Vízügyi Közlemények, 1972 (54. évfolyam)

4. füzet - Rövidebb közlemények és beszámolók

(83) the problem oí the economic filter length emerged. The corresponding studies were started by the theoretical examination of pressure distribution in the layer and the casing. The first results of this study are summarized in the present paper. it will be readily perceived that energy is consumed in conveying the discharge entered from the layer. Over the section above the top edge of the filter this effect is allowed for readily by modifying the depression created using a head loss calculated from the well yield and the characteristic parameters of the blinde casing of the well. Over the length of the filter the phenomenon is substantially more difficult to describe analytically, since allowance must be made for the change in the rate of flow along distance. The other difference over the computation methods applied in conventional pipe hydraulics is that the resistance to flow along the filter pipe differs drastically from that in smooth pipes. The reason for this is the flow across the well screen, which is perpendicular to the main direction of flow. The development of a boundary layer along the pipe wall is prevented thereby and the radial flow tends to make movement appreciably more turbulent than would be the case in closed conduits. Owing to the much higher resistance along the well screen the difference in pressure at different heights of the screened strech may be considerable. Since the rate of flow entering from the layer through the screen is proportionate to the difference in pressures prevailing at the external boundary surface of the layer and within the well screen, proceeding downward, a section may be reached where the sum of resistances acting along the overlying screen section becomes large enough to balance the full depression and thus — in the absence of any differential pressure across the layer — no inflow is likely to occur. The expressions relating head loss, specific inflow and the total well yield are written first for filters of infinite length. This approach is then repeated to find methods for describing the case of layers of finite thickness, or filters of finite length. The method can even be extended for estimating the hydraulic parameters of wells drawing water from several layers. For being able to describe the process under consideration the knowledge of resistances along the screens is of paramount importance. For taking these into consideration the use of a combined, so-called extended resistance coefficient is suggested. Laboratory experiments have been started for determining the magnitude of this coefficient. It is expected that the results thereof will become shortly available to supplement the present paper and thus the design problem mentioned in the ntroduetion will be accessible to a numerical solution. BRIEF PUBLICATIONS AND BEPORTS 1. Bendefy, László, Civ. Engr., Dr. : Surveys over llie Hungarian reach oi the Danube and the results of the latest river survey (For the Hungharian text see pp. 446) The first acceptable, formally correct map of the Danube was constructed between 1510 and 1520, while the first general map has been traced at the beginning of the 18th century by army engineers (Marsigli). A hydrographie survey was run between 1773 to 1788 by the order of emperor Joseph II, under the name "Präliminaire Flussaufnahme". The first survey in the modern sense was started by the Hungarian engineer M. Huszár, who traced 2441 maps to 1:3600 scale which are still conserved at the National Archives, Budapest. The next survey from 1890 to 1904 was completed by the Hungarian Hydrographic Institute already. Since the beginning of the present century several detail surveys have been run in response to actual demands. The latest comprehensive hydrographie survey was completed between 1968 and 1970 by the Hungarian Besearch Institute for Water Besources Development, the results of which were published in 8 volumes. It is of interest to note that this publication marked the 450th, 250th, 200th, 150th and 75tli anniversaries, respectively of earlier maps. In the first three volumes successive stretches of the Danube are represented, giving the following details: