Vízügyi Közlemények, 1934 (16. évfolyam)
4. szám - XI. Szakirodalom
21 erosions. These latter are to be seen near the village of Ráckeve, and are very typical. The stretching direction of the sand is NW — SE, which corresponds to the main direction of wind ; but it is remarkable that the direction of the dunes is the same, which is in contradiction with the general law set up by M. E. Cholnoky, according to which the direction of dunes is perpendicular of that of the main direction of wind. Dr. L. Kádár, geographer of the L. E. Almássy expedition to Lybia in 1933, made similar observations. He found the stretching direction of the sand and the dunes in the Lybia desert to be the same as the direction of the dominant N-S wind. The explanation of this is still to seek, and only exact research can discover it. The sand in the Csepel Island has been bound by afforestation, by planting vineyards and orchards and by human settlement. The general view of the island does not show any old geologic formations of marine origin, only the work of the river and the wind is apparent, to which human activity, in the form of river improvement and sand-stabilisation, has added. VII. PROTECTION OF THE REAR APRON OF OVERFLOW DAMS. BY L. LÁSZLÓFFY. This article is a historical review dealing with the development of this problem. The mechanism of dam-undermining was first observed by G. de Candemberg in 1824—26 (fig. 1), but before laboratory research began, preventive measures consisted only in filling up the eroded cavities with stone, or paving the downstream bottom on a section as long as possible. Talented hydraulic engineers designed good rear aprons, but even these did not prove effective in the hands of imitators. As early as 1904, M. B. de Kenessey, a Hungarian engineer, pointed out the fact that the rear apron should be constantly below the tail water-level, and that its primary duty is not to give protection against the impulse of water, but to divert the water jet and produce internal whirls. He stated objections against constructing long rear aprons, which by removing the water jet from the toe of dam are apt to bring about a shooting stream of water, and thus destroy the rear apron, great erosion being formed at its extremity. Experiments begun in 1905 at Karlsruhe, in 1914 at Graz and in 1919 at Brünn, and since then carried on in many other laboratories, have helped much in clearing up the problem, but the validity of Kenessey' s statement has not been altered. Dr. Einivachter, in Hungary, whose work was published in volume II. 1932 of this periodical, rendered great service in throwing light on this problem. Systematic experiments have proved that the question of the protection of a rear apron can only be solved by means of preliminary tests on models. Figure 7 shows such a test carried out in the hydraulic laboratory, of which Prof. Rohringer is Chief Director, at the Budapest Technical University.
