Spur Dykes Research Articles

STUDY ON LOCAL SCOUR AND VARIATION OF BED COMPOSITION AROUND NON-SUBMERGED SPUR DYKE

Several studies have been made on local bed scour by movable bed experiments. Researches taking into account non-uniform sediment transport are still very few, in particular in case of the involvement of spur dykes. In this paper, we carried out movable bed experiments around an impermeable spur dyke under non-submerged condition. Both uniform and non-uniform sediment beds with similar mean diameter used and the changes of the bed configuration and bed surface composition have been considered. In addition, we used color sediment which is divided in three ranges of diameter with different colors in order to clarify the spatial differences of bed surface composition through pictures and visual observation. We focused on the relation between initial grain-size distribution and the final scour hole depth and the influence of vortices around a spur dyke on bed surface composition.

Experiment and simulation of turbulent flow in local scour around a spur dyke

The turbulent flow in the local scour hole around a single non-submerged spur dyke is investigated with both experimental and numerical methods. The experiments are conducted under clear-water scour regime with an impermeable spur dyke. The scour geometry and flow velocities are measured in details with a high-resolution laser displacement meter, electro-magnetic velocimetries and PIV (Particle image velocimetry). A 3D non-linear k-ɛ model is developed to simulate the complex local flow field around the scour area. The numerical model is formulated using FVM (Finite volume method) on a collocated unstructured mesh, capable of resolving complex geometries and boundaries. It is found that the simulation results are reasonably consistent with those of the experimental measurements. Based on the study results, the nature of the flow structure around a spur dyke with local scour hole is analyzed.

Scour in hydraulic engineering

As a result of damage and failures due to undermining of foundations and the blockage of water flow caused by sediment aggradation downstream of scour holes, scour in hydraulic engineering has become an important issue in recent decades. This paper presents recent research on: (a) plunge pool scour in which a variety of effects were investigated; (b) bridge piers for which spatial and temporal scour development was described using particle image velocimetry for a test case; (c) the failure of riprap-protected spur dykes in a straight river reach. In all three fields, the main features of scour are described based on similitude according to Froude—that is, using the densimetric Froude number of the approach flow as the dominant parameter. The large number of parameters involved were controlled with suitable experimental setups allowing for the determination of the governing effects of a scour problem. Results are presented that readily apply to river engineering problems.

Morphodynamic river processes and techniques for assessment of channel evolution in Alpine gravel bed rivers

Over the past 10 years many restoration projects have been undertaken in Austria, and river engineering measures such as spur dykes and longitudinal bank protection, which imposed fixed lateral boundaries on rivers, have been removed. The EU-Life Project “Auenverbund Obere Drau” has resulted in extensive restoration on the River Drau, aimed to improve the ecological integrity of the river ecosystem, to arrest riverbed degradation, and to ensure flood protection. An essential part of the restoration design involved the consideration of self-forming river processes, which led to new demands being imposed on river management. This paper illustrates how model complexity is adapted to the solution and evaluation of different aspects of river restoration problems in a specific case. Point-scale monitoring data were up-scaled to the whole investigation area by means of digital elevation models, and a scaling approach to the choice of model complexity was applied. Simple regime analysis methods and 1-D models are applicable to the evaluation of long-term and reach-scale restoration aims, and to the prediction of kilometre-scale processes (e.g. mean river bed aggradation or degradation, flood protection). 2-D models gave good results for the evaluation of hydraulic changes (e.g. transverse flow velocities, shear stresses, discharges at diffluences) for different morphological units at the local scale (100 m–10 m), and imposed an intermediate demand on calibration data and topographic survey. The study shows that complex 3-D numerical models combined with high resolution digital elevation models are necessary for detailed analysis of processes (1 m–0.01 m), but not for the evaluation of the restoration aims on the River Drau. In conclusion, model choice (complexity) will depend on both lower limits (determined by the complexity of processes to be analysed) and upper limits (field data quality and process understanding for numerical models).

