Controlling floods in Colombo city through rain water harvesting

Controlling floods in Colombo city through rain water harvesting

Eng. M. Chandrasekera, CEng FIE(Sri Lanka), MICE(UK)

Lusail City, Qatar

25^th January 2013

I find the above Article which appeared in The Island on January 4, 2013, written by Dr Tanuja Ariyananda, Director of the Lanka Rain Water Harvesting Forum (www.lankarainwater.org) is thought provoking.

Rainwater harvesting by itself is a good concept especially as a low cost, viable option for obtaining safe drinking water for peasants in the arid zones of the country.

In urban areas too harvested rainwater could be used to cut down the cost of pipe borne water by those who occupy significant building spaces/paved areas.

However, the Writer’s concept needs rationalization/enrichment if it is to be considered as a flood control measure in Colombo CMC area.

According to ‘buildsrilanka’ Web Site of the UDA, the Land use Distribution in the CMC area is as follows.

Land Use Distribution (1977 – 2010)

Activity	Extent Ha.
	Year 1977	Year1996	Year 2010
Residential	1,687	1402	780
Commercial	201	225	1052
Institutional	465	411	238
Industrial	151	149	-
Transport	610	650	700
Port Related	-	-	506
Open Spaces	142	262	355
Water	98	98	98
Others	375	532	-
Total	3729	3729	3729

Source:            1977 - 1985 City of Colombo Development Plan
1996 - Land Use Survey, 1996 UDA,  2010 -  Projected by UDA

From the above table it is evident that out of the total extent of 3729 ha of the CMC controlled area, about 2000 ha is occupied by any form of buildings.

While the 60% value the Writer has given for the run off factor  is applicable for non built up areas, the runoff factor for the built up areas will be  much higher (in excess of 90%).

Further, there need to be more justification for using of 440 mm of rainfall for the calculation

However, as we have observed rainfalls in excess of 400 mm rainfall in Colombo on two occasions in the recent past (1992 and 2010) and also another event of 200 mm of rainfall occurred in 2005, let us consider 400 mm as the threshold rainfall value for the sake of argument.

With  a 400 mm rainfall, the two land use categories (open/non – built up and the built up) will produce approximately the following run off volumes for a rainfall of 0.400 mm falling within a short duration (say, within 6 – 12 hours assuming that there is no outflow of any water during this short interval).

1. For open land:       (3729 – 2000) ha x 10000 x 0.4 x 0.6 = 4,000,000 cubic meter
2. For built up land/buildings: 2000 ha x 10000 x 0.4 x 0.9 = 7,000,000 cubic meter

Out of the above values, it is the second category which contributes to flooding of the roads/drains due to quicker release of water, from the developed properties.

It is very clear from above that for ‘the harvested rain’ to cause any meaningful reduction of floods, the volume of water created by the second category above should be brought (as far as practicable) towards zero.

The water thus retained could be used for non- drinking purposes as suggested by the Writer or be slowly released to the public drains during the non rainy times.

The suggestion of the Writer is obviously very good but how practical it is?

Could the CMC and the UDA make it mandatory for the property owners to provide in-house retention tanks (on their roof tops, basements or in their compounds) and also ensure that the tanks are kept empty ahead of the rainy seasons?

What incentives will have to be given, what regulation should be brought in, to compel the property owners to fall in line with this requirement?

As a start, CMC/UDA may have to include it in the Planning Regulation for all future Property development by including a clause such as:

 For each square meter of the space covered by buildings and for each square meter of the paved areas in the compounds, a rain water retention volume of 0.4 cubic meters shall be provided. The roof tops and the paved areas shall be sloped to ensure collection of rainwater into the Retention Tank.

The Retention tank shall (preferably by gravity or by electric pump) should be kept emptied before the onset of rainy periods in accordance with the Weather forecasts of the Meteorology Department.

Those who violate this Regulation shall be liable for a cash fine/drainage levy of Rs. ………. or shall be liable for punishment by the ……. Courts.

We are aware that currently a very ambitious Project is under implementation with the money borrowed from the World Bank and driven by the enthusiasm of the Defense Secretary to alleviate the flooding problem in the CMC area.

However, to my knowledge, that Project does not include the restoration of the detention areas which were present in the CMC area during the pre 1980s.

No Drainage Project will succeed in Colombo without restoration of the temporary detention space due to following limitations in the Drainage system in Colombo:

1. Terrain in the flooding parts of Colombo is very flat.
2. They are all within just 2 to 3 meters above the mean sea level
3. There are only two drainage streams to drain the entire area: Kittnpahuwa ela in the north and Wellawatta canal on the south. Both have very flat gradients and their bed levels are about the same /or less than the Mean sea level.
4. Flow in Kittanpahuwa ela is also subject to variations depending on the water level in the Kelani River.

In view of above, the Writer’s proposal (although needs a long term sustained effort), has much merit for consideration since it is about the creation of an ‘at source temporary detention’ option.

Let us invite more contributions from the knowledgeable Engineers on this subject.

Eng. M. Chandrasekera, CEng FIE(Sri Lanka), MICE(UK)

Lusail City, Qatar

25^th January 2013