{"id":244980,"date":"2025-12-20T15:04:27","date_gmt":"2025-12-20T09:34:27","guid":{"rendered":"https:\/\/www.colombotelegraph.com\/?p=244980"},"modified":"2025-12-29T01:05:37","modified_gmt":"2025-12-28T19:35:37","slug":"why-sri-lankas-major-reservoirs-cannot-prevent-floods-during-a-severe-rainfall-event","status":"publish","type":"post","link":"https:\/\/www.colombotelegraph.com\/index.php\/why-sri-lankas-major-reservoirs-cannot-prevent-floods-during-a-severe-rainfall-event\/","title":{"rendered":"Why Sri Lanka\u2019s Major Reservoirs Cannot Prevent Floods During A Severe Rainfall Event"},"content":{"rendered":"

By Thushara Dissanayake<\/a> –<\/b><\/span><\/p>\n

\"\"

Eng. Thushara Dissanayake<\/p><\/div>\n

Our recent experience with the massive disaster brought by Cyclone Ditwah<\/a><\/span> clearly indicated how vulnerable the country is to an extreme rainfall event. The impacts fell mainly into two categories: flooding across the major river basins and numerous landslides in the hill country. Many major river basins, including the Mahaweli, Kelani, Kalu, Maha, Attanagalu, and Deduduru, overflowed, affecting thousands of households and many cities and suburbs. Many criticisms were directed at the organisations responsible for reservoir management regarding how spill gates were operated during the event. While flood events are not new to the country, the question is whether we can cope with a similar or even more severe event in the future, especially given the finite reservoir storage available. This article discusses why Sri Lanka\u2019s major reservoirs cannot prevent floods during severe rainfall.<\/p>\n

Reservoirs are not designed primarily for flood control<\/b><\/span><\/p>\n

Sri Lanka\u2019s multipurpose reservoirs, mainly built by the Accelerated Mahaweli Development Program, and other irrigation reservoirs have not been designed as flood control reservoirs, though occasionally they perform that task on a minor scale. For their primary functions, irrigation and hydropower generation, the storage levels are maintained as much as possible by capturing rainwater to guarantee the supply of water adequately for the intended purposes. Thus, these reservoirs will accommodate sudden floodwater only if they are below the full supply levels at the time of flood. On the contrary, if the authorities receive accurate forecasts about the time and quantity of expected rainfall, there is a chance of lowering these reservoirs by releasing water before the event. However, they will reach their full capacity quickly, forcing spill releases to protect the dam if the rainfall event is severe. Hence, our reservoirs cannot fully prevent river floods in extreme events, unlike dedicated flood control dams available in countries such as Japan.<\/p>\n

Extreme rainfall volumes far exceed reservoir storage<\/b><\/span><\/p>\n

Extreme rainfall events produce catchment runoffs that far exceed reservoir storage itself. For instance, the recent Ditwah cyclone generated runoff flows in Deduru Oya, which is equivalent to several times of Deduru Oya reservoir storage. Hence, reservoirs can capture only a fraction of floodwater even if they are fully empty at the onset of the event. In other words, reducing the water levels of reservoirs before a severe flood event has no significant advantage.<\/p>\n

Multiple tributaries create runoff that reservoirs cannot regulate<\/b><\/span><\/p>\n

Owing to the topography, many major rivers of Sri Lanka consist of several tributaries with larger catchments. During an extreme event where the whole basin gets equally extreme rainfalls, these tributaries generate runoff in parallel. Many major reservoirs are located at places upstream of the connecting points of certain tributaries with the main river in certain river basins. As a result, runoff of certain tributaries flows bypassing the reservoir. For instance, all the major reservoirs in the Mahaweli system, namely Kothmale, Victoria, Randenigala, and Moragahakanda, are situated in the upstream catchment. Hence, runoff of other tributaries of the Mahaweli, such as the Uma Oya, Badulu Oya, and\u00a0 <\/span>Loggal Oya, bypasses these reservoirs. Consequently, low-lying areas in the downstream, Mahiyanganaya and Polonnaruwa, have a higher chance of getting flooded by extreme runoffs of these tributaries if severe rainfall is received by their catchments.<\/p>\n

Not all the river basins have reservoirs<\/b><\/span><\/p>\n

Many major river basins like Kelani, Attanagalu, Kalu, Gin, and Nilwala are not equipped with major reservoirs at all. The Kelani River has only minor reservoirs like Castlereagh and Maussakelle in the upstream, designed for hydropower rather than flood control. Therefore, heavy runoff in the Kelani always leads to flooding in Colombo and the suburbs, and only the flood bunds can control the flood risk to several areas. A similar situation can be observed in the Kalu, Gin, Nilwala, and Attanagalu basins as well.<\/p>\n

The need to take urgent action on dam safety<\/b><\/span><\/p>\n

Extreme rainfall events can lead to dam overtopping unless spill gates are operated in time. A dam failure would cause a massive disaster due to the sudden release of a large volume of water at a higher velocity.\u00a0 <\/span>As a result, a destructive flood wave will be generated, which is capable of sweeping away buildings, bridges, and other properties adjacent to the river, with heavy land erosion. Hence, it goes without saying that the damage would be manyfold compared with a flood event. Further, the disruption of the agricultural and power generation activities that depended upon the reservoir will add to the total economic loss in the long run.<\/p>\n

Hence, reservoir operators have to take action with utmost care in order to protect the dam. They must monitor catchment rainfall and consequent inflow to the reservoir. If the inflow is too high, they must release water irrespective of the fact that the downstream area has a flood risk. The only thing they can do is to give a timely warning to downstream residents to either evacuate or follow suit. Most of the time, these are not individual decisions but instructions given in reservoir operation guidelines by the respective authority. The reservoir operators have to follow these instructions to ensure the safety of the dam.<\/p>\n

Climate change is causing high-intensity rainfalls<\/b><\/span><\/p>\n

So far, Sri Lanka has dealt with monsoonal rains with random occurrences of low-pressure regions in the Bay of Bengal, resulting in heavy downfalls and, in some cases, cyclones.\u00a0 <\/span>However, with climate change, rainfall events of 200-400 mm within a day are likely. It is high time that we analyze possible rainfall intensities in the future with the help of climate change models to strengthen disaster preparedness and mitigation activities. Therefore, operating our major reservoirs based on such data will have to be reconsidered to utilize their potential in flood mitigation, if possible.<\/p>\n

Conclusion<\/b><\/span><\/p>\n

Sri Lanka\u2019s major reservoirs have been designed primarily for irrigation and hydropower generation. consequently, their capacity to mitigate floods during extreme rainfalls is inherently limited. Flood mitigation must therefore adopt different approaches, including the construction of dry dams and flood bunds, the development of efficient and reliable early-warning systems, and public awareness and preparedness programmes. Accordingly, flood risk reduction needs an integrated, multi-faceted strategy that goes beyond existing reservoir operations, which incorporates both structural and non-structural measures.<\/p>\n

*The writer is a chartered Civil Engineer specializing in water resources engineering<\/b><\/em><\/span><\/p>\n","protected":false},"excerpt":{"rendered":"

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