, we show that it adequately describes the temporal variation in the bed level that is measured experimentally. The advection velocity of the HPDE can be used to calculate M from the energy slope and the Shields number, enabling the calculation of the spatial and temporal variations of M in the experiments. The formula for M showed that the magnitude of M is three to four orders of magnitude smaller than the velocity of the uniform flow. Then, we show that M obtained from the proposed formula is in agreement with those obtained from an instability analysis. Furthermore, we demonstrate that the proposed formula is applicable to actual rivers, in which the scale and conditions differ from those in the experiments.

Practical Data‐Driven Flood Forecasting Based on Dynamical Systems Theory

AbstractData‐driven flood forecasting methods are useful, especially for rivers that lack information required for building physical models. Although the former methods can forecast river stages using only past water levels and rainfall data, they cannot easily predict unprecedented water levels and require a large amount of data to build accurate models. We focus on phase‐space reconstruction approaches based on dynamical systems theory and develop a practical data‐driven forecasting method to overcome existing problems. The proposed method can predict unprecedented water levels owing to a proposed correction term, and provide forecasts using only a small number of water level increase events. We applied the proposed method to data from actual rivers and it achieved the best forecast performance among existing data‐driven methods, including a multilayer perceptron, and a conventional method based on phase‐space reconstruction. In addition, the proposed method forecasted the exceedance of the evacuation warning level 6 h earlier for small and steep rivers. Given its performance and maintainability, the proposed method can be applied to many gauged rivers to facilitate early evacuation.

Additions to the itinerary of King Sigismund’s travels along the Rhine (1414–1418)

It is a well-known fact in research that medieval German rulers also used river routes during their travels. In recent years, however, there has been an increasingly in-depth and comprehensive investigation of the actual rivers to which and the periods of time from which the imperial river cruises can be traced. The question of whether the waterway or land route was preferred when approaching the riverbankshas also been raised by a more recent researchers. Obviously, late medieval source materials provide more information on this issue, thus the reign of Sigismund of Luxembourg can also be covered by this field of research. By examining King Sigismund's travels on the Rhine it can be stated that the monarch took a boat wherever he could on the middle and upper reaches of the Rhineland, regardless of the direction of the river, unless his governmental policy required him to leave the Rhine. On the Constance-Basel route, however, he did not necessarily take advantage of the opportunities for navigation, and it seems that he sometimes opted for land travel. However, the characteristics of sources from the Sigismund period does not allow us to perform quantitative studies that would allow us to show the percentage difference between the frequency of land and water transport. The meeting of the royal couple in 1414 - following the realisation of Sigismund's political intentions - was also organised in the Rhine region, and instead of the monarch waiting for the arrival of his wife from the Kingdom of Hungary in Nuremberg, they started to travel together only from the Middle Rhine region. Furthermore, it appears that their 'joint' journeys were often combined with parallel journeys, and that the royal couple might have been further apart from one another for a certain period of time as they opted for different transport arrangements. It is assumed that in such cases the queen remained on the water for as long as possible. The meeting place could then either be the end of the river cruise or, as in the case of Frankfurt, a station of major importance from the poin of view of the representation of the monarch.

Long-term modelling of fluvial systems at the watershed scale: Examples from three case studies

