Vízügyi Közlemények, 1955 (37. évfolyam)

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

(20) Along the Hungarian section of the Danube Valley the subsoil is composed everywhere of highly pervious sandy gravel covered by 1—6 m of silt or clay. The Danube River has cut its channel into this perviuos layer, so that its water is in direct contact with it. Average dimensions of the Danube levees are shown in Fig. 1. Let process taking place during the flood on the geological section of Fig. 2. be examined from soil-mechanical point of view and be supposed that the pervious layer has been saturated at the beginning of the flood already. It is evident that in the instant when the water stage has reached and surpassed the level of the bottom plane of the upper layer, the latter is put under hydrostatic pressure and its saturation starts upwards from below. After some time the semi-pervious layer is soaked and the water may appear on the landside ás seepage or piping on the • surface. It is evident that much less time is required for soaking a surface layer of some meters thickness, interwoven with roots and holes of vermin than for saturating a levee of 15—20 m width of compacted material. Soaking of the toe of the levee on the land side takes placé mostly through the pervious and the surface layer, that is, upwards from below. (Fig. 3.) •The examination of water pressures developed on the bottom plane of the surface layer on the land side (Fig. 4.) leads to the statement that, as long as there is no or very slight water flow through the pervious layer, the water pressure on the bottom plane of the surface layer is theoretically everywhere uniform, however far be the levee from the riverbank and whatewer be its width. "With the saturation of the surface layer begins the water flow in the pervious layer and the hydrostatic pressure on the bottom plane of the surface layer diminishes. Progressing frofn the land side toe of the levefe the hydrostatic pressure diminishes, and there is a certain distance where it will be insufficient even to force the water through the upper layer to the surface. Independently of the soaking process of the top layer the flow lines, and also the pressure distribution in the pervious layer, may essentially change, when the water can rise to the surface somewhere at the rear of the levee. Water flow will then start toward such places of seepage and the hydrostatic pressure will greatly diminish in their surroundings. The thickness of the surface layer and its permeability, however, vary everywhere. Water will rise to the surface first where the resistance of the surface layer is smallest. At the places of surge stream lines crowd and the water may rise to the surface at considerable velocity. Therefore, in places where the surface layer is not soaked, safety against floods is determined by the resistance of the surface layer to hydrostatic pressure.; in places where the water has already risen to the surface, the safety factor depends on the velocity of surge of the water still permissible without harm to the layer structure. Where flood protection works are built on pervious subsoil the safety factor against floods and methods of protection must not be based on the assumption of an impervious foundation or on experience gathered relative to the latter. The pressure developed in the pervious layer, depending on dimensions and quality of the pervious and of the surface layer, decreases with distance from the levee. In every section of the flood protection works there is a distance behind the levees, beyond which pressure exceeding the load capacity of the surface layer can no more develop. This distance as the ,,design width" of the protection works characterizes subsoil conditions from the point of view of safety against floods. On good subsoil the design width may be less than the width of the base of the levee, under poor conditions of stratification it may be even a multiple of the latter. In the latter case the subsoil has also to be consolidated within the design width. Consolidation can be executed by several methods depending on the level and duration of floods: a) First of all, in any case, a longer path of percolation has to be imposed un water in the previous layer already, in order to reduce the hydrostatic pressure