Senior Engineers of the Irrigation Department have made recommendations over many decades to limit the damage caused to life, property and crops by floods occurring over the more vulnerable areas of the Island. Most recently, retired Senior Deputy Director Eng Anton Nanayakkara has written to the Press (SUNDAY TIMES of 2 January 2011) on the subject of flooding in Colombo. He has pointed out that the excavation of “lakes” to retain flood waters would be a “national crime” as these would not be a viable solution for the minimisation of the inundation of low-lying lands in the Colombo region. His views, and those of his predecessors on similar lines, have fallen on deaf ears because it is only short-gestation-period projects (such as the construction of the proposed lakes that could be fitted into the 5-year period between Parliamentary elections) which are attractive to Ministers and MPs alike. Politicians look for quick rewards – of various kinds – and one or more of them seem to have latched on, after the last floods in the Colombo area, to the profitable idea of excavating several lakes, ostensibly to hold back some of the runoff which would otherwise build up into floods.
Apart from stating bluntly that these lakes will not help control the floods expected, Eng Nanayakkara has also decried the damage that would be caused to the existing landscape and vegetation by carrying out these “wasteful experiments”.
In the interests of members of the public who do not have a close acquaintance with this subject, the Citizens’ Movement for Good Governance (CIMOGG) sets out below, in simple layman’s language, the basic technical aspects of the construction of lakes for flood retention as there is a dire need to dampen the excessive expectations placed in these lakes by those who claim that such lakes will help to mitigate the high flood flows which affect some localities badly during heavy thunderstorms.
Almost every adult member of the public and even most children in their teens are aware that storms occur frequently during certain seasons of the year. Although the first storm of the rainy season could sometimes turn out be the worse than the ones that follow, the more general pattern is that some rainfall would be experienced over a number of days or a few weeks before the heaviest storms of the season occur. Where the soil is sandy or gravelly, the smaller rainfalls which precede the heavier storms would percolate fairly easily through the soil towards the nearest drainage channels, streams, rivers, lakes or the sea without raising ground water levels greatly or leaving an excess for surface runoff. However, in certain soils, especially those which one encounters in the lower reaches of valleys or other low-lying lands, the soil contains high proportions of clay, silt and decayed organic matter, all three of which tend to slow down the percolation of water, thus causing the water table to rise. Whereas the steepness of the topography at higher elevations facilitates the rapid outflow of surface water, leaving little room for flooding, a rapid build-up of water levels will be experienced in the bottoms of valleys and in low-lying land. Hence, one may expect that, after a few days of even moderate rainfall, the levels of water in any flood retention lakes and the ground water in the surrounding areas will rise simultaneously, with the result that the flood-absorbing capacity (“live storage”) left in the lakes to deal with the storms to follow is going to be minimal and, of course, far less than during the dry season.
The functioning of these flood retention lakes is not at all comparable to the effect of a dam built across a waterway, where excess runoff and percolation from the surrounding soils can be stored for a relatively long period, well above the dry season ground water levels, thus allowing time for the rains to decrease in intensity until the stored water could be released slowly thereafter for power generation or irrigation during the succeeding drier season. In short, if one were to excavate a lake during the dry season in low-lying land so as to act as a flood retention aid, one would only be fooling oneself because the early rains, coupled with the poor percolation characteristics in this type of situation, will result in a rapid loss of the available live storage capacity, which is essential to deal with big storm flows.
There is another point that also requires study. What does one do with the fine soils which are removed to form the ponds? If they are to be dumped at a higher elevation, one would have to sacrifice good high ground to save poor, low-lying land. How does one safeguard the dumped soil from liquefying into a dangerous slurry during heavy rain and flowing back into the nearest low-lying area? Growing grass, shrubs and trees on the surface will not always work because we know that areas of weak soils and landfills, even if covered by dense vegetation, are known to “landslide” in adverse weather conditions.
Did anybody carry out technically realistic calculations of the topography, precipitation, percolation, runoff, ground water levels, extreme storm runoffs etc and then compute the usable depths and areas of lakes required, and their probable live storage, not to mention the preparation of proper documentation and the calling of open, competitive tenders, before giving out the recent contracts? The answer would undoubtedly be a “No”.
CIMOGG has, for several years, been trying to persuade government institutions, before they initiate any kind of project, to furnish the public with timely basic information on (a) the details of the problem or need for which a solution is being sought, (b) the nature of the investigations carried out to help formulate a plan to solve the problem or to meet the identified need, (c) a comparison of the effectiveness and costs of alternative proposals studied, (d) an estimate of the time required for implementation, and so on. Whatever be the nature of the proposals put forward, there is every likelihood that there are well-experienced members of the public who would be qualified and want, as responsible citizens, to contribute to the further improvement of all basic proposals and, perhaps, even the abandonment of unworkable ones provided that the requisite information is made available to them.
Instead, what happens generally is that almost every scheme that involves the expenditure of large sums from the public purse is hatched surreptitiously and the relevant contracts awarded, through “professional” middlemen, to selected suppliers or contractors. Various excuses are offered to justify the need for secrecy and the hurried finalisation of contracts, which are often not in the public interest. CIMOGG continues to plead that politicians and bureaucrats keep foremost in their thoughts that they should be mindful of the fact that they hold positions requiring strong adherence to the doctrine of public trust, which means that they should not squander public funds and assets as they have been doing for the past four decades with inceasing impunity and a near-total lack of accountability.
Leaving aside any question of financial wrongdoing, about which CIMOGG has no specific information in relation to this latest civil engineering adventure, its position is that the construction of flood retention ponds was a matter that should have been put up for a thorough public discussion before implementation. Meanwhile, further construction should be brought to a halt.