Impacts of the TGP project on the Yangtze River ecology and management strategies

TGP project ‐ the Three Gorges Project on the Yangtze River ‐ is the largest hydro‐project in China. This paper outlines the environmental and ecological impacts of the project. Cumulative sedimentation in the reservoir will reach an equilibrium in 100 years with a total volume about 16 billion m3. The planned Xiangjiaba and Xiluodu reservoirs, which will be located at 1020 km and 1180 km upstream of the TGP dam, can be used to trap sediment and thus reduce the sedimentation rate of TGP reservoir in the first 90 years. Experimental studies showed that impoundment of TGP reservoir will cause sediment deposition at the apron of the Chongqing harbor and even change the stem flow from the west channel to the east channel. Building spur dykes and groins to regulate the flow can solve the problem. Calculation with 1‐D models show that the released water from the dam will scour 2.5 billion tons of sediment from the downstream channels in 40 years, which will result in channel degradation. The flood stage will then be reduced by about 3 m at Shashi and 0.75 m at Wuhan for a flood of discharge 30,000 m3/s. Studies indicate that the reservoir will not necessarily cause earthquakes, but will induce more landslides, which will threaten little the safety of the dam. It is reported that the impoundment of the reservoir so far has not obviously affected the water quality. The reservoir operation will change the hydrologie conditions, which may affect the life cycle and habitats of fishes and reduce the fishery harvest. The regulation of the flow for power generation and flood control may change the fluvial processes and some meandering and braided sections may become unstable, which will undermine the habitats of some endangered species. The longer the time of channel bed stability, the more species of benthic invertebrate may live and propagate on the riverbed. The number of benthic invertebrate will reduce because the erosion and sedimentation will make the channel bed unstable. New resettlement policy has been implemented with overall arrangements for production and living conditions for the emigrants. The farmland to be inundated would be about 23,000 hectares. Experiments in the reservoir area indicate that about 27,000 barren slope‐land may be transformed into a terraced citrus orchard and it is able to turn out an output value equal to as much as those by the inundated land.

PREDICTION OF 3D FLOW FIELD AND LOCAL SCOURING AROUND SPUR DYKES

Numerical investigation of the 3D flow and sediment transport around a series of non-submerged spur dykes has been presented. The complex velocity field and bed shear stress are predicted by the widely used κ-e model in combination with the wall function approach. Coupling the mass conservative equation of the sediment with an empirical formula for the bed load transport, a scour model has been put forward. The effect of the local bed slope is taken into account by correcting the threshold condition for the sediment entrainment and incorporating the particle gravity as a part of the effective shear stress. A sand slide process is introduced to avoid the excess slope over the angle of sediment repose. The model result has been compared with the laboratorial experiment. It is shown that both the velocity field and the local scouring predicted by the current model are in reasonable agreement with those of the experiment

VELOCITY MEASUREMENTS AROUND NON-SUBMERGED AND SUBMERGED SPUR DYKES BY MEANS OF LARGE-SCALE PARTICLE IMAGE VELOCIMETRY

Field observations on the surface flow were carried out in a straight reach of the Uji River. Velocity distributions on the water surface were obtained based on the captured video images by means of large-scale particle image velocimetry (LSPIV). Flow structures around spur dykes are studied for both non-submerged and submerged cases.It is clearly seen that, for both cases, the dykes are working as an additional roughness. Their effect on channel conveyance capacity is estimated in terms of the effective channel width calculated from the velocity distributions. It is also of interest that the maximum velocity filament appears at a different position as the water depth varies in case of the channel with single-side dykes such as studied here. In addition, large-scale boils shedding from the submerged dykes are clearly observed and their origin and advection mechanisms are also evaluated.

A trench test on formative proess of the riverbank of the Tone River

This study aims to develop a renewal or conservation method of the riverbank with or without bank protection works. As the first step, a trench test was carried out at the natural riverbank of the Tone River, where the riverbank was formed by the installation of several spur dykes in the 1960s, in order to investigate formative process of the natural riverbank. It is concluded through this field investigation that formative process and the developing velocity of riverbank strongly depend on changes in the water and bank lebel, and the enhancement of depositional effect of fine sediments due to the growth of vegitation on the riverbank.