Definition, Causes and Control of Floods

Definition of Flood

Indonesia is a country that has quite high rainfall. During the rainy season, almost all regions in Indonesia receive high intensity rain. The rainy season usually lasts for about 4 months. This situation is actually beneficial because droughts rarely occur in Indonesia. Water is very important in life, both as a consumption material and for agricultural irrigation.

According to Chow (1956), floods are relatively high flows that exceed the capacity of natural channels for water flow. A similar opinion was also expressed by Rostvedt et al. (1968), that floods are high river flows that exceed natural or artificial river banks. Ward (1978) also states that a flood is an overflow of water that fills land that is not usually submerged.

Based on BNPB data, floods were the most common natural disaster in Indonesia in 2020, reaching 36.5% of the total disasters. Apart from that, there were also tornadoes (29.80%), landslides (19.60%), and forest and land fires (11%). The four provinces with the highest number of disaster incidents were West Java (623 incidents), Central Java (522 incidents), East Java (408 incidents), and Aceh (264 incidents).

Specifically for floods, there are three types. First, when the river body is unable to accommodate the water so it overflows. Second, high rainfall in upstream areas has an impact on the city below. Third, tidal floods or sea water entering land.

One of the important findings from this research is that the majority of respondents use groundwater to meet daily needs, reaching 79.70%. Only 19.70% of respondents used water from PDAM. Water needs are met through deep groundwater (48.60%) and shallow groundwater (31.40%) (Nuswantoro, 2021).

Floods in Indonesia (Source : Liputan6.com)

Rapid development, especially in urban areas, has resulted in a reduction in open land. The construction of office buildings, shopping centers, residential areas and parking areas, all of which use concrete as a construction material, results in reduced absorption of rainwater into the ground. Apart from that, the lack of green open space also makes the water absorption capacity of the area poor. Throwing rubbish into waterways is also a factor that worsens the condition of waterways. With less rainwater being absorbed by the ground and water flow being obstructed, flooding occurs. Floods in Indonesia are often considered a common disaster. People are no longer surprised by floods, especially in densely populated urban areas.

Flood management and control must be seen from various aspects simultaneously and comprehensively, considering the complexity of the problems related to flood management. Flood management plans in river basins must seek optimal solutions and serve as a reference for the various parties involved. In addition, the plan must be supported by regional regulations. However, it should also be noted that limited funds for development are something that needs to be considered in its implementation.

Causes of Floods

Supporting Factors for Flood Intensity

The actual causes of flooding are not only related to extreme rainfall and weather factors. There are several other factors that contribute, such as high water discharge in upstream areas, reduced capacity of reservoirs and lakes as flood water reservoirs. Apart from that, narrowing and shallowing of rivers due to sedimentation and rubbish, tidal waves due to sea tides, and land subsidence (subsidence) which increases the risk of waterlogging are also factors that cause flooding (Chandra and Supriharjo, 2013). However, extreme rainfall remains the main cause of flooding. All parties and the public are advised to remain alert to the potential for high rainfall, especially because the peak rainy season is estimated to occur from February to March. Apart from that, there is still the possibility of the phenomenon of equatorial atmospheric waves or Madden-Julian Oscillation (MJO) and cold surges as climate variations this rainy season. The Meteorology, Climatology and Geophysics Agency (BMKG) defines the peak of the rainy season as the period where the accumulated rainfall reaches the highest amount in one basic period for each season zone.

Flood Control

Flood control is basically a complex matter. In the engineering aspect, this involves various engineering disciplines, such as hydrology, hydraulics, watershed erosion, river engineering, river sedimentation morphology, flood control system engineering, urban drainage systems, and water structures. However, the success of a flood control program also depends greatly on other aspects involving social, economic, environmental, institutional, legal, and so on. The political aspect is also important, often the most crucial factor. Strong political support from various agencies, both the executive government, legislative (DPR/DPRD), and judiciary, will greatly influence solutions for flood management in urban areas.

Basically, flood control activities include various activities as follows:

a. Structure

The following are several ways to control floods using structural methods:

1. Build artificial water storage retention buildings to accommodate and control excess rainwater flow.
2. Build sediment control structures upstream of rivers, such as sabo dams/sand pockets, to prevent sediment from entering the reservoir, increase water absorption into the ground, protect the dam from damage, and prevent postal flooding.
3. Strengthen river banks by using rivetments (river bank protection) to reduce cliff damage, such as landslides, which can damage vegetation along the river flow. Build reservoirs and dams to regulate the flow of water through rivers.
4. Create a weir to monitor and regulate the volume of water flowing through the river, thereby preventing the flow of water at high volumes and speeds in downstream areas.
5. Build embankments (berms) on the left and right sides of the river to reduce the risk of river water runoff into the surrounding area when the river water level reaches its maximum point.
6. Implementing a polder system, namely a closed water management system in an area with elements such as embankments, pumps, water channels and retention ponds.
7. The polder system allows temporary storage of rainwater in a separate area from the main drainage, then the water is pumped out when the intensity of the rain decreases and the main drainage channel returns to normal.
8. Building groundsills, namely buildings made on river beds to stabilize the slope of the river and reduce the speed of water flow and maintain the strengthening of the river bed.
9. Create a canal as an open channel to accommodate water discharge from residents’ drainage along the canal.
10. Carrying out normalization by increasing the cross-sectional size of the drainage channel (redesign) and dredging the river bed (dredging).
11. Establish flood disaster detection posts in several river basins that have the potential to experience flooding.
12. This can be strengthened with an Information Technology (IT) based system which includes early warning alarms and dissemination of information via social media.

By implementing these structural methods, it is hoped that the risk and impact of flooding can be significantly reduced.

b. Non-structural

The following are several ways to control floods without using control structures that can have a positive influence on river regimes:

1. Pay attention to the preservation of forests and vegetation in upstream areas and along river flows to maintain the condition of water absorption areas and reduce the speed of river flow from upstream.
2. Conduct outreach to the public about the importance of awareness of environmental cleanliness, especially the cleanliness of drainage and rivers.
3. Building organic and inorganic waste disposal facilities in the community’s residential environment, as well as socializing the importance of its application and benefits.
4. Provide strict sanctions for parties who damage the environment.
5. Implement strict regulations to limit the construction of high-rise buildings in dense urban areas and prohibit construction in water catchment areas or green open spaces (RTH).
6. Regulate illegal settlements along river banks so that the cross-sectional capacity of the river is not disturbed.

River Basin Management (DPS) is closely related to regulations, implementation and training. Land use activities in DPS management aim to save and store water and carry out soil conservation. Some activities in DPS management include:

1. Maintenance of vegetation in the upstream part of the DPS.
2. Planting vegetation to control water flow speed and soil erosion.
3. Maintaining natural vegetation or planting water-resistant vegetation along drainage embankments,
4. channels, and other areas to control excessive flow or soil erosion.
5. Special arrangements for flood control structures, such as check dams, along streambeds that are easily eroded.
6. Special management to anticipate sediment flows resulting from volcanic activity.

The main target of DPS management activities is to achieve the following conditions:

1. Reducing flood discharge in downstream areas.
2. Reduces soil erosion and sediment load in rivers.
3. Increasing agricultural production through structuring land use and water protection.
4. Improve environmental conditions in DPS areas and along rivers.

Source :

Book of Teknik Dan Pengelolaan Sumber Daya Air

Writer : Dra. Fauzia Mulyawati, ST, MT; Dr. Agung Wahyudi Biantoro, ST, MT.