Abstract:This paper describes delta development processes with particular reference to Cimanuk Delta in Indonesia. Cimanuk river delta, the most rapidly growing river delta in Indonesia, is located on the northern coast of Java Island. The delta is subject to ocean waves of less than 1 m height due to its position in the semi-enclosed Java Sea in the Indonesian archipelago. The study has been carried out using a hydrodynamic model that accounts for sediment movement through the rivers and estuaries. As an advanced approach to management of river deltas, a numerical model, namely MIKE-21, is used as a tool in the management of Cimanuk river delta. From calibration and verification of hydrodynamic model, it was found that the best value of bed roughness was 0Ð1 m. For the sediment-transport model, the calibration parameters were adjusted to obtain the most satisfactory results of suspended sediment concentration and volume of deposition. By comparing the computed and observed data in the calibration, the best values of critical bed shear stress for deposition, critical bed shear stress for erosion and erosion coefficient were 0Ð05 N m 2 , 0Ð15 N m 2 , and 0Ð00001 kg m 2 s 1 , respectively. The calibrated model was then used to analyse sensitivity of model parameters and to simulate delta development during the periods 1945-1963 and 1981-1997. It was found that the sensitive model parameters were bed shear stresses for deposition and erosion, while the important model inputs were river suspended sediment concentration, sediment characteristics and hydrodynamic. The model result showed reasonable agreement with the observed data. As evidenced by field data, the mathematical model proves that the Cimanuk river delta is a river-dominated delta because of its protrusion pattern and very high sediment loads from the Cimanuk river. It was concluded that 86% of sediment load from the Cimanuk river was deposited in the Cimanuk delta.

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One of the potential hazards that can cause major problems in coastal areas is erosion. In order to solve this problem, the notion of sustainable coastal development has garnered growing attention around the world. Coastal zone management mechanisms aim to ensure the sustainability of resources and the environment. For example, natural coastal protection using vegetation such as mangrove trees is currently preferred in many places in the world. However, there are challenges in the development of this natural form of coastal protection, e.g. mangrove seedling trees can be damaged by the waves or the current before they grow strong enough, and thus require appropriate protection until at least two years after planting. To solve this problem, a natural coastal protection system that combines a main natural protection and a temporary man-made structure is proposed. After a process of weighing temporary man-made-structure alternatives, the geobag dyke was selected. This study aimed to quantify the effects of various geobag dyke configurations and geobag unit weights on wave height reduction. Laboratory experiments were conducted on a narrow wave flume using a mangrove model as main natural protection and geotextilegeobag models as temporary man-made structures. Various wave conditions were generated during the laboratory tests. This paper focuses on the experimental results of wave transmission through the protection system in order to determine the most effective geobag dyke configuration to reduce the wave height. Based on experimental modelling, the most effective geobag dyke configuration uses relatively heavy geobag units with a moderate dyke slope (1:1.5).