YOGYAKARTA - Flood is a natural phenomenon that often occurs and faced by all countries in the world. This phenomenon occurs due to water bodies or rivers that cannot accommodate water so that the water overflows to the surrounding area. In recent years, flood tends to increase with higher intensity and magnitude of flooding also increases.
In Semarang city, flooding is not only caused by water overflow from drainage channels due to high rainfall but also affected by flash flood.
Lecturer of Department of Geography, Semarang State University, Purwadi Suhandini, said the flash flood has occurred several times in Semarang. Based on research, flash flood has ravaged Garang Watershed for five times, namely in 1963, 1990, 2000, 2002 and 2008. It is possible that another flash flood will occur in the future along with increasing rainfall in the upstream of Garang watershed. "It is unfortunate that flash flood has not been studied and addressed by the local government," Purwadi said defending his dissertation in doctoral exam at Faculty of Geography, Saturday (31/12).
From his research, the main factor causing flash floods in the Garang watershed is rainfall. Meanwhile, change in land use had no significant effect to the flood. According to Purwadi, big flood and very big flood occur when rainfall is high in the Upper and Middle Garang watershed in the form of very heavy rainfall exceeding 100 mm/ day, which occurs at the same time. He noted about 90% of flash floods in the Garang watershed is in form of small flash flood and medium flash flood, while the other 10% is big and very big flash floods. "The flash flood in the Garang watershed tends to be more dangerous because peak discharge tends to increase while time to reach the flash tends to be shorter," he said.
According to Purwadi, the current capacity of the dike is still able to accommodate peak flash flood discharge of 15-year period, but not able to accommodate peak discharge of a 50-year-period which is very big.
To anticipate big and very big flash floods, he suggested the government to create a flood control dam in Kreo River and Kripik River. It is because significant rainfall affects flash flood in Garang watershed. He also suggested that the government increases the capacity of river channel downstream because now it is only able to cope with a 15-year period flash flood. "The increase in channel capacity can be done with the elevation of the flood levee of at least 15-25 cm," he said.
No less important, he added, the government needs to disseminate early warning model, especially for flash flood, because it is very big and catastrophic.