H D Tjia


Coastal accretion has been found to be important for Sutnatra's East coast and Java's North coast, rcspectively amounting to 60-50O m/yr and 55 - Zl4 m/yr near the mouths of large streams. Elsewhere on the same coasts the yearly accretion rates are respectively 15 m and less than 30 m. The annual accretion in the vicinity of Padang (Sumatra's West coast) is less than 10 m. In the Landak region of West Kalimantan
a rate of 110 m/yr is known. Data are wanting for other coasts of Indonesia but it seems hardly probable that higher accretion rates are present, except presumably for the Mahakam river, East Kalimantan. Comparing accretion rates on Java's North coast from maps surveyed in the period around 1850 till 1946 (aerial photographs), it was found that aggradation rates most commonly have been accelerating since 1920- This rise is undoubtedly due to an increase in denudation rate through vast. deforestations by the growing population and the partial clearance of estates during the Pacific War and the ensuing armed revolution It may also indicate an absolute lowering of sea level and/or tectonic uplift of the land. Positively favourable for coastal growth are the
following factors : High denudation rate in the drainage basins through scanty vegetation, high relief, heavy rainfall and the presence of easily erodible rock like marl; tectonic uplift; clastic volcanic activity; rhizophora growth in the coastal swamps; low springtides; quiet and shallow sea. In studying the beach ridges of the Tjiasem Bay area (West Java) it was found that the older generation of beach ridges is invariably of larger s:ze and was formed before the turn of the century, while since then only smaller-sized ridges have been developed. The same relations are also
valid for other beach ridge series in Java, i.e. the older ones are larger than the younger ridges. The latter were probabl.v formed because of an important climatic change around 1900 - if the age - data inferred from accretion rates in the Tjiasem Bay area are right - and,/or a eustatic lorvering of sea level.

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