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Prof.dr. P. Hoekstra, Dr. T. Hoitink IMAU WT 77-190

Morphodynamic processes and ecosystems along the East Kalimantancoast

The aim of the pilot study is to characterise the hydrodynamic and sedimentaryregimes of the Berau and Mahakam delta regions. Regarding the main project,basic information will be gathered to be able to choose between the two deltasystems and to sharpen the research objectives.

 AdjustmentsSome methodological adjustments were required. The draft of the utilizedresearch vessel (Geomarin), which is approximately 5 m, appeared too large formany purposed in the Berau region. Therefore, speedboats were hired severaltimes.

Scientific resultsOur project has yielded understanding of the hydrodynamic and sedimentaryregimes of the Berau and Mahakam delta regions. In both Delta regions ourfocus was threefold. First, estuarine processes were analyzed, addressingupriver tidal attenuation, turbidity response to current variation and the relativeimportance of river discharge with respect to the tidal motion. By combining ourown measurements with information provided by local institutions, the tidaldeformation along the Mahakam river was established. The suspended sedimentload in the Mahakam appeared less dramatic than what is suggested in theliterature, as the suspended sediment concentrations rarely exceed 150 mg/l. Inthe Berau, the larger portion of the freshwater leaves the estuary at the northernbranch. In the thalweg of the northern branch, an estuarine turbidity maximumwas established.

Second, river mouth processes were investigated, addressing the competitionbetween mixing and stratification, settling of suspended sediment from riverplumes and the dispersal of buoyancy. Both in the Mahakam and in the Berauour anchor stations were well chosen, in the sense that they were located closeto the region where the maximal mixing appeared to be. The data from theMahakam resulted in a clear image of the development of a freshwater pulseover a tidal cycle. At the river mouth connected to the northern branch of theBerau delta, where the larger part of the freshwater leaves the estuary, tidalcurrents are weak which allowed for a stably stratified water mass.

Third, the hydrography of the adjacent shoreface waters was explored. At theshoreface connected to the Mahakam delta three distinct water layers could bedistinguished. A thin top plume layer in the order of 0.5 m was separated fromthe underlying waters, and persisted over the entire 50 km that the shorefacemeasurements spanned. Up to approximately 20 km offshore, oceanic waters

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that penetrate the shelf as a high-salinity, low-temperature tongue could bedistinguished. The intermediate water layer is relatively well-mixed. In the Berauregion it was assessed that the juxtaposition of coral reefs forming the Berau reefcomplex is indeed subject to buoyant discharges from the estuary. However, asthe suspended sediments in the estuary are sandy, they have settled from

suspension before reaching the reefs. This holds at least during the dry seasonwhen the measurements took place.