Floods of meadows in the delta of the river Nemunas contribute to the deposition and retention of sediments and nutrient that would otherwise deposit in the Curonian Lagoon. In grassland area of the Nemunas delta the formation of alluvial soil occurs according to the flood dynamic rules: water discharging into the valley leaves suspended sediments on the soil surface. By mathematical modelling it was established that about 35% of suspended sediments inflow deposited there. Due to sand particles deposited during the study period (1950-1991), the natural river bank levee rose by 0.3 m. Fine clay and silt particles deposited uniformly within the entire model area and formed a 4-6 mm thick layer there. It contained about 50-60 t/ha of silt deposits. The valley soils were naturally fertilized with 250 t of potassium, 950 t of phosphorus, 38,000 t of calcium, and even 147,000 t of organic matter saturated with nitrogen. Certain amount of heavy metals also deposited there. The deposition process has not yet been adequately investigated under the conditions of flow bed covered by grass. It was established that grass cover intensified the sediment deposition in the floodplain. Considering calculation results and measurement data, the process of suspended sediment deposition was analysed and new formulas were derived. It was established that in order to increase sedimentation in the valley, it would be necessary to increase water discharge overflowing from river bed into the delta valley.
In spite of many investigations performed on turbulent flows, their structure has not yet been sufficiently explored. The difficulty is that, when a detailed picture of the velocity field is necessary, the widely employed Particle Image Velocity (PIV) method can provide photos covering only a short interval of flow, which cannot include the largest structures of turbulent flow, and consequently these structures cannot be investigated. In this study, the author tried to obtain necessary data about the processes occurring in the flow by analyzing instantaneous velocity measurements carried out by 3D means. A measurement at the points of a flow cross-section takes at least 1 minute. During this time all vortex structures, including the largest, occur repeatedly many times and can be studied. The analysis of such measurements was the aim of this article. The process of the generation of vortices at the bottom and their further development, including the conditions of the development of the largest vortices, has been investigated. The results of these investigations are discussed in this article.
 bstract Flooded river valley meadows are ve^ important for river ecology, as they entrap sediments and ŝ ediment-bound nitrogen or heavy metals. Nevertheless, the grass-covered fioodplains, particularly in river j deltas, are often separated from the rivers by dykes. Wtien such systems are designed, it is necessary to model sediment deposition in these separated areas. However, the existing calculation methods and models are g adapted for flow over the sandy bottom. In the meadow flows there are other boundary conditions more > favourable for sediment deposition. £The focus of our studies was the processes of suspended sediment deposition in the flooded delta ot the fP River Nemunas. Calibration results of the mathematical model with common sediment deposition formulae for r iverbed flows did not correspond to the data of measurements. The differences were obvious. Consequently, it S was necessary to study the peculiarities of How and sediment motion under these conditions, as well as to work *« out formulae suitable for calculations.
The deposition of sediment in the inundated fioodpiains of rivers, particularly in their deltas, improves the water quality of the rivers themselves and of their receivers. It would be very useful if ways to increase the amount of sediment deposition could be identified. An investigation into a number of possibilities was carried out in the delta of the River Nemunas. Various factors can influence the increase or decrease of sediment deposition, e.g. the increase of water discharge flowing through the river valley, the growth of bushes on the floodplain, the building of road banks across the valley and changes in growth stages of grass (which affect its ability to entrap sediments). The effects of these factors are discussed, and the most successful ways for improvement in the Nemunas delta are proposed. Similar methods may also be applicable in other river deltas.
In spite of the many investigations that have been conducted on turbulent flows, the generation and development of turbulent vortices has not been investigated sufficiently yet. This prevents to understand well the processes involved in the flow. That is unfavorable for the further investigations. The developing vortex structures are interacting, and this needs to be estimated. Physical summing of velocities, formed by all structures, can be unfavorable for investigations, therefore they must be separated; otherwise bias errors can occur. The difficulty for investigations is that the widely employed Particle Image Velocity (PIV) method, when a detailed picture of velocity field picture is necessary, can provide photos covering only a short interval of flow, which can’t include the largest flow structures, i.e. macro whirlpools. Consequently, action of these structures could not be investigated. Therefore, in this study it is tried to obtain the necessary data about the flow structure by analyzing the instantaneous velocity measurements by 3D means, which lasts for several minutes, therefore the existence and interaction of these structures become visible in measurement data. The investigations conducted in this way have been already discussed in the article, published earlier. Mostly the generation and development of bottom vortices was analyzed. In this article, the analysis of these turbulent velocity measurements is continued and the additional data about the structure of turbulent vortices is obtained.
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