MECHANISM AND SIMULATION OF THE EXPANSION AND EXTINCTION OF STABLE VEGETATION AREAS IN A GRAVEL-BED ALLUVIAL FAN RIVER

Abstract

Significant and monotonous expansion in luxuriant, stable vegetation areas like wood-lands has been observed in gravel-bed reaches of the Tama River. Changes in vegetation, substrate and channel shape in micro- and macro-scale was traced carefully together with hydrologic and hydraulic conditions experienced. A scenario for the expansion was obtained, where the positive effect of floods by forming a “top fine-sediment layer” necessary for the stable vegetation growth was emphasized, as well as their negative effect by disturbing and removing a gravel surface with vegetation. A simple but comprehensive simulation model based on the scenario was developed and used for identifying hydrologic, geomorphic and fine-sediment-supply conditions encouraging or discouraging the growth of stable vegetation.