 |
|||||||
| About UNEP | UNEP offices | News centre | Publications | Calendar | Awards | Milestones | UNEP Store |
|
|||||||
|
 |
|
|
|
||
| UNEP>DTIE>IETC | |
|
|
<Technology Needs for Lake Management in Indonesia - Investigation of Rawa Danau and Rawa Pening, Java> B. Impact of Drainage of the Lake 1. Multiple effects of drainage of the lake Drainage of the lake has also led to the partial draining of the swamp forest, a process which is likely to be one of the main causes of the gradual deterioration and loss of biodiversity in the swamp forest. Draining of Rawa Danau began over 150 years ago. This would have started a series of processes which continue at the present time. These processes have affected the ecology of the area at present, and will affect the ecology of the area in the future. 2. Effect of exposure of soil to the air The original soil of the caldera probably had a high proportion of organic material, derived, during hundreds of years, from the swamp vegetation. Exposure to the air of a previously waterlogged soil with a high proportion of organic matter leads to rapid decomposition of the organic content of the soil, particularly under tropical conditions of temperature and humidity. Decomposition is carried out by bacteria and insects. These depend on air for their survival. While the soil is flooded, there is little oxygen in solution, and the small amount of dissolved oxygen in the water is rapidly consumed at the surface of the waterlogged soils by bacteria and benthic organisms. Thus at a depth of about 2 cm or less below the surface of the waterlogged soils, the conditions are anaerobic and bacterial action is greatly reduced, being confined to anaerobic bacteria. There will be no insects at all, since these would be dependent on access to oxygen in the surface water. Following exposure to the air, the composition of the soil would change rapidly as the proportion of organic matter decreased, and the overall depth of the soil would decrease, as the organic component was removed by decomposers. The original soil of the caldera as exposed at the surface would probably have been poor soil for growing rice as very loose organic soils are not an ideal medium for rice plants. After some years the change in soil composition towards a soil with a higher content of clay and silt would lead to improved rice production. It is uncertain to what extent the soils of the caldera are waterlogged at present. This would call for further investigation. Such investigation is probably unnecessary, as far as is known at present, for the formulation of management policies. The results would be of much biological interest. 3. Water storage in the soil in the caldera One would expect that, despite the removal of the rock lip at the exit, there would be an appreciable variation in water level in the caldera between the wet season and the dry season. There would therefore also be an equivalent variation in the water level in the soil in the caldera. As the organic content of the soil decreased, the percentage water content in the soil would also decrease. Initially, the soil would hold a lot of water and be very easily drained. After some years the rate of drainage from the soil each dry season would become slower, but the percentage water content of the soil would also decrease. At the present time, during the dry season, there is clear evidence of drainage of water from the soil of the former swamp into streams within the caldera. Water can be seen draining into streams such as the Ci Kalumpang from a height of 10 to 20cm above the existing water level in the streams. Water can also be seen draining from the exposed and eroded river banks into the Ci Danau river at the point where it leaves the caldera. Here the water may be draining from about half a metre above the river level into the river. On the Ci Bojong at Kampung Tambakan, water was seen draining into the stream from over one metre above the level of the stream. This means that the soil near the banks of the streams is drying out to depths of over 2 metres near Rancasumur and Tambakan, to depths of 1 metre half way to Ci Danau from Rancasumur, and to depths of at least 1 metre at the exit from the caldera. There is no information as to how far from the banks of the streams and rivers this drainage extends. It may be a localised effect confined to 20 or 30 metres from the streams or it may be that, during the dry season, there is widespread drainage of soils throughout the whole caldera. Superficial inspection of surface soils suggests that drainage is widespread in the western section of the caldera and much less extensive in the eastern section. 