Bed Level Variations Research Articles

沖縄本島億首川における出水時の河床変動に着目したマングローブ林の生育地評価

Influence of hydraulic conditions on the regeneration of mangrove forest was clarified at the Okukubi River in Okinawa Island, Japan. Field surveys for identifying structure of mangrove forest were conducted at 8 plots, where Bruguiera gymnorrhiza (L.) LAM. and Kandelia candel (L.) DRUCE. dominate. While bed-level variation, absolute bed-level fluctuation and non-dimensional shear stress during a flood were estimated by two-dimensional bed deformation analysis. Then relationship between density of seedlings and saplings of mangrove species and hydraulic conditions was evaluated from regression analysis. The density of maturing trees of mangrove species positively correlates with absolute bed-level fluctuation and non-dimensional shear stress. The result represents that regeneration of mangrove community is occurred at sites where sediments of bar surface is exchanged during the flood. Soft soil, which is easy to stick for mangrove seeds, is needed as the first stage of regeneration, and hence sediment movement is essential for forming regeneration site of the mangrove species.

Analysis of Bed Load Equations and River Bed Level Variations Using Basin-Scale Process-Based Modelling Approach

Bed load transport is a key process in maintaining the dynamically stable channel geometry for restoring the form and function of river ecosystems. Bed load consists of relatively large sediment particles that are moved along the streambed by rolling, sliding or saltation. Currently, various empirical correlations are used to estimate bed load transport rates since no single procedure, whether theoretical or empirical, has yet to be universally accepted as completely satisfactory in this aspect. Bed load particles are primarily sourced from river bed materials or banks. The amount of bed load and its spatial distribution contributes significantly to river bed level changes. Hillslope sediment contribution, mostly available to the river in the form of suspended load, also plays an important role in river bed level changes. This study aims to analyse different bed load equations and the resultant computations of river bed level variations using a process-based sediment dynamic model. Analyses have revealed that different bed load equations were mainly deduced from the concept of relating bed shear stresses to their critical values which are highly factored by the slope gradient, water discharge and particle sizes. In this study, river bed level variations are calculated by estimating total surplus or deficit sediment loads (suspended loads and bed loads) in a channel section. This paper describes the application of different widely used bed load equations, and evaluation of their various parameters and relative performances for a case study area (Abukuma River Basin, Japan) using a basin-scale process-based modelling approach. Relative performances of river bed level simulations obtained by using different bed load equations are also presented. This paper elaborates on the modelling approaches for river bed load and bed level simulations. Although verifications were not done due to unavailability of field data for bed load, qualitative evaluations were conducted vis-a-vis field data on flow and suspended sediment loads as well as the bed loads presented in different past studies.

BED ELEVATION CONTROL IN CURVED CHANNEL BY REDUCING WIDTH WITH ERODIBLE EMBANKMENT

Riverbed aggradation in front of the power plant outlet causes the decrease of power generation efficiency. Although bed excavation and embankment construction are conducted to keep riverbed elevation, sediment inflow occurs by embankment collapse caused by snowmelt runoff. In this study, hydraulic model experiments and 2-D numerical simulation were conducted in order to investigate embankment and river bed level variation. It was clarified that erodible embankments composed of bed material could concentrate flow on inner bank of river bend and restrain riverbed aggradation.

Test case for bed morphology computations

The current research work presents experimental measurements to produce data base intended as support in the development of new and the refinement of existing codes for computing free-surface flows with movable beds, specifically around bridge abutments and generally on bed level evolution due to sediment transport. All experimental test case results described in this work are relevant to unsteady two-dimensional, quasi or fully three-dimensional flow calculations for viscous flows in open channels. These data are believed to represent the only experimental test case available today for bed morphology code validation. The experimental procedure was carried out in a laboratory open channel flume using a trapezoidal abutment. The bottom of the flume was covered with fine material. For different flow conditions, predetermined by numerical simulations, water and bed level were measured in the tested section, especially in the vicinity of the abutment. The expected bed erosion and the maximum scour depth at the upstream sloped face of the construction are satisfactorily simulated by the experimental procedure. Finally, the computational data on the bed level variation were compared with the laboratory measurements and the results were satisfactory. The experimental results are graphically presented and can be used by other researchers in order to validate their numerical simulations.

