Challenges in dam safety and extreme rainfall estimation in relation to Sri Lanka

Abstract

Ensuring the safety of our dams in a changing global climate is of paramount importance to Sri Lanka. The devastating floods of January – February 2011 caused an enormous Rs. 66 billion in damages which included a massive Rs. 13 billion loss in the rice crop alone. A million people were rendered homeless and 62 lives were lost. Many major reservoirs spilled, while a few medium sized and several hundred minor tanks breached. Batticaloa experienced record rainfall, while Senanayake Samudra the country’s largest reservoir was spilling after 16 years. Sri Lanka has over 300 large and medium sized dams. These could experience overtopping due to extreme rainfall, the consequences of which would be disastrous. Hence there is a necessity to regularly assess the adequacy of present spillways using detailed hydrological studies which include the estimation of probable maximum precipitation (PMP) and probable maximum flood (PMF). The estimation of extreme rainfall is a highly controversial issue but nonetheless critical in relation to dam safety. While overestimation of extreme values would be uneconomical, underestimation could be disastrous. Thus arriving at realistic estimates while being aware of inherent risk and uncertainty, is a challenge in hydrologic design. The paper presents 24-hour point PMP values for several meteorological stations in Sri Lanka, using the hydro-meteorological and statistical procedures. The work which is based on extensive studies done by the authors during the past several years, and published both locally and internationally, could be effectively used in current and future dam safety studies.