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<Technology Needs for Lake Management in Indonesia - Investigation of Rawa Danau and Rawa Pening, Java> M. Available Technology which chould be Employed in Rehabilitation of the Lake 1. Construction of check dams There is inadequate information at present to enable any decision to be made on the suitability of particular river entrances in Rawa Pening for the construction of check dams. This would call for two sets of data. First, there is a need for information on the silt and bed-load carried by each of the major rivers and streams entering Rawa Pening. This should be in the form of monthly data and should be related to water flow in the appropriate rivers. This information could be obtained by the use of small sediment traps, water level indicators and automatic recorders. Secondly, there is a need for detailed large scale maps of the entrances of each of the major rivers entering Rawa Pening. A check dam will only be cost effective if the topography is appropriate for easy and cheap construction. Equally, a check dam will only be effective if it is large enough to retain an appropriate quantity of silt. This is determined by the nature of floods coming down the river, the volume of water per second in a flood, the potential volume of the check dam, the average residence time of water in the check dam, the bed load of the river and the volume of silt and clay in suspension. It would be expected that, in the area of Rawa Pening, most of the silt, clay, and bed-load would come downstream during flood episodes, that the proportion of bed-load would be substantial, that floods would be sudden, carrying a large volume of silt and clay, and involve a large volume of water in a short period of time. This appears to be the probable situation following the large-scale development of tourist facilities, agricultural production and associated de-forestation on steep hillsides of unstable volcanic rock and unconsolidated tufa. This is supported by the presence of large basalt boulders in the stream beds of rivers along the edge of Rawa Pening and particularly at points where even relatively small streams cross below the road from Ambarawa to Salatiga on the South side of the Lake. The catchment area round Rawa Pening is noted on land use maps and land capability maps as being liable to extreme erosion hazard or severe erosion hazard. Thus it would be expected that, while check dams might be desirable on principle, they would only be cost effective in terms of retention of sediment, in a few of the rivers flowing into the lake, where the local topography was particularly suitable for the construction of large check dams. The question of the cost effectiveness of check dams depends on the benefits expected to be obtained from their construction. In this case, it is not clear what value could be put on the benefit from construction of the check dams. This follows from the fact that no person or organisation pays for the water from Rawa Pening. The water is seen as a free resource. If reduction of sediment input to Rawa Pening is advocated by any organisation, or if removal of existing deposits from Rawa Pening is advocated by any organisation, then that organisation should be in a position to produce data showing the benefit to be obtained from the check dam relative to the cost of construction of the check dam. Such data would be likely to be in financial terms. Such data does not exist at present as far as can be determined, yet suggestions are made that check dams should be constructed on incoming rivers. Technological information required for construction of check dams is as follows. Large scale mapping of river entrances is essential. Monthly records using automatic recorders of river flows, monthly data on sediment load in each river flowing into the lake, monthly assessment of bed load and average size of bed load, assessment of average flood flows, and structure of floods in time, crest height in floods. Some of the assessments of flood properties could not be obtained by automatic recorders but have to obtained by staff in the field during floods. Compilation of the data to produce a Cost/Benefit analysis would be difficult in the absence of any figures for the actual value of the water in the lake. A figure could be substituted using the value, for instance, of power produced per cubic metre by PLN. Similarly a value could be substituted using the value of rice produced per cubic metre of water used in irrigation. It would be still more difficult to nominate a value for water used in recreation and fisheries. In another similar river system in East Java, investigation revealed a situation where 30 check dams existed on one river system. Of these, at the time the system was examined, about 19 were filled with sediment and non-functional, while 11 were functional. In each year about 4 of the dams were emptied and sediment removed. The managers suggested that more check dams were desirable, but could produce no data to support this conclusion. The suggestion that it might be more effective to spend the available money in clearing sediment from more of the existing dams had apparently not been considered. In Java at present there appears to be a belief that check dams are a "Good Thing", and should be constructed whenever possible. A well established cost-benefit basis for construction of check dams should be formulated. Admittedly, in cases such as Rawa Pening where the benefits of the water storage are not recognised or assessed in financial terms, this becomes more difficult. 