4. Rate of flow of water in the swamp The normal rate of flow of water in a swamp would be expected to be slow, or even undetectable, particularly during the dry season, when water levels are at their lowest. The flow in the upper Ci Kalumpang was slow, at less than 0.2 metres per second. As canoes moved down the Ci Kalumpang the rate of flow increased until at the junction with the Ci Danau the flow was surprisingly fast at over 1.0 metres per second. The flow was so strong that it was not possible to go downstream along the Ci Danau to the exit from the swamp, as the canoes could not be paddled back against this current. Along the Ci Bojong river, the flow was slow near the road at the foot of the hills, at an estimated 0.3 metres per second. At Kampung Tambakan, half way to the main Ci Danau river, the Ci Bojong had cut a gully with precipitous sides about 4 metres deep through the sediments and was running at about 2.0 metres per second. The depth of the gully and the rate of flow show that this not a normal stream in a swamp. At the Kampung Kajaroan, about 1.5km from the exit from the caldera, on the flat surface of the swamp sediments, the Ci Bojong and the Ci Danau meet. There is evidence at the junction of massive soil erosion, with several metres of swamp deposits missing, and soil layers exposed on banks. Both rivers had become broad wide and rather slow flowing at less than 0.5 metres per second, but the volume of water flowing was now many times larger than at any previous site. It appears that additional water has appeared in the Ci Danau between this point and the junction with the Ci Kalumpang. This could come from drainage of ground water from the sediments and from springs. There are reports of several hot springs and other springs in the area of the caldera. 5. Erosion of the original sediments in the caldera near Rancasumur Along the length of the Ci Kalumpang river from Rancasumur to the Ci Danau river, the banks of the stream are eroded. At Rancasumur, the banks are effectively vertical, unstable, and lacking any vegetation cover. Further down this side stream (the Ci Kalumpang), the banks become less steep and are covered with plant growth of annuals, mainly weeds. In places the bank has collapsed. In other places, the stumps and roots of massive forest trees are exposed in the banks of the stream, but usually at some depth below the soil surface. These features suggest that in the past, forest trees, perhaps swamp forest trees, covered much of the land along the banks of the Ci Kalumpang from Rancasumur to the Ci Danau. Later, these trees were destroyed or died, and were buried under sediments. Recent erosion of the stream banks has exposed some of the stumps of these ancient forest trees and some have fallen into the stream. In the case of the erosion on the Ci Kalumpang near Rancasumur, it is likely that this is caused by increased run-off following deforestation of hillsides and by run-off from roads, "kampung" (village) roofs and "pasars" (local markets). Increasing urbanisation of the margins of the caldera are likely to have led to increased erosion of the sediments near the margins of the basin formed by the caldera. The main focus of urbanisation on the floor of the caldera is the complex of Padarincang through which the Ci Kalumpang flows. 6. Soil erosion at the exit from the caldera on the Ci Danau river In the limited areas which have been inspected, massive soil erosion is also apparent at the exit from the caldera and on the Ci Danau river within the caldera for at least several hundred metres upstream from the actual exit, and probably much further. In the case of erosion of the soil at the exit from the caldera, this is most likely to be caused by the increased flow of water from the caldera following the removal of the rock lip. The river banks are at least two metres high in this area and are steep or even undercut. The Ci Danau flows strongly over the stony bed of the river between walls formed by the sediments of the caldera. It is probable that before the removal of the rock lip, there was no comparable river flowing through the sediments. The river has been created as a result of water cutting down through the sediments as it flowed towards the new, lower, exit from the caldera. The creation of this lower river bed has been a crucial feature in allowing increased drainage from the soils of the caldera into the river and its tributaries far upstream. It is necessary to establish exactly how far upstream this drainage has extended. This can be done by exact topographic survey using lasers. It is necessary to establish accurately the height of the bed of the river and of the banks of the river. The drainage system in the sawahs (rice fields) in the south west of the caldera has led to further drainage of the lake sediments. The relatively fast flow of water in the Ci Danau near Kampung Baru may be a result of the increased gradient in the bed of the Ci Danau following this erosional downcutting to form a river bed leading to the exit. This suggests that the effects of the deepening of the exit from the caldera has affected erosion throughout at least half of the caldera.
|
|
|
|||
Table of Contents 
 |
© United
Nations Environment Programme | privacy
policy | terms
and conditions |contacts|support
UNEP | UNEP
sitemap
|