Vízügyi Közlemények, 1963 (45. évfolyam)

4. füzet - VIII. Könyvismertetés

"(47) The methods developed and currently practiced in Hungary for protecting the flood levees are reviewed critically in this paper and the principles of simpler, up-todate and economical methods are established. After discussing in detail the main characteristics of flood levees, two concepts are introduced with the help of which various phenomena can be given a more correct interpretation. One of these is the critical flood wave, i.e. the flood level and flood duration, at which the stability of the levee is still ensured. This flood level and duration can be related to the shape, dimensions, material and structural changes occuring in the course of time — to ageing — of the levee. The stability of the levee is ensured as long as the actual flood wave is less severe than this critical one. The second is the effective height of the levee, which is the total height minus the 0,5 — 0,6 thick top layer made permeable by plant roots, burrowing animals and atmospheric effects (Fig. 3.). The task of levee protection is to provide temporary securing works and emergency reinforcement for the levee wherever the actual, or potential flood wave is higher than the critical wave for the particular levee section. Flood levees must be protected against: 1. Flood waves higher than the effective height of the levee ; 2. wave action attacking the water-side slope of the embankment; 3. soaking, sliding of the slope on the protected side, internal erosion and subsequent failure of the entire levee section, and 4. hydraulic soil failure and erosion in the soil under and behind the levee. The actual protection of levees can be classified into two phases. One of these comprises the active protection of the levee against the destructive action of water at times of floods, the other is the prophylactic protection, consisting of the realization and reinforcement of the levee and the installation serving the protection thereof. The methods of levee protection are discussed in the above order. 1. Prophylactic protection against floods exceeding the effective height (the height of the undisturbed solid core

) of the levee involves the periodical overturning and compaction of the loose layer overlying the effective solid embankment body, while active protection can be carried out by sealing the loose layer, or by raising somehow the levee crest. Water overtopping the levee crest tends to erode the air-side slope and results in erosion which proceeds from the protected side towards the water-side. Protection consists of preventing flow over the crest by raising somehow the levee crest. This kind of protection is, however, always questionable, since the condition of the upper layer of the levee is unknown. Raised crests — small dykes on the main > levee crest — to be effective must be securely founded in consideration of the loose upper part of the embankment, must not be excessively long and must be reinforced continuously under careful inspection. Crest dykes are usually made of earth, preferably in burlap bags filled to threequarter capacity and placed in a closely packed pattern. Where no bags are available the crest dyke may be constructed of earth, but must be protected on the water-side, e.g. by a brushwood fence secured by stakes. 2. For protecting the levee against wave action, wave conditions at the particular site must be explored first. Wave action can be made harmless partly in the forefield of the levee, by reducing wave amplitudes, or by making the wave to breake, and partly by the use of materials on the slope which are capable of absorbing the mechanical wave energy. Both kinds of wave protection have been realized by conventional materials, mainly various ligneous materials gained on or in the vicinity of the levees itself (brushwood mattress, straw). Methods of protection are illustrated in Figs. 15-20All these methods call for a large stock of ligneous material, which must be continuously renewed. Brushwood cropping necessitates special forestry methods. Further, more, the cropping, storage but actual flood protection itself requires considerable manual labour. The most urgent task is to provide permanent protection of the highest possible extent by the gradual reshaping or creation of wave damping woods, or occasionally by the realignment or revetment of the levees. Where this is impracticable, protection against wave action can be realized by damping, or breaking blankets before