The cyclic development of a macrotidal mudflat on varying timescales

The development of a mudflat at Peterstone Wentlooge on the north shore of the Severn Estuary, UK, was observed over a two year period on a multi-timescale basis. The mudflat is located in an area with a macrotidal regime, strong tidal currents, exposure to short period wind waves and high turbidity. The mudflat consists of two Holocene sediment layers: relict clay extending over the entire width of the intertidal, overlaid by modern sediment located between the salt marsh edge and mean sea level. A combination of episodic and periodic mudflat development was observed on decadal, seasonal, spring-neap and tidal timescales. There was a clear seasonal development of the modern layer in terms of bed level, morphology and composition, with accretion during spring and summer and with erosion during autumn and winter. This periodic development was superimposed on a longer-term trend of continual erosion of the relict clay when its surface was exposed. The mean seasonal variation in bed level of the modern layer was in the order of 10 cm and was due to the presence or absence of short period waves. On the spring–neap and tidal timescales the development of the mudflat, due to tidally driven processes, was frequently modulated by episodic wave events. Variations of bed level during tidal inundation were observed autonomously and compared well with manual observations taken during exposure on a daily basis. Bed level variations of 1 cm were observed during single periods of tidal inundation and daily through measurements taken during mudflat exposure.

The effects induced by a new embankment at the confluence between two rivers: TELEMAC results compared with a physical model

With the aim of evaluating the hydraulic risks connected with the construction of a new embankment at the confluence of the Magra Rivera and the Bagnone Torrent, an experimental analysis was conducted using an undistorted 1:55 scale physical model. A parallel study using the software TELEMAC-2D applied to the same domain as the physical model and with the same boundary conditions was also carried out. The use of a numerical model enabled comparison of the results obtained from the physical model and a test of the reliability of the numerical model itself in terms of accuracy, stability and instability. In order to estimate the effects induced by the construction of a new embankment, two sets of experiments were carried out; in the first set bed and bank topography before constructed were modelled, and in the second set the actual situation with the embankment was considered. Comparison of data obtained from numerical simulations with measurements from the physical model showed a good agreement from a qualitative point of view. However, some differences were noticed in terms of local values, particularly in the bed morphology evolution where estimation of bed level variations are strongly dependent on the local sediment transport dynamics. Copyright © 2000 John Wiley & Sons, Ltd.

Sediment Flux and Bed Level Measurements from a Macro Tidal Mudflat

A detailed analysis is presented of the variations in suspended sediment flux and bed level, measured for one year in the Spurn Bight, Humber Estuary (U.K.). Suspended sediment concentrations were correlated to flow speed, and wave activity dominated the erosion processes and governed the residual tidal flux. An onshore flux and gradual accretion of a few millimetres of sediment were typical of calm hydrodynamic conditions. In contrast, large waves caused the erosion of several centimetres of sediment and prevented deposition over slack water, producing significant seaward transport. This eroded material could be replenished in just a few days under calm conditions. The mudflat level was continually changing in order to reach some equilibrium in balance with the wave climate, tidal cycle, river runoff and seasonal effects. Large differences in the mudflat morphology rapidly occurred when there were significant deviations from the modal forcing conditions. These relatively short term events tended to cancel out with time, and the net seasonal changes in bed level variations were about a few millimetres. Multiple regression analysis was used to produce empirical expressions predicting the residual sediment flux and bed level from readily observable environmental parameters. The results quantified the dominant physical processes affecting the residual tidal flux and established some site-specific relationships that provide a basis for more general flux predictions.

Physical factors affecting to distribution of plant communities on a sand bar

The purpose of this paper is to clarify the influence of physical conditions on the distribution of riverine plant communities. Full vegetation survey was performed in 1994 and 1997, on an alternating bar formed in a downstream reach of the Yoshino River, Shikoku, Japan. Comparison of two vegetation maps for the two years, including two woody and fifteen herbaceous communities, reveals characteristic changes of plant communities. Five factors, namely bed-level variation, absolute bed-level fluctuation, relative bed-level from the surface of low-water stream, duration of inundation and shear velocity of flooding flow, were defined and evaluated to describe the physical conditions at many reference points distributed on the bar. Community-specific spatial averages of the five factors are not uniquely related with individual plant communities, however their combinations appear to be characteristic of some ecological groups of plant communities. It is suggested that the five factors will be effective for distinguishing the physical conditions to sustain the ecological groups.

