Estuarine hydraulics in a shallow deltaic estuary with reference to the Kali Garang Estuary Semarang, Indonesia

Suripin (1992) Estuarine hydraulics in a shallow deltaic estuary with reference to the Kali Garang Estuary Semarang, Indonesia. Masters thesis, Memorial University of Newfoundland.

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Abstract

Most of the Indonesian rivers are characterized by the formation of deltas at the mouths. The estuaries are usually shallow and the longitudinal bed slope low. The rivers debouch into an ocean with low to moderate marine energy. The river flow fluctuates seasonally in accordance with the monsoon season. During the wet season, from November to April, floods may occur due to the intense rainfall. The river flow becomes quite small during the hot, dry seasons. -- Naturally, the rivers bring a lot of sediments which originate from volcano ashes and erosion of the catchment basin. Most sediments flow down to the estuaries and near-shore areas during the high river flows of the rainy season. In most cases, many hydraulics structures have been built along the river systems. They are intended for sediment control, flow regulation and flood control. -- The fluctuation of river flow is considered to be a major factor in determining estuarine circulation patterns and delta formation. Typically, the circulation pattern shifts from that of a highly stratified estuary during the high river flow of the rainy season to that of a well-mixed estuary during the low river flow of the dry season. Furthermore, the quantity and variation of discharge in association with wave effects results in various delta geometries ranging from river-dominated to wave-dominated deltas. River-dominated deltas are characterized by highly irregular and protruding shorelines and by sediment bars deposited parallel to the direction of river flow. Wave-dominated deltas are characterized by straight shorelines with sediment barriers deposited parallel to the shorelines. -- To understand the contribution of river flow and marine forces to the behaviour of estuaries and the geometry of deltas in Indonesia, the Kali Garang Estuary has been chosen as a subject of study. The Estuary consists of two branches: Kali Semarang, and the West Channel. Tidal excursion, salinity distribution and circulation patterns are used to represent the estuarine behaviour. The Discharge Effectiveness Index is used to evaluate the relative contribution of river flow versus marine forces to the development of deltas. Field observations of water level, salinity, temperature, currents, and sediments were made during the dry season of 1991. Existing data sets related to the study were also collated. -- The study shows that the Kali Garang Estuary experiences a small-diurnal tide-range (≈ 65 cm). In the manmade West Channel the tide propagates up the estuary as far as the weir. The tidal range decreases landward and the fresh water discharge flows seaward over the saline water that intrudes landward below it. In the Kali Semarang, the tide propagates as far as its mid length. Fresh water mixes directly with seawater, and a weak vertical stratification is experienced. The saline water moves up and down the estuary due to the flood and ebb tides. -- The coast where the estuary discharges to the sea is subject to a very moderate wave climate. Maximum wave power occurs in phase with maximum river discharge and this results in the formation of a wave-dominated delta. The high river sediment during the rainy season is spread out by waves generated by the west monsoon. This material is then redistributed along the shoreline by waves generated by the east monsoon during the low freshwater flow of the dry season.

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