Vízügyi Közlemények, 1961 (43. évfolyam)
4. füzet - IX. Képek a Föld különböző részeinek vízépítési munkáiról
(13 > RESISTANCE OF THE GRASS COVER IN WATER CONVEYING REDS Ry Z. Babos (For the Hungarian text see pp. 65) Increased demands on the water conveying capacity of earth beds of watercourses called for larger bed dimensions, which result in additional loss of crop area and in increased costs. The only possibility for minimizing these consists in the simultaneous improvement of the water conveying capacity, which however is not possible unless the beds are lined. The costs thereof being usually prohibitive, the most simple solution is a carefully planted, or constructed and continuously maintained grass cover, completed wherever indicated by a rubble, -— or concrete lining around the low water surface. Whenever the velocity of flow exceeds a certain limit, the stability of the bed material is impaired in ail extent depending upon the properties of the bed-, or lining material. One consideration to be observed in designing the bed is to limit the velocity of flow. The critical condition beyond which the material of the bed, or of the lining is already attacked by the velocity of flow, is usually specified and described in handbooks^ on the one hand by the highest limit flow velocity, and, on the other by the permissible value of the tractive force acting along the bottom. For grass covered surfaces these values are compiled in Tables I to III. As revealed by the tabulated values, the flow velocity that can be assumed for the bed material is general roughly doubled by the presence of a grass cover. The relevant investigations and studies are incomplete and the number of theoretically uncleared factors is still very great. Therefore, the values computed by these formulae should expediently be compared to such established in the field empirically. As indicated by these latter, the resistance against the damaging effect of extreme flow velocities of grass protected slopes and with roots firmly interwoven bottoms of natural watercourses exposed to the severe conditions of nature, is appreciably higher than the values contained in the table, expecially if the duration of exposure is relatively short. Actual conditions that can be observed at water beds under natural circumstances are illustrated by the example of two watercourses selected at random, the bed cross section of which is shown in Fig. 1, while the mean velocity and the corresponding tractive force is represented as a function of head in Table IV. As to be seen therefrom the mean velocity at the passage of highest flood waves increases to as far as 4,5 m/sec, and still no damages could be observed on the grass covered side slopes of the beds. The same experience could be gained with a number of other watercourses, further with the natural grass covers of very steep hillsides and valleys. Relating to the two above examples the corresponding values of gradient, hydraulic radius and flow velocity are shown in a more detailed manner in Figs. 2. and 3. As will be perceived it would be of appreciable significance if, relying on favourable experiences, the limit velocity established for grass covered surfaces would be increased by 50, or even 100 per cent. Roth the gradient, and the hydraulic radius (dimensions of the bed cross section) could be materially increased thereby, whereas a bed section considerably smaller than hitherto applied could be used for the same gradient. The beneficial consequences would be particularly great in the case of relatively smaller beds, inasmuch as a considerable amount of earthwork could be saved in addition to the great number of costly structuies (weiis for reducing the surface slope, etc.). In order to promote the widespread application of grass covers — the cheapest method of bed protection —it would be desirable to increase the limit velocities to be considered for these by at least 50 per cent. (Authors' summary translated by Z. Szilvássy, struct, engr.)
