New Dam Break Risk Assessment Method in Fuzzy Framework

Dams are useful for mitigation of floods, and at the same time there is a risk of dam breach or failure from floods, apart from seismic hazards and factors such as ageing of dam material. In recent decades, there is an alarming increase in dam breach events. This has drawn the attention of hydrologists to have a relook at methodologies being considered for dam risk analysis. Effective risk analysis requires accounting for both failure probability of dam and dam break consequences. There are numerous factors which effect the consequences, and there is considerable amount of uncertainty, vagueness and ambiguity among them due to lack of data and knowledge. To address this, we propose a new dam break risk assessment method in fuzzy framework. It considers fuzzy hierarchical model for risk assessment based on combination of static and variable fuzzy set theory. A hierarchical structure is devised for various factors influencing dam break consequences. Furthermore, weights are assigned to the factors using Fuzzy Analytical Hierarchy Process (FAHP). Thereafter, weighted information of different factors is comprehended to arrive at estimate of a risk index. The effectiveness of proposed method is demonstrated through case study on Hemavathi dam located in upper reaches of Cauvery River basin, India. It is a composite dam with masonary spillway and earthen flanges. The catchment area of river up to the dam site is 2904 sq. Km. Furthermore, height of dam above riverbed level is 44.5 m, and its gross storage capacity is 1047 Mm³. As per Bureau of Indian Standards (BIS) the dam is classified as large dam and therefore qualifies for Probable Maximum Flood (PMF) as design flood. Breach analysis of Hemavathi dam was performed using 1D-2D coupled HEC- RAS model to map the extent of flooding downstream of the dam using PMF (corresponding to 2-day PMP) as inflow and maintaining initial pool level in reservoir at maximum water level (MWL). For comprehensive risk assessment, life loss, economic loss, and social and environmental influence caused by dam break are considered in the model.