Hydrodynamic Simulation and Flood Inundation Analysis for Framing Robust Flood Management Strategies: Insights from the 2018 Kerala Floods

Ernakulam district in Kerala, India, has experienced frequent flooding in recent years due to a combination of natural and human-induced factors. Heavy monsoon rainfall often overwhelms the district’s drainage systems, resulting in widespread flooding. The low-lying terrain, with many areas below sea level, further exacerbates the issue. The district’s coastal location exposes it to storm surges, tidal flooding, and sea-level rise. High sea levels and storm surges can physically block rivers and streams from discharging water into the ocean, compounding the flooding problem. Rapid urbanization and infrastructure development have significantly altered the district’s landscape. The construction of buildings, roads, and other structures has obstructed natural drainage channels, while deforestation and land-use changes, such as converting wetlands and paddy fields into residential or commercial areas, have diminished natural flood buffers. Additionally, poorly maintained or clogged drainage systems hinder efficient water flow. Climate change is projected to increase the frequency and intensity of extreme weather events, including heavy rainfall, making the district even more vulnerable to future flooding.

The 2018 Kerala floods severely affected Ernakulam district, triggered by heavy rainfall, dam releases, and other factors. To analyze the flood inundation dynamics, a hydrodynamic simulation was conducted using the HEC-RAS software developed by the US Army Corps of Engineers’ Hydrologic Engineering Center (HEC). The study focused on a segment of the Periyar River Basin between Kalady and Mangalapuzha. The simulation incorporated the basin’s physical, hydrological, and operational attributes, such as inflow sources, tributaries, seasonal flow patterns influenced by monsoon rainfall, and the generation of a Digital Elevation Model (DEM) for delineating the watershed and river network. Hydrodynamic models are based on the numerical integration of momentum and mass conservation equations, describing the physical processes in the basin (World Meteorological Organization, 2009). These models, such as HEC-RAS, are powerful tools for predicting water levels, current velocities, waves, and sediment transport, particularly in regions with sparse field measurements. Using the Saint-Venant equations, the HEC-RAS model accounts for factors like travel time between two points along the river, slope, cross-section, water flow, and dynamic velocity. The equations are solved using the four-point implicit box finite difference scheme to estimate discharge and water surface elevation at specific points. Observed rainfall and discharge data from peak flood events during the 2018 monsoon were used for the simulations. On July 16, 2018, the peak discharge at the Kalady station (upstream) was recorded at 5107.89 m³/s. The downstream station at Mangalapuzha, located approximately 22 km away, also observed significant discharge levels. A key finding from the flood simulation was the complete inundation of the Cochin International Airport (CIAL), situated on the outer banks of the river. The airport’s runway, aligned roughly parallel to the river, was submerged during the flooding. The recurrence of similar rainfall events, coupled with flood-induced river discharges, poses a persistent threat to critical infrastructure such as CIAL. Hence, the Government of Kerala must develop and implement effective flood mitigation strategies to minimize future risks and damages.