To date, many numerical approaches are available in studying water flow and sediment dynamics along rivers from the reach to the watershed scale, based on various simplifications. The aim of the present research is to demonstrate how the so-called “morphodynamic quasi-equilibrium hypothesis” is effective in modelling riverine landscape morphodynamics evolution at the watershed scale, focussing on the long-term changes of longitudinal profile and bed grainsize composition of large alluvial rivers. A lumped 0-D, two-reach, two-grainsize model based on the Local Uniform Flow hypothesis and originally developed for reproducing fluvial changes at the historical time-scale, has been applied to three large watercourses to reproduce their long-term evolution, performing a sensitivity analysis based on the present conditions. Three morphometric parameters were analysed, aiming to describe the evolution of the longitudinal profile (concavity and aggrading) and the grainsize composition of the river bed (fining). Though the 0-D approach does not allow for a spatial distribution of the input parameters, namely the liquid and solid discharge, the modelling outcomes show reasonably good qualitative trends. At the scale of analysis (centuries to millennia) and for the chosen large sedimentary systems (thousands of kilometres long), which show high inertia to geomorphological changes likely owing to their longitudinal scale, the model can be helpful in detecting where the present conditions reflect a big disturbance to the “natural” trend. This initial detection method can provide additional insights in evaluating the capability of actual rivers to respond to the present external forcing, eventually reaching a quasi-stable configuration.

Analysis on Scour Characteristics under River Bed Variation in Weir Downstream

Various river projects are underway in small rivers in Korea that typically have natural flows. However, recent findings have shown that damages could be aggravated by structures such as weirs and drop structures during flood incidents. Experimental studies for securing the stability of flood control for these artificial structures have been insufficient, and designs applying the existing domestic design standards would not be suitable for the steep flow sections such as the actual small rivers, possibly aggravating the damages. The present study involved hydraulic model experiments conducted in a laboratory to investigate the surrounding flow patterns according to the river bed slope at the downstream part of the weir model. Further, the scour characteristics in the apron section during the overflow of the structure were analyzed to determine the appropriateness of the apron length. Thus, as the upstream river bed slope gradually increased, the experimental scour length deviated more from the design criteria formula. The results suggest that both the formula suggested by the National Construction Research Institute and Bligh’s formula presented in the River Design Criteria are not suitable for steep-slope rivers, such as small rivers in Korea, because both formulas were proposed based on the seepage line distance and river bed materials without considering the slope of the river bed. Thus, in designing the apron and bed pitching of weirs and drop structures, the river bed slope, scour characteristics of weir overflow, and existing design factors should be comprehensively considered to devise a design formula appropriate for environment of the small rivers in Korea.

Correlation analysis of spatio-temporal images for estimating two-dimensional flow velocity field in a rotating flow condition

Summary Flow velocity estimation in actual rivers using image processing technique has been highlighted for hydrometric communities in the last decades, and this technique is called Large Scale Particle Image Velocimetry (LSPIV). Although LSPIV has been successfully tested in many flow conditions, it has addressed several limitations estimating mean flow field because of difficult flow conditions such as rotating, lack of light and seeds, and noisy flow conditions. Recently, an alternative technique named STIV to use spatio-temporal images based on successively recorded images has been introduced to overcome the limitations of LSPIV. The STIV was successfully applied to obtain one-dimensional flow component in the river for estimating streamflow discharge, where the main flow direction is known. Using the 5th order of central difference scheme, the STIV directly calculated the mean angle of slopes which appeared as strips in the spatio-temporal images and has been proved to be more reliable and efficient for the discharge estimation as compared with the conventional LSPIV. However, yet it has not been sufficiently qualified to derive two-dimensional flow field in the complex flow, such as rotating or locally unsteady flow conditions. We deemed that it was because the strips in the given spatio-temporal images from not properly oriented for main flow direction are not narrow enough or clearly visible, thus the direct estimating strip slope could give erroneous results. Thereby, the STIV has been mainly applied for obtaining one-dimensional flow component. In this regard, we proposed an alternative algorithm to estimate the mean slope angle for enhancing the capability of the STIV, which used correlation coefficient between odd and even image splits from the given spatio-temporal image. This method was named CASTI (Correlation Analysis of Spatio-Temporal Image). This paper described the step-by-step procedure of the CASTI and validated its capability for estimating two-dimensional flow field in the rotating flow. For this purpose, we generated artificial images for lid-driven cavity flow where true velocity field can be manipulated and validated with respect to the proposed CASTI algorithm. As a result, the CASTI was successfully performed to capture two-dimensional velocity vector in the given cavity flow.

