Vízügyi Közlemények, 1997 (79. évfolyam)

3. füzet - Rövidebb tanulmányok, közlemények, beszámolók

A folyómeder kolmatálódásra hajlamos részeinek lehatárolása... 401 Identification of river channel areas inclined for colmatation, based on the analysis of bed material by Dr. László RÁKÓCZI С. E. CSc. The water yielding capacity of bank-filtered wells can be reduced temporarily or for a longer period of time due to the clogging effect of sediment particles that enter the pores of the bed material and to that of the benthic plants and animals that are attached to the surface of the channel bed. These two processes are called the mechanical and biological colmatation. Due to the turbidity of flowing waters these processes can only be observed in the near-bank shallow zone and/or periods when the water contains but little suspended sediments (using underwater video cameras). No undisturbed samples can be taken from gravel channel beds. The relevant literature reports only a single case, that of the River Rhine, when the process of colmatation of gravel channel bed has been observed, using a special vessel. From this observation vessel a room-size diving-bell has been submerged in the water onto the river bed, down to 10 meter depth (.

Figure I.). During these in situ investigations it was found at a series of bank-filtered drinking water wells that under the stabilized (armoured) gravel channel bed surface there was a 1—10 mm thick muddy-silty layer (Figure 2.) that made the river bed practically impermeable. Rich benthic layer was also observed on the surface of the channel bed. Results of a detailed channel survey, in the section of a well, is shown in Figure 3. This indicates the colmatation of the armoured gravel channel bed. Although flow velocity would not allow the deposition of suspended solid particles at this site, the suction effect, the depression of water withdrawal from the well, initiated the colmatation process. There was no colmatation observed on the armoured channel part, which is located to the left from the deepest point of the section, because depression (suction) was not strong enough to suck suspended sediment particles into gaps of the bed material. On the left side of this section lose, moving sediment was observed, which do not allow neither mechanical nor biological colmatation. In the lack of the excellent possibilities offered by the di vingbell one should find other solution for identifying those parts of the river bed which are inclined for clogging temporarily or for elongated period of time. In Hungary the relationship between the particle size composition of the bed material and the factors of channel dynamics (deposition, scouring etc.) are known for about 20 years. Figure 4. shows the main types of particle size composition curves. In addition to the integrating curves, usually considered for practical purposes, the corresponding differential curves are elso shown in this figure. Type "a" corresponds to the dynamic equilibrium of the stream. Type "b" characterizes selective erosion, when sediment moving forces are big enough to move certain particle size fractions only. Type "c" refers to a later stage of the earlier one, when certain sediment fractions have been washed off already from the surface of the channel bed. Type "d" corresponds to armoured channel bed, where the roughest particles of the original channel bed remained there only, while types "c" and "f ' refer to the deposition of the sediment. From the view point of colmatation armoured channel reaches are the most important ones. Plotting the particle size distribution curves of samples taken at several points of a cross-section (from 7 sections as shown for the River Danube in Figure 5.) in the same diagram one can identify whether there are any armoured portions of the channel and if yes what is their width. The lasting character of armoured channel can be detected by taking repeated samples from selected points of the cross-section. Disruption of armoured channel surface upon the effect of a flood hydrograph or the refilling of the gaps during the recession of the hydrographs can be detected by this method. Since colmatation can occur only at lastingly stable river beds, the scouring of the bed material would be indicated by the abrupt increase of the water yielding capacity of the wells, while the rearmouring is indicated by decreasing discharges. From the Hungarian point of view it is worthwhile to note that according to the German experience the dredging of the colmated zone in front of a series