Numerical method for two-dimensional bed-level variation (Part-1)

Numerical method for two-dimensional bed-level variation (Part-1)

Numerical method for two-dimensional bed-level variation (Part-2)

Numerical method for two-dimensional bed-level variation (Part-2)

Numerical method for one-dimensional bed-level variation in steep streams-part one

Numerical method for one-dimensional bed-level variation in steep streams-part one

Numerical method for one-dimensional bed-level variation in steep streams-part two-

Numerical method for one-dimensional bed-level variation in steep streams-part two-

Study on modelling the morphology of torrents on volcano slopes

An exploratory study is done to modelling of rivers on volcano slopes. Beside the occurrence of mud flows or lahars these rivers are characterised by unsteady supercritical flow and large suspended-sediment concentrations. A fully coupled 1D model is presented for the rivers, provided that lahars do not occur. It can simulate unsteady supercritical flow (Froude numbers between about 1 and 1.8) with volumetric sediment concentrations less than about 10%. The redistribution of the suspended-sediment concentration (i.e., the lag between concentration and transport capacity due to sediment relaxation) causes instabilities of the bed conformable to antidunes. Calculations with a numerical model using Preissmann's implicit scheme, and applied to a river on the Kelud Volcano in Indonesia, show the rapidity of bed-level variations and the upstream propagation and growth of bed disturbances. These results offer a deeper understanding in the problems of the rivers on the Kelud. Furthermore it yields a base for inve...

Fan Formation by the Sediment Runoff Due to River Bed Erosion

We had a localized torrential downpour near the Fukui-Gifu prefectural border in September, 1965 (we call this Okuetsu Rainstorm). Especially, Nishitani village in Fukui prefecture (after this disaster, residents in this village moved to another place and now this district is included in Ohno city) suffered crushing damage from this storm. Namely, in Nakajima district which was the main part of the Nishitani village, before the disaster the number of households was 154, but after the disaster, 58 households were swept away and 86 were buried. A distinguishing feature of this disaster was that the bulk of the houses in the main part of the Nakajima district situated on the debris fan which had been formed by the sediment runoff from the Kamata river was buried second floor deep in sediment but not swept away. Judging from the fact-finding survey and data analysis, these houses were buried by the sediment which outflowed and deposited from the Kamata river taking the type of the sediment flow rather than the debris flow.In this paper, in order to reproduce the burying process of the Nakajima district by the sediment flow, sediment hydrograph of the Kamata river is evaluated by the calculation of the bed level variation at first, then by using this sediment hydrograph as a input boundary condition, simulation of the horizontally twodimensional bed level variation in Nakajima district is carried out, and it is found that the calculated results are comparatively in good agreement with actual phenomena.

Quelques travaux de correction sur le cours de l'Isère

The river Isere winds its way down the Gresivaudan valley to Grenoble and intersects a cross-valley at Voreppe just after leaving the city. A number of river training schemes have been implemented since the mid-nineteenth century (e.g. canalisation and bank protection works), which were governed by liberal policies along the river above the city and a more directorial outlook downstream, in accordance with prevailing tendencies at the various times they were put into effect. Major bend-cutting and dyke reinforcement work has been carried out since the middle of the twentieth century, including correction of the river's longitudinal profile, some sections of which were tending to build up by sediment deposition, whilst gravel extraction from others was unduly increasing their depth. Work has just started on irnplementation of a new river flood protection programme for Grenoble. Because of considerable past and predicted river bed level variations, this programme has required more detailed research and testing than earlier schemes, which has involved fresh determination of water levels associated with the predicted rates of river flow, up to and including for the hundred-year peak flood discharge. For the necessary tests, a 1:100 scale model of an 8 km stretch of the river Isere through Grenoble was designed and run in accordance with Froude similarity laws. In addition, a mathematical model of the part of the Isere from its confluence with the river Arc to the sill at Saint Gervais, i.e. about 80 km in length, was subsequently designed and operated. In designing this model, several very complicated computer calculations had to be carried out. In addition to dyke protection and provision of a channel down the middle of the river bed, a number of sub-river sills are to be installed as a means of stabilising the river and its aquifers. Preservation of present flood areas to attenuate major flood peaks above Grenoble is also being envisaged. In view of the highly complex problems involved on both horizontal and longitudinal scales, the physical and mathematical models in use for the study are invaluable assets as regards accurate prediction of data for present and future river engineering projects.