Power Law Exponents for Vertical Velocity Distributions in Natural Rivers

While log law is an equation theoretically derived for near-bed region, in most cases, power law has been researched by experimental methods. Thus, many consider it as an empirical equation and fixed power law exponents such as 1/6 and 1/7 are generally applied. However, exponent of power law is an index representing bed resistance related with relative roughness and furthermore influences the shapes of vertical velocity distribution. The purpose of this study is to investigate characteristics of vertical velocity distribution of the natural rivers by testing and optimizing previous methods used for determination of power law exponent with vertical velocity distribution data collected with ADCPs during the years of 2005 to 2009 from rivers in South Korea. Roughness coefficient has been calculated from the equation of Limerinos. And using theoretical and empirical formulae, and representing relationships between bed resistance and power law exponent, it has been evaluated whether the exponents suggested by these equations appropriately reproduce vertical velocity distribution of actual rivers. As a result, it has been confirmed that there is an increasing trend of power law exponent as bed resistance increases. Therefore, in order to correctly predict vertical velocity distribution in the natural rivers, it is necessary to use an exponent that reflects flow conditions at the field.

藻波発生時の柔軟植生の変形解析と流れ抵抗変化に関する研究

A lot of aquatic plants are observed in actual rivers and they have a potential to improve water quality. A large-scale coherent vortex is generated near the vegetation edge, which dominate the momentum and scalar transport. Thus, estimating the flow resistance of vegetated flows is of great importance in river management. In such vegetated open-channel flows, both the geometry of the vegetation elements (shape, size, flexibility and vegetation density) and turbulence characteristics affect the hydrodynamic resistance significantly. However, any important relation between the vegetation motion and the flow resistance property have not been yet established. Therefore, in the present study, we highlighted these important topics and measured the instantaneous velocity structure and coherent motion in open-channel flows with flexible vegetation by using PIV-PTV technique. As the results, the hydro-mechanic interaction between the flow and flexible plant motion was revealed.

Habitat Evaluation on Scour Hole Downflow Low Drop Structure Types Using Large-Scale Experiment

This study sought to review whether large-scale experimentation can apply to actual rivers concerning the effect of topographical change by scour in the downstream area of natural-type low drop structures on the fish habitat. The large-scale applicability experiment performed in this study installed low drop structures as the study object within the experiment channel and precisely surveyed topography in the downstream area of drop structures along with the hydraulic amount including water level and flow velocity under certain flow conditions. Based on topographical data acquired through the survey after the experiment ended, this study reviewed the change of suitability index and difference of weighted usable area by performing 2D habitat simulation. Ultimately, through the habitat simulation results in cases of considering and not considering scour in the downstream area of drop structures, this study analyzed the effects of topographical change in actual rivers on the habitat.

小規模構造物の埋め込み深さが橋脚周辺の洗掘に及ぼす影響

By using three-dimensional numerical model, numerical analysis about local scouring around a pier was carried out. The method of the analysis is three-dimensional LES. The main objectives are to obtain the data of innovative prevention measure that is effective for the actual rivers. The prevention measure is to attach or bury the small-scale structures at the front base of the pier, and to change the current and reduce the scouring depth. Before now, many investigations are carried out, and several important results for the abatement of local scouring were clarified. However, they were the effects under the limited conditions in experiments and numerical analysis. In present paper, the small scale structure is attached or buried in front of the bridge pier and the effect of prevention measure to local scouring is considered. The scouring depth that the small scale structure is attached and buried is smaller than it that the small structure is only attached.

