Vízügyi Közlemények, 1958 (40. évfolyam)

4. füzet - VII. Kisebb közleménye

(37) The différent systems of intricate passage-ways dug by rodents are equally dangerous, the passage dug by a mole may exceed in length 30 to 50 metres, while systems of water rats may attain 100 and even 200 metres in length. The labyrinth of a hamster may extend over 6 to S, that of a suslik over 8 to 12 scj. metres a I a maximum depth of 2 to 3 metres. Rodents show preference to the airside and the crown of the levees. Moles living around the toe of the embankment and being driven by the rising water level have been observed to dig higher and higher into the levee and even into freshly built emergency dykes. Duets created by roots of plants

, living or decaying, left-over stumps of cut trees constitute a considerable hazard. The danger is limited to individual spots in case of lone-standing trees. Logged land or forests still standing on areas adjacent to the levee are an even more serious source of danger. Stumps and roots neglected during superficial deforestation in building the foundation for the levee, form an irrevocable defect and contribute to the complete saturation of the levee at times of extended floods (Pictures 1 and 2). Roots from forests in flood areas usually extend into the subsoil and body of the levee, owing to more favourable humidity conditions prevailing there. The network formed by these roots spreads, beside the core to the slope, and even to the crown of the levee ( Picture 3). Roots often extend beyond the centerline of the levee, while smaller levees may be penetrated entirely by roots striving towards the protected side. Water seeping into ducts created by decayed roots, and into loose soil surrounding living ones may, at times of extended floods, saturate the entire levee body and eventually lead to breaks. The saturated levee is then washed away at the level of entwined roots penetrating into the levee at the toe (Picture 4). Roots of trees growing on the airside (Picture 5), resp. waterside (Picture (!) slopes of the levee have been traced all oyer the levee body from toe to crown, and found to form an unpredictable pattern with the resulting duct-system constituting an ever-present source of danger. Roots of grass- and weed-cover protecting the slopes against the scouring action of waves tend to loosen the soil in the top layers, vet deep roots may extend down even into the core of I he levee. In order to avoid the development of cavities and root-ducts the use of soils highly contaminated with organic matter should be dispensed with for levee-construction; soils containing reed-roots are especially dangerous (Picture 7). The evaluation of these experiences leads to the following conclusions: /. With respect to floods of long duration, which are likely to occur under Hungarian conditions, the provision of protecting belts along the levees is deemed advisable. No trees or deep-rooted plants should be tolerated within these belts. 2. In designing new levees, the top-soil layer most exposed to harmful effects of vegetation and animal life should be subtracted from the useful levee cross-section and the remaining core and base width alone should be introduced into calculations. •1. Herms should be omitted from the standard cross-section and a milder slope should be used instead, on the protected side. 4. The possibility of applying internal cutoff walls should be investigated to effectively prevent the development of ducts caused either by animals or by vegetation. (Author's summary translated hy 7.. Szilmssy c. engr.) GEFÄHRDUNG DER HOCHWASSERSCHUTZDEICHE DURCH PFLANZEN UND KLEINTIER von /.. Babos u. AI. Iiokor (Bilder siehe Seite 100- 177 des ungarischen Textes) DK. 627.514 Das Donau-Sommerhochwasser 1954 und das Eishochwasser des Frühjahrs 1956 erreichten in Ungarn katastrophale Ausmaße. Die Erfahrungen der langanhaltenden Abwehrarbeiten lenkten die Aufmerksamkeit unter anderen auch auf die sehr schädliche Einwirkung der Pflanzen und Tierwelt. Die durch Wurzeln und Gänge der Wühlliere unterstützte vollkommene Durchweichung der Dämme führte in vielen Fällen zu Dammbrüchen. Die Ausdehnung der Gangsysteme der Sch'idlingstierc ist zwar sehr verschieden, doch sind dieselben durchwegs sehr gefährlich. Die Länge der Maulwurfgänge erreicht 30—50 m, die der Feldmaus- und Wasserrattengänge kann 100—200 m