Coastal hydrodynamics, geomorphology and sedimentary environments of two major Javanese river deltas. Program and preliminary results from the Snellius-II expedition (Indonesia)

Abstract

River outflow, sediment transport, depositional processes and facies, as well as delta morphology of the rivers Solo and Porong (East Java, Indonesia) have been studied in detail during the Indonesian-Dutch Snellius II expedition (1984–1985). The humid tropical climate of Java in combination with the mountainous relief and the presence of easily erodible volcanic soils in relation to an intensive agriculture, results in a tremendous soil erosion and denudation rates are extremely high. Since river flow in the dry season is strongly reduced, sediment transport is mainly restricted to the wet season. Every wet season large volumes of sediment are carried towards the sea and the surplus of sediment in the coastal zone has resulted in the formation of two major river deltas. The Solo delta, a single-delta, is made up of one straight major channel with only few and small natural crevasses. Prominent natural levees are present; a consequence of the fact that in the wet season maximum outflow of the river coincides with high water. The development of this delta is considered to be a direct response to the high outflow velocities of the river, the huge mudload in the wet season and the low current and wave action. The Porong delta is a half-circular, lobate delta with a multidistributary network of channels which resulted in a regular distribution of sediment. A small part of the delivered sediments in both deltas escapes to sea and is seized by offshore, monsoon-induced coastal currents. The monsoon-type climate has a significant influence on depositional processes by determining river regime and sediment transport, by modifying river outflow and generating a coastal drift of water and suspended sediments.