Design Flood Revisions and Spillway Adequacy in Steep Tropical Catchments: A Multi-Dam Reassessment from Kerala, India

Kerala, on the windward side of the Western Ghats in southern India, receives about 3000 mm of annual rainfall under a tropical monsoon climate, driven by orographic south-west monsoon rainfall. The state has a high population density of about 859 persons per square kilometre and a limited geographical extent, with settlements concentrated in river valleys and downstream of reservoirs. These physiographic and socio-hydrological conditions make flood events critically important from both hydrological and societal perspectives. The catastrophic flood of 2018 further emphasized the need for an updated hydrological reassessment of existing dams and their spillway performance, and reservoir rule curves in Kerala.
Kerala has 53 large dams, of which 30 dams distributed across nine river basins are analysed in this study. The selected catchments are characterized by short hydrological response lengths and steep terrain, with longitudinal bed slopes ranging from 20 to 80 m km⁻¹. The Western Ghats rise sharply from near sea level to elevations of approximately 2500 m, promoting intense orographic rainfall, short travel times, and rapid runoff concentration. For the 30 dam catchments, the Time of Concentration (Tc) varies between 0.7 and 5 h, indicating fast-rising floods with minimal natural attenuation. Several catchments exhibit high hydrological response, with specific flood exceeding 13 m³ s⁻¹ km⁻². Most dams are located within 100 km of the Arabian Sea coastline and occur in serial or cascade arrangements along the same river valleys, a configuration that is hydrologically relevant for upstream–downstream flood interactions.
The study reassesses the Inflow Design Flood (IDF) and spillway adequacy of the selected dams. Of the 30 projects, 20 dams were completed before 1985, before the Bureau of Indian Standards (BIS) issued Indian Standard IS 11223:1985, which formally introduced IDF categories such as the Probable Maximum Flood (PMF), Standard Project Flood (SPF), and 100-year flood. In projects commissioned before 1985, spillway capacities were generally fixed using prevailing hydrological practices, limited storm data, and engineering judgment.
In the present reassessment, IDF estimation is carried out in accordance with BIS guidelines using a hydro-meteorological approach, and unit hydrograph parameters are derived from the Flood Estimation Report. Storm parameters are derived from the Probable Maximum Precipitation (PMP) Atlas for the West-Flowing Rivers of the Western Ghats, published by the India Meteorological Department (IMD) and the Central Water Commission (CWC), which compiles major historical storm events from 1905 to 2010. The revised design floods are compared with existing spillway capacities, and the analysis also examines relationships with Tc, gross storage, specific flood, and year of dam completion.
Results indicate that 26 out of the 30 dams show spillway inadequacy under the revised IDF. In several projects, design flood exceedance exceeds 200%, and in some cases, reaches more than 300%. Spillway inadequacy is more frequent in short-response catchments with lower Tc and higher specific flood values. This study offers a comparative hydrological perspective for steep tropical catchments in Kerala. It may support an informed, evidence-based reassessment of existing dams using updated datasets and contemporary analytical practices for prioritization of dam safety.