CLASTERING OF EXTREME RAINFALLS AND RECOVERY OF GEOMORPHIC SYSTEMS

Extreme event means an event during which the threshold of varies processes is passed and this leads to the transformation of equilibrium of slope or/and river channel systems. The frequency of extreme rainfalls fluctuate in space and time. In the aride zone it may happen vary rare, sometime once in century or millennium. In temperate and mediterranean zone the recurrence interval varies from several years to decades. In extreme tropics such extremes repeat every year or even several times in one year. In most climatic zones it is enough time for relaxation and recovery of the system.Only in the extreme tropics like in Cherrapunji it is no time for relaxation and new equilibrium of the system has been formed. In the nature we observe also the clustering of extreme events. It happens when in some periods the events repeat every 2-3 years, every year or several times in one year, when due to missing of time for relaxation it follows the disturbance of slope or fluvial system and finally new equilibrium creates. Such circumstances have been observed in last decade in Central Europe and at the margin of the Bhutanese Himalaya. The detail examination of rainfall and flood records from the Little Ice Age as well from the phase 8.5 -8.0 kyr C14 BP help to distinguish frequent clusters of extreme events, which jointly form the phase with higher anomalies and humidity. This clusterings seem to be responsible for the transformation of natural systems characteristic for the wetter phases of the Holocene, for cut and fill sequences in alluvia or glacial advances and debris flow activity in high mountains.