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<Technology Needs for Lake Management in Indonesia - Investigation of Rawa Danau and Rawa Pening, Java> Rawa Danau A. Present Environment and Past Manipulation 1. Location This lake lies in an ancient caldera in the province of West Java, about 100km west of Jakarta. Administratively it lies within the "Kabupaten (Regency) of Serang." Geographically it is located at 6 11' South and 105 59' East. Ecologically, it is impossible to discuss functionally the area of swamp and lake separately from the catchment. The catchment includes the steep hills and mountains forming the wall of the caldera. To the south-east a range of low hills allows easy access to the caldera floor. Rawa Danau is commonly quoted as being between 80 and 200 metres above sea level. The visible floor of the caldera lies at or above 87 metres. The floor of the caldera rises to the south and south-east, at the foot of the hills, to heights of more than 200 metres, the great majority being between 87 metres and 100 metres in height. 2. Topography and history of the caldera The lake does not exist at present. Two hundred years ago there was a substantial lake in the caldera. The size of the original lake is not known. Information on the circumstances which led to the disappearance of the lake comes from the paper by Endert (1932) published in Tectona. In that paper he refers to the reports of Brumond (1840), Schram (1874), and Koorders (1912). It has not been possible to locate copies of the papers by Brumond or Schram, and that by Koorders is not relevant here. A translation from the Dutch of relevant sections of Endert's paper is given below. "Brumond reports that already, during the reign of the Governor General Van den Bosch, attempts had been made to drain the lake and reclaim the land for agricultural purposes, by lowering the base of the outlet channel. This attempt was not totally successful, although those engaged in this effort succeeded in lowering the water level of the lake by two metres." Schram, Inspector of B.O.W., who was ordered to ascertain and find out to what extent the Rawa Danau lake could be used for irrigation in areas outside the Rawa Danau area, submitted a report (probably unpublished), in 1874. I found this report with the manuscript of the data written by Koorders." Endert then quotes sections from Schram's report. This includes the following: "A narrow rocky chasm, named the Pasang Tenang, provides the means for drainage of water from the lake. In this chasm, the water from the lake forms a forty foot high waterfall, the Tjoeroek Betoeng, and continues its journey under the name of the Pasang Tenang for another five "palen", until it finally enters the Soenda Strait, south of Anyer." Further investigation in the Encyclopaedie van Nederlandsch-Indie, 1917, Volume I, Second Impression, 1917, page 571, to which Endert refers, reveals the following statements: "Dano (Danoe): Lake in the Tjiomas District of the Pandeglang Division of the Banten Residency, occupies a few square kilometres, and is the deepest part of a crater bottom, which is largely occupied by a swamp. In 1910 this covered approximately 50 square kilometres." "In the past the waterfall (the Tjoeroek Betoeng) appeared to have been even higher, but in 1835 there was an attempt to deepen the chasm to drain the swamp. This was only successful in the south, which is now occupied by padi fields and villages." "Between 1907 and 1910 the drainage channel was widened and deepened again, to enlarge the area of padi fields. The Northern part is still a morass and regarded as an important nature reserve because of its unique flora." "The lake itself has a depth of up to 10 metres; some gases bubble up to the surface, spewing mud occasionally." Notes - B.O.W. - Bijzondere Openbare Werken or Extraordinary Public Works. Tjoeroek Betoeng - now Curug Betung Palen- One "Paal" is 1506.94 metres. Soenda - now Sunda Dano is an alternative spelling for Danoe and Danau and means "lake". Tjiomas - now Ciomas 3. Evidence of higher water levels in the past Water-worn rocks are exposed on the south bank of the exit from the caldera at a height of approximately two metres above the present dry season water level. These rocks are part of the bedrock and are water worn on the sides as well as on top. It is not possible for them to have been worn away by foot usage, as part of a former path. From this it appears that, in the past, there was a strong flow of water for long periods over these rocks at a height of two metres above the present water flow. It seems likely that this was the level of the outlet from the caldera for a long period, and that a gap of 12 to 15 metres wide was cut in the rim of the caldera to a depth of 2 metres in the period before 1920. This may have been done in several stages over many years. The original natural outlet was probably a broad shallow lip about 200 metres wide stretching from side to side across the small valley, on the floor of which the present water gauges are situated. This outlet probably had a distinctive fauna which has now vanished. This fauna probably included numerous small lamellibranchs (? Corbicula sp). The narrow channel of fast-flowing stream water, which replaced the original outlet, leads directly to the waterfall but may not have improved its scenic value. 4. Possible drainage ditches in the caldera Examination of one of the small rivers draining the swamp suggest that these have been altered and are not entirely natural. It appears that these were originally constructed to form drains by the Dutch Governor at the same period in which the exit from the swamp was lowered. It is very unusual for a river in a swamp to run in a straight line. Three streams draining the south west corner of the swamp run parallel to each other at a distance of 1km approx. All are remarkably straight, over distances of two kilometres. When they change direction, they tend to do so in parallel with each other. In the Ci Bojong river the original path of the drainage channel may have been affected by rapid erosion in the past. The unusually deep channel of the stream at Tambakan appears to be larger than the flow of water at present would suggest would be likely. There is a possibility that the original course of the upper Ci Kalumpang was altered by people to flow, as it does at present, through the large "kampungs" (villages) of the Padarincang area, and in particular Sukacai. It seems possible that it was diverted above Ci Bojong from the Ci Bojong river to the Ci Kalumpang at some period in the recent past. The combined flow of these two relatively large streams in the Ci Bojong channel would have led to rapid erosion of the original banks of a drainage canal during the dry season, and to the development of irregularities in the path of the river, as well as to a deepening of the channel, as is the case at present. The main Ci Danau river runs in a noticeably straight line past the large hill (Gunung Jamungkal) on the north, two km east of the confluence of the Ci Kalumpang and Ci Danau rivers. This effectively straight line continues for four kilometres. From this evidence it seems likely that the account given by Endert (1932) is correct. It is probable that, in addition to lowering the base of the exit from the caldera, drains were dug or deepened to speed up the drying of the land surface. At the present day these have become, effectively, deeply incised erosion channels through the layers of soil in the caldera. 5. Recent alterations to the exit from the caldera The first lowering of the outlet was followed, some time later, by the creation of two channels in the long cut in the rock wall of the caldera leading eventually to the Curug Betung waterfall. The foundations of the two channels can be seen in the rock of the stream bed starting 40m downstream from the gauge stations. It is not known how much of the long cut was made before 1850 and how much was made at a later date. In the Encyclopaedie of 1917 it is suggested that further attempts to lower the exit were made in the period 1910-1917. From the residual structures exposed in the bed of the cut, it appears that this is the foundation for a gravity fed draw off of water from the Ci Danau after the river has left the caldera. It is unlikely that this gravity fed draw-off was ever completed. For present purposes these points are immaterial. Such a draw-off, as it is at present, would not have affected the level of water in the caldera. The objective of this draw-off was either to supply water to Cilegon for domestic use and for industrial use in steel making, or for hydro-electric power production. Staff of Krakatau Steel deny that this was constructed by Krakatau Steel and draw attention to concrete markers on the land at the exit from the caldera labelled "BB". This refers to the steel-making Company Besi Baja which preceded Krakatau Steel. There are also markers on the same land with "KS" for Krakatau Steel. The option for a gravity fed intake pipe to Cilegon was abandoned later in favour of the storage reservoir on the Ci Danau near the coast, and the use of a shorter pipeline along which water was pumped. Because these structures as seen at the present day are not be likely to have affected the water levels in the caldera, they are not considered further here.
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