Vízügyi Közlemények, 1932 (14. évfolyam)
2. füzet - XII. Kisebb közlemények
25 3-8 million hectares (9-4 million acres) of arable lands. When the country had been insured against the destructive effects of the waters, their utilization was begun, just before the world war broke out : it was a question of producing water power of 2-8 million HP and of increasing the net-work of waterways from 2500 kilometres (1550 miles) to 4200 kilometres (2600 miles). The Peace Treaty of Trianon frustrated these schemes, by dividing the Carpathian basin among five countries, and bringing to an end the possibility of uniform utilization of waters. 11-15 per cent of pre-war Hungary was flood-controlled, whilst in mutilated Hungary the areas exposed to danger of floods amount to 22-3 per cent of the total territory. Of 79 levee and drainage companies, 14 have been cut into two parts and 2 into three parts. The natural possibility of uniform water utilizations has been destroyed and the systematic work of a period of 100 years has been sacrified to political interests. PROTECTION OF OVERFLOW DAMS AGAINST EROSION. The transmission of electric energy to great distances has considerably furthered the construction of water power plants. The cost of building overflow dams being high, attempts have been made to discover methods which, besides being satisfactory from the point of view of statics, should be economical, requiring a minimum of maintenance. Now that we are able to observe, by means of experiments with models, the processes taking place in the interior of a flowing body of water, it is not difficult to find for the dam to be constructed a proper adjustment well corresponding to the nature of the river. However, we may save much useless effort and expense, if we ascertain when designing, what kind of conditions will prevail at the flow of water over dam, without which knowledge the planning would of course be based on mere conjecture. It is a well-known fact that the water, which has overflowed a dam, or has flowed under pressure through an orifice, has a velocity of retreat greater than the wave velocity (j/^gt) i.e. below the dam the stream becomes a „shooting flow", then, on the apron of the dam, or farther below in the river-bed, after striking against the relatively quiet tail water and producing the phenomenon of the hydraulic jump, it changes to the more placid „streaming flow". As to its external form, the hydraulic jump may take place as a free jump without forming a water cylinder, or it may form a free jump with a top cylinder, or again it may be a backed-up jump with a top cylinder (see Figure 6). The nature of the form assumed depends solely upon the relative position of the tail water, supposing a constant q and t. The height of the hydraulic jump with a free top cylinder may be computed on the basis of the law of momentums by means of the formula : Dams with an apron placed high may produce the hydraulic jump in yet another form. (Figs. 11c —d). For instance, if in the case of a downflow with a backed-up cylinder, the tail water is backed up gradually, then at a certain limit of depth the jet is suddenly pushed up from its low position to the surface, where it flows forth in III. By : Dr. JÓZSEF EINW ACHTER. 10a