2. Possible aeration of the lake centr This possibility might arise if there were any risk of de-oxygenation of all or part of the lake. This is a shallow lake, with a relatively large surface area relative to its volume. Nevertheless a problem could develop if the input of organic material were sufficiently large to support such numbers of bacteria that the oxygen consumption by the bacteria on the bottom of the lake exceeded the oxygen available in solution in the water. The rate of oxygen availability (speed of solution of oxygen at the surface) depends on the surface turbulence of the water and the temperature. Surface turbulence depends on the average wind speed, the fetch, the proximity of bottom irregularities in shallow water, the rate of circulation of the water, and the surrounding land topography. The warmer the water the lower the quantity of oxygen which can be dissolved in the water. The rate at which organic matter accumulates in the lake is another crucial factor. The possibility has been raised earlier that this lake has such a high input of organic matter, in addition to the organic matter which formed the bottom of the original swamp, that there is a chance that the lake could become wholly or partly de-oxygenated. The organic matter comes into the lake from towns such as Ambarawa and Salatiga, in the form of domestic waste, run-off from streets and street markets, run-off from padi fields, organic matter carried in floods from hills, and ash and charred organic matter from fires on hillsides. The input of fish-food as bait for fish appears to be excessive, and is considered to be a potential source of problems. An unknown quantity of organic matter is deposited from mats of water hyacinth, floating on the surface. Other unquantified sources of organic matter are the beds of Hydrilla and the other littoral vegetation growing around the lake. The main source of oxygen for the water in the lake is diffusion from the surface of the lake where oxygen dissolves in the water. The rate of solution is increased by surface turbulence. A large but variable proportion of the surface of the lake is covered by floating mats of water hyacinth. Estimates have been made that between 40% and 60% of the lake may be covered by mats of water hyacinth at any one time. It would be very desirable to have further details of the nature of the movement of the water hyacinth mats over the lake surface in relation to the direction and strength of the wind. It is very desirable to investigate in what way the fishing platforms follow the movement of the mats. This data would be best obtained by regular monthly aerial surveys of the lake with aerial photographs. It is not clear whether satellite imagery would be able to provide this information. It is recorded that water under these mats may be completely de-oxygenated, even near the surface. Thus one can conclude that even under existing conditions a substantial proportion of the water in the lake may be de-oxygenated at any one time. This proportion might be thought to be at least 25%. The effect of large scale or total de-oxygenation of the water in the lake would be generally disastrous as it would become cumulative and a long time would be required to restore the original oxygenated conditions. Animals in general cannot live without oxygen. If de-oxygenation became general, fish would die in large quantities, and their bodies would add greatly to the amount of organic matter available for decomposition by anaerobic bacteria. Along with fish, organisms such as crustacea, insects, and protozoa would be killed by the lack of oxygen. These would form an enormous biomass of animal protein suitable for decomposition by anaerobic bacteria. Animal protein contains relatively large quantities of sulphur compounds. Under anaerobic conditions, these sulphur compounds are decomposed by anaerobic bacteria to form mercaptans and sulphuretted hydrogen. These are toxic gases, and water containing them is quite unusable for domestic purposes or for drinking by stock. Such water released from a dam into a river would be toxic and would kill fish and stock drinking from the river. The natural fauna of the river would be wiped out and would take months or years to recover. It may become necessary or desirable to aerate the water in parts of Rawa Pening artificially using pumps which force air through pipes to the bottom. Aeration of the water is achieved by diffusion from the air bubbles rising to the surface, and also by creating an upward movement of the water from near the bottom, where any tendency for de-oxygenation is likely to be maximal. It is worth investigating the effectiveness of a small aeration apparatus, based on bubbling air from a plastic pipe or similar device near the bottom of the lake. The effects of such a limited experiment on water quality in the lake could be monitored by students from Universitas Kristen Satya Wacana. Recently, work has been undertaken near Bogor by staff from the Institut Limnologi on aeration of a small lake (Bojongsari) near Bogor (Hartoto and Sulastri, 1990). It would not be relevant at this stage to attempt to aerate the whole of Rawa Pening, but investigations should be undertaken to assess the efficiency of different methods of aeration in the Rawa Pening environment. The actual type of apparatus used to drive air through the water could have very different effects in different areas depending on the degree to which layers of organic matter on the bottom of the lake were stirred up. Every effort should be made to use, and if necessary design, an apparatus for aeration which caused minimal stirring of the bottom deposits. The rate of consumption of oxygen in the lake will be greatly increased if the bottom deposits, which are anaerobic, are stirred into suspension in the lower layers of the lake water. The effectiveness of this type of equipment has to be checked in operation on site. Measurements of oxygen concentration in the water in the lake have to be taken at regular intervals from a number of marked sites at known distances