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 2002. 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
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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 2. 1976 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.
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Figure 3. 1-2002 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
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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 2,000m3/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,
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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 1961. 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 1991. 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
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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 3,900,000m3 in the year 2000.
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
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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 1990.
Assuming that an estimated 100 to 300 million m3 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.
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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.
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