Vízügyi Közlemények, 1955 (37. évfolyam)

1-2. füzet - XIV. Lászlóffy Woldemár: Az árvíz előrejelzése

(21) under the surface layer. In planned protection works the width of the flood area has to be adequately chosen ; with old works it is to be examined whether borrow pits, abandoned stream channels etc. in the flood area cannot be filled to attain the design width (seepage path) required in the flood area already. b

) If part of the „design width" falls on the land side, the hydrostatic pressure under the surface layer will probably be maximum at the land side toe of the levee. The surface layer will be soaked there fastest and water will rise to the surface first at the places where the surface layer is thinnest. Soaking destroys the stability of the layer and may cause slip of the embankment or — what is worse — the velocity of the water gushing to the surface may grow to a degre to produce hydraulic ground failure. Conventional widening of levees helps only exceptionally against this danger ; a safer method is to build a gravel banquette with filter at the places in danger which by its weight protects against embankment or ground failure and by its filter prevents fine grains from flowing to produce piping (Fig. 5/a and b

). According to the aforesaid the most advantageous protection work is built if the „design width" falls entirely on the river side, and the foot of the levee on the land side is protected by a gravel banquette with filter. (Fig. 7.) c) If the major part of the „design width" falls on the land side and the pervious layer is thin, sheetpiling, cutoff walls or injection of an impervious screen, that is, reduction of the stress on the surface layer, may be effective. In case of a pervious layer of greater thickness: loading of the surface layer on the land side, that is, increasing the resistance of the surface layer may be useful. Along longer sections of protection works the above mentioned methods are generally very costly. In the neighbourhood of places, where the water has already risen to the surface, the hydrostatic pressure decreases and the stress on the surface layer is also reduced. From the point of view of safety against floods it is advantageous if the water can rise to the surface in as many places as possible. Such „undangerous" seepage may be made use of to increase the safety against floods. On this basis behind the levees pressure reducing wells or drains, but also additional dams can be built to keep an adequate ljead of water to counteract water pressure. (Fig. 6.) No general rule can be set up for either of the procedures. On every flood protection line to be reinforced the stratification of the subsoil is to be explored, and the modes and proportions of protection are to be fixed for each section according to exploration results. WIEDERHERSTELLUNG DER SCHUTZLINIE Yon J. Sik (Abbildungen und Tabelle siehe Seite 172—185. des ungarischen Textes) DK. 627.517 Das Julihochwasser 1954 verursachte auf der ungarischen Donaustrecke an 4 Stellen Deichbrüche (Siehe Beilage III im Anhang). Ursache der Deichbrüche war in drei Fällen Grundbruch. Bei Ásványráró, wo der erste Deichbruch erfolgte, schwemmte das auf das Inundationsgebiet strömende Wasser, infolge ablenkender Wirkung eines Altufers um 800 m stromabwärts, die landseitige Böschung hinweg und es entstand ein vierter Bruch (Abb. 1). An den Durchbruchsstellen entstanden 5 — 9 m tiefe Kolke. Zur Verhinderung der Erweiterung der Bruchstellen wurden 12 400 m

3 Steinwurf verwendet. Zur Zurückleitung des ausgetretenen Wassers wurden die Schutzdeiche an zwei Stellen in 100, bzw. 560 m Länge durchstochen. Infolgedessen musste nach Abfluss des Hochwassers an sechs Stellen für die Wiederherstellung gesorgt werden. Im Laufe der Herstellungsarbeiten wurden die Kolke zugeschüttet, die Dicke der undurchlässigen Deckshichte wurde in grösserer Breite erhöht und die Deichprofile wurden mit stärkeren Abmessungen als vorher errichtet