from the aeration equipment. It is necessary to assess the extent of aeration in the water, the depth of the aerated water, and the extent to which the aerated water spreads out from the point of aeration. The objective in the first instance should be to ensure that a substantial volume of water in the lake will remain aerated, irrespective of weather conditions, and irrespective of any increase in the input of organic matter to the lake. If successful and leading to an improvement in water quality over a period of several months, this could become a standard policy. If introduced as a normal policy, then several such pieces of equipment should be used and maintained in good working order. There is at present no organisation with funds for this purpose. The major problem in implementation of this or any similar technology aimed at water quality management is that there is no organisation at present which acknowledges responsibility for water quality management of the water, or indeed which accepts any responsibility for management of any aspect of the lake. 3. Use of suction dredges to remove sediment The use of dredges has been proposed in order to remove sediment from the lake. There is some concern that the large quantities of silt, clay, sand and gravel carried into the lake may reduce significantly the volume of water which can be stored in the lake. This is particularly relevant in view of the increased building activity in the catchment and the general absence of any attempt to control soil loss or restrict erosion from construction sites. Mechanical dredging is unlikely to be successful as a large area of the bottom of the lake is covered with sub-fossil tree stumps and branches from the original swamp forest which existed in the basin for centuries before the construction of the dam. A suction dredge might be effective if appropriate measures were taken to limit the size of material which could pass through the pipe from the apparatus. The problem with a suction dredge is the disposal of the material removed from the lake. It would seem likely that the material would have to be disposed of in the valley of the Tuntang river. Unfortunately this river has a narrow, steep gorge immediately below the dam and there appears to be no obviously suitable land area on which sediment could be deposited for drying out and subsequent removal. This sediment could have commercial value if it were available, as there is a commercial operation involved in the removal of the rich organic sediment from parts of the lake bed exposed during the dry season. This material is mixed with limestone and sold as fertiliser for use on crops and on gardens. 4. Extraction of organic (peat) soils from Rawa Pening A small industry exists at present, removing the organic soils from parts of Rawa Pening during the dry season, when the water level is low. The soil is dried, mixed with limestone and sold for fertiliser. About 20 people are employed in this industry by one company, PT. Siti Rawa, near Ambarawa. The soil is dug out by hand by labourers and rowed to the shore in small flat-bottomed wooden boats. On the shore it is removed by shovel from the boats and dumped on the shore. From the dumps the material is shovelled into trucks and taken to a large drying area where the material is spread on a flat concrete surface to dry. After drying it is mixed with limestone and the mixture is put in bags and sold. Obviously this is quantitatively inefficient. The material is handled by shovels at least four times. This is only feasible in view of the low wages for manual workers in Java. One truck carries 5 cubic metres of wet soil, where 1 cubic metre weighs 1 tonne. After drying, the 5 cubic metres become 2 cubic metres of peat. The total truck load of 5 cubic metres then weighs 2 tonnes. Each day during the dry season, the trucks make an average of 1 trip. Thus 50 cubic metres of soil is extracted by this company alone each year. The bags when sold weigh 50kg, and are sold for Rp 10,000 each. The bags are trucked to the Dieng Plateau area (about 100km distant) for sale to mushroom growers, to the Department of Forestry for nursery cultivation of seedlings, and to various flower growers in West Java and Jakarta (distances of over 300km). A licence for this operation is issued by the Department of Mines in Salatiga. The company pays tax to the Governor of the Province (at the rate, it is claimed, of 6% of the overall profit). Since the tax paid in 1995 was Rp 80,000 this suggests that the profit which is made is about 1.3 million rupiah. This represents a surprisingly low profit level for the operation, considering the capital investment in trucks and boats involved. 5. Potential mechanical harvesting of water hyacinth plants A mechanical harvester is available on the lake to facilitate removal of water hyacinth from the lake, if required. This machine is operated, if required, by the Department of Public Works. In discussion it appears that the machine is expensive to operate and the Water Resources section of Public Works does not have enough funds to justify operation of the machine. The machine is really a large mechanical chain grab operated from a barge with a substantial engine. The efficiency of the equipment is not high, as it can only clear a limited amount of water weed in the course of one day. To be effective, several such machines would be needed on the lake and all would have to be operational at one time. If 5 such machines were operating for 5 days each week throughout the year, then their collective impact on the water hyacinth population might be significant. Since, at present, there is no measurable economic value in removal of water hyacinth from the lake, there seems little point in having such a machine on the lake.
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