CHANGES OF RUNOFF MECHANISM OF THE BRANTAS RIVER OVER THE PAST 30 YEARS

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1 CHANGES OF RUNOFF MECHANISM OF THE BRANTAS RIVER OVER THE PAST 30 YEARS TOSHIKATSU OMACHI Infrastructure Development Institute KATUMI MUSIAKE Fukushima University The Brantas River in the East Java originates in the mountain ranges of 3,000m above sea level where the sediment yield is very high. In the center of the basin, Mt. Kelud erupts at 15 years intervals supplying large amount of volcanic ash. Aggradations of the riverbed used to be a chronic problem of the river in the flood control and water utilization. However, in the recent three decades, due to check of sediment by reservoirs and excessive extraction of the riverbed materials, violent degradation of riverbed has occurred and this trend is ongoing. These changes resulted in the runoff mechanism of floods in the basin. In this study, change in flood hydrographs has been examined. The storage capacity of the basin halved during the period from 1976 to Although the risk of flooding due to overtopping of levees has been reduced, the measures to stabilize the riverbed must be most urgently taken in order to protect embankments, revetments, bridges, siphons and other structures in the river. Figure 1. The Brantas River Basin 1

2 OUTLINE OF FLOODS Flood in March 1976 In March 1976, the Brantas River experienced its first major flood after the independence. The rain from 9th until 13th of March amounted to approximately 120mm. The flood discharge was measured over the entire duration. Three to four flood peaks formed in the hydrographs of the upstream tended to break down in the downstream. This tendency is illustrated in the gently sloped flood curves shown in the figure-2. In those days, the riverbed was high and floodwater was retarded in various locations such as non-levee channels, confluences of tributaries, and the marshes of Tulungagung and the Widas River basin. The 1976 flood caused extensive damages over the entire basin. Although the Porong River suffered from leakages and cracks due to continued high water, frantic efforts of authorities concerned and local people narrowly prevented the levee from collapsing. Figure Flood Discharge Hydrograph Flood in January 2002 In January 2002, there was a rainfall of about 130mm in the upper reaches and about 100mm in the middle to lower reaches of the Brantas. The rain brought about the largest peak discharge in the post-independence period. The flood inflicted heavy damages on upstream tributaries such as the breach and/or failures of embankments and washing of bridges. In Surabaya, many parts of the city were inundated and the people s daily life sustained major damages. Although the flood hydrographs peaked sharply, the mainstreams of the Brantas and Porong Rivers hardly suffered damages. 2

3 Figure Flood Discharge Hydrograph COMPARISON OF FLOOD DECLINING RATE Changes in the flood runoff mechanism have been verified by using flood declining rates of the 1976 and 2002 floods. The comparison of the rates in %/hr after the flood peaks at the selected stations indicates that the figures of the 2002 flood are almost twice as large as those of the 1976 flood. Since the two-fold increase in the flood declining rate in a single tank model is equivalent to half the basin storage capacity, this illustrates that flood retarding capacity of the basin was reduced by half during this period. Figure 4. Flood Discharge Declining Rate 3

4 Figure 5. Schematic Representation of Reduced Basin Storage Capacity CHANGES OF RIVER CHANNEL The riverbed of the Brantas has continued to lower since the 1980s presumably due to the sediment deposition in upstream dams and sand extraction in the middle and lower reaches. The degradation of the riverbed has led to increased discharge capacity of the channel, but it has brought about a substantial reduction in the retarding capacities of the mainstream, tributaries and the middle and downstream sections of the Widas River. Stage-discharge curves at major observation stations were examined. Figure-6 shows that the water levels corresponding to 2/3 of the design flood discharge fell by 1.8m to 3.4m along the mainstream. Figure 6. Changes of Water Stage at 2/3 of Design Flood Discharge Flood discharge capacities at all these stations except Porong are above 2,000m 3 /sec at the highest water levels of the 1976 flood. This is an indication that the discharge capacities had made a phenomenal expansion. The discharge capacity at Porong close to the mouth of the Porong River, 4

