Controlling floods in Colombo city through rain water harvesting


By Dr Tanuja Ariyananda
Lanka Rain Water Harvesting Forum ( )

Floods are regarded as number one natural disaster in Sri Lanka, mainly due to excessive rainfall during monsoons. The highest rainfall of 805 mm within 24 hours was experienced in 1897 and a great flood submerged the capital of Colombo on 5th June 1992 due to a rainfall of 494mm. The parliament was submerged in water and Parliamentary officials said the cost of the damage caused by the flooding was over Rs. 200 million (1.8 million U.S. dollars), with carpets, computers and furniture all being damaged. In addition flood- waters destroyed power and communication lines and this in turn, disrupted road transport and communications and seriously hampered rescue and relief efforts. More recently on the Thursday 11th of November 2010, the city recorded 440 millimetres of rain which is the second highest rainfall since the 492 millimeters recorded in June 1992. .

Rapid expansion of city during the last four decades ( According to Dept. of Census and Statistics, the number of households recorded in Colombo district has increased from 272, 489 in 1981 to 555,926 in 2011) has led to an increase in the water- impermeable surface due to buildings , asphalt and roads. Filling of marshy land where flood water was retained, and encroachment of canal banks by unauthorized constructions, obstruction of the drains by dumping of waste have aggravated poor drainage. All these increased the speed and volume of storm water run-off.

The Colombo municipality has an area of 37 sq km. i.e 37 million sq. m (1 sq km is equal to 1 million sq. meters). A rain fall of 440 mm, as experienced in Nov. 2010,will generate 16,280,000 m3 of run off water. It is estimated that 60% of the rain falling on Colombo municipality area goes as run off. This would amount to a volume of 9,768,000 m3 from a 440 mm of rainfall. If our existing drainage system and retention system were able to cope with 40% of run off volume, then we need to deal with only 20% of the run off, which amounts to a volume of 3,256,000 m3, to prevent flooding.

On- site detention of run-off water, is a common feature of modern urban drainage system adopted by some countries, to cope with under capacity street drainage and for flood and erosion control. Rainwater harvesting tanks though primarily used for water supply can be used for on-site detention . It is reported that there are more than 100,000 buildings with in Colombo municipality, which house schools, hospitals, commercial, industrial institutes. Most of these buildings would have large roof areas, much more than 100 m2. The volume of water run off from roof area of 100 m2 from a rain fall of 440 mm is 35 cubic meters. (Run-off roof water = rain fall x roof area x run off co -efficient

= 0.44 m x 100 x 0.8

= 35 m3

(Note; run off co-efficienct value for different roof types varies between 0.9-.07)

If only 20 m3 volume of water is collected from these building this would amount to

= 20 m3 x 100,000

= 2, 000,000 m3

According to Dept. of Census and Statistics in the year 2011 there are 555,926 households in Colombo District. If 10% of the households who would be having roof area more than 100 m2 collect at least 5m3 per household then it would be possible to retain 277, 960 m3 (5 m3 x 55,592 ) of water. This would be 8.5 % of the flood water from 440 mm rainfall

If all the large buildings and 10% of the households in Colombo retain/collect 20 m3 and 5 m3 of water respectively, it would reduce the flooding caused by unusual large rain fall of 440mm to 70%. In addition, the stored water can be used as a water source for washing, toilet flushing, gardening etc.

Cost: To store 1 m3 ( cubic metre ) of water, it would cost approximately Rs. 10,000 (for large volumes it would be less). Therefore, the total cost would be approximately Rs. 22,779,600,000

( 2,000,000 m3 + 277,960 m3) x Rs. 10,000

If the Colombo municipality/UDA is to subsidize 10% of this cost then it would cost the municipality Rs. 2,277, million. ( US$ 17, milion). This is a small proportion of the US$ 223 million provided by The International Bank for Reconstruction and Development (IBRD) of the World Bank to stop flooding and improve storm water drainage in the capital Colombo. (

The present legislation on UDA drainage law ( Gazette 1597/8 17th April 2009) states that all new building above a certain roof area should incorporate rain water harvesting systems in their plan, and the volume of water collected will depend on the building category, roof area and the rain fall band of the location. For example an Institutional building in Colombo district is required to retain/recharge 10 m3 from each 100m2 of roof area. The old buildings are to adopt this 3years after the passing of the regulation which was to be in April 2012.

Other methods that can be adopted to retain and reduce storm water run off are

1.Introduce permeable paving in public car parks, pavements, walkways etc.( this could be easily adopted since UDA is on a drive to beautify Colombo by paving the pavements)

2.Create and cleaning existing retaining ponds at different levels in the city

3.Introduce swales and porous drains to increase infiltration

4.Promote ground water recharging

5.Use of play grounds, parks, car parks as infiltration enhancement facilities


Example from other countries

Japan: In the Kokugikan area located at Ryogoku within the Sumida city, an enormous Kanto earthquake occurred on the 1st of September, 1923. Forty thousand residents of Sumida City were burned to death. The water supply systems were also heavily dam aged. In response, a rainwater harvesting project was started in Kokugikan. Innovative rainwater projects were undertaken for flood control, water shortage mitigation and disaster /prevention 23 years ago. In 1982, the City directed the Japanese Sumo Association to stock rainwater for mitigating urban flooding, and to use stocke d rainwater for city water conserva tion and emergency water supply. This was done through harvesting rainwater falling on the roof (8400 m2) into a base ment tank with a capacity of 1000 m3, this tank functions as a life point. The water is currently used for flushing toilets, cooling towers, and irrigating plants. At the community level, a unique rainwater utilization facility, "Rojison" has been set up by local residents. In this system, 72 rainwater facilities have been combined with a capacity of 10,000 m3. To date, 50 local governments have a guideline for rain water utilization, while the other 36 have a subsidy system for installing rainwater tanks.

Korea: The Seoul Metropolitan Government (SMG) is a 600 year old city with a population of 12 million. Studies of rainfall trends over the last five to thirty years depict a recent increase in amounts. Rainfall is concentrated in the summer months with higher intensities. The traditional approach for coping with urban flooding has been "Linear". This means that all the rainwater that has fallen over a wide area is conveyed to and managed by sewers or rivers by upsizing of the sewer, widening of the rivers or increasing the pumping capacity. Due to heavier and unpredictable rainfall, the water conveyance system in the whole city is limited, thus requiring upgrading. However, this costs a lot of money and time .A new "Areal" approach of managing rainwater has been tried by storing rainwater in small but numerous tanks, peak runoff is reduced while the time to peak flow is retarded. After the rainfall, the relatively clean water stored in the tanks is used for non-drinking purposes resulting in water conservation.

SMG recently designed special mandates that requires a rainwater harvesting system for newly constructed buildings larger than 5000 m2 and existing official buildings larger than 3000 m2. The water level of each tank is monitored at a disaster prevention center through the internet. The public can have access to the information at any time and place. Such information can be transferred to the mobile phone, local station and central station of the people who control the system. The building owner is instructed to empty the tanks depending on the weather forecast. Financial incentives or punishments are given to the building owner who does or does not obey the rule.

Climate Change: Recent climate change prediction studies have indicated that Sri Lanka will experience high variability of rain fall. Extreme weather events are becoming more frequent and intense. In order to cope with the urban flooding caused by climate change, it is necessary to shift the rainwater management para digm, from the traditional "Linear" approach to the new "Areal" approach. Monitoring and management of multi ple rainwater tanks in a whole city area can be coupled with modern IT and used as a powerful solution to mitigate the damage caused by urban flooding.

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