Veress Márton: A Bakony természettudományi kutatásának eredményei 23. - Covered karst evolution... (Zirc, 2000)

KARSTIFICATION

The percolating waters of the background area flow over the clay series of the cover sediments towards the depression. Seepage is mostly expected during the summer half-year in background areas of vegetation cover and less impermeable sediments. Intermittent springs may form in depressions where channels incising into the cover sediments expose impermeable series. In the case of hidden activity infiltrated water reach karst passages without coming to the surface. Intermittent springs and particularly hidden activity prolong corrosional development. While inflow from the surface is restricted to some minutes' time, water recharge from unconsolidated sediments may last for several days or weeks. The dissolving power of solutions passing through unconsolidated sediments and enriching in biogenic C0

2 is higher than that of surface runoff. A phase of activity is the time interval during which activity is observed. During an activity period - a rainy or snowmelt period - one or more phases of activity may take place. The amount of water input of the depression depends on the size of the catchment (background area) and on the runoff coefficient. It is also influenced, for instance, by the vegetation of the background area. (In treeless areas with wind of favourable speed and direction, snow accumulates in the depression.) The sediment accumulation from waters flowing towards the depression as well as of the lithology of cover sediments may modify water input. The water input of the conduit may differ from that of the surface depression. While the overflow from flood ponds reduces the water input of the passage, during hidden activity water from the background may reach the conduit in an increasing amount. In addition to water from activity, the amount of actual activity also includes water from intermittent springs and from hidden activity. The duration of activity (mostly accumulation) and of hidden activity (corrosion) may be different or vary from year to year for the same depression. The evolution of the individual depressions is specific and its rhythm is variable. If in a covered karst depression the rate of water recharge exceeds the rate of water conduction to depth, flood ponds develop. Their diameters and depths are some metres. The intermittent ponds influence karstification on the covered karst in two ways. The pond may cause the covering and plugging of the chimney and triggers infilling of the depression. (In the infilling depression the pond survives for longer periods and more sediments are deposited.) On the other hand, the deepening fill increases the intensity of solution (more biogenic C0

2 is produced) since it takes longer to transfer water into the karst. Consequently, the sediments reaching the covered karst depression through flood ponds do not increase erosion but maintain or even increase corrosional development. By the duration of their existence the ponds are referred into the following types: - Short-term ponds form in depressions with open conduit. Water transfer into the karst takes place through conduction. The dropping of their water table is rapid (the pond formed during one phase of activity does not survive until the next) and continuous. Water flow in conduits is probably turbulent. - The sediments of ponds of longer existence (Picts. 26,27) partly accumulate in depressions with impounded conduits. They are slowly drained partly by seepage and partly by flow. If they are not emptied before the next phase of activity, the rate of dropping water table may reduce in the wake of repeated water recharge. (The dropping of water table may stop and even a rise may follow for the same pond.) - Long-term ponds (Pict. 28) form in depressions with filled conduits. The ponds loose part of their water by seepage and another part by evaporation. The life-time of such ponds may be longer than an activity period.