5 however, remained at the level of the design flood discharge despite a notable degradation of the riverbed. CHANGES IN RIVER BASIN Tulungagung Area During the Dutch colonial rule the confluence of the Ngrowo River with the mainstream of the Brantas River was controlled by weirs and floodgates. After the Dutch withdrawal, inadequate maintenance and management of the river facilities combined with the aggradation of the mainstream riverbed caused the Tulungagung area to suffer from chronic inundations. The city of Tulungagung was the center of this area and about 10 km southwest of the city there laid a large marshland which stretched over an area of 3,000ha in the dry season and expanded to 28,000ha during the wet season. To remedy this situation, the Tulungagung South Drainage Tunnel was completed in In consequence, the 28,000 ha marshland where rice could not be cultivated has almost ceased to experience inundations and the 3,000 ha marsh was also reduced to one half in area. The project also benefited the local communities by almost getting rid of malaria hazards in the area, thereby making a significant contribution to the promotion of the health of the residents. Subsequently, the second drainage tunnel was completed in 1984 and the third one was built in Thus while flood retarding functions were reduced considerably, the drainage conditions of the area have improved to a greater extent. Widas River Basin The Widas River is one of the three major tributaries of the Brantas River and the large flatland lying between the Widas and the Brantas functioned as a retarding basin for years. However, the degradation of the mainstream riverbed has contributed to the substantial reduction of flooded area and the frequency of inundations, which has in turn helped to increase the value of agricultural lands in this region. Currently, improvement works on the Widas River are in progress and it is expected that floodwater will reach the mainstream Brantas River even more rapidly. Confluence of Tributaries The upstream levees of tributaries as viewed from the mainstream remained at a somewhat lower level than the downstream levees and the triangle zone formed by the mainstream and tributaries functioned as a retarding basin in times of floods. However, since the mainstream bed has lowered in meters, the retarding function presumably has been reduced substantially. Extraction of River Sand The riverbed of the Brantas is composed mostly of sand and silt, while gravel and cobble stone as bed material are limited to the upstream sections of the mainstream and its tributaries. For this reason, stones for construction purposes are generally expensive in the region. In revetment and retaining wall construction, mortar masonry is in wider use than gabions or stonework to save higher costs of stones. Fine aggregates for mortar production are in greater demand. Sand and silt are also used in larger quantities as fill material in the preparation of housing sites. In the Brantas 5

6 River basin, therefore, readily obtainable river sand has come to account for an important portion of the construction materials required in the region. In or around 1976, river sand was obtained by manual labor and the extraction of river sand was considered favorable, since the riverbed aggradation posed a serious problem at that time. Later, with a rapid rise in the demand for construction materials in the East Java, the quantity of sand extracted continued to increase so sharply that the provincial government had to issue a ban on the sand extraction, but apparently it had less effect on the operations. According to an estimate, in the Brantas River basin area river sand was extracted in an annual quantity of approximately 3,900,000m 3 in the year Obviously, this situation has not changed since then. Forestry Many of the experts concerned ascribed the increasing sediment and flood discharges to illegal deforestation. In Java, notably the Brantas River basin, however, gradual extension of cultivation to the higher possible mountain hillside is not a recent practice. According to information obtained through interviews with parties concerned, illegal felling of trees is centered on such species of high economic value as mahogany and teak. In the Brantas basin, natural forests, if not at all, are very limited in extent. Apart from the mahogany forests in the basin area for which enough information about their actualities is not available, all the teak forests found in the area are artificial forests (plantations), which have existed since the colonial days. Because of their nature, the trees in the plantations are fated to be felled when they have grown enough to have commercial value. During the authors visit to one of the plantation forests, they saw saplings planted in the site of regular deforestation and were informed that these young trees would grow to a certain height in two or three years. For the sake of forest conservation, illegal cutting in teak and rubber plantations, which occupy a substantial area should be controlled and the operators of these plantations should be induced to shift to environmentally low impact operations by such means as thinning and selective cutting. During the site visit, the authors found thick growths of tall slender teaks, which suggested that no adequate care had been extended to them. As a matter of fact, the forests account for 26% (in 1990), a relatively small portion, of the entire area of the Brantas River basin. Consequently, the contribution of the deforestation in this area to the flood occurrence is considered to be relatively limited when compared with that of the drastic changes of the retarding function. In regard to sediment discharge in the Brantas basin, it is predominantly produced by the volcanic activities of such mountains as Mt. Sumeru and Mt. Kelud. Therefore, sediment discharge from the forest may have relatively less effects on the basin. Effects of Mt. Kelud and Mt. Sumeru and Erosion Control The two active volcanoes, Mt. Kelud and Mt. Sumeru, are most characteristic of the Brantas River basin. In particular, Mt. Kelud has erupted at intervals of about 15 years and in each eruption lake water and volcanic ash from the crater ran down the slopes in pyroclastic flows, causing heavy damage in neighboring areas. Large quantities of light and highly mobile volcanic ash reached the mainstream in a relatively short time and induced a riverbed rise. Thus traditionally the aggradation of the riverbed has posed a major problem to the Brantas. It was for these reasons that major improvement works consisting primarily of dredging have been executed in the section 6