Improving student learning by addressing misconceptions

Students—and often those who teach them—come to class with preconceptions and misconceptions that hinder their learning. For instance, many K–12 students and teachers believe groundwater exists in the ground in actual rivers or lakes, but in fact, groundwater is found in permeable rock layers called aquifers. Such misconceptions need to be addressed before students can learn scientific concepts correctly. While other science disciplines have been addressing preconceptions and misconceptions for many years, the geoscience community has only recently begun to concentrate on the impact these have on students' learning. Valuable research is being done that illuminates how geologic thinking evolves from the “novice” to “expert” level. The expert is defined as an individual with deep understanding of Earth science concepts. As research progresses, geoscientists are realizing that correcting preconceptions and misconceptions can move teachers and students closer to the “expert” level [Libarkin, 2005].

Turburence structure and coherent motion in vegetated canopy open-channel flows

A lot of aquatic plants are observed in actual rivers and they have significant effects on hydrodynamic characteristics such as the velocity distributions, turbulence and coherent structure, as well as mass and momentum exchanges between the vegetated and non-vegetated zones. Aquatic vegetation canopies are responsible for water quality, nutrient and particulate removal, and can reduce turbidity and sediment transport significantly. In the present study, we investigated turbulence structures and coherent motion in vegetated canopy open-channel flows, on the basis of non-intrusive LDA and PIV measurements as well as LES calculations. Section 2 deals with the theoretical consideration on submerged canopy open-channel flows, in which the importance of double-averaging techniques for both time-average and space-average was highlighted. Section 3 describes experimental techniques. Section 4 deals with mean flow structure within and over the canopy, in which the dispersive properties were highlighted. Section 5 considers turbulence structure, in which Reynolds stress, turbulence intensities and turbulent kinetic energy (TKE) budget were discussed. Further, quadrant conditional analysis and two-point space-time correlation revealed the importance of coherent structures near the vegetation edge. Section 6 examines the effects of submergence depth on coherent structure and compares them with terrestrial canopy flows. The whole flow region was divided into three sub-zones, i.e., the emergent zone, the mixing-layer zone, and the log-law zone. In particular, coherent eddies such as sweeps and ejections were highlighted on the instantaneous contour plane of Reynolds stress and vorticity. Finally, Section 7 deals with some LES simulations, which predicted large-scale coherent eddies reasonably in aquatic canopy flows.

EXPERIMENTAL STUDY ON TURBULENT DIFFUSION IN VEGETATED OPENCHANNEL FLOWS BY USING LASER INDUCED FLUORESCENCE METHOD

In actual rivers, many aquatic plants are often observed and submerged canopy flow appears in floods. Submerged plants create complex flow patterns, in which wakes behind vegetation and coherent motions are generated. In particular, mass and momentum transfers and the mixing process are promoted significantly between the in-canopy layer and over-canopy layer. It is, therefore, of great importance for river ecosystem to reveal turbulent diffusion in submerged vegetated open-channel flows. In this study, both of PIV and laser induced fluorescence (LIF) measurements were conducted simultaneously with a pair of high-speed cameras in order to evaluate the relation between the turbulent diffusion property and the coherent motions.

LARGE EDDY SIMULATION ON COHERENT VORTICES IN VEGETATED CANOPY OPEN-CHANNEL FLOWS

In actual rivers, many aquatic plants are often observed and they have significant effects on hydrodynamic properties. In such vegetated open-channel flows, velocity profiles are largely changed in the vertical and spanwise directions. The vegetated canopies generate coherent turbulent motions that promote momentum and mass transports between over and within canopy. It is thus needed to investigate these turbulence characteristics and coherent motions in hydraulic engineering and river environment. In the present study, we conducted a large eddy simulation in vegetated open-channel flows, and revealed numerically the turbulence structure and correlation properties in the whole depth region including within-canopy and over-canopy layers.

風波・植生開水路流れの組織乱流構造に関する実験的研究

Enviromental problems about aquatic plants are now increasing in actual rivers. Coherent turbulence events such as ejections and sweeps are generated by the shear instability near the boundary between above-canopy layer and within-canopy layer. When wind-induced water-waves appear on the free-surface flows, a large amount of oxygen gas may be transported toward the vegetation layer, and it is infer red that the mass and momentum exchanges are promoted significantly by the coherent motions. It is thus very important in river environment to investigate these turbulence structure and coherent motions in such flows. In this study, we conducted turbulence measurements by using a high-speed PIV technique. It was found from time-averaged velocity profiles that some wave effects appear even in the canopy layer. The correlation analysis was also conducted in order to examine turbulence transport process promoted by the wind-induced air/water interface. Furthermore, relation between the coherent event and waves were revealed on the basis of the present PIV data.

SPACE- TIME CORRELATION PROPERTIES IN MEANDERING COMPOUND OPEN-CHANNEL FLOW BY USING MULTI-LAYER SCANNING PIV

It is necessary to investigate coherent turbulence structure in meandering compound open-channel flows for predicting the transport and distribution of suspended sediment in actual rivers. However, in previous studies, flow-visualization techniques such as PIV and PTV, have not been applied sufficiently, and thus, there is almost no detailed information of coherent turbulence in meandering compound open-channel flows.Therefore, in this study, we conducted innovative flow-visualization measurements by using a multi-layer scanning PIV system, and could analyze instantaneous velocity vectors on horizontal planes at several different elevations simultaneously.As the results, the correlation properties between the different elevations were revealed in the meandering compound open-channel flow.

A103 植生キャノピー開水路流れの乱流構造の可視化計測

It is important in river environment and hydraulic engineering to reveal the hydrodynamic properties in vegetated open-channel flows, in which velocity profiles are largely changed in the vertical and spanwise directions. In particular, vegetated canopies generate coherent turbulent motions (such as sweeps and ejections) which promote erosions and depositions of sediment on beds.Therefore, it is needed to investigate these turbulence mechanism and coherent vortices in order to maintain the vegetations in actual rivers. In this study, we conducted turbulence measurements by using PIV technique, and revealed the turbulence structure in open-channel flows with vegetated canopies.

風波発生下における植生キャノピー開水路流れの乱流構造について

It is very important in liver environment to reveal the hydrodynamic properties in open-channel flows with vegetated canopies, in which velocity pmfiles are largely changed in the vertical direction. In paiticular, windinduced water-waves generate strong down-bursting phenomenon which promotes mass, momentum and gas transfer toward the vegetation region across the air-water interfere. Therefore, it is reeded to investigate these turbulence mechanism and coherent motions in onzler to maintain the aquatic eco-system in actual rivers. In this study, we conducted turbulence measurements by using two-component laser Doppler anemometer (LDA), and revealed the turbulence structure in the vegetated open-channel flows with wind-induced water waves.

CRANK FLOW AND BANK ATTACKING CONCENTRATED FLOW AT SHARP ANGLE IN A RIVER CHANNEL OF DIVERGENCE-CONVERGENCE MEANDERING SYSTEM

This paper deals with crank flow and bank attacking concentrated flow at sharp angle in a river channel which has divergence-convergence meandering system. Since bank erosion due to crank flow has recently become urgent problem in actual rivers, practical consideration was made based on field data of Kinu-river and Watarase-river. And development of crank flow was discussed in accordance with the channel meandering pattern especially of divergence-convergence system. Results of this study is expected to act an important role on practical application and to suggest to further studies.

C202 多断層スキャニングPIVの開発と蛇行複断面流れの3次元構造の解明

It is very important to reveal coherent horizontal vortices, in meandering compound open-channel flows for predicting the transport and distribution of suspended sediment in actual rivers. However, in previous studies, flow-visualization techniques such as PIV and PTV, have not sufficiently been applied to such flows, and thus, there is almost no detailed information on coherent turbulence in meandering compound open-channel flows. Therefore, we conducted innovative flow-visualization measurements by using a multi-layer scanning PIV system, and analyzed instantaneous velocity vectors on horizontal planes at several different elevations simultaneously. As the results, the time-space correlations between the different elevations, the vortexcore motion and instantaneous velocity properties were revealed in the meandering compound open-channel flows.