Rainwater Harvesting to Prevent Flooding

According to the lecturer of Engineering Faculty at Gadjah Mada University (Universitas Gadjah Mada/UGM) Dr.Ing.Ir. Agus Maryono, the flood prevention with rain harvesting can be done by several methods. First, the method of harvesting rain with revitalization lakes, and ponds. Second, rain harvesting with landscape modification. Third, rain harvesting by developing groundwater protection area. Fourth, harvesting rain with forest preserve.

Agus explained, the condition of the lake, the lake and there in various places in Indonesia worsened. Carrying capacity are drastically reduced due to sedimentation, the numbers drastically reduced as much covering, and plundered to serve residential areas.

"Methods of rainwater haversting do to revive lakes, and ponds with the eco-hydraulic concept, repair and nourish the entire components of the lake, and the lake there is concerned, so it can serve to accommodate and absorb rainwater to be used for drinking water purposes and replenishment of ground water, and hold rainwater runoff to prevent flooding," he said.

Indonesians wade through flood in Bandung, West Java, Indonesia. (Picture from: http://www.commercialappeal.com/)

To harvest the rain with landscape modification, Agus said, the method is currently being done in many developed countries, such as in Canada, Germany, and Japan. One is to replace the drainage network with the notches in different places or landscape modifications, so that rain water will be collected in the basin location.

"With the way this turned landscape modification can reduce the cost of construction of a regional drainage network. In Indonesia this method has not been developed at all, so the urge to do the study and intensive pilot project," he explained.

Indonesian capital Jakarta hit by deadly flooding at January 2013. (Picture from: http://www.bbc.co.uk/)

Then the third rain harvesting methods that can be used to prevent flooding by developing groundwater protection area. How, government and the community seek a particular region or area specifically designated as an area of ​​rainwater harvesting or rainwater catchment vegetation and guarded diversification should not be built any construction on the area.

"For this purpose have been areas that have a high prescription kepasitas and free from contamination pollutants. Concept is not widely known in Indonesia, each region needs to immediately find a location or area that can be developed into a nature preserve rain water infiltration," said Agus again.

Diagram of rainwater harvesting system. 
(Picture from:  http://www.ppcb.gov.in/)

The fourth method that can be used to prevent the flooding, that harvesting rain with forest preserve. Forests can be used as a component for water harvesting by maintaining forest sustainability. Recent research in the Amazon jungle, Latin America mentions that the actual rain forest can recycle up to 75 percent, and the remaining 25 percent flows to the downstream and seep into the ground.

Recycling mechanism rain began to evapotranspiration, cloud formation and cloud forest in the region is falling back in the form of rain, and so on. Recycling is a mechanism to harvest the forest in the rain. With 75 percent of the water circulating in the rain forest, then the frequency of precipitation in the region is relatively high and regular and relatively long rain season.

Rain with a high frequency of this will not cause flooding because 75 percent evaporates and only 25 percent flows to downstream. Drought will not happen, because 25 percent of the water supply to downstream is obtained almost continuously throughout the year. Seeing the forest functions as a component of rainwater recycling, then to the front of the forest should be regarded as fixed or capital assets, not as capital moves.

"Be aware that the price of timber produced from forests penetrated no more than 7 percent when compared to the price of the integral functions of forests as forests, water storage, flood control, drought control, erosion control, temperature stabilizers, micro and macro ecosystem conservation and oxygen suppliers , "said Agus again. ***

Professor of Hydrology and Water Resources at Bogor Agricultural University (Institut Pertanian Bogor/IPB), Prof. Hidayat Pawitan also revealed that match. The technology to harvest rain in the upstream can reduce or mitigate flooding downstream, which tend to be urban. Moreover, with the development of urban areas often filled with rows of buildings like shophouses or model townhouse that, when they designed the compensation system of water catchment or catchment systems or polders, will result in flooding.

According to him, a measure to assess the feasibility of flooding is runoff coefficient, which states the percentage of rainfall that becomes part of runoff or flooding. For natural forests as the ideal natural conditions, the runoff coefficient is 0.1 percent, or at least to secondary forests coefficient is less than one percent. For agricultural crops or food, the value of this coefficient increased to 0.2 to 0.3, and if the land use multiple single residential home or landed houses to double the number of shops or townhouse that runoff coefficient increased to 0.5 to 0.9.

"The mean normal for urban areas could be 0.7. Just imagine if it rained 100 mm per 24 hours, land cover is filled with houses double as shop, then 70 percent of the rain water will instantly be flooded melimpas. This is equal to 70 mm runoff which is equivalent to a volume of 700 cubic meters of water per hectare or 70 thousand cubic meters per square kilometer, "said Hidayat.

For the Bandung area, he added, the average rainfall of 100 mm per 24 hours also occur every year and in the wet can occur several times per year. "Just imagine for a region you know, whether there are shelters or polder system in such capacity?, And by carrying out the rain and runoff harvesting technology, if its effectiveness could be 50 percent, it reduces flooding, not only the intensity, but also the frequency," said Hidayat again . *** [FEBY SYARIFAH | PIKIRAN RAKYAT 03012013]

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