7 downstream of the Kediri City on the heels of the improvement projects for the two rivers of Porong and Surabaya. However, as noted earlier, the Brantas riverbed has shown a tendency toward degradation despite the eruption of Mt. Kelud in Assuming that an estimated 100 to 300 million m 3 of material produced by each eruption is deposited in the Brantas River of 200 km in total length and 200 m in width, the riverbed will rise by 2.5 to 7.5 m. Actually, the volcanic eruption of Mt. Kelud in 1990 filled the Wlingi Dam with sediment and the dam reservoir had to be dredged. The Brantas faces the major problem of ensuring the maintenance of its discharge capacity and stabilization of the channel while meeting the active demand for river sand supplies and coping with the hazards of periodical eruptions of Mt. Kelud. On the other hand, it is also a major problem how to reduce direct damages from debris flows in the Brantas River basin. In connection with the brisk volcanic activities of Mt. Kelud and Mt. Sumeru, both characterized by steep topographies, the sediment deposition in upstream reservoirs, such as Sutami (Karangkates) dam, which plays a vital role in the flood control of the Brantas River, is also a major problem demanding early solution. CHANGES OF FLOOD DAMAGE In the preceding section, the floods of March 1976 and January 2000 were compared in hydraulic and hydrological terms and the comparison indicated that floods in the Brantas River basin have shown a new pattern of faster and larger flood runoff. Such changes of flood runoff pattern have been observed in many countries including Japan. In many cases it can be attributed to the urbanization and deforestation of river basins and the conversion of rivers into straight channels and the tendency toward concentrated flood runoffs has led to reduced security against floods. In the Brantas River, however, river channel improvements and the degradation of riverbed due to sand extraction is directly responsible for the change in the flood runoff mechanism. The river is a rare example of phenomena in which flood concentration takes place side by side with an increase in the discharge capacity. In the flood of March 1976, rising flood levels resulting from the aggradation of the riverbeds of the Brantas and Porong combined with their overage embankments and revetments to drive these structures to near collapse as played up in the national newspapers. Collapse of levees was narrowly avoided by enthusiastic efforts of flood-fighting. In the January 2002 flood, on the other hand, the peak discharge was 1.3 to 1.8 times larger than that of the 1976 flood, but the maximum water levels at different points of the Brantas River were lower than those in the 1976 flood and the mainstreams of both the Brantas and Porong suffered almost no damage. This is attributable to a substantial expansion of the effective discharge capacities of both rivers realized through their lowered riverbed as discussed. According to information obtained during the authors visit to the sites and newspaper reports regarding the flood of January 2002, residents in the neighborhoods of Mojokerto and Jombang along the mainstream were concerned about the possible collapse of their nearby embankments and revetments, but these structures escaped damages thanks to the operations of the weir at Menturus and the floodgate at Mlirip. 7

8 Evidently, the flood damages in the Brantas River basin have now shifted from damages due to overtopping and/or collapse of the mainstream embankment to failures of levees in tributaries, debris flows in mountain regions and inundation in urban areas. CONCLUSION The results of the studies can be summarized as follows: (1) The riverbed degradation in the past 30 years was over 3 m in the section between the confluences of the Konto River and the Widas River and over 2 m in other sections of the mainstream. (2) It seems that the degradation was due to the sediment trap at reservoirs and riverbed excavation. (3) The riverbed degradation in the Brantas and Porong rivers increased their flood carrying capacities. (4) The degradation resulted in a substantial reduction in the flood retarding capability of the basin, particularly in the Tulungagung area and the Widas River basin. (5) As a consequence, the basin storage capacity of the Brantas River basin has almost halved in the past 30 years, which resulted in a faster and higher runoff peak. (6) For these reasons, the risk of overtopping of levee has been reduced, but the risk of damage to the embankments, revetments, bridges, weirs, siphons and other structures in/across the rivers is increased substantially. Hence the stabilization of the riverbed is an urgent necessity in the Brantas River. ACKNOWLEDGEMENT The Perusahaan Umum Jasa Tirta provided the hydrological data used in this study to the authors during their field surveys. The authors express their deep appreciation to the Perusahaan Umum Jasa Tirta for providing valuable data and also to the Japan Bank for International Cooperation for giving the opportunity